CHD Case hardening depth (CHD)
Case hardening depth (CHD) · Tratamiento térmico · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “CHD 0.6-0.8 mm at 550 HV” states a case hardening depth (chd) requirement.
What controls
Depth, measured from the surface inward, at which the part stops being considered hardened according to a defined criterion: usually a limit hardness value, and in some cases a chemical content. The figure means nothing without that criterion: the same part gives different depths if measured to 550 HV or to 500 HV. It is a result requirement, and what makes it verifiable is the depth-criterion pair, not the process used to achieve it.
How to verify
The reference method is destructive: the part, or a witness coupon treated in the same batch, is sectioned, a metallographic cross-section is prepared and a microhardness traverse is run from the surface inward until it crosses the limit value on the drawing. Macro-etching is used to see the extent of the hardened zone in localized hardening. Non-destructive eddy-current or ultrasonic methods are valid as production checks once correlated, but metallography remains the reference.
Workshop guideline values
- General
- Diffusion cases in the order of 0.5–1.0 mm and localized hardening cases of 1–3 mm.
- Accuracy
- Tight ranges such as 0.6–0.8 mm, with defined hardness criterion and measurement location.
- High precision
- Thin nitrided layers measured in microns, with metallography and microhardness certified per batch.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Specifying a case depth without stating the hardness it is measured to, which is what makes it verifiable.
- Comparing depths obtained with different criteria and concluding that the part has got worse.
- Forgetting to subtract the material that will be removed in subsequent grinding.
How to get it in the workshop / if it comes out of tolerance
When planning subsequent machining, remember that everything you grind off comes out of the case: if the drawing calls for 0.6 to 0.8 mm and you are going to remove 0.15 by grinding, the case has to come out of the furnace designed for that. On gear teeth, depth is measured at a specific point on the flank defined by the drawing, and it is not the same as at the tooth root. Always ask where it is measured before arguing about a result.
Case too shallow: insufficient cycle, or too much material removed in subsequent machining. Uneven case between areas of the part: in localized hardening it is a matter of fixturing and path; in diffusion treatments, of atmosphere circulation and how the furnace was loaded. Before rejecting, check that the measurement was taken at the point shown on the drawing and with the correct hardness criterion.
Related symbols
Other symbols in Heat treatment
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