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Top1. Introduction
Aggressive mass reduction trends in the automotive industry have spurred the development of suspension springs that can withstand high stresses at a reduced section size (Wise, Spice, Davidson, Heitmann, & Krauss, 2001). During the last two decades, considerable efforts have been made in the development of high-performance spring steels to meet the needs for the weight and cost savings in the automotive industry (Lee, Lee, Li, Yoo, & Nam, 1988). At present, a major application of high-quality spring steels is used in high-speed railway. Major applications of spring steel are in Railway coach axles, Crank pin on heavy machines, Crank shafts, Spline Shafts, leaf spring likewise (Cui, Liu, Pan, & Gao, 2008). Grain refining is an effective method to improve the strength and toughness of spring steels. Its microstructure varies with the reheating temperature in the course of making spring, which directly influences its final mechanical properties (Das, Dutta, & Ray, 2009; Baldissera & Delprete, 2009).
Spring steel is the category of medium carbon steel. These steels have high hardness and can be produced by working, quenching or precipitation hardening. A study has been made to investigate the fatigue properties of high-strength spring steel in relation to the microstructural variation via different heat treatments (Wilson & Mintz, 1972; Shin, Lee, & Ryu, 1999). In order to increase the hardenability, elements such as chromium, manganese and silicon are added to these steels. Furthermore, silicon retards the conversion of the carbide to cementite during tempering. It refines the carbides and improves the sag resistance significantly (Nam, Lee, & Ban, 2000). Heat treatment creates a permanent change in the material that alters many characteristics such as fatigue strength (Ardehali Barani, Ponge, & Raabe, 2006). Spring steels are used in the quenched and tempered condition which gives optimum strength and toughness, vibrational damping (Datta, 1981). The change in microstructure and strength after the heat treatment process depends on the cooling rate obtained during quenching (Murakami, Takada, & Toriyama, 1988). Due to operational safety, springs have to meet increasing performance requirements, which concern mechanical properties, Tribological properties as well as fatigue strength (Bensely, Shyamala, Harish, Mohan Lal, Nagarajan, Krzysztof, & Rajadurai, 2009).
In the manufacturing process of mechanical springs, high tensile residual stresses are generated which reduces considerably the spring strength and service life. These unfavorable residual stresses are partially eliminated by the heat treatment (Melander & Larsson, 1993). In this process, the spring is heated uniformly below the material transformation temperature (Carneiro, Pereira, Darwish, & Motta, 2002). An experimental investigation has been conducted to assess the stress relief influence on helical spring fatigue properties (Del Llano-Vizcaya, Rubio-Gonzalez, Mesmacque, & Banderas-Hernández, 2007).