The Importance of Temperature Control in Plasma Nitriding

posted On Monday, February 20, 2023 in Blog

This article was originally published January 2023 by Industrial Heating. View the original article here.

Plasma nitriding, also called ion- or glow-discharge nitriding, is an advanced surface engineering technique that allows for treatment of many different engineering components made of ferrous and titanium alloys [1-2]. The process is quite complex and its control requires an understanding of the phenomena occurring at the interface of the glow discharge and the surface of the cathode/nitrided object [1-6]. The knowledge of heat exchange in the vacuum vessel is needed [6]. Many large parts made of cast iron s are nitrided at Advanced Heat Treat Corp. (AHT), (Fig. 1 & 2).

Figure 1 Importance of Temperature Control in Plasma Nitriding

Figure 1

Stamping dies during plasma nitriding. The bar in the center of the picture is the central anode.

All ferrous alloys, including stainless steels can be nitrided without any special preparation/activation steps necessary with other nitriding methods. In plasma nitriding, simple mechanical masking of certain surfaces can be used. This might be necessary when parts with threaded holes and other areas need to stay soft after the treatment. This a great advantage of the method as compared to gas nitriding. Plasma nitriding is the low-nitriding potential method, allowing a thin compound zone of nitrides to be produced without sophisticated control methods [1].

Good and full control of the process requires proper control of the electrical plasma parameters involving frequency and voltage, but the temperature control of individual parts is a fundamental issue that most affects the final outcome.

Figure 2 Importance of Temperature Control in Plasma Nitriding

Figure 2

Plasma nitriding of the gas compressor cylinders. Note thread-masking bolts. The The bar at the center is the central anode.

Temperature Control in Plasma Nitriding

Although control of plasma nitriding can be quite challenging, this article will concentrate only on temperature errors. Nitriding's kinetics rate is dependent on temperature and is exponentially affected by it (Fig. 3) [1].

An illustration of a practical effect of an error in temperature control during nitriding is shown in Fig. 4. The graphs illustrate how important temperature is in forming total case depth in the example of nitriding steel Nit135M. An error of 50°F (28°C) in temperature measurement may require an additional 20 hours of processing to achieve the desired total case depth of 0.020 inch (0.5mm). Errors like that could be caused by factors primarily related to the emittance effects and thermocouple problems.

Figure 3 Importance of Temperature Control of Plasma Nitriding

Figure 3

Effect of temperature on the case-depth formation for 42CrMo4 (4140) steel.

Figure 4 Importance of Temperature Control of Plasma Nitriding

Figure 4

Kinetic rates of nitriding for Nit135M steel at two different temperatures. The graphs were made based on Stange Elektronik software.

Emittance of the object changes during plasma nitriding (Fig. 5.). This leads to a drop of its temperature if the thermocouple is located tin the old thermocouple controlling block. When the controlling thermocouple is placed in a previously nitrided, geometrically identical object as the treated parts, this will result in the temperature of the fresh parts being higher than the high-emittance object and higher than needed (Fig. 6) [4-6].

Figure 5 Importance of Temperature Control of Plasma Nitriding

Figure 5

Changes of the emittance and temperature of the fresh steel object during plasma nitriding. The thermocouple controlling the load was located in the high-emittance, nitrided block of steel. Picture adopted from E. Rolinski and others [6].

There are many ways of locating thermocouples in the objects controlling the load. One way, used for over 40 years in cold-wall vessels, is protecting the wire from the high-voltage plasma by locating it in the ceramic tubes inserted into the treated part or a thermocouple block [7]. Unfortunately, this method is sensitive to the insertion depth, and the real temperature of the object is higher than indicated by the thermocouple (Fig. 7).

Figure 6 Importance of Temperature Control of Plasma Nitriding

Figure 6

Temperature profile of the low (0.23) and high (0.55) emittance blocks of steel during plasma nitriding in a mixture of 30% N2 and 70% H2 and final pressure of 2.5 mbar (1.9 torr).

Thermocouple wires, which are at high voltage and under glow discharge used in modern hot-wall vessels, are not so sensitive to the insertion depth. Their accuracy is much greater than the accuracy of others. Nevertheless, the effect of the parts' emittance in those systems versus the emittance of the parts/blocks with a thermocouple in it still has a very important meaning [4].

Figure 7 Importance of Temperature Control in Plasma Nitriding

Figure 7

Temperature errors caused by the insertion depth of the thermocouples with ceramics (per Klockner Ionon Gmbh).

Plasma nitriding is an excellent method in treating parts made of a variety of different ferrous alloys. However, it is very sensitive to many control factors that need to be properly understood before the method is applied.

Want to learn more about temperature control in ion / plasma nitriding? View other nitriding articles here.

References

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, “Nitriding of Titanium Alloys”, ASM Handbook, ASM International Vol. 4E, Heat Treating of Nonferrous Alloys, Volume Editor, G. E. Totten, 2016, pp.604-621.
E. Rolinski, G. Sharp, “Controlling Plasma Nitriding”, ASTM International, Materials Performance and Characterization, Vol. 6, No 4, 2017, pp.698-716, https://doi.org/10.1520/ MPC20160051. ISSN 2370-1365.
E. Rolinski, J. Machcinski and G. Sharp, “Heating in Plasma Nitriding Furnaces”, Industrial Heating, Jan, 2014, pp 27-29.
E. Rolinski, J. Machcinski, T. Larrick and G. Sharp, “Effect of Cathode Workpiece Surface Emissivity on Temperature Uniformity in Plasma Nitriding”, Surface Engineering, 2004, Vol. 20, No 6, pp 426-429.
E. Rolinski and J. Machcinski, “Heating in Plasma Nitriding: The Effect of the Workpiece Emittance”, in Encyclopedia of Iron, Steel, and Their Alloys”. DOI: 10.1081/E-EISA-120052080, 2015, Ed. By Taylor & Francis. pp 2580-2590.
W. Oppel, Internal Technical Report, Klockner Ionon GmbH, Cologne, 1977.