Correlation of Local Constitutive Properties to Global Mechanical Performance of Advanced High-Strength Steel Spot Welds

Metallurgical and Materials Transactions A - Recent results have shown that the strength of resistance spot-welded joints made from advanced high-strength steels (AHSS) do not increase linearly...     

Microscale-calibrated modeling of the deformation response of low-carbon martensite

Micropillar compression tests were used to determine the uniaxial compressive stress–strain response of martensite blocks extracted from a low-carbon, fully lath martensitic sheet steel, M190, with the nominal composition C = 0.18, Mn = 0.47, P = 0.007, S = 0.006, Si = 0.18, Al = 0.06, Ti = 0.045, B = 0.0014 and balance Fe (all in wt.%). Specimens with a diameter exceeding ～1 μm and consisting of a single martensite block showed elastic–nearly perfectly plastic behavior with a yield stress of the order 1200 MPa. Similar specimens which contained multiple martensite blocks showed pronounced strain hardening, arising from the geometrical constraint produced by the interface(s). No size dependence of flow stress was observed in micropillars with diameters exceeding 1.0 μm, but a significant scatter in strength and hardening rate was observed in micropillars with smaller diameters. Flow data for micropillars in the size-independent regime were used to determine parameters in a crystal-plasticity-based model of martensite. Full three-dimensional crystal plasticity simulations, with material properties determined from micropillar tests, were then used to predict the macroscopic uniaxial stress–strain behavior of a representative volume element of martensite. The predicted stress–strain behavior was in excellent agreement with experimental measurements, and demonstrates the potential for micropillar tests to determine material parameters for individual phases of a complex microstructure.     

Static recovery of tempered lath martensite microstructures during long-term aging in 9-12% Cr heat resistant steels

Static recovery of tempered lath martensite microstructures of strength enhanced ferritic steels has been investigated during very long-term aging up to 2 × 10⁴ h at 650 °C for 3 steels containing 9 to 12% Cr. Static recovery of tempered lath martensite microstructure occurs after an incubation period for 1-2 × 10³ h in the steels. The static recovery is controlled by the loss of strengthening due to M₂₃C₆ precipitates, and disappearance of MX carbonitrides cannot be the main cause of the static recovery.