Abstract
In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.
Highlights
Tempered martensitic steels are used in virtually all kind of engineering applications, which imply cyclic deformation at high load levels, e.g., power train in transportation, gear boxes or crack shafts in power generation
In the case of the bainitic microstructure, the ferrite lancets are separated by thin cementite films, while plate-shaped cementite precipitates of 100-200nm length are formed 60° tilted towards the lancet axes
In the case of the martensite microstructure, 1μm sized cementite plates are formed along the martensite block boundaries, while tiny (
Summary
Tempered martensitic steels are used in virtually all kind of engineering applications, which imply cyclic deformation at high load levels, e.g., power train in transportation, gear boxes or crack shafts in power generation. The excellent combination of strength and ductility can be attributed to (i) an optimum in size and distribution of carbides, (ii) a small prior austenite grain size and (iii) a sufficient ductility of the martensitic bulk material. These parameters are determined by the way of heat treatment. The defects are surrounded by a socalled fine-granular or optically dark area (FGA or ODA) The origin of this area is attributed to the accumulation of cyclic plasticity leading to local grain refining (polygonization, [3]) and/or hydrogen concentration [4]. From the correlation of different steel heat treatments with the respective fatigue properties it is
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