Research on the influence of heat treatment temperature on bearing capacity of high steel grade pipeline during in-service welding

Abstract

During the welding of in-service pipeline, natural gas is continuously transported in the pipeline, which maintains a high gas pressure. Therefore, the welding process is completed under strong cooling conditions, and welding delay crack is easy to occur. Preheating before welding and heat treatment after welding can effectively control the hardened microstructure, reduce the residual stress and ensure the welding quality. In this study, the influence of heating temperature on the bearing capacity of high steel grade pipe during in-service welding repair was studied. The high temperature tensile test was used to simulate the bearing capacity of the pipe under heating and high temperature environment. It is found that when the heating temperature is below 400°C, the pipe strength remains at the original level. With the increase of heating temperature, when the test temperature is higher than 400°C, the yield strength and tensile strength of the pipe decrease significantly. When the test temperature is 450°C, the yield strength and tensile strength of the material decrease by 15.8% and 11.1%, respectively, compared with the normal temperature, which are lower than the pipe standard requirements. Therefore, it is suggested that when the heat treatment temperature is higher than 400°C during in-service welding repair, it is necessary to consider reducing the pipeline pressure. At the same time, the box furnace heat treatment method was adopted to heat treat the pipe, and the tensile properties of the pipe after heat treatment were tested to analyze the bearing capacity of the pipe after heat treatment. It is found that when the heating temperature is higher than 700°C, the tensile properties of high grade pipeline steel pipe decrease sharply. It is suggested that when the heat treatment temperature is higher than 700°C, the risk assessment of the service safety of the heat treated pipeline should be carried out.

Study on Distribution of Heat Treatment Temperature Field of Long distance Oil and Gas Pipeline with High Steel Grade

“…During the welding process of the reinforced pipe, the natural gas in the pipe keeps flowing, and the high gas pressure is maintained, resulting in the completion of the welding process under a strong cooling condition. The welding process is different from the static welding process under the action of forced heat transfer between the natural gas in the pipe and the inner wall of the pipe, which will have a serious influence on the welding process, the heat transfer, the formation of temperature field, the distribution of stress field and the welding quality in the pre-welding and post-welding heat treatment [9][10]. This study simulates the laboratory's forced cooling process of pre-welding and post-welding heat treatment.…”