Fatigue Behavior of Ultrafine-Grained Medium Carbon Steel with Different Carbide Morphologies Processed by High Pressure Torsion

Christoph Ruffing,Eglantine Courtois-Manara,Yulia Ivanisenko,Christian Kübel,Aaron Kobler,Robby Prang,Eberhard Kerscher

doi:10.3390/met5020891

Abstract

The increased attention ultrafine grained (UFG) materials have received over the last decade has been inspired by their high strength in combination with a remarkable ductility, which is a promising combination for good fatigue properties. In this paper, we focus on the effect of different carbide morphologies in the initial microstructure on the fatigue behavior after high pressure torsion (HPT) treatment of SAE 1045 steels. The two initial carbide morphologies are spheroidized as well as tempered states. The HPT processing increased the hardness of the spheroidized and tempered states from 169 HV and 388 HV to a maximum of 511 HV and 758 HV, respectively. The endurance limit increased linearly with hardness up to about 500 HV independent of the carbide morphology. The fracture surfaces revealed mostly flat fatigue fracture surfaces with crack initiation at the surface or, more often, at non-metallic inclusions. Morphology and crack initiation mechanisms were changed by the severe plastic deformation. The residual fracture surface of specimens with spheroidal initial microstructures showed well-defined dimple structures also after HPT at high fatigue limits and high hardness values. In contrast, the specimens with a tempered initial microstructure showed rather brittle and rough residual fracture surfaces after HPT.

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

Introduction

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

Results

Discussion

Conclusion