VHCF Strength of Helical Compression Springs - Influence of Heat Treatment Temperature before Shot Peening

Abstract:

Previous fatigue tests show that the heat treatment temperature has a significant influence on high cycle fatigue behaviour of helical compression springs. In order to investigate the effect of the heat treatment temperature on the fracture behaviour and the cyclic life, fatigue tests in the very high cycle regime (VHCF) were conducted.The tested springs were manufactured from oil hardened and tempered SiCr-alloyed valve spring steel wire with a diameter of d = 1.6 mm. After winding and grinding of the spring endings, the springs were heat treated at either 360°C or 400°C for 30 minutes. In order to generate compressive residual stresses in the surface area, the springs were shot peened. After shot peening, the springs were again annealed at 240°C for 30 minutes.Fatigue tests were conducted at 40 Hz using a special spring fatigue device. Up to 900 springs were tested simultaneously at various stress levels to 5∙10⁸ or 10⁹ cycles. Fractured springs were investigated by means of a stereomicroscope as well as a scanning electron microscope to analyse the fracture behaviour and failure mechanisms. The vast majority of the springs show crack initiation at the surface at the inner side of the coil. Less frequently, crack initiation occurs at subsurface locations. Our results show that heat treatment at a temperature of 360°C leads to four times more subsurface cracks than at a temperature of 400°C and reduces the overall fatigue life time.

• Periodical: Key Engineering Materials (Volume 664)
• Pages: 140-149
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.664.140
• Online since: September 2015
• Authors: Isabell Brunner*, Desislava Veleva, Jörg Beyer, Matthias Oechsner
• Keywords: Heat Treatment Temperature, Helical Compression Springs, SiCr-Alloyed Valve Spring Steel, Very High Cycle Fatigue Test

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