Failures Induced by Abnormal Banding in Steels
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FEATURE
Failures Induced by Abnormal Banding in Steels F. D’Errico
Submitted: 19 February 2010 / in revised form: 25 June 2010 / Published online: 16 July 2010 Ó ASM International 2010
Abstract In industrial applications failures of mechanical parts made of carbon and alloyed steels may develop either during heat treatment steps or final finishing operations. Such failures have high impact costs for manufacturers, since heat treated steel products, in general, are high value products which increase in value with each step in the production process until the final life-cycle manufacturing steps are completed. This work highlights the selection of steels to avoid premature ruptures developing during either the heat treatment steps or finishing operations with emphasis on the role of banding in the failure process. Failure does not have to involve fracture but may simply imply a decrease in performance of surface treated components as consequence of surface properties, even in the presence of correct heat treatment parameters. The root causes for banding in steels, are described in literature, and banding has major effects on final product properties (and causal relationships). Therefore, the causes of banding are studied and classified. Conclusions suggest that microstructural defects such as (micro)segregation bands and other defects such as slag and oxides inclusions are developed in the early fabricating cycle steps and can cause premature failure of either semi-finished or finished products. Keywords Segregation Heat treatments Toughness Banding Anisotropy Martensite Transformation plasticity Failure Austenite
F. D’Errico (&) Department of Mechanical Engineering, Politecnico di Milano, Milan, Italy e-mail: [email protected]
Introduction Basically the ultimate quality of steel products is determined from the steelmaking technological cycles and the casting process technologies employed to fabricate raw products. The blooms, billets, bars, and slabs that are used as input to the forging, rolling, or machining operations are used to finally shape the product and the post shaping heat treatments and finishing all impact component quality. While the chemical composition of a steel may be defined, in practice a solid product may not reach uniform chemistry over its entire section and the chemical composition varies, as does the final microstructure. Thus, the mechanical properties, which are related to material chemistry and microstructure, can vary greatly and product quality is ultimately influenced by chemistry and microstructural variations. A homogeneous distribution of alloying elements in steels can be easily achieved by stirring while steel is in a liquid state. At the end of steelmaking process, liquid phase is essentially uniform in chemistry. On the other hand, solidification processes generally develop macroscopic and microscopic partitioning of chemical elements between parent liquid and growing solid crystals. This fact causes a non-uniform distribution of chemical elements to
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