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Heat Treating (Nitriding) Nitralloy 135M Steel & Ferrous Alloys for Surface Hardness

Authored by Dr. Edward Rolinski

posted On Thursday, July 6, 2023 in Blog

Nitriding is a common thermochemical process for surface hardening of engineering components and some tools [1-6]. Enhanced tribological properties are accomplished with plasma or gas methods, depending on specifications and the application, Fig. 1.

Plasma nitriding stamping dies

Fig. 1. Plasma nitriding of large stamping dies. Picture taken at Advanced Heat Treat Corp. in Monore, Michigan.

Nitriding does not affect core properties or dimensions of the products since it is carried out below their tempering or stress relieving temperature. Thanks to the high surface hardness achieved after this treatment, wear resistance as well as the bending fatigue strength of the component is significantly increased.

Nitralloy 135M Hardness After Nitriding

After nitriding, the hardness in Nitralloy 135M steel increases significantly. This specific steel nitrides very well and its highest, near-surface hardness readings exceed a value of 65HRc equivalent and the total case depth is in the range of 0.005-0.025”. An example of the hardness graph after a typical 36 hours cycle is shown in Fig. 2.

Nitralloy chart

Fig. 2. Hardness profile of Nitralloy 135M sample after a typical long nitriding cycle.

All steels containing nitride-forming elements such as chromium, molybdenum and aluminum or titanium can get a high surface hardness level. This leads to a significant increase of their wear resistance. Even thin layers, below 5 mils have very good and linear wear characteristics at high contact stresses (unit load) of 200 MPa (29 ksi). Fig. 3.

Nitriding Nitralloy Steel

Fig. 3. Linear wear depth vs. time of friction at different unit loads of nitrided layer produced in Nitralloy 135M by controlled gas nitriding at 530°C (985°F) for 6 hours. Total case depth 0.10 mm (3.9 mils), compound layer was 0.009 mm (0.35 mils), and surface hardness was 1300 HV0.5. Linear wear at 50 to 200 MPa (7 to 29 ksi) was 4.3 to 7.6 mm, and wear intensity was 0.03 to 0.072 mm/min. Adopted from [1].

The increase of hardness after nitriding is also related to formation of compressive stress in this near-surface area and this results in a significant enhancement of the bending fatigue strength of the component.

Want to learn more about nitriding Nitralloy 135M and other ferrous alloys? Check out our MANY nitriding articles.


  1. gas nitriding
  2. nitriding
  3. plasma nitriding
  4. wear resistance