Effect of segregations on mechanical properties and crack propagation in spring steel

Abstract

Considerable efforts have been made over the last decades to improve performance of spring steels, which would increase the service time of springs and also allow vehicles weight reduction. There are different possibilities of improving properties of spring steels, from modifying the chemical composition of steels to optimizing the deformation process and changing the heat treatment parameters. Another way of improving steel properties is through refining the microstructure and reducing amount of inclusions. Therefore, the focus of the current investigation was to determine the effect of more uniform and cleaner microstructure obtained through electro-slag remelting (ESR) of steel on the mechanical and dynamic properties of spring steel, with special focus on the resistance to fatigue crack propagation. Effect of the microstructure refinement was evaluated in terms of tensile strength, elongation, fracture and impact toughness, and fatigue resistance under bending and tensile loading. After the mechanical tests the fracture surfaces of samples were analyzed using scanning electron microscope (SEM) and the influence of microstructure properties on the crack propagation and crack propagation resistance was studied. Investigation was performed on hot rolled, soft annealed and vacuum heat treated 51CrV4 spring steel produced by conventional continuous casting and compared with steel additional refined through ESR. Results shows that elimination of segregations and microstructure refinement using additional ESR process gives some improvement in terms of better repeatability and reduced scattering, but on the other hand it has negative effect on crack propagation resistance and fatigue properties of the spring steel.

Fatigue Behaviour of Tempered and Isothermal Heat Treated Aisi 5160 Leaf Spring Steel

“…As previously shown in Figure 6, the decline was owing to a modest decrease in the hardness values from 62 HRC to 57 HRC. Spring steel samples had a more substantial decrease in stress to failure from 740 MPa to 599 MPa as tempering temperatures were increased from 450 to 600 °C, which was about 0.809 % reduction in the stress to failure, and this decrease in stress to failure was also attributable to a decrease in spring steel hardness from 57 HRC to 51 HRC as well as due to the dislocations coalesces at high tempering temperatures [3]. The stress to failure of spring steel samples that were isothermally heat-treated at 830 °C increased by 878 MPa, which is a considerable increase.…”

“…That instance, when tempered at 440 °C, the S-N curve shows a continuous decrease, but when tempered at 400 °C, the S-N curve shows a continuous increase, and fatigue failure is predominantly caused by massive oxide inclusions. Despite the fact that specimens tempered at 400 °C have greater fatigue strength than specimens tempered at 440 °C [3]. The effect of Pb % in SAE5160 steel was measured after introducing the temper temperature and employing the cooling oil at 66 Co. Charpy experiments revealed that the presence of phosphorus causes embrittlement even at low levels of phosphorus and causes intergranular fracture.…”

Fatigue Crack Propagation of 51CrV4 Steels for Leaf Spring Suspensions of Railway Freight Wagons

“…Via scanning electron microscopy (SEM), transgranular fracture surfaces were observed. In these fractures' surfaces, secondary cracks and fracture micro-mechanism, especially due to facet cleavage, were observed [7,11,16]. Additionally, reduced-sized ductile dimples were observed.…”

“…The investigation regarding fracture resistance properties in leaf springs is of great importance, not only due to their quasi-brittle behaviour [4], but also because thermal and mechanical treatments can significantly change these properties. For example, the number of cycles for fatigue failure tends to be higher when the tempering temperature increases [4,7]. These variations may be greater if a superficial shot-peening treatment is applied.…”

Fatigue Crack Propagation of 51CrV4 Steels for Leaf Springs Suspensions of Railway Freight Wagons

“…Regarding the development of fatigue striations, their visualization is not frequently observed, however, when they were observed in martensitic steels, on a very small scale. According to [7], the fatigue striations had a quite reduced spacing (around 1 µm).…”