Microstructural evolution in 12% Cr heat-resistant steel during compression deformation at 650°C

ABSTRACT

The 12% Cr heat-resistant steel used for critical components of fossil-fired power plants works in high-temperature and high-pressure steam environments for a long time and inevitably undergoes a certain amount of plastic deformation during service. This plastic deformation will affect the microstructure and hot ductility of the heat-resistant steel and ultimately influence its service life. Therefore, the effect of hot compression deformation on microstructural evolution at 650°C was investigated in this work. As the amount of hot compression deformation increased, the number of subgrains significantly increased, and their sizes gradually decreased. When the amount of deformation reached 50%, dynamic recrystallization occurred, forming a preferred orientation in the rolling direction. <111> and <001> fiber texture appeared in the microstructure when the deformation amount reached 8%, and the intensity of <111> texture was significantly higher than that of <001> texture. Their intensity values further increased with the increase of the amount of deformation. As the amount of hot compression deformation increased, the deviation angle of the Kurdjumov–Sachs (KS) orientation relationship between M₂₃C₆ particles and α-Fe gradually increased. Their KS orientation relationship began to be disrupted at the deformation amount of 6% and was completely destroyed at the deformation amount of 20%. With the amount of deformation increasing up to 50%, there were no micropores at the M₂₃C₆/matrix interfaces, and no hot cracking occurred. This was mainly attributed to the occurrence of dynamic recrystallization which alleviated stress concentration at the particles /matrix interfaces. The mechanisms of microstructural evolution involving subgrains, grain boundary characteristics, texture evolution, strain distribution, and orientation relationship were discussed in detail.

KEYWORDS:

• Heat-resistant steel
• hot compression deformation
• subgrain
• texture
• orientation relationship

Introduction

In China's energy structure, coal accounts for a large proportion of all energy consumption. Although the country has been vigorously developing renewable energy, thermal power will still dominate the power structure for a long time in the future [1]. The ultra-supercritical thermal power units consume less coal and produce less pollution, exhibiting green and energy-efficient advantages, which is of great significance for reducing carbon dioxide emissions and achieving a double-carbon policy, and is an inevitable trend of the development of thermal power units in China [2].

12% Cr heat-resistant steel, as an important material in ultra-supercritical thermal power units, has been used for rotor, boiler pipe, steam turbine and other important components. Many researchers studied the microstructural evolution during heat treatment process and long-term service of this steel. The growth behavior of austenite grain, isothermal transformation kinetics, and martensitic transformation and their effects on the mechanical properties of 12% Cr steel during heat treatment process were investigated [3,4]. It was demonstrated that coarsening of M₂₃C₆ particles and the precipitation of new phases such as Laves phase and Z phase occurred during long-term thermal ageing or creep [5,6].

During long-term service, Cr-rich M₂₃C₆ particles lead to a relatively high content of W and Mo around M₂₃C₆, thereby promoting the formation of Laves phase [6,7]. Due to the rearrangement of alloying elements, the growth of Laves phase particles will consume M₂₃C₆. Xu et al. [8–10] proposed two nucleation mechanisms of Laves phase, namely, independent nucleation at subgrain boundaries and nucleation near M₂₃C₆ particles. It was found that with increasing ageing time, due to the segregation of Si and P at the M₂₃C₆/ferrite interfaces, Laves phase particles nucleating in the vicinity of M₂₃C₆ particles tended to directly swallow adjacent M₂₃C₆ particles, which induced the formation of coarse and irregularly shaped Laves phase particles and resulted in severe strain localization and deterioration of ductility. Therefore, M₂₃C₆ particles play an important role in the service performance of 12% Cr steel.

The 12% Cr steel bars are produced by multiple passes and large deformation in the forging process, thus requiring high hot ductility. However, when the forging temperature of 12% Cr steel drops to a certain critical temperature, it is susceptible to hot cracking. In addition, the 12% Cr steel used in ultra-supercritical thermal power units works in high-temperature and high-pressure steam environments for a long time and inevitably undergoes a certain amount of local plastic deformation during service. Moreover, it also experiences severe plastic deformation before creep failure. This plastic deformation at service temperature will negatively affect the microstructure and mechanical properties of the material, which ultimately influences the safe service life of the equipment. Therefore, it is necessary to study the influence of plastic deformation on microstructural evolution at service temperature, and establish the relationship between the degree of plastic deformation and microstructure evolution. This will provide theoretical support for the safe operation and life assessment of the equipment.

In this work, the effect of hot compression deformation on the microstructural evolution at 650°C is investigated. The microstructural features, including subgrains, grain boundary characteristics, texture evolution, strain distribution, grain orientation, and orientation relationship, will be analyzed in depth, and the mechanisms of microstructural evolutions at different deformation conditions will be revealed.

Materials and methods

The material used in this work is 12% Cr ultra-supercritical rotor steel, which is a typical martensitic/ferritic heat-resistant stainless steel. The measured composition of this investigated material is Fe-0.13C-0.06Si-0.42Mn-0.012P-0.003S-10.46Cr-0.76Ni-1.01Mo-0.19V-0.05Nb-1.04W-0.06N (wt.%). The as-received hot forged steel bars were treated by a three-stage heat treatment process, first normalized at 1050°C for 21.5 h (oil quenched), then tempered at 570°C for 21 h (air cooled), and finally tempered at 690°C for 23 h (air cooled).

After heat treatment, the tested material was cut into Φ 8 × 12 mm cylindrical samples which were subjected to axial high-temperature compression testing on a Gleeble 3500 thermal simulation machine in a high-purity argon environment. Prior to compression testing, the samples were ground and polished to remove the surface oxide layer. The service temperature of 12% Cr heat-resistant steel is in the range of 600–650°C, so the temperature of hot compression deformation was selected as 650°C. The hot compression test process is as follows: heating at a rate of 10 °C/s to 650°C, holding for 5 min to ensure uniform temperature of the sample, and then conducting compression tests under different deformation conditions with a deformation rate of 1 s⁻¹, the amount of hot compression deformation are 3%, 6%, 8%, 12%, 20% and 50%, respectively. After deformation, the samples were water-cooled to room temperature.

EDAX-TSL electron backscattered diffraction (EBSD) combined with the Hitachi S3400N scanning electron microscope was used to analyze the grain boundary characteristics, grain size, recrystallization behavior, and the orientation relationship between different phases. The EBSD test parameters are an acceleration voltage of 20 kV, a working distance of 15 mm, and a step size of 0.05μm.

Results

Evolutions of size and number of subgrains

As shown in Figure 1, the number of subgrains increased significantly as the amount of compression deformation increased, and the size of the subgrains gradually decreased. At a deformation amount of 6%, the morphology of martensite laths was very obvious and the number of subgrains was small. When the deformation amount reached 8%, a large number of subgrains appeared inside the laths. When the deformation amount reached 20%, the laths were almost completely broken, and the morphology of laths was gradually blurred, but the original orientation of the laths still existed. When the deformation amount increased to 50%, dynamic recrystallization occurred in the microstructure, the original orientation relationship of the laths completely disappeared, and the preferred orientation was formed along the rolling direction.

Figure 1. EBSD orientation map of 12% Cr steel after compression deformation at 650°C. (a) 6%, (b) 8%, (c) 20%, (d) 50%.

Figure 2 shows TEM images of 12% Cr steel after compression deformation at 650°C. It was noted that M₂₃C₆ particles mainly precipitated along lath boundaries and MX particles mainly formed in lath interior. At a deformation amount of 20%, the morphology of martensite laths gradually disappeared, and a large number of subgrain boundaries occurred, which were formed by dislocations pile-up and rearrangement. When the deformation amount reached 50%, significant dynamic recrystallization took place, and the dislocation density around grain boundaries and within grains decreased.

Figure 2. TEM images of 12% Cr steel after compression deformation at 650°C. (a) 6%, (b) EDS results corresponding to (a), (c) 20%, (d) 50%.

Evolution of grain boundary characteristics

Martensite laths were gradually divided into many subgrains and the number of martensite subgrains increased with the increase of the amount of compression deformation, as shown in Figure 3. When the deformation amount reached 20%, the part of the prior austenite grain boundary disappeared, however, the martensite laths still existed. When the deformation amount increased to 50%, the prior austenite grain boundary and martensite laths completely disappeared, and the grain orientation was gradually consistent with the rolling direction.

Figure 3. EBSD inverse pole figures (IPF) and grain boundary distribution images of 12% Cr steel after compression deformation at 650°C. (a) 3%, (b) 20%, (c) 50%.

Figure 4 exhibits the proportion of low-angle boundaries changing with the amount of compression deformation. When the deformation amount increased from 2% to 20%, the proportion of low-angle boundaries increased continuously, and this proportion value reached its maximum (58.3%) at the deformation amount of 20%; as the deformation amount increased from 20% to 50%, the proportion of the low-angle boundaries began to decrease.

Figure 4. Evolutions of the proportion of low-angle grain boundaries with different amounts of compression deformation at 650°C.

During the hot compression deformation process, the dislocation density increased and dynamic recovery gradually occurred, forming subgrain boundaries, which were low-angle boundaries (Figure 2). The increase in the amount of compression deformation led to an increase in the number of subgrains, ultimately resulting in an increase in the proportion of low-angle boundaries. When the deformation amount increased from 2% to 20%, only dynamic recovery occurred in the microstructure, so the proportion of low-angle boundaries continued to increase. However, when the deformation amount reached 50%, dynamic recrystallization occurred, and the misorientation of low-angle boundaries gradually increased and evolved into high-angle boundaries. Therefore, the proportion of low-angle boundaries decreased.

Figure 5 shows the distribution of grain boundary misorientation under different amounts of hot compression deformation. It can be observed that as the hot deformation amount increased, the proportion of 2° to 5° grain boundaries first increased and then decreased. When the deformation amount increased from 2% to 20%, only dynamic recovery took place in the microstructure, which resulted in the increase in the number of subgrains and the proportion of 2° to 5° grain boundaries. When the deformation amount reached 50%, dynamic recrystallization appeared in the microstructure, which led to increase of the misorientation of low-angle boundaries and decrease of 2° to 5° grain boundaries. The proportion of 5°–52° grain boundaries remained basically unchanged when the deformation amount did not exceed 20%, and it significantly increased when the deformation amount increased to 50%, which was mainly attributed to the occurrence of dynamic recovery and dynamic recrystallization.

Figure 5. Evolutions of the distribution of grain boundary misorientation under different amounts of hot compression deformation at 650°C.

There were three mechanisms of dynamic recrystallization, namely discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and geometric dynamic recrystallization (GDRX). Traditional DDRX appeared in the form of nucleation and growth of new grains. However, due to the high stacking fault energy of ferritic stainless steel, dislocations tended to cross slip and climb. Therefore, it was generally believed that the possibility of DDRX occurring during hot deformation was very low. GDRX generally took place during high deformation strain, often in the form of original grains splitting into new grains [11]. Due to the relatively low deformation strain in this work, GDRX was unlikely to occur.

CDRX was usually considered to consist of three stages, namely, strain strengthening, dynamic recovery, and high-angle boundary migration. In the primary stage of CDRX, a large number of dislocations were generated during the deformation process. Due to dynamic recovery, a large number of dislocations formed low-angle boundaries; as the deformation process continued, the misorientation of low-angle boundaries gradually increased and evolved into high-angle boundaries. Finally, the migration of high-angle boundaries caused the annihilation of dislocations in new grains [12]. In this study, as the amount of compression deformation increased, the subgrain size decreased and the misorientation of subgrain boundaries increased, all of which were consistent with the basic characteristics of CDRX [13]. Therefore, when the hot compression deformation of 12% Cr steel were in the range of 3–50%, CDRX played a dominant role.

Evolution of texture

As shown in Figure 6, when the deformation amount was less than 8%, there was no obvious fiber texture. When the deformation amount reached 8%, <111> and <001> fiber texture appeared, and the intensity of <111> texture was significantly higher than that of <001> texture. As the amount of compression deformation increased, the intensity of the <111> and <001> textures increased correspondingly. When the deformation amount reached 50%, the maximum intensity values of the <111> texture and <001> texture were 8.555 and 6.290, respectively. From Figure 7, it was noted that with the increase of compression deformation amount, the maximum intensity of <111> texture and <001> texture increased correspondingly, and the maximum intensity of <111> fiber texture was significantly higher than that of <001> fiber texture.

Figure 6. Texture images of inverse pole figures (IPF) of 12% Cr steel under different amounts of compression deformation. (a) 3%, (b) 6%, (c) 8%, (d) 20%, (e) 50%.

Figure 7. Changes of the intensity of <111> texture and <001> texture as a function of compression deformation amount at 650°C.

Dillamore et al. [14] reported that crystal rotation occurred along with slip deformation, gradually forming <001> and <111> fiber texture, and the intensity of <111> fiber texture was higher than that of <001> fiber texture during severe compression deformation of Fe-C alloys, which were in accordance with the research results of this study. This was mainly because there was basically no lattice bending between the <001> direction and the <111> direction. Subgrains grew faster inside the <111> direction grains than inside the <001> direction grains, and the <111> direction grains existed more stably after recrystallization, which could also be used to explain the phenomenon that occurred in this work well.

Evolution of strain distribution

As an analytical tool in orientation imaging microscopy (OIM), the Kernel Average Misorientation (KAM), mainly obtained by calculating the average value of the orientation difference between the center point of the kernel and the nearest adjacent point, is often used to evaluate the local strain distribution [15].

Figure 8 shows KAM distribution under different amounts of hot compression deformation. KAM values are divided into different ranges and represented by different colors. Blue represents the lowest KAM value and red represents the highest value. When the deformation amount increased from 3% to 20%, the KAM value of most grains gradually increased. With the deformation amount increasing from 20% to 50%, the KAM values of most grains increased dramatically. According to the KAM values, it was found that the strain concentration was mainly distributed at the vicinity of the grain, lath and subgrain boundaries, and the overall strain distribution was relatively homogeneous, which indicated that greater deformation incompatibility occurred at these positions during deformation.

Figure 8. KAM images of 12% Cr steel under different amounts of compression deformation. (a) 3%, (b) 6%, (c) 8%, (d) 20%, (e) 50%.

The number of lath subgrains gradually increased from 3% deformation amount to 20% deformation amount, which caused the increase of the KAM value at most grains. With the deformation amount increasing from 20% to 50%, the dynamic recrystallization and the grain refinement occurred, the number of grains increased significantly, and the corresponding grain boundaries increased, which resulted in a sharp raise of the KAM value at most grains.

Evolution of the orientation relationships between M₂₃C₆ and α-Fe

M₂₃C₆-type particles were mainly precipitated along the prior austenite grain boundaries and the martensite lath boundaries. There existed an orientation relationship between M₂₃C₆-type particles and the ferrite matrix, namely {011}_Ferrite∥ {111}_M23C6, <111>_Ferrite∥ <011>_M23C6, which was the famous Kurdjumov–Sachs (KS) orientation relationship. Generally, in the absence of deformation, the deviation angle of the ideal KS orientation relationship was mainly concentrated at 1° to 2° [16–18]. Figures 9 and 10 present the EBSD orientation map and the deviation angle distribution map of KS orientation relationship at the compression deformation of 6%, respectively.

Figure 9. EBSD orientation map of 12% Cr steel at the compression deformation of 6%. (a) IPF, (b) Enlarged local image of region b in a, (c) Phase images with the deviation angle of the KS orientation relationship.

Figure 10. The deviation angle distribution map of the KS orientation relationship at the compression deformation of 6%.

From Figure 9, the morphology of M₂₃C₆-type particles was mainly short rod-shaped, which was basically consistent with the morphology in undeformed samples. As shown in Figure 10, the deviation angle of the KS orientation relationship between M₂₃C₆ and α-Fe was mainly concentrated in 2° to 6°, with a small amount distributed at 8° to 16°. Compared with the undeformed samples, the deviation angle of the KS orientation relationship increased significantly at a deformation amount of 6%, and the KS orientation relationship between M₂₃C₆ and α-Fe began to break.

Figures 11 and 12 show the EBSD orientation map and KS orientation relationship deviation angle at compression deformation of 50%, respectively. It can be seen that the morphology of M₂₃C₆ particles was almost an equiaxed shape, which indicated that M₂₃C₆ particles gradually evolved from short rod-shaped to equiaxed shape as the deformation amount increased. As shown in Figure 12, the deviation angles of KS orientation relationship between M₂₃C₆ and α-Fe were primarily concentrated at 11°–25°, with a small amount distributed at 3–10°. Compared with a deformation amount of 6%, the deviation angle of the KS orientation relationship increased sharply at a deformation amount of 50%, and the KS orientation relationship between M₂₃C₆ and α-Fe was completely destroyed. When the deformation amount exceeded 20%, even at a deformation amount of 50%, there were no micropores at the M₂₃C₆ particle interfaces, and 12% Cr steel did not exhibit the phenomenon of hot cracking. Combined with the TEM results in Figure 2, it was found that under a deformation amount of 50%, there didn't exist obvious dislocation pile-up at the M₂₃C₆ particle interfaces, mainly because the dynamic recrystallization process absorbed the dislocations around the interfaces and subgrain boundaries [19], which significantly relieved the deformation stress concentration at the particle interfaces.

Figure 11. EBSD orientation map of 12% Cr steel at the compression deformation of 50%. (a) IPF, (b) Enlarged local image of region b in a, (c) Phase images with the deviation angle of the KS orientation relationship.

Figure 12. The deviation angle distribution map of the KS orientation relationship at the compression deformation of 50%.

Figure 13 presents the distribution of the deviation angle of the KS orientation relationship between M₂₃C₆ and α-Fe under different amounts of hot compression deformation. It was found that the deviation angle of the KS orientation relationship gradually increased as the amounts of compression deformation increased. When the deformation amount increased from 6% to 8%, the deviation angle of KS orientation relationship increased significantly. Under the deformation amount of 20%, the deviation angle increased sharply, and the KS orientation relationship between M₂₃C₆ and α-Fe was completely destroyed. And the deviation angle increased slightly when the deformation amount increased from 20% to 50%.

Figure 13. The deviation angle distribution map of the KS orientation relationship under different amounts of compression deformation.

During the hot compression deformation, the deviation angle of the ideal KS orientation relationship is mainly caused by the uncoordinated strain of the two phases at the phase boundary. The hardness of M₂₃C₆ is dramatically higher than that of the ferrite matrix, and it is difficult to deform during the deformation process. If a deformed matrix contains some precipitate particles which are difficult to deform, the local strain incompatibility produced during deformation will gradually be relieved by creating the geometrically necessary dislocation (GND) at the interface between the particles and the matrix [20,21]. The sluggish accumulation of GND at the interface between the particle and the matrix will lead to an increase in the mismatched angle of the dislocation array around the particles. Finally, the spatial lattice of the crystals (mainly M₂₃C₆ particles) on both sides of the phase boundary rotates and the energy of the phase boundary per unit area increases. As the amounts of compression deformation increased, the dislocation accumulation near the phase boundary intensified, which caused the increase in the tilt of the crystal. Therefore, the deviation angle of the KS orientation relationship between M₂₃C₆ and α-Fe gradually increased.

Li et al. [22] investigated the hot plastic behavior of 12% Cr martensitic heat-resistant steel in the range of 600–1150°C. At temperatures of 700–900°C, micropores mainly formed at grain boundaries, while at temperatures of 600°C and 650°C, micropores mainly formed inside the grains. This indicated that as the temperature increased, grain boundaries became weaker than inside the grains, and micropores formed at grain boundaries rapidly developed along the grain boundaries, ultimately leading to intergranular fracture. The intergranular cracks formed at high temperatures led to hot embrittlement, which was attributed to the sliding of grain boundaries. Grain refinement could effectively suppress the propagation of intergranular cracks and improved ductility [22]. In martensitic heat-resistant steel [23], due to the stress concentration at the interfaces of coarse M₂₃C₆ particles and its inherent brittleness during deformation, microcracks were easily formed at the interfaces of coarse particles, which significantly reduced the toughness and plasticity of heat-resistant steel.

In this work, although the M₂₃C₆ particles had a coarse size (500–1000 nm) and completely lost its KS orientation relationship with the matrix when the deformation amount exceeded 20%, even at a deformation amount of 50%, there were no micropores at the M₂₃C₆ particles interfaces, and 12% Cr steel did not exhibit the phenomenon of hot cracking. Dynamic recrystallization always occurs preferentially at grain boundaries and can suppress the formation and development of intergranular cracks [24]. In this paper, dynamic recrystallization also preferentially occurred at the M₂₃C₆ particles interfaces, it could significantly relieve the stress concentration induced by deformation at the interfaces, which suppressed the initiation and propagation of cracks at the grain boundaries and particles interface and improved the hot ductility of martensitic heat-resistant steel.

Conclusions

In this work, the effect of hot compression deformations on microstructural evolution at 650°C was investigated. The mechanisms of microstructural evolutions involving subgrains, grain boundary characteristics, texture evolution, strain distribution and orientation relationship were revealed. Several conclusions can be drawn as follows:

1. As the amount of hot compression deformation increased, the number of subgrains significantly increased, and the size of subgrains gradually decreased. When the deformation amount reached 50%, dynamic recrystallization occurred in the microstructure, a preferred orientation formed along the rolling direction. <111> and <001> fiber texture appeared in the microstructure when the deformation amount reached 8%, and the intensity of <111> texture was significantly higher than that of <001> texture, their intensity values increased correspondingly with the increase of the deformation amount.

2. When the deformation amount increased from 3% to 20%, the number of lath subgrains gradually increased, which caused the increase of the KAM value at most grains. With the deformation amount increasing from 20% to 50%, the dynamic recrystallization and the grain refinement occurred, the number of grains increased significantly, and the corresponding grain boundaries increased, which resulted in a sharp raise of the KAM value at most grains.

3. In the absence of deformation, there existed an orientation relationship between M₂₃C₆-type particles and the ferrite matrix, namely {011}_Ferrite∥ {111}_M23C6, <111>_Ferrite∥ <011>_M23C6, which was the famous Kurdjumov–Sachs (KS) orientation relationship. As the amount of hot compression deformation increased, the deviation angle of the Kurdjumov–Sachs (KS) orientation relationship between M₂₃C₆ particles and α- Fe gradually increased. Their KS orientation relationship began to be disrupted at the deformation amount of 6% and was completely destroyed at the deformation amount of 20%. With the amount of deformation increasing up to 50%, there were no micropores at the M₂₃C₆/matrix interfaces, and no hot cracking occurred. This was mainly attributed to the occurrence of dynamic recrystallization which alleviated stress concentration at the particles /matrix interfaces.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available from the corresponding author upon request.

Additional information

Funding

The authors are grateful to the financial support of the National Key R&D Program of China [grant number 2021YFB3702401], the National Natural Science Foundation of China [grant numbers 52101145, 51831002] and Major Program of the National Natural Science Foundation of China [grant number 52293394], the Shanghai ‘Super Postdoctoral’ Incentive Plan and New Young Teachers Launch Plan of Shanghai Jiao Tong University.