Fracture Toughness Evaluation of Powder Metallurgical ASP2030 High-Speed Steels Using Flexural Specimens and Finite Element Method

In the present study, the fracture toughness of hardened and tempered powder metallurgical (PM) high-speed steel ASP 2030 was investigated using notched and unnotched bending specimens and the finite element method. The normal flexural strength of notched and unnotched specimens marquenched by austenitizing at 1150, 1170, and 1185°C, followed by quenching to room temperature is measured after triple tempering at 560°C for 2 h. The finite element method (FEM) analysis is performed to observe the true stress distribution and calculate the critical fracture stress in the specimens under the experimental conditions of the bending test. The microstructural features of the specimens were investigated by X-ray diffraction (XRD) and a field emission scanning electron microscope (FESEM) with an electron backscatter detector (EBSD). No retained austenite was detected in the tempered specimens, and according to the results of the EBSD analysis and XRD tests, the microstructure of the matrix consists of martensitic ferrite laths. It can be observed that with the increase of austenitizing temperature from 1150 to 11850C, the normal flexural strength of the specimens decreases. The decrease in flexural strength of the specimens is due to the increase in the prior austenite grain size and consequently the martensitic ferrite laths after tempering. In addition, as the austenitizing temperature increases, the volume fraction of the undissolved carbides decreases, which causes the size of the undissolved carbides to increase and the flexural strength to decrease. According to FEM, the critical crack length calculated from the critical fracture stress is approximately equal to the average diameter of undissolved carbides.