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One Minute Mentor: Variations in Cooling Rate
November 20, 2018
Source: ASM International
Variations in cooling rate, from point to point, can exaggerate the production of residual stresses because, in some instances, it is possible for austenite transformation products to be different in sections with different cooling rates. For example, in rapidly cooled sections in moderately alloyed irons, it is possible for the austenite to transform to martensite, a very hard and brittle microconstituent that expands a different amount than the normal pearlitic or bainitic constituent when it forms from austenite.
This difference in expansion rates between differing transformation products adds to differences in contraction rates due to differences in section sizes. Typical cast shapes that could be expected to have different cooling rates from point to point are illustrated in the diagram.
The diagram shows four cast shapes and corresponding causes of residual stresses in castings.
(a) Differences in cooling rates due to shape.
(b) Nonuniform metal sections.
(c) Mechanical mold restraint.
(d) Surface peening during cleaning. Diagram courtesy of the American Foundry Society
Materials Processing and Treatment | Heat Treating
Metals and Alloys | Cast Irons