Do cold bending specimens of steel plates for pressure vessels need to simulate post-weld heat treatment?
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Technical requirements of the design institute: The tensile and impact specimens of the steel plates used for containers must undergo the maximum and minimum simulated post-weld heat treatments； Cold-formed specimens require minimum simulated welding heat treatment. Knowledge points found online: The cold bending test can reflect the plastic deformation at the bent portion of the specimen, and it can reveal whether there are defects such as uneven internal structure, internal stresses, and inclusions in the steel. The cold bending test can also conduct a thorough inspection of the welding quality of steel, revealing defects such as lack of fusion, cracks, and impurities on the bent surface of the welded parts. In my opinion, for steel plate raw materials, the cold bending test should be conducted in the state in which the steel plates are supplied, without the need to simulate post-weld heat treatment, as the cold forming of the cylinder and head takes place before post-weld heat treatment. For welding procedure qualification, cold-formed specimens shall undergo minimum simulated post-weld heat treatment ； For product test plates, the cold-formed specimens shall undergo post-weld heat treatment along with the product. For welds, the properties after post-weld heat treatment should be checked. I ask fellow sailors for their corrections.
In my opinion, it should be done after simulating post-weld heat treatment. Reason: 1. This cold bending test is used to evaluate the properties of the raw material after simulated welding heat treatment. It is not used to determine whether cold forming is possible; the degree of deformation in cold forming is much smaller than that in cold bending tests. 2. The typical delivery condition for sheets is normalized or normalized + tempered. Verification: If the condition at delivery is found to be satisfactory, it does not guarantee that the condition after simulated welding will also be satisfactory. Therefore, after heat treatment for recovery properties following hot working, it can only prove that the condition at this point is satisfactory; it cannot guarantee that the condition simulating the post-weld state is also satisfactory. However, such equipment must undergo overall or partial post-weld heat treatment after being manufactured. Whether in the overall structure or in individual components, some of the base material must undergo post-weld heat treatment and is not used in its as-delivered state. It can only be guaranteed that it is in good working condition. I’m not sure if what I’ve said makes sense.
For national standard container plates such as Q345R and 15CrMoR, if they are purchased in stock, their cold bending tests are usually conducted in their as-delivered condition. If cold bending in the formed state is required, this must be specified in the contract when placing an order for custom production. As for American standard container plates and European standard container plates, such as SA516Gr70 and P265GH, since the standards do not specify cold bending, no cold bending tests are conducted. If a cold bending test is required, it must be specified in the custom order; the test condition can be either the as-delivered condition or the molded-welded condition. Since the post-weld heat treatment condition represents the actual usage condition of the steel, it is representative to perform cold bending in the as-welded state. Wugang Panglong
Under normal circumstances, post-weld heat treatment at temperatures below the lower transition temperature improves impact toughness and plasticity, but excessive treatment time will lead to a deterioration in these properties. It can’t be generalized. It is also related to the temper brittleness of the material.
It was previously believed that post-weld heat treatment would improve the impact properties of the material. After reading a paper by Sui Yi from Ansteel titled \"Study on Simulated Post-Weld Heat Treatment of Steel SA516Gr70 for Medium and Low Temperature Pressure Vessels\", it was found that after simulated post-weld heat treatment, both the strength and toughness of 70-mm-thick SA516Gr70 steel plates decreased to varying degrees: For SPWHT, at the specified temperature and time, Rp0.2 and Rm were 46℃ lower; the values according to the material standard were ≥260 for Rp0.2 and 485–620 for Rm, with ≥20 for KV2. In the case of no SPWHT treatment, the corresponding values were 357 for Rp0.2, 544 for Rm, and 208 for KV2. At 610℃ for 9 hours, the values were 315 for Rp0.2, 485 for Rm, and 173 for KV2; at 690℃ for 6 hours, they were 335 for Rp.0.2, 505 for Rm, and 192 for KV2. After simulated post-weld heat treatment, the cementite in the steel underwent varying degrees of spheroidization, but no significant growth occurred. The author believes that the main reasons for the decrease in strength are the spheroidization of cementite in the steel and the reduction in dislocation density. Based on the data, the effect of heat treatment time on strength is greater than that of temperature.