There have been more and more studies around the world (Japan, Germany, France, etc.) concerning carbonitriding to a case with a high amount of nitrogen characterized by adding ammonia to the process gas at a higher rate than the usual 3-5%.[3,4,8,9] Most of this work focuses on the impact of nitrogen on the lifetime of carburized parts exposed to Hertzian pressure or fatigue such as gears, sprockets, bearings, crankshafts, etc.
The emergence of the downsizing concept is aimed at reducing the mass of an engine while maintaining performance. Fuel consumption is reduced while engine power is increased in various industrial sectors (automotive, aerospace, etc.). This has the effect of:
- Reducing the contact surfaces
- Increasing contact pressures and forces applied
- Increasing torque transmission
Having a look at the first two points, we can observe that in working condition the contact temperature will increase for Hertzian fatigue (Fig. 1.)
From a metallurgical viewpoint, by reflecting on where metallurgy and mechanics are closely related, if one refers to figure 1 we can observe that between 2.5 and 3 GPa there are several substantial changes.
- A change in the metallurgical structure
- A reduction in hardness obtained after heat treatment if the working temperature is higher than the tempering temperature (better to stay 122°F/50°C below tempering temperature if parts are exposed to mechanical stresses for a long time, an effect described by Jaffe & Hollomon)
- A significant reduction in residual stresses caused by thermo-kinetic exposure associated with hardening and phase transformations
We quickly understood that these mechanical and metallurgical changes will have the effect of reducing the fatigue life of the stressed parts (reduction of local strength and residual compressive stresses in the surface layer). To overcome this effect, which is similar to softening while tempering, nitrogen has been identified as an interesting research subject for many years. Nitrogen allows tightening of the Fe-C alloys in different ways (e.g., participation in the martensitic transformation, increasing hardness by solid solution and precipitation of hard phases). One can find this effect directly in some stainless steels of the “high-nitrogen steels” family or by performing nitrogen-inducing thermochemical treatments such as nitriding and carbonitriding.
In this context, we present mechanical and metallurgical “high-nitrogen” (HN) carbonitriding interests while having a closer look at its control in industrial high-volume production.
Consequently, this article will:
- Demonstrate the benefits of carbonitrided layers rich in nitrogen
- Explain the driving technique of the atmosphere of nitrogen-rich production carbonitriding (HN system)
- Promote this new family of thermochemical treatments (whether atmospheric or at low pressure) in our various business sectors, considering the benefits it can bring to our products
Successful Examples of Nitrogen-Rich Carbonitriding
In this section, we discuss the effect of nitrogen on the fatigue behavior of mechanical components and present different examples. One is contact fatigue on carburizing steel, carbonitrided and shot peened; the second is the fatigue of low-pressure carbonitrided gears; and the last is nitrogen-enriched bearings made from 100Cr6 steel.
Contact Fatigue of Carburizing Steel, Carbonitrided and Shot Peened
The team of Professor Y. Watanabe worked with Nissan on the development of HN carbonitriding associated with a shot-peening operation on case-hardening steels (CrMo). Contact-fatigue tests were made on hardened (%C=0.7) and carbonitrided (%C= 0.8%, %N=1.1) specimens, which were all blasted with cast-iron balls (hardness: 700Hv/recovery rate >300%).
The results show a strong advantage for carbonitrided and shot-peened specimens (Fig. 2).
Fatigue on Low-Pressure-Carburized Gears
The ECM group with automaker PSA performed low-pressure carbonitriding and gas quenching of gears between 356-392°F (180-200°C). They observed a 34% life increase in fatigue resistance of gears (PSA tests) comparing carbonitrided parts to parts carburized and hardened in hot oil. This process has been identified as very promising for the future development of high-torque transmissions.
Nitrogen-Enriched Surface on 100Cr6 Bearings
A German team showed interest in imparting a flash of nitrogen while austenitizing bearings made from 100Cr6 (using nitrogen/methanol carrier gas adding ammonia). Figure 3 summarizes the results obtained on tracks with and without a Brinell indentation mimicking pre-damaged raceways.
The difference in life is largely in favor of nitrogen-containing cases. In addition, any spalling observed on carbonitrided raceways is very small in size compared to the carburized samples (Fig. 4).
Major Metallurgical Structure Changes for Nitrogen-Rich Carbonitriding
In the thesis of Ms. Loukachenko, we can see an example of carbon and nitrogen profiles (Fig. 5) obtained by the thermal treatment developed by Watanabe, injecting 8% ammonia by volume.
The micrographic analysis performed at this time shows us that the quenched metallurgical structure is composed of martensite, retained austenite and CrN.
Given the amount of nitrogen introduced (>0.7% by mass in the surface and 0.1% by mass in a depth of 275 µm) in samples of 27MnCr5 and 27CrMo4, the structural information can be completed as follows:
- The hardenability of the enriched layer is higher.
- Retained austenite, obtained “in larger quantities” (not systematic ~ 15%), is stabilized by having carbon and nitrogen in solid solution.
- Quenched martensite is rich in carbon but also nitrogen.
- New phases of nitrides may occur, but we can also consider formation of carbonitrides.
- Residual stresses are more thermally stable as long as the stresses are not caused by martensitic transformation.
- Treatment may cause pores by recombination of atomic nitrogen in molecular nitrogen (not the case in the thesis of 2006).
The set of points previously mentioned (except the last one, which will have a very adverse effect on the fatigue strength of the stressed parts) will contribute to a mechanical reinforcement of the surface and a better stability of the structure at “high temperature” between 392-662°F (200-350°C). These two effects will improve the resistance to Hertzian fatigue and resistance to abrasive wear of the treated parts.
But what happens when ammonia is injected in a carburizing atmosphere?
Effect of Ammonia on the Process Atmosphere
HN carbonitriding, like standard carbonitriding (%NH3 ⇐ 3 vol%), is performed in conventional carburizing furnaces of very different technologies (e.g., N2/CH3OH or endothermic gas). The atmosphere will be governed by the main reactions establishing equilibrium: the dissociation of ammonia, the nitriding reaction and the carburizing reaction accompanied by additional secondary reactions (Fig. 6).
We find that the injection of ammonia at its dissociation will have an effect on the equilibrium carburizing reactions and vice versa. Indeed, the total amount of hydrogen is influenced by dissociation of ammonia but also by cracking of CH4 and the water-gas reaction. It will therefore create a balance between the two families of reactions through hydrogen. We understand fairly quickly that one of the keys to controlling HN carbonitriding properly will be the measurement of this gas.
Impact of Nitrogen on the Diffusion Layer
Diffusible nitrogen in solid solution in austenite will have an effect on the extent of phase boundaries in the iron-carbon phase diagram. Having a strong effect on austenite, nitrogen will decrease the AC1 temperature (first appearance of austenite phase) and shift the eutectoid point to the left (Fig. 7).
With the atmosphere in equilibrium with the surface of the workpieces, we will also observe a change in the carbon and nitrogen activities (aC and aN) of the steel since the interstitials, once adsorbed, are alloying elements that must be taken into account in calculating the alloy factors.
Once in the material, nitrogen will have an impact on the solubility limit of carbon in austenite. This should be followed carefully if we are to avoid the formation of soot and cementite.
It is the same for carbon, which will have an impact on the solubility limit of nitrogen in austenite. It will influence the formation of nitrides and carbonitrides.
Based on initial experiments in HN carbonitriding, nitrogen also appears to have an effect on the kinetics of carbon diffusion. Indeed, it is possible to obtain deeper hardened cases than obtained by only carburizing at identical treatment times.
Measurement, Control of HN Carbonitriding
The cracking reaction or dissociation of ammonia is never completed. There is still some residual ammonia in the exhaust. By examining figure 8, we see that the nitriding effect is directly related to the amount of undissociated ammonia. These results were obtained in stable conditions (temperature, pressure and volume of carrier gas injected).
However, we see in figure 9 that the relationship between the ammonia injected into the furnace and residual ammonia measured in the exhaust is strongly influenced by the actual condition of the furnace.
Therefore, the key point for controlling carbonitriding processes is based on the fact that the percentage of adsorbed nitrogen (Nad) depends on the amount of undissociated ammonia (residual) and not on the amount of ammonia injected.
We can control the nitriding effect of the process atmosphere by measuring the volume of residual ammonia and the amount of hydrogen, and we can control the potential by varying the flow of injected ammonia (Fig. 10).
For the carbon potential, we control the atmosphere in the same way as in a carburizing atmosphere (oxygen sensor, plus CO analyzer or CO/CO2 analyzer) down to a certain temperature (<1616°F/880°C for an N2/CH3OH system), where we begin to find ourselves in a nonequilibrium situation: carbon activities and carburizing reactions are out of phase.
Therefore, if the amount of not-completely reacted CH4 or C3H8 becomes significant, measuring and taking into account the residual methane will correct the carbon potential calculated from a state of equilibrium.
Conclusions and Prospects
High-nitrogen carbonitriding rich in nitrogen is characterized by:
- Percentage of nitrogen in the surface >0.6%
- Nitrogen diffusion depth >0.6 mm
- Residual austenite and martensite rich in nitrogen
It can meet two interrelated technical issues: the downsizing and increasing applied stresses associated with a rise in contact temperature.
There are several solutions on the market. We must be able to adapt “traditional carbonitriding” furnaces to measure and control the nitrogen and carbon potentials in the furnace, however, to ensure quality treatment (free of porosity) per CQI-9, RQP1 and Nadcap. Nitrogen and carbon potentials are measured based on their chemical activities and the effect of addition to the elements in a carbonitrided alloy.
To control and regulate a carburizing and nitriding atmosphere, it is necessary to extend the equipment usually found in a carburizing plant (CO/CO2 analyzer and O2 probe) associated with:
- An ammonia analyzer
- A hydrogen sensor (Fig. 11)
- A CH4 analyzer
All these tools are, of course, suitable for a traditional carburizing furnace.
Work is currently under way to optimize the simulation system for the enrichment of nitrogen and carbon (HT-Tools™) to obtain a calculated hardness profile that takes proper account of the impact of nitrogen and carbon on the evolution of the metallurgical structure. We currently have good correlation between the measured carbon and nitrogen and calculated profiles for 20MnCr5 carbonitrided at 1700°F (930°C) for 240 minutes under addition of fixed ammonia. These results are summarized in figure 12, which was provided by IWT Bremen in Germany.
For more information: Contact Patrick Torok, vice president sales & marketing – Heat Treat North America; United Process Controls Inc., 8904 Beckett Rd., West Chester, OH 45069; tel: 513-772-1000; e-mail: Pat.Torok@group-upc.com; web: www.group-upc.com
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