Deformations and Dimensional Changes During Heat Treatment

During the heat treatments of special steels – such as quenching, tempering, or annealing – it is common to observe deformations and dimensional changes. Understanding their causes is essential to prevent them and to obtain functional, reliable, and project-compliant components.

What do we mean by:

• Deformation: alteration of the geometry of a part (e.g., warping, out-of-square, ovalisation, torsion, etc.)
• Dimensional change (or more precisely, volumetric change): variation in volume without a geometrical deformation.

Although these are different phenomena, they often occur together. Understanding their origin allows you to control them through appropriate design and heat treatment choices.

Geometrical deformations are mainly caused by internal stresses resulting from:

• Temperature differences between core and surface
• Non-uniform microstructural transformations
• Residual stresses from mechanical machining (work hardening)
• Positioning errors inside the furnace

Key principle:
The lower the ΔT (temperature difference) between the core and the surface, the lower the thermal stress—and therefore the deformation—generated.

Residual stresses from machining
Operations such as milling or grinding induce stress states that add to those generated by heat.

Thermal inhomogeneities
Uneven preheating
Excessively fast cooling
Sections with varying thickness
Directionality of quenching jets

Positioning in the furnace
The way the part is placed greatly affects heat distribution and the resulting stress pattern.

Volumetric change is directly linked to the phase transformations of steel during rapid cooling (quenching).

Each phase change involves a volume variation that adds up to deformation caused by temperature gradients.

An industrial component before and after quenching may show:

• Angular shifts near sharp edges
• Longitudinal torsion
• Local dimensional deviations from 0.1 mm up to several millimetres, depending on material, section geometry, and the treatment applied