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Ph.D. Eng. Periklis Christodoulou ‒ FLSmidth Ludowici, Pinkenba, Australia, Ph.D. Eng. Renata Chylińska, Ph.D. Eng. Małgorzata Garbiak (malgorzata.garbiak@zut.edu.pl), Prof. bogdan Piekarski ‒ Institute of Materials Science, West Pomeranian University of Technology, Szczecin, Poland

PERIKLIS CHRISTODOULOU, RENATA CHYLIńSKA, MAŁGORZATA GARbIAK, bOGDAN PIEKARSKI

Microstructure and microhardness of annealed austenitic cast steel

INTRODUCTION

Microhardness of a material reflects the elastic-plastic properties around the point of measurement. It is also a very sensitive indicator of changes in both the microstructure and properties due to a heat treatment or mechanical treatment, and the process of aging that oc-curs in materials during operation [1÷6]. Microhardness measure-ment taken under the conditions of low testing loads allows making a number of indentations in selected areas of the sample. For ex-ample, in the material with a multiphase structure, like the creep-resistant austenitic cast steel, the size of the indentation (depending also on the amount of load applied) represents, in most cases, the average microhardness of a matrix with the precipitates of different origin (Fig. 1). Microstructure of this material is characterised by a relatively small volume content of large and hard precipitates, and a soft matrix with fine secondary precipitates [4].

When the random microhardness measurements are taken at dif-ferent points on the sample cross-section, a map of the microhard-ness values can be created in the form of a microhardness distribu-tion density function reflecting local microstructure of the examined material. This means that to various peaks in this distribution one can assign the presence of different structural components, such as a relatively homogeneous matrix, matrix with increased number of fine precipitates, and matrix with large precipitates [3÷6].

The aim of this study was to use the results of the random HV0.01 microhardness measurements to describe changes in the microstructure of 0.3C-18Cr-30Ni cast steel with an addition of ti-tanium, where the said changes have occurred as a result of anneal-ing at a temperature of 800°C.

EXPERIMENT

Studies were conducted on the creep-resistant austenitic cast steel (Tab. 1) melted in an open induction furnace with acid lining [5].

Test ingots were annealed in air at 800°C for 10, 100, 500 and 1000 hours. Then, from these ingots, specimens with dimensions of ø10×10 mm were cut out. The front faces of the specimens were prepared for the metallographic examinations and microhardness measurements by grinding them with 500, 700, 1200 grit papers and 3 μm diamond paste, and with further polishing in an Al2O3 slurry.

Microhardness was measured on unetched polished sections, using an LM247AT microhardness tester and a load of 10 G load applied for 10 seconds ‒ HV0.01. Measurements were taken at ran-dom, imposing each time a grid with 200 points onto the surface of the polished specimen [4].

To determine the microhardness distribution density, a “densi-ty” function, forming part of the S-Plus programme for statistical analysis, was used [3, 7]. Based on the preliminary analysis, it was decided to determine the density of the probability distribution of

a given value of HV0.01 (Fig. 2) using 25 adjacent measurement points [4].

Metallographic examinations were performed on polished sec-tions etched with the Mi19Fe reagent (PN-61/H-04503). Micro-structure of cast steel after the annealing process (Fig. 3÷6) con-tained the identified titanium carbides TiC of a primary origin present in the interdendritic areas, fine secondary precipitates of the Cr23C6 chromium carbide distributed along the boundaries and in-side the dendritic cells, and a G phase formed around the particles of titanium carbide [4, 5, 8]. To the G phase, the symbol of Ni16Ti6Si7 can be assigned [4, 5].

RESULTS AND DISCUSSION

Graphical presentation of the results of the random microhardness measurements is shown in Figure 2. It contains the following in-formation: – the dotted line shows the microhardness distribution density

function determined on the basis of two hundred measurements. The distribution components were determined with a PeakFit programme [9, 10],

– the continuous line approximating the microhardness distribu-tion density function (dotted line) is the sum of all the peaks of the distribution components. In the ideal case, the continuous and dotted lines coincide,

Fig. 1. Microstructure of the 18Cr-30Ni-Ti cast steel after annealing at 600°C for 500 hrs with visible microhardness indentation [4]Rys. 1. Mikrostruktura staliwa 18Cr-30Ni-Ti wyżarzonego w temperatu-rze 600°C przez 500 h z widocznymi odciskami wgłębnika [4]

Table 1. Chemical composition of the tested cast steel, wt %Tabela 1. Skład chemiczny badanego staliwa, % mas.

C Si Mn P S Cr Ni Nb Ti

0.31 1.95 1.05 0.018 0.01 18.3 29.6 0.03 0.93

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– figures above the peaks correspond to the microhardness at which the peak reaches its maximum; under these figures, the percent fraction of the peak area in the total distribution is given.Full discussion of the strategy of the approximation of the micro-

hardness distribution curves is presented in [3÷5]. The interpreta-tion of the data presented in Figure 2 is as follows.

The first five peaks represent the microhardness distribution in a matrix with the varying content of secondary carbides. The sum of the areas under the peaks is equivalent to a volume content of the matrix in an alloy (the sum of all peak areas is equal to one). It should be noted that microhardness in different areas of the ma-trix depends not only on the number of secondary precipitates, but also on their spatial distribution and distribution of the size values. Other peaks correspond to the matrix areas with other precipitates. These can be the precipitates of increased microhardness, or hard and large-sized precipitates (Fig. 1).

As regards the size of the indentations (Fig. 1), it is assumed that most of the obtained results of the measurements give us infor-mation about the averaged microhardness values in the area of the microstructure comprising mainly the matrix and secondary pre-cipitates, or the matrix and primary precipitates. This conclusion is confirmed by the fact that none of the microhardness measurements taken during these studies have corresponded, even approximately, to the microhardness of carbides present in cast steel [5]. There-fore, the interval of (100, 300) HV0.01 reflects the distribution of microhardness in a matrix with the varying content of precipitated phases. For this interval of the HV0.01 values, the components of the microhardness distribution density function were compared for the cast steel after different times of annealing (Fig. 2).

The analysis of the cast steel microstructure (Figs. 3-6) and in-terrelating this image with the processed results of microhardness measurements (Fig. 2) suggest the following description.

Annealing at 800°C/10 hours. Microstructure of the cast steel after short-time annealing is far from the state of thermodynamic equilibrium. It can generally be assumed that peaks in the interval of (0, 300) HV0.01 (Fig. 2a) refer to the matrix. Their width indi-cates the range within which microhardness of the matrix changes as a result of local differences in the content of secondary precipi-tates, mainly the Cr23C6 carbides [4, 5].

The highest peak (the maximum value of 233 HV0.01, Fig. 2a) describes changes in the microhardness of the matrix (austenite su-per-saturated with carbon and alloying elements) containing single fine precipitates of secondary carbides (Fig. 3). In the vicinity of this peak, there are two smaller peaks. Most likely, they characterise the areas:

– inside the dendritic cells without precipitates visible un-der the light microscope (the peak with a maximum value of 159 HV0.01),

– at and on the cell boundaries with the growing number of fine precipitates (the peak with a maximum value of 300 HV0.01).The extreme left-side peak (122 HV0.01) represents the

measurements which have yielded low values due to the presence of various structure defects such as: – heavy microsegregation of alloying elements in austenite, – presence of grain boundaries free from the precipitates, – presence of impurities or micro-discontinuities (micropores)

caused by the dendritic structure of material [11].To the latter two factors can be assigned all low values of HV

(below 150 HV0.01), also in other distributions (Fig. 2c and d).In this case, the approximate matrix content is 94% (1% + 4% +

82% + 7%, Fig. 2a), and it is characterised by large local microhard-ness variations, indicating the presence of areas characterised by different microstructure and phase composition. If, by assumption, peaks 159, 233, 300 HV represent these areas, then the conclusion is that the matrix is composed of three different regions, whose vol-ume content in the alloy amounts to 4%, 82% and 7%, respectively. In this case, the weighted average microhardness of the matrix will amount to about 235 HV0.01.

Fig. 2. Microhardness distribution density function in cast steel after annealing at 800°C for: a) 10, b) 100, c) 500, d) 1000 hrsRys. 2. Funkcja gęstości rozkładu mikrotwardości staliwa wyżarzonego w temperaturze 800°C przez: a) 10, b)100, c) 500, d) 1000 h

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Annealing at 800°C/100 hours. The matrix area still shows some differences as regards the relative volume content of the second-ary Cr23C6 carbides (Fig. 4). Meanwhile, matrix content in the al-loy has increased by 2%, i.e. up to 96% (70% + 26%, Fig. 2b). At the same time, close analysis of the peak distribution (Fig. 2b) allows judging that the matrix has undergone a significant degree of homo genisation, showing now the presence of two subregions only. The weighted average microhardness of the matrix has not changed, and by amounting to about 230 HV0.01 confirms the ex-pected homogeniza tion of the cast steel matrix.

Absence of the peak observed previously (with a maximum val-ue of 159 HV0.01) proves: – matrix homogenization as a result of the nucleation of fine sec-

ondary chromium carbide precipitates within the entire volume of dendritic cells (Fig. 4), and/or

– disappearance of the effect of microsegregation of the alloying elements in austenite, observed during the solidification and cooling of ingots.Annealing at 800°C/500 hours. The next step in prolonging the

time of the cast steel annealing has brought further changes in the microstructure (Fig. 5).

Now, at the grain/dendritic cell boundaries (where single pre-cipitates existed previously), continuous chains of fine particles of the chromium carbide appear with a well-visible, surrounding them, narrow area free from the precipitates. The content of the G phase has increased quite notably. Its precipitates in the form

of massive envelopes around the TiC carbides are observed on the metallographic section. The growth and coagulation of secondary precipitates of the Cr23C6 carbide are detected in the border area and inside the dendritic cells, where their increased concentration has occurred earlier.

On the cross-section of a dendritic cell, at least three areas/zones are visible; they differ from each other in the phase composition, and in the number and size of the precipitates (Fig. 5). Peaks of the following maximum values are assigned to them: – 178 HV0.01 ‒ areas inside the matrix, free from the secondary

precipitates, visible under a light microscope (these are the areas of a microhardness corresponding to the microhardness of pure austenite [5]),

– 235 HV0.01 ‒ areas with single globular secondary precipi-tates of the Cr23C6 carbide, grouped on cell boundaries and around them,

– 257 HV0.01 ‒ areas with a high relative volume content of glob-ular secondary precipitates of the Cr23C6 carbide, present around the phase boundaries/dendritic cell boundaries.Attention also deserves the fact that (Fig. 5):

– in the vicinity of the G phase, surrounding the precipitates of TiC, the Cr23C6 carbides of a globular or elongated shape have appeared,

– free from the visible secondary precipitates are both the matrix areas (borders are now decorated with single fine Cr23C6 carbides only) and areas near the large precipitates of TiC-G.

Fig. 3. Microstructure of the cast steel after annealing at 800°C/10 hrsRys. 3. Mikrostruktura staliwa wyżarzonego w 800°C/10 h

Fig. 4. Microstructure of the cast steel after annealing at 800°C/100 hrsRys. 4. Mikrostruktura staliwa wyżarzonego w 800°C/100 h

Fig. 5. The microstructure of the cast steel after annealing at 800°C/500 hrsRys. 5. Mikrostruktura staliwa wyżarzonego w 800°C/500 h

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Fig. 6. The microstructure of the cast steel after annealing at 800°C/1000 hrsRys. 6. Mikrostruktura staliwa wyżarzonego w 800°C/1000 h

The volume content of the matrix has practically remained un-changed (about 96%, Fig. 2c), but the weighted average matrix microhardness increased to about 241 HV0.01. The number of the matrix subregions has increased to three, including pure matrix (peak 178 HV0.01, 11% content in the matrix) and two subregions of probably different content of the hard phases, mainly chromi-um carbides.

Annealing at 800°C/1000 hours. After the longest time of an-nealing, in the cast steel microstructure, further increase has oc-curred in both the relative volume content and size of secondary precipitates of the M23C6 carbide, mainly as a result of the overlap-ping processes of precipitation and coagulation (Fig. 6). This con-firms a general decrease of the matrix content in the alloy to about 88% (1 + 36% + 43% + 8%, Fig. 2d).

Prolonged annealing of cast steel has increased the content of the matrix with secondary carbide precipitates as compared to the matrix free from the precipitates. This is also the theoretically ex-pected effect of the prolonged time of annealing at a temperature at which the precipitation processes occur most intensively. Two facts confirm this statement:

– further increase in weighted average microhardness of the matrix to about 249 HV0.01 (Fig. 2d),

– obvious disappearance of “pure” matrix (peak 178 HV0.01) and instead the appearance of areas with increased matrix microhard-ness (peak 267 HV0.01, Fig. 2d).The process of the G phase formation is still going on, although

now at a much slower pace (Fig. 6), probably due to increased thick-ness of the envelope of this phase forming around the TiC particles.

Examinations of microstructure indicate that the precipitation of chromium carbide particles takes place within the whole volume of dendritic cells. Carbides are fine-dispersed due to previous changes in the chemical composition of the matrix, mainly its impoverish-ment in carbon and chromium.

Now, the microhardness distribution density function (Fig. 2d) is described with peaks that represent:

– matrix area with an average microhardness of 203 HV0.01, con-taining fine-dispersed particles of the Cr23C6 carbide,

– matrix area with an average microhardness of 231 HV0.01, con-taining single globular secondary precipitates of the Cr23C6 car-bide, clustered at the cell borders and around them,

– matrix area with an average microhardness of 267 HV0.01, with-in which are distributed only the largest particles of the Cr23C6 carbide, formed as a result of the ongoing process of coagulation,

– matrix area with an average microhardness of 305 HV0.01, con-taining high amount of the globular particles of the Cr23C6 car-bide.

Comparing the obtained distributions of the cast steel micro-hardness values (the result of annealing for a time from 10 to 1000 hours), it can be assumed that within the time of up to 500 hours of annealing, the amount, size and distribution of particles in the matrix are determined by the processes of precipitation. After this time, their role in forming an image of the microstructure is taken over by the processes of coagulation.

It is quite obvious that this is a very simplified analysis. It does not cover all factors (they are not included in the studies) that are respon-sible for the microhardness of the material, to mention for example:

– diffusion processes associated with the formation of G phase and changes in the chemical composition of the phases already exist-ing,

– nucleation processes on dislocations of the particles of high dis-persion, or

– fields of structural stress changing in the matrix as a result of the processes of precipitation and coagulation of the particles.

CONCLUSION

The above described analysis of changes in the microstructure of titanium-containing 0.3C-18Cr-30Ni cast steel as a result of an-nealing and in the related changes in microhardness accompanying this treatment has as the main aim to derive a relationship between peaks observed in the developed microhardness distribution density function and selected areas of the microstructure directly observed under the microscope. The analysis does not attempt at a more com-prehensive description of the quantitative and qualitative processes that accompany these changes and at a determination of their nature.

It should be noted that the main advantage of a research of this type is the multifaceted interpretation of the results due to: – application and study of one type of measurements (hardness de-

scribed with the distribution density function) and factors char-acterising the cast steel microstructure (the distribution of the content and size of phases, local changes in the segregation of phases in the solid solution, micro-defects),

– fully quantitative (statistical) description of each technological state of the cast steel owing to the sufficient number of measure-ments of a random character.This procedure allows searching for relationships and depend-

encies in the following cause-and-effect sequence: microstructure and condition of the material → the results of microhardness meas-urements → useful functions and features of the material (e.g. me-chanical properties [3], resistance to corrosion and abrasion etc.).

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