12 1. NATURE AND SOURCE OF RESIDUAL STRESSES
1.3.6 CHEMICAL AND PHASE CHANGE
is source of residual stress occurs when a chemical reaction and/or phase change takes place
within part of a material whose products have different dimensions than the source, thereby cre-
ating a misfit and associated residual stresses. Phase changes commonly occur during heat treat-
ment of metals. Additional thermally induced residual stress may also be created by the rapid
quenching sometimes involved. In most cases, the resulting development of residual stresses is
undesirable. However, occasionally material phase changes are deliberately introduced to form
desired residual stresses. Figure 1.9 shows an example of a steel circular saw that has been “heat-
tensioned” by localized induction heating near the tooth-line. Alternatively, oxy-acetylene or
laser heating are also used in practice. e material in the heated area transforms from a marten-
site to a bainite structure and thereby undergoes a small reduction in volume and consequent
circumferential tension. is tooth-line tension has the same beneficial effect on circular saw
cutting stability as blade tightening has on a conventional hacksaw.
Figure 1.9: Circular sawblade heat-tensioned by induction heating.
ese four sources of residual stress are just general categories. Residual stress examples
could easily belong to more than one of them, depending on process details and which aspect
of the residual stress production process is emphasized. For example, coated materials formed
by electroplating, chemical vapor deposition, plasma deposition or glazing, typically have large
residual stresses within the coated layer. e stresses in these coatings could be caused by phase-
based effects during deposition, or by thermal expansion mismatch. ese stresses can cause
curvature in thin components; measurement of that curvature forms the basis of Stoney’s method
for plating stress evaluation.
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