Metallurgical and corrosion characterization of a ...

UNLV Retrospective Theses & Dissertations

1-1-2005

Metallurgical and corrosion characterization of a martensitic Metallurgical and corrosion characterization of a martensitic

stainless steel as a function of silicon content stainless steel as a function of silicon content

Harish Krishnamurthy University of Nevada, Las Vegas

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METALLURGICAL AND CORROSION CHARACTERIZATION OF A

MARTENSrriC STAINLESS STEEL AS A FUNCTION OF

SILICON CONTENT

by

Harish Krishnamurthy

Bachelor of Engineering in Mechanical Engineering University of Madras, India

May 2003

A thesis submitted in partial fulfillment of the requirements for the

Master of Science Degree in Mechanical Engineering Department of Mechanical Engineering

Howard R. Hughes College of Engineering

Graduate College University of Nevada, Las Vegas

December 2005

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

UMI Number: 1435611

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uNiy Thesis ApprovalThe Graduate College University of Nevada, Las Vegas

N nvom hor 1 7. 200 5

The Thesis prepared by

H a r is h K rish n a m u rth y

Entitled

Metallurgical and Corrosion Characterization of a Martensitic Stainless

S t e e l a s a F u n c t i o n n f S i l i c o n C o n t e n t

is approved in partial fulfillment of the requirements for the degree of

M a ste r o f S c ie n c e in M e ch a n ica l E n g in e e r in g __________

Examination Committee Member

Examination Committee Membe.

\L VGraduattCollkge Faculty Representative

Examirmion Committee Chair

Dean of the Graduate College

11


ABSTRACT

Metallurgical and Corrosion Characterization of a Martensitic Stainless Steel as afunction of Silicon Content

by


Dr. Ajit K. Roy, Examination Committee chair Associate Professor of Mechanical Engineering

University of Nevada, Las Vegas

T91 grade steels containing 1 and 2 weight percent (wt %) silicon (Si), custom-melted

by vacuum-induction melting practice, were tested for evaluation of their metallurgical

and corrosion properties. The results of tensile testing indicate that both the yield strength

and the ultimate tensile strength were gradually reduced with increasing temperature

(ambient to 550°C. For material containing 2 wt % S i , the failure strain was significantly

enhanced at 550°C, indicating enhanced ductility. No failures were observed in an acidic

solution at constant-load irrespective of their Si content. The magnitude of all parameters

obtained from the slow-strain-rate testing was gradually reduced with increasing

temperature. However, T91 grade steel containg 2 wt % Si showed better SCC resistance

in terms of its ductility possibly due to the formation of more protective oxide layers due

to higher Si content. In general, the corrosion potential became more active at higher

temperatures. T91 grade steel containing 1 wt % Si showed relatively more brittle failures

compared to the higher Si-containing alloy.

Ill


TABLE OF CONTENTS

ABSTRACT............................................................................................................................ iii

LIST OF TABLES..................................................................................................................vi

LIST OF FIGURES................................................................................................................vii

ACKNOWLEDGEMENTS..................................................................................................viii

CHAPTER I INTRODUCTION....................................................................................1

CHAPTER 2 MATERIALS AND TEST SPECIMEN...................................................82.1 Test Materials............................................................................................................... 82.2 Test Specimens........................................................................................................... 102.3 Test Environment.......................................................................................................12

CHAPTER 3 EXPERIMENTAL PROCEDURES.......................................................143.1 Tensile Testing........................................................................................................... 143.2 Stress Corrosion Cracking Tests at Constant-Load................................................. 173.3 Slow-Strain-Rate SCC Testing..................................................................................193.4 Cyclic Potentiodynamic Polarization Testing......................................................... 253.5 Metallographic Evaluation by Optical Microscopy................................................. 283.6 Fractographic Evaluation by Scanning Electron Microscopy................................29

CHAPTER 4 RESULTS....................... ......................................................................... 314.1 Metallographic Evaluations....................................................................................... 314.2 Tensile Properties Evaluations..................................................................................334.3 SCC Evaluation at Constant-Load............................................................................ 384.4 SCC Evaluation under SSR Condition..................................................................... 394.5 Localized Corrosion Evaluation............................................................................... 444.8 Fractographic Evaluations......................................................................................... 46

CHAPTER 5 DISCUSSION..........................................................................................54

CHAPTER 6 SUMMARY AND CONCLUSIONS..................................................... 58

CHAPTER 7 SUGGESTED FUTURE WORK........................................................... 60

APPENDIX A OPTICAL MIROSCOPY DATA..........................................................61

IV


APPENDIX B LOCALIZED CORROSION DATA....................................................64

APPENDIX C SCANNING ELECTRON MICROSCOPY DATA................................... 69

BIBLIOGRAPHY..................................................................................................................83

VITA........................................................................................................................................86


LIST OF TABLES

Table 2.1 Tensile Properties of Modified 9Cr-IMo Steel...................................................9Table 2.2 Chemical Compositions of Four Heats of T91 Grade Steel (wt % )..................10Table 2.3 Chemical Composition of Test Solution (gm/liter).......................................... 13Table 4.1 Results of Tensile Testing..................................................................................36Table 4.2 Results of CL Testing.........................................................................................39Table 4.3 Results of Slow-Strain-Rate Technique............................................................41Table 4.4 Results of CPP Testing...................................................................................... 46

VI


LIST OF FIGURES

Figure LI Overall Transmutation Process......................................................................... 2Figure 2.1 Configuration of the Smooth Cylindrical Tensile Specimen.................. .....11Figure 2.1 Configuration of the Polarization Specimen..................................................12Figure 3.1 MTS Test Setup................................................................................................15Figure 3.2 Instron Test Setup.......................................................... 16Figure 3.3 Constant-load Test Setup.................................................................................18Figure 3.4 A T ypical Calibration Curve for a Proof Ring...............................................19Figure 3.5 CERT Machines for SSR Testing...................................................................20Figure 3.6 SSR Test Setup................................................................................................ 21Figure 3.7 Load Frame Compliance Test......................................................................... 22Figure 3.8 CPP Test Setup................................................................................................ 26Figure 3.9 Luggin Probe Arrangement............................................................................ 27Figure 3.10 Standard Potentiodynamic Polarization Plot.................................................27Figure 3.II Leica Optical Microscope............................................................................... 29Figure 3.12 Scanning Electron Microscope.........................................................................30Figure 4.1 Optical Micrograph of T91 grade steel containing 1% S i.............................32Figure 4.2 Optical Micrograph of T91 grade steel containing 2% S i.............................32Figure 4.3 s-e Diagram vs. Temperature................................................... 34Figure 4.4 s-e Diagram vs. Temperature............................................................................34Figure 4.5 YS versus Temperature....................................................... 36Figure 4.6 UTS versus Temperature............................................................................... 37Figure 4.7 %E1 versus Temperature..................................................................................37Figure 4.8 %RA versus Temperature.............................................................................. 38Figure 4.9 s-e Diagram versus Temperature...................................................................40Figure 4.10 s-e Diagram versus Temperature................................................................... 40Figure4.II %E1 versus Temperature...................................................................................42Figure 4.12 %RA versus Temperature.............................................................................. 42Figure 4.13 TTF versus Temperature.......................................... 43Figure 4.14 Of versus Temperature.................................................................................... 43Figure 4.15 CPP Diagram of T91 Grade Steel Containing I wt % S i ..............................44Figure 4.16 CPP Diagram of T91 Grade Steel containing 2 wt % Si.............................. 45Figure 4.17 SEM Micrograph of T91 Grade Steel (1% Si), 500X...................................47Figure 4.18 SEM Micrograph of T91 Grade Steel (2% Si), 500X................. 48Figure 4.19 SEM Micrograph of T91 Grade Steel (1% Si) at RT, 500X.........................50Figure 4.20 SEM Micrograph of T91 Grade Steel (1% Si) at 60°C,.500X......................50Figure 4.21 SEM Micrograph of T91 Grade Steel (1% Si) at 90°C, 500X......................51Figure 4.22 SEM Micrograph of T91 Grade Steel (2% Si) at RT, 500X.........................51Figure 4.23 SEM Micrograph of T91 Grade Steel (2% Si) at 60°C, 500X......................52Figure 4.24 SEM Micrograph of T91 Grade Steel (2% Si) at 90°C, 500X......................52

vn


ACKNOWLEDGEMENTS

This work was performed under the able guidance of my advisor, Dr. Ajit K. Roy. It

was my privilege and pleasure to work with him on this project and I would like to

express my deepest gratitude to him for his contributions.

I would like to thank Dr. Anthony E. Hechanova, Dr. Samir Moujaes and

Dr.Venkatesan Muthukumar for their valuable suggestions and support throughout this

investigation. I would like to acknowledge the help extended by my friends, especially

Debajyoti Maitra, Pankaj Kumar, Subhra Bandyopadhyay, Joydeep Pal, Lalitkumar

Savalia, Venkataramakrishnan Selvaraj and Vikram Marthandam at Materials

Performance Laboratory, UNLV.

I would also like to express my appreciation to my parents, Mr. N. Krishnamurthy

and Mrs. Rajivi Krishnamurthy for their persistent support and the confidence they had

in me. Their encouragement and support always kept me going. Finally I would like to

acknowledge the financial support of United States Department of Energy (USDOE).

Vlll


CHAPTER 1

INTRODUCTION

The Yucca Mountain site, located ninety five files northwest of Las Vegas, Nevada

has recently been proposed to be the nation’s first geologic repository to contain

approximately 77,000 metric tonnes of spent nuclear fuel (SNF) and high-level

radioactive waste (HLW) without any reprocessing. [1] However, the amount of

SNF/HLW is expected to increase in the near future resulting from spent fuel to be

generated from the existing nuclear power plants. SNF contains several unstable nuclei in

sufficient quantities to render them radioactive for a prolonged time period. In view of

this rationale, the geologic repository at the Yucca Mountain site is being designed to

dispose of SNF/HLW for a period of million years. In order to circumvent the drawback

associated with the long-term disposal of future nuclear waste, the United States

Department of Energy (USDOE) has been considering an alternate method known as

transmutation to reduce the radioactivity of SNF for prospective disposal in the geologic

repository for shorter durations.

Transmutation refers to the transformation of minor actinides and fission products

from SNF by a process, in which the nucleus of an atom changes due to natural

radioactive decay, nuclear fission, nuclear capture or other related processes. Thus, this

process results in the transformation of long-lived isotopes to species with short half-lives


and reduced radioactivity. This process involves bombarding SNF with neutrons

generated by projecting protons from an accelerator or a reactor onto a target material

such as molten lead-bismuth-eutectic (LBE), which can also act as a blanket coolant to

minimize the heat generated during this process. The transmutation process is illustrated

in Figure 1.1. The molten LBE will be contained inside a vessel made of a suitable

material such as a martensitic stainless steel, which is also referred to as target structural

material.

AcceleratorProtons

< Transmutation system s >'——i-r —

% Spallation neutron target

High-level radio-active wastes neutrons

Without transmutation

w .

Stable nuclides.Long-lived nuclides Nuclear fission short-lived nuclides

ConbAm^

Geok^cal(isposal

Radiotoxicity reduced to 1/200

«Geologicaldisposal

Figure 1 .1 - Overall Transmutation Process

Since, the molten LBE will be contained in a vessel made of a suitable structural

material, the metallurgical characteristics of the structural material and the surrounding

environment will play significant roles in influencing the performance of these alloys.


Thus, an environment-assisted degradation such as liquid metal embrittlement will

influence the structural stability of the containment material. This type of degradation is

significantly different from the environment-assisted degradations such as stress

corrosion cracking, hydrogen embrittlement and localized corrosion in the presence of an

aqueous solution. For liquid metal embrittlement to occur, the interatomic bond on the

metal surface will be significantly impaired due to the decohesion of the protective

surface layers of the structural material. The extent of degradation in the presence of

molten metal will be dependent on the protectiveness of these layers. It was also found

that the addition of controlled amounts of oxygen to LBE can suppress the corrosion of

engineering materials by LBE. The beneficial effect of oxygen can be attributed to the

formation of a more protective and adherent oxide layer on the metal surface, thus

reducing the extent of degradation due to the presence of molten metal. Thus, the

selection of a suitable material having resistance to the penetration of liquid metal

through this protective layer will provide sufficient resistance to degradation due to the

molten LBE. [2]

Since the target structural material will be subjected to the presence of a molten metal

such as LBE, a research project was initiated to evaluate the corrosion behavior of a few

candidate structural materials in the presence of both molten LBE and aqueous solutions

of varying pH. The corrosion studies in molten LBE was planned to be performed at the

Los Alamos National Laboratory (LANL) using its LBE loop. However, this study could

not be performed at LANL due to some scheduling and technical problems associated

with the presence of high oxygen content in the molten LBE loop. Instead, extensive

corrosion studies have been performed in aqueous solutions of different pH values at


ambient and elevated temperatures involving three martensitic alloys, namely Alloy EP-

823, Alloy HT-9 and Type 422 stainless steel. The results of metallurgical and corrosion

studies performed by previous investigators [3-9] at UNLV involving all three alloys

have been presented and published earlier.

The susceptibility of Alloys EP-823, HT-9 and 422 to stress corrosion cracking

(SCC) has most recently been determined at UNLV in neutral and acidic aqueous

solutions at ambient and elevated temperatures using numerous state-of-the-art

experimental techniques. The SCC behavior of all three alloys has been determined using

both constant load (CL) and slow-strain-rate (SSR) testing techniques. Further, the

localized corrosion behavior of these alloys has been evaluated by electrochemical cyclic

potentiodynamic polarization (CPP) technique. The effect of applied potentials on their

cracking susceptibility has also been investigated in previous studies. The overall data

based on the most recent investigations [3, 8, 9] indicate that Alloy EP-823 exhibited the

best metallurgical and corrosion performance. The superior performance of Alloy EP-823

has been attributed to the presence of sufficiently high silicon (Si) content [10] compared

to the nominal Si content in other two martensitic alloys. A similar beneficial effect of

high Si content on the corrosion resistance of martensitic stainless steel containing 10-12

weight percent (wt %) chromium (Cr) has been reported elsewhere. [11]

It is well known that the presence of high Si content in steels can promote the

formation of protective oxide layers such as silicon dioxide (Si02) [12] which can

minimize or prevent the dissolution of a structural material to be used as a containment

material during the transmutation process. Martensitic chromium-molybdenum (Cr-Mo)

alloys have been extensively used in nuclear applications. [13] The Cr-Mo steels have


been modified through addition of Niobium (Nb) and Vanadium (V) to enhance their

corrosion resistance and metallurgical stability at temperatures relevant to the nuclear

operations. These types of martensitic alloys are commonly referred to as T91 grade

stainless steels. Since the beneficial effect of Si has been demonstrated in previous

investigations [14], the conventional T91 grade steel has been modified in the current

investigation by adding Si of different concentrations.

Martensitic T91 grade steels containing high Cr have been used in core components

of fast breeder reactors as well as for fusion reactor structures. These fully-martensitic

materials are very promising candidates due to their high stability of physical and

mechanical properties under irradiation as well as under thermal aging. The presence of

high Cr content in these steels provides sufficiently high strength at elevated

temperatures, low thermal stress and low corrosion rate in the presence of a liquid metal.

In view of these superior properties, martensitic stainless steels containing 9-13 wt % Cr

and 1% Mo are being considered as structural materials to contain high power spallation

targets [15-16] such as molten LBE.

The conventional 9Cr-lMo steels have recently been modified through addition of Nb

and V to enhance its strength, thermal conductivity and corrosion resistance, and to

reduce the thermal expansion. This modified grade of Cr-Mo stainless steel, commonly

known as the T91 grade steel, is currently being proposed for both the window separating

the accelerator area from the liquid metal spallation target and the target container in the

accelerator driven systems (ADS). Because of the high hardenability, the tempered

martensitic microstructure of the T91 grade steel can provide excellent strength and

toughness properties, thus, making this steel a suitable candidate for use in the


temperature regime of 400-600°C, which is similar to the operating temperature range

during the transmutation process. The T91 grade steel has also been known to provide

superior welding characteristics. [17-21] Based on these rationales, T91 grade steel has

been selected as the primary material for the liquid metal container of the target for the

megawatt pilot experiment (MEGAPIE) [22] to be performed at the Swiss Spallation

Neutron Source (SINQ).

In view of the superior performance of high Si containing Cr-Mo steel, discussed

earlier, experimental heats containing four different levels of Si content have been melted

by a vacuum-induction-melting practice. These experimental heats contained 0.5, 1.0, 1.5

and 2.0 wt% of Si in addition to approximately 9% Cr and 1% Mo. Further, Nb and V

have been added to these experimental heats so that the basic compositions of these

alloys were very similar to that of the martensitic T91 grade steel. This investigation is

aimed at evaluating the effect of 1 and 2 wt% Si content on the metallurgical and

corrosion behavior of T91 grade steel, custom melted specifically for this study.

Since the candidate target structural material will be subjected to an operating

temperature ranging between 400 and 550°C during the transmutation process,

experimental heats of T91 grade steel containing 1 and 2 wt % Si have been tested for

tensile properties evaluation in this temperature regime. The corrosion behavior of these

alloys in the presence of molten LBE is yet to be evaluated either at UNLV or LANL

depending on the availability of proper testing facilities. Meanwhile, extensive corrosion

studies have been performed in an acidic aqueous solution at ambient and elevated

temperatures using state-of-the-art experimental techniques. The susceptibility of these

alloys to SCC has been determined under CL and SSR conditions as a function of the Si


content. The localized corrosion behavior of both heats of the T91 grade material has

been investigated in a similar environment by electrochemical CPP technique. The

metallographic evaluations of the test materials have been performed by optical

microscopy (OM) to determine their metallurgical microstructures. The extent and

morphology of failure in specimens used in tensile and SCC testing have been

determined by scanning electron microscopy (SEM). The comprehensive test results are

presented in this thesis.


CHAPTER 2

MATERIALS AND TEST SPECIMENS

2.1 Test Materials

The materials tested in this investigation were modified 9Cr-lMo stainless steels

having variable Si content. Modified 9Cr-lMo is commonly known as T91 grade steel.

As mentioned in the previous section, the presence of high Si content has proved to be

beneficial both from the metallurgical and corrosion points of view for numerous energy

applications. The basic 9Cr-lMo steel was originally developed for the oil industry to

provide superior oxidation resistance compared to that of 2.25 Cr-lMo steel. This type of

Cr-Mo steel has been known to perform satisfactorily due to its stable microstructure,

high oxidation and sulphidation resistance, high thermal conductivity and low thermal

expansion compared to the conventional austenitic stainless steels. In order to enhance

the elevated temperature tensile properties of conventional 9Cr-lMo steel, Nb and V

have been added to provide high tensile and creep strength. The typical tensile properties

of modified 9Cr-IMo steel are given in Table 2.1. [23]


Table 2.1 Tensile Properties of Modified 9Cr-lMo Steel

Ultimate Tensile Strength 85 ksi, (586 MPa), (minimum)Yield Strength, 60 ksi, (414 MPa), (minimum)% Elongation 20, (minimum)% Reduction in Area 40, (minimum)Hardness 248 Hb, (minimum)

The experimental heats of modified 9Cr-lMo (T91 grade) steel containing four

different levels of Si were melted by a vacuum-induction melting (VIM) practice. They

were subsequently processed into flat products of different dimensions. These processes

included forging and hot rolling, followed by proper thermal-treatment. They were

austenitized at 1850°F (1110°C) for one hour, followed by an oil-quench, which resulted

in the formation of hard but brittle martensite. In order to enhance the ductility of this

martensitic microstructure, the austenitized materials were further tempered at 1I50°F

(621°C) for one hour, followed by air-cooling. The purpose of quenching and tempering

of all four heats of steel was to achieve a fully-tempered and fine-grained martensitic

microstructure without the formation of any retained austenite. The chemical

compositions of all four VIM heats are given in Table 2.2. This investigation was focused

on the characterization of metallurgical and corrosion properties of only two heats

containing 1 and 2 wt % Si (heat number 2404 and 2406, respectively).


Table 2.2. Chemical Compositions of Four Heats of T91 Grade Steel.

Material/ Heat No.

Elements (wt %)

c Mn P S Si Ni Cr Mo A1 V Cb N(ppm) Fe

T91/2403 .12 .44 .004 .003 .48 .30 9.38 1.03 .024 .23 .91 570 Bal

T91/2404 .12 .45 .004 .003 .02 .30 9.61 1.03 .025 .24 .89 529 Bal

T91/2405 .11 .45 .004 .004 .55 .31 9.66 1.02 .024 .24 .085 485 Bal

T91/2406 .11 .45 .004 .004 .88 .31 9.57 1.01 .029 .24 .087 302 Bal

Bal: Balance

2.2. Test Specimens

Smooth cylindrical specimens having 4-inch (101.4 mm) overall length, 1-inch (25.4

mm) gage length and 0.25-inch ( 6.35 mm) gage diameter of T91 grade steels containing

1 and 2 wt % Si were machined from the heat-treated flat bars in such a way that the gage

section was parallel to the longitudinal rolling direction. A gage length (1) to gage

diameter (d) ratio (1/d) of 4 was maintained for the cylindrical specimens according to the

ASTM Designation E 8. [24]. The pictorial view and the detailed dimensions of the

cylindrical specimens used in the tensile and SCC testing are shown in Figure 2.1. The

specimens tested for the evaluation of localized corrosion susceptibility by the CPP

technique were smaller cylindrical specimens, shown in Figure 2.2.

10


(a) Pictorial View

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• a 4 3 6 l â g gtwapoiifti

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BOTH EMUS (OPnONAL)

L a b e l b e r th e n d s o f t h e sp e c im e n o c c o rd in o t o a t t a c h e d s p e c if i c a t i o n

D raw C y lin d ric a l T h r e a d e d Dob B one T e n s ile Specim en

UNLVH r. Bov

(b) Schematic Diagram

Figure 2.1 Configuration of the Tensile Specimen

11


(a) Pictorial View

A

(b) Schematic Diagram

Figure 2.2 Configuration of the Polarization Specimen

2.3. Test Environment

As mentioned in the preceding section, martensitic stainless steels containing Cr and

Mo have been suggested to be the primary structural materials to contain the molten

LBE, which will act as a target material during the transmutation process. The operating

temperature for the transmutation process may range between 400 and 550°C. Therefore,

the use of modified 9Cr-lMo steel is justified for application in this temperature regime

12


due to the presence of Nb and V. It should, however, be noted that very little work has so

far been performed in the United States in the presence of molten LBE involving T91

grade steel. Even though the current investigation was aimed at evaluating the

metallurgical and corrosion characteristics of T91 grade steel in the presence of molten

LBE, such facilities are yet to be developed at UNLV. While facilities to perform the

LBE testing are being designed, the susceptibility of both heats of T91 grade steel to SCC

and localized corrosion has been evaluated in an acidic solution having a pH in the

vicinity of 2.0. The chemical composition of the tested environment is given in Table 2.3.

Both SCC and localized corrosion studies have been performed in this environment at

ambient temperature, 60°C and 90°C.

Table 2.3. Chemical Composition of the Tested Solution (gm/liter)

Environment(pH) CaClz K2SO4 MgS04 NaCl NaNOs Na2SÛ4

Acidic (2.0-2.2) 2.77 7.58 4.95 39.97 31.53 56.74

13


CHAPTER 3

EXPERIMENTAL PROCEDURES

As indicated earlier in this thesis, the purpose of this investigation was to evaluate the

tensile properties of T91 grade material martensitic steels containing 1 and 2 wt %

Silicon (Si) at temperatures ranging from ambient to 550°C. The susceptibility of both

materials to stress corrosion cracking (SCC) has been determined under both constant

load CL) and slow-strain-rate (SSR) conditions in an acidic solution at ambient and

elevated temperatures. The localized corrosion behavior was studied by an

electrochemical method known as cyclic potentiodynamic polarization (CPP). In

addition, optical microscopy (OM) and scanning electron microscopy (SEM) were used

for metallographic and fractographic evaluations, respectively.

3.1 Tensile Testing

The tensile properties of both heats of T91 grade steel were evaluated at ambient and

elevated temperatures using an axial/torsional servo hydraulic MTS machine and a

screw-driven INSTRON equipment, shown in Figures 3.1 and 3.2, respectively. The

tensile properties including the yield strength (YS), ultimate tensile strength (UTS),

percent elongation (%E1) and percent reduction in area (%RA) were evaluated according

to the ASTM Designation E 8-04 [24] involving smooth cylindrical specimens of both

materials. A minimum of two specimens were tested at a strain rate of 10' ̂per second at

14


each temperature and an average value of each tensile parameter was determined. The

experimental data such as the load, time and extensometer reading were recorded in the

data file at the rate of 100 per second. The engineering stress versus engineering strain (s-

e) diagrams were developed using these data. The magnitude of YS was determined by

the point of intersection of a line drawn parallel to the linear portion of this curve at a

strain offset value equivalent to 0.2 % of the strain. The magnitudes of the UTS, %E1 and

%RA were also determined from these plots and the dimensions of the cylindrical

specimens before and after testing.

Figure 3.1 MTS Test Setup

15


Figure 3.2 INSTRON Test Setup

16


The MTS model 319.25 unit had an axial loading capability of 250 kN. On the other

hand, the model 8862 INSTRON equipment had a maximum load-bearing capability of

100 kN. The cylindrical specimen was moimted between two wedge-grips and was pulled

by a movable actuator. The load cell, contained in the crosshead measured the applied

force on the test specimen. The movement of the crosshead relative to the locked

crosshead generated the strain within the specimen, and consequently the corresponding

load. Both MTS and INSTRON equipments had a customized ceramic-lined heating

chamber to perform elevated temperature testing in the presence of an inert atmosphere

such as nitrogen. However, the INSTRON equipment was capable of performing tensile

testing at a maximum temperature of 1500*̂ C, while the MTS was capable of performing

tensile testing at a maximum temperature of 600°C.

3.2 Stress Corrosion Cracking Tests at Constant Load

A common method of evaluating the SCC susceptibility of a structural material is the

use of a constant tensile load that can act as a driving force for cracking to occur in the

presence of a potent environment. Therefore, smooth cylindrical specimens were used to

load them at various levels of stress according to the ASTM Designation G 49 [25]. A

calibrated proof ring, manufactured by the Cortest Inc. was used to perform the SCC

testing at CL to satisfy the requirements of the National Association of Corrosion

Engineers (NACE) standards [26]. The CL test setup used in this study consisted of a

calibrated proof ring, proof ring base, specimen grips, environmental chamber,

microswitch, dial indicator, thermocouple, temperature and time controller as shown in

Figure 3.3.

17


A - Dial Gage B - Proof Ring C - Test Specimen D - KnviTonmcnial Chamber

Figure 3.3 Constant-Load Test Setup.

Each proof ring was provided with a calibration curve, showing the load versus

displacement of the ring, which was used to determine the applied load during the CL

testing. A typical calibration curve for a proof ring is shown in Figure 3.4. Micrometers

with a dial indicator were used to measure the ring deflection. The operation of the proof

ring was based on the ability to transfer the load of a deflected proof ring to a tensile

specimen to obtain a sustained loading. The load was applied on the proof ring by using a

standard wrench on the tension-adjusting screw and lock-nut. A thrust bearing was used

to distribute the load and, thus prevent seizure.

The specimen grips on the proof ring were made of stainless steel to be fully-resistant

to the testing environment. A Hastelloy C-276 vessel was used for testing both at ambient

and elevated temperatures. This environmental chamber was firmly secured by using O-

ring seals to prevent any leakage. For high temperature testing, a heating cartridge was

connected to the bottom of the environmental chamber, and a thermocouple was used in

the top cover to monitor the testing temperature by use of the temperature controller. The

18


test specimens were electrically isolated by means of nylon bushing and all tube fittings

were wrapped with Teflon tapes to prevent any leakage.

The amount of deflection needed to apply the desired load was obtained from the

calibration curve of each proof ring. The specimens were loaded at stress levels

equivalent to the different percentages of the materials’ room temperature YS value, and

the corresponding time-to-failure (TTF) was recorded. The determination of the SCC

tendency using this technique was based on the TTF for the maximum test duration of

thirty days. The cracking susceptibility in the CL testing was expressed in terms of a

threshold stress (% ) for a specific testing environment, below which the specimen did

not exhibit any failure during the maximum test period of thirty days.

gJI

0 0.02 0.04 0.06 0.08 0.1 0 .1 2 0.14 0.16

D eflection (in)

Figure 3.4 A Typical Calibration Curve for a Proof Ring

3.3. Slow-Strain-Rate SCC Testing

SSR testing used in this investigation was performed in a specially-designed system

known as a constant-extension-rate-testing (CERT) machine, shown in Figure 3.5. This

equipment allowed testing to simulate a broad range of load, temperature, pressure.

19


strain-rate and environmental conditions. These machines, designed and manufactured by

the Cortest Inc., offered accuracy and flexibility in testing the effects of strain rate,

providing up to 7500 lbs of load capacity with linear extension rates ranging from 10' ̂to

1 0 ' ̂in/sec.

To ensure the maximum accuracy in the test results, this apparatus was comprised of

a heavy-duty load-frame that minimized the system compliance while maintaining

precise axial alignment of the load train. An all-gear drive system provided consistent

extension rate. This machine provided the maximum flexibility and working space for

test sample configuration, environmental chamber design, and accessibility. An added

feature included in this model (model# 3451) for ease of operation was a quick-hand

wheel to apply a pre-load prior to the operation.

Constant Extension Rate Testing Machines

I

A - LVDT B - Top Actuator C - Environmental Chamber D - Bottom Actuator

Figure 3.5 CERT Machines for SSR Testing

20


The SSR test setup used in this study consisted of a top-loaded actuator, testing

chamber, linear variable differential transducer (LVDT) and load cell, as shown in Figure

3.6. The top-loaded actuator was intended to pull the specimen at a specified strain rate

so that the spilled solution, if any, would not damage the actuator. A heating cartridge

was connected to the bottom cover of the environmental chamber for elevated-

temperature testing. A thermocouple was connected on the top cover of this chamber to

monitor the inside temperature. The load cell was intended to measure the applied load

through an interface with the front panel user interface. The LVDT was used to record the

displacement of the gage section during the SSR testing.

/Stepper m otor pow er driveLoad cell

LVDT Field pointTherm oouple Front panel 'user Interface

'Testing c h a m b e rleating coll

'Specimen

Figure 3.6 SSR Test Setup

Prior to the performance of SCC testing by the SSR technique, the load-frame-

compliance factor (LFCF- the deflection in the frame per unit load), was determined by

using a Type 430 ferritic stainless steel specimen. The generated LFCF data are shown in

21


Figure 3.7. These LFCF values were inputted to a load frame acquisition system prior to

the SCC testing. The resultant LFCF values are also shown in Figure 3.7.

0.70

000

000

lo.«}

I" 03 0

070-

0 .1 0 -

000

Fi3re-1 (LFCF=<fe-6)

y= 4E-06X+OJS38

Frare-3 0-FCF=5e-6) y= SEOex+0.2309

1

7 “f FrarFrare-2 (LFCF=6e-8)

y=5&0&(+0.1248

1000 3000 3000 4000

Load (lb)5000 6000 7000 8000

Figure 3.7 Load Frame Compliance Test

A strain rate of 3.3x10'® was used during the SSR testing. This strain rate was

selected to optimize the combined effect of the applied stress and the testing environment

to promote cracking. It is well known that the SCC occurrence is an effect of two

significant factors such as the applied/residual stress and a susceptible environment. If the

stress is applied at a very fast rate to the test specimen, while it is exposed to the aqueous

environment, the resultant failure may not be different from the conventional mechanical

22


deformation produced without an environment. On the other hand, if the strain rate is too

slow, the resultant failure may simply be attributed to the corrosion damage due to the

environmental interaction with the material, thus, causing breakdown of the protective

surface film. In view of this rationale, the SSR testing un this investigation was at a strain

rate of 3.3x10 ® that provided the most effective contributions of both the mechanical and

environmental factors in enhancing the environment-induced cracking susceptibility of

modified T91 grade steels using the SSR testing.

During SCC testing by the SSR method, the specimen was continuously strained in

tension until fracture, in contrast to more conventional SCC tests conducted under

sustained loading conditions. The application of a slow dynamic straining during the SSR

testing to the specimen caused failure that probably might not occur under a constant load

or might have taken a prohibitively longer duration to initiate cracks in producing failures

in the tested specimens.

Load versus displacement, and stress versus strain curves were plotted during these

tests. Dimensions (length and diameter) of the test specimens were measured before and

after testing. The cracking tendency in the SSR tests was characterized by the TTF, and a

number of ductility parameters such as %E1 and the %RA. Further, the maximum stress

(O m ), and the true failure stress (O f) obtained from the stress-strain diagram and the final

dimensions were used. The magnitudes of %E1, %RA, Gm and Of were calculated using

the following equations:

El =Lf-Lp

V ^0 yX100 ; Lf >Lo (Equation 3.1)

23


% RA =- A, ̂

V Ao jxioo ;Ao>Af (Equation 3.2)

On =f A.(Equation 3.3)

ny~D„(Equation 3.4)

^ / = -n y . D f (Equation 3.5)

Where,

Ao = Initial cross sectional area

Af = Final cross sectional area

Pm= Ultimate tensile load

Pf = Failure load

Lo= Initial length

Lf= Final length

Do= Initial Diameter

Df= Final Diameter

24


3.4. Cyclic Potentiodynamic Polarization Testing

The susceptibility of the modified T91 grade steels of different Si content to pitting

and crevice corrosion was determined by performing Cyclic Potentiodynamic

Polarization (CPP) experiments in an identical acidic aqueous solution using a GAMRY

potentiostat. This CPP testing is based on a three-electrode polarization concept, in which

the working electrode (specimen) acts as an anode and two graphite electrodes (counter

electrodes) act as cathodes, as shown in Figure 3.8.

The reference electrode was made of Ag/AgCl solution contained inside a Luggin

probe having the test solution that acted as a salt bridge. The tip of the Luggin probe was

placed at a distance of 2 to 3 mm from the test specimen, as shown in Figure 3.9. At the

onset, the corrosion or the open circuit potential (Ecorr) of the test specimen was

determined with respect to the Ag/AgCl reference electrode, followed by forward and

reverse potential scans at the ASTM specified [ 2 7 ] rate of 0 .1 7 mV/sec. An initial delay

of 5 0 minutes was given for attaining a stable Ecorr value. The magnitudes of the critical

pitting potential (Epit) and protection potential (Eprot), if any, were determined from the

CPP diagram.

25


Graphite Counter Electrodes

Reference Electrode

Electrode Specimen)

Figure 3.8 CPP Test Setup

Before conducting the CPP test, the potentiostat was calibrated according to the

ASTM Designation G 5 [10]. Calibration of the potentiostat was performed to generate a

characteristic potentiodynamic polarization curve (Figure 3.10) for ferritic Type 430

stainless steel in IN (1 Normal) H 2S O 4 solution at 30°C. Small cylindrical specimens

made of T91 grade steels were used to generate the calibration curve.

26


WCfk-nq fcîect^ocie

R efe rence ïi loct rod<

Lugyin Capil'ary

2-3 mm

Figure 3.9. Luggin Probe Arrangement

.00

o .s o

0 .6 0

0 .4 0

= 0 . 2 0

E c o r r

10 ' Î0100 . 1 ri

CUfiRKN"' oSN Sirv

Figure 3.10. Standard Potentiodynamic Polarization Plot (ASTM G 5)

27


3.5 Metallographic Evaluation by Optical Microscopy

It is very important to ensure that sample preparation was carried out with care to

produce high quality and useful micrographs. During the sample preparation, care was

taken to ensure that the material was sectioned at a proper location for characterization of

a specific feature. The sample was mounted using the right ratio of epoxy and hardener.

Steps were taken to ensure that the mounted specimen had the appropriate thickness to

prevent rocking during grinding and polishing. The edges of the mounted specimen were

rounded to minimize the damage to the grinding and polishing discs.

The mounted specimens were ground with rotating discs of abrasive paper. The

grinding procedure involved several stages using a finer paper each time. This was done

to remove scratches resulting from the previous coarser paper. The removal of scratches

was done by orienting the sample perpendicular to the previous scratches. The polished

sample was then washed with deionized water to prevent contamination. Finally, etching

was done by using Fry’s reagent to reveal the microstructure of Alloy T91 using standard

etching procedures. Care was taken to ensure that the specimen was not over-etched. The

specimen was then immediately washed with deionized water and subsequently dried

with acetone and alcohol.

The metallographic evaluations of the mounted specimens were performed by using a

Leica optical microscope with a magnification of up to 500X. The optical micrographs

showing the microstructure and secondary crack, if any, were obtained in both as-

polished and the etched conditions.

28


Figure 3.11 Optical Microscope

3.6 Fractographic Evaluation by Scanning Electron Microscopy

The fractographic evaluations of the primary fracture surface of cylindrical

specimens of both heats of T91 grade steel were performed by using a JOEL Model 5600

scanning electron microsope (SEM). This equipment is capable of analyzing specimens at

magnifications up to 100,000X. The SEM uses a beam of highly energetic electrons to

examine objects on a very fine scale. Such examination can yield information on

topography, morphology, composition and crystallography.

The sectioned cylindrical specimens, cut to a specified length was placed on a sample

holder to accommodate multiple specimens for fractographic evaluations. The SEM test

setup used in this investigation is illustrated in Figure 3.12.

29


Figure 3.12 Scanning Electron Microscope

30


CHAPTER 4

RESULTS

The conventional T91 grade steels, modified through additions of 1 and 2 weight

percent (wt %) Si, have been tested for evaluation of their metallurgical and corrosion

properties. The tensile properties of both materials have been determined at temperatures

ranging from ambient to 550°C. The susceptibility of these materials to stress corrosion

cracking (SCC) has been evaluated in an acidic aqueous solution at ambient and elevated

temperatures using constant-load (CL) and slow-strain-rate (SSR) testing techniques. The

localized corrosion behavior of these materials has been determined by an

electrochemical method known as cyclic potentiodynamic polarization (CPP).

Metallographic and fractographic evaluations of the tested specimens have been

determined by optical microscopy (OM) and scanning electron microscopy (SEM),

respectively. The comprehensive test results are presented in this chapter.

4.1 Metallographic Evaluations

The metallurgical microstructures of both heats of T91 grade steel in the etched

condition are illustrated in Figures 4.1 and 4.2, respectively. The etchant used

31


in this evaluation contained 5 gms of cupric chloride (CuCla), 40 ml of hydrochloric acid

(HCl), 25 ml of ethanol (C2H5OH), and 30 ml of deionized water. This etchant is known

as Fry’s reagent. An examination of these micrographs indicates that both heats exhibited

fine-grained tempered martensitic microstractures having some delta ferrites. These types

of microstructures are very common with quenched and tempered martensitic chromium-

molybdenum (Cr-Mo) steels. [28]

Figure 4.1 Optical Micrograph of T91 grade steel containing 1% Si

Figure 4.1 Optical Micrograph of T91 grade steel containing 1% Si

32


4.2 Tensile Properties Evaluations

As indicated in the previous section, the target structural materials will be subjected

to the operating temperatures of molten lead-hismuth-eutectic (LBE), which may range

between 420 and 550°C. In view of this rationale, both heats of T91 grade steel were

tested in an MTS machine in this temperature regime. The results of tensile testing at

different temperatures are superimposed in an engineering stress versus strain (s-e)

diagram as a function of the testing temperature, as shown in Figures 4.3 and 4.4. As

expected, the magnitude of yield strength (YS) and ultimate tensile strength (UTS) was

gradually reduced at higher testing temperatures. However, the failure strain was not

significantly influenced by the variation in the testing temperatme for steel containing 1 %

Si. On the other hand, the failure strain for the higher Si-containing material was

gradually reduced up to 400°C, followed by significant increase at 550°C. The reduced

magnitude of ef with increasing temperatures (ambient to 400°C) may be attributed to the

dynamic strain ageing effect.

33


160

140-

120 -

100 -

8 0-

ê 60 -

40

20

0 -

-20

T91 Steel (1% Si), Heat No. 2404

RT

150°C

550°C

— I—

0.00 0.05— I—

0.10— I—

0.15 0.20—I

0.25

Strain

Figure 4.3 s-e Diagram versus Temperature

RT: Room Temperature

T91 Steel, (2% Si) Heat No. 2406160-

140-

RT120 -

150°C - 300°C

■ 400°C

100 -

1 80-

6 0 -

550°C40-

2 0 -

0 -

-200.0 0.1 0.2 0.3

Strain



34


The magnitudes of the tensile parameters, determined from the s-e diagrams and the

dimensions of cylindrical specimens, before and after testing, are given in Table 4.1. This

table shows the average values of these parameters based on duplicate testing. The

variations of YS, UTS, percent elongation (%E1) and percent reduction in area (%RA)

with temperature for both alloys are illustrated in Figures 4.5 through 4.8. An evaluation

of Figures 4.5 and 4.6 reveals that the magnitude of both YS and UTS were significantly

reduced at 550°C, suggesting enhanced plastic flow for both alloys at this temperature.

Figures 4.7 and 4.8 illustrate the effect of testing temperature on %E1 and %RA,

respectively for both alloys. It is interesting to note that the ductility in terms of both

these parameters of T91 grade steel containing 1% Si was not influenced by the variation

in temperature, even at 550°C. However, the T91 grade steel containing 2% Si showed a

significant enhancement in both parameters at 550°C. The magnitude of %E1 and %RA at

this temperature was increased by about 1 0 and 11 points, respectively, as shown in

Figures 4.7 and 4.8.

35


Table 4.1 Results of Tensile Testing

Material/HeatNo.

Temperature(T ) YS, ksi (Mpa) UTS(ksi) ef %E1 %RA

T91(l%Si)/2404 RT 125.66 (8 6 6 ) 143.36 0 .2 0 2 1 65.66

T91(l%Si)/2404 150 118.25 (815) 134.54 0 .2 1 2 1 6 6 .1

T91(l%Si)/2404 300 107.83 (743) 127.26 0.195 2 0 66.36

T91(l%Si)/2404 400 106.78 (736) 124.47 0 .2 1 2 1 66.13

T91(l%Si)/2404 550 94.46 (651) 98.20 0 .2 2 2 1 67.64

T91(2%Si)/2406 RT 130.73 (901) 149.97 0.25 26 76.19

T91(2%Si)/2406 150 119.42 (823) 133.27 0.245 24 72.85

T91(2%Si)/2406 300 108.44 (747) 129.74 0.23 23 71.48

T91(2%Si)/2406 400 92.80 (639) 130.03 0.23 2 2 71.70

T91(2%Si)/2406 550 70.52 (486) 77.42 0.34 32 83.03

140n- A - Heat # 2406 (2%Si) - ■ - H e a t # 2404 (1%Si)130-

1 20 -

o 1 1 0 -jgw 100-

90-

80-

70-

5000 100 200 400 600300

Temperature (°C)

Figure 4.5 YS versus Temperature

36


160n- A - Heat # 2406 (2%Si)

Heat # 2404 (1%Si)150-

140-

130

110

100

90-

80

100 200 300 400 500 6000

Temperature (°C)

Figure 4.6 UTS versus Temperature

32

30-j

28

Ü3 26-

24-

22

20

-A -H eat#2406(2% Si) - ■ - H e a t # 2404 (1%Si)

1 0 0 200 300 400 500 600

Temperature (°C)

Figure 4.7 %E1 versus Temperature

37


84 -,

- A - Heat # 2406 (2%Si) - ■ - H e a t # 2404 (1%Si)

82

80

78-

70-

68 -

66 -

100 200 300 400 500 6000

Temperature (°C)

Figure 4.8 %RA versus Temperature

4.3 SCC Evaluation at Constant-Load

Smooth cylindrical specimens of T91 grade steels containing 1 and 2 wt % Si were

subjected to SCC testing at constant-load (CL) in an acidic aqueous solution at ambient

temperature, 60 and 90°C using calibrated proof rings, as described in the previous

section. These specimens were loaded at 95% of the materials’ YS values, and the

corresponding TTF was recorded. The results of these tests indicate that no failures were

observed in either alloy at the applied stress level, irrespective of the testing temperature.

Since no failures were observed in thirty days, it can be construed that the threshold stress

for SCC in these two alloys may lie in the vicinity of 0.95YS. The results of SCC testing

are shown in Table 4.2.

38


Table 4.2 Results of CL Testing

Material/Heat No. Environment / Temperature Oa Result

T91 (l% Si)/2404

AcidicSolution*

RT 0.95YS (YS=126 ksi) NF60°C

90°C

T91 (2%Si) / 2406RT

0.95YS (YS=131 ksi) NF60°C

90°C


Ga; Applied stress

NF: No Failure

Composition of acidic environment is shown in Table 2.3

4.4 SCC Evaluation under SSR Condition

The results of SCC testing involving smooth cylindrical specimens of two heats of

T91 grade steels in a similar environment using the SSR technique at ambient and

elevated temperatures are illustrated in Figures 4.9 and 4.10, respectively. The

magnitudes of %E1, %RA, TTF and Of values, determined from these s-e diagrams and

the dimensions of the tested specimens, are shown in Table 4.3. An evaluation of these

data indicates that all four parameters were gradually reduced with increasing

temperature indicating enhanced cracking susceptibility at elevated temperatures. It

should however be noted, that the reduction in Of for T91 grade steel containing 1% Si,

was insignificant between 60 and 90°C. For T91 grade steel containing higher Si content,

the magnitude of Of was, however, reduced drastically in this temperature regime.

39


J2

I

T91 (l% Si)/2404 Acidic environment (pH-2.2)

160-1

140-RT

60“C120 -

100 -

80-

60-

40-

2 0 -

0.00 0.05 0.10 0.15 0.20

Strain


140-

120 -

1 00 -

Ù 60 0040

2 0 -

0

T-91-2406 (2% Si)Acidic environment (pH 2.2)

RT

60°C

lksi=6.895 Mpa

— I--------- '----------1--------- '--------- 1--------- :--------- 1--------- '--------- 1-----------'-------- 1—0.00 0.05 0.10 0.15 0.20 0.25

Strain


40


Table 4.3 Results of Slow-Strain-Rate Technique

Material/HeatNo.

Environment/Temperature %E1 %RA TTF

(hrs) Of, ksi (MPa)

T91 (l% Si)/ 2404

AcidicSolution*

RT 21 40.31 19.34 139.850(964)60°C 16 38.98 16.32 137.981(951)90°C 11 26.34 13.20 137.596(945)

T91 (2%Si) / 2406

RT 23 43.46 21.53 163.165(1125)60°C 17 40.85 17.25 162.659(1121)90°C 13 29.69 14.89 143.864(992)


*Composition of acidic environment is shown in Table 2.3

The effects of temperature on all four parameters, shown in Table 4.3, are illustrated

in Figures 4.11 through 4.14. These figures also illustrate the effect of silicon content on

each parameter. An evaluation of the overall data indicates that the susceptibility of T91

grade steel to SCC was more pronounced in the material containing 1% Si compared to

that with 2% Si. Thus, the beneficial effect of higher Si content on the cracking resistance

of T91 grade steel in the presence of an acidic solution is clearly demonstrated. The

enhanced cracking resistance of the T91 grade steel containing 2 wt % Si is the result of

thicker oxide layer formed on the specimen’s surface.

41


24 -,

- A - Heat # 2406 (2% Si) - ■ -H e a t #2404 (1%SI)22

20

18-

5 16-

14

12 -

30 40 50 60 70 80 90

Temperature ( C)

Figure 4.11 %E1 versus Temperature

6 0 -,- A - Heat # 2406 (2% Si) - ■ - Heat # 2404 (1% Si)5 5 -

5 0 -

4 5 -

<oc 4 0 -

3 5 -

30 -

2 5 -

80 90SO 60 7030 40

Temperature ( C)

Figure 4.12 %RA versus Temperature

42


- A - Heat # 2406 (2% Si) Heat # 2404 (1% Si)22

20

18-

t 16-

14

30 40 50 60 70 80 90Temperature ( C)

Figure 4.13 TTF versus Temperature

- A - Heat # 2406 (2% Si) Heat # 2404 (1% Si)

190-

180-

Iksi=6.895 Mpa

170-

160-

e150-

140-

1308030 40 50 60 70 90

Temperature (”C)

Figure 4.14 True Failure Stress versus Temperature

43


4.5 Localized Corrosion Evaluation

The results of localized corrosion studies by cyclic potentiodynamic polarization

(GPP) technique indicate that both heats of T91 grade steel exhibited active-passive

behavior, as illustrated in Figures 4.15 and 4.16. A classical positive hysterisis loop was

observed with both heats at all three testing temperatures. It is, however, interesting to

note that the critical pitting potential (Epit) could not be detected in CPP tests performed

at 60 and 90°C for either alloy.

Environment : Acidic Solution (pH ~ 2.2) Temperature : 30°C

'corr

1800mA

Figure 4.15 CPP Diagram of T91 Grade Steel Containing 1 Wt % Si

44


Environment : Acidic Solution (pH~2.2) Temperature : 30°C

Epit O

ioCURVE (T9i-2£06-3tN»l .OTA}: |» -2 2 ,ia u A .y -^ .B m V i

Figure 4.16 CPP Diagram of T91 Grade Steel Containing 2 Wt % Si

The magnitudes of the corrosion potential (Ecorr) and Epit, if any, determined from

these CPP diagrams are given in Table 4.4. An evaluation of the overall data indicates

that for the T91 grade steel containing 1% Si, the Ecorr value gradually became more

active (negative) with increasing temperature, as expected. However, the 2 wt % Si-

containing T91 grade steel showed some discrepancy in that, the Ecorr value was

relatively more noble (positive) at 90°C compared to that at 60°C. With respect to the Epu

value, no significant difference in its magnitude was observed in T91 grade steel as a

function of the Si content. Similarly, the variation in Si content did not significantly

influence the Ecorr value at each testing temperature.

45


Table 4.4 Results of CPP Testing

Material/Heat No. Environment / Temperature (°C)

Avg. Ecorr, mV ( A g /A g C I )

Avg. Ecorr, mV (Ag/AgCl)

T91 (l% Si)/ 2404

AcidicSolution

RT -524 -36260 -562 N/A90 -590 N/A

T91 (2%Si)/ 2406

RT -528 -37460 -571 N/A90 -530 N/A

RT : Room Temperature

N/A: Not Applicable

Composition of acidic solution is shown in Table 2.3

4.6 Fractographic Evaluations

The results of fractographic evaluations of the primary fracture surface of cylindrical

specimens used in tensile testing, by SEM, are illustrated in Figures 4.17 through 4.18 for

both alloys. An evaluation of these micrographs indicates that the T91 grade steel

exhibited dimpled microstructures at different testing temperatures irrespective of the Si

content. The presence of dimpled microstructures signified ductile failures. As

anticipated, cleavage failures were observed at ambient temperature near the edge of the

fracture surface, as shown in Figures 4.17 and 4.19 for T91 grade steel containing 1 and 2

wt % Si, respectively.

46


(a)RT

(b) 550°C

Figure 4.17 SEM Micrographs of T91 Grade Steel (1% Si), 500X

47


(a)RT

(b) 550°C

Figure 4.18 SEM Micrograph of T91 Grade Steel (2% Si), 500X

48


The fractographic evaluations of the primary fracture surface of cylindrical specimens

tested by the SSR technique indicate that the T91 grade steel containing 1 wt % Si

suffered from brittle failure when tested in an acidic solution at ambient temperature, 60

and 90°C. Trans granular brittle failure was observed at ambient temperature, as shown in

Figure 4.19. At 60°C, the morphology of failure in this alloy consisted of both

intergranular and transgranular cracks as illustrated in Figure 4.20. Finally, solely

intergranular cracks were solely observed in the SEM micrographs of this alloy at 90°C,

as shown in Figure 4.21.

With respect to the fracture morphology of the T91 grade steel containing 2 wt % Si,

ductile failures characterized by dimpled microstructures were observed at ambient

temperature and 60°C.However, a combination of ductile and brittle failures was

observed at 90°C. The SEM micrographs of T91 grade steel containing 2 wt % Si tested

in an identical acidic solution at three different temperatures are shown in Figures 4.22

through 4.24. The results of SEM study indicate that the presence of 2 wt % Si in T91

grade steel may be beneficial to prevent brittle failure at ambient temperature and 60°C

from the SCC point of view. However, at a higher temperature (90°C), the high Si content

was not effective to prevent brittle failure when exposed to an acidic solution in a

stressed condition. The synergistic effect of the high temperature and aggressive chemical

species can, thus, promote a combined intergranular and transgranular brittle failures

along with some dimples despite the presence of higher Si content in T91 grade steel.

49


Figure 4.19 SEM Micrograph of T91 Grade Steel (1% Si) at RT, 500X

Figure 4.20 SEM Micrograph of T91 Grade Steel (1% Si) at 60°C, 500X

50



Figure 4.22 SEM Micrograph of T91 Grade Steel (2% Si) at RT, 500X

51


• m .»



52


SEM : Scanning Electron Microscope

RT : Room Temperature

53


CHAPTER 5

DISCUSSION

This investigation was focused on evaluating the beneficial effect of Si content on

both the metallurgical and corrosion behavior of modified 9Cr-lMo steel, also known as

T91 grade steel. Two custom-melted heats of T91 grade steel were tested for evaluation

of their tensile properties at temperatures ranging from ambient to 550°C.

Simultaneously, the susceptibility of both heats of alloys to stress corrosion cracking

(SCC) was evaluated in an acidic solution at ambient and elevated temperatures under

constant-load (CL) and slow-strain-rate (SSR) conditions. The susceptibility of these

materials to localized attack in an identical aqueous solution was determined by cyclic

potentiodynamic polarization (CPP) technique. The fractographic evaluations of the

tested specimens were performed by scanning electron microscopy (SEM). The

metallurgical microstructures were evaluated by optical microscopy (OM).

The results of tensile testing indicate that the magnitudes of both YS and UTS were

gradually reduced with increasing temperature, as expected. However, the extent of

reduction in tensile strength was more pronounced with T91 grade steel containing 2 wt

% Si. It is also interesting to note that the failure strain (ef) was not significantly

influenced by the variation in temperature in the case of T91 grade steel containing I

wt% Si. But in the case of higher Si-containing alloy, the failure strain was reduced with

54


increasing temperature up to 300°C followed by an increase in ef at temperatures up to

550°C. Such reduction in Cf at temperatures between ambient and 300°C may be

attributed to a phenomenon called dynamic strain ageing (DSA) in susceptible materials.

It is interesting to note that the magnitude of ef in T91 grade steel containing 2 wt % Si

was significantly enhanced at 550°C, demonstrating an enhanced ductility in terms of

both %E1 and %RA. In the presence of acidic environment, the T91 grade steel

containing 2 % Si was better in terms of ductility whereas the lower Si-containing alloy

was better in terms of strength at the tested temperatures.

DSA is the phenomenon of interaction between diffusing solute atoms and mobile

dislocations during plastic deformation and depends on deformation rate and temperature,

which govern the velocities of mobile dislocations and diffusing solute atoms,

respectively. [29] The occurrence of DSA can be manifested by the anomalous features

of material behavior including serrated yielding, variation in total elongation and fracture

strain. The presence of interstitial solute elements such as carbon and nitrogen in the

vicinity of the grain boundaries of a martensitic material such as T91 grade steel can

impede the movement of dislocations at some critical temperature regime (ambient to

300°C). Once the temperature is increased beyond this range, the plasticity of the material

may be enhanced, thus showing enhanced failure strain as observed in this investigation.

The resultant enhancement in ductility in terms of the ef, %E1 and %RA, as seen with T91

grade steel containing 2 wt % Si is consistent with the observations made by other

investigators on martensitic stainless steels. [30]

The results of SCC testing at CL indicate neither heat of T91 grade steel exhibited

failure at applied stresses equivalent to 95% of their room temperature YS value. As to

55


the SSR testing results, the magnitude of TTF and two ductility parameters (%E1 and

%RA), was gradually reduced with increasing temperature when tested in an identical

acidic solution. The cracking susceptibility of T91 grade steel containing 1 wt % Si in

terms of Gf was not influenced by the testing temperature. However, an enhanced

cracking tendency in terms of Gf was noted in T91 grade steel containing 2 wt % Si due to

a change in temperature from 60 to 90°C, suggesting a detrimental effect of higher Si

content from the SCC point of view. Nevertheless, the presence of 2 wt % Si content

exhibited a beneficial effect on the cracking resistance of T91 grade steel in terms of

%E1 and %RA, in both tensile and SSR testing.

As to the localized corrosion susceptibility, both heats of T91 grade material

exhibited active-passive behavior in an acidic solution at all three testing temperatures.

However, Epu could not be detected at 60 and 90°C, with either alloy. The higher Si-

containing alloy was more resistant to generalized and localized corrosion. In general, the

magnitude of Ecorr became more active at higher testing temperatures except at 90°C,

where there was an anomaly in the case of T91 grade steel containing 2% Si. Such an

anomaly can be explained by the variation in physical characteristics of the surface film

as a function of temperature. This result could have been more accurate if more samples

had been available for testing. Other investigators had earlier encountered a similar

situation when conducting tests at a temperature of 90°C.

The cylindrical specimens tested for tensile properties evaluation exhibited dimpled

microstructures at ambient and elevated temperatures, irrespective of the Si content.

However, the morphology of failure was different in T91 grade steel containing different

Si-content when subjected to SCC testing by the SSR technique. The SEM micrographs

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obtained with lower Si-containing alloy exhibited a combination of intergranular and

transgranular brittle failure. The severity of brittleness in this alloy was increased with

increasing temperature. On the other hand, the higher Si-containing T91 grade steel

showed ductile failures at ambient temperature and 60°C. But a combination of brittle

(intergranular and transgranular) and ductile failures was observed in the 90°C acidic

solution in the SSR testing. These results clearly demonstrate the beneficial effect of

higher Si content in enhancing the corrosion resistance, when tested in an acidic solution

at intermediate temperature. As to the metallurgical microstructures, both heats exhibited

fully tempered martensitic microstructures, typical of quenched and tempered martensitic

alloys.

Even though, SCC testing involving T91 grade steel with two levels of Si content

could not be performed in molten LBE, due to the lack of infrastructure at UNLV, it can

be concluded that the higher Si-containing steel could provide better resistance to

cracking even in the presence of molten metal. This inference is made in view of better

resistance of T91 grade steel with 2 wt % Si to SCC in an acidic solution by exhibiting

superior ductility in terms of %E1, %RA and ef. It is anticipated that the relatively thicker

surface film due to the higher Si content in this alloy will enhance the resistance to

penetration of molten LBE through these surface layers. However, additional studies

need to performed to characterize the extent of penetration due to the molten metal by

performing experimental work in a prototypic environment, should such a facility be

available in the near future.

57


CHAPTER 6

SUMMARY AND CONCLUSIONS

T91 grade steels containing I and 2 weight percent (wt %) Si were subjected to

tensile properties and stress corrosion cracking (SCC) evaluations using state-of-the-art

experimental techniques. The susceptibility to localized corrosion of these alloys was

determined by an electrochemical technique. Microscopic evaluations of the tested

specimens were performed by optical microscopy and SEM. The key conclusions derived

from this investigation are given below.

• The magnitude of YS and UTS was gradually reduced with increasing

temperature for T91 grade steels containing I and 2 wt % Si.

• The failure strain (ef) was reduced at 300°C in both alloys. Further, a significant

enhancement in ef was noted with the higher Si-containing alloy at 550°C,

suggesting an enhanced ductility due to higher Si content.

• Neither alloy exhibited cracking in constant-load SCC testing. The magnitude of

%E1, %RA, TTF and Of was reduced at higher temperature m SSR testing.

However, the reduction of Of at 90°C in T91 grade material containing 2 wt % Si

was substantially higher.

• Both alloys exhibited active-passive transition in the CPP testing. In general, the

Ecorr became more active at higher temperatures.

58


• The SEM evaluations of the primary fracture surface of the cylindrical specimens

tested under the SSR condition revealed brittle failures in steel containing 1 wt %

Si. The presence of higher Si content resulted in ductile failures at ambient

temperature and 60°C. But, brittle failure characterized by intergranular cracking

were observed in steel containing 2 wt % Si in 90°C SSR testing.

• Fine-grained fully tempered martensitic microstructures were observed in optical

micrographs of both alloys.

59


CHAPTER 7

SUGGESTED FUTURE WORK

Tensile Properties evaluations in T91 grade steel containing 0.5 and 1.5 wt % Si.

SCC evaluations in steels containing 0.5 and 1.5 wt % Si under CL and SSR

conditions.

Impact resistance evaluations of steels containing 0.5,1.0,1.5 and 2.0 wt % Si by

charpy testing method.

Characterization of defects by transmission electron microscopy to analyze

reduced failure strain at some critical temperatures.

Comparison of microstructures and failure morphology as a function of four

levels of Si content.

Comparison of critical potentials by CPP technique as a function of four levels of

Si content.

• SCC evaluations using self-loaded specimens (C-ring, U-bend and double

cantilever beam).

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APPENDIX A

OPTICAL MICROSCOPY DATA

a»

%« : '-■

Optical Micrograph of T91 Grade Steel Containing 1% Si-500X

•-i * . <

HR»:

e : : '

^ L


61



62


APPENDIX B

CPP DATA

0 .0 mVIOJMmA

lm<A)

CPP Diagram of T91 Grade Steel Containing 1 wt % Si at 30°C

im(A)


63


h» (A)



64




65




66



oirve * * OTA)


67


Cl Tensile Micrographs

APPENDIX C

SEM DATA

SEM Micrograph of T91 Grade Steel containing 1 % Si at RT, 35X


68


y .


' V ' ' " ' 4 -

, V


69


SEM Micrograph of T91 Grade Steel containing 1 % Si at 300°C, 35X


70




71



%

a r * •


72


m m

*r%&



73




74



m m .

> *


75


iL -<

^ w



76


mmMm.SEM Micrograph of T91 Grade Steel containing 2 % Si at RT, 500X

4» %»


77


,*# Î

; ysp^

r>* # #<

* r,. .*k


wm:mrn


78


C2 Slow-Strain-Rate Data

SEM Micrograph of T91 Grade Steel Containing 1% Si at RT, 35X

SEM Micrograph of T91 Grade Steel Containing 1% Si at 60°C, 35X

79


.vf̂ T ' /

3% :':

SEM Micrograph of T91 Grade Steel Containing 1% Si at 90”C, 35X

SEM Micrograph of T91 Grade Steel Containing 1% Si at RT, 35X

80




81



82


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85


VITA Graduate College

University of Nevada, Las Vegas


Address:4214 Grove Circle, Apt # 1 Las Vegas, NV 89119

Degree:Bachelor of Engineering, Mechanical Engineering, 2003 University of Madras, India

Special Honors and Awards:• Member, American Nuclear Society Student Chapter• Member, National Dean's List• Member, UNLV Materials Society• Awarded graduate research assistantship at UNLV to pursue M.S. program in

Mechanical Engineering.

Selected Publications:

• Pankaj Kumar, Harish Krishnamurthy and Debajyoti Maitra, “Effect of Silicon Content on the Metallurgical and Corrosion Properties of a Martensitic Stainless Steel”, American Nuclear Society Student Conference, Columbus, Ohio, April 2005.

Thesis Title: Metallurgical and Corrosion Characterization of a Martensitic Stainless Steel as a function of Silicon Content.

Thesis Examination Committee:Chairperson, Dr. Ajit K. Roy, Ph. D.Committee Member, Dr. Anthony E. Hechanova, Ph.D.Committee Member, Dr.Samir F.Moujaes, Ph. D.Graduate College Representative, Dr. Venkatesan Muthukumar, Ph. D.

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