Research on the Collaborative Effect of Plastic Deformation and Solution Treatment in the Intergranular Corrosion Property of Austenite Stainless Steel

Abstract:

AISI 304 austenite stainless steel was applied extensively in the modern industry due to its good properties on mechanics and corrosion resistance. However, there is severe intergranular corrosion when the AISI 304 was working at the temperature 420~850 ℃ called sensitizing temperature. This phenomenon was more obvious with increase of strain. In addition, this effect can not be removed completely even with the heat treatment subsequently. In present work, the influence of solution treatment and plastic deformation on the intergranular corrosion property of AISI 304 was investigated. The specimens subjected to different strain were obtained by the uniaxial tensile tests at room temperature. XRD was used to measure the fraction of martensitic phase which was induced by deformation. Optical metal lographic microscope was applied to observe the evolution of microstructure. The influence of various deformation values, solution temperature and holding time on intergranular corrosion was quantitative analyzed by electrochemical potentiodynamic reactivation (EPR) method. Experimental results showed that the degree of the intergranular corrosion increased with the increase of deformation, and with the decrease of solution temperature and holding time. It is indicated that since the solubility of carbon in martensite and austenite is discrepant, the content of carbon in the grains recrystallized is discrepant too. The more martensite is transformed, the more chromium carbide is formed in the grain boundary after sensitization. This phenomenon causes poor intergranular corrosion resistance due to the lack of chromium. In addition, the carbon segregation which is caused by plastic deformation will relieve with the rise of solution temperature and holding time. It is because that the carbon atom is more active at higher temperature, and the distribution of carbon is more homogeneous with the extended holding time. Then the quantity of chromium carbide will decrease in solution treatment process. Consequently the chromium depletion will be mitigated. From the above, a uniform solution treatment condition is not suitable for austenite stainless steel with the effect of martensitic transformation in cold working. Flexible scheme can be employed to insure better combination property of products.

Key words: austenite stainless steel    plastic deformation    martensitic transformation    solution treatment   sensitization    intergranular corrosion

Xiaosong ZHANG1,Yong XU1,2,3(,Shihong ZHANG1,Ming CHENG1,Yonghao ZHAO2,Qiaosheng TANG3,Yuexia DING3
1 Institute of Metal Research,Chinese Academy of Sciences, Shenyang 110016, China
2 School of Materials and Engineering, Nanjing University of Science and Technology, Nanjing 210016, China

Austenitic stainless steels have many excellent properties such as high strength, weldability, corrosion resistance, and good formability [1, 2]. They are used in aerospace, chemical, automotive, food machinery, pharmaceuticals, instrumentation, energy, and construction. Widely used in industries such as decoration. The reason why it can obtain high strength and excellent plasticity is due to deformation-induced martensitic transformation during room temperature deformation, and at the same time can bring about phase change plasticization effect, namely TRIP (transformation induced plasticity) effect [2 ~5], so cold forming, deep drawing, stamping, and hydroforming can be easily performed. The excellent corrosion resistance of austenitic stainless steel is due to the Cr element contained in the composition, which results in a dense passive film on the surface of the steel to prevent the matrix from being damaged [6]. However, when austenitic stainless steel is used in the sensitization temperature range of 420~850 °C, C in the steel is easily precipitated in the form of Cr-rich carbide, so that the Cr content in the grain boundary and adjacent areas is reduced, forming a Cr-poor zone. If corrosive medium is encountered at this time, corrosion may occur along the grain boundary or near the grain boundary, so that the crystal grains lose their binding force and form localized damage. This phenomenon is called intergranular corrosion. Intergranular corrosion starts from the surface of the material and develops along the grain boundary to the inside, greatly weakening the bondability between the grains. The stainless steel subjected to this local corrosion does not change significantly in appearance and size, but the strength and plasticity of the steel and The drastic decrease in toughness [7] severely affected the service life of stainless steel products.
Many scholars have studied the effects of solution treatment and sensitizing temperature on the intergranular corrosion properties of austenitic stainless steels [8-12]. Zhang Genyuan and Wu Qingfei [13] used electrochemical potentiodynamic reactivation (EPR) to study the intergranular corrosion sensitization, sensitization time, and sensitization of 304 stainless steel at two solution temperatures of 950 and 1050 °C. As a result of the relationship between temperatures, it was found that the resistance to intergranular corrosion of AISI 304 at 1050 °C in solution was better than that at 950 °C. Sun Xiaoyan et al. [14] studied the effect of solution temperature at 1100 °C and 0.25-2 h on the intergranular corrosion of 316L austenitic stainless steel by soaking method, and observed the microstructure and corrosion of 316L stainless steel with different heat treatment conditions by optical microscope. According to the evolution of the morphology, it is considered that the sample with a holding time of 0.5 to 1 h has good overall performance at 1100 °C. However, deformation-induced martensitic transformation not only affects the weldability of the formed stainless steel products, but also increases the intergranular corrosion tendency. Moreover, the content, structure, and distribution of the deformation-induced martensite phase vary with the amount of deformation. The adoption of a unified solution treatment system does not ensure the complete elimination of the structural changes caused by the phase transformation, and thus the subsequent sensitization behavior. And corrosion performance can have a significant effect. Garcia et al. [15] studied the effects of deformation on the intergranular corrosion properties of 304 stainless steel. However, the solid solution conditions used in the experiment were relatively simple. The sample was only solution treated at 1050 °C for 15 min. The time is short and the processing is not enough. There is still a lack of systematic research on the effect of solution treatment of austenite stainless steels after plastic deformation on intergranular corrosion performance. Therefore, this work takes AISI 304 austenitic stainless steel as an example to comprehensively analyze the effect of solution temperature and holding time on the degree of sensitization under different deformation amounts, and further clarifies the system of plastic deformation and solution treatment for austenitic stainless steel. Microstructure evolution and intergranular corrosion performance synergistic mechanism.
1 Experimental method
The experimental material was AISI 304 austenitic stainless steel supplied by Goodfellow Cambridge, UK, which was supplied in a cold-rolled annealed condition with a thickness of 0.7 mm. The chemical composition (mass fraction, %) is: C 0.05, Cr 18, Ni 8.07, Mn 1.12, Si 0.05, P 0.02, S 0.01, N 0.043, Fe balance. In order to ensure that the stainless steel used for the experiment is austenitic, the sample is subjected to high-temperature solution treatment at 1100 °C for 1 h before the relevant experiment.
The tensile test equipment adopts MTS 5105 microcomputer control electronic universal testing machine. The tensile samples are prepared according to GBT228-2002. The experimental temperature is 20 °C, the strain rate is 10-3 s-1, and the deformation amount is 0 (undistorted). ), 30% and 50%.
Martensitic phase content in austenitic stainless steels was measured on a D8 advance X-ray diffractometer (XRD) using Cu Kα, 40 kV, 300 mA, a maximum output of 18 kW, and an X-ray source width of approx. It is 5 mm and the diffraction angle is 2θ=40°~100°[16]. The horizontal projection of the X-ray incident light is parallel to the direction of the tensile load. Before the experiment, the strained layer introduced into the surface of the sample during the mechanical thinning process was removed by electrolytic polishing. Electrolytic polishing was performed using a 10% HClO4 alcohol solution with a voltage of approximately 35 V and a current of 1 mA.
The parallel section of the deformed tensile specimen was taken and the sampling area was 10 mm × 10 mm. The material was then treated with solution and solution for different temperatures and times. The solution temperatures were 950, 1000, 1050, 1100 and 1150 °C, respectively. The soaking time was 0.5 and 1 h respectively. After water cooling, the sensitization treatment was performed. The sensitizing temperature was 650 °C for 2 h and then air-cooled. Heat treatment process shown in Figure 1.
The quantification method of the intergranular corrosion tendency is the electrochemical potentiodynamic reactivation (EPR). The experimental equipment is PATSTAT4000 electrochemical workstation. Electrochemical experiments using three-electrode electrolytic cell system, in which the working electrode is an experimental AISI 304 stainless steel electrode, the auxiliary electrode is Pt, and the reference electrode is a saturated calomel electrode (SCE). The reference electrode is connected to the research system via a saturated KCl solution and a salt bridge. The salt bridge in the electrolyte solution is close to the electrode surface. The electrolyte solution used in this experiment was 0.01 mol/L KSCN and 0.5 mol/L H2SO4 solution.
In order to observe the microstructure of stainless steel, the specimens were cold-set, mechanically polished, electropolished, electrolytically etched with 10% oxalic acid alcohol solution, and then the metallographic structure of the specimens was performed using an Axovert 200 MAT metallographic microscope (OM). Observed.
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Fig.1 Schematic of heat treatment process method

2 experimental results
2.1 Change of martensitic phase transformation at different deformation rates
Fig. 2a shows the XRD spectrum of AISI 304 stainless steel under uniaxial tension. To further quantify the change in martensite content, a direct comparison method was used to quantify the diffraction peaks in the graph [17]. Assuming that the grains are all randomly oriented, the volume fraction Vi of each phase i can be calculated based on the integrated intensity I of its diffraction peaks, as follows [18]:

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In the formula, n is the total number of diffraction peaks of each phase, including austenite phase γ, martensite phase α′ and ε; Iji is the integrated intensity of the (hkl) crystal face of i phase; Rji is the phase of the i phase [hkl The theoretical calculation strength of the crystal plane can be calculated using Equation (2) [19]:

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In the formula, V is the unit cell volume, γ, α′ and ε are 45.88×10-3, 23.64×10-3 and 21.57×10-3 nm3 respectively [20]; F is the structural factor, which is different The specific atomic scattering coefficient f of the crystal plane [hkl] is calculated, and f is calculated based on the composition of the 304 stainless steel [21]. The values are shown in Table 1. P is a multiplicity factor for a specific crystal. The number of crystal planes included in the family of faces is shown in Table 2. The angle θ is the diffraction angle; exp(-2M) is the Debye-Waller factor (DWF) [19], where the M factor is based on Debye. Function, element characteristic temperature and atomic mass are calculated and can be obtained in the related literature [21]. DWF is used to eliminate the offset error caused by X-ray induced atomic vibrations. The values of different crystal orientations are shown in Table 3.

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Fig.2 XRD spectra of AISI 304 under uniaxial tensile experiment (a) and volume fraction of martensite in the deformation (b)

The martensitic phase can be divided into bcc type α′ martensite and hcp type ε martensite according to the different crystal structure. At room temperature stretching, studies [19] show that γ austenite and α′. The diffraction peaks of martensite can be clearly observed, but almost no diffraction peak can be observed in ε martensite. The XRD spectrum obtained in this experiment also does not show the diffraction peak corresponding to ε martensite. This is due to At high strain, ε martensite as the mesophase is rapidly converted to α′ martensite [22], so Ijε = 0 in the formula. The relationship between the volume fraction of martensite and the deformation amount is calculated, as shown in Fig. 2b. As can be seen from Figure 2b, as the amount of deformation increases, the content of martensite phase gradually increases. When the material is not deformed, the content of martensite inside the fully solution treated sample is very low, close to zero. When the deformation amounts were increased to 30%, 40%, 50%, and 60%, respectively, the calculated martensite content was 21.0%, 29.2%, 36.7%, and 42.5%, showing a monotonous increase.

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Table 1 Atomic scattering factors (f) of AISI 304 under X-Ray

2.2 changes of sensitization degree under different deformation quantities
In the EPR experiment, the activation potential was scanned at a constant rate from the open circuit potential (open circuit potential, OCP) at a constant rate, and then the potential was immediately controlled from the passivation potential to the activation area. The activation peak and reactivation peak appeared on the scanning curve. Figure 3 was the original intention of the experiment, and the maximum active anode was expressed in the positive scanning with Ia. Scanning current, Ir is the maximum reactivation scanning current in reverse scanning, and reactivation rate is marked with Ra, which is used to evaluate the sensitivity of intergranular corrosion. The higher the value, the higher the sensitivity of intergranular corrosion. Ra is called the sensitization degree [23]. The expression is as follows:
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Fig. 4 shows the polarization curves and sensitization of stainless steel specimens at different temperatures of 1000 h and 1150 C at 1 h and 650 C sensitized 2 mm respectively. It can be seen that with the increase of deformation, the activation current changes little, and the reactivation current shows a significant difference. Under the condition of solid solution temperature of 1000 and holding time of 1 h, the sensitivities of samples with 30% deformations and 50% deformations were 22.63% and 25.86%, respectively. The sensitivity of the undeformed specimen for reference is only 17.53%. Under the condition of solid solution temperature of 1150 and holding time of 1 h, the sensitivities of samples with 30% deformations and 50% deformations were 10.08% and 21.45%, respectively. The sensitization of undeformed specimens for reference is only 9.50%. The result is due to the uniform corrosion of the surface of the electrode in the process of activation. With the increase of the voltage, the surface of the specimen is induced to form a passivation film, which reduces the current gradually. In the process of reactivation, the passivation film formed during the positive scanning process on the surface of the sample will break up and dissolve due to the poor Cr of some regions. With the increase of the poor Cr degree, the corrosion rate of the region increases simultaneously, that is, the peak current of the reverse sweep is increased. It is clearly observed from the diagram that the reactivation current increases with the increase of the deformation amount, and the sensitivity increases with the increase of the deformation amount, and the intergranular corrosion tendency of the sample increases with the increase of the deformation amount.

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Fig.3 Method to read and calculate the sensitization (Ra) using the electrochemical potentiodynamic reactivation (EPR) curve (OCP—open circuit potential, Ia—activation current, Ir—reactivation current, Ra=Ir/Ia)

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Table 2 Multiplicity factors (P) for the phases present in AISI 304

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Table 3 Debye-Waller factors (DWF) of AISI 304

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Fig.4 Polarization curves of specimen under different quantity of deformation
(a) 0 deformation,1000 ℃ for 1 h 
(b) 30% deformation,1000 ℃ for 1 h
(c) 50% deformation,1000 ℃ for 1 h 
(d) 0 deformation,1150 ℃ for 1 h
(e) 30% deformation,1150 ℃ for 1 h 
(f) 50% deformation, 1150 ℃ for 1 h
2.3 Changes in the degree of sensitization under different solution treatments
Due to the large number of experimental conditions, the polarization curves of individual samples are not elaborated one by one. Only the susceptibility change trends of the sample under different conditions of solution temperature are shown, as shown in Figure 5. From Figure 5, it can be observed that when the deformation is 30% and the holding time is 1 h, the reactivation current shows a gradual decrease trend. After the solution temperature exceeds 1050 °C, the decreasing trend begins to slow down, at 950, 1000, 1050, The sensitizations at 1100 and 1150 °C were 26.91%, 22.63%, 11.09%, 10.21% and 10.08%, respectively. The sensitization of the sample under other conditions also showed this trend. It can be seen that the overall tendency of the sensitization of the sample decreases with the increase of the solution temperature. When the deformation is 30%, the sensitization decreases at the solution temperature of 1050 °C. When the amount is 50%, the rate of decrease of the sensitization starts to slow down when the solution temperature reaches 1000°C.
In Fig. 5, 950 and 1150 °C solution treated samples were selected for analysis. Comparing the four curves of the two groups, it can be seen that as the holding time is extended from 0.5 h to 1 h, the specimens under each deformation amount are The sensitization degree showed a declining trend. When the deformation was 30%, the solution temperature at 950 °C decreased from 37.79% to 26.91%. The sample at 1150 °C decreased from 14.74% to 10.08%. The deformation was 50%. At 950 °C, the solution temperature was reduced from 56.21% to 49.07%, and that at 1150 °C was reduced from 32.38% to 21.45%. Overall, the effect of the soaking time was not significantly affected by the solid solution temperature.
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Fig.5 Sensitization degree under different solution treatment temperatures and times

3 Analysis and discussion
3.1 Effect of plastic deformation on intergranular corrosion of AISI 304 stainless steel
The deformation-induced martensitic transformation of AISI 304 austenitic stainless steels occurs at room temperature. The more obvious the effect is, the more martensite content is contained in the specimens. Studies [24, 25] have shown that in the solution treatment process, misfits are higher and recrystallization occurs first at sites with higher lattice distortion energy. The difference in nucleation time will lead to uneven grain size. The grains of the core are larger. The results in Fig. 2b confirm that in the AISI 304 austenitic stainless steel, as the amount of deformation increases, the content of the martensite phase also increases. Therefore, the effect of the deformation amount on the intergranular corrosion tendencies is mainly reflected in the deformation-induced Martensitic transformation. The relative recrystallization effect of the body.
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Fig.6 OM images of samples under 0 (a), 30% (b) and 50% (c) deformations at solution treatment temperature 1000 ℃ for 0.5 h

Figure 6 shows OM images of 0, 30%, and 50% deformation samples at a solution temperature of 1000 °C for 0.5 h. It can be seen that when the deformation amount is different, the grain size and distribution of the sample without deformation are relatively uniform and the grain boundary is smooth when the holding time is short (Fig. 6a). With the increase of the deformation amount to 30%, obvious grain size has already appeared in the sample, the grain size is 20~80 μm, and the size of coarse grain and fine grain is different by 3~5 times (Fig. 6b) ). When the amount of deformation reached 50%, the grain size inhomogeneity further increased, and a more concentrated fine grained area appeared (Fig. 6c). This shows that after a large plastic deformation of the solution treatment process, the degree of grain growth is not consistent, the nucleation of new crystals, that is, recrystallization does not occur simultaneously in all places.
At the junction of the martensite and austenite phases, dislocations cannot migrate through the phase boundary. Therefore, the dislocation migration during deformation will form a large number of entangled and mismatched structures at this point, and the lattice distortion energy High, so that the new grain nucleation earlier. Because the solubility of C element in martensite is much lower than that of austenite, and the deformation-induced martensitic transformation is a non-diffusion phase transition process, so the martensite formed in the deformation process is the supersaturation of C element. Solid solution. In the early stage of high temperature solution heat treatment, the martensite phase has not undergone a high temperature reverse phase transformation. The crystal defects in the two phases such as dislocation cells have not been completely eliminated. At this time, the C atoms in martensite are subjected to high temperature and deformation stress field. The passage through these defects will rapidly diffuse into the austenite phase, which is much faster than the elimination of defects in the atomic rearrangement of the crystal and the formation of new austenite phases. They form Cottrell pinning dislocations in the high-density dislocation areas at the phase boundary, which makes the content of C at the phase boundary higher [26], so the content of C elements in the recrystallized grains here is also Higher. With the prolongation of the holding time, the adjacent crystal cores of the same size continue to merge with each other, and the newly formed crystal grains begin to grow. The degree of growth increases with the increase of the holding time, so the first nucleated crystal The grain size is larger. In the original parent austenite austenite, the formation of recrystallized nuclei is relatively late, and the growth process is also carried out later, while some have not yet grown up with each other[25,27]. When the above factors work together, the grain boundary formed by recrystallization is mainly a continuous network structure composed of high-energy free grain boundaries. The content of C on this grain boundary is relatively high.
The intergranular corrosion of austenitic stainless steel has very obvious characteristics compared to other corrosion. During the sensitization process, C tends to form a complex carbide Cr23C6 second phase with Cr, which precipitates in the supersaturated austenite and distributes on the grain boundary. The Cr content of this carbide is much higher than that of the mother. Phase, therefore, causes the Cr content to rapidly decrease at the grain boundary, while at the same time in the austenite phase, the more active C atoms diffuse faster, and the Cr atoms have slower diffusion rates and cannot be added in time to the Cr at the grain boundaries. Together, the Cr content at the grain boundary of stainless steel will decrease, resulting in Cr depletion. Studies [12] have shown that when the Cr content at the grain boundary is lower than 12%, the passivation film formed on the surface will be broken and dissolved in the corrosive medium, and the passivation state will be destroyed. The large cathode and small anode will form between the grain and the grain boundary. The galvanic cell accelerates the corrosion of the grain boundary region, thereby causing damage to the grain boundary of the sample, and cutting the contact between the grains, resulting in embrittlement of the stainless steel. As shown in Fig. 7, compared with the undeformed condition (Fig. 7a), the carbide grains in the grain will be precipitated along the subgrain boundary in a bead-like manner. As the deformation increases, the subgrain boundary in the grain increases. The more carbide particles, the larger the more obvious, the more the number, the more dense the distribution, the more severe the grain boundary corrosion, the smaller the bead-like particle gap, and finally the formation of a very thick black trench at the grain boundary (Figure 7b and c). Under this effect, the connection between the grains is completely separated, which is also the root cause of the intergranular corrosion that causes the embrittlement of the stainless steel.
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Fig.7 OM images of samples under 0 (a), 30% (b) and 50% (c) deformations at solution treatment temperature 1000 ℃ for 0.5 h, then sensitization at 650 ℃ for 2 h

3.2 Effect of Solution Temperature and Holding Time on Intergranular Corrosion of AISI 304 Stainless Steel
Fig. 8 shows the OM image of the specimen at different solution temperature and holding time with 30% deformation. It can be seen that as the solution temperature and holding time increase, the grain size of stainless steel increases. When the solution temperature is 950 °C and the holding time is 0.5 h, the average grain size is 23.25 μm, and the solution temperature When the temperature was increased to 1150 °C and the holding time increased to 1 h, the average grain size reached 102.45 μm.
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Fig.8 OM images of samples under 30% deformation at different solution treatment temperatures and times (a) 950 ℃ for 0.5 h (b) 950 ℃ for 1 h (c) 1150 ℃ for 0.5 h (d) 1150 ℃ for 1 h

Studies [23] have shown that grain size is an important factor affecting intergranular corrosion of materials. The grain size and the time required to achieve full sensitization can be expressed by the following formula:
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In the formula, d1 and d2 are the diameters of two different crystal grains. tmax, 1 and tmax, 2 are the time required for the samples with different grain sizes to reach full sensitization. At any point in the sensitization phase, the concentration of Cr at grain boundaries is related to tmax, and its specific expression is:
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In the formula, C is the concentration of Cr at the grain boundary at any sensitization point, C0 is the initial concentration of Cr, t is the sensitizing time, tmax is the time required to achieve complete sensitization, and k is a constant.
From the above two formulas, it can be concluded that when t<tmax, the concentration of Cr at the grain boundary of the sample with a large grain size is higher than the concentration of Cr at the grain boundary of the sample with a small grain size, indicating the grain size. A large sample precipitates less precipitated carbide than a small sample and has a lower degree of sensitization. This is because although with the increase of the grain size, the reduction of the surface area of the grain boundary causes the precipitation of carbides per unit area, but at the same time as the grain size increases, the distance of the C atoms to the grain boundary will be longer. In the case of a short sensitization time, this will be a very important factor, so that the sample with a large grain size will precipitate less carbide than the sample with a small grain size, resulting in a large sensitization degree of the grain. Smaller than the crystal grain.
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Fig.9 OM images of EPR samples under 30% deformation at different solution treatment temperatures and times, then sensitization at 650 ℃ for 2 h
(a) 950 ℃ for 0.5 h (b) 950 ℃ for 1 h (c) 1150 ℃ for 0.5 h (d) 1150 ℃ for 1 h
Fig. 9 shows the OM images of the specimens etched by the EPR test at different solution temperature and holding time at 30% deformation. It can be observed that when the solution temperature is 950 °C and the holding time is 0.5 h, the grain boundary width is 6~8 μm, and when the solution temperature is 1150 °C, the holding time is 1 h. Corrosion grain boundary width is 2~3 μm. The grain boundary width of the sample with smaller grain size is significantly higher than that of the grain with larger grain size. This shows that under the effect of longer holding time and higher solid solution temperature, the crystallographic corrosion tendency of the sample with larger grains is smaller.
In the solution process, the migration of the new grain boundary formed by recrystallization will be hindered by the pinning of carbides, and the dissolution of carbides will take some time. Therefore, it will take a certain period of holding time before the crystals can be better promoted. Boundary migration, thereby optimizing grain boundary feature distribution and reducing the number of high-energy free grain boundaries [27]. At the same time, during the loading process, the crystals undergo severe plastic deformation, and a large number of subgrain boundaries are generated inside. When the holding time is short, a large number of subgrain boundaries may react with other grain boundaries in the future, and distortion at these subgrain boundaries is also high, which also strengthens the tendency of intergranular corrosion of AISI 304 stainless steel. On the other hand, although the C atoms diffuse uniformly during the high temperature of the solution treatment, the line defects and point defects in the crystals are greatly reduced after a certain period of time. The diffusion of the C element can only rely on a small number of Frenkel defects. As a result, homogenization of the C element requires a longer holding time. When the solution temperature is low, due to the lower atomic activity, the migration rate of C atoms is slower. When the solution temperature increases, the diffusion motion of C atoms is accelerated. The longer the holding time, the more fully the diffusion of C atoms.
The recrystallization behavior occurs in the material during the solution treatment. The change of the lattice structure in the process is a non-diffusion phase transformation process [26]. Under the effect of high temperature, the martensitic phase of the bcc structure passes the recrystallization. Austenite phase converted to fcc. At this time, the uniform diffusion of the C element takes a long time. According to the EPR experiment, the sample under a small amount of deformation of 30% has a solution treatment of 1050 °C for 1 h and the sensitization degree has dropped to below 11.09%, which is very close to the complete solution treatment condition at 1150 °C. The 9.50% of the original undeformed sample shows that under the solution conditions, the diffusion of C element has basically reached homogeneity. For samples with deformation of up to 50% and solution conditions of only 950 °C for 0.5 h, due to the uniform diffusion of C element, the sensitization degree is as high as 56.21%, and the intergranular corrosion tendencies are quite serious.
In summary, the effect of deformation amount on intergranular corrosion is mainly reflected in the change of the microstructure inside the crystal caused by the deformation-induced martensitic transformation, and the diffusion distribution of C element in the early stage of solid solution, thus the mechanism of recrystallization, especially the new The effect of grain boundary generation mechanism, and the effect of solution temperature and holding time mainly affect the intergranular corrosion performance of AISI 304 stainless steel by affecting the recrystallization, grain growth, and diffusion rate of C element.
Therefore, in practical industrial applications, for austenitic stainless steels with deformation-induced martensitic transformation effects during cold working, the solid solution treatment system cannot adopt uniform standards, and the quantitative heat treatment process design should be combined with the degree of deformation of the parts. Ensure that the final part achieves excellent overall performance.
4 Conclusion
(1) As the amount of deformation increases, the amount of martensite phase induced by deformation in the austenitic stainless steel AISI 304 increases. After the solution treatment and sensitization treatment, the intergranular corrosion tendency of the sample increases. It is the solubility of C element in martensite and austenite. In the initial stage of the solution heating process, C element has a concentrated distribution at the phase boundary, so that the content of C in the grains formed by recrystallization is slightly increased. Differently, the higher the content of martensite, the more carbides are formed in the second phase, and the more grain boundaries where Cr is depleted, the worse the intergranular corrosion performance.
(2) With the increase of the solution treatment temperature and holding time, the sensitization degree of the modified AISI 304 sample gradually decreased, and the intergranular corrosion performance improved. The reason is that due to the increase of temperature, the higher the activity of C atom and the faster the diffusion rate, and the longer the holding time, the more uniform the diffusion of C atoms, and the segregation of C element will decrease as the solution temperature and holding time increase. This reduces the concentration of concentrated carbides and reduces the Cr-depletion tendency of grain boundaries.
(3) The solution treatment can reduce the intergranular corrosion tendency of AISI 304 due to plastic deformation to some extent, but it cannot completely eliminate the microstructure changes caused by the deformation process.
The authors have declared that no competing interests exist.

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http://www.ams.org.cn/CN/10.11900/0412.1961.2016.00284

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