PRINCIPLES of GAS NITRIDING Nitriding Involves Enriching He World Is Looking for Better There Are Two Common Methods of Products Produced at Lower Gas Nitriding

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PRINCIPLES OF GAS NITRIDING Nitriding involves enriching he world is looking for better There are two common methods of products produced at lower gas nitriding. The first method is the the surface of ferrous parts costs. Nitriding has been and so called single stage nitriding. In this with nitrogen via gas, salt continues to be one of the two method, a work piece is heated to be- T main diffusion processes in the tween 925 to 975ºF (495 to 525ºC) in an bath, and plasma nitriding heat treating industry (carburizing is atmosphere which has an ammonia the other). To meet the more grueling dissociation rate of between 15 and methods, resulting in a part environment that designs demand, ni- 30%. The resulting work piece contains having increased surface triding practices and requirements a series of phases of progressively de- have and continue to evolve. creasing nitrogen content from the sur- hardness, wear and Nitriding involves enriching a fer- face to the core. At a temperature corrosion resistance, and rous part’s surface with nitrogen and above 850ºF (450ºC), nitrogen intersti- results in a part with increased surface tially dissolves into a part until the con- fatigue strength. hardness, wear and corrosion resist- centration reaches 0.1%. Then, gamma ance, and fatigue strength. Nitriding prime ( ’) nitride (Fe4N) begins to form Patrick Weymer* has traditionally been accomplished at grain boundaries with a nitrogen using gas nitriding, salt bath nitriding, concentration of between 5.7 to 6.1%. BeaverMatic Inc. and plasma nitriding. The focus of this When the nitrogen concentration ex- Rockford, Ill. article is gas nitriding, which is com- ceeds 6%, a compact layer of ’ nitride monly accomplished in a retort furnace or compound layer begins to form on in an ammonia, dissociated ammonia, the parts surface. As the process con- hydrogen, and nitrogen atmosphere. tinues, new nitrides form in a diffusion zone below the compound or white Nitriding Process Description layer and towards a parts core. In nitriding, atomic nitrogen forms The compound layer causes the re- via a catalytic decomposition of am- sulting part to be brittle and could spall monia (NH3) on the metal surface of a in service. For these reasons, it was in work piece. This decomposition obeys some cases desirable to minimize the following reaction: white layer thickness. This led to the development of the double stage or NH3 —> [N] + 3/2H2 Floe process (U.S. Patent No.2,437,249). The first stage of the Floe process con- The rate that nitrogen diffuses into a sists of heating a work piece to 925 to part is dependent upon the processing 975ºF (495 to 525ºC) and the ammonia *Member of ASM International and member, temperature, the parts alloy content, and dissociation rate is controlled to be- ASM Heat Treating Society amount of nitrogen that is available. tween 25 to 25%. During the second stage, the temperature can be main- tained or raised to between 1,025 and 1,050ºF (550 and 565ºC). In addition, the dissociation rate is raised to between 75 and 80% by reducing the ammonia flow rate and diluting with nitrogen, dissociated ammonia or hydrogen. Keep in mind that raising the temperature during the second stage, lowers the resulting case hardness, increases the case depth, and may lower the core hardness and effective case depth. It is important to note that a work piece must be austenitized, hardened and tempered before beginning nitriding. Tempering is usually per- formed at 30ºF (16ºC) above the nitriding temperature to provide a stable BeaverMatic nitriding furnace installation. core structure during nitriding. In 12 HEAT TREATING PROGRESS • JULY/AUGUST 2009 some materials, the case hardness re- %[N] duces as the core hardness reduces. 10 2 0.5 Fully Automated Nitriding Furnaces 0.0 1 As is the case with all processing, it 5.5

2 6.0 / 3

is desirable to minimize the cycle time. ) 3 ) 2 2 7.5 0

1 H ( In nitriding, this can be accomplished H p p / / 3 2 by increasing the processing tempera- ) 3 H ’ N ture and ammonia flowrate. However, -11 N p H p ( = p

in nitriding, higher processing temper- p n g l atures and higher ammonia flowrates K 0.1 -22 lead to an increase in the growth rate of the white layer. In some cases, it can -33 be acceptable to have white layer on a resulting part. Since the white layer can 0.01 -44 be very hard and brittle, it may be de- 450 500 550 600 650 700 sirable to minimize or completely elim- Temperature, °C inate it. This can be accomplished by controlling the nitriding potential Lehrer diagram for pure iron. during the nitriding process. Nitrided 31CrMoV9V In contrast to the carburizing gas, ring gears. the nitriding atmosphere is not in equi- librium since the ammonia flow rate is too high to allow it to fully dissociate to nitrogen and hydrogen during the process. For this reason, the furnaces exhaust gas consists of ammonia, nitrogen, and hydrogen. It is customary Microstructure of to use a burette to determine the per- nitrided 31CrMoV9V centage of ammonia dissociation. Am- ring gears showing monia is the only constituent that is white layer. soluble in water. A graduated burette filled with water can be used to measure the ammonia dissociation rate of the furnace exhaust gas. Since this method is not continuous and is manual, it introduces operator induced variability, which makes it difficult to nology. Integration of these devices repeat the process. along with smart flowmeters into con- For this reason, work began on a trol systems with recipe management new control parameter, nitriding po- systems and programmable logic con- tential as a means to reduce the white trollers now enables a fully automated layer thickness or eliminate the white process and the results have shown to layer altogether. Nitriding potential, be repeatable. Bibliography Kn, is based upon the partial pressure Now that nitriding potential-control • ASM Handbook: Heat Treating, Vol 4, p of the ammonia still present in a fur- systems are more common and are 263, 387-388, 1991. nace (the amount of ammonia that has proven, there have been specifications • D. Pye, A Gas Nitriding Review, Indus- not yet dissociated) and the partial written that require nitriding potential trial Heating Online, Dec. 23, 2008. • T. Lotze, Gas Nitriding, Application Bul- pressure of hydrogen (H2) that has al- control in critical applications where no white layer or a specific amount of letin, Sept. 9, 2003. ready dissociated from ammonia. Kn • E. Lehrer, Z Electrochem, Vol. 36, p 383 - is calculated as follows: white layer is required. This develop- 392, 1930. ment has led to heat treating specifica- • SAE International Aerospace Material 3/2 Kn = pNH3/pH2 tions like SAE International AMS Specification: AMS 2759/10, May 1999. 2759/10 to require control to specific • P. Weymer, Close Process Control Yields If we take Kn as a value for nitride nitriding potential values. Nitriding Consistency, Repeatability, Heat forming, we obtain the diagram de- Treating Progress, March/April, 2008. veloped by Lehrer. Although the Conclusion • T. Bell, et.al., Controlled Nitriding in Am- Lehrer diagram only applies to pure Equipment developments like sen- monia-Hydrogen Mixtures. Heat Treat- sors, computer control systems, and ment '73, The Metals Society, Book 163, p iron, it can be used as a good approx- 51-58, 1975. imation for steels that are not too flow control devices have been highly alloyed. proven invaluable for nitriding. These For more information: Patrick Weymer, Means of collecting the required developments have increased the ac- BeaverMatic Inc., 424 Buckbee St., data and calculating the nitriding po- curacy of the resulting work pieces Rockford, IL 61104; email: Patrick. tential have been introduced and im- and the ease of reproducing the final [email protected]; Web site: plemented into current furnace tech- results. HTP www.beavermatic.com HEAT TREATING PROGRESS • JULY/AUGUST 2009 13