Regarding the issue of heat-treated base metal test plates

Thread Content

I would like to discuss with all of you the issue related to heat treatment of the base material test plates: A company is manufacturing a vessel made of 13MnNiMoR material. The material was supplied in a heat-treated state of “normalizing + tempering”; the head of the vessel was formed by hot pressing, which required a heat treatment to restore its original state. After the head was formed, it underwent another “normalizing + tempering” treatment. The entire vessel needed to undergo stress-relief heat treatment at 620°C after welding. The base material test plates were used to track all the heating processes undergone by the material, and the following results were obtained: after hot pressing and the “normalizing + tempering” treatment, mechanical property tests were conducted on the test plates, and their strength met the requirements. However, after the final stress-relief heat treatment, further tests were carried out on these plates, and it was found that the tensile strength had decreased by 100 MPa compared to the strength of the raw material. Question 1: I believe that, as a test plate for heat treatment of base materials, its role is merely to determine whether the material used is consistent with its condition at the time of delivery. In other words, after the head is formed and subjected to \"forward + reverse\" heat treatment, if the mechanical tests on the test plate show satisfactory results, then its function is fulfilled; there is no need to monitor its subsequent exposure to heat (since stress-relief heat treatment does not alter the material’s condition at the time of delivery). In other words, it is not necessary to conduct testing after the final stress-relief heat treatment of the test piece. Question 2: Why does the tensile strength of the material show little change after the \"forward + reverse\" heat treatment of the head, whereas it decreases by more than 100 MPa after the full-body stress-relief heat treatment (620°C)? Question 3: When all the operational process parameters are normal, the tensile strength of the material still decreases by more than 100 MPa. Can it still be used normally? If not, how can it be restored to normal functionality?