Origin of hard and soft zone formation during cladding of austenitic/duplex stainless steel on plain carbon steel

Abstract

Weld cladding of corrosion resistant materials is an attractive alternative to bulk usage. Such processes are generally associated with potential deterioration of mechanical properties at the clad/metal interface, often attributed to the formation of hard and soft zones which is usually minimised by control of dilution and use of a buffer layer. To ascertain the origin of the hard and soft zones, weld cladding of austenitic stainless steel (309L) was attempted on plain carbon (0.28 wt-%) steel base metal at various heat inputs, and correspondingly different dilution percentages for the weld metal were obtained. The formation of hard and soft zones was observed mainly on the austenitic stainless steel (i.e. weld metal) side. Typically close to the welding interface (i.e. visible fusion line) a hard zone was observed, followed, but only at higher dilution, by a soft zone. In general, with increasing dilution, the peak hardness and width of the hard zone increased, and the chromium and nickel concentrations of the soft zone significantly decreased. The hard zones were martensitic regions which, at higher dilution, contained chromium carbide precipitates. These precipitates may explain the chromium depletion in the adjacent soft zone. The nickel depletion may be a result of the positive interaction coefficient between nickel and carbon. Since hard/soft zone formation was least at lower dilution, cladding of duplex and superduplex stainless steel was conducted on the same plain carbon base with and without a 309L buffer at an optimum dilution of about 15%. Even at this dilution, direct cladding always resulted in hard and soft zone formation, which was significantly reduced or eliminated by indirect cladding. The hard zones were identified as regions with extensive carbide precipitation (no evidence of martensite formation was found), which occurred in duplex/superduplex grades even at 15% dilution.