An assessment of vacuum-heat-treated H11 hot-work tool steel using the Klc/HRc ratio

Charpy V-notch (CVN) impact-test values are widely used in toughness specifications for AISI H11 hot-work tool steel, even though the fracturing energy is not directly related to the tool design. K-lc, the plain-strain stress-intensity factor at the onset of unstable crack growth, can be related to the tool design; however, K-lc test values are not widely used in toughness specifications. This is surprising since to the designer K-lc values are more useful than CVN values because the design calculations for tools and dies of high-strength steels should take into account the strength and the toughness of materials in order to prevent the possibility of rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture-toughness testing (ASTM E399-90), which is difficult to perform reliably for hard materials with a low ductility, could be replaced with a so far non-standard testing method. A particular problem is that the manufacture of the fatigue crack samples is difficult and expensive, and this has promoted the search for alternative fracture-toughness testing methods. One of the most promising methods is the use of circumferentially notched and fatigue-precracked tensile specimens. With this technique the fatigue crack in the specimen is obtained without affecting the fracture toughness of the steel, if it is obtained in soft annealed steel, i.e., prior to the final heat treatment. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell-C hardness - Fracture toughness K-lc - Tempering temperature) for some AISI H11 hot-work tool steel delivered from three steel plants. On the basis of the combined tempering charts the influence of the processing route on the mechanical properties was investigated. In the same way, vacuum-heat-treated tool steels were assessed and their properties expressed as a ratio of the fracture toughness to the hardness (K-lc/HRc).