Indian Journal of Engineering & Materials Sciences

Vol. 18, April 2011, pp. 119-124

Cold compaction of ODS 9Cr martensitic steel powder synthesized by

mechanical alloying

T S Kavithaa, R Subramanian & P C Angelo*

Department of Metallurgical Engineering, PSG College of Technology, Coimbatore 641 004, India

Received 24 June 2009; accepted 1 March 2011

Oxide dispersion strengthened (ODS) steels are advanced class of nanoscale composite materials whose microstructure

can be controlled through incorporation of oxide particles by mechanical alloying (MA). This paper deals with the

methodologies attempted for cold compaction followed by subsequent sintering of ODS steel of composition Fe-9.2Cr-2W-

0.2Ti-0.28V-0.01Mn-0.01Si-0.152C-0.02Ni-0.008Mo-0.33Y2O3 synthesized via mechanical alloying of elemental powders.

Microstructural studies of the compacted and vacuum sintered specimens after solutionizing heat treatment are reported. As-

cold compacted specimens had a green density of 82% TD. Hardness of the heat treated samples is also reported.

Keywords: Oxide dispersion strengthening (ODS), Mechanical alloying (MA), Cold compaction, Energy dispersive

analysis of X-rays (EDAX)

Development of low or reduced-activation materials

for fusion reactors has its focus on the issue of

radioactive waste disposal/recycling of materials

from fusion power plant components after they have

reached the end of their service lifetime1. Hence, the

choice of viable candidate materials for structural

applications in a nuclear power plant has been

narrowed down to (i) ferritic-martensitic steels (ii)

SiC/SiC composites and (iii) vanadium alloys. Of

these potential candidates, SiC/SiC composites and

vanadium alloys are yet to reach the

commercialization stage in comparison to steels and

require considerable research to reach engineering

feasibility. Among the steels, 9 and 12% Cr steels

possess improved creep-rupture strengths combined

with good oxidation and corrosion resistance at

elevated temperatures compared to 2.25%Cr steels2-

4. Based on a series of irradiation tests, 8-9% Cr

instead of 12%Cr was selected as an optimum level

for advanced power plants, since lower Cr content

avoids severe hardening and embrittlement problems

associated with the formation of fine dispersions of

the ¢ phase5,6

. Within the ferritic/martensitic steel

class, analogs of commercial steels with Cr, W, V

and Ta as alloying elements have been developed

with both low activation potential and better

radiation resistance than the parent compositions1,7,8

.

Increasing the operating temperature of these alloys

to 873 K or 923 K would expand the design window

of these alloys, making them more attractive for

advanced power systems. However, solid solution

strengthening results in alloys with limited ductility,

while precipitation strengthening results in

coarsening of precipitates at higher temperatures. To

overcome these limitations, Oxide dispersion

strengthening (ODS) of the alloys with ultra fine

oxide particles by mechanical alloying (MA) appears

to be the most feasible route to produce

technologically superior materials for applications at

higher temperatures9,10

. The present study aims at the

consolidation of ODS 9Cr martensitic steel using

elemental powders prepared via MA by cold

compaction followed by sintering. Microstructural

characterizations of the compacted specimens are

reported.

Experimental Procedure

Synthesis of ODS alloy powders

ODS alloy powders were synthesized using a

double cone blender by MA of elemental metal

powders to arrive at the following chemical

composition (in wt%): Fe-9.2Cr-2W-0.2Ti-0.28

V-0.01Mn-0.01Si-0.152C-0.02Ni-0.008Mo-0.33Y2O3.

The ball to charge ratio was maintained as 10:1 and

1 wt% stearic acid was used as the process control

agent. Steel bearing balls of 6 mm diameter were —————

*Corresponding author. (E-mail: angelopsg@gmail.com)

INDIAN J. ENG. MATER. SCI., APRIL 2011

120

used as the grinding media. The blender was

operated at 40 rpm and the steel powder charge

was blended for a duration of 25 h. The XRD

analysis of the milled powders showed that all

the individual constituents have gone into solution

(Fig. 1). Detailed characterization studies of the

synthesized steel powders and the subsequent

optimization of the milling parameters have been

reported elsewhere11-13

.

Cold pressing of ODS alloy powders

The mechanically alloyed powders were

thoroughly mixed with 0.6 wt% of lubricant

containing 2.1% Zn for achieving good green

strength. MA powders along with lubricants were

blended for 40 min at 20 rpm. The blended powders

were cold compacted employing a DAEWHA

(Korean make) press into specimens which had 82%

TD along with good surface finish (Fig. 2).

One of the cold compacted specimens was then

vacuum sintered at 10-5

Torr vacuum for 210 min at

1273 K. Three cold compacted samples were sintered

at 1073 K for 180 min. After sintering of the three

specimens, one each was subjected to furnace cooling,

air cooling and quenching respectively.

Microstructural characterizations of the three

specimens were carried out employing a Nikon

optical microscope and JEOL JSM 6360 scanning

electron microscope (SEM). Microhardness

measurements of the cold compacted, sintered,

solution treated and heat treated samples were carried

out employing a Mitutyo microhardness tester with

diamond indenter using a load of 50 g.

Results and Discussion

Characterization of vacuum sintered specimens

The optical micrographs of vacuum sintered ODS

steel specimens indicate the formation of a two-phase

microstructure with a uniform distribution of the

second phase in the matrix (ferrite). Figure 3 depict

the unetched and etched microstructures of the

vacuum sintered ODS steel specimens respectively.

The SEM/EDAX analysis indicated a substantially

uniform two-phase microstructure with significantly

lower fraction of second phase containing slightly

higher chromium content. (Table 1) Area analysis

employing EDAX confirmed the presence of the

various alloying elements of the ODS steel (Fig. 4).

Characterization of sintered and furnace cooled specimens

The EDAX results of the cold compacted, sintered

and furnace cooled specimen is given in Table 2.

Typical elemental X-ray distribution images of the

same specimen showing the presence of yttrium along

with tungsten are shown in Fig. 5.

Characterization of sintered and air cooled specimen

The SEM examination of the sintered and air cooled

specimen indicated the formation of a two-phase

microstructure. Area analysis by EDAX gave the

composition of the individual alloying elements as

shown in Fig. 6 and it can be seen that in spite of the

two phase structure the concentration of the various

elements reasonably correspond to the starting

composition. X-ray distribution images indicated the

Fig.1—XRD of milled steel powder

Fig. 2—Image of cold compacted ODS steel specimens

Table 1—EDAX results of the cold compacted and vacuum

sintered specimen

wt % Sl.

No

Analyzed

Region Fe Cr Y W Ti V

1 Area (Matrix) 89.18 7.21 0.29 2.87 0.41 0.05

2 Spot (2nd phase) 88.18 11.49 - 0.3 - 0.03

Fig. 3—Optical micrographs of cold compacted and vacuum

sintered ODS steel specimens (a) unetched and (b) etched

KAVITHAA et al.: COLD COMPACTION OF ODS 9Cr MARTENSITIC STEEL POWDER

121

Fig. 4—Area analysis employing EDAX, confirms the presence of all elements

Fig. 5—X-ray images of sintered and furnace cooled specimen

Table 2—Combined EDAX results of the sintered and furnace

cooled specimen

wt % Analyzed

region Fe Cr W Y Ti V

Area 80.68 15.34 2.98 0.5 0.25 0.26

Area 82.71 14.65 1.78 0.63 - 0.22

Table 3—Combined EDAX results of sintered and air cooled

specimen

wt % Analyzed

Region Fe Cr W Ti V Y

Area (Matrix) 86.73 9.21 2.34 1.06 0.47 0.19

Area (Matrix) 88.91 8.42 2.19 0.15 0.26 0.7

Area (Matrix) 89.11 8.19 2.41 - - 0.28

INDIAN J. ENG. MATER. SCI., APRIL 2011

122

presence of tungsten, vanadium, titanium and yttrium together in certain areas as shown in Fig. 7. EDAX results obtained from three different regions of the specimen are given in Table 3.

Characterization of sintered and quenched specimen

The SEM/EDAX analysis of the cold compacted and quenched specimen shown in Fig. 8 indicates the

desired starting composition of the alloying elements

after consolidation. Figure 9 represents the X-ray

distribution images of the same specimen wherein, the

presence of yttrium along with tungsten is evident.

The results show that the area analysis gives

concentrations of various elements corresponding to

the starting composition.

Fig.6—Area analysis by EDAX of the composition of the sintered and air cooled specimen

Fig. 7—X-ray distribution images indicating the presence of tungsten, vanadium, titanium and yttrium together in certain areas

KAVITHAA et al.: COLD COMPACTION OF ODS 9Cr MARTENSITIC STEEL POWDER

123

Preliminary mechanical property evaluation of ODS steel

specimens

The microhardness measurements recorded for

the cold compacted, vacuum sintered specimen and

cold compacted heat treated specimens are given in

Table 4. Hardness values indicate that quenched

specimen shows the highest value while furnace

Table 4—Micro hardness measurements of the ODS steel

specimens

Specimen Condition Micro hardness of the

matrix phase (VHN)

Cold compacted vacuum sintered 70

Air cooled 80

Furnace cooled 74

Quenched 115

Fig. 8—SEM/EDAX area analysis of the sintered and quenched specimen

Fig. 9—X-ray distribution images of the area shown in Fig. 8

INDIAN J. ENG. MATER. SCI., APRIL 2011

124

cooled specimen shows the lowest value which can be

attributed to the relatively faster cooling rates in

quenching as compared to annealing.

Conclusions

The following conclusions can be drawn from this

study:

ODS 9Cr martensitic steel successfully prepared by

mechanical alloying of elemental powders has been

cold compacted and sintered in vacuum.

The compacts have been sintered at 1073 K for 3 h

and were subjected to furnace cooling, air cooling and

quenching.

The SEM/EDAX analysis of the above samples

showed two-phase microstructure, while the sintered

and air cooled and quenched specimens gave the best

elemental distribution corresponding to the starting

composition.

Microhardness measurements show that the

quenched specimen records highest hardness.

Acknowledgement One of the authors (TSK) thanks DST, New Delhi

for financial support (SR/WOS-A/ET-15/2005) under

Women Scientists Scheme (WOS-A). The authors

would like to thank PRICOL, Coimbatore for the help

received in cold pressing experiments. Thanks are due

to the Principal, PSG College of Technology for

providing infrastructural facilities.

References 1 Baluc N L, Status of R&D Activities on Materials for Fusion

Power Reactors, presented at Fusion Energy Conference,

Chengdu, China, OV/5-4, 2006.

2 Kohyama A, Kohno Y, Asakura K & Kayano H, J Nucl

Mater, 212-215 (1994) 684-689.

3 Klueh R L, Gelles D S, Jitsukawa S, Kimura A, Odette G R,

vander Schaaf B & Victoria M, J Nucl Mater, 307-311

(2002) 455-465.

4 Chaouadi R, J Nucl Mater, 360 (2007) 75-91.

5 Muroga T, Gasparotto M & Zinkle, S J, Fusion Eng Des, 61-

62 (2002) 13-25.

6 Baldev Raj, Mannan S L, Vasudeva Rao P R & Mathew M

D, Sadhana, 27 (2002) 527-558.

7 Bloom E E, Zinkle S J & Wiffen F W, J Nucl Mater, 329-

333 (2004) 12-19.

8 Phil Maziasz, ORNL Rev, 35(2002) 3.

9 Zinkle S J, Advanced materials for fusion technology,

presented at 23rd Symp on Fusion Technology, Italy, 2004.

10 Chen Y L & Jones A R, Metall Mater Trans A, 32A (2001)

2077.

11 Kavithaa S, Subramanian R, Angelo P C & Shankar P, Nat J

Technol, 5 (2009) 81.

12 Kavithaa S, Subramanian R, Angelo P C & Shankar P, J

Manufact Eng, 4 (2009) 86.

13 Kavithaa S, Subramanian R & Angelo P C, Trans Indian Inst

Met, 63 (2010) 67.