Nt Nitriding
Nitriding · Tratamiento térmico · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “Nitride 0.4 mm · 700-900 HV” states a nitriding requirement.
What controls
Diffusion of nitrogen into the surface at a relatively low temperature, with no subsequent quench, which creates a very hard, wear-resistant layer with minimal distortion. Since there is no phase change or rapid cooling, the part barely moves, which allows already finished parts to be nitrided. The trade-off is that the layer is thin and does not tolerate generous grinding: removing a few hundredths can take away much of what was achieved. It only works well on steels with the appropriate alloying elements.
How to verify
Surface hardness with a low-load hardness tester or microhardness tester, because a high load punches through the layer and actually measures the core: this is the classic mistake when verifying nitriding. Case depth and white layer thickness are determined by metallography on a witness coupon from the same load, with a microhardness traverse from the surface. The heat treatment certificate must record the cycle, hardness and thicknesses.
Workshop guideline values
- General
- Case depths on the order of 0.2–0.4 mm on parts with moderate wear.
- Accuracy
- Case depths on the order of 0.3–0.6 mm with surface hardness specified in HV.
- High precision
- White layer controlled to a few microns and case depth certified per load.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Measuring the hardness of a nitrided part with the usual load of a Rockwell test: the indenter punches through the layer and the reading is invalid.
- Grinding a nitrided part as if it had a carburized case: here, hundredths count.
- Nitriding a steel that is not alloyed for it and expecting catalog hardness values.
How to get it in the workshop / if it comes out of tolerance
It is the treatment for parts that cannot distort: lead screws and spindles, molds, liners, precision shafts. The part is machined to final size, except for a light subsequent polish, and areas that must not harden are masked. Check that the steel is suitable for nitriding; on an ordinary steel the result is poor. Keep in mind that a very thin, brittle compound layer, the so-called white layer, usually forms on top of the diffusion zone, and the drawing may limit its maximum thickness.
Low measured hardness with correct case: almost always a method problem, from using too much load on the hardness tester; repeat with microhardness. White layer above the limit: cycle adjustment at the heat treater, or light subsequent polishing if the drawing allows. Spalling or flaking of the layer: excessive white layer or poorly prepared surface before nitriding. If grinding is needed afterward, it must be accounted for in the specified depth.
Related symbols
Other symbols in Heat treatment
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