Case Hardening of Stainless Steel Using Nitrogen

Using nitrogen in the formation of the hard case overcomes the problems associated with case carburizing including depletion of matrix chromium and carbon with accompanying lower hardness and corrosion resistance.

The desired surface content of nitrogen is obtained at a given alloy composition, process temperature and nitrogen pressure, and gradually decreases toward the core.

Case hardening is a thermochemical heat treatment transferring carbon from a carburizing atmosphere into the surface of a low-carbon, low-alloy steel, austenitized at a temperature of, for example, 950 C (1740 F), which upon quenching, leads to a hard martensitic case surrounding a softer core. Without the high load-bearing capacity and the compressive residual stresses of the case, gears in motor vehicles would have a reduced service life, and so would have many other machine parts without case hardening.

There appears to be a need to extend this process to stainless steels, not necessarily to gears, but for example, to tooling used in the polymer and food industries and to many stainless steel components subjected to wear. Respective efforts have, however, shown that one ends up with the precipitation of chromium carbides, depleting the matrix of both chromium and carbon, which impairs the corrosion resistance as well as the hardness of martensite.

Using nitrogen in the formation of the hard case offers a way to overcome this problem. Like carbon, this element is interstitially dissolved in the austenite; that is, in the intersticies of the gamma iron face-centered cubic (FCC) lattice, and upon quenching, increases the hardness of martensite to the same extent as carbon [1]. Work on high-nitrogen stainless steel [2] has revealed that compared with carbon, nitrogen widens the austenite phase field, enhances the solubility of nitrogen and retards the precipitation of chromium nitrides. Therefore, if case hardening of stainless steel (for example, AISI 420, or X20Cr13) is carried out using nitrogen instead of carbon, it is reasonable to expect a higher interstitial solubility and better corrosion resistance. The methodology and results are discussed below.

Fig 1 Phase diagram of stainless steel AISI 420 (X20Cr13) calculated by Thermocalc including isobars of equilibrium N2 pressure. At a temperature of 1100¿C and P(n2) just below 1 bar, for example, a nitrogen content of 0.35% is expected in the steel surface which, together with 0.2% C in the bulk material, is sufficient to produce a surface hardness of about 59 HRC.

The SolNit(r) process

Carburizing is carried out in equilibrium with a carbonaceous gas atmosphere to reach a desired surface carbon content. The same concept is applied to nitriding, which is called solution nitriding (SolNit) to distinguish it from conventional nitriding carried out at temperatures <600 C (1110 F). The nitriding atmosphere consists of pure nitrogen (N2), which is a protective gas below 900 C (<1650 F), but becomes active by thermal dissociation at higher temperatures.

The solution nitriding temperature, T(n), is selected between 1050 and 1150 C (1920 and 2100 F). This increase over the temperature level of conventional case hardening is necessary because of the high chromium content, which retards diffusion. AISI 420 (X20Cr13) stainless steel contains 13% Cr and 0.2% C. After selecting T(n) and the alloy chemical composition, the third process variable, the nitrogen pressure P(n2), controls the process to obtain the desired surface content of nitrogen, which gradually decreases as you proceed from surface toward the core. The case depth increases with T(n) and the solution nitriding duration, t(n), but so does the grain size, just as in conventional case hardening. A nitrogen penetration or case thickness, s(n), of 2.5 mm (0.1 in.) was achieved, but s(n) often is smaller than 1 mm (0.04 in.) to keep t(n) below 4 hours.

As nitrogen is a volatile element, a pN2 on the order of 100 bar would be required to force it into the surface of a low-alloy case-hardening steel, which naturally, nobody wants to engage in. Chromium, however, attracts nitrogen atoms and lowers P(n2), which is further reduced as T(n) is lowered. Therefore, solution nitriding of high-chromium stainless steels may be carried out at 0.2 to 2 bar, a pressure which is easy to handle and to control.

A vacuum furnace (after a few adjustments) is best suited to carry out the process. The near-net shape parts are heated in N2 gas to allow convection. When T(n) is reached, P(n2) is adjusted to give the required nitrogen content in the surface. The adjustment is made on the basis of thermodynamic calculations, an example of which is given in Fig. 1. After t(n), the parts are quenched using N2 gas at a pressure of 5 to 10 bar.

Fig 2 Part of a polymer extruder (a) made of stainless steel AISI 420 (X20Cr13), SolNit-M(r) treated to a surface hardness of 59 HRC (photo courtesy of Gerster AG, Egerkingen); and hardness profile (b).

Application of SolNit-M(r)

A part of a polymer extruder treated using SolNit-M (M = martensitic case) is shown in Fig. 2a. The hardness profile of the tool from the surface toward the core is depicted in Fig. 2b. Due to the strengthening effect of chromium, the hardness of the martensitic core is higher than in a low-alloy, case-hardening steel containing 0.2% carbon. To lower the core hardness, a ferritic-martensitic stainless steel such as AISI 429 (X10Cr13) may be chosen, or you can use a grade having a higher chromium content (Fig. 3). However, an alloy content that is too high results in retained austenite (RA) in the case, which lowers the hardness and may require a cryogenic treatment (deep freezing).

Fig 3 Flanges (a) made of free machining stainless steel X14CrMoS17 having a chemical composition of 0.14% C, 17% Cr, 0.5% Mo and 0.2% S, SolNit-M(r) treated to a surface hardness of 58 to 59 HRC (photo courtesy of Gerster AG, Egerkingen); and hardness profile (b).

Other applications are stainless files, knives, chains, medical instruments and wear parts. With a surface hardness of close to 60 HRC, SolNit-M treated parts are harder than through-hardening stainless steels such as X46Cr13 or the AISI 440 series, for example. In addition, the corrosion resistance is improved by nitrogen.

Fig 4 Impeller (a) and disk (b) castings made of duplex stainless steel G-X 3CrNiMoCuN26-6-3-3 having a chemical composition of greater or less than 0.03% C, 26% Cr, 6% Ni, 3% Mo, 3% Cu and 0.2% N (photos courtesy of KSB, Pegnitz); and mass loss after 1,842 hours service in a recirculation loop of a sewage digestion plant at 37 C (c). The increase of service life of SolNit-A(r) treated parts compared with solution-annealed parts is estimated at 60%.

Extending the treatment

What can be done about austenitic and duplex stainless steels? After solution nitriding and quenching, a high-strength, yet ductile, austenitic case is formed. Instead of calling the process case hardening, we refer to this SolNit-A(r) process as case strengthening.

The austenitic stainless steel AISI 304 (X5CrNi18-10) dissolves about 0.45% N, which more than doubles its yield strength. Because of their higher Cr/Ni ratio, the ferritic-austenitic duplex stainless steels AISI 329 and X2CrNiMoN22-5-3 dissolve 0.8% N, which leads to a fully austenitic case.

SolNit-A increases the resistance of duplex stainless steels to cavitation by about an order of magnitude. Resistance to erosion is improved as well. Therefore, applications could be possible in the field of fluid-flow machinery, such as pumps, turbines, valves, elbows, etc. (Fig. 4). Again, the corrosion resistance is distinctly enhanced by nitrogen (Fig. 5). In thin parts made of sheet or tube, the nitrogen content may be increased throughout the cross section to increase strength and corrosion resistance. For example, diaphragms were cold formed in the soft solution-annealed condition, and then strengthened using the SolNit-A process.

Fig 5 Polarization curves of SolNit-A(r) treated and solution annealed duplex stainless steel having a chemical composition of 0.02% C, 22% Cr, 5% Ni, 3% Mo and 0.2% N, and tested in aqueous 0.2 M H2SO4 + 3% NaCl at 25¿C. Treating using the SolNit-A(r) process reduces the active current density by about one third, and increases the breakthrough potential by about 0.4 V.

Further advantages

The superior properties induced by solution nitriding are not just accidental, but are based on fundamentals, such as for example, the increase of free electrons in the lattice caused by the dissolution of nitrogen, which leads to ordering, strengthens the metallic character of interatomic bonding and stabilizes the austenite [2]. Recent studies [3, 4] show that the concentration of free electrons is highest if the stainless steel contains both, carbon and nitrogen. This is used within the case of AISI 420 (X20Cr13) where 0.2% carbon in the steel is met with 0.3% nitrogen introduced by solution nitriding. As a result, the austenitic phase field is widened and the interstitial C+N content is readily dissolved without promoting grain boundary precipitates. Unfortunately, this beneficial C+N effect cannot be extended to austenitic steels, because usually they contain little carbon.

The thermodynamics of SolNit are given in [5] as well as some kinetic data. This information assists in the selection of suitable stainless steels [6]. The resistance to wear and corrosion is discussed in [7]. The treatment is further defined in patents [8]. Based on these and other references, the user may appreciate that the new heat treatment process is well understood.

Advantages of the process in addition to those mentioned above include:

• In comparison to CO, CH4, NH3 gases of other thermochemical heat treatments, N2 is neither toxic nor explosive.
  
• There is no inner oxidation along grain boundaries as in conventional case hardening and the parts stay rather bright.
  
• An oxygen cell is superfluous as the nitriding equilibrium is governed by pN2.
  
• The thermal dissociation of N2 allows for recombination N2 2N, which keeps the atmosphere fresh and active. Therefore, no gas flux is required, which also helps to save energy.
  
• Narrow gaps and blind holes are evenly solution nitrided, and the furnace may be loaded as densely as allowed by the quenching requirements. Gas quenching reduces distortion.
  
• Because stainless steel parts have to be hardened or solution annealed anyway, only the cost of additional furnace time may be allotted to solution nitriding. Therefore, thin parts having a shallow case depth s(n), after a duration t(n) in the one-hour range, are especially cost effective.
  
  

At this stage, the SolNit process is offered by Ipsen International (Kleve) and Gerster AG (Egerkingen).

References

• Mittemeijer, E.J., Tempering of Iron-Nitrogen Martensite, Z. Metallkd., 74, 7, p 473-483, 1983
  
• Gavriljuk, V.G., Berns, H., High Nitrogen Steels, Springer Verlag, Berlin-New York, 1999
  
• Gavriljuk, V.G., Berns, H., Precipitates in Tempered Stainless Martensitic Steels Alloyed with Nitrogen, Carbon or Both, in: High Nitrogen Steels '98, Eds. H.H¿inen, S.Hertzman, J.Romu, Proc. of 5th Intl.Conf. on High Nitrogen Steels, Espoo, Finland, Stockholm, Sweden, Trans. Tech. Publications Ltd., Switzerland-Germany-UK-USA, p. 71-80, 1999
  
• Shanina, Bela D., Gavriljuk, V.G., Berns, H. and Schmalt, F., Concept of a new high-strength austenitic stainless steel, Steel Research, 73, 3, p 105-113, 2002
  
• Berns, H., Juse, R.L., Bouwman, J.W. and Edenhofer, B., Solution Nitriding of Stainless Steels - A New Thermochemical Heat Treatment Process, Heat Treatment of Metals, 2, p 39-45, 2000
  
• Berns, H., Stainless steels suited for solution nitriding, Mat.-wiss. u. Werkstofftech, 33, p 5-11, 2002
  
• Berns, H., et al., Solution Nitriding of Stainless Steels for Process Engineering, Mat.-wiss. u. Werkstofftech. 31, p 152-161, 2000
  
• Patents DE 40 33 706 and DE 43 33 917