Manufacturing of High-Performance Forging Dies by Ausforming

Abstract

During forging, dies are subject to a complex load collective caused by combined thermally and mechanically induced stresses. Crack formation and deformation on tool surfaces, as a result of low fatigue resistance, lead to tool failure and high process costs. Grain refinement is regarded as a method to improve fatigue resistance due to enhanced ductile material properties. To generate a fine-grained microstructure in the die material, increased deformation can be applied in the metastable austenite phase, also known as ausforming. In this study, the thermo-mechanical treatment ausforming will be used to form the final contour of forging dies. For this purpose, an analogy study was performed in which a preform is ausformed. It is investigated to what extent a fine-grained microstructure can be achieved in the final forming stage. The hot-working steel X37CrMoV5-1 (AISI H11) was used as specimen material. The developed sample geometry represents the inner contour of a highly mechanically loaded forging die. To achieve optimal properties, process routes with different cooling strategies and two defined true plastic strains were examined in metallographic analysis and hardness measurements according to EN ISO 6507-1 (HV1). It is shown that, after complete austenitisation, the highest hardness values can be achieved by applying a water-air spray cooling with subsequent forming. This could be demonstrated without material failure in the samples even with a high true plastic strain.

Adjusting Mechanical Properties of Forging Dies Produced by Ausforming

“…Furthermore, different martensite structures occurred in the radius and in the centre of the bottom region. The centre region shows no material flow during forming, leaving the martensite structure similar to that of a simply tempered material [8]. The fatigue tests have shown that higher fatigue life is achieved with lower deformation during ausforming.…”

“…The preformed workpieces were made of hot-working tool steel X37CrMoV5-1 (AISI H11) by die forging at 1060°C above Ac3-temperature. This steel grade contains austenite stabilizing elements such as Cr, Mo, V and is particularly suitable for the ausforming process due to its slower transformation kinetics in the austenite phase [8]. The forging tools were also made of X37CrMoV5-1 (AISI H11) and were lubricated with Con Traer G 300 which was applied on the tools before positioning the warm preforms.…”

“…The temperature at the workpiece surface is monitored by means of a thermal imaging camera, since undercooling can lead to premature phase transformation of the material and consequently to failure due to cracking [8]. However, it is not possible to cool the entire forging to a forming temperature, since other material areas are not actively cooled and heat is only extracted by tool contact.…”

“…Preliminary tests have already shown suitable pressure parameters for producing a uniform conical spray pattern with a controllable cooling rate. Spray parameters from preliminary tests [8] are used to prevent damage during forming.…”

“…Therefore, the authors investigated the ausforming process by means of upsetting tests in the metastable austenite phase and thus lower forming temperature in preliminary studies [8]. It could be shown that a longer holding time turns out to shift higher hardness values towards the center.…”