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Nitriding Powder Metal (PM) Components

Advanced Heat Treat Corp. (AHT) senior scientist Edward Rolinski explains why low temperature ion/plasma nitriding works well on powder metals.

posted On Monday, July 8, 2019 in Blog

This article originally appeared on Industrial Heating's website on May 29, 2019. It was written by AHT employees Edward Rolinski and Mikel Woods. See the complete article here

Nitriding of Powder-Metal (PM) Components

The application of ion/plasma nitriding to powder-metal (PM) products is recommended over the gas nitriding method. Ion/plasma nitriding results in surface hardening while gas nitriding causes unintended through-nitriding, making the treatment not suitable for a majority of engineering components made of PM.

Production of mechanical components such as gears and armatures made of powdered (sintered) metal/steels often requires secondary heat treatment in order to meet the requirements for high-performance applications. In many situations, the application of quenching and tempering is not sufficient if fatigue strength and wear resistance of the product needs to be enhanced. Therefore, either carburizing or nitriding/nitrocarburizing is often used.

The application of gas methods of nitriding or ferritic nitrocarburizing (FNC) in those situations is quite complicated because of the porous nature of PM, resulting in through-hardening and potential for embrittlement and “swelling” Liquid methods result in salt becoming trapped in the porosities of PM alloy, which can be difficult to remove.

Ion/plasma nitriding proves to be the most efficient method of the low-temperature treatments applied to PM since it does not cause penetration of internal porosities and cavities present in the low-density products. Therefore, the ion/plasma nitriding process is used for surface hardening of many automobile parts produced in large quantities. The method can easily be used for nitriding low-alloy and high-alloy stainless steel PM such as those made of 310 stainless steel.

Gas vs. Ion/Plasma Nitriding

Gas nitriding with ammonia results in nitriding of the entire thickness of the product, thanks to the presence of interconnected porosities. In fact, the compound layer (white layer) composed of Fe2 (NC)/Fe4N nitrides is found inside 3- to 5-mm-thick samples. The explanation of this phenomena is simple: Molecules of ammonia do not crack or dissociate completely at the surface but are able to penetrate deeper into the structure since their mean free paths are significant and therefore cause internal nitriding. Through-nitriding of the structure affects fracture toughness of the component, which is well-demonstrated in the presence of cracks in metallographic samples.

To the contrary, ion/plasma nitriding is limited to the surface of low-density PM components. This is due to the fact that the mechanisms of the ion/plasma nitriding process is different than it is in gas nitriding. Ions of nitrogen and nitrogen-hydrogen radicals are accelerated in the electric field toward the cathode (part) and, after collisions with it, are implanted or release free nitrogen atoms reacting with the surface. The mean free path of those species is short, as compared with the ammonia molecules, and that is why their penetration is limited to the surface. It should also be noted that a minor concentration of ammonia is generated by plasma due to the mixture of nitrogen and hydrogen used in the ion/plasma nitriding process and may result in occasional partial nitriding of the open porosities present at the surface. However, this phenomena can be controlled by adjusting physico-chemical parameters of the process.

Examples of gas and ion/plasma nitriding of a low-density PM demonstrate that the ion/plasma nitriding process is superior over the gas process in this specific application. Therefore, the ion/plasma process is recommended for surface hardening of components made of a variety of sintered metal steels. The process can be used as pure ion/plasma nitriding or nitrocarburizing if the specific tribological or contact/bending requirements demand for certain properties to be especially enhanced.  

Learn more about Powder Metals and AHT's capabilities here:

 view AHT's Powder metals webpage

Nitriding of Powder Metals (PM) References

A. Flodin, “Powder Metal through the Process Steps,” Gear Technology, September/October 2018, pp. 50-55.

S. Neilan, R. Warzel, B. Hu and B. Aleksivich. “Examining the influence of tempering parameters on the microstructure and mechanical properties of heat-treated, low-alloy PM steels,” Gear Solutions, October 2018. pp. 31-37.

H. Danninger and M. Dlapka, “Heat Treatment of Sintered Steels-what is different?” HTM J. Heat Treatm. Mat. 73 (2018) 3 pp. 117-130.

H. Ferguson, “Steam Sealing for Nitrogen Treated Ferrous Part,” US Patent no 4,738,730, Apr. 19, 1988.

E. Rolinski,” Plasma Assisted Nitriding and Nitrocarburizing of Steel and other Ferrous Alloys.” Chapter 11 in Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014, pp 413-449.

E. Rolinski, G. Sharp, “When and Why Ion Nitriding/Nitrocarburizing Makes Good Sense,” Industrial Heating, Aug. 2005, pp 67-72.

E. Rolinski, G. Sharp, “Ion Nitriding and Nitrocarburizing of Sintered PM Parts,” Industrial Heating, Oct. 2004, pp 33-35.

E. Rolinski, G. Sharp, K. Brondum and N. Peterson, “P/M Turbo Charger and Armature Components: Plasma Nitriding and Nitrocarburizing to Reduce Friction Wear,” 2005 SAE World Congress Detroit, Michigan, April 11-14, 2005, Paper 2005-01-0722, Ed. SAE International, Warrendale, PA 15096-0001-USA.

Hudis M, (1973), "Study of ion-nitriding," Journal of Applied Physics, 44, 1489-1496.

Skalecki, M.,-G., Klümper-Westkamp, H., Hoffmann, F., Zoch, H., W., Bischoff, S., Rohde, J., "Plasma nitriding potential and a new modeling approach for plasma nitriding process control,” Proceedings of the 28th ASM Heat Treating Society Conference, October 20–22, 2015, Detroit, Michigan, USA, pp 325-328.

E. Rolinski, G. Sharp, K. Brondum and N. Peterson, “P/M Turbo Charger and Armature Components: Plasma Nitriding and Nitrocarburizing to Reduce Friction Wear,” 2005 SAE World Congress Detroit, Michigan, April 11-14, 2005, Paper 2005-01-0722, Ed. SAE International, Warrendale, PA 15096-0001-USA.

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