Temperature Dependence of Fracture Characteristics of Variously Heat-Treated Grades of Ultra-High-Strength Steel: Experimental and Modelling

Abstract

The temperature dependence of tensile characteristics and fracture toughness of the standardly heat-treated low-alloyed steel OCHN3MFA along with three additionally heat-treated grades was experimentally studied. In the temperature range of ⟨−196; 22⟩ °C, all the additional heat treatments transferred the standard steel from a high- to ultra-high strength levels even with improved tensile ductility characteristics. This could be explained by a reduction of the inclusion content, refinement of the martensitic blocks, ductile retained austenite content, and homogenization of the shape ratio of martensitic laths as revealed by metallographic, X-Ray, and EBSD techniques. On the other hand, the values of the fracture toughness of all grades were found to be comparable in the whole temperature range as the cause of a high stress triaxiality in the pre-cracked Charpy V-notch samples. The values of the fracture toughness of the standard steel grade could be predicted well using the fracture model proposed by Pokluda et al. based on the tensile characteristics. Such a prediction failed in the case of additionally heat-treated grades due to the different temperature dependence of the fracture mechanisms occurring in the tensile and fracture-toughness tests. While the tensile samples fractured in a ductile-dimple mode at all temperatures, the fracture-toughness specimens exhibited a transition from the ductile to quasi-brittle fracture mode with decreasing temperature. This transition could be interpreted in terms of a transfer from the model proposed by Rice and Johnson to the model of Tvergaard and Hutchinson.

Dynamic Fracture Characteristics of High-Strength Steel

“…7. Dynamic KId-values obtained in the range 〈55.8, 77.5〉 MPa.m 1/2 are, in average, somewhat higher than the static KIc and KIC values in the range 〈56.4, 73.1〉 MPa.m 1/2 reported in [1] for an equal temperature range 〈-60, -20〉 °C. This relationship is also reflected by temperature dependence of DFT in Fig.…”

“…In recent years, an extensive investigation of the temperature dependence of tensile and fracture characteristics of standard heat-treated steel grade and some differently treated grades has been carried out. The results obtained from the tensile tests and the static fracture toughness tests were reported elsewhere [1] together with detailed microstructure analyzes. This article is devoted to impact bending and dynamic fracture tests, and the related values of absorbed impact energy (AIE) KV and dynamic fracture toughness (DFT) KId are presented for the standard heat-treated steel grade.…”