What is the reason for the cracking of 304 austenitic stainless steel pipe welded joint?
At room temperature, the microstructure of austenitic stainless steel is stable austenite, non-magnetic as a whole, with high toughness, high plasticity and good corrosion resistance, which is widely used in nuclear power plant equipment, pipeline and support structure. Compared with other types of stainless steel, austenitic stainless steel has no phase transformation, is not sensitive to hydrogen, is easier to weld, and the welded joint has better plasticity and toughness in as welded state. The microstructure and mechanical properties of the welded joint directly affect the mechanical properties of the whole weldment, but hot cracks and intergranular corrosion are easy to occur in the weld of austenitic stainless steel.
Intergranular corrosion is a kind of corrosion extending to the interior along the interface between metal grains. The main reasons are the difference between the chemical composition of the grain surface and the interior, as well as the existence of grain boundary impurities or internal stress. Intergranular corrosion can reduce the bonding force between grains, and then greatly reduce the strength of metal, while the outer surface of the material with intergranular corrosion has almost no change. Due to the significant reduction of the bonding force between grains, the mechanical properties of the material are reduced. If the material is subjected to internal and external stress, cracks can be generated by slight bending in the light, and can be broken into powder in the heavy. Therefore, intergranular corrosion is the most dangerous form of failure of stainless steel. In addition, intergranular corrosion is not easy to be detected, which brings potential safety hazard to the use of equipment.
A cracked 304 stainless steel pipe is a tap water inlet pipe, the water temperature in the pipe is normal temperature, and it is used in outdoor atmospheric environment, and the pipe diameter is small ϕ In order to find out the cracking reason of the stainless steel pipe welded joint, Cao Longtao, Gong Lanfang and Chen Zhijiang from Zhuzhou Electric Locomotive. analyzed the chemical composition, fracture morphology and microstructure, and put forward the corresponding improvement measures, In order to provide help for the relevant practitioners.
Physical and chemical test
The macroscopic appearance of the cracked stainless steel pipe joint is shown in Figure 1. The stainless steel pipe on one side close to the connecting valve is marked as 1, and the stainless steel pipe on the other side is marked as 2. The cracking position is circled as shown in Figure 1. It can be seen that the cracking position is on one side of stainless steel pipe 1, close to the heat affected zone of weld.
Figure 1 macroscopic appearance of cracked stainless steel pipe
The macro morphology of the fracture on the 1 side of stainless steel pipe is shown in Fig. 2. It can be seen that the fracture is rough and severely rusted, without obvious plastic deformation, and the overall fracture is brittle.
Fig. 2 macroscopic fracture morphology of stainless steel tube 1 side
SEM and EDS analysis
After ultrasonic cleaning, the fracture surface of stainless steel tube 1 side was observed under tungsten filament scanning electron microscope (SEM). The SEM morphology is shown in Fig. 3 and Fig. 4. It can be seen that most of the fracture surface is crystal sugar like intergranular fracture morphology, and there are a lot of granular corrosion products on the grain surface.
Fig. 3 intergranular fracture morphology of stainless steel tube 1 side fracture
Fig. 4 morphology of corrosion products on 1 side fracture of stainless steel pipe
The inside and outside of the stainless steel tube 1 side fracture were observed respectively. The SEM morphology is shown in Fig. 5 and Fig. 6. It can be seen that the number of corrosion products inside the tube is more and the corrosion degree is more serious. Therefore, it is judged that the corrosion starts from the inside of the tube.
Fig. 5 SEM appearance of the inner side of stainless steel tube with 1 side fracture
Fig. 6 SEM appearance of stainless steel tube 1 side fracture
X-ray energy dispersive spectrometer (EDS) was used to analyze the micro composition of the fracture of stainless steel tube 1 side. The results of EDS analysis of granular corrosion products on the grain are shown in Table 1. It can be seen that there are mainly iron, oxygen, carbon and other elements.
Table 1 energy spectrum analysis results of corrosion products on side 1 fracture of stainless steel pipe (mass fraction)
Chemical composition analysis
The chemical composition of stainless steel tube 1 and stainless steel tube 2 were analyzed by direct reading spectrometer. The results show that the chemical composition of stainless steel tube 1 does not meet the technical requirements of 304 stainless steel in GB / t20878-2007 “grades and chemical composition of stainless steel and heat resistant steel”, and the chemical composition of stainless steel tube 2 basically meets the requirements of the standard.
The sample is taken from the longitudinal section of the fracture on the 1 side of the stainless steel tube, and the sampling position is shown in Fig. 1. After inlaying and polishing, the sample is placed under the optical microscope for observation, and the microstructure morphology is shown in Fig. 7 and Fig. 8. It can be seen from Figure 7 that the crack propagates along the austenite grain boundary, intergranular separation occurs in local austenite, and there is obvious intergranular corrosion near the fracture area; It can be seen from figure 8 that part of the grains dissolved.
Figure 7 morphology of intergranular corrosion and secondary crack near fracture
Figure 8 grain dissolution morphology near fracture
After the metallographic sample is etched by ferric chloride hydrochloric acid aqueous solution, the microstructure far away from the fracture is shown in Fig. 9. It can be seen that the microstructure of stainless steel tube 1 is austenite. According to the technical requirements of GB/T 6394-2017 method for determination of average grain size of metals, the grain size is 6.5.
Figure 9 microstructure of stainless steel tube 1 far away from fracture
Analysis and discussion
From the above physical and chemical test results, it can be seen that the chemical composition of stainless steel pipe 1 selected by the construction company is inconsistent with the design drawing, with high carbon content and low chromium and nickel content. In stainless steel, nickel can change the crystal structure of stainless steel, so it is called austenite forming element. Adding nickel to stainless steel can significantly improve the corrosion resistance, plasticity and welding performance of stainless steel. In addition, nickel can expand the passivation range of stainless steel in non oxidizing medium and effectively improve the passivation ability of stainless steel. Secondly, the mass fraction of chromium in stainless steel tube 1 is 12.30%. For austenitic stainless steel, the necessary condition for high corrosion resistance is that the mass fraction of chromium must be greater than 12% (generally not less than 17%). When the mass fraction of chromium is less than the critical value of 12%, the material will lose its corrosion resistance. Under the action of corrosive medium, intergranular corrosion is easy to occur in the chromium poor area of grain boundary. Finally, the carbon content of stainless steel tube 1 is 0.20%. With the increase of carbon content in stainless steel, the chromium carbide generated at the grain boundary also increases, which increases the chance of forming chromium poor zone at the grain boundary and the tendency of intergranular corrosion. Therefore, carbon is the main element of intergranular corrosion. Generally, stainless steel controls the carbon content below 0.08%. It can be seen from the above three points that the chemical composition of stainless steel pipe 1 does not meet the requirements, which greatly reduces the corrosion resistance and weldability of the steel pipe, which is the main cause of intergranular corrosion of austenitic stainless steel pipe.
In the process of steel pipe welding, when the temperature of heat affected zone of stainless steel pipe is 450 ~ 850 ℃, the diffusion rate of carbon in austenite is faster than that of chromium in austenite. When the mass fraction of carbon in austenite stainless steel exceeds 0.02% ~ 0.03%, the excess carbon will continue to diffuse to austenite grain boundary, Because the diffusion speed of chromium atoms in the crystal is smaller than that of carbon atoms, the chromium atoms can not diffuse to the grain boundary, which leads to a significant reduction of chromium content near the grain boundary and the formation of chromium poor zone, which reduces the corrosion resistance and leads to intergranular corrosion.
In the process of using stainless steel pipe, the circulating tap water provided chloride corrosion environment and accelerated the crack propagation, which was consistent with the SEM analysis of fracture surface.
To sum up, stainless steel pipe 1 material does not meet the requirements, and the heat affected zone is sensitized during the welding process, which leads to a significant decline in corrosion resistance, and then leads to intergranular corrosion. The chloride corrosion environment provided by tap water accelerates the crack propagation, and finally leads to the crack failure near the stainless steel welded joint.
Conclusions and suggestions
(1) The cracking property of stainless steel tube is intergranular fracture caused by intergranular corrosion, and the fracture position is the welding heat affected zone of stainless steel tube 1.
(2) The main reason is that the chemical composition of stainless steel tube 1 does not meet the requirements, the carbon content is too high, and the chromium and nickel content is too low. During the welding process of stainless steel tube, when the temperature of heat affected zone is in the dangerous intergranular corrosion temperature zone of 450 ~ 850 ℃, the welding sensitization phenomenon occurs, and the chromium poor zone is formed at the grain boundary, which reduces the corrosion resistance of the joint, and then produces cracks. The chloride corrosion environment provided by tap water promotes the crack propagation.
In view of the cracking reasons of stainless steel pipe, the following suggestions are put forward.
- (1) Strengthen the incoming inspection of incoming stainless steel pipes to ensure that the chemical composition, mechanical properties and microstructure meet the requirements of the standard.
- (2) Mark the qualified materials and make records to prevent the misuse of materials due to unclear identification.
- (3) Considering that if the stainless steel pipe stays at 450 ~ 850 ℃ for a long time in the welding process, it may produce sensitization phenomenon, it is suggested to adopt small current, high welding speed and other methods in the welding process, so as to shorten the stay time of the welded joint in the intergranular corrosion dangerous temperature zone; At the same time, the stabilization heat treatment after welding makes the chromium atoms in austenite diffuse to the grain boundary, increases the chromium content at the grain boundary, and avoids intergranular corrosion.
Authors: Cao Longtao, Gong Lanfang, Chen Zhijiang
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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