Andreas Stieben

Microalloyed Engineering Steels with Improved Performance - an Overview

Abstract Microalloying elements are added to a wide range of steels for improving microstructure, properties, processing or general performance. A survey is given on the various reasons for microalloying of engineering steels. In case hardening steels microalloying improves toughness and fatigue properties mainly due to a smaller and more homogeneous prior austenite grain size. In forging steels, microalloying controls the phase transformation during cooling after forging enabling shorter process routes. Furthermore, microallying contributes to the microstructural refinement improving the balance of fatigue, cyclic behavior and strength. Recently, microalloying supports the interface engineering in air hardening medium Mn steels and prevents embrittlement.

18CrNiMo7-6 with TRIP-Effect for Increasing the Damage Tolerance of Gear Components — Part I: Alloy Design

High performance components such as gear wheels shall be resistant to rolling-contactfatigue. This type of failure is usually caused by effects occurring on a microscopic scale, such ascrack initiation at non-metallic inclusions. Much effort has been invested so far in improving thesteel cleanliness. However, these high performance components often do not reach the desiredservice life. Preliminary failure within the guarantee terms still occurs which leads to high warrantycosts. Alternative to improving steel cleanliness, the damage tolerance of high performancecomponents could be increased by inducing the TRIP-effect around the crack tip. Due to high localstrain hardening, martensite transformation occurs. The high compressive stresses related to it coulddelay or stop crack propagation by reducing stress concentrations via plastic deformation. As aresult, rolling-contact fatigue resistance of carburized steels may be increased and preliminaryfailure may be avoided. Part I of this study focuses on modifying the chemical composition ofconventional 18CrNiMo7-6 steel with Al to develop a high-strength, yet ductile matrix with a highwork hardening potential. Dilatometric tests on laboratory melts analyze the possibility of adjustinga microstructure able to produce a TRIP-effect. Both isothermal annealing and Quenching andPartitioning (Q&P) are used to stabilize residual austenite and optimum process routes areidentified.

18CrNiMo7-6 with TRIP-Effect for Increasing the Damage Tolerance of Gear Components — Part II: Microstructure and Mechanical Properties

High performance components such as gear wheels shall be resistant to rolling-contact fatigue. This type of failure is usually caused by effects occurring on a microscopic scale, such as crack initiation at non-metallic inclusions. Much effort has been invested so far in improving the steel cleanliness. However, these high performance components often do not reach the desired service life. Preliminary failure within the guarantee terms still occurs which leads to high warranty costs. Alternative to improving steel cleanliness, the damage tolerance of high performance components could be increased by inducing the TRIP-effect around the crack tip. Due to high local strain hardening, martensite transformation occurs. The high compressive stresses related to it could delay or stop crack propagation by reducing stress concentrations via plastic deformation. In part II of this study, the microstructures and mechanical properties of the steels modified via Al-alloying and heat treated in process routes according to part I are compared to conventional 18CrNiMo7-6. Special interest is paid to the stability of the residual austenite as well as to the change in strain hardening rate under tension.