Effect of post-forging heat treatment on stress corrosion behavior of nuclear grade 316LN stainless steel in boiling MgCl2 solution

Effect of post-forging heat treatment on stress corrosion behavior of nuclear grade 316LN stainless steel in boiling MgCl2 solution

Effect of post solution- and stress relief-treatment on the stress corrosion cracking (SCC) resistance via U-bend specimens test in 42% boiling MgCl2 solution, as well as the microstructure, residual strain and mechanical properties of the forged 316LN stainless steel was studied. Results showed that the yield stress was reduced and the residual strain was eliminated through post solution-treatment for the forged steel. After immersion in boiling MgCl2 solution for 24, 48 and 72 h, respectively, all the U-bend specimens of either the solution-treated or the stress relief-treated steels suffered from clearly transgranular stress corrosion cracking (TGSCC). Furthermore, of which all the stress relief-treated specimens were entirly cracked, while the solution-treated specimens were only locally cracked after immersion for 72 h, suggesting higher SCC resistance for the forged steel after a proper post solution-treatment. Finally, the mechanism of the effect of post-heat treatments on the SCC resistance was discussed in terms of the residual strain and yield stress of the forged steel.

Key words:  stainless steel    nuclear materials    heat treatment    stress corrosion cracking    magnesium chloride solution   fractography

1 Introduction

Austenitic stainless steel has excellent corrosion resistance and excellent comprehensive mechanical properties and processing properties, and is widely used in the nuclear industry [1, 2]. Laboratory research [3-5] and nuclear power plant operation site [2,6] show that stress corrosion cracking (SCC) is the main failure mode of nuclear and electrical structural materials such as stainless steel and nickel-based alloys, and the damage is large. The anti-SCC properties of nuclear power structural materials. At present, some researches have been carried out on the SCC behavior of stainless steel in simulated nuclear power operating conditions at home and abroad [2, 3, 6]. Meng et al. [3] pointed out that cold-processed 316NG stainless steel occurs along the crystal in the primary water of simulated pressurized water reactor. Stress Corrosion Cracking (IGSCC), the crack growth rate (CGR) is related to the amount of hydrogen dissolved in the solution, the degree of cold working of the sample, and the sampling direction. Andresen et al [7] systematically studied the CGR of non-sensitized stainless steel in high temperature and high pressure water, and proposed the PLEDGE model to predict the effect of yield strength, corrosion potential, alloy composition and other factors on CGR.
The third-generation nuclear power technology AP1000 main pipeline adopts the manufacturing method of integral forging and processing (bending and pressing) forming, and for large forgings such as main pipes, heat treatment is generally required after forging to further improve the structure and eliminate residual stress/strain. . The difference in heat treatment process, the mechanical properties, microstructure and residual stress/strain of the material may affect the corrosion resistance and SCC properties of the material [8-10]. Garcı́a et al. [10] pointed out that the cold-processed 304 stainless steel undergoes mixed SCC cracking in boiling MgCl2 solution, and with the increase of cold working degree, transgranular stress corrosion cracking (TGSCC) gradually becomes the main cracking mode, and solid The solution treated 304 stainless steel mainly produces IGSCC, and the sensitization treatment of stainless steel can also affect its SCC sensitivity.
Austenitic stainless steel has a large SCC sensitivity in boiling MgCl2 solution, which is a method to quickly evaluate the sensitivity of stainless steel SCC [11-13], but the sensitivity of SCC for 316LN stainless steel for nuclear power main pipeline at home and abroad. Not enough yet. In this paper, the rapid evaluation method is used to study the effect of post-forging heat treatment process (solution treatment and stress relief treatment) which is often used in actual production on the sensitivity of SCC for 316LN stainless steel. From the perspective of environmental corrosion resistance and environmental cracking, it is large. Data support is provided for the optimization of the production process of the thick-wall primary circuit.

2 Experimental methods

The experiment uses 316LN stainless steel for the main pipeline of nuclear power plant. Its chemical composition (mass fraction, %) is: C 0.010, Cr 17.07, Ni 12.87, Mn 1.35, S 0.003, P 0.023, N 0.12, Si 0.26, Cu 0.06, Mo 2.21, Fe balance. First, the material is multi-directional forged, the forging ratio is 4, the initial forging temperature is 1130 °C, the final forging temperature is 900 °C, and the air is cooled to room temperature after the forging is completed. Then they were solution treated (10 h at 1070 °C, water-cooled) and de-stressed (10 h at 950 °C, furnace cooling). The specific forging and heat treatment process is shown in [14]. The sample after the solution treatment was labeled as S41, and the sample after the stress treatment was labeled as S42. The room temperature mechanical properties of the material were tested on an AG-100KNG electronic material testing machine.
The sample of 10 mm×10 mm×2 mm was cut by wire cutting method, and the surface of the sample was polished to 2000# by metallographic sandpaper; then polished by mechanical polishing to 2.5 μm with diamond polishing paste. After that, it was electrolytically etched with a 10% aqueous solution of oxalic acid, and then the microstructure of the sample was observed by a metallographic microscope, and the inclusions in the grain boundary and the inside of the grain after etching were observed by a FEI XL30FEG scanning electron microscope (SEM). The EDAX Genesis XM2 Energy Scattering Spectrometer (EDS) analyzes the chemical composition of inclusions. Electron backscatter diffraction (EBSD) was used to observe the microstructure and measure the residual strain of the metal microdomain [15]. The preparation and experimental operation of the EBSD sample are described in the literature [14].
The SCC sensitivity of 316LN stainless steel in boiling MgCl2 solution was measured using a U-bend sample according to the national standard YB/T 5362-2006 [16]. The sample size before U-bending is 50 mm × 15 mm × 2 mm. First, the six surfaces of the sheet sample were polished to 400 # step by step using water sandpaper. Then, the sample was bent into a U shape using a fatigue tester and fastened with bolts of 316L stainless steel to keep the arms of the U-shaped sample parallel. Finally, all samples were thoroughly ultrasonically cleaned in an acetone-alcohol biphasic solution and then dried for use. For the experiment, 42% (mass fraction) MgCl2 solution was prepared with analytically pure MgCl2·6H2O plus deionized water, heated and adjusted to have a boiling point of (143±1) °C, and equipped with a vertical glass reflux condenser with sufficient cooling capacity. Prevent concentration of the experimental solution. The experiment was carried out in 3 batches, and the experimental solutions were in boiling state for 24, 48 and 72 h, respectively. At the end of the experiment, the sample was removed and rinsed with deionized water. The surface morphology of the U-shaped sample was observed by SEM, and then the sample was broken along the main crack in the air, and the macroscopic and microscopic fracture morphology of the sample were observed by stereoscopic optical microscopy and SEM, respectively.

3 Experimental results

3.1 Room temperature mechanical properties

The room temperature mechanical properties of the two materials are shown in Table 1. The yield strength (Rp0.2) and tensile strength (Rm) of S41 are both smaller than S42, while the elongation after break (A) of S41 is greater than S42, which indicates that the solution treatment can more effectively eliminate the 316LN stainless steel in forging processing. During the work hardening in the process, the area shrinkage ratio (Z) of S41 and S42 is not much different. Figure 1 shows the tensile fracture morphology of 316LN stainless steel at room temperature. Both S41 and S42 show obvious fracture morphology of the dimple, indicating that both S41 and S42 have ductile fracture.

Table 1 Mechanical properties of 316LN stainless steel at room temperature

Fig.1 SEM fractography images of S41 (a) and S42 (b) samples after tensile tests at room temperature

3.2 Microstructure and residual strain

Figure 2 is a microscopic image of 316LN stainless steel after different post-forging heat treatments. The 316LN stainless steel is a single-phase austenitic stainless steel. The microstructures of S41 and S42 do not show significant differences, the grain size is also similar, and the grain size inside the material is not uniform, resulting in “mixed crystal” phenomenon. The grain size distributions of S41 and S42 counted by EBSD are shown in Fig. 3, and the average grain size is 144.86 and 136.92 μm, respectively. Therefore, solution treatment or stress relief treatment after forging has no significant effect on the microstructure of 316LN stainless steel. Fig. 4 is a distribution diagram of inclusions inside S41 and S42. It can be seen that the number of inclusions inside the two samples is very small and the size is about 5 μm. The EDS results of the inclusions are shown in Table 2. It can be seen that the inclusions mostly contain Al oxides, and some of the oxides contain elements such as Ca.

Fig.2 Microstructures of S41 (a) and S42 (b) samples

Fig.3 Grain size distribution of S41 (a) and S42 (b) samples

Fig.4 Inclusions inside S41 (a) and S42 (b) samples

Figure 5 is a partial mean orientation difference (LAM) distribution obtained from EBSD, which can quantitatively characterize the microscopic residual strain and its distribution inside the sample [15]. The average LAM values of S41 and S42 were 0.404 and 0.795, respectively. It can be seen that compared with S41, the residual strain inside S42 is higher and the distribution is less uniform. In addition, the microhardness values of S41 and S42 are 157.3 and 168.5, respectively [14]. Carlsson et al. [17] proved that the strain hardening degree of the material has a good correspondence with the microhardness. Therefore, the solution treatment after the forging can be performed. More effective elimination of residual strain inside the 316LN stainless steel.

Fig.5 EBSD LAM images of S41 (a) and S42 (b) samples

3.3 SCC sensitivity

The physical map of the U-shaped sample before the SCC experiment is shown in Figure 6a. The bottom of the sample undergoes severe plastic deformation, and its surface topography is shown in Fig. 6b, in which the solid arrow indicates the scratch introduced by the sample grinding, and the dotted arrow indicates the slip band generated by the surface during the bending of the sample.

Fig.6 Image of the U-bend specimen before SCC test (a) and SEM image of the sample bottom surface (b)

Significant SCC occurred in S41 and S42 after soaking for 24, 48 and 72 h in boiling MgCl2 solution. Figure 7 shows the SCC crack morphology of the bottommost surface of the sample after immersion in the experimental solution for 72 h. All of the samples were cracked substantially in the middle of the bottommost surface, and the SCC cracks all expanded from the outer surface to the inner surface. In addition, the crack morphology of U-shaped samples with other soaking times (24 and 48 h) was not significantly different from that of the samples soaked for 72 h and was not listed. Lu et al [18] measured the maximum residual tensile stress at the bottom of the U-shaped sample, and the SCC sensitivity was the most, which was consistent with the experimental results in this paper. At the same time, in addition to some obvious cracks, a large number of “streaks” are formed on the surface of the sample. After zooming in, it can be seen that these “stripes” are also SCC cracks (Fig. 7c), that is, there are many places on the bottom surface of the U-bend sample. Cracking indicates that 316LN stainless steel has a large SCC sensitivity in boiling MgCl2 solution.

Fig.7 SEM images of the SCC cracks on the bottom surface of the U-bend S41 (a) and S42 (b) samples after immersion in boiling MgCl2 solution for 72 h, and the magnified image of square area in Fig.7a (c)

The macroscopic SCC fracture morphology of all samples is shown in Figure 8. Among them, the darker part is the area where SCC is sprouted and expanded, and the brighter part is the area broken in the air. For U-shaped samples that were immersed in boiling MgCl2 solution for 24 and 48 h, respectively, the crack length could not be accurately counted due to the different SCC initiation and cracking speeds at different positions inside the sample, and thus the SCC sensitivity difference between S41 and S42 could not be accurately compared. . However, for U-bend samples after 72 h of soaking, S42 has completely cracked (Fig. 8e), while only some areas of S41 have SCC (Fig. 8f), so the SCC sensitivity of S42 in boiling MgCl2 solution is significantly higher than that of S41. of.

Fig.8 Macro fracture morphologies of S41 (a, c, e) and S42 (b, d, f) samples after SCC tests in boiling 42%MgCl2 solution for 24 h (a, b), 48 h (c, d) and 72h (e, f)

Figure 9 shows the SCC fracture morphology of a U-shaped sample after immersion in boiling MgCl2 solution for 72 h. Both S41 and S42 showed obvious cleavage fractures, indicating that TGSCC occurred in both S41 and S42 under the experimental conditions.

Fig.9 SEM fracture morphologies of U-bend S41 (a) and S42 (b) samples after SCC tests in boiling MgCl2 solution

4 Discussion

4.1 SCC Mechanism of 316LN Stainless Steel in Boiling MgCl2 Solution

The SCC mechanism of metals can be divided into two types: anodic dissolution type and hydrogen induced cracking type [19]. Early studies [20, 21] believed that hydrogen induced cracking in molten MgCl2 solution, and some studies [22, 23] considered the anodic dissolution mechanism. The researchers [12,24,25] systematically studied the SCC behavior of austenitic stainless steels (304, 310 and 316) in boiling MgCl2 solution, and believed that in the cathode potential interval, 316 stainless steel undergoes hydrogen-induced cracking, while in the anode potential interval, Anodically soluble SCC occurs in 316 and 310 stainless steel. At the same time, Wang et al [19] study that H is almost not involved in the SCC process of 310 stainless steel in boiling MgCl2 solution.
Alyousif et al [12] pointed out that 304 and 316 stainless steels produced TGSCC in saturated MgCl2 solution at 143 and 155 °C. Cracks preferentially nucleate at the sliding step and expand under stress; MgCl2 solution below 135 °C The occurrence of IGSCC occurs because strain-induced martensite transformation along the grain boundary produces hydrogen embrittlement, which reduces the strength of the grain boundary. In this paper, the U-bend experiment is used to obtain the TGSCC fracture morphology (Fig. 9), which is consistent with the results obtained by Poonguzhali et al. [13] using the slow strain rate stress corrosion test. In addition, the sensitized austenitic stainless steel grain boundary forms a chromium-depleted grain boundary due to precipitation of Cr carbides, and IGSCC is prone to occur in corrosive media [11], and ultra-low carbon (C<0.03%). ) 316LN stainless steel is not prone to metal sensitization [11], and there are very few inclusions inside the material (Fig. 4), which may be an important reason for 316LN stainless steel to generate TGSCC in the experimental solution without IGSCC. .
According to the related literature [11-13, 19] and the experimental results herein (Figs. 7 and 9), it can be considered that 316LN stainless steel undergoes anodic dissolution type SCC in a 42% boiling MgCl2 solution. 316LN stainless steel undergoes severe slip deformation during the preparation of U-shaped samples, resulting in a large number of slip bands, while the number of neighboring atoms of some atoms on the slip surface is less than that of atoms without slippage, immersed in boiling MgCl2 The atoms on the back slip surface of the solution are more soluble and enter the solution, that is, active dissolution occurs at the sliding step, causing crack initiation and expansion, and TGSCC occurs [26]. Meng et al [27] also found that a large number of sliding steps were generated on both sides of the surface of the 690TT alloy due to plastic deformation, and preferential dissolution occurred in 0.1 mol/L HCl+0.1 mol/L H2SO4 solution. The slip zone dissolves the sequence.
In addition, the degree of deformation of the bottom of the U-shaped sample from the outer surface to the inner surface is different, and the closer to the outer surface, the greater the degree of deformation, and thus the SCC crack spreads from the outer surface to the inner surface (Fig. 8). At the same time, as the crack gradually spreads to the inner surface, the degree of deformation decreases, and the SCC expansion rate also decreases. This may be the reason why the macroscopic SCC crack length does not differ significantly after the sample is immersed in boiling MgCl2 solution for 24 and 48 h.

4.2 Effect of post-forging heat treatment on SCC sensitivity

The experimental results in this paper (Fig. 2~4) show that there is no significant difference in the inclusions and grain sizes inside S41 and S42, but the yield strength (Table 1) and microscopic residual strain (Fig. 5). Both showed significant differences, resulting in differences in SCC sensitivity between S41 and S42 in boiling MgCl2 solution.
Yield strength is an indicator of the macroscopic mechanical properties of materials and has important engineering significance. Shoji et al. [28] pointed out that the increase in yield strength of stainless steel (304L, 316L and 348) caused by cold rolling and hot rolling caused an increase in the IGSCC crack growth rate (CGR) in high temperature and high pressure water. When the yield strength of these three kinds of steels reaches 750 MPa, the CGR is also close. N as an alloying element can significantly increase the strength of stainless steel by solid solution strengthening [1]. At the same time, Poonguzhali et al [13] found that the increase of N content in 316LN stainless steel (0.07% < N < 0.22%) caused the decrease of SCC sensitivity and was mainly caused by the increase of yield strength. The effect of yield strength on SCC sensitivity is as follows [13, 28-30]: When the crack tip is equal to or greater than the effective yield stress, the material will yield and plastically deform, thereby relaxing the stress at the crack tip region. According to the relevant literature [29], the formula for calculating the maximum plastic dimension R is as follows:

Among them, α=1 (plane stress), α=2 2√ (plane strain), KIC is fracture toughness, σs is effective yield strength, so when the material σs is small, and KIC is larger, the maximum plastic zone size R is large, causing passivation of the crack tip, thereby reducing the rate of expansion of the SCC crack. For the nuclear industry, irradiation, cold working and local deformation will cause the increase of the yield strength of nuclear power structural materials. The actual nuclear engineering should consider the influence of the change of yield strength on the sensitivity of SCC.
Residual stress and residual strain on the surface and inside of the material are one of the important factors causing SCC [2, 4, 31]. Acharyya et al. [31] considered that the grinding or machining treatment caused severe plastic deformation on the surface of 304L stainless steel, resulting in a surface hardened layer, which caused an increase in SCC sensitivity in boiling MgCl2 solution. Hou et al. [32] considered that the different cold rolling deformation modes (1D, 2D and 3D) affected the grain boundary strain concentration and grain boundary characteristic distribution of 690 alloy, which caused its SCC sensitivity in high temperature alkaline solution. s difference. In the process of forging, stainless steel undergoes work hardening caused by softening and deformation caused by recrystallization, so that there is a certain degree of residual strain inside the stainless steel after forging. Compared with the stress relief treatment, the solution treatment is more effective in eliminating the residual strain generated during the forging process (Fig. 5), thereby reducing the SCC sensitivity of the stainless steel. Therefore, the actual nuclear power main pipeline is recommended to be solution treated after forging to reduce the SCC sensitivity of the material.

5. Conclusion

(1) Compared with the stress-relieving treatment, the solution treatment after forging can further effectively reduce the yield strength of 316LN stainless steel and eliminate the residual strain during forging, but the difference in grain size of 316LN stainless steel is not obvious after the post-forging heat treatment process. Impact.
(2) Significant TGSCC occurred in 316LN stainless steel after immersion in boiling MgCl2 solution for 24, 48 and 72 h. Compared with the stress relief treatment, the solution treatment after forging can improve the anti-SCC performance of 316LN stainless steel.
The authors have declared that no competing interests exist.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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