Blowout preventer is one of the important well control equipment, which is used to control the wellhead pressure during drilling, workover, oil testing, and other processes. The oil and gas wellhead blowout preventer is a core component of oil and gas deep well drilling, and its material is medium high carbon low alloy high-strength steel. By adjusting the composition and adopting appropriate heat treatment processes, the mechanical properties of steel castings can be met. However, as the main pressure bearing component, if there are defects in the steel casting body for repair, the incorrect welding process will cause a serious decrease in the hardness of the metal, coarse grains, and a decrease in mechanical properties, which will affect the safety of well control equipment use.
According to the heat treatment process of steel castings, two welding process evaluations were completed: the welding process for quality heat treatment after welding and the welding process for stress relief heat treatment after welding in the quenched back state. The reasonable welding process was determined through the direction of material weldability, welding method, welding process, etc. under different states and heat treatment processes. This study verifies through process evaluation that the welding joint performance obtained under this process can meet relevant requirements and meet the requirements of API 16A and NACEMRO175/ISO 15156. It is allowed to be used for repairing welding defects in steel castings.
1) When performing quality heat treatment on steel castings after welding, select welding materials that match the composition of the base material. When performing stress relieving heat treatment after welding, the main consideration is to have welding materials that are equivalent to or higher than the CE standard of the base metal.
2) After the weld of the steel casting is quenched, the tempering cooling speed should be slow enough to avoid internal stress and reduce the SCC resistance of the weld and its Heat-affected zone.
3) When the steel casting has only minor defects, process 2 can be selected for repair, and stress relief heat treatment shall be carried out after welding. Welding processes such as thin electrode backing, multi-layer and multi pass welding, and low interlayer temperature can effectively prevent the formation of brittle structure and grain growth in the Heat-affected zone, and obtain higher impact toughness in the Heat-affected zone; When there are significant defects in the product, the welding process of quality heat treatment after welding of steel castings is chosen for repair. After welding, quality heat treatment is carried out, and the microstructure of the weld zone is refined. The hardness range of the three zones is small, and the toughness of the weld zone is significantly improved, which can achieve higher impact values. Both processes can obtain high-quality welded joints, ensuring qualified performance of steel castings.