Thermal deformation behavior and microstructure analysis of the as-cast super austenitic stainless steel Sanicro35

Thermal compression experiments on the super austenitic stainless steel Sanicro35 were carried out using a Gleeble 3800 thermal simulation laboratory machine to investigate its thermal deformation behavior at different deformation temperatures (900 °C–1150 °C) and strain rates (0.001–10 s−1). The microstructure of the large deformation zone of the specimen was investigated using electron backscatter diffraction (EBSD). The results show that the thermal compression rheological stress of the super austenitic stainless steel Sanicro35 decreases with increasing temperature and decreasing strain rate. Dynamic recrystallization (DRX) is the main softening mechanism for this material. The morphology characteristics, recrystallization fraction, dislocation density and twin grain boundary distribution of the microstructure were analyzed by EBSD. With the increase of deformation temperature, the higher grain boundary mobility contributed to the growing of DRX grains. As the strain rate increases, the larger deformation storage energy provides sufficient activation energy for DRX grain nucleation, and the nucleation of DRX grains becomes denser. The twin boundaries are mainly distributed within the DRX grains. The smaller the grain size of DRX, the denser the nucleation of twin boundaries, and the generation of twins can promote the development of DRX. The softening mechanism under most deformation conditions is discontinuous dynamic recrystallization (DDRX). However, at 10 s−1, the high strain rate causes microbands to be generated within the deformed grains, and the microband boundaries evolve toward the high-angle grain boundaries (HAGBs) with increasing temperature, which promotes the occurrence of Continuous dynamic recrystallization (CDRX).


Introduction
Compared with conventional austenitic stainless steel, super austenitic stainless steel has a higher content of chromium, nickel, molybdenum, nitrogen and other elements.It has excellent corrosion resistance and comprehensive mechanical properties and is widely used in the chemical industry, petrochemical industry, aerospace, marine engineering and other fields [1][2][3].Sanicro35 is a new super austenitic stainless steel developed by Sandvik.Its composition system is 27Cr-35Ni-6.5Mo-0.3N, in the traditional 6Mo stainless steel 254SMO and N08367 based on improving the Cr, Ni, and N content.The combination of nitrogen and chromium can significantly improve the pitting and interstitial corrosion resistance of austenitic stainless steel, giving it excellent comprehensive mechanical properties [4,5].
The mechanism of thermal deformation behavior is very complex.In order to control thermal deformation behavior, it is necessary to conduct in-depth research and analysis on the properties and treatment conditions of the material.Zhao [6] optimized the hot working process of high molybdenum and high nitrogen super austenitic stainless steel through the evolutionary law of microstructure and hot working diagrams and determined that the optimal process window for hot working was a deformation temperature of 1150 °C-1250 °C and a strain rate of 0.001-0.01s −1 .Ren [7], obtained the thermal deformation activation energy of 254SMo by thermal compression test, analyzed the recrystallization mechanism and established the Z-Hollomon parametric equation for 254 SMo.By calculation, Yang [8], obtained the apparent activation energy (Q) of high-temperature deformation of high-nitrogen austenitic stainless steel of 350.1 kJ mol −1 and established the Arrhenius ontological relationship.The thermal processing diagram showed that 1050 °C-1100 °C, 0.001-0.1 s −1 , was its optimal thermal processing window.By analyzing the thermal processing map and microstructure of the super austenitic stainless steel 24Cr-22Ni-7Mo-0.4N, Zhao [9] found that the reasonable thermal processing region was the deformation temperature of 1150 °C-1200 °C, the strain rate of 0.1-1 s −1 , and that the softening mechanism under most of the deformation conditions was discontinuous dynamic recrystallization (DDRX).The super austenitic stainless steel Sanico35 exhibits prominent grain size and highly uneven distribution in the as cast microstructure under a low magnification microscope, resulting in poor overall performance of the original as cast microstructure.Dynamic recrystallization (DRX) generation can refine the grain.The hot deformation process and dynamic recrystallization behavior of this steel are investigated to find a reasonable hot working region and improve its comprehensive mechanical properties.
This paper used thermal simulation compression experiments on the super austenitic stainless steel Sanicro35 and analyzed the thermal deformation behavior at different temperatures and strain rates.Electron backscatter diffraction (EBSD) technology was used to analyze the microstructure and morphology of the central deformation zone of the specimen after hot compression, elucidating its deformation mechanism and dynamic softening mechanism, providing a reliable theoretical basis for its research.

Experimental method
The cast super austenitic stainless steel was homogenized at 1210 °C for 48 h and solution treated at 1250 °C for one hour to obtain a homogeneous single austenitic organization.The compositions of super austenitic stainless steels are shown in table 1. Cylindrical hot compression specimens with a diameter of 8 mm and a height of 12 mm were cut by wire-cutting.
Uniaxial thermal compression experiments were performed on a Gleeble-3800 thermal simulation tester.A type k thermocouple was first spot-welded in the middle of the cylindrical surface to control the thermal cycle by monitoring the temperature change through a feedback system.Graphite lubricant and tantalum sheets were used to minimize friction at both ends of the specimen in contact with the platen before the experiment [10].The experimental temperatures ranged from 900 °C to 1150 °C, and the strain rates ranged from 0.001 s −1 to 10 s −1 .The detailed thermal compression process was shown in figure 1, where the specimens were first heated to 1250 °C at a rate of 20 °C s −1 and held for 6 min to arrive at the uniformity of microstructure.After that, the specimens were cooled to different deformation temperatures (900 °C, 1000 °C, 1050 °C, 1100 °C, and 1150 °C) at a rate of 5 °C/s.After holding the specimens in this temperature range for 1 min, compressive deformation was started with a deformation degree of 60%.After heat deformation, the compressed specimen was rapidly water-cooled to room temperature.
After the thermal compression experiment, the specimen was cut into two halves along the direction of the compression axis, one half of the specimen was selected for mechanical grinding and polishing, and the samples were subjected to electrolysis for EBSD observation, with an electrolytic solution of 10% perchloric acid, voltage of 15 V.This paper used EBSD to scan the central large deformation zone.It mainly scans the region that produces dynamic recrystallization.Then, the EBSD data were analyzed and processed by using the post-processing software to obtain the morphological characteristics of the microstructures, the distribution of recrystallization, the density of dislocations, and the distribution of twinned grain boundaries.

Flow characteristics and deformation mechanisms
Figure 2 shows the true stress-strain curves of super austenitic stainless steel at different temperatures and strain rates.Strain rate and temperature are critical factors affecting rheological stress, which decreases with increasing temperature and decreasing strain rate.The change of rheological stress is divided into three stages [11].In the first stage, dislocations continue to produce and accumulate so that the work hardening continues to rise, resulting in the beginning of the rapid increase in rheological stress; in the second stage, dynamic recrystallization and dynamic recovery reduce the work hardening rate so that the growth of the rheological stress slows down so that it reaches the peak and the performance of single-peak characteristics [12].In the third stage, as the strain increases, work hardening is continuously counteracted, resulting in a decrease in work hardening rate and a decrease in rheological stress.When the rheological softening and hardening reach equilibrium, the rheological curve gradually becomes steady [13].
The upward trend of the flow curves remained at high strain rate conditions and became more pronounced at lower temperatures (figure 2(d)).The phenomenon may be because the high strain rate cannot provide enough time for the nucleation and growth of dynamically recrystallized grains, and work-hardening dominates, which leads to an increasing trend in the rheological curve [14].Figure 2(f) shows the maximum stress for different deformation conditions, which decreases with increasing temperature and increases with increasing strain rate.When the strain rate is kept constant, the faster rheological curve peaks and the more pronounced the downward trend as the temperature increases.It suggests that DRX is the primary softening mechanism of super austenitic stainless steel, and it is more likely to occur at high temperatures [15].The strain and stress at the peak of the rheological curve are more significant under low temperature and high strain rate conditions, which produce more significant strain inhomogeneity in the specimen.In order to obtain a realistic stress-strain curve for sanicro35 during thermal compression experiments using the Gleeble-3800 thermal simulator, it is necessary to friction-correct the raw stress-strain data to minimize stress distortions caused by frictional factors during compression.This correction process can be realized by equation (1) [16].
In which, σ f represents the stress after friction correction; σ is the true stress; ε is the true strain; f is the friction coefficient; b is the obstacle parameter; h 0 and r 0 are the original height and radius of the sample; h and r are the height and average radius of the deformed sample; r M and r T respectively denote the maximum radius and top radius of the deformed sample [17].The true stress-strain curve after friction correction is shown as the dashed line in figure 2.

Microstructure analysis
Figure 3 shows the EBSD microstructure of super austenitic stainless steel under different deformation conditions.Under the 1 s −1 strain rate conditions, the necklace-like distribution of fine DRX grains along the grain boundaries in figure 3(a).It indicates that at this stage mainly proceeds as DDRX [18,19].The increase in temperature provides energy for grain boundary migration, and the fine grains distributed along the grain boundaries show a growth trend (figure 3(b)).When the temperature rises to 1150 °C, the size of the necklacelike distributed DRX grains increases significantly (figure 3(c)).Under the 10 s −1 strain rate condition, the faster compression rate and lower temperature elongate the pristine grains along the direction perpendicular to the compression axis, which results in a flattened structure of the deformed grains (3(d)).Compared with figure 3(a), the nucleation rate and coverage of DRX are significantly improved, which is attributed to the higher deformation storage energy at a strain rate of 10 s −1 , providing sufficient driving force for DRX nucleation and promoting its nucleation.The deformed and DRX grains are alternately arranged in a chain-like structure, and typical DDRX also occurs (figure 3(e)).When the temperature rises to 1150 °C, some DRX grains have developed into equiaxed crystals (figure 3 At a deformation temperature of 1150 °C, the larger size of the DRX grains in figure 3(g) is due to the lower strain rate, which allows sufficient time and conditions for the DRX grains to grow.As the strain rate increases, the shortening of the deformation time leads to the inability of the DRX grains to grow sufficiently.The DRX grain size tends to decrease (figure 3(h)).When the strain rate increases to 0.1 s −1 , the more significant strain rate provides sufficient activation energy for the DRX grains, making the DRX grain nucleation denser.
In conclusion, as the strain rate increases, the austenite grains tend to flatten.The smaller the DRX grain size, the denser the nucleation.As the temperature increases, the DRX grains tend to grow.The higher DRX fraction for deformation conditions with a strain rate of 10 s −1 indicates that DRX develops faster and denser at higher strain rates.It is due to the high strain rate with considerable deformation storage energy and dislocation density, which provides sufficient driving force for DRX nucleation [20].The DRX grains in all microstructural regions are necklaced along the grain boundaries of the deformed grains.It indicates that DDRX is the primary softening mechanism in super austenitic stainless steels [21].The presence of DRX grains within the deformed grains at both 1 and 10 s −1 , especially at 10 s −1 , suggests that CDRX is prone to occur under high strain rate conditions, but is mainly dominated by DDRX.
Figure 4 shows the distribution of grain boundary orientation differences under different deformation conditions.On the one hand, the average value of grain boundary orientation difference (θ Ave ) increases from 7.5°to 35.2°with increasing temperature when the strain rate is 1 s −1 (figures 4(a)-(c)).θ Ave increases from 8.6°t o 44.7°with increasing temperature when the strain rate is 10 s −1 (figures 4(d)-(f)).On the other hand, the percentage of LAGBs decreases while the percentage of HAGBs increases with increasing temperature (figures 4(j)-(k)).It is due to the higher driving force for grain boundary migration with increasing deformation temperature, which promotes sub-grain transformation and DRX grain growth [22].
When the deformation temperature is 1050 °C, θ Avg decreases from 48.6°to 23.6°with increasing strain rate (figures 4(g)-(i)).Contrary to the pattern in figures 4(j)-(k), the percentage of HAGBs decreases from 73.8% to 57.4% with increasing strain rate, while the percentage of LAGBs increases from 26.2% to 42.6% (figure 4(l)).The distribution of LAGBs at the grain boundaries of the deformed grains and within the grains is caused by the rapid accumulate of dislocations, strong interactions between dislocations, and limited recovery at high strain rates [23].As the strain rate increases, the DRX grains continuously nucleate to produce fine DRX grains, so the percentage of low-angle grain boundaries is increasing.
The generation of twins promotes DRX nucleation, while the formation of twins is an important nucleation and growth mechanism for DDRX.The current results show that the process of DRX occurs with the formation of twin crystals in materials with low-layer dislocation energy, and most of the twins are generated during the growth of DRX grains [24].Austenite with low dislocation energy is prone to twinning, which promotes DRX nucleation and is one of the reasons for the high DRX fraction at 10 s −1 strain rate.
Figure 5 shows the distribution of twin boundaries in the microstructure under different deformation conditions.Under the condition of 1 s −1 strain rate, the twin boundaries are mainly distributed in the fine DRX grains.With the increase of temperature, the DRX grains grow continuously, and the twin boundaries distributed in the DRX grains become more and more obvious (figures 5(a)-(c)).Under the condition of a 10 s −1 strain rate, the twin boundaries distributed within the fine DRX grains are denser compared with figure 5(a) (figure 5(d)).As the temperature increases, it can be observed that the smaller the DRX grains, the denser the distribution of twin boundaries (figure 5(e)).However, it is worth noting that the twin boundaries are sparse when the temperature increases to 1150 °C because the DRX grains develop into equiaxed grains (figure 5(f)).At a deformation temperature of 1050 °C, the lower strain rate results in essentially no formation of twin boundaries within the fully grown DRX grains (figure 5(g)).As the strain rate increases, many twin boundaries appear within the DRX grains, mainly distributed within the newly nucleated DRX grains (figure 5(h)).When the strain rate increases to 0.1 s −1 , the larger deformation energy storage drives the continuous nucleation of DRX grains, and the distribution of twin boundaries becomes denser.
Figure 5(j) shows the percentage of twin boundaries under different deformation conditions.When the strain rate is 1 s −1 , the percentage of twin boundaries increases with temperature raise.Because high temperature promotes grain boundary migration and nucleation occurs at twin boundaries.Notably, when the strain rate is 10 s −1 , the percentage of twinned grain boundaries first increases and then decreases.This is because the increase in temperature causes the DRX grains to grow and develop into equiaxed grains, which leads to a decrease in the percentage of twin boundaries.The increase in strain rate provides sufficient driving force for DRX nucleation, but the shorter deformation time leads to insufficient time for DRX grain growth, resulting in small and dense DRX grains.The percentage of twin boundaries increases with the increase in strain rate.Figures 5(k)-(l) shows the local magnification of figures 5(h) and (i), respectively, from which the twins that are nucleating and those that have nucleated can be observed.The nucleating twins are mainly distributed in the newly nucleated DRX grains, and the denser the newly nucleated DRX grains are, the denser the twin boundaries are.The nucleated twins are mainly distributed in the larger DRX grains after nucleation.Figures 5(k)-(l) shows that twinning can promote DRX nucleation.
In conclusion, the twin grains are mainly distributed within the DRX grains, and the smaller the DRX grains are, the denser the twin boundaries are.Meanwhile, the generation of twin grains can promote the development of DRX.The percentage of twin boundaries increases continuously with increasing temperature.However, when the temperature increases to the extent that the DRX grain develops into an equiaxed grain, the percentage of twin boundaries within the DRX grain decreases significantly.As the strain rate increases, the DRX grains are more likely to nucleate, so the twin boundaries distributed within the DRX grains are denser.
Figure 6 shows the recrystallization diagram of the microstructure under different deformation conditions.Under 1 s −1 strain rate conditions, the larger strain and lower temperature transformed pristine grains by deformation into deformed grains with a coverage of 90.7%.The DRX grains do not have sufficient time and conditions to grow up, so the DRX grains account for a relatively small percentage (figure 6(a)).When the temperature increases from 1050 °C to 1150 °C, the proportion of DRX and sub-grains increases continuously, At 1050 °C, the lower strain rate causes the primary grains to develop into sub-grains at a percentage of 71.2%.Sufficient deformation time makes the DRX grains grow sufficiently, so the DRX grain size is large (figure 6(g)).When the strain rate increases from 0.01 s −1 to 0.1 s −1 , the deformation time shortens the pristine grains into deformed grains, so the percentage of deformed grains keeps increasing.The increase in strain rate provides enough activation energy for the nucleation of DRX grains, so the percentage of DRX grains and the nucleation rate keep increasing (figures 6(h)-(i)).Figures 6(j)-(l) shows the statistics of recrystallization fraction under different deformation conditions.The DRX fraction increases from 5.2% to 29.2% and 19.4% to 47.8% at strain rates of 1 s −1 and 10 s −1 , respectively (figures 6(j)-(k)).The DRX fraction increases with the strain rate from 16.1% to 35.3% (figure 6(l)).
Dislocation mobility plays a vital role in the dynamic recrystallization nucleation process.KAM stands for Kernel Average Misorientation, closely related to the dislocation density and its evolution concerning grain orientation.Therefore, KAM plots are often used as an essential method to study dynamic recrystallization processes.High KAM regions usually indicate the presence of dense dislocations associated with large amounts of deformation energy.Figure 7 shows the dislocation maps of microstructure regions under different deformation conditions.It is evident from figure 7 that many dislocations exist inside the deformed grains and at the grain boundaries, while there are almost no dislocations inside the newly nucleated DRX grains.
Under 1 s −1 strain rate conditions, as the DRX grains grow with increasing temperature, the driving force to produce DRX decreases the dislocation density, leading to a significant decrease in the KAM value (figures 7(a)-(b)).The locally enlarged image in figure 7(b) shows that green lines gradually form DRX grain boundaries within the deformed grains, indicating that CDRX is occurring in this region.DDRX occurs along the fine DRX grains at the grain boundaries of deformed grains.It indicates that DDRX and CDRX coexist in this region, but mainly in DDRX.When the temperature rises to 1150 °C, the dislocations are mainly distributed at the grain boundaries of the deformed grains.While the dislocation density inside the deformed grains is low (figure 7(c)).This is because the driving force for recrystallization is considered to be the difference in dislocation density between the deformed grains and the DRX grains.Therefore, the dislocation density of the deformed grains significantly reduces when this region is transformed into DRX grains [25].
When the strain rate increases from 1 s −1 to 10 s −1 , the dynamic restitution weakens, the dislocation densities and deformation storage energy increase, resulting in a higher KAM value.The high strain rate causes microbands to be generated within the deformed grains (figure 7(e)).At the same time, it can be seen from figure 4 that most of the microband boundaries transform from LAGBs to HAGBs with increasing temperature, indicating that the promotion of microband generation can facilitate the occurrence of CDRX [26].DRX grains are generated within the deformed grains (figures 7(d), (e)).This is because the sub-grains within the deformed grains continuously absorb dislocations during rotation from LAGBs to HAGBs, which are eventually transformed into newly nucleated DRX grains, and typical CDRX occurs [27].When the temperature rises to 1150 °C, the higher temperature and strain energy increase the dislocation slip, and some of the DRX grains have developed into equiaxed grains with a significant decrease in dislocation density (figure 7(f)).Microbands are present in the deformed grains, indicating that CDRX occurs in this region.This condition favors CDRX development and DRX grain growth.It is shown that high temperatures and high strain rates favor CDRX in super austenitic stainless steels [28].As can be seen from figures 7(d)-(f), under the deformation condition of strain rate of 10 s −1 , the necklace-like distribution of DRX grains at the grain boundaries of the deformed grains and the generation of microbands within the deformed grains indicate the coexistence of DDRX and CDRX.
At a deformation temperature of 1050 °C, the lower strain rate and higher temperature cause the DRX grains to grow sufficiently, while the DRX grains continue to absorb dislocations as they grow, which results in a low dislocation density (figure 7(g)).When the strain rate increases to 0.01 s −1 , sufficient strain energy is available to generate many dislocations within the deformed grains (figure 7(h)).When the strain rate increases to 0.1 s −1 , the dislocation density inside the deformed grains and at the grain boundaries increases significantly (figure 7(i)).The dislocation density inside the deformed grains increases with the strain rate (figures 7(g)-(i)), which is consistent with the distribution law of equivalent strain and confirms that the degree of strain determines the defect density [29].The KAM plots of microstructure under different deformation conditions shown in figure 7 can confirm the strain inhomogeneity of the whole specimen to a large extent.
Figure 8 shows thermal deformation behavior characteristics of super austenitic stainless steel in microstructure.DDRX is the primary softening mechanism in austenitic stainless steels.DDRX and CDRX coexist under 10 s −1 strain rate conditions.A high strain rate has considerable deformation storage energy and dislocation density, and the grain boundary mobility increases with temperature.Hence, the strain rate of 10 s −1 has a high DRX fraction and twin nucleation rate.Under low temperature and high strain rate conditions, the DRX grains are distributed in a necklace-like pattern at the grain boundaries of the deformed grains, indicating the occurrence of typical DDRX.The high temperature and low strain rate provide sufficient time and conditions for the DRX grains to grow, and the DRX grains grow faster and have larger grain sizes under these conditions.

Conclusion
The rheological curves and microstructures of the super austenitic stainless steel Sanicro35 at different temperatures and strain rates are obtained utilizing thermal compression experiments and EBSD characterization.Its thermal deformation behavior and dynamic softening mechanism are analyzed, and the reasonable thermal processing region concludes as follows: the deformation temperature is 1050-1150 °C, and the strain rate is 10 s −1 .
(1) The rheological stress decreases with increasing temperature and decreasing strain rate.The three stages of the rheological curves indicate that DRX is the primary softening mechanism for super austenitic stainless steels.
(2) As the temperature rises, the higher grain boundary mobility drives the DRX grains to grow.As the strain rate increases, the more considerable deformation storage energy provides sufficient activation energy for DRX grain nucleation, and the denser the DRX grain nucleation becomes.Dislocations are mainly distributed in the interior of deformed grains and at grain boundaries, and the driving force that generates DRX reduces the density of dislocations.
(3) The high fraction of DRX grains at 10 s −1 strain rate is because a high strain rate has more considerable deformation storage energy and dislocation density, which promotes DRX grain nucleation.At the same time, the generation of twins also promotes the nucleation of DRX grains.The smaller the DRX grain, the denser the twin boundaries within the DRX grain.
(4) DDRX is the primary softening mechanism of sanicro35.When the strain rate is 10 s −1 , the large deformation can generate microbands within the deformed grains, and the microband boundaries evolve toward HAGBs with increasing temperature, promoting the occurrence of CDRX.The softening mechanism is mainly the coexistence of DDRX and CDRX.

Figure 1 .
Figure 1.Heat deformation process and deformation area distribution.

Figure 2 .
Figure 2. True stress-strain curves of Sanicro35 under different temperature and strain rate conditions (a) 0.001 s −1 (b) 0.01 s −1 (c) 0.1 s −1 (d) 1 s −1 (e) 10 s −1 (f) Maximum stress (The solid line is the original curve and the dashed line is the friction correction curve).