Advances in Thermochemical Diffusion Processes

Thermochemical diffusion processes like carburizing and nitriding play an important part in modern manufacturing technologies. They exist in many varieties depending on the type of diffusing element used and the respective process procedure. The most important industrial heat treatment process is case hardening, which consists of the thermochemical diffusion process of carburizing or its variation, carbonitriding, followed by a subsequent quench. The latest developments of using different gaseous carburizing agents and increasing the carburizing temperature are one main area of this paper. The other area is the evolvement of nitriding and especially the ferritic nitrocarburizing process by improved process control and newly developed process variations using carbon, nitrogen and oxygen as diffusing elements in various process steps. Also, special thermochemical processes for stainless steels are discussed.

In thermochemical diffusion processes, elements like carbon, nitrogen or boron are diffused into metal surfaces in order to enhance the surface properties and the strength or all-metal components.

In modern heat treatment furnaces, the diffused elements usually originate from gases reacting at high temperatures with the all-metal surfaces. This can be a pure thermal and chemical reaction as a consequence of the thermal dissociation of the gases. An increase of the reaction velocity can be achieved in utilizing an electric field in order to ionize the reaction gas (plasma) resulting in largely increased mass transfer.

The industrial thermochemical diffusion processes existing today are known as carburizing and nitriding. Though they have existed for many decades, they have evolved with time into more precisely controlled and reliable processes as part of the total manufacturing process of metal, especially steel components.

In the last few years, a number of new developments and improvements in different areas have helped to increase the importance of diffusion processes, leading to all-metal components with higher endurance capability.

1. Carburizing

The dominating carburizing technology today is the gaseous carburizing process which uses endothermic gas, carrier gas and a hydrocarbon gas, such as natural gas, propane, LPG or others, as enrichment gas for achieving high carbon potentials. Also, methanol diluted with nitrogen can be fed into the furnace, which at elevated temperatures creates a carrier gas inside the furnace similar to endothermic gas.

The most economical gassing process is the direct feed of a fuel (hydrocarbon gas) plus an oxidizing gas (air, carbon dioxide or water) into the furnace and creating a CO- and H2- containing carburizing atmosphere inside the furnace [1].

Figure 1 Comparison of gas consumption values for a pusher

Figure 1 Comparison of gas consumption values for a pusher

Certain requirements like sufficiently high furnace temperatures, strong gas circulation or furnace muffle, need to exist in the furnace for a successful utilization of this in-situ gassing technique called Supercarb® [2]. Therefore, years ago, this process was limited to batch furnaces like pit furnaces and sealed quench furnaces. In the meantime, the Supercarb process is used also in all types of continuous furnaces like mesh-belt furnaces, rotary hearth furnaces and, in the last four years, specially adapted pusher furnaces [3]. The savings in gas consumption using Supercarb can be very high, as the example of a pusher furnace in figure 1 shows.

1.1 Low-Pressure Carburizing

Even more process gas can be saved when hydrocarbon gases, totally without an oxidizing gas, are directly introduced into carburizing furnaces. In this case, the carbon transfer is a direct result of the decomposition of the hydrocarbon into free carbon and hydrogen. Because of the high carbon availability of hydrocarbon gases, such a process only works with a high dilution of the hydrocarbon gases or a utilization of the hydrocarbon gases at low pressures. The last version is the well-known low-pressure carburizing process.

In the eighties and nineties, the main hydrocarbon gas used for low-pressure carburizing was propane, despite its inherent deficiencies of furnace sooting and non-uniform carburizing [4, 5].

Figure 2 Mean carbon flux values (g/m2h) for different carburizing processes

Figure 2 Mean carbon flux values (g/m2h) for different carburizing processes

In the last five years, the hydrocarbon gas acetylene has taken the dominant role in low-pressure carburizing. Acetylene has an average of 10% more carbon transferred compared to propane, increased carburizing capability, especially on complicated work piece geometries, and does not show any soot formation if run at a pressure below 10 mBar [5,6]. This gives acetylene extraordinary carburizing power.

The main advantages of low-pressure carburizing are the increased mass transfer resulting in reduced process times, improved layer uniformity, no internal oxidation, increased stress resistance and better surface quality (in connection with gas quenching) [7].

1.2 Low-Pressure Carbonitriding

Until recently, a deficiency still existed, and this was the inability to do a carbonitriding process at low pressure.

With plasma carburizing, it has been possible for about 30 years to carbonitride using methane or propane in the boost phases and nitrogen gas in the diffuse phases [8]. This procedure is not possible with low-pressure carburizing, as nitrogen gas starts to dissociate thermally only above 1832°F (1000°C).

Figure 3 Cycle for low-pressure carbonitriding

Figure 3 Cycle for low-pressure carbonitriding

Lately, however, a method was developed using ammonia at low pressures in the diffuse phases, or in most cases in the last diffuse phase, in order to transfer nitrogen next to carbon into the steel surface (Fig. 3).

Figure 4 Carbon and nitrogen profiles of a steel 30CrMo4 after low-pressure carbonitriding at 880°C

Figure 4 Carbon and nitrogen profiles of a steel 30CrMo4 after low-pressure carbonitriding at 880°C

Figure 5 Carbon and nitrogen profiles of a steel 15CrNi6 after low-pressure carbonitriding at 930°C/820°C

Figure 5 Carbon and nitrogen profiles of a steel 15CrNi6 after low-pressure carbonitriding at 930°C/820°C

Adjusting the time and temperature ratio of the ammonia utilization against the acetylene utilization allows for the production of defined carbon and nitrogen surface contents. In this way, relatively low surface nitrogen contents (e.g., 0.3 wt.-%, as in Fig. 4) or very high surface nitrogen contents (e.g., 0.7 wt.-%, as in Fig. 5) can be produced [9].

The advantage of carbonitriding versus carburizing is that a carburized microstructure with an increased content of nitrogen has a higher temperature resistance, an increased hardenability, improved wear resistance and, in some instances, a higher load carrying capability [10].

Figure 6 Influence of temperature and carbon potential on carburizing depth and cycle duration

Figure 6 Influence of temperature and carbon potential on carburizing depth and cycle duration

1.3 High-Temperature Carburizing

Another trend in the last few years is the increased utilization of higher carburizing temperatures with the main goal to reduce cycle times and, thus, save costs. Fig. 6 shows curves of carburizing depths versus carburizing times for four different carburizing temperatures of 1616°F, 1706°F, 1796°F and 1922°F (880°C, 930°C, 980°C and 1050°C) for gaseous carburizing in endothermic gas and different carbon potentials. In this diagram, the time saving for different carburizing depths in using higher carburizing temperatures can be seen. For example, a carburizing depth of 1.2 mm, the holding time on temperature can be reduced from 400 minutes to 220 minutes to 115 minutes by increasing the carburizing time (on temperature) from 1706°F to 1796°F and further to 1922°F  (930°C, 980°C and 1050°C, respectively).

Figure 7 Industrial applications of high-temperature Carburizing furnaces

Figure 7 Industrial applications of high-temperature Carburizing furnaces

Naturally, the utilization of higher carburizing temperatures of above 1832°F (1000°C) can also be done with low pressure carburizing, as can be seen in Fig. 2. Vacuum furnaces for low-pressure carburizing are even more adapt for higher temperatures because the material used for the furnace lining and the furnace heating elements is usually graphite, which has very high temperature resistance. But also atmosphere furnaces for gas carburizing are today increasingly used for high-temperature carburizing, as the table in Fig. 7 shows.

Thus, even in sealed quench furnaces, temperatures of 1859°F and 1868°F (1015°C and 1020°C) are used today industrially, and also pusher furnaces have gone up to 1796°F (980°C) [11].

This is due to the increased use of newly developed silicon carbide materials for hearth, muffles and especially radiant tubes.

The main problems remaining with high temperature carburizing is the grain growth of existing case hardening steels and the reduced lifetime of grids and baskets.

2. Nitriding

2.1 Control of the Nitriding Potential

The state of the art of nitriding in ammonia or diluted ammonia gas is to control the nitriding potential. The nitriding potential is defined as:

KN  =  p(NH3)
          p(H23/2)

This definition is a direct consequence of the ammonia dissociation reaction:

NH3  ↔ [N] + 3/2 H2

By choosing the respective nitriding potential, nitrogen-rich compound layers of the s-nitride, nitrogen-poor compound layers of the γ-nitride as well as totally compound- layer-free nitrided surfaces can be produced.

The so-called Lehrer diagram also gives good guidelines for industrial steels and what type of compound layer to expect for the respective nitriding potentials controlled in the furnaces [12].

For controlling the nitriding potential, it is necessary to measure either the ammonia content or the hydrogen content of the atmosphere. This can be done with infrared or other gas analyzers. The state of the art is, however, to measure the nitriding potential continuously online directly inside the furnace with a hydrogen sensor called HydroNit® [13]. This sensor, the scheme of which is shown in Fig. 8, is capable of directly measuring the partial pressure of hydrogen inside the nitriding furnace using a measuring tube of a special material capable of being permeable only to hydrogen gas.

Figure 8 Principle of the HydroNitÆ-Sensor

Figure 8 Principle of the HydroNitÆ-Sensor

2.2 Ferritic Nitrocarburizing

In ferritic nitrocarburizing, both nitrogen and carbon are transferred into the steel surface to produce a nitrogen and carbon containing ?-compound layer.

The gas used for this process, therefore, is a mixture of ammonia gas and a carbon carrying gas. Standard industrially-used gases are a mixture of ammonia and endothermic gas (50:50) or a mixture consisting of ammonia plus CO2 (5%) and nitrogen gas (45%) [14, 15].

In these gas mixtures, the nitrogen transfer depends on the ammonia dissociation just like in nitriding. The carbon transfer is caused by the CO-hydrogen reaction: 

CO + H2 ↔ [C] + H2O

with gases with high CO-content (endothermic gas) delivering much more carbon than those with low CO-content (CO2).

Figure 9 Chemical composition of ?-compound layer produced by different nitriding and carburizing potentials [16].

Figure 9 Chemical composition of ?-compound layer produced by different nitriding and carburizing potentials [16].

The main problem with the nitrocarburizing atmospheres produced by these two gas mixtures is that the carbon content and the nitrogen content in the compound layer cannot be adjusted independently of each other. The carbon transfer increases with higher hydrogen content, which at the same time, however, lowers the nitriding potential. Thus, automatically compound layers produced in ferritic nitrocarburizing with the two gas mixtures mentioned above will have a low carbon content if a high nitrogen content is produced, and vice versa (Fig. 9) [16].

With a new method developed in the last few years, it is possible to produce ?-compound layers in ferritic nitrocarburizing which have at the same time a high nitrogen content as well as a high carbon content. J. W½nning had already in 1977 shown that the strongest carbon transferring gases in nitrocarburizing next to endothermic gas are hydrocarbon gases, and especially propane [17].

Figure 10 Special two-step FNC cycle resulting in ?-layers with large nitrogen and carbon content

Figure 10 Special two-step FNC cycle resulting in ?-layers with large nitrogen and carbon content

The new method developed [18] splits the nitrocarburizing cycle in two parts with the first part run in ammonia plus CO2 and nitrogen in order to produce a high nitrogen content in the compound layer. The second part is run in a gas mixture consisting of ammonia and propane (plus nitrogen) (Fig. 10).

2.3 Ferritic Oxi-Nitrocarburizing

Oxi-nitriding has also been known since the 1970's and was noted for faster surface reactions and higher nitrogen transfer [19]. It never gained much importance, as in pure classical nitriding in ammonia gas the growth of the compound layer was already sufficiently fast, and the goal in those days was more to restrict its thickness than to improve it.

With the short time cycles of ferritic nitrocarburizing and the problem with sometimes bothered surface reactions due to passive oxide layers on the surface of the steel components, the importance of the utilization of oxygen in a first part of an fnc cycle was noticed about three years ago [20].

This led to the development of the ferritic oxi-nitrocarburizing process with air being added to the nitriding atmosphere inside the furnace during the last part of the heating cycle and the first part of the nitrocarburizing cycle. H.-J. Spies examined this effect and found, that high oxidizing potentials are needed in order to transform the passive oxide layer into a nitrogen permeable layer of iron oxide [21].

Figure 11 Structure and hardness profile of an oxi-nitrocarburized austenitic stainless steel X5CrNi 18-10 (DIN 1.4301)

Figure 11 Structure and hardness profile of an oxi-nitrocarburized austenitic stainless steel X5CrNi 18-10 (DIN 1.4301)

The ferritic oxi-nitrocarburizing treatment is favorably used for higher alloyed materials (e.g., hot and cold working tool steels and also especially stainless steels), as the example of the steel X5CrNi 18-10 (DIN 1.4301) in Fig. 11 demonstrates.

3. Special Processes for Stainless Steels

Stainless steels, if treated with normal nitriding or carburizing processes, lose most of their corrosion resistance due to the formation of chromium nitrides or carbides.

By the development of new low-temperature or high-temperature processes, this deficiency can be overcome.

3.1 Plasma-Carburizing of Austenitic Steels

Figure 12 Microstructure of the steel X2CrNiMo 18-14-3(DIN 1.4435) after plasma-carburizing at 662°F (350°C) [22]

Figure 12 Microstructure of the steel X2CrNiMo 18-14-3(DIN 1.4435) after plasma-carburizing at 662°F (350°C) [22]

Lowering the carburizing temperature to values, which prohibit the formation of chromium carbides (Cr23C6) (i.e., to temperatures below 752°F [400°C]), can produce a thin shallow surface layer supersaturated with carbon with a large hardness increase and basically no loss of corrosion resistance.

Fig. 12 shows as an example the microstructure of the steel X2CrNiMo 18-14-3 (DIN 1.4435) after plasma carburizing for 96 hours at 662°F (350°C), having produced a carburized layer of 25 ?m thickness with a hardness of approx. 1150 HV and a carbon content of approximately 3 wt.-% [22]. Because of the low temperature, there are hardly any dimensional changes involved with this process.

Figure 13 Microstructure of the steel 314L after plasma-nitriding at 752°F (400°C) [23]

Figure 13 Microstructure of the steel 314L after plasma-nitriding at 752°F (400°C) [23]

The formation of the chromium nitrides CrN and Cr2N can be avoided by nitriding at temperatures below 878°F (470°C) leading to a shallow (10-30 ?m) nitrogen super-saturated diffusion layer of high hardness (approximately 1100 HV) [23].

The structure of such a layer produced on the steel 314L after plasma-nitriding at 752°F (400°C) is shown in Fig.13 [23]. This process is used in different areas of food processing equipment, chemical industry, nuclear power plants, etc.

3.3 Solution Nitriding

The low-temperature processes of plasma-carburizing and plasmanitriding have the disadvantage that the thickness of the diffusion layers produced are extremely shallow, in economical times reaching not much above 20 JTM.

A new developed process is able to overcome this deficiency and to produce hardened layers on stainless steels with thicknesses of up to 1 and even 2 mm without any loss of corrosion resistance.

This process uses the capability of stainless steels to dissolve nitrogen at temperatures above 1832°F (1000°C) to a large extent without formation of chromium nitrides.

This process was developed theoretically and in the laboratory by Professor H. Berns [24]. The industrialization of the solution nitriding technology SolNit® was done in a joint co-operation between Professor Berns, Ipsen International and H"rterei Gerster AG, Switzerland [25, 26].

4. Innovative Equipment for Thermochemical Diffusion Processes

In the past, thermochemical diffusion processes were carried out in batch furnaces (pit, bell or chamber) or stepped respectively continuous furnaces with limited process and quenching flexibility.

In the last few years, an innovative cell concept of furnaces integrating atmosphere, vacuum (low pressure) and plasma processes into one heat treatment line and leaving each load the choice for quenching in oil, water, polymer or gases, was developed [30]. Fig. 15 shows such a multiple cell system called mult-i-cell®, where a type of shuttle system called Vac-Mobil® (a travelling vacuum furnace in itself) transfers the load from cell to cell, until a whole heat treatment sequence (e.g., preheating, austenitizing, quenching, tempering, nitriding (gas or plasma) and cooling) is finished.

At the end of the heat treatment sequence, the load has passed through a plurality of furnace cells (six in the example mentioned above) without ever having been in contact with air and without any necessity of the subsequent load to pass through the same cells or same sequence. Thus, an ultimate flexible heat treatment installation is now available for high-quality industrial manufacturing of all-metal components.

5. Conclusions

The examples mentioned above of carburizing, carbonitriding and ferritic nitrocarburizing represent only a limited amount of the development work on thermochemical diffusion processes of the last few years. They prove, however, that thermo-chemical diffusion processes are clearly on the advance. Only due to their increased capabilities, the development of higher stressed motor engine components, car suspension parts, drive shafts and gear components is made possible, frequently in conjunction with a respective wear resistant or low-friction surface coating produced in pvd-installations [31].

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