Effect of heat input on weld microstructure and toughness of laser-arc hybrid welding of martensitic stainless steel

[Display omitted] •The inter- and intra-granular carbides in CGHAZ play a big role in the toughness.•The variations of CGHAZ microstructure and Erichsen value are consistent.•An equivalent grain size is proposed to establish a new Hall-petch relationship.•The threshold of equivalent grain size is calculated to be 11μm.•Below the threshold, the weld toughness is acceptable to meet rolling requirement. Laser-arc hybrid welding of AISI 420 martensitic stainless steel (MSS) was carried out, and the microstructure-toughness relationship was discussed and established. The coarse-grained heat affected zone (CGHAZ) is the weakest area of the weld. The CGHAZ microstructure and the toughness assessed by Erichsen cupping test had a good agreement with the increase of heat input, indicating that the toughness was not only dependent on the grain size but also the contents of inter- and intra- granular carbides within the CGHAZ. By introducing the effects of the carbides, an equivalent grain diameter (De) was proposed to establish a new Hall-petch relationship between the CGHAZ microstructure and the toughness, whose fitting degree was increased from 0.677 to 0.974. The weld toughness was acceptable to meet the rolling requirement when the De was smaller than 11μm, under which the weld fractured along the base metal rather than the CGHAZ.