Abstract: Cracks occur near the welds of stainless steel pipes such as exhaust pipes and cooling water pipes for automobile chassis. The causes of cracks are analyzed by macroscopic observation, chemical composition analysis, metallographic inspection and other methods. The results show that compared with 06Cr19Ni10 stainless steel, 1Cr14Mn10Ni1 stainless steel pipes are more likely to precipitate Cr23C6 carbides near the welds, and local Cr depletion occurs. The Cr depletion area corrodes and cracks along the grain boundaries.
Keywords: stainless steel pipe; weld; local Cr depletion; cracking
1 Physical and chemical inspection
1.1 Macroscopic observation
After 1~2 years of operation, exhaust pipes and cooling water stainless steel pipes for automobile chassis often crack near the welds, and the cracking area is 3~10mm away from the weld edge. The macroscopic morphology of the cracked part near the weld of the stainless steel pipe is shown in Figure 1. It can be seen from Figure 1 that there is no plastic deformation at the fracture of the exhaust pipe, the fracture is rough, and no fatigue source and fatigue extension marks are found. The material of the cracked stainless steel pipe is 1Cr14Mn10Ni1 steel.
1.2 Chemical composition analysis
The chemical composition of the cracked exhaust pipe and cooling water pipe was analyzed, and the results are shown in Table 1. It can be seen from Table 1 that the chemical composition of the cracked stainless steel pipe meets the technical requirements.
1.3 Metallographic inspection
Samples were taken near the cracks of the cracked exhaust pipe and cooling water pipe welds, respectively. After the samples were corroded with aqua regia, they were placed under an optical microscope for observation. The results are shown in Figure 2. It can be seen from Figure 2 that obvious black corrosion products and cracking phenomena along the grain boundaries appeared near the cracks.
1.4 Corrosion test verification
Samples were taken from the vicinity of the welds of steel pipes welded with two stainless steel materials, 1Cr14Mn10Ni1 and 06Cr19Ni10. The specifications of the two materials were 75 mm×1.5 mm (outer diameter × wall thickness) and 89 mm×1.5 mm (outer diameter × wall thickness), respectively. After being corroded with aqua regia, they were placed under an optical microscope for observation. The results are shown in Figure 3. As shown in Figure 3, no black corrosion products and cracking along the grain boundaries were found near the welds.
The welded steel pipes of two stainless steel materials, 1Cr14Mn10Ni1 and 06Cr19Ni10, were placed in a neutral salt spray test chamber. The welded steel pipes were taken out after 5 months. Their macroscopic morphology is shown in Figure 4. As shown in Figure 4, there is cracking near the weld of 1Cr14Mn10Ni1 stainless steel pipe. When the cracked part is opened, there is no plastic deformation at the fracture, the fracture is rough, and no fatigue source and fatigue extension traces are found; there is no cracking near the weld of 06Cr19Ni10 stainless steel pipe.
Samples were taken from the welds of the above two 1Cr14Mn10Ni1 and 06Cr19Ni10 stainless steel pipes that have undergone neutral salt spray tests. After being corroded with aqua regia, they were placed under an optical microscope for observation. The results are shown in Figure 5. As shown in Figure 5, black corrosion products along the grain boundaries appear near the cracks of 1Cr14Mn10Ni1 stainless steel pipe, and no black corrosion products along the grain boundaries appear near the welds of 06Cr19Ni10 stainless steel pipe.
1.5 Comparison of local Cr depletion
1Cr14Mn10Ni1 stainless steel and 06Cr19Ni10 stainless steel welded steel pipes were corroded with 6% FeCl3 solution by volume. The local Cr depletion near the weld was different, resulting in obvious differences in their corrosion resistance. Therefore, this method can also be used to intuitively and quickly identify the local Cr depletion near the weld.
Two specifications of 1Cr14Mn10Ni1 stainless steel pipes were welded using four different welding processes. After 24 hours of corrosion in 6% FeCl3 solution by volume, the 3~5mm area near the weld was disconnected. Two specifications of 06Cr19Ni10 stainless steel pipes were welded using four different welding processes. Even after 72 hours of corrosion in 6% FeCl3 solution by volume, only slight pitting corrosion occurred on both sides of the weld. The results are shown in Table 2. The macroscopic morphology near the weld of the stainless steel pipe after corrosion by 6% (volume fraction) FeCl3 solution is shown in Figure 6.
2 Comprehensive analysis
Austenitic stainless steel contains a small amount of carbon, which forms carbide Cr23C6 with chromium. When heated to high temperature, the carbide dissolves in the γ phase. The higher the temperature, the more carbides dissolve. Then, this state is preserved to room temperature by rapid cooling to form a supersaturated solid solution (solution treatment). During the slow cooling process, in order to maintain equilibrium, carbides will precipitate from the solid solution. The supersaturated solid solution is unstable. When reheated at low temperature (400~850℃), carbides will precipitate (sensitization treatment). Carbides usually precipitate preferentially along the grain boundaries. This change causes austenitic stainless steel to have an intergranular corrosion tendency. During welding, the temperature near the weld can reach 400~850℃. Therefore, the welded structure of austenitic stainless steel materials may be damaged by intergranular corrosion [1].
During sensitization treatment, the diffusion rate of carbon to the grain boundary is greater than that of chromium. Cr23C6 precipitates at the grain boundary, and chromium is depleted at the grain boundary and its adjacent area. When the chromium content is reduced to below the limit of chromium content required for passivation, a micro-battery is formed, which accelerates the corrosion along the grain boundary.
When the austenitic stainless steel welded joint is rapidly cooled after welding, the carbon in the austenite structure is supersaturated. Once it encounters a heating temperature of 400~850℃ and an appropriate retention time, the diffusion rate of chromium atoms in the grain is lower than that of carbon atoms. The chromium atoms have no time to diffuse to the grain boundary, resulting in a significant decrease in the chromium content near the grain boundary, forming a chromium-depleted area, reducing the corrosion resistance of the material, and then causing intergranular corrosion [2].
1Cr14Mn10Ni1 and 06Cr19Ni10 are both austenitic stainless steel materials. The upper limit of the mass fraction of carbon in 1Cr14Mn10Ni1 stainless steel is 0.15%, and the lower limit of the mass fraction of chromium is 13.00%; the upper limit of the mass fraction of carbon in 06Cr19Ni10 stainless steel is 0.08%, and the lower limit of the mass fraction of chromium is 18.00%. 1Cr14Mn10Ni1 stainless steel has a high carbon content and is more likely to form Cr23C6 carbides. At the same time, the mass fraction of chromium is low, and the mass fraction of chromium in the "locally Cr-poor" area is more likely to be lower than the limit value of the mass fraction of chromium required for passivation; different specifications of 1Cr14Mn10Ni1 and 06Cr19Ni10 stainless steel pipes use different welding processes. After being corroded by 6% (volume fraction) FeCl3 solution, the fracture tendency near the weld of 1Cr14Mn10Ni1 stainless steel pipe is serious, which has also been further verified by experiments.
After 5 months of long-term neutral salt spray corrosion, cracks appeared near the weld of 1Cr14Mn10Ni1 welded pipe. The common cracks near the weld are 1Cr14Mn10Ni1 stainless steel pipes. The black corrosion products appearing at the grain boundaries near the cracks are intergranular corrosion caused by the micro-battery of large cathode and small anode formed by "local Cr deficiency". There are almost no cracks near the weld of 06Cr19Ni10 stainless steel pipe, indicating that 1Cr14Mn10Ni1 welded pipe is more prone to intergranular corrosion, which is the main cause of cracking failure.
The main measures to prevent and control intergranular corrosion of stainless steel welds are: using low-carbon stainless steel materials, performing post-weld heat treatment, adding strong carbides, etc. Due to the limitations of the design structure, cost, manufacturing capacity and other comprehensive factors of stainless steel pipes such as exhaust pipes and cooling water pipes for automobile chassis, it is difficult to perform post-weld heat treatment. The strong carbide welded joints with titanium and niobium elements are subjected to the welding heat cycle, and TiC and NbC dissolve in the overheating zone. At this time, the stabilizer Ti and Nb elements have lost the function of stabilizing carbon. If the joint is heated to 400~850℃ again or works at this temperature, there is a risk of knife-shaped corrosion [3], and the material cost is high. It is recommended to use low-carbon or even ultra-low-carbon stainless steel materials such as 06Cr19Ni10.
3 Conclusions
(1) During the welding process of 1Cr14Mn10Ni1 stainless steel exhaust pipes and cooling water pipes, Cr23C6 carbides are easily precipitated near the grain boundaries, resulting in the appearance of "local Cr-poor" areas near the weld, forming a micro-battery of large cathode and small anode, forming intergranular corrosion, resulting in the deterioration of local material properties near the weld, and even early fracture.
(2) The corrosion of stainless steel welded samples with 6% (volume fraction) FeCl3 solution can identify the area and degree of "Cr-poor" near the weld.
(3) The use of low-carbon stainless steel materials such as 06Cr19Ni10 can effectively inhibit intergranular corrosion near the weld of steel pipes.
