Influence of a Thermo-Mechanical Treatment on the Fatigue Lifetime and Crack Initiation Behavior of a Quenched and Tempered Steel

Abstract

A thermo-mechanical treatment (TMT) at the temperature of maximum dynamic strain aging has been optimized and performed on quenched and tempered steel SAE4140H (German designation: 42CrMo4) in order to improve the fatigue limit in the high cycle fatigue (HCF) and and very high cycle fatigue (VHCF) regimes. Fatigue tests, with ultimate cycle numbers of 107 and 109, have shown that the TMT can increase both the fatigue lifetime and the fatigue limit in the HCF and VHCF regimes. The increased stress intensity factors of the critical inclusions after the TMT indicate that the effect can be attributed to a stabilized microstructure around critical crack-initiating inclusions through the locking of edge dislocations by carbon atoms during the TMT.

Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment

“…In this state, the material exhibits a 0.2% yield strength of about 1500 MPa and ultimate tensile strength of 1900 MPa [29], which correlates to a hardness of 594 ± 5 HV 0.5. The hardness reduction due to the TMT, which takes place at 265 • C, is not significant [22]. The fatigue tests as well as the TMT were conducted on a servohydraulic push-pull testing machine with a capacity of 100 kN.…”

“…The force was measured with a 100 kN force transducer. The TMT was conducted at a temperature of 265 • C, which was identified as temperature where the DSA effects are most pronounced [22]. The specimens were heated inductively to this temperature at zero stress and were kept in this state for 15 s soaking time.…”

“…Besides approaches to enhance the purity of steels and minimizing critical inclusion formation during steel production, a thermomechanical treatment (TMT) in the temperature range of maximum dynamic strain aging (DSA) is another approach to increase the fatigue strength of steels by strengthening the microstructure around the inclusions after the steel production [20,21]. Increasing the fatigue limits of SAE4140 in the HCF and VHCF regimes by a TMT was recently shown in [22]. It is assumed that during the TMT, plastic deformation in the temperature range of DSA introduces a more stable dislocation structure around the non-metallic inclusions, which delays or prevents crack initiation thus resulting in higher HCF lifetimes and increased fatigue strengths [20].…”

“…In this study, we systematically investigate the influence of shape, area and position of critical non-metallic inclusions on the high cycle fatigue lifetime of the steel SAE4140 in a quenched and tempered state. In a second step, we study whether a TMT, which increases the fatigue limit and the fatigue lifetime in the HCF regime [22], affects the shape, area and position of critical inclusions and whether the effectiveness of the TMT is influenced by these parameters. The goal of the study is to gain a better understanding of how the TMT and the inclusion parameters interact.…”

Analysis of Surface State after Turning of High Tempered Bearing Steel

“…The deep insight into thermal cycle during formation of WL was reported by Hosseini et al [13] as well. It should be mentioned that the surface state of quenched steel remarkably affects its functional properties as was reported in new studies [14][15][16]. Contribution of WL depends on the manner in which surface after hard turning is exploited [17,18].…”