The structure of nodular cast iron end cover casting for train engine is shown in the figure. The material brand is QT500-7. The casting structure is complex and the casting is difficult. It has high requirements for the quality of sand core and process control in the. No defects are allowed to be exposed on all machining surfaces and key areas. As the performance of the casting itself requires the tensile strength to be greater than 500 MPa, the tensile strength after welding at the defect position shall not be less than 80% of the performance of the casting body. In order to reduce the loss of waste products caused by casting defects that cannot be repaired, the defective parts shall be repaired by welding under specific conditions to make the castings meet the final assembly and use requirements.
Due to the high content of C, s and P impurities, low strength and poor plasticity in cast iron, the welding performance of cast iron is poor and the sensitivity of cracks in the welding area is strong. The welding repair of cast iron can usually adopt cold welding and hot welding. Due to the complex structure and small wall thickness of the end cover casting, it is easy to cause casting deformation and increase thermal stress when using thermal welding. Arc cold welding can save energy consumption, improve working conditions, reduce welding repair cost and shorten welding repair cycle. When the weld is cast iron, the cold welded joint is easy to produce white structure, hardened structure and cold crack. For example, the chemical composition and mechanical property requirements of QT500-7 are shown in the table. In order to ensure the reliability of the casting, the mechanical properties of the welding area are required to reach at least 80% of the properties of the base metal. Therefore, higher requirements are put forward for the welding repair of nodular iron castings.
Through the determination of welding repair process WPS, WPQR welding process identification and selection of relevant parameters of welding process, the control process and requirements of welding process are standardized, the control points and precautions of welding process are clarified, the one-time welding repair qualification rate of nodular iron castings is improved, and the performance and service reliability of castings after welding are guaranteed.