Effect of heat treatment on mechanical properties of H11 tool steel

Journal of Achievements in Materials and Manufacturing Engineering 01/2009; 35(2).
Source: DOAJ


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.

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Available from: Sayyad Zahid Qamar, Sep 09, 2014
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    • "Metallic materials consist of a microstructure of small crystals called grains or crystallites. The grain sizes and composition which make up the nature of the grains are one of the most effective factors that determine the overall mechanical behaviors (hardness, tensile strength, toughness, wear resistance) of metals (Qamar, 2009). An efficient way to manipulate the properties of metals via grain structure manipulation by controlling the rate of diffusion and that of cooling is that of heat treating procedures. "
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    • "Alumina (Al 2 O 3 ) based ceramics are considered to be the most suitable tool materials, among others, for machining hardened steels because of their high hot hardness, wear resistance and chemical inertness (Dewes & Aspinwall 1997). Qamar (2009) has studied mechanical testing of H11 samples. His results revealed that with increasing temperatures: (a) hardness increases to a maximum and then gradually decreases; (b) impact toughness decreases to a minimum and then increases; (c) yield strength decreases, then increases, and then increases again; (d) ultimate strength increases to a maximum and then steadily decreases; and (e) ductility (% elongation) gradually decreases till 600 • C, and then increases rather sharply. "
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    ABSTRACT: This experimental study is conducted to determine statistical models of cutting forces in hard turning of AISI H11 hot work tool steel (∼ 50HRC). This steel is free from tungsten on Cr–Mo–V basis, insensitive to temperature changes and having a high wear resistance. It is employed for the manufacture of highly stressed diecasting moulds and inserts with high tool life expectancy, plastic moulds subject to high stress, helicopter rotor blades and forging dies. The workpiece is machined by a mixed ceramic tool (insert CC650 of chemical composition 70%Al2O3+30%TiC) under dry conditions. Based on 33 full factorial design, a total of 27 tests were carried out. The range of each parameter is set at three different levels, namely low, medium and high. Mathematical models were deduced by software Minitab (multiple linear regression and response surface methodology) in order to express the influence degree of the main cutting variables such as cutting speed, feed rate and depth of cut on cutting force components. These models would be helpful in selecting cutting variables for optimization of hard cutting process. The results indicate that the depth of cut is the dominant factor affecting cutting force components. The feed rate influences tangential cutting force more than radial and axial forces. The cutting speed affects radial force more than tangential and axial forces. Keywords.Hard turning; ceramic tool; AISI H11; cutting force; ANOVA; RSM.
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    ABSTRACT: The importance of various form of heat treatment operations on medium carbon steel in order to forester the problem that may arise in making a wrong choice of these steel materials or faulty heat treatment operations which may give rise to serious disruption in terms of human safety, higher cost and untimely failure of the machine components is of great concern. The mechanical properties such as ductility, toughness, strength, hardness and tensile strength can easily be modified by heat treating the medium carbon steel to suit a particular design purpose. Tensile specimens were produced from medium carbon steel and were subjected to various forms of heat treatment processes like annealing, normalizing, hardening and tempering. The stiffness, ductility, ultimate tensile strength, yield strength and hardness of the heat treated samples were observed from their stress-strain curve. The value of the yield strength (σ y) was observed to be higher for the tempered specimen possibly as a result of the grain re-arrangement, followed by the hardened, normalized and annealed specimens. The value of the ultimate tensile strength (σ u) were also observed to be in the order; hardened> tempered>normalized>annealed.
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