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topic 18276

Case depth in carburizing or nitriding

A discussion started in 2002 but continuing through 2020


Q. Can you please inform me where can I find national or international standards online that talk about case depth in nitriding or carburizing?

George Bougioutakis
- Leeds, UK


A. Here are some American standards of possible relevance. Links to ISO, British and MIL-S-12515C SURFACE HARDENING: FLAME AND INDUCTION (FOR FERROUS ALLOYS) [inactive]

MIL-STD-1878A [affil. link to spec at Techstreet] CARBURIZING, GASEOUS ATMOSPHERE, PROCESS FOR [Canceled]
ASTM A534-01 Standard Specification for Carburizing Steels for Anti-Friction Bearings
ASTM A255-02 Standard Test Method for Determining Hardenability of Steel
ASTM E1077-01e1 Standard Test Methods for Estimating the Depth of Decarburization of Steel Specimens
ASTM G79-83 (1996)e1 Standard Practice for Evaluation of Metals Exposed to Carburization Environments
ASTM A37 Recommended Practice for Carburizing and Heat-Treatment of Carburized Objects -withdrawn 1936
ASTM A355-89(2000) Standard Specification for Steel Bars, Alloys, for Nitriding

Search ISO specifications at
British Standards (BS) at
British DEF, DTD, NES standards at and DIN standards at

I am not sure exactly what you are seeking. Case [depth] is typically considered as the portion of a ferrous alloy, extending inward from the surface, in which the hardness is greater than that of the core.*  However, it is common to specify a minimum hardness at a given depth, and this can be achieved by various combinations of temperature, time, surface carbon activity, and initial alloy composition. Carburizing and nitriding occur by non-steady-state diffusion described by Fick's second law, with a concentration curve that decays from the surface into the bulk [Mathematical solution requires diffusion coefficients and the erf function, and is covered in metallurgy/ materials science courses]. To avoid extreme brittleness, an upper limit is sometimes placed upon the surface hardness.

*ASM Handbook, Volume 5, Surface Engineering, p. 948 (1994).

Ken Vlach
- Goleta, California

contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.

Replacement for MIL-STD-1878

January 9, 2020

Q. Hi all,

I have 2 questions:
1) Which standard replaces the MIL-STD-1878 [affil. link to spec at Techstreet]?
2) Can I use this standard for SAE 4340 steel?


Tomer Sonnenschein
- Israel

January 27, 2020

A. Tomer,
According to
there was no replacement named at the time the standard was cancelled. Though Ken Vlach's reply above suggests some alternatives.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
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McHenry, Illinois
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