PSI - Issue 68

S.R. Raghuraman et al. / Procedia Structural Integrity 68 (2025) 769–775 S.R. Raghuraman et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction According to the International Energy Agency (IEA), steel industry generates 1.3 tonnes of CO 2 per tonne of steel produced. Apart from that, forty percent of the demand for new steel results from the need to replace damaged steel components during service. Within their research, Cooper et al. (2014) had shown that the majority of steel components are scrapped or recycled before the end of their structural integrity is reached. Maximum material utilization before scrapping and subsequent recycling can be achieved in many cases in terms of optimal reconditioning and subsequent re-use, which leads to a sustainable and efficient use of resources. Reconditioning requires initial identification of areas that were damaged during primary loading. Through targeted mechanical and/or thermal treatment, the material can be reconditioned and therefore be prepared for its second use. In order to differentiate between surface and volume damage, a comprehensive understanding of the damage mechanisms is required. Considering the High-Cycle-Fatigue (HCF) loading, fatigue damage is predominantly characterised by the occurrence of persistent slip bands (PSB) and resulting micro-cracks on the surface. Irreversible processes within these slip bands are associated with increasing roughness of the surface in form of intrusions and extrusions, which represent a possible starting point for the formation of micro-cracks, as the stress concentration is locally increased. These mechanisms have been the subject of research for decades and the dislocation mechanisms in the slip bands are well documented by Radaj et al. (2007) and Polák et al. (1985). Several references including Radaj et al. (2007) and Haghshenas et al. (2019) indicate that the fatigue life of metallic materials can be significantly extended by removing these near-surface microstructural features. The present study contributes to a project which aims to develop a model for determining the re-use potential RP of steels based on the example of a quenched and tempered steel SAE 4140. In addition to extensively instrumented fatigue tests in terms of load increase tests (LIT) as well as constant amplitude tests (CAT), analytical methods based on light microscopy (LiMi), scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to evaluate the RP . These findings can be correlated in order to serve as input variables concerning model development. 2. Materials and methods The focus of the presented research is on a quenched and tempered SAE 4140 steel (1.7225, 42CrMo4). As part of the heat treatment, specimens were austenitized ( T Aus = 860 °C) and then quenched in oil to room temperature. This was followed by a tempering process ( T Temp = 550 °C and t Temp = 120 min.), resulting in a distinctive microstructure regarding tempered steels consisting of tempered martensite and precipitated carbides (Cr, Mo) shown in Fig. 1 (a).

Fig. 1: Microstructural investigations for the quenched and tempered SAE 4140 steel (initial condition) electron back scatter diffraction. (a) band contrast image to visualize the general martensitic microstructure; (b) EBSD-pole figure mapping

This microstructure leads to a preferred combination of high strength and toughness and makes the material suitable for crankshafts, gear wheels and other technical applications that require high load resistance. Fig 1 (b) contains the result of the Electron Backscatter Diffraction (EBSD) investigation and illustrates the orientation distribution of former austenite grains.