Last week, we began our discussion of the factors that cause parts to change size during heat treatment. Let’s now look at some specific examples.
Experimental work has been done on many materials to show the effects of heat treatment on size change. The effects are different for every material grade. For example, a 3.15-inch cube of D-2 tool steel during hardening grew by 0.08% in one dimension, while shrinking in the other two dimensions. In this instance, the problem boiled down to knowing the part orientation from the mill-supplied bar, which was important in trying to plan for size change during heat treatment.
In another example, slitter blades of D-2 tool steel are notorious for size change during tempering – and can shrink or grow depending on the tempering temperature. D-2 is a transformation-hardening tool steel that requires both a hardening and tempering step during the heat-treating process. The dimensional changes on hardening and tempering must be added together when trying to estimate total size change. Final part hardness is determined by tempering temperature, and the hardness requested by the drawing specification or end-user may have a drastic effect on size change since it dictates final tempering temperature.
In another example, according to a Latrobe Steel data sheet, 17-4 precipitation-hardening stainless steel can typically be expected to shrink by 0.0004-0.0006 inch/inch when aging from Condition A to Condition H-900 and 0.0018-0.0022 inch/inch when aging from Condition A to Condition H-1150.
Communication with the heat treater, experimentation and process control can help to provide fairly accurate, consistent/repeatable size-change estimates. Do your homework on dimensional change and you will be handsomely rewarded!