The Role of Microstructure on Crack Growth in Carburized Steels

Colorado School Of Mines, Golden CO

Investigators

Abstract

Carburization is used extensively in the transportation and heavy equipment industries to harden the surfaces of steel parts such as gears and bearings. The process results in a gradient of residual stress in the part that provides enhanced resistance to wear and damage. Understanding the underlying phenomena that control this enhanced resistance can lead to new technologies for carburized steel parts. This award supports fundamental scientific research to uncover the mechanisms that link processing and structure to properties and performance in these materials. The new knowledge will allow the development of heat treatment strategies to refine the surface structure produced through carburization to improve performance, and to build models to predict materials performance based on processing and surface structure. This new knowledge has numerous applications including gear boxes, which see higher loads as vehicle lightweighting improves. Student trainees will engage in valuable educational opportunities through participation in this research. The objectives of this work are to characterize the interaction of fatigue cracks with carburized steel microstructures that contain combinations of retained austenite and plate martensite, particularly focusing on processing strategies to refine the microstructure, and to utilize the observations to inform physically-based fatigue life prediction models in these complex microstructures. The work will be accomplished by designing variations in carburizing processing routes, including novel heat treatment processes intended to refine the carburized steel microstructure. Then, fatigue crack growth studies will be performed with in-situ and interrupted test analysis of the crack interaction with the microstructure. Multi-scale characterization will be utilized to quantify microstructural evolution in the vicinity of the fatigue crack, particularly with respect to retained austenite stability against transformation and microscale residual stresses present due to the transformation. Finally, the fatigue crack growth data and characterization will inform models that predict the influence of microstructure on fatigue crack growth rates based on the effective driving force for crack growth.