On the Thermo-Mechanical Behaviour of SA508 Grade 4 Ferritic Steel

Abstract

The development of residual stresses in the Heat Affected Zone (HAZ) during welding of a ferritic steel can be critical to weld structural integrity. The Prior Austenite Grain Size (PAGS), the thermo-mechanical properties of the phases that develop during phase transformation, and the transformation strains are some of the key parameters that can alter residual stress development during welding. Understanding the trend in variation of these parameters is crucial for Finite Element (FE) modelling of residual stress development in weld. In this study, the effect of PAGS on the phase transformation in SA508 grade 4 was determined. For this purpose, samples were heated up to 900, 1050, 1250, and 1350°C and held for various time intervals to produce different austenite grain sizes. The measured austenite grain sizes were then used to fit parameters in an exponential equation implemented in an FE User MATerial subroutine (UMAT) for the modelling of welds. With performing various free dilatometry experiments, it is shown that the only phase that austenite transforms to upon cooling is martensite. In addition, the mechanical properties of as-received material, austenite, and martensite as a function of temperature were measured. Also, various uni-axial loads were applied during cooling cycles, and before the onset of phase transformations, to measure the evolution of transformation strain to generate an empirical formulation for numerical modelling.