The size effect of precipitates on microstructure evolution during high-temperature deformation

Cr-Ni-Mo steel, as a hot-work die steel, has become increasingly demanding of service temperature and strength. This study investigated the effect of precipitates on the deformation behavior of tempered martensite at 700 °C. Tempering specimens with different dislocation densities leads to different behavior due to the size and volume fraction of precipitates. This decreases dynamic recrystallization and interfacial mobility, resulting in higher interfacial energy. Meanwhile, the precipitates boost strength and thermal stability of the tempered martensite.


Introduction
The Cr-Ni-Mo martensitic steel is of great interest in hot-work die steel due to the high strength martensite.Martensite enables the strengthening through interstitial solid solution with an excess of dissolved carbon.During tempering, carbides form which can cause secondary hardening or a slow drop in strength due to precipitation.Molybdenum carbides maintain material strength at high temperatures because they do not coarsen quickly.Therefore, Cr-Ni-Mo martensitic steels have high-temperature strength, which makes them suitable for use at higher temperatures [1] .
Dynamic recovery (DRV) and dynamic recrystallization (DRX) happen in the Cr-Ni-Mo martensitic steel during high temperature deformation, affecting its rheological behavior and mechanical properties.Ferrite is less likely to recrystallize due to the higher stacking fault energy.Barnett et al. [2] had studied the ferrite DRX of low carbon steel and found that recrystallization only occurred at 923 K and 0.01 s - 1 .In calculating the hot work diagram of low carbon steels in the ferrite zone, Li et al. [3] found that at a strain less than 0.3, recrystallization only occurred at high temperatures (close to 700 ℃) and low strain rates.Several researchers [4][5][6] have reported on the DRX of ferrite, which only takes place at high temperatures, low strain rates, and above a critical strain value.However, no one has reported the occurrence of DRX at high temperatures and high strain rates.
Precipitation pinning on the boundary retards recrystallization.Zhang et al. [7] found that Cr23C6 on the boundary effectively prevents DRX during the high temperature deformation process of 0.1C-18Cr-1Al-1Si ferritic stainless steel.In a study by Liu et al. [8] on 19Cr2Mo ferritic stainless steel, the addition of W reduced the size of the Laves phase and increased the amount of precipitation, and the smaller and more precipitation on the boundary effectively prevented DRX.
While ferrite has been studied, martensite with medium carbon and high dislocation density lacks reports on its DRV and DRX.In this study, we used various dislocation densities to achieve different precipitate sizes and study the effect of precipitation on deformation at a low strain rate at 700 ℃ in Cr-Ni-Mo martensitic steel.

Materials and experimental methods
The chemical composition of the steel is shown in Table 1.The material was smelted into 15 kg ingots in a vacuum furnace and then forged into round bars of Φ200 mm after austenitizing at 1200 °C for two hours.Then a specimen of Φ100mm × 100 mm was taken from the center of the bar for subsequent heat treatment.The test steel was subjected to two heat treatment processes: for one specimen, it is annealed at 600 °C for 10h, austenitized at 980 °C for 2h and water quenched, and then tempered at 580 °C for 2h (AQA); the other specimen was directly austenitized at 980 °C for 2h and water quenched, and then tempered at 580 °C for 2h (QA).
Table 1 The chemical composition of the Cr-Ni-Mo steel used.
Figure 1.XRD patterns obtained from AQA and QA specimens.Figure 1 shows two separate XRD patterns of the α-Fe's five peaks after quenching.The dislocation densities of the materials were estimated using the half-peak width data to be 0.47 × 10 14 m -2 for the AQA specimen and 1.06 × 10 14 m -2 for the QA specimen [9] .The precipitated carbides of two specimens after tempering were obtained by electrolytic extraction with an electrolyte of 1% tetramethylammonium chloride + 10% acetylacetone + methanol.High-temperature tension was carried out on a NAK GNT300 high-temperature testing machine with a rate of 0.0004 s -1 at 700 ℃.SEM and EBSD were performed on a Zeiss Supra 55 field emission scanning electron microscope with a symmetry probe.TEM was performed on a JOEL 2100F transmission electron microscope.Figure 2 shows the true stress-strain curves of both specimens at 700 ℃.The QA specimen exhibits a tensile strength of 462 MPa at 0.05 strain, followed by a rapid decrease in stress with increasing strain, and finally fractures at 0.13 strain, with a section shrinkage of 25%.The AQA specimen exhibits a tensile strength of 127 MPa at 0.03 strain, followed by a long dynamic hardening and dynamic softening process and with a section shrinkage of 80%.The QA specimen exhibited a high strength and poor toughness, while the AQA specimen exhibited a low strength and good toughness.in tempered specimens.Figure 3 (a, b) shows SEM images of the two tempered specimens.Both specimens maintain a grain size of around 30 μm, with blocks distributed within the grains.Long strips and granular carbides precipitate at the block boundaries, and fine precipitates within the blocks, which is typical of tempered martensite.

Microstructure of the tempered specimens
The carbides are shown in Figure 3 I and are mainly M3C (i.e.Fe3C) and M2C (i.e.Mo2C) types.The electrolytic extraction showed that QA has 0.855 wt% carbide, mostly M2C, and AQA has 0.627 wt% carbide, mostly M3C.patterns of carbides in tensile deformed.Figure 4 shows the microstructure of the two specimens near the fracture of the necking area after deformation at 700 ℃. Figure 4 (a) shows that the QA specimen retains its tempered martensitic microstructure even after high-temperature deformation.Fine precipitates with an average size of 100 nm can be seen at the block boundaries and inside the blocks.From Figure 4 (b), the results show that the AQA specimen has an equiaxed grain shape after high-temperature deformation, with a grain size of about 1 μm and a larger average precipitate size (400 nm) at the grain boundaries compared to the QA specimen.The XRD analysis of the carbide powders from both specimens showed a transformation from M3C+M2C carbides to mainly M2C carbides after deformation at 700 ℃.In Figure 5 (a, b), the distribution of grain boundaries in the necking zone of the two specimens after deformation at 700 °C is shown.The red line corresponds to misorientations of boundaries between 5° and 15°, the black line corresponds to misorientations of boundaries between 15° and 50°, and the green line corresponds to misorientations of boundaries >50°.Martensite is generated by the phase transformation of austenite, so martensite has the external boundary of pre-austenite and the internal block and packet formed by many pairs of variants.As misorientations between martensitic variants are more likely to be either less than 15° or greater than 50° in alloyed steel [10], the boundaries between 15° and 50° are most likely to be prior austenite grain boundaries [11].In the QA specimen the preaustenite boundaries were clearly visible and there are martensite blocks wrapped by large angle grain boundaries; however, the AQA specimen had equiaxed crystalline microstructure of around 1 μm in size, with some small angle grain boundaries formed internally due to continued deformation.Figure 5 (c, d) shows the fraction of recrystallization in both specimens, the AQA specimen recrystallized in more zone.The QA specimen maintains their basic morphology of tempered martensite well during the high temperature deformation process, while the DRX occurred in the AQA specimen.Figure 6.CSL distribution of specimens after deformation at 700 ℃. Figure 6 shows the histogram of the CSL distribution of both specimens after 10% deformation at 700 ℃.The AQA sample has more Σ3, Σ11 and Σ33c type boundaries, while the QA sample has more Σ17b high energy boundaries.The finer precipitates have a stronger pinning effect on the boundaries and lower the mobility of the boundaries, increasing the interfacial energy of the material and making DRV as well as DRX more difficult to occur.Figure 5 shows the recrystallization process occurring in the AQA specimen as the geometrically dynamic recrystallization during the high temperature process when the grain boundary width is reduced to sub-crystalline size.Figure 7 shows a TEM image of a heavily deformed region in the QA specimen.The fine polygonal M2C carbides are pinning the boundaries, preventing the occurrence of DRX.

Evolution of precipitates during deformation at 700 ℃
The martensitic steel is heat treated by quenching and tempering to obtain higher strength with optimum strength and toughness matching.In Cr-Ni-Mo martensitic steel, the carbide precipitation sequence is M3C→M2C→M23C6 or M6C.The carbides in the tempered state of the specimen mainly comprises M3C carbides at the boundaries and needle-like M2C carbide inside the blocks.After deformation at 700 ℃, due to the small coarsening rate of the Mo and V carbides, the needle-like M2C carbides generated in the tempered state hardly changed, but the M3C carbides in the tempered state dissolved at the same time as the polygonal M2C carbides precipitated.
The QA specimen produced more M2C carbides than the AQA specimen due to higher dislocation density.The QA specimen had fewer alloying elements for the polygonal M2C carbides to form at the boundaries after 700 ℃ deformation, resulting in smaller carbides compared to the AQA specimen.

The size effect of precipitates on deformation behavior at 700 ℃
The fine precipitates also have a Zener pinning effect on the grain boundaries.When the volume fraction of precipitates becomes smaller or the size becomes larger, the pinning pressure decreases.This pinning force can be expressed by the following equation [12] : where  is the boundary surface energy per unit area,  is the substructure size,  and  are the volume fraction and average diameter of precipitates, respectively.
Following this, the Zener pinning force is 19.6 MPa for the QA specimen and 4.3 MPa for the AQA specimen.Obviously, the pinning force of precipitates to the grain boundary is lower in the AQA specimen than in the QA specimen.
The different behaviors of the two specimens during high-temperature deformation are due to carbide distribution as explained in section 4.1.The AQA specimen has a lower work hardening rate and less hindrance to DRV due to fewer precipitates in the laths.The larger polygonal M2C carbides have less pinning effect on the lath boundary, so the mobility of interface is higher during high-temperature deformation.Then the DRV leads to the stress concentration zone coordinated.After a larger deformation, the DRX occurs.The QA specimen exhibits high work-hardening rate and strength due to the higher volume fraction of M2C carbides inside the laths, which have strong interaction with dislocations due to their small sizes.Meanwhile, the fine carbides are also very effective in preventing the occurrence of DRV.The smaller polygonal M2C carbides at boundaries have a good Zener-pinning effect on the lath boundaries, so the QA specimen is very effective in retaining the tempered martensitic microstructure with a large density of dislocations.Therefore, the QA specimen exhibits higher strength.

Conclusion
QA specimen exhibits higher strength at 700 ℃ and AQA specimen exhibits better toughness.The lower dislocation density of the AQA specimen results in fewer nucleation of precipitates, which precipitate at the boundaries and are larger in size.The larger precipitates result in less Zener pinning forces in the AQA specimen, and DRX occurs during high-temperature deformation.However, the continuous dynamic recrystallization of the QA specimen occurs only at the pre-austenite boundary, while the smaller precipitates lead to strong Zener pinning, leaving the microstructure internally only in the form of an increased Σ17b interface, reducing the interfacial migration rate and improving the interfacial energy.

Figure 2 .
Figure 2. True stress-strain curves of both specimens at 700 ℃.Figure2shows the true stress-strain curves of both specimens at 700 ℃.The QA specimen exhibits a tensile strength of 462 MPa at 0.05 strain, followed by a rapid decrease in stress with increasing strain, and finally fractures at 0.13 strain, with a section shrinkage of 25%.The AQA specimen exhibits a tensile strength of 127 MPa at 0.03 strain, followed by a long dynamic hardening and dynamic softening process and with a section shrinkage of 80%.The QA specimen exhibited a high strength and poor toughness, while the AQA specimen exhibited a low strength and good toughness.

Figure 3 .
Figure 3. SEM images of tempered specimens (a) QA (b) AQA and (c) XRD patterns of carbides in tempered specimens.Figure3 (a, b) shows SEM images of the two tempered specimens.Both specimens maintain a grain size of around 30 μm, with blocks distributed within the grains.Long strips and granular carbides precipitate at the block boundaries, and fine precipitates within the blocks, which is typical of tempered martensite.The carbides are shown in Figure3I and are mainly M3C (i.e.Fe3C) and M2C (i.e.Mo2C) types.The electrolytic extraction showed that QA has 0.855 wt% carbide, mostly M2C, and AQA has 0.627 wt% carbide, mostly M3C.

3. 3 .Figure 4 .
Figure 4. SEM images of specimens after deformation at 700 ℃ (a) QA (b) AQA and (c) XRDpatterns of carbides in tensile deformed.Figure4shows the microstructure of the two specimens near the fracture of the necking area after deformation at 700 ℃.Figure4 (a)shows that the QA specimen retains its tempered martensitic microstructure even after high-temperature deformation.Fine precipitates with an average size of 100 nm can be seen at the block boundaries and inside the blocks.From Figure4(b), the results show that the AQA specimen has an equiaxed grain shape after high-temperature deformation, with a grain size of about 1 μm and a larger average precipitate size (400 nm) at the grain boundaries compared to the QA specimen.The XRD analysis of the carbide powders from both specimens showed a transformation from M3C+M2C carbides to mainly M2C carbides after deformation at 700 ℃.

Figure 5 .
Figure 5. EBSD maps of specimens after deformation at 700 ℃ (a) QA(BC+GB) (b) AQA(BC+GB) (c)QA(REX) (d)AQA(REX).In Figure5 (a, b), the distribution of grain boundaries in the necking zone of the two specimens after deformation at 700 °C is shown.The red line corresponds to misorientations of boundaries between 5° and 15°, the black line corresponds to misorientations of boundaries between 15° and 50°, and the green line corresponds to misorientations of boundaries >50°.Martensite is generated by the phase transformation of austenite, so martensite has the external boundary of pre-austenite and the internal block and packet formed by many pairs of variants.As misorientations between martensitic variants are more likely to be either less than 15° or greater than 50° in alloyed steel[10], the boundaries between 15° and 50° are most likely to be prior austenite grain boundaries[11].In the QA specimen the preaustenite boundaries were clearly visible and there are martensite blocks wrapped by large angle grain

Figure 7 .
Figure 7. Fine carbides pinning the lath boundaries in QA specimen.Figure5shows the recrystallization process occurring in the AQA specimen as the geometrically dynamic recrystallization during the high temperature process when the grain boundary width is reduced to sub-crystalline size.Figure7shows a TEM image of a heavily deformed region in the QA specimen.The fine polygonal M2C carbides are pinning the boundaries, preventing the occurrence of DRX.