Study on Austenite Grain Growth Behavior of GCr15 Bearing Steel

The effects of austenitizing temperature and holding time on the austenite grain growth behavior of GCr15 bearing steel are reflected in this paper. SEM and EBSD deeply reveal the effect of austenite grain size on the misorientation angle of grain boundary. The results show that both the holding time and the austenitizing temperature will make the austenite grain size grow to a certain extent. However, the change of temperature will cause a greater change in grain size. The grain size will also have different growth trends in different austenitizing temperature ranges. The grain size of Gcr15 test steel is combined with the Sellars model to derive its growth kinetics model.


Introduction
In recent years, although China 's bearings have achieved remarkable results in all aspects, and some fields have approached or reached the level of the world 's advanced bearing enterprises, China 's bearing industry still has related problems such as backward manufacturing technology, poor performance consistency and low life reliability.
As a high carbon chromium bearing steel, GCr15 steel has an indispensable position in bearing materials [1].However, in today 's era of highly developed industry, there are higher standards for the performance of bearing steel.This requires that the microstructure of bearing steel should have finer prior austenite grains to increase the performance and life of bearing steel.At this stage, in order to make the bearing steel structure have a finer and more uniform structure, in addition to having a suitable heat treatment process, the study of the role of grain size changes during the period is also a top priority.The influence of grains on the heat treatment effect of the bearing is not one-sided.If the final formed bearing has idealized grains, the performance of the bearing is also reliable.The finer the austenite grain of [2,3] bearing steel is, the higher the strength and the better the toughness will be after heat treatment.On the contrary, the crack propagation work of bearing steel with coarse austenite grains is significantly reduced and the bearing steel has a large cracking tendency and other unfavorable factors.
In order to study the relationship between holding time and temperature and austenite grain size of GCr15 steel, a series of experiments were carried out in this paper, and the mathematical model of austenite grain growth of GCr15 steel was deduced according to the relationship between the three.Based on this theory, it will help the production of the product to control the performance of the product more clearly.

Test Materials and Methods
Table 1 shows the chemical composition of the hot-rolled GCr15 test steel.Several metallographic samples of 20mm × 20mm × 10mm were taken along the rolling direction on the test material.The sample was heated in a box furnace after reaching the target temperature, and the sample was taken out immediately after reaching the holding time for oil quenching to room temperature.Figure 1 shows the hot working process of this experiment.The sample was washed and dried after grinding and polishing with anhydrous ethanol.The samples were observed and photographed by 4XC optical microscope, and 10 photos were taken for each sample under 200 times field of view.The work of grain size statistics is handed over to the high-profile IPP software.
The samples with austenitizing temperature of 950 °C and 1100 °C for 30 min were selected and named as No1 and No2.After mechanical grinding, polishing and electrolytic polishing, the No1 and No2 samples were placed in the Zeiss Ultra-55 scanning electron microscope equipped with Oxford-EBSD imaging system for crystallographic data acquisition.The scanning steps of No1 and No2 samples were set to 0.20 μm and 0.25 μm, respectively.The EBSD test data of the samples were processed by CHANNEL-5 software, and the crystallographic characteristics of the quenched structure were analyzed by Matlab programming.

Effect of Heating Temperature on Austenite Grain Size of GCr15 Bearing Steel
In order to study the role of several heating temperatures here, the sample of 30 min is taken, and the prior austenite grain structure is shown in figure 2. At the same time, it also reflects the change rule of austenite grain to a certain extent.The grain sizes were 9.2μm, 22.2μm, 47.3μm, 56.7μm, 83.1μm and 128.7μm respectively when the heating temperature was from 850 ℃ to 1100 ℃.In figure 3, the variation trend between the average grain size of the test steel and another parameter is shown.This key parameter is the heating temperature.If figure 2 shows that the heating temperature affects the grain size of the test steel, then figure 3 further highlights its importance.And with the heating temperature of 1000 ℃ as the boundary, before 1000 ℃, The austenite grains of the test steel show a lower growth trend with the increase of temperature; after 1000 ℃, the change of temperature has a more obvious effect on its size.The increase of austenitizing temperature will promote the continuous growth of austenite grain size.This experiment conforms to this rule, but the austenite growth trend in different temperature ranges is obviously different.The particular observation is that as the temperature of austenitizing increases, the tendency for grain growth in austenite becomes more pronounced.
At 850 ℃, because the Acm of GCr15 bearing steel is about 900 ℃, some of the carbides are not dissolved, and the Ac1 temperature of GCr15 bearing steel is 750 ~ 760 ℃, so some austenite precipitates first.In figure 2 (a), some austenite grains are coarser, while some austenite grains are finer.
Compared with before, at 900 °C, the austenite grains grow further and achieve higher uniformity.When the temperature is heated to 950 ℃, it can be seen that the grains are not uniform and are growing.
With the addition of external temperature, the energy in the test steel is also increasing.One phenomenon that spontaneously occurs to reduce the internal energy is austenite grain growth through grain boundary migration [4].In this experiment, with the increase of temperature, this phenomenon is more and more obvious.
In the range of 850 ℃ ~ 1000 ℃, the grain growth law is relatively stable, showing a gradual increasing trend.After the heating temperature reaches 1000 ℃, it shows a rapid growth trend in the range of 1100 ℃, and some grain sizes are different.

Effect of Holding Time on Austenite Grain Size of GCr15 Bearing Steel
The content of figure 4 is the relationship between the grain size of the sample and the temperature and time from another perspective.Obviously, the high temperature of the same temperature will also cause the grain size to grow.With the increase of temperature, the austenite grains grow rapidly.Considering the temperature and time, the extension of time at high temperature will have a greater impact on the austenite grain size.
The influence of simple temperature or holding time on grain size growth is strong or weak, which is shown in figure 3 and figure 4.

Kinetic Model of Austenite Grain Growth of GCr15
The process of spontaneous formation of austenite grains occurs through the migration of boundaries between the grains.The main factor affecting the grain growth rate is the migration rate of grain boundaries.In the case of GCr15 bearing steel, the migration rate of the grain boundaries during the austenitizing process is closely linked to both the austenitizing temperature and the volume fraction of carbides present.The increase of austenitizing temperature will increase the dissolution degree of carbides，that is ，the pinning effect of undissolved carbides with smaller volume fraction on grain boundaries is greatly reduced and is beneficial to the migration of grain boundaries.The following equation can perfectly show the growth behavior of austenite grains at different austenitizing temperatures [5,6]: where A is a constant; Q is the activation energy of grain boundary migration (J•mol -1 ) ; R is the gas constant (8.31J/(mol•K)) ; T is the absolute temperature (K) ;    and   0  are the average diameter of austenite grains (μm) at t and t0, respectively.n is the time index.
Equation (1) shows in a more detailed way than figure 3 and figure 4 that the grain size increases exponentially with temperature and also grows with the duration of heating.
It can be seen from equation (1) that n, A and Q are required to obtain the specific equation suitable for this experiment.
As far as the austenite grain size after quenching is concerned, D0 has little effect here, so the influence of D0 on the model results can be eliminated here.Then the equation can be transformed into: It can be seen from equation ( 2) that there are three parameters to be fitted in the Sellars model: n, A and Q.The Beck equation has made outstanding contributions to improving the accuracy of the Sellars model here [7]: At this time, the equation is converted into a linear function of lnD with respect to lnt.After the corresponding processing of the experimental data, the linear fitting is performed as shown in the figure 5.
After taking the mean value of the obtained n value, take n = 6.5336.
Here, the exponential form in equation ( 4) can be transformed into the following form: The linear relationship between lnk and 1/T is confirmed in equation ( 6).The slope of the linear function is -Q/R and the intercept is lnA.The values of n and k obtained in equation ( 5) and the experimental values are brought into equation ( 5) for calculation to obtain the values of parameters A and Q.
Finally, n = 6.5336,A = 1.2985 × 10 39 , Q = 7.59617 × 10 5 were obtained.As the final parameters are brought into the Sellars model, the grain growth model of GCr15 bearing steel is also unraveled: The content in figure 6 is the comparison between the prediction model and the experimental value.The predicted model is in good agreement with the experimental values in the figure, indicating that the Sellars model can well predict the change law of austenite grains of GCr15 bearing steel under the two parameters of temperature and time, which is irreplaceable for the application value in practical engineering.

Analysis of Prior Austenite Grain
The SEM images of samples No.1 and No.2 are clearly presented in figure 7.With the increase of heating temperature, the martensite lath in the test steel becomes coarser and more directional, and the prior austenite grain boundary [8] becomes clearer.In figure 8, the prior austenite (PA) grain boundaries attract most of the grain boundaries with misorientations of 15 ° -45 °, Packet and Block grain boundaries are mostly grain boundary orientation differences greater than 45 °, which are large-angle grain boundary; the martensite lath boundary orientation difference is 5 ° ~ 15 °, which is a low-angle grain boundary.The increase of austenitizing temperature makes the crystallographic block morphology of the test steel change from small size block to long strip, which is also consistent with the literature [9], as shown in figure 7 and figure 8.The increase of temperature also affects the distribution of different misorientation angle of grain boundarys.The specific performance is that the high-angle grain boundary changes from a more uniform distribution to a more concentrated distribution, and the distribution of small-angle grain boundaries becomes uneven.
High-angle grain boundaries can more effectively the propagation of separated microcracks [10,11], which will make the test steel have better toughness.There are many reasons for the formation of high-angle grain boundaries.The main reason is that the adjacent orientation of bainite or martensite phase is different from that of retained austenite in ferrite matrix.The low-angle grain boundary exists in the large grain in an approximately parallel manner with the adjacent structure [12].Compared with the two austenitizing temperatures, the higher austenitizing temperature has a higher proportion of lowangle grain boundaries, because it is accompanied by more large-sized grains.Among the two samples, the prior austenite grains of the sample with lower austenitizing temperature are smaller, and it has more large-angle grain boundaries with uniform distribution, which has higher ability to inhibit crack propagation and better toughness.

Conclusion 1)
The austenitizing temperature is the main factor affecting the austenite grain size, and it is consistent with the holding time in terms of the positive correlation with the austenite grain size.
2) In addition to the proportion of grain boundary misorientation angle is affected by the prior austenite grain size, it also affects the distribution of misorientation.When the prior austenite grain is small, there are more large angle grain boundaries with uniform distribution, which have higher ability to inhibit crack propagation and better toughness.
3) The austenite grain growth law of GCr15 steel under different heating temperature and holding time can be described by the following equation:

Figure 3 .
Figure 3.The change trend of average austenite grain size of test steel with austenitizing temperature.

Figure 4 .
Figure 4. Change rule of austenite grain size of GCr15 bearing steel.
Q with parameter k can avoid the problem of uneven distribution of parameters A and Q caused by insufficient test data and reduce the error of the model.In equation (3), only the parameters n and k are unknown.The following results appear only after taking logarithms on both sides of equation (3

Figure 5 .
Figure 5.The relationship between lnD and lnt of test steel at different heating temperatures.

Figure 6 .
Figure 6.Comparison of measured and calculated average diameter of austenite grains.

Figure 7 .Figure 8 .
Figure 7. Scanning electron microscope image of quenching structure of sample.(a) No.1, (b)No.2.The IPF (inverse pole figure) diagram and grain boundary distribution of the No.1 and No.2 samples are shown in figure 8.In figures 8 (b) and (d), tThe red line and the black line represent the grain boundaries with misorientation between 5 ° -15 ° and 15 ° -45 °, respectively, and the misorientation greater than 45 ° is represented by yellow lines.