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Materials Research

Print version ISSN 1516-1439On-line version ISSN 1980-5373

Mat. Res. vol.5 no.3 São Carlos July/Sept. 2002

http://dx.doi.org/10.1590/S1516-14392002000300025 

Interaction Between Second-Phase Particle Dissolution and Abnormal Grain Growth in an Austenitic Stainless Steel

J.C. Dutra*a, F. Siciliano Jr.b and A.F. Padilhab

aDepartment of Metallurgy, Faculty of Industrial Engineering
Av. Humberto de Alencar C. Branco, 3972, 09850-901 São Bernardo do Campo SP, Brazil

bDepartment of Metallurgical and Materials Engineering, University of São Paulo,
Av. Prof. Mello Morais, 2463, 05508-900 São Paulo - SP, Brazil

*e-mail: jdutra@cci.fei.br

Received: September, 27, 2001; Revised: July 10, 2002

 

The continuing development of stainless steels has resulted in complex steel compositions with substantial amounts of alloying elements. The benefits of such additions invariably come attached to unavoidable disadvantages. One of the most critical item is the potential microstructural instability of the material. Alloying elements may be in a supersaturated solid solution, in which the precipitation of carbides, nitrides, borides and intermetallic phases occurs in a wide range of temperatures. In order to dissolve the mentioned precipitates, solution annealing is commonly performed. However, at the temperature range in which this treatment is carried out, the onset of abnormal grain growth can occur. The interaction between the dissolution of these second-phase particles and the occurrence of abnormal grain growth is investigated in this work. This study also shows that the thermodynamics and the kinetics of dissolution of precipitates may be used to predict whether abnormal grain growth takes place.

Keywords: abnormal grain growth, stainless steels, precipitates.

1. Introduction

There are basically two modes of grain growth: normal and abnormal1-5. During normal grain growth, sometimes called "continuous grain growth", the distribution of grain sizes remains approximately constant and the grains increase their diameter continuously and gradually, displaying a monomodal grain size distribution. During abnormal grain growth or secondary recrystallization (sometimes called "discontinuous grain growth"), particular grains grow exaggeratedly. The mean size of the others remains constant and the grain size distribution becomes very inhomogeneous. In other words, secondary recrystallization requires that the normal grain growth be strongly impeded, with the exception of a few grains, which act as nuclei.

Abnormal grain growth has been reported predominantly in alloys containing particles. When specimens containing a considerable amount of dispersed particles is annealed at a temperature close to the solvus point, in which part of the second-phase is dissolved, very large grains are formed by discontinuous grain growth2.

Solution annealing heat treatments are usually performed to dissolve precipitates. Precipitates such as carbides, nitrides, sulphides, borides and several types of intermetallic phases are often present in the microstructure of austenitic stainless steels. Nitrides and sulphides are very stable and are not able to be dissolved in the solid state. On the other hand, carbides like M23C6 and M6C as well as intermetallic phases may dissolve completely, depending on both the annealing temperatures and times. Carbides of the MC type can be only partly dissolved at usual annealing temperatures. These particles play a major role in both the grain growth inhibition and onset of abnormal grain growth6.

The aim of the present study is, therefore, to investigate the interaction between second phase particles dissolution and abnormal grain growth during solution annealing heat treatments in a titanium stabilised austenitic stainless steel.

2. Experimental

The material chosen for this study was a stainless steel DIN X 10 NiCrMoTiB 15 15, Werkstoff-Nr. 1.4970. The chemical composition (in wt.%) is the following: 0.09%C, 14.6%Cr, 15%Ni, 0.46%Si, 1.70%Mn, 0.003%P, 0.004%S, 0.013%Co, 1.25%Mo, 0.46%Ti, 0.07%Cu, 0.0045%B, <0.05%Nb and 0.01%N. Several 10.5 mm-diameter bars were obtained by swaging and solution treatment. 10 mmheight specimens were cut off and submitted to heat treatments at temperatures of 1090, 1110, 1130, 1150, 1200, 1250, and 1300 °C for 5, 10, 15, 20, 25, 60, 125, 625, 3125 min. Additional specimens were aged at the temperatures 600, 700, 800, and 900 °C for 1000 h to evaluate the effect of the type of second-phase particles on grain growth. After these ageing treatments, they were treated at the temperatures 1090, 1130, and 1300 °C for 0.5, 5, and 25 h to provoke abnormal grain growth.

The metallographic preparation was performed by using standard procedures, while the final polishing was done by using Minimet®, with a solution of colloidal silica of 0.25 mm, for 45 min. The electrolytical etching used was a solution of 70% vol. nitric acid and 30% vol. distilled water, under a tension of 0.75 V and current density of 50 mA for 15 min. This etchant7 allows to reveal only the grain boundaries necessary to measure the grain size. This procedure does not reveal annealing twins, which is good when grain size measurements are involved.

Grain size measurements were conducted according to ASTM E-112-82. Twenty fields were measured for each specimen. When abnormal grain growth took place, the volumetric fraction of gross grains was measured by quantitative analysis.

The second-phase particles were extracted by dissolving the matrix with a Berzelius solution8, followed by a vacuum filtration with a membrane filter with a porous size of 0.1 mm. The residues obtained with this technique were analysed by X-ray diffraction in a Guinier-Jagodzinski apparatus with a monochromatic radiation of Cuka1. This was done to identify the type, structure and lattice parameters of the second-phase particles9.

3. Results and Discussion

At the ageing temperature of 600 °C, the precipitation of M23C6 took place at grain boundaries. At 900 °C, there was exclusively the precipitation of (Ti0.92Mo0.08)C in both the interior of grains and at their boundaries. At 700 and 800 °C there was the presence of both precipitates mentioned above. The average grain sizes of the samples treated at 1090 and 1130 °C for all periods of time were basically the same: 44.7 and 42.9 mm, respectively. These average grain sizes were measured for grains subjected to normal grain growth inhibition only. Such procedure could not be done for samples treated at 1300 °C because of their low level of heterogeneity. In fact these samples presented only normal grain growth.

The evolution of abnormal grain growth was quantified by measuring the volume fraction of gross grains, as can be seen in Fig. 1 and Table 1. This figure shows that the onset of abnormal grain growth occurs earlier in those samples aged at 600 °C for 1000 h at 1090 or 1130 °C.

The analysis of Fig. 1 shows clearly that there is a distinct behaviour for the samples aged at 600 °C for 1000 h from the other ones. This figure shows lines of iso-percentage of gross grains for several temperatures and times and different previous ageing treatments. For similar periods of time, the percentage of gross grains is always higher in samples previously aged at 600 °C as compared to other samples, which means that the presence of M23C6 particles has an enhancement effect on the onset of abnormal grain growth. The tendency in the other cases is, therefore, to retard the onset of abnormal grain growth. This may also be concluded from the results presented in Table 1.

 

 

The tendency for second-phase particles dissolution may be analysed by the solubility product. The solubility products were calculated by a linear regression of the logarithm of the weight concentrations of the respective elements in solid solution in austenite as a function of the inverse of the absolute temperature. Supposing that the particles are (Ti0.92Mo0.08)C, the following equation can be obtained10:

where Ti, Mo, and C represent the weight percentages of these elements and T, the absolute temperature.

The solubility product of M23C6 particles can be calculated by the following equation:

It is worth mentioning that the latter solubility product was experimentally determined within the range 750 to 1100 °C11.

As an example, the temperature in which all M23C6 particles may be in solid solution can be calculated, by supposing that all the carbon contained in this steel is combined with Cr. Assuming this, the temperature obtained is 1029 °C. The same calculation for (Ti0.92Mo0.08)C particles gives an equilibrium temperature of 1169 °C, considering in this case that only half of the carbon is in solid solution in austenite. This temperature is higher than the one calculated for the M23C6 particles, which shows that (Ti0.92Mo0.08)C particles are more stable and, therefore, may only be completely dissolved in higher temperatures. This fact brings consequences to grain growth behaviour. Figure 1 shows the volumetric fraction of gross grains versus the type of second-phase particles and time for the annealing treatments at 1090 °C. It shows that abnormal grain growth occurs more extensively for the samples aged at 600 °C for 1000 h. These samples have predominantly M23C6 precipitated at grain boundaries. As these particles are less stable than (Ti0.92Mo0.08)C ones, although they impose grain growth inhibition in some regions, in others where dissolution takes place, abnormal grain growth may occur.

However, the kinetics of dissolution of these particles must also be analysed. Aaron and Kotler's model can be used to study this aspect12-14. The solution time can be calculated by the following equation:

where:

r0 is the second-phase particle radius;

DV is the volumetric diffusion coefficient;

k is the coefficient of supersaturation:

where:

CI is the concentration of solute atoms in equilibrium at the interface between matrix and particle at solution temperature TS;

CP is the concentration of solute atoms in equilibrium in the particle;

CM is the concentration of solute atoms in equilibrium in the matrix at the solution temperature TS.

By using the values of pre-exponential coefficient and the activation energy from the diffusion of chromium and titanium in austenite15, it is possible to evaluate the time for the solution of the M23C6 and (Ti0.92Mo0.08)C particles, respectively, at various temperatures. The supersaturation coefficient of 0.1 is typical for common alloys12. The results of this analysis is shown in a graphical form in Fig. 2a, which presents a few curves showing the solution time for several M23C6 particles of different radii as a function of time and temperature. For instance, a 0.4 mm-diameter particle takes 10 s to reach complete dissolution at 1300 °C. Figure 2b, on the other hand, shows the dissolution curves of (Ti0.92Mo0.08)C particles of different radii at various temperatures.

 

 

The calculated times for dissolution of M23C6 and (Ti0.92Mo0.08)C particles can be seen in Table 2. The diameters considered are typical of the samples used in this work. This table shows that the times for dissolution of M23C6 precipitates are very short.

 

 

A comparison between the calculated dissolution times and the time for the onset of abnormal grain growth is seen in Table 3. The average particle size considered for this analysis is 2.5 mm because it is the closest value to the analysed samples. This table shows that abnormal grain growth takes place in shorter times than the total dissolution of the particles. This analysis can be used to predict Fig. 3, which was produced after examining many samples treated at different times and temperatures. As a consequence of the partial dissolution of particles, some grain boundaries can be free for moving, and this effect leads to a preferential growth, which initiates abnormal grain growth.

 

 

 

 

 

 

4. Conclusions

The following conclusions can be drawn from this work:

•Within the temperature range of abnormal grain growth occurrence, M23C6 particles precipitated predominantly at grain boundaries, causing an earlier onset of abnormal grain growth as compared to samples in which precipitation occurred in the interior of the grains (at lowest temperature, 1090 °C). This effect is associated to the stability and kinetics of dissolution of M23C6 particles;

•The initiation and the kinetics of abnormal grain growth can be predicted by analysing both the thermodynamics and kinetics of second-phase particle dissolution.

Acknowledgements

The authors are grateful to the agencies CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico and FAPESP for sponsoring this work.

References

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