The Toughness of a Hot Work Die Steel

Abstract

The literature dealing with the practical use of fracture mechanics in the better understanding of crack propagation characteristics of industrially used steel products, with particular reference to hot work die steels, has been reviewed. The effect of forging reduction and heat treatment on the fracture toughness of B.S.S. 224 No. 5. nickel-chromium—molybdenum hot work die steel has been determined in the ambient to 500°C. temperature range. Arising from these tests the expected rise in fracture toughness from ambient temperature to 100°C. occurs, which is maintained up to 300°C;thereafter a decay takes place and the fracture toughness at 500°C. is nearly identical to that at room temperature. Temper embrittlement and the possibility of dynamic strain ageing appear to contribute to theloss of elevated temperature toughness. Stereoscan fractrographic analysis showed little variation between maximum and reduced toughness when examined at relatively low magnifications. However, at high resolutions it was observed that a cleavage pattern was present with maximum toughness, whilst a dimple structure surprisingly was associated with falling toughness. Upset forging appears to offer superior fracture toughness and mechanical properties compared to direct forged die block material. Increasing forging reduction did not affect properties measured in the longitudinal direction : transverse properties were impaired by heavier amounts of forging. Drop forge Works trials of No. 5. die steel inserts, used in conjunction with a 4-ton counter-blow hammer, manufacturing crankshafts, were assessed for die production life. The insert which produced the largest number of drop forgings also possessed superior fracture toughness at room temperatures. It was unfortunate that in assessing toughness, validity criteria were not met. However, comparison between inserts was still possible. A possible practical and economic consideration has arisen from these results in that the drop-forger, by selecting a die block pre—heating temperature of 100°C. (this being the temperature at which maximum crack resistance occurs), and then by the judicious application of coolant to ensure that the die block temperature does not exceed 300°C.a more consistent and improved die life may result. The use of fracture toughness data to predict the most suitable operating temperature conditions has demonstrated the limitations of using conventional notch toughness assessment, which should now be replaced by substituting crack propagation testing techniques. Additionally, any die steel development should take cognizance of the fracture toughness parameter which should be included in any property assessment requirement for hot work die steels.