Oxygen-free, nitrogen-hydrocarbon heat-treating atmospheres can offer improved part quality and cost benefits compared to conventional endothermic atmosphere and vacuum processes.

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Fig. 1. Weight gain of iron coupons exposed to different N2- hydrocarbon blends, a TGA study

Oxygen-free, N 2-HC heat-treating atmospheres have been a topic of industrial interest for over a quarter century for a combination of reasons. These include: significantly reduced toxicity, explosivity and environmental impact; flexibility in selecting flow rates and executing on-demand start-ups/shutdowns; low capital cost of the atmosphere supply system; elimination of process control and maintenance issues associated with endo/exo-generators; and most importantly, improvements in product quality due to elimination of intergranular-oxidation defects formed under carbon-monoxide atmospheres in steels containing reactive alloying additions such as Mn, Cr, Si, Al or V.

The main impediment limiting a broader use of N2-HC atmospheres – namely, poor dissociation kinetics of hydrocarbons – has been addressed in the vacuum-furnace heat-treating area by using the least stable, yet expensive compounds: acetylene, propylene, propane or cyclohexane. Air Products recently explored the feasibility of using low-cost N 2-CH4atmospheres in applications including carburizing, neutral-carbon annealing, carbonitriding and nitrocarburizing of alloy and stainless steels by activating the gas stream entering simple, 1-atm-pressure treatment furnace with so-called cold-plasma discharge. Work results confirmed expectations that the new method offers quality and cost benefits in a range of treatments. This article highlights the development of the activated carburizing technology.

Fig. 2a. Electric discharge activation (gold plasma) non-thermal, glow-to-arc discharge mode

Background

The early studies of Kaspersma and others[1-3] of N2-HC blends demonstrated that due to thermochemical stability acceptable rates are obtained only by using complex N2-H2-HC atmospheres involving hydrocarbons heavier than methane and at temperatures higher than in traditional carburizing. Similar findings in the field of vacuum carburizing resulted in a gradual replacement of CH4with acetylene, propane-hydrogen or ethylene blends.[4-5] Our own thermogravimetric (TGA) data (Fig. 1) shows that the lower the temperature, the more indispensible the use of expensive acetylene.

An important advantage of hydrocarbon atmospheres over the endothermic and methanol atmospheres is the absence of intergranual oxidation (IGO) of Mn, Si and, in the case of alloyed or microalloyed steels, Cr, Al, Ti or Zr.[8-11] Since IGO reduces fatigue strength, steel parts processed under CO-rich atmospheres require longer carburizing cycles, deeper cases and more stock to be ground off in the subsequent machining. As illustrated in Fig. 2, an electric discharge activation of inexpensive CH4enables carburizing and eliminates surface defects.

Fig. 2b. Electric discharge activation - activated carburizing

Cold Plasma Carburizing System

As described elsewhere,[12] when operating in the non-thermal plasma mode (Fig. 2a), electric-discharge injectors comprise high-voltage DC or AC electrodes positioned across the stream of gas directed into the furnace. As illustrated in Fig. 2b, the product includes ions, radicals, atoms and excited molecules, e.g., C2H2, C3H6, C2H4, CH3, CH2, C3H8, H2, H and N[14-17] readily carburizing metal. The injectors can be retrofitted to various types of furnaces, and the carburizing is controlled as in a vacuum furnace using the CHTE-WPI model (Fig. 3).[18]

Fig. 3. Carbon transport in activated atmosphere carburrizing

Experiments and Results

Carburizing experiments were run in a semi-production scale, electrically heated box furnace. In the first tests, AISI-1010 shim stock coupons were used to measure carbon flux and H2released from an activated CH4feed (Fig. 4). In further tests, AISI-8620 production parts, rings (R) and shafts (S) were plasma carburized, oil quenched and tempered to compare them to the endothermically carburized, conventionally processed parts. AC and DC plasma power modes were tested with the AISI-8620 treatments, but only AC was used for the shim treatments. A laser gas analyzer was used to monitor composition of gases at the inlet and outlet of the furnace.

Just like in low-pressure (vacuum) carburizing and, to a large extent, endo- or methanol-atmosphere carburizing, carbon mass flux is the most reliable measure of the plasma-activated carburizing process, enabling the operator to compare various conditions and predict outcomes. Results showed that the plasma-activated conditions T2-T4 produced at least as high a flux as the endo-atmosphere carburizing evaluated by Linde Gas[20] and the vacuum-furnace carburizing with propane.[21]

The non-activated, purely thermal condition T1 resulted in an unacceptably low carbon flux, even though its carbon activity, ac, and apparent carbon potential, Cp*, values were high. Carbon activity in the gas phase was evaluated per a procedure[19] using measurements of H2and CH4concentrations in the flue stream for the carburizing reaction of CH4= C + 2H2, ignoring activation effects. Outokumpu HSC Chemistry® was used for activity calculations. Apparent carbon potential of N2-CH4atmospheres was evaluated from eq. 5 in Fig. 3. In the plasma runs T2-T4, the carbon flux scaled more with the electric energy absorbed by incoming gas, Eactiv, (eq. 4 in Fig. 3) and furnace temperature than with the degree of thermal dissociation indicated by H2concentration. This indicates that formation of active CmHngroups plays a role in plasma carburizing.

Fig. 4 Carburizing tests on AISI 1010 shim stock, 0.015 inches thick

R-ring and S-shaft AISI-8620 were carburized, oil quenched and tempered with the AC and DC cold-plasma system. For the boosting and diffusing steps, the flue stream analysis was as follows: CH4≤4.4 vol%, H2<1.0 vol%, CO2<0.1 vol%, CO<0.05 vol%, H2O<50 vppm (dew point below -48oC = -54oF) and O4below our detection limit of 50 vppm. Oxygen adsorbed on furnace walls, oxide films and air leakage was the likely O2source. The low level of the gases required for the endo carburizing provided no support for the conventional acand Cpcalculations proposed first by Collin et al.[22] and indicated that the primary carburizing reaction, CmHn= m C + n/2 H2, was operational. The H2release scaled with the furnace temperature and plasma input throughout the treatment cycle. Microhardness profiling of parts R and S is shown in Fig. 5.

The DC and AC plasma-carburized parts displayed higher hardness (higher carbon) going deeper into the material. This type of hardness profile is valuable, especially in the case of parts requiring post-machining of the surface to restore dimensional accuracy. Another investigation indicated that the effective case depth (ECD) for 50 HRC (converted from Vickers) in the plasma-carburized parts was 0.700 mm (0.0276 inches), i.e. somewhat larger than in the comparable endo-atmosphere carburized parts.[23]

Cross-sectional secondary electron images (SEI) of endo- and plasma-carburized parts S, and elemental maps of Mn, Cr and Si were acquired using SEM energy-dispersive X-ray spectroscopy (EDS), Fig. 6. The endo-carburized part revealed a clear, intergranular oxidation to the depth that agrees with the diffusional calculations and experiments presented by Chatterjee-Fisher.[8] The enrichment of oxidized grain boundaries in Mn, Cr and Si was explained by the higher affinity of those alloying elements for oxygen and oxygen-containing gases (CO, CO2and H2O) always present in endothermic and methanol atmospheres.[11] In contrast, no IGO effect was observed in the AC and DC plasma-carburized samples exhibiting a vacuum-furnace quality of surface and subsurface material.

Fig. 5. Microhardness profiles for endo-atmosphere, DC-plasma and AC-plasma activated N2-CH4 atmosphere carburized 8620 steel parts (oil quenched and tempered)

Conclusion

AC and DC electric-discharge (cold-plasma) gas injection systems were explored in carburizing of steels at 1-atm pressure using N2-CH4blends. Easy to install in many types of heat-treating furnaces, these new cold-plasma systems can minimize atmosphere toxicity and environmental impact while offering an instant turn-on/turn-off capability.

Evaluations of carburizing showed that metal surface reaction with excited hydrocarbons dominated the activated process. Carbon flux, potential and activity in gas phase indicated that the rates were comparable to those of low-pressure (vacuum) carburizing systems. The carburizing process control is mass-flux-based, i.e. all low-pressure control models are applicable.

Fig. 6. Intergranular oxidation of AISI 8620 steel parts carburized in an IQ furnace using endo-atmosphere in a box furnace using activated AC-plasma (N2-CH4) stream (2% nital)

Carburizing effects were compared for AISI-8620 steel rings and shafts processed with the plasma-activated N2-4.5 vol% CH4 blends and the conventional endothermic atmosphere using the same heat-treatment schedule. Plasma-processed parts were IGO-free and revealed a somewhat deeper effective carburized depth. The microhardness profile directly under the metal surface was relatively flat, which is beneficial from the post-machining and fatigue-strength perspective. The endo-carburizing resulted in predictable IGO defects.

Given the advantages observed with this cold-plasma-activated atmosphere technology, carburizers using endothermic atmosphere or those considering vacuum furnaces for carburizing can realize a range of benefits that eliminate the issues with endothermic atmospheres or the expense of vacuum furnaces.IH

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Acknowledgments
The authors would like to thank Messrs. J. Conybear and D.J. Bowe for valuable industrial suggestions, Dr. S.P. Gangoli for plasma technology contributions and Prof. R.D. Sisson for access to CarbTool diffusion software.

Extracted from article first presented at ASM Heat Treating Society’s 25th Conference and Exposition. Reprinted with permission of ASM International. All rights reserved.