Yield Strength of Hot Forging Die Steels at Working Temperatures

Abstract

One mode that limits the usefulness of hot forging die steels is localized plastic deformation in regions of high pressure. To understand this behavior the yield strength of the steel needs to be measured at working temperatures in order to determine the likelihood of localized plasticity. One of the issues in using die steels for hot forging applications is that they are initially tempered to a hardness value when put into service. As the die is used to produce forged components, the contact with the hot forging causes the die to continue to temper and hence soften with continued used. To explore these issues three different die steels were obtained and tested experimentally. Experimental compressive yield strengths were determined for the three die steels (FX, 2714 and WF). The die steels were tempered to various hardness values prior to compression testing. The five room-temperature hardness values after tempering ranged from 20 to 38 HRC. The five temperatures for compression testing ranged from 593 to 704 °C (1100 to 1300 oF). From these tests a good characterization of the high temperature plastic behavior of each steel was obtained. It was found that the WF steel which had the highest alloy content was the strongest of the three steels under all test conditions. The FX and 2714, which had similar alloy contents (with FX having slightly less carbon, nickel and vanadium), had yield strengths that were close to each other at the intermediate temperatures, but at the high and low end of the testing range for temperature the FX was stronger than the 2714. Hence, to obtain the greatest resistance to localized plastic deformation during operations the choice of die steel should be WF, followed by FX and then 2714.