The Doctor has always been fascinated with anything involving case hardening. Gas nitriding is no exception; a seemingly old and well-established technology that changes so little that it begs the question, “What’s new?” Well, as it turns out, quite a bit. Let’s learn more.
Gas nitriding of iron and steel is a thermochemical process dating back well over 100 years. Dr. Adolph Machlet (U.S.) of the American Gas Company of Elizabeth, N.J., in 1913 patented the first gas nitriding process utilizing an atmosphere of ammonia. He also patented, in 1914, a process for nitrocarburizing using an ammonia and hydrocarbon gas mixture. This was followed in 1924 by a patented process developed by Dr. Adolph Fry (Germany) of Krupps Steel Company, who also employed an atmosphere of pure ammonia. Fry’s investigative work on the nitriding process took place at roughly the same time as Machlet’s but was interrupted by World War I. Fry’s work also led to the development and patenting of steels specifically engineered for nitriding – Nitralloy® steels containing aluminum and chromium.
These single-stage nitriding processes were carried out at maximum nitrogen potential and resulted in brittle cases with high porosity as well as networks of iron carbonitrides at the grain boundaries. For most component parts, this less-than-optimal microstructure required removal of the “white layer” produced, typically by grinding.
Starting in 1922, work was under way at the U.S. Bureau of Standards on nitriding specimens with the goal of understanding the relationship between iron and nitrogen. This work was supported by C. B. Sawyer and Fry, who were both actively working on this subject around 1923 and offered versions of an iron-nitrogen phase diagram. These efforts culminated in the establishment of an updated iron-nitrogen phase equilibrium diagram in 1929.
In the 1930s, E. Lehrer (Germany) introduced, for the iron-nitrogen (Fe-N) system, a diagram relating temperature and nitrogen potential. This allowed one to accurately determine the phase boundaries in the iron-nitrogen (temperature-composition) system. The Lehrer diagram shows the solubility of nitrogen in ferrite as a function of the nitriding potential. To this end, thin iron foils were homogeneously nitrided in flowing gas mixtures comprised of ammonia and hydrogen. This allowed the nitriding process and resultant case microstructure to be better understood.
Work was then undertaken to reduce the nitriding potential and the problematic white layer and culminated in 1943 with a process patented by Carl Floe (U.S.) using a two-stage nitriding process in an atmosphere of ammonia and dissociated ammonia. The Floe (pronounced “flow”) process created a nitrogen-rich layer (in the first stage), then diffused this layer (in the second stage) to produce a more optimal case microstructure and reduce the need for post-heat-treatment grinding. This process helped more accurately develop a targeted case depth and microstructure.
Single-stage and two-stage gas nitriding, as originally developed, is still in use today. By the 1990s, however, Leszek Małdziński (Poland) modified the Lehrer diagram to incorporate iso-concentration curves after analysis of the results of experiments also involving thin iron foils (Fig. 1).
In simplest terms, these curves represent constant nitrogen concentration in iron (both on the surface and at the interphase boundary) as a function of temperature and nitriding potential. This overcame a shortcoming in the Lehrer diagram, which predicts the phase structure of the nitrided layer but does not provide information about the concentration of nitrogen (in α, γ′ and ε phases) as a function of temperature and nitriding potential. This is the basis for understanding the kinetics of nitrided layer growth.
Thus, not only can we determine the nitriding potential at any given temperature, but we can also predict the microstructural phases that will be produced, paving the way for practical process simulators. Hence, modern gas nitriding was born, and all-new equipment, processes and controls in use today are based on this diagram.
Putting Theory into Practice
Nitriding is a gaseous diffusion process in which nitrogen is absorbed into the surface of the steel (Fig. 2). The process begins at the surface of the part with the formation of a compound (aka white) layer, which depends to a large extent on the composition of the steel (and, in particular, the carbon content).
The phases formed within this surface layer are the epsilon (ε) phase and the gamma-prime (γ′) phase. Each has attributes that enhance the mechanical properties of the case and can be varied in percentage as a function of temperature and gas chemistry. The thickness of the compound layer is a function of material (plain-carbon steel forming a thicker compound layer than alloy steels), temperature, time and the process-gas composition.
Underneath the compound layer is the diffusion layer, where the diffusion of nitrogen away from the surface results in the formation of nitrides and, in some instances, carbonitrides. In general, the compound zone will have high hardness, good wear resistance, improved corrosion and other tribological properties. The diffusion zone improves fatigue resistance and other mechanical properties.
The parameter that defines the rate of dissociation of ammonia is the nitriding potential (KN). In practice, the nitriding potential is selected so as to produce the required case depth and microstructure. The higher the nitriding potential, the higher the nitrogen concentration will be both at the surface and in a particular phase (α, γ′ or ε). Thus, in order to maintain a given nitriding potential, one must increase the rate of the flow of ammonia. Clearly, the higher the nitriding potential needed, the greater the flowrate of ammonia. The same is true of temperature; as it increases so too does the ammonia flowrate.
Today, advanced nitriding processes control the ammonia flowrate into the furnace retort at a given temperature and, as a result, are capable of producing single (α), dual (γ′+α) or three-phase (ε+γ′+α) compound layers.
Some practical lessons have been offered to the industry and may be documented as follows:
- Increasing the temperature will increase the case depth and increase the white layer, provided an atmosphere allowing for formation of a white layer is not controlled.
- Set the nitriding potential to match the desired phase on the parts’ surface. Carbon will shift the boundary to the epsilon phase to lower nitriding potentials; increasing amounts of nitride-building elements will shift the boundary to higher nitriding potentials.
- Nitride-building elements have a high impact on the nitrogen flux needed to saturate the structure. Therefore, diluting the nitriding atmosphere with nitrogen or treating the part at low pressures will stop a proper nitriding of high-alloy steels earlier compared to low-alloyed or carbon steels.
- Increasing the furnace pressure will increase the growth of the white layer, but this effect will slow down by increasing the nitride-building alloying elements.
Gas nitriding has returned to its roots, requiring only ammonia as a process gas in systems specifically designed to vary or stop the gas flow to achieve the desired metallurgical microstructure and hardness. Flowrates are minimized and controlled by process simulators and simple sensors. Emissions from these systems are extremely low as well, so no longer can we tell if nitriding is being done in the heat-treat shop by the odor of ammonia in the air.
- Steel Heat Treatment Handbook, George E. Totten and Maurice A, H. Howes (Eds.), Marcel Dekker, Inc., 1997
- Patent No 85924, “Method of Gas Nitriding,” Poland, 11.06.1977
- “ASM Handbook, Heat Treating, Volume 4, ASM International, 1991
- Zys´k J., “Rozwój azotowania gazowego stopów z·elaza” (in Polish), Institute of Precision Mechanics, Warsaw, 2008
- Epstein, S., et. al, “Observations on the Iron-Nitrogen System,” Bureau of Standards Journal of Research, 1929
- “An Introduction to Nitriding,” Practical Nitriding and Nitrocarburizing, ASM International, 2003
- Hofman, Agnieszka et. al, “Consumption of Ammonia in the Zero Flow® Regulated Gas Nitriding and the Processes Used to Date,” Conference Proceedings, SECO/WARWICK 19th Heat Treatment Technical Seminar on New Trends in Heat Treatment, 2016
- Winter, Karl-Michael, “Gaseous Nitriding: In Theory and In Real Life,” United Process Controls white paper, 2009