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Metal Casting Technologies : September 2006
42 www.metals.rala.com.au Optimize Core Performance TECHNICAL FEATURE INTRODUCTION ot Work Tool Steels are the most commonly used materials for High Pressure Diecasting (HPDC) dies, but alternatives such as Maraging Steels and special high temperature resistant materials are becoming more frequently used where service conditions are more demanding. These special materials include Nickel base alloys, Tungsten, Molybdenum, Nickel Aluminides, and ceramic type materials. Increasing use of Surface Engineering is also being made to improve the performance of conventional die steels using techniques such as Nitriding, Carbo-nitriding, and Physical and Chemical Vapour Deposition (PVD and CVD) to produce controlled depth surface regions containing hard constituents such as carbides and nitrides to increase wear resistance (1). The material requirements for HPDC dies can be summarized as follows: ■ Resistance to wear and attack by liquid metals, entrained oxides and dross. ■ High thermal and mechanical fatigue cracking resistance in order to resist fine cracking, chipping damage and gross fracture (See Figure 1) ■ High strength ■ Structural and dimensional stability during service ■ Resistance to softening in service ■ Ability to be machined or ground during die making ■ Ability to be electro-discharge machined (EDM) to complex shapes. ■ Ability to be surface treated in order to reduce friction and increase wear resistance. To meet the hardness, wear and other mechanical property and dimensional requirements the alloy steels used for dies must have these key heat treatment characteristics: ■ High hardenability for through hardening on heat treatment except where incomplete through hardening may be needed for impact resistance ■ High hardness after tempering ■ Freedom from distortion and cracking during hardening. Since die makers are often involved in producing dies for diecasting and for plastic injection moulding it is relevant to comment here that, in addition to the above characteristics, the Die Steels used for the moulding of plastics must also possess: ■ Very good polishing ability to provide satisfactory surface finishes on moulded parts - this requires very clean steel with high homogeneity. ■ Resistance to corrosion because some plastic moulding compounds are chemically aggressive. This short feature outlines some of the basic metallurgy of the Hot Work Die Steels most commonly used for HPDC dies. Non-metallurgist readers may obtain some useful background by referring to the metallurgical and practical heat treatment aspects of Plain Carbon and Alloy Steels that have been outlined in earlier MCT papers (2-4). COMPOSITION OF HOT WORK DIE STEELS Plain C Steels cannot be used for hot working dies since they have limited hardenability and lose their strength at only moderate temperatures. They can only be fully hardened in small sections, and even then, only by rapid quenching in water giving an increased chance of distortion and cracking. Even when fully hardened Plain C Steels are not temper resistant and hence lose their strength and hardness after relatively short exposure times at temperatures over 200oC. For die steels that work at higher temperatures, as diecasting or extrusion dies for example, alloying elements are needed to: ■ Control hardenability to obtain fully through hardened structures in the required section thickness using slower cooling rates during quench hardening to minimize the danger of distortion and cracking. ■ Provide resistance to softening during tempering treatments and during subsequent service. ■ Produce secondary hardening of the microstructure via the precipitation of fine stable alloy carbides. Hardenability describes the ability of a given steel to be hardened by cooling to form a hard microstructural constituent It s Hot Work for Die Steels H By Dr. John Pearce Figure 1. The effects of thermal fatigue: severe heat checking in a HPDC die.