Investigating the Effect of Tempering Temperature on the Microstructure, Hardness and Burst Pressure of 34CrMo4 Steel

Introduction and Objectives: 34CrMo4 steel has the ability to increase its strength through quenching and annealing heat treatment due to its moderate carbon content. The aim of the present study is to obtain the optimal annealing temperature in order to achieve optimal structural and mechanical properties, including hardness and burst pressure.Materials and Methods: In the present study, 34CrMo4 steel was used as the starting material. After going through the stages of the natural gas tank construction process and performing heat treatment, hardness testing, hydraulic pressure testing, burst pressure testing, and microstructural studies were performed to investigate the microstructural and mechanical properties of the produced tanks.Results: The results showed that in the annealing temperature range of 505 to 585°C, the hardness decreased from 392 to 310 Brinell and the breaking pressure decreased from 620 to 500 bar. Compared to the national standard (ISIRI 7598), the samples made at temperatures of 505, 525, and 545°C do not meet the standard conditions and the optimum heat treatment temperature is in the range of 565 to 585°C for compressed natural gas tanks.Conclusion: With increasing tempering temperature, the strength of the material decreases and bursting occurs with less pressure. With increasing annealing temperature, the fracture surfaces will progress from brittle fracture to quasi-brittle and ductile fracture. The annealing temperature range at which optimal mechanical properties will be achieved in accordance with the National Standard 7598 will be between 565-585°C.