Understanding the Role of Initial Microstructure on Intercritically Reheated Heat-Affected Zone Microstructures and Properties of Microalloyed Steels

It is well known that during multipass welding of low carbon steels significant reduction in toughness is observed at intercritically reheated heat-affected zone (HAZ) regions of the weld. This drop in toughness at the HAZ is attributed to the formation of martensite-austenite (MA) constituents that act as potential crack initiation sites. Most previous research has focused on understanding the effect of single weld thermal cycle (heating rate, peak temperature, cooling rate) on the formation of these MA constituents. However, the effect of initial microstructure on the development of MA constituents during multipass welding has not been reported. Studies have shown initial microstructure of the steel has a prominent effect on the re-austenitization kinetics, which could hence have a potential influence on the formation of MA constituents during welding.

In this work, the effect of different initial microstructures on the final microstructure and properties of inter critically reheated HAZ is studied on two microalloyed steels.  Each steel considered had two different initial microstructures (ferritic or bainite/martensitic), which were subjected to a two-pass weld HAZ thermal cycle with peak temperature of 920 ⁰C (>AC₃) and 860 ⁰C (Ac₁<860⁰C <Ac₃) and Δt_8/5 ~43 ⁰C/s. Preliminary analysis of the HAZ microstructure revealed a difference in MA constituents between steels with different initial microstructures. The effects of these MA constituents on the mechanical properties are being evaluated with hardness mapping, notched tensile and Charpy v-notch testing. Finally, a hypothesis to explain the observed difference in the MA constituents of the HAZ formed from different initial microstructures is being developed based on austenite nucleation and growth.