Modern naval applications require alloys with high strength, good toughness, and sufficient weldability. To keep pace with advancements in materials technologies, the U.S. Navy is researching the next generation of high-performance steels. A recently designed austenitic stainless steel has the potential to meet these criteria due to its unique combination of γ’ precipitation strengthening and use of the transformation induced plasticity (TRIP) effect. TRIP occurs through a strain induced transformation from austenite to martensite which extends uniform ductility and toughness by continuously strain hardening the material over the course of deformation. The TRIP effect is a highly sensitive mechanism that depends on precise manipulation of the matrix composition to control the austenite stability. While initial studies of this alloy show promise in terms of meeting mechanical property requirements, no studies conducted thus far have evaluated weldability. The objective of this work was to assess the effects of welding on the microstructure, and weldability of this alloy.
Microstructural characterization of the fusion zone in autogenous welds were performed via scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Differential thermal analysis (DTA) was used to determine the phase formation temperatures and resulting solidification temperature range of the weld metal. Electron probe microanalysis (EPMA) was conducted to assess the extent of solute segregation within the weld metal. Changes in the solute partitioning coefficients over the duration of solidification were tracked to gain further insight into the weld metal solidification path. Varestraint testing was then used to measure the susceptibility to solidification cracking and to compare the general weldability of the TRIP steel to similar commercial alloys.
Results and Discussion:
Varestraint testing revealed that the alloy was extremely susceptible to solidification cracking. Even in the absence of an external applied strain, large center line solidification cracks were observed. SEM and EDS showed the cracks propagated along low melting point secondary phases and sulfides that formed at the end of solidification. EPMA indicated that these secondary phases formed because of extensive segregation of solute elements such as Ni, Si, Al, and Ti to the liquid at the terminal stages of solidification. The DTA data showed that these phases formed at 1163 °C on cooling, which extended the solidification temperature range (STR) of the weld metal to 213 °C. Large STR’s have been shown to increase solidification cracking susceptibility in fusion welds. In an effort to improve resistance to cracking, adjustments to the alloy’s nominal composition were necessary to reduce the STR while maintaining the TRIP and precipitation properties of the alloy. A python script was written which utilized ThermoCalc to conduct thermodynamic simulations on a variety of alloy compositions which systemically deviated from the original nominal composition. Composition ranges were selected which allowed changes in the solidification path from primary austenite to δ-ferrite, which has been widely demonstrated to improve resistance to solidification cracking. The solidification path, STR, phase fractions, γ’ precipitation behavior, and austenite stability were modelled to find compositions with optimal combinations of weldability and mechanical properties. The influence of various alloy elements on the solidification behavior and concomitant STR will be described.
Solidification cracking caused by the large STR rendered the TRIP alloy unweldable. To improve resistance to cracking, changes to the alloy’s nominal composition were explored using an iterative, thermodynamic modelling approach to find compositions which balanced weldability, γ’ precipitation, and the TRIP effect. Changes in composition which allow for primary δ-ferrite formation are expected to reduce the susceptibility to solidification cracking while leaving the base metal mechanical properties intact.