Peter Wong, YungJoon Jung
Date of Award
Doctor of Philosophy
Department or Academic Unit
College of Engineering, Department of Mechanical and Industrial Engineering
materials science, carbide coarsening, carbide coarsening simulation model, carbide dissolution, carbide dissolution simulation model, martensitic stainless steel, quantitative carbide analysis
Materials Science and Engineering | Mechanical Engineering
Cutting blades and knives in various forms are manufactured from martensitic stainless steel strips. The manufacturing process of these cutting knives comprises a hardening heat treatment, cutting edge formation, and shaping into product dimensions. In a production environment, the hardening heat treatment is typically carried out continuously using an in-line heat treatment system. Such a heat-treatment process enables high production speed and efficient through-put. However, a high speed in-line heat-treatment process is very sensitive to raw material variations. Such variations may arise from differences among the manufacturing processes employed at raw material suppliers as well as shipment to shipment quality variations from a supplier. Some of these variations can be very subtle and might not have been fully understood by conventional material characterization techniques. The subtle material variations could cause differences in the response of the materials to the heat treatment, thereby potentially impacting the downstream manufacturability as well as the performance of the finished products. In addition, with the increasing demand for higher through-put production, optimizing the process parameters has become even more crucial. Therefore, the purposes of this work were to study the physical metallurgy of the hardening process and ultimately develop a simulation model to predict the kinetics of secondary carbide dissolution and coarsening during the austenitizing treatment of martensitic steel.
The steel studied in this work mainly contains 0.7 wt. % C and 13% of Cr, which is a non-AISI standard martensitic stainless steel. Detailed material characterization was carried out using advanced quantitative metallographic techniques to characterize the subtle materials variations. The secondary carbide size distributions before and after the hardening process were characterized by scanning electron microscopy (SEM) and analyzed by computer assisted image analysis techniques. It was found that both the volume fraction and number of the secondary carbides decreased during the hardening treatment process, while the mean diameter remained nearly unchanged, indicating critical effects of Ostwald ripening on the final carbide size distribution. Historically, however, studies on the heat treatment of this martensitic stainless steel focused mainly on secondary carbide dissolution, while little attention is paid to carbide coarsening.
To better understand and ultimately provide a tool for the simulation of the concurrent occurrence of carbide dissolution and coarsening, mathematical carbide dissolution and coarsening model was developed incorporating a metallurgical kinetic theories of dissolution and Ostwald ripening. This was justified since most previous models were developed to predict only the mean carbide diameter and as such does not address the change in secondary carbide size distributions caused by concurrent dissolution and coarsening.
Comparison of simulated distributions with those determined experimentally indicates that both dissolution and coarsening indeed occur concurrently during the hardening process. It was found that during austenitization the average radius of carbide particles increases quickly as small carbide particles dissolve in the austenite, but increases only slowly once small particles have disappeared. The nearly constant carbide radius maintained after the disappearance of small particles reflects comparable rates of carbide dissolution and coarsening. The cumulative amount of carbide dissolution increases while the average radius remains nearly constant.
Ming Laura Xu
Xu, Ming Laura, "Secondary carbide dissolution and coarsening in 13% Cr martensitic stainless steel during austenitizing" (2012). Mechanical Engineering Dissertations. Paper 23. http://hdl.handle.net/2047/d20002442
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