title = "Distortion During Heat Treatment: Causes and Prevention"

content = """
Distortion during heat treatment is a dimensional change in a part caused by the thermal, phase transformation, and stress-relief events that occur during the process. Minimizing and predicting distortion is one of the most challenging aspects of heat treatment at Ridgecrest Metalworks, particularly for precision components where machining operations have already brought dimensions close to final.

Distortion has three primary causes. Thermal distortion results from differential thermal expansion when different sections of the part heat or cool at different rates, generating non-uniform strain. Transformation distortion occurs when martensite forms during quenching—the FCC austenite transforms to BCT martensite with approximately 4% volumetric expansion; if this transformation occurs at different times in different zones, the unequal expansion causes permanent shape change. Stress relaxation distortion results when pre-existing residual stresses (from prior machining, cold work, or welding) relax at elevated temperature, releasing the stored elastic strain as dimensional change.

Factors that increase distortion: rapid quenching (high thermal gradients), asymmetric part geometry, asymmetric loading or stacking in the furnace (causing uneven support during plastic-temperature hold), prior residual stresses from machining or cold work, and hardenability variation in the material.

Prevention strategies include: using the slowest quench medium that achieves the required hardness (oil or polymer vs. water for steels with sufficient hardenability); marquenching (quenching into a salt bath above the martensite start temperature Ms, equalizing temperature before air cooling—allowing transformation to occur uniformly throughout the section); austempering (isothermal quench to produce bainite rather than martensite, with lower transformation stresses); stress relieving before hardening to remove machining stresses; symmetrical part support in the furnace (vertical hanging for shafts, flat trays for discs); and press quenching (holding the part in a die during quenching) for thin gears and discs.

Designing in distortion allowance: when distortion cannot be fully prevented, parts are designed with additional stock on critical surfaces that are finish-ground after heat treatment to achieve final tolerances. Distortion direction and magnitude must be predictable and repeatable for this approach to work reliably.
"""

version = "0.0.1"
