Influence of surface condition, cycling frequency and ferritic zones on the high and very high cycle fatigue properties of a pearlitic steel

Abstract

In the present work, the high and very high cycle fatigue (HCF/VHCF) properties of pearlitic steel R350HT are investigated. Servo-hydraulic (cycling frequency between 15 Hz and 60 Hz) and ultrasonic (19 kHz) fatigue tests were performed with different surface conditions (fine turned, mechanically polished and electropolished). At a load ratio of R = −1, solely failures in the HCF regime were found for all surface conditions, although runout specimens were loaded with more than 1010 cycles, and crack initiation was exclusively at the surface. At R = 0.1, in contrast, mechanically polished specimens showed internal failures for lifetimes between 106 and 109 cycles. The origin of fracture in the interior was identified as distributed ferritic phases embedded in the harder pearlitic microstructure. Crack initiation from ferritic zones could be explained by their larger size relative to inherent defects such as nonmetallic inclusions and precipitates. The absence of turning grooves in combination with compressive residual stresses near the surface of mechanically polished specimens led to an increase in HCF strength compared with fine-turned and electropolished specimens and facilitated VHCF failure from the interior. No frequency effect was observed when crack initiation originated from turning grooves or artificial surface defects, but slightly higher fatigue strengths were determined for electropolished specimens at 19 kHz than at conventional cycling frequency. This can be explained by the strain-rate sensitivity of ferrite where fatigue cracks preferentially initiate. Additional tests with specimens containing artificially introduced micro-holes were performed to systematically study material's defect tolerance and to quantify the mean-stress sensitivity.