The Influence of the Microstructure on the Fracture Toughness of Cold Work Tool Steels

摘要

The microstructure of cold work tool steels produced via the powder metallurgy (PM) proce consists of hard globular carbides embedded in a softer martensitic matrix. Together wi high hardness and wear resistance these steels should possess a sufficient toughness to preve catastrophic failure. While hardness and wear resistance can be improved by increasing the amount of carbides, toughness decreases by taking these measures. In this work the influence of the microstructure on the toughness is investigated and quantified by plane strain fracture toughness tests using three point bending specimens. The amount, size and distribution of the carbides is characterized by quantitative image analysis on sections oriented parallel to the fracture surface. The mechanical in-situ properties of the matrix are estimated by indentation tests using an Ultra-Micro indenter in a scanning electron microscope. Additionally, the size of the plastic zone in front of the crack tip is calculated and compared with the mean distance between the carbides. Toughness is governed mainly by the amount, size and distribution of the carbides. Their chemical composition is of less importance, since the carbides are always much harder than the matrix irrespective of their composition.