Effect of solution temperature on 316L austenitic stainless steel δ Effect of ferrite transformation

The 316L austenitic stainless steel containing residual ferrite was solution treated at different temperatures. The microstructure, texture and precipitates of the test samples were analyzed by SEM, EBSD, TEM and microhardness techniques. The results show that there are three phases of austenite, ferrite and sigma in austenitic stainless steel after solution treatment at 900-1100 ℃ for 30 min and water quenching. Sigma phase is formed in the range of 900-1000 ℃, which will improve the hardness of the matrix. Sigma phase mainly consists of residual δ Ferrite decomposition, {001} < 110 > and {001} < 100 > oriented δ Ferrite is preferentially transformed to Sigma phase. With the increase of temperature, the content of Sigma decreases, the average grain size of austenite increases, and the hardness decreases gradually. When the solution temperature exceeds 1050 ℃, the Sigma phase is completely dissolved into the austenite, the average grain size of the austenite grows significantly, the hardness value decreases rapidly, and the {001}<110>and {001}<100>textures in the residual ferrite are re strengthened. After solution treatment at 1100 ℃, the residual ferrite content decreased to 0.2%.

Austenitic stainless steel has excellent forming properties, pitting resistance, intergranular corrosion resistance and high temperature mechanical properties, and is widely used in pipelines, heat exchangers, medical materials and welding. Usually, the austenitic stainless steel after hot rolling is subjected to solid solution treatment, the purpose is to make the carbide, intermetallic phase and residual δ ferrite precipitated during hot rolling re-solid solution in austenite at high temperature, using rapid cooling, to obtain a single austenite organization and reduce the tendency of intergranular corrosion cracking. The solid solution temperature is too low, the precipitates can not be solved back in austenite; too high temperature, resulting in austenite grain growth, mechanical properties reduced. Austenitic stainless steel in the residual δ ferrite solid solution and phase transformation at high temperatures, the transformation of the intermetallic phase is usually rich in Cr and Mo, common intermetallic phases are σ phase, χ phase and η phase, as shown in Table 1.
Table.1 Precipitation phases in austenitic stainless steel

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Sigma phase is a tetragonal crystal structure, hard and brittle, Ohmura et al. used nanoindenter to determine the hardness of Sigma phase in 316 stainless steel up to 17 GPa. Sigma phase is one of the most important precipitation phases in stainless steel, its appearance is likely to lead to alloy embrittlement, reducing corrosion resistance. sigma phase nucleation requires a high-energy interface, usually in the island ferrite and grain boundaries in the form of block Villanueva et al. found that the presence of ferrite in austenitic stainless steels promotes the formation of the Sigma phase. During high-temperature phase transformation, δ ferrite undergoes co-precipitation reaction to decompose into γ austenite and Sigma phase. Therefore, it is necessary to reduce the ferrite content in 316L bars to avoid its transformation into the hard and brittle Sigma phase in the high temperature service condition, which leads to material failure.
The Chi phase is an intermetallic compound with a body-centered cubic structure, which is present in austenitic stainless steels containing Mo and is usually formed at grain boundaries, twin grain boundaries and intracrystalline. The formation of Chi and Laves phases is always followed by carbide and Sigma phases, and they are often post-generation phases.
The presence of these hard and brittle intermetallic phases will lead to the reduction of the matrix corrosion resistance and mechanical properties, therefore, the need to design a reasonable solution treatment temperature to avoid reducing the δ ferrite content and the precipitation of intermetallic phases. At present, the detailed identification and characterization of the four precipitated phases of austenitic stainless steel solid solution treatment have rarely been reported systematically. In this experiment, the heat treatment test of hot rolled bar of 316L austenitic stainless steel containing a small amount of residual δ ferrite with different solid solution temperatures is carried out to study the phase transformation pattern, microstructure, weave and hardness evolution, which is important for developing a reasonable production process and reducing the transformation and precipitation hazards during its processing and use.

1. Experiment

The material used is a 22 mm diameter 316L hot-rolled bar with the chemical composition shown in Table 2.
The equilibrium phases in the alloy were first calculated using JMatPro thermodynamic software to analyze the phase classes and phase transformation patterns of the alloy during solidification. The samples were heated to 900,950,1000,1050,1100°C, respectively, and then quenched in water after holding for 30 min. The samples before and after solid solution treatment, after inlaying, grinding and electrolytic polishing, were analyzed by EBSD in Jeol-7001F scanning electron microscope, and the EBSD data were processed by Channel-5 software. For the samples before and after solid solution treatment, transmission samples were prepared by electrolytic double spraying respectively, and STEM microscopic morphology observation and selected area electron diffraction (SAED) physical phase identification were performed using Jeol-2100F transmission electron microscope. Hardness measurements were performed using a Tukon-2500 fully automatic Vickers hardness tester.
The microstructure and antipodal diagram of the original sample are shown in Fig. 1. The original sample is an austenitic stainless steel containing a small amount of residual δ ferrite, with a large number of {111} twins in the austenitic grains and a twin boundary length of 33.2% of the total grain boundary length. Analysis of the orientation distribution function of δ ferrite, 2 = 45 ° ODF cross-sectional map shows that the maximum value of the polar density is 5.79, located around the {001} <110> rotational cubic weave, while there is a strong {001} <100> cubic weave, indicating that the residual ferrite has a strong meritocratic orientation.
Table.2 Chemical composition of the original sample

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Fig.1 Microstructure and 2=45° ODF cross section of the original sample

2. Results and Discussion

2.1 Calculation of thermodynamic equilibrium phase diagram

Figure 2 shows the relationship between the precipitation phase and temperature of 316L austenitic stainless steel in thermomechanical equilibrium. As shown in Figure 2a, the alloy starts to generate δ-ferrite at 1445°C during solidification and austenite at 1424°C. δ-ferrite transforms to austenite as the temperature decreases, and the transformation is complete at 1150°C. As the temperature decreases further, the Sigma phase forms at 955°C, the M23C6 phase forms at 917°C, the Chi phase forms at 911°C, and the Laves phase forms at 728°C. Figure 2b shows a local enlargement of Figure 2a between 800 and 1200°C. As shown in the figure, the content of the precipitated phases is low in this temperature range, and the content of Sigma phase reaches a maximum of 2.66% at 910°C, while the content of Chi phase and M23C6 phase is only 0.51% and 0.08% at 900°C. This shows that when solid solution treatment of 316L austenitic stainless steel is carried out in the range of 900 to 1100°C, the possible precipitation phases are Sigma phase, Chi phase and M23C6 phase, while the Laves phase transition temperature is low and difficult to generate.

2.2 Phase identification and content statistics

Figure 3a shows the scanning transmission electron microscopy bright field image of the solid solution treated sample at 900°C for 30 min, which shows that the ferrite (bcc) and Sigma phases are distributed adjacent to each other and twins exist in the austenite (fcc) matrix, and the number of internal dislocation defects in the grains is low after the heat treatment recovery process in the matrix. Selected area electron diffraction calibrations were performed on the matrix and precipitated phases, respectively, and the results are shown in Figures 3b to 3d.

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Figure.2 Precipitation phase content of the original material versus calculated temperature

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Figure.3 Scanning transmission electron microscopy bright field image of the sample after solid solution at 900°C and selected area electron diffraction of the corresponding area
Figure 4 shows the results of the EDS surface scan analysis of the energy spectrum containing the three phase regions, indicating that the austenite, ferrite and Sigma phases contain Fe, Cr and Ni elements, while Mo elements are enriched in the Sigma phase.
The same transmission electron microscopy analysis of the original sample and other solid solution temperature samples showed that the second phase of 316L austenitic stainless steel appeared before and after solid solution, mainly δ ferrite and Sigma phase, neither Chi phase nor M23C6 phase was found. After exceeding 1050°C, there is only δ ferrite 1 second phase in the sample. As shown in Table 2, the low carbon content (0.022%) in the raw material caused difficulties in the formation of the M23C6 phase. Combined with the analysis of equilibrium phase diagram in Fig. 2b, Chi phase and M23C6 phase exist below 910°C and the phase content is very small at 900°C. The limited time of solution heat treatment and the restricted atomic diffusion distance, the non-equilibrium heat treatment process reduces the possibility of generation of these two phases in the actual solution heat treatment process.

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Figure.4 EDS surface scan of the energy spectrum of three phases in solid solution samples at 900°C
The results of EBSD analysis are shown in Fig. 5, where the austenite, ferrite and Sigma phases are marked in blue, red and green, respectively, and the black lines characterize the large-angle grain boundaries. The Chi phase and M23C6 phase were also not found in the physical phases before and after the solid solution treatment. Figure 6 shows the statistical analysis of the content of several phases generated by solid solution treatment, and the results show that: the original sample contains austenite, residual ferrite (2.57%) and a very small amount of Sigma phase (0.03%); after solid solution treatment at 900, 950 and 1000°C, the residual ferrite undergoes eutectic decomposition and generates austenite and Sigma phases, the ferrite content decreases and the Sigma phase The content of ferrite decomposes further and the content of Sigma phase decomposes significantly after the temperature exceeds 1050°C. The Sigma phase generated by the decomposition completely solidifies in the austenite phase under the action of high temperature.
Some scholars found that the Sigma phase in austenite precipitation is very difficult, preferential nucleation at the ferrite / austenite interface, the finer the grain, the more large angle grain boundaries, the more likely to generate Sigma phase. Austenitic steels with high carbon content form more carbides, which are favorable to the nucleation of Sigma phase. In this study, the carbon content of the original sample is low, and no precipitation of carbide is found in the solid solution treatment, so the nucleation and growth of Sigma phase is mainly decomposed by δ ferrite.
The analysis results show that: δ ferrite decomposes rapidly by eutectic decomposition at 750~870℃ to generate Sigma phase, and with the increase of solid solution temperature, the diffusion activity of alloying elements increases, and the Sigma phase solidifies into austenite, and the residual Sigma phase content decreases with the increase of temperature after solid solution treatment. When the temperature reaches 1050℃, the diffusion activity of alloying elements is active, and the Sigma phase can be completely solidified into the matrix within 30min, and the ferrite content continues to decrease. After solid solution at 1100°C, the value of ferrite content, decreases to 0.2%.

2.3 Microstructure, weave and hardness evolution

The average grain size of austenite grains of the samples before and after solid solution in Fig. 5 was counted and microhardness measurements were performed, and the results are shown in Table 3. The grains grew slightly during the solid solution treatments at 900,950 and 1000°C; the grain growth was significant at temperatures up to 1050°C, with the average grain size reaching 10.15 μm, and at a temperature of 1100°C, the average grain size reached 16.20 μ The statistical results of the {111} twin boundary length to the total grain boundary content show that the {111} twin boundary content of the solid solution treated samples at different temperatures fluctuates from 29.4% to 33.4%, indicating that the solid solution process has no significant effect on the formation of annealed twins.
Figure 7 shows the weaving evolution of the ODF cross-sectional view of δ ferrite 2=45° in the solid solution treated samples. Figure 1 shows the typical {001}<110> rotational cubic weave and {001}<100> cubic weave in the original sample. After solid solution treatment at 900, 950 and 1000°C, the initial {001}<110> and {001}<100> weakened significantly and the polar density maximum shifted to around the γ orientation line, indicating that the δ ferrite with the initial two crystal orientations preferentially transformed to the Sigma phase. With the increase of the solid solution temperature, the polar density maxima returned to the {001}<110> and {001}<100> weaves at 1050 and 1100°C solid solution treatments, indicating that after the complete solid solution of the Sigma phase into austenite, the other oriented grains in the residual δ ferrite have a higher priority than the {001}<110> and {001}<100> oriented grains to back dissolve into austenite. The {001}<110> and {001}<100> weaves were re-enhanced.
The results show that the austenite weaving in the initial samples and the samples treated with solid solution at different temperatures are random, indicating that the grain orientation of austenite grains is mostly random and no obvious weaving is formed during the solid solution heat treatment process, and the austenite grain growth mechanism is random. Due to the random distribution of austenite weave, the residual δ ferrite content is low, and the hardness is less affected by the weave, and is mainly affected by the average grain size and the number of Sigma phases together.

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Figure.5 EBSD phase distribution before and after solid solution treatment

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Figure.6 Phase content before and after solid solution treatment
The hardness results are shown in Table 3: After solid solution treatment at 900, 950 and 1000°C, the hardness of the matrix was increased due to the formation of Sigma hard and brittle phase, and the hardness tended to decrease gradually with the increase of temperature, as the Sigma phase content decreased and the average grain size increased. The hardness values are only affected by the growth of the average grain size and show a rapid decrease relative to the original sample.

Table.3 Average grain size and hardness of austenite before and after solid solution treatment

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Figure.7 Evolution of the δ-ferrite structure of the solid solution treated sample

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Figure.8 Typical austenite structure of solid solution treated samples

3. Conclusion

  • 1) The austenitic stainless steel 316L containing a small amount of residual δ ferrite was quenched in water after solid solution treatment for 30 min at 900-1100°C. The austenite, ferrite and Sigma phases existed, and the Sigma phase was mainly generated by the co-precipitation decomposition of residual δ ferrite.
  • 2) Sigma phase is generated in the range of 900-1000°C, and its content decreases with the increase of temperature. the formation of Sigma hard and brittle phase increases the hardness of the matrix, and with the increase of temperature, the content of Sigma phase decreases, the average grain size increases, and the hardness shows a gradual decrease, and reaches above 1050°C, the Sigma phase disappears and the hardness value decreases rapidly.
  • 3) At 900,950 and 1000℃ solid solution treatment, {001}<110> and {001}<100> oriented δ ferrite preferentially transformed to Sigma phase, after the temperature exceeded 1050℃, Sigma phase completely solid solved into austenite, the residual δ ferrite content decreased, and {001}<110> and {001}<100> weave was re-enhanced. 1100℃ After solid solution, the ferrite content is reduced to 0.2%.

Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

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