Low-temperature fracture toughness of electron-beam welded low-carbon martensitic-austenitic steel

Good toughness, i.e., resistance against crack initiation and propagation, is extremely important for automotive parts that must endure a wide range of loading conditions and operational temperatures. With modern advanced (ultra)high-strength steels (AHSS) used in the welded load-bearing members, this requirement can be challenging to be met. In addition to the challenges with achieving sufficient properties in the weld metal and the heat-affected zone (HAZ), the heat-input in welding or post-weld heat treatment (PWHT) can impair the originally good mechanical properties of the metastable microstructures of the base materials. A mitigating solution to this is to use welding methods that limit heat-input, e.g., electron-beam welding, which can produce even-strength welded joints for strength classes of S1100 and above. In this study, we report fracture toughness properties of electron-beam welded 0.2C-1.5Mn-0.5Si-0.8Al-1.1Cr-0.8Ni (wt.%) martensitic-austenitic direct-quenched and partitioned AHSS (DQ&P), focusing on the low-temperature properties of the base materials, weld seam, and HAZ tested according to the standard ASTM E1921 with 9 mm thick SENB specimens. The effect of low-temperature PWHT done at the partitioning temperature of 275 °C is evaluated as well. The results show that despite the weld seam with ~3 vol.% of retained austenite is marginally the weakest region, it can match low-temperature toughness of the DQ&P base material, both reaching good fracture toughness reference temperature T0 levels around -40 °C. Both average KJc and provisional T0Q indicate that PWHT treatment impairs toughness properties in all the conditions, possibly eliminating the need for PWHT, which would preserve the base material better as well.