Reasons for unqualified low temperature impact performance of G18NiMoCr3－6 cast steel welded joints

The low temperature (－40 ℃) impact performance of a G18NiMoCr3－6 cast steel welded joint was unqualified. The reasons for the unqualified low temperature impact performance of welded joints were analyzed using methods such as mechanical performance testing, macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination and tempering process test. The results show that when the weld metal was tempered at 560 ℃,Fe₃C thin shells precipitated at the original austenite grain boundaries, reducing the grain boundary bonding force, resulting in high－temperature tempering brittleness of the material. The ductile－brittle transition temperature of the material increased, ultimately led to low temperature brittle fracture of the material.