Austenitic stainless steel is mainly composed of Cr and Ni elements. It has high corrosion resistance, good plasticity, easy processing and forming, and good welding performance. It is currently the most important acid-resistant stainless steel. Austenitic stainless steel welded pipe is a steel pipe made by curling and welding austenitic stainless steel strip. The production process of austenitic stainless steel welded pipe is relatively simple, with high production efficiency, superior product performance, and a wide range of applications, such as pipes for chemical equipment, pipes for heat exchange equipment, decorative pipes, etc. Once cracks occur in stainless steel welded pipes during use, it may cause very serious consequences, so after the welded pipes are processed, they are usually tested for sealing, such as water pressure testing. Researchers use macroscopic observation, chemical composition analysis, metallographic inspection, scanning electron microscopy (SEM) and energy spectrum analysis to analyze the causes of cracks to prevent such problems from happening again.
01 Physical and chemical inspection
1.1 Macroscopic observation
In recent years, some welded pipe customers have reported that cracks appear in stainless steel welded pipes after the sealing test. Unlike common welded pipe cracks, which usually occur at the weld position, this type of crack does not occur at the weld, but at the parent material position of the welded pipe, and is a transverse crack perpendicular to the weld. The crack is short, usually less than 3mm in length, and sometimes as small as a needle tip (see Figure 1). This type of crack has been found in austenitic stainless steels such as 304, 316L, and 321. The parent material samples with defects are all thin plates with a thickness of less than 1.0mm.
The macroscopic morphology of the crack fracture is shown in Figure 2. The middle is the crack area, and the left side is the area where the cracks are artificially broken.
1.2 Chemical composition analysis
The sample matrix is 316L stainless steel, and the chemical composition of the parent material is shown in Table 1.
1.3 Metallographic inspection
A metallographic sample was cut from the parent material and placed under an optical microscope for observation. The results are shown in Figure 3. As shown in Figure 3, the parent material structure is austenite and the grain size is 9.0. The specification of the welded pipe (outer diameter × wall thickness) is 25mm × 0.5mm. The welding and post-weld heat treatment are completed on the automatic production line, and then the water pressure test is performed. No obvious cracks were found after welding, but after the water pressure test, a large number of transverse short cracks perpendicular to the weld were found on the parent material of the welded pipe.
1.4 Scanning electron microscopy and energy spectrum analysis
The crack fracture sample was mechanically cut, ultrasonically cleaned, cleaned with ethanol, and dried, and then placed under SEM for observation, and the energy spectrum analysis was performed. The results are shown in Figure 4 and Table 2.
02 Comprehensive analysis
According to the above analysis results, it is found that the cracked specimens have the following characteristics: (1) The cracking position is in the non-weld area and does not extend to the weld; (2) The cracking is intergranular and the fracture is rock candy-like; (3) The copper element is enriched along the grain boundary at the crack.
The above characteristics are consistent with the characteristics of copper contamination cracks (CCC). CCC is a liquid metal embrittlement (LME) phenomenon, that is, when a solid metal material with good plasticity comes into contact with a specific liquid metal, the bonding force between atoms decreases. After being subjected to tensile force, the strength and plasticity of the metal material decrease significantly. This usually occurs between two metals with a large difference in melting points. For austenitic stainless steel, the common low-melting-point metals that can cause LME phenomenon during welding are Cu and Zn. The crack position is enriched with a large amount of Cu element. The melting point of pure copper is 1083℃. As the purity decreases, the melting temperature decreases, which is much lower than the melting point of stainless steel. During the welding process, if the heat-affected zone is contaminated by copper, LME phenomenon may occur. The reason why the parent material is contaminated by copper is that it undergoes post-weld heat treatment. The post-weld heat treatment temperature of stainless steel is usually selected to be greater than 1000℃. During heat treatment at this temperature, the copper contaminated on the surface becomes liquid, and the thermal expansion coefficient of austenitic stainless steel is relatively high. Under the action of capillary force, liquid Cu will penetrate inward along the austenite grain boundary, thus forming grain boundary penetration, destroying the continuity of the grain boundary, causing grain boundary embrittlement, decreased bonding force, and cracks under the action of tensile stress. If the material is thin, a penetrating crack is formed, so the crack is confirmed as CCC.
Copper added as an alloy generally does not cause copper contamination cracks. The most likely place to introduce copper contamination is the automatic processing production line of welded pipes, on which there are many copper workpieces, such as fixtures and contacts. The most effective way to prevent CCC is to avoid scratches and residues between the material and copper. In most cases, attention should be paid to welding fixtures, conductive nozzles or other copper-based parts that may contact the surface of the material.
An investigation of the pipe production process found that when the pipe is made by tungsten inert gas welding (TIG) welding, in addition to the normal shielding gas at the welding gun position, an internal shielding gas is set inside the pipe. The user uses a copper pipe to transport the internal shielding gas. It can be found on site that the copper pipe contacts the inner wall of the stainless steel and scratches during production. The copper pipe has also been worn to a certain extent. This confirms that the copper pipe that transports the internal shielding gas is the source of CCC. After the user replaced the gas supply pipe, the cracking problem no longer occurred.
03 Conclusion
(1) The cracks in 316L stainless steel welded pipes are copper contamination cracks, which are caused by the scraping residues of the copper pipe that transports the welding internal shielding gas on the inner wall of the welded pipe.
(2) During the processing and use of welded pipe materials, they should be kept away from low-melting-point substances such as copper, zinc, and aluminum, or try to avoid scratching and friction with such substances, so as to prevent such microcracks from occurring at the root.
(3) After the user replaced the stainless steel shielding gas delivery pipe, the welded pipe no longer cracked.
