Alkali induced stress corrosion cracking of stainless steel materials, referred to as “alkali embrittlement” or “alkali cracking”, has been reported in many literatures, but most of the research focuses on high temperature conditions, while the related research on alkali embrittlement of stainless steel under medium and low temperature conditions is less.
The cases of alkali embrittlement failure of stainless steel also occur in high temperature environment, and the failure cases at medium and low temperature are relatively rare.
The storage tank of hydrogen generator in a domestic nuclear power plant is made of 316L austenitic stainless steel, the medium in the tank is KOH solution, and the working pressure is 700kPa~800kPa.
After 8 years of service, the lower head of the tank cracked.
The physical and chemical inspection and stress state analysis of different areas of the tank were carried out, the causes of the tank cracking were studied, and suggestions for improvement were put forward.
1. Physical and chemical inspection
1.1 Macroscopic observation and penetrant testing
The leakage tank is welded by the cylinder and the elliptical head, and the head can be divided into straight edge section and curved section, as shown in Fig. 1a).
The through crack on the outer wall is located at the straight edge of the head.
The upper tip of the crack is about 8mm from the fusion line, and the lower tip is about 13mm from the fusion line.
The total length of the crack is about 5mm.
Liquid penetrant testing was carried out on the tank, and the results showed that no other cracks were found on the outer wall of the tank except the through cracks;
Many cracks were found on both sides of the inner wall near the weld, including axial cracks perpendicular to the weld and circumferential cracks parallel to the weld, as shown in Fig. 1b).
The axial cracks are only located in the straight edge section of the head within 13mm from the weld fusion line, with uniform circumferential distribution and different lengths.
The upper tip of the longer crack is 1-2 mm from the fusion line, and the lower tip is about 13 mm from the fusion line;
The upper tip of the shorter crack is about 4mm away from the fusion line, and the lower tip is about 10mm away from the fusion line.
This crack is recorded as a type A crack, and the through crack is a type A crack.
Circumferential cracks are located at both sides of the weld 1~3mm from the fusion line.
Cracks on the cylinder side are recorded as Class B1 cracks, and cracks on the head side are recorded as Class B2 cracks.
The macroscopic diagram of crack distribution of leaking tank is shown in Fig. 2.
1.2 Chemical composition analysis
The chemical composition of cylinder and head base metal is analyzed by spark direct reading spectrometer.
Both chemical compositions meet the requirements of ASTM A473-2017 standard.
1.3 Metallographic inspection
Take samples from the barrel and head base metal, and conduct metallographic inspection with the optical microscope.
The microstructure of the base metal of the tank is shown in Fig. 3.
The microstructure of the barrel base metal is austenite+a small amount of annealed twins, and the grain size is grade 6;
The microstructure of head base metal is austenite+a lot of deformation twins and slip bands, and the grain size is 3.5.
1.4 Hardness test
Use a digital Vickers hardness tester to test the hardness of various parts of the tank.
The hardness of barrel and head base metal is 165 HV and 248 HV respectively.
The hardness of weld, cylinder side heat affected zone and head side heat affected zone is 171 HV, 188 HV and 165 HV respectively, and the average thickness of cylinder side and head side is 3.71 and 4.24mm respectively.
The cylinder is 316L steel in the normal solution annealed state.
The hardness of 316L steel is not specified in ASTM A473-2017 standard, but according to the standard Cold Rolled Stainless Steel Plates and Strips (GB/T 3280-2015), the hardness of 316L steel is required to be no more than 220 HV, so it can be seen that the hardness of the head is high, which is related to the fact that there are a lot of deformation twins and slip bands in the structure, and it is 316L steel in the cold work hardening state.
1.5 Analysis of crack morphology
1.5.1 Crack surface analysis
On the inner wall of the tank, the surface of Class A and Class B cracks shall be sampled for analysis. The sampling location is shown in Fig. 4.
After the sample is flattened, polished and etched by the arc surface, it is observed by the optical microscope. The microscopic morphology is shown in Fig. 5.
It can be seen that the two types of cracks both extend along the crystal on the surface.
The central position of Type A crack is wide, and the two ends are thin.
The heat affected zone at the head side has obvious coarse grain zone and fine grain zone, with a total length of about 4mm;
The heat affected zone on the cylinder side only has coarse grain zone, about 0.8mm long, and no fine grain zone is found.
The base metal at the head side contains a large number of deformation twins and slip bands, with high deformation and distortion. Static recrystallization occurs when welding is heated.
Due to the high temperature near the weld, grain growth occurs after recrystallization, forming a coarse grain area.
Only static recrystallization occurs in the area far from the weld, and the grain does not grow to form a fine grain area.
The base metal at the side of the cylinder is in the solution annealed state, with poor deformation and distortion, and insufficient recrystallization driving force.
Due to the high temperature near the weld, grain growth directly occurs, forming coarse grain zone;
As the temperature of the area far from the weld is lower than the grain growth temperature, only recovery occurs without crystallization, and there is no fine grain area similar to the head side, so it is impossible to directly judge the scope of the heat affected zone.
The barrel and head parent materials are 316L stainless steel, with the same thermal conductivity. The scope of the heat affected zone on both sides of the weld is basically the same.
Inferred from the scope of the heat affected zone on the head side, the width of the barrel heat affected zone is about 4mm.
It can be seen that the tip of one side of some Class A cracks is located in the heat affected zone, the tip of the other side is located in the straight edge section of the head, and the center is located in the straight edge section of the head;
The other part of Class A cracks are located at the straight edge of the head;
All Type B cracks are located in the heat affected zone on both sides of the weld.
1.5.2 Crack section analysis
Figures 6 and 7 show the microstructures of the two types of cracks in the wall thickness direction.
Type A cracks extend from the inner wall of the tank to the outer wall along the crystal, with different depths.
The severe parts almost run through the full wall thickness of the tank, the crack tip is bifurcated, and the grain boundary is not sensitized.
It has typical characteristics of intergranular stress corrosion cracking.
Type B1 and B2 cracks are mainly located in the heat affected zone on both sides of the weld.
The cracks extend along the grain, the tip is bifurcated, and the grain boundary is not sensitized. They have typical characteristics of intergranular stress corrosion cracking.
The microhardness of type A, B1 and B2 cracks is 242 HV, 171 HV and 157 HV respectively.
The reason for the sharp decrease of hardness in the B2 type crack zone is that the static recrystallization occurs after the welding of the original deformed austenite grains.
In order to further analyze the origin position of Class A crack on the inner wall of the tank, along the length direction of the same crack, dissect and measure its depth at the center and both sides. The results are shown in Fig. 8.
The middle part of the crack is the deepest along the wall thickness direction, which indicates that the origin of Class A crack is in the middle of the crack length direction, extending from the inner wall surface to both sides.
1.6 Residual stress analysis
The residual stress analyzer is used to test the residual stress of the cylinder and the head respectively with the weld as the boundary.
Each position is tested in two directions: 0 ° (parallel to the weld direction) and 90 ° (perpendicular to the weld direction). The test results are shown in Fig. 9.
The 0 ° and 90 ° residual tensile stress zones on the cylinder side are about 20 and 12 mm away from the weld centerline respectively;
The residual tensile stress zones at 0 ° and 90 ° directions at the head side are about 17 and 15 mm away from the weld centerline respectively.
Type A crack and type B crack are located in the residual tensile stress zone.
2. Comprehensive analysis
Type A crack and type B crack are located in the residual tensile stress zone of the tank, and both extend along the wall thickness direction from the inner wall to the outer wall along the crystal.
Type A cracks originate in the base metal area of the head and extend to both sides on the surface perpendicular to the weld;
Type B cracks are located in the heat affected zone on both sides of the weld and extend parallel to the weld on the surface.
The manufacturing process of the head is cold stamping.
The straight edge of the head is formed by “flanging” the edge of the original sheet inwards, which will produce large plastic deformation and residual tensile stress.
Under the long-term effect of the original cold working residual stress, temperature of 65~70 ℃, and KOH alkaline solution service conditions, intergranular stress corrosion cracking perpendicular to the weld was generated.
Due to the recovery and recrystallization of austenite grains after welding in heat affected zone, the original residual stress basically disappears.
Due to the cooling shrinkage of austenite grains, welding residual tensile stress is generated in the heat affected zone.
The residual stress is mainly perpendicular to the weld. Under the long-term effect of the service conditions of KOH alkali solution at 65~70 ℃, intergranular stress corrosion cracking parallel to the weld is generated.
The cracking mechanism of the tank can be explained by the membrane cracking theory of alkali induced stress corrosion cracking.
In KOH alkali solution environment, a passive film is formed on the inner wall surface of the tank, and the passive film breaks under the action of high residual tensile stress.
After the passivation film breaks, a passivation film is not formed on the metal surface in the fracture area in time.
The bare metal contacts KOH lye. OH – concentrates in the surface fracture area, and then reacts with the bare metal.
The bare metal reacts with the concentrated lye to form a metal oxide film.
This oxide film breaks again under the effect of stress, and then the passivation fracture passivation fracture cycle repeats, and the crack continues to expand and extend, eventually, the tank cracked and leaked.
3. Conclusions and Suggestions
(1) The circumferential cracks and axial cracks on the inner wall of the storage tank are alkali induced stress corrosion cracking.
The circumferential cracks are mainly affected by the welding residual tensile stress, while the axial cracks are mainly affected by the cold working residual tensile stress on the straight edge of the head.
(2) Control welding heat input and reduce welding residual stress to avoid circumferential cracks;
After the cold forming of the head, the stress relief process is added to reduce the residual stress of cold working to avoid axial cracks.
(3) In order to ensure the safe operation of equipment, non-destructive testing measures such as penetrant testing should be strengthened during operation.