Effect of heat treatment on mechanical properties of H11 tool steel

Abstract

Purpose: AISI H11 is a special alloy steel, categorized as chromium tool steel. Because of its high toughness and hardness, it is well suited for hot work applications involving very high loads. Typical applications are hot-work forging and extrusion dies, helicopter rotor blades, etc. For longer life and higher design accuracy, properties of this type of tool steel can be improved by various types of heat treatment. Current work reports and analyzes results of mechanical testing performed on variously heat treated H11 steel samples, to arrive at an optimum heat treatment strategy for hot work applications. Design/methodology/approach: Tensile and impact test specimens were fabricated using precision milling and EDM. These samples were subjected to various heat treatment sequences, consisting of annealing, hardening, air and oil quenching, and tempering at different temperatures. Heat treated samples were then mechanically tested for hardness (Rockwell), impact toughness (Charpy), and tensile properties (yield strength, ultimate strength, ductility). Findings: Mechanical testing of H11 samples revealed that with increasing temper temperatures: (a) hardness first increases to a maximum and then gradually decreases; (b) impact toughness first decreases to a minimum and then increases; (c) yield strength first decreases, then increases, and then increases again; (d) ultimate strength first increases to a maximum and then steadily decreases; and (e) ductility (% elongation) gradually decreases till 600oC, and then increases rather sharply. Practical implications: Though a very promising candidate for hot-work applications, H11 steel is not commonly used for die and tool making. Results of this study can provide die designers and users in the metalworking industry with good guidelines to select proper heat treatment strategies to use H11 steel for various applications. Originality/value: Very little information is available in published literature about mechanical properties of H11 steel, especially after different types of heat treatment. Results from this study can fill some of this gap.