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 T 0 levels around -40 °C. Both average K Jc and provisional T 0Q 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.