Loading...
Study of fatigue properties, residual stresses and fine microstructural features of induction and furnace tempered bar steels
Parthasarathi, Venkkateesh
Parthasarathi, Venkkateesh
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2014
Date Submitted
2014
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2015-05-01
Abstract
Load-controlled, fully reversed, cantilever bending fatigue testing was conducted on specimens of induction hardened and tempered 1045 and 10V45 steels. Three different tempering treatments consisted of induction tempering treatments for either 5 s at 270 °C, or 4 min at 220 °C, and a furnace tempering treatment of 10 hr at 215 °C. Surface hardness for all heat-treat conditions and both steels was approximately 55 HRC, and the effective hardened depth was about 2.5 mm (to a depth of 50 HRC). Fatigue specimens were notched to a depth of 0.2 mm to promote surface nucleated cracks. Endurance limits ranged from approximately 470 to 605 MPa (68-88 ksi) for the three tempered conditions of the 1045 steel. For the 10V45 steel, endurance limits ranged from 495 to 515 MPa (72-75 ksi). During induction hardening, both steels possessed a prior austenite grain (PAG) size of about 10 [mu]m. X-ray diffraction (XRD) revealed similar surface residual stress values (about -750 MPa) and similar residual stress gradients. For all six conditions, the transition from compressive to tensile residual stress occurred at about 2.5 mm from the surface, which coincided with a change in microstructure from martensite in the case to the original ferrite-pearlite in the core. Surface nucleated cracks were verified in all conditions, and differences in inclusion populations were deemed to be of secondary importance. Scanning-electron microscopy (SEM) revealed transgranular failure of the martensitic case microstructure in 1045 and intergranular failure at prior austenite grain boundaries in the 10V45 steel. This difference was attributed to different phosphorus (P) levels, 0.006 wt pct P and 0.015 wt pct P for the 1045 and 10V45 steels, respectively. Microscopic investigation of the early stages of crack nucleation and small-crack growth was not possible because the nucleation sites on the fracture surfaces were obliterated at the end of the fully-reversed bending fatigue tests. Analyses of ancillary specimens in the form of thin disks of heat treated 1045 and 10V45 steels showed that vanadium (V) increased the tempering resistance in for a martensitic microstructure. Tempering at 400 °C for times of 30 min and longer showed that softening of the 10V45 steel was retarded compared with 1045 steel. Transmission electron microscopy (TEM) revealed the presence of finer iron-carbide precipitates in tempered martensite of 10V45 steel as compared with 1045 steel, as well as a greater propensity for intralath carbides (10V45) rather than carbides at lath boundaries (1045). Near-surface specimens from fatigue-tested specimens showed complex mixtures of transition carbides and cementite for all conditions. Compared to the 1045 steel, the 10V45 steel exhibited a higher ratio of fine transition carbide precipitates to coarser cementite precipitates. The 1045 steel also exhibited more regions of comparatively lower dislocation density. Based on observations from previous publications, it was hypothesized that the 10V45 steel would have exhibited endurance limits in the 550 to 600 MPa range if the P level had been 0.006 wt pct (as for the 1045 steel) rather than 0.015 wt pct. The higher variability in endurance limit for the 1045 steel compared with the 10V45 steel is possibly related to a higher and a more-variable rate of tempering. Alternatively, the low variability of the 10V45 steel is related to a lower and a less-variable rate of tempering owing to a direct influence of V on carbide precipitation and dislocation annihilation during tempering. A hypothesis was provided that suggests that adding 0.1 wt pct V to 1045 steel reduces the propensity and/or magnitude for hardness/strength gradients, strain gradients, and localized regions of concentrated plastic deformation.
Associated Publications
Rights
Copyright of the original work is retained by the author.