C Carburizing / case hardening
Carburizing / case hardening · Tratamiento térmico · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “Carburize CHD 0.8-1.0 mm · 58 HRC surface” states a carburizing / case hardening requirement.
What controls
Treatment in which the surface layer of a low-carbon steel is enriched with carbon at high temperature and then quenched, so that the surface becomes hard and wear-resistant while the core retains toughness. The drawing must give three things for it to be verifiable: surface hardness, case depth measured to a defined hardness value, and core hardness. Without the hardness criterion to which the depth is measured, the case depth figure means nothing.
How to verify
The reference method is a metallographic section of a part or of a witness coupon treated in the same load: the section is polished, a microhardness traverse is run from the surface inward, and the depth at which the drawing hardness value is reached is determined. Surface hardness is checked with a hardness tester on the specified scale, and core hardness on the section. Non-destructive methods are useful for production control, but case depth is certified by metallography.
Workshop guideline values
- General
- Case depths on the order of 0.5–1.0 mm on general-purpose gears and shafts.
- Accuracy
- Case depths on the order of 0.8–1.5 mm with a tight range and controlled surface hardness.
- High precision
- Thin cases below 0.5 mm or deep cases above 2 mm, both with per-load metallographic control.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Stating a case depth without saying to what hardness it is measured: the same part gives different figures depending on the criterion.
- Grinding after treatment without deducting the material removed from the hardened case.
- Assuming surface hardness guarantees case depth: they are two independent requirements and are verified separately.
How to get it in the workshop / if it comes out of tolerance
The part is machined before treatment and ground afterward, leaving stock on the tight dimensions; but be careful: grinding eats into the case, so the stock must be accounted for in the specified depth. Areas that must not harden (threads, fastening holes, faces to be machined later) are protected with stop-off paste or left with extra stock to be removed. It is a treatment that distorts: long or thin parts come out warped.
Case below requirement: insufficient time or carbon potential in the cycle, or too much material was removed in grinding. Low surface hardness with correct case: a problem with the subsequent quench or with decarburization. Distortion: machine with stock, stress relieve before treatment and, on delicate parts, consider a lower-distortion treatment. Hardening in areas that were supposed to stay soft: the masking failed.
Related symbols
Other symbols in Heat treatment
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