In Situ Synchrotron X-ray Diffraction Analysis During Post-Weld Heat Treatments of Supermartensitic Stainless Steel Weld Metal

The objective of the present work is to analyze "in situ" the phase transformations associated to martensite-to-austenite and vice versa during post-weld heat treatments (PWHTs) of the weld metal of supermartensitic welding consumable, considering two different initial microstructures. For this purpose, a coupon was welded with 1,7 kJ/mm of heat input, using the semiautomatic arc welding process with Ar + 5% He as shielding gas. The filler metal was a 1.2-mm diameter metal-cored tubular wire, which contains 12.5%Cr, 6.7%Ni, and 2.5%Mo (% by weight). Two initial microstructural conditions were analyzed: as welded (AWC) and as solubilized at 1000 degrees C, 60 minutes (ASC). Under these conditions, two different PWHTs involving simple (intercritical: 665 degrees C, 15 minutes) and double tempering (intercritical: 665 degrees C, 15 minutes + subcritical: 590 degrees C, 15 minutes) were carried out. To study the microstructural evolution during PWHT, "in situ" x-ray diffraction was performed using synchrotron radiation, measuring every 3.15 seconds during heating, holding, and cooling. For intercritical tempering, the solubilization increased the start austenite temperature during heating from 618.7 to 665 degrees C, and increased the start martensite temperature during the cooling from 128 to 248 degrees C. The reformed austenite at room temperature was higher for the AWC condition than for the ASC condition for both intercritical tempering (25.7 and 9.6%, respectively) and second subcritical tempering (46 and 26%, respectively). Nevertheless, a higher amount of both reverted and reformed austenite was formed during the heating at 590 degrees C for the ASC. This is correlated to the lower macrostrain of the reformed austenite for ASC, suggesting that formation modes of reverted austenite are different. Furthermore, although for both conditions, the kinetic exponent n of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model indicates diffusion-controlled unidimensional growth with decreasing nucleation rate, both the n parameter and equilibrium volume fraction of transformed austenite are higher for the ASC condition than for the AWC condition.