Dimensional Distortion

Heat Gradient

Heating a part unevenly creates temperature differences that cause different sections of the metal to expand at different rates. If the yield strength of the material is exceeded at these temperatures, dimensional distortion becomes a permanent feature of the geometry. Residual stresses from previous machining operations often contribute to this movement when the part enters the furnace.

Phase Change

Volumetric shifts associated with the transition from austenite to martensite alter the physical envelope of the workpiece. Because the crystal lattice occupies more space in its hardened state, dimensional distortion is an inherent risk in the hardening of tool steels. Non-uniform cooling rates across a part with varying cross-sectional thicknesses exacerbate these size changes.

Control Measure

Stress relieving before final machining helps stabilize the material against unexpected movements during subsequent hardening steps. To minimize dimensional distortion, heat treaters use controlled heating rates and uniform quenching techniques such as vacuum gas cooling. Precision is restored.

Precision grinding after the hardening process provides the final correction needed to bring the component back into the required dimensional specification. Fixturing the parts during the tempering cycle also prevents warping by applying physical constraints as the metal structure stabilizes.