Microstructures and mechanical properties of 30Cr-4Mo ferritic stainless steel joints produced by double-pulsed gas metal arc welding

Abstract

Joining of medium-thick ferritic stainless steel (FSS) traditionally follows the protocol of multi-layer and multi-pass welding to avoid massive grain coarsening in the heat-affected zone (HAZ). In this study, an attempt has been made to join 5-mm-thick 30Cr-4Mo FSS with single pass using double-pulsed gas metal arc welding (DP-GMAW) process employing ERNiCr-3 Ni-based filler. Defect-free joints were made under optimized welding parameters. Microstructural examination elucidated that mean grain size number of HAZ was successfully controlled above 3 due to the relatively low heat input of DP-GMAW. A linear relation between width of HAZ and heat input was found and given. In HAZ, no sigma (σ) phase, chi (χ) phase, or Laves phase was detected, and even-distributed (Ti, Nb)(C, N) precipitates presented mostly inside grains, which significantly kept the integrity of grain boundaries to preserve ductility. Ni-based filler greatly improved the ductility of welds as well. In the fusion zone (FZ), some dispersed white Nb-rich carbides have been observed and columnar dendrites turned into equiaxed dendrites near the centerline of FZ. All specimens qualified the bending test indicating favorable ductility of welded joints.