GlossaryKind of term: Manufacturing termHeat treatment
What is heat treatment?
Heat treatment changes the properties of metal by controlled heating and cooling, as in hardening, quenching and tempering, nitriding and stress relieving.
Heat treatment is the umbrella for processes in which metal is heated and cooled along a set temperature and time profile to change its hardness, strength, toughness or internal stress. For steel these include hardening (austenitising and quenching, followed by tempering), quenching and tempering (hardening plus tempering at a higher temperature for a strong, tough condition), surface hardening such as induction hardening, case hardening and nitriding, and stress relieving. Aluminium has heat treatments of its own, such as solution treatment and ageing.
What does the choice of heat treatment depend on?
Which treatment fits depends on the material and its hardenability, the requirement on the drawing and the geometry. Through hardening changes the whole cross-section, as far as the steel hardens through; case hardening, induction hardening and nitriding give a hard surface layer over a tougher core. The requirement is given as hardness in HRC, HV or HB, with a case depth for surface hardening, and sometimes as tensile strength. Stress relieving is done below the transformation temperature and, for quenched and tempered steel, below the tempering temperature, so hardness stays almost unchanged. Hardening and case hardening usually cause more distortion than nitriding, which runs at a lower temperature without quenching.
How does heat treatment work in practice?
A heat treatment in the middle of the route splits it into a part before and a part after, for example rough milling, stress relieving and finish milling, or turning, hardening and grinding. That usually calls for stock allowance, extra setups and re-aligning to a datum. Quenched and tempered steel at moderate hardness is machined normally. After hardening to a high hardness, conventional machining with standard tools is usually no longer practical; grinding, hard milling or hard turning with suitable cutting materials, and wire EDM, are the usual next steps. The work mostly goes to a heat treater, with a price per batch or per kilo, a minimum charge and a lead time of its own.
What is heat treatment often confused with?
Hardening is not stress relieving: hardening raises hardness and strength, stress relieving reduces internal stress and leaves hardness almost untouched. Hardness is the property, hardening the process. And hardening does not necessarily change the whole material: with surface hardening the core stays tougher than the case.
How do you see heat treatment in Blake?
If the drawing calls for a heat treatment such as hardening, it appears in Blake as a proposal with the part. You confirm or correct the proposal; the name and time of that change stay visible. Subcontracted hardening becomes its own line in the estimate, priced from your subcontract library, and as long as that price is missing, the line stays open. More on estimating.
Frequently asked questions
Why stress relieve between rough and finish milling?
Roughing removes a lot of material, which releases internal stresses and can make the part warp. Stress relieving after roughing lets most of that distortion happen before the sizing cuts. Finish milling then reaches the final dimensions with less risk of them moving later.
Can I still mill after hardening?
After hardening to high hardness, usually not with standard tooling; quenched and tempered steel at moderate hardness, yes. Hard milling and hard turning can work, depending on hardness, machine, cutting material and tolerance. Grinding or wire EDM often follows; do most work before hardening.
What should the drawing state for hardening?
The process and the hardness as a range, for example 58 to 62 HRC, and for surface hardening the case depth. Also state which faces must stay soft and whether dimensions apply before or after the treatment.