As heat treaters, we often ask ourselves the question, “How much size change – if any – can I tolerate?” While we would love to be able to answer this question each and every time we run a load of parts, in relativity (volumetric) size change during heat treatment cannot be predicted well enough to allow for final machining and/or grinding to final tolerances prior to most heat treatments.

The answer to size change is an extremely complicated one, involving literally hundreds of variables. Heat treating often contributes to dimensional change, adding its own unique set of process (and, in some cases, equipment) variables, all of which affect part size change. While a precise forecast of a part’s size change cannot be expected, a heat treater is usually able to provide a reasonable estimate by considering each of these potential factors so as to help the manufacturing area prepare for post-heat-treat operations such as machining.

Variables Affecting Dimensional Change

• Material grade
• Variation in material properties (chemistry, homogeneity, grain size, number and type of inclusions, and hardenability)
• Part orientation vs. grain orientation
• Starting microstructure
• Heat treatments performed at the mill (dozens of variables)
• Manufacturing process and sequence of operations
• Residual stress state (from manufacturing)
• Heat treatment after component manufacturing (dozens of variables)
• Hardness range (initial, final)

Variables Related to Heat Treating

• Type of process selected (annealing, hardening, nitriding, carburizing, etc.)
• High-heat process (anneal, normalize, austenitize), temperature and soak times
• Low-heat process (age, temper, stress relief), temperature and soak times
• Furnace temperature uniformity
• Furnace repeatability
• Quenchant type
• Quench rate
• Part size
• Load size
• Load configuration
• Part orientation/fixturing
• Type of furnace atmosphere
• Deep freezing or cryogenic processing
• Number of tempering cycles

Next week we’ll finish this discussion, providing size-change examples.