Steel Surface Hardness After Heat Treatment - 8620, 15-5 PH, Nit135M & More

posted On Monday, January 8, 2024 in Blog

Commercial steel-treating processes are based on decomposing austenite, the high temperature phase of iron with carbon, in several different ways [1]. The stress heat treating processes are:

• Annealing
• Quenching
• Interrupted Quenching
• Tempering and
• Austempering.

The resulting properties significantly affect the strength of the steel product; however, many failures of machinery/mechanical components and tools start from the surface. This is typically caused by significant wear and abrasion, rolling contact (RCF) or bending fatigue (BF) generated by the operation and occasionally corrosion from the work environment.

Heat treating materials used for such devices or final treatments of the finished components has a tremendous effect on improving their longevity. Commercial heat treating processes, including quenching with tempering and martempering, are started at very high temperatures exceeding 1600°F and therefore are done for raw steels or unfinished components because they may cause significant dimensional changes. When the traditional quenching process is applied, end-quench hardenability curves are of great importance since they can demonstrate the hardness and depth that can be achieved when the cooling rate is known [1].

In many applications, the surface condition of the steel part is critical. Therefore, the parts are subjected to additional thermochemical treatments such as carburizing followed by quenching or to a much lower-temperature nitriding process. Selection of the treatment depends on a required general mechanical strength of the selected steel. Carburizing is used for low-carbon, low-alloy steels and therefore their tensile and yield strength are not as high as the medium carbon, low alloy steels used for nitriding. Nevertheless, the specific selection of steel and the treatment of it depends on the designing/metallurgical engineers.

Surface Properties of Carburizing and Nitriding

There are numerous applications of the diffusion processes, carburizing and nitriding. In carburizing, a low-carbon steel which has relatively good toughness but is soft, is heated in a carbon-containing atmosphere so that carbon diffuses into the steel, producing a carbon-enriched case [1, 2]. Further quenching increases the hardness of the layer by forming the martensitic structure. The core stays soft and ductile. An example of such treatment is shown in Fig. 2.

The same 8620 steel subjected to nitriding had a layer that was not as hard and as deep as the carburized layer, see Fig. 3.

It should be noted that although the carburizing process is much faster than the gas nitriding process, its temperature is very high and the required quenching after diffusion of carbon is complete, may cause significant distortion of the treated components and therefore in many situations, it cannot be considered as the final operation. The opposite is true for nitriding - both gas and ion methods - and therefore this process is almost always used as a final operation for the manufactured product.

In many applications, not only are increased tribological surface properties required, but also very high yield/tensile strength of the part. In these situations, highly alloyed steels are used. Nitriding produces a very high hardness and this operation does not cause any distortion or dimensional changes of the treated part; it is a final operation. Fig. 4 demonstrates how high hardness, >1000HV/HK (> 65HRc equivalent) can be achieved.

When nitriding is applied, it is also possible to harden bearing steels such as, for example, M-50 and maintain a very high core hardness of the part at the level of 64-65 HRc (above 800 HV), see Fig. 5.

Typical nitriding steels such as Nit135M develop a very high hardness and deep case depth to endure very high tribological properties while maintaining a high strength of the steel, see Fig. 6.

There are, of course, many scientists and engineers in the country who are materials specialists wholly involved in materials science and engineering and who control heat treating processing in commercial environments [1]. Advanced Heat Treat Corp. is very well-staffed and well-equipped to provide the best heat treating processes appropriate for various applications. This is achieved with excellent communication with our customers.

Questions about Surface Hardness & the Heat Treating of 8620, 15-5 PH, 4340, 4130, M-50 and other Steels? 

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References for "Steel Surface Hardness After Heat Treatment - 8620, 15-5 PH & More"

1. Van Vlack, “Elements of Materials Science & Engineering”, Forth Edition, Addison-Wesley Publishing Company, 1980.

2. “Thermochemical Surface Engineering of Steels”, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014, pp 413-449.

1. carburizing
2. edward rolinski
3. nitriding
4. stainless steel