Abstract
The increased attention that ultrafine grained (UFG) materials have received over the last decade has been provoked, not least, by their high strength in combination with remarkable ductility. The main focus of our investigation was the evaluation of the effect of different carbide morphologies in the initial microstructure on the fatigue behavior after high pressure torsion (HPT) treatment of SAE 1045 steel. In our case HPT increased the hardness by a factor of 1.75 - 3.2 compared to the initial states. The achieved hardness maximum was 726 HV. The amount of increase depended on the initial carbide morphology. By stress controlled cyclic four point bending tests with a load ratio of 0.1 endurance limits were determined for the initial and HPT states. The endurance limit increased linearly with hardness until 500 HV and independently of the carbide morphology. All fracture surfaces were investigated by SEM after the fatigue tests. They revealed pretty flat fatigue fracture surfaces with crack initiation at the surface or rather at non-metallic inclusions for the UFG states. Morphology and crack initiation mechanisms were changed by severe plastic deformation compared with the coarse grained initial state. Residual fracture surfaces with a spheroidal initial microstructure showed well-defined dimple structures also after HPT at high fatigue limits and high hardness values. In contrast, the specimens with initial tempered microstructure showed rather brittle and rough residual fracture surfaces.
Highlights
Despite the progress achieved over the last fifty years in the development of new steel grades and thermal- and thermomechanical treatments, the quest for novel processing routes allowing further enhancement of mechanical properties remains of great current interest
We present fatigue properties of ultrafine-grained medium carbon steels with two different carbide morphologies
The soft annealed state contains spheroidal carbides distributed in a uniform coarse grained microstructure with well-defined grain boundaries (Figures 2a and 3b)
Summary
Despite the progress achieved over the last fifty years in the development of new steel grades and thermal- and thermomechanical treatments, the quest for novel processing routes allowing further enhancement of mechanical properties remains of great current interest. It was proposed that grain size refinement could be the most promising way to improve the fatigue life of steel because it allows obtaining high strength in combination with good ductility values [1,2]. Severe plastic deformation (SPD) of metals and alloys is a well-established method to obtain ultrafine-grained structures, or phase compositions that are impossible to obtain by conventional thermal treatment. Essential for SPD is the combination of a high hydrostatic pressure, to avoid crack initiation, and an enormous shear strain. The most developed SPD techniques are Equal Channel Angular
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
