Advances In Thermo-Chemical Diffusion Processes

Thermo-chemical diffusion processes like carburizing, nitriding and boronizing 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 thermo-chemical diffusion process 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 boronizing and special thermochemical processes for stainless steels are discussed.

In the Thermo-chemical Diffusion processes elements like carbon, nitrogen or boron are diffused into metal surfaces in order to enhance the surface properties and the strength or metallic components.

In modern heat treatment furnaces, the diffused elements usually originate from gases reacting at high temperatures with the metallic 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 thermo-chemical diffusion processes existing today are known under the names carburizing, nitriding and boronizing. They exist since many decades and have evolved with time to 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 metallic components with higher endurance capability.

1. CarburizingThe dominating carburizing technology today is the gaseous carburizing process using endothermic gas carrier gas and a hydrocarbon gas, like 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, creating at elevated temperatures 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].

Fig. 1: Comparison of gas consumption values for a pusher.

Certain requirements like sufficiently high furnace temperature, strong gas circulation, furnace muffle, etc. 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 also in 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 CarburisingEven more process gas can be saved when hydrocarbon gases totally without an oxidizing gas are directly introduced into carburising 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 di-lution of the hydrocarbon gases, or a utilization of the hydrocarbon gases at low pressures. The last version is the well-known lowpressure carburising process.

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

Fig. 2: Mean carbon flux values (g/m2h) for different carburising processes.

In the last five years, the hydrocarbon gas acetylene has taken the dominant role in low-pressure carburising. The reason is its extraordinary carburising power with on the average 10% more carbon transferred compared to propane (see figure 2) as well as its much more uniform carburising capability, especially on complicated work piece geometries, and finally the fact that vacuum furnaces run with acetylene do not show any soot formation if run at a pressure below 10 mbar [5, 6].

The main advantages of low-pressure carburising 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 CarbonitridingUntil recently, a deficiency still existed, and this was the inability to do a carbonitriding process at low pressure.

With plasma carburising, 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 carburising, as nitrogen gas starts to dissociate thermally only above 1000°C.

Fig. 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 (figure 3).

Fig. 4: Carbon and nitrogen profiles of a steel 30CrMo4 after low-pressure carbonitriding at 880ƒC.

Fig. 5: Carbon and nitrogen profiles of a steel 30CrMo4 after low-pressure carbonitriding at 880ƒC.

Adjusting the time and temperature ratio of the ammonia utilization vs. the acetylene utilization allows to produce defined carbon and nitrogen surface contents. In this way, relatively low surface nitrogen contents of e.g. 0.3 wt.-% (figure 4) or very high surface nitrogen contents of close to 0.7 wt.-% (figure 5) can be produced [9].

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

Fig. 6: Influence of temperature and carbon potential on carburising depth and cycle

duration.

1.3 High-Temperature CarburisingAnother trend in the last few years is the increased utilization of higher carburising temperatures with the main goal to reduce cycle times and thus save costs. Figure 6 shows curves of carburising depths versus carburising times for four different carburising temperatures of 880, 930, 980 and 1050°C for gaseous carburising in endothermic gas and different carbon potentials. In this diagram, the time saving for different carburising depths in using higher carburising temperatures can be seen. E.g. for a carburising depth of 1.2 mm, the holding time on temperature can be reduced from 400 to 220 to 115 minutes by increasing the carburising time (on temperature) from 930 to 980 and further to 1050°C.

Fig. 7: "Industrial applications of high-temperature carburising furnaces.

Naturally, the utilization of higher carburising temperatures of above 1000°C can also be done with low pressure carburising, as can be seen in figure 2. Vacuum furnaces for low-pressure carburising 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 carburising are today increasingly used for high-temperature carburising, as the table in figure 7 shows.

Thus, even in sealed quench furnaces, temperatures of 1015°C and 1020°C are used today industrially, and also pusher furnaces have gone up to 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 carburising is the grain growth of existing case hardening steels and the reducedlifetime of grids and baskets.

2. Nitriding2.1 Control of the Nitriding PotentialThe 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(H2

3/2)

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 gives good guidelines also for industrial steels, what type of compound layer to expect for the respective nitriding potentials controlled in the furnaces [12].

Fig. 8: Principle of the HydroNitÆ-Sensor.

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 analysers. The state of the art is, however, to measure the nitriding potential continuously on-line directly inside the furnace with a hydrogen sensor called HydroNit® [13]. This sensor, the scheme of which is shown in figure 8, is capable of measuring directly 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.

2.2 Ferritic NitrocarburisingIn ferritic nitrocarburising, both nitrogen and carbon are transferred into the steel surface to produce a nitrogen and carbon containing ε-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).

Fig. 9:Chemical composition of ?-compound layer produced by different nitriding and carburising potentials [16].

The main problem with the nitrocarburising 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 nitrocarburising with

the two gas mixtures mentioned above will have a low carbon content if a high nitrogen content is produced, and vice versa (figure 9) [16].

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

Fig. 10:Special two-step fnc cycle resulting in ?-layers with large nitrogen and carbon content.

The new method developed [18] splits the nitrocarburising 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) (figure 10).

2.3 Ferritic Oxi-NitrocarburisingOxi-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 nitrocarburising 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-nitrocarburising 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 nitrocarburising 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].

Fig. 11: Structure and hardness profile of an oxi-nitrocarburised austenitic stainless steel X5CrNi

18-10(DIN 1.4301).

The ferritic oxi-nitrocarburising treatment is favourably used for higher alloyed materials, like 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 figure 11 demonstrates.

3. Special Processes for Stainless SteelsStainless steels, if treated with normal nitriding or carburising processes, loose most of their corrosion resistance due to the formation of chromium nitrides or carbides.

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

3.1 Plasma-Carburising of Austenitic Steels

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

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

Fig. 12: Microstructure of the steel X2CrNiMo 18-14-3(DIN 1.4435) after plasma-carburising at

350ƒC [22].

3.2 Plasma-Nitriding of Austenitic Steels

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

The structure of such a layer produced on the steel 314L after plasma-nitriding at 400°C is shown in figure13 [23]. This process is used in different areas of food processing equipment, chemical industry, nuclear power plants, etc.

3.3 Solution NitridingThe low-temperature processes of plasma-carburising and plasmanitriding have the disadvantage that the thickness of the diffusion layers produced are extremely shallow, in economical times reaching not much above 20 j.tm.

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 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 Härterei Gerster AG, Switzerland [25, 26].

4. BoronizingIn boronizing, boron atoms diffuse into the surface of steels and form compound layers of FeB and Fe2B of appreciable thickness (50-200 μm).

The industrial utilization of boronizing is mainly done with boron containing pastes or granules [27] with limited importance because of the labour intensive procedure of paste application and removal.

Gas boronizing and plasma boronizing promised to be much cleaner and more economical processes.Their industrial utilization, however, is almost non-existent due to the poisonous character of the gaseous donor media, like diborane, borontrichloride or boron-trifluoride.

Because of this, a development project was started a few years ago at the Institut f¸r Werkstofftechnologie Bremen (IWT) to develop a novel boronizing process using harmless boron containing precursors. Successful results were achieved using trimethylborate B(OCH3)3 and exciting it with a

Fig. 13: Microstructure of the steel 314L after plasma-nitriding at 400ƒC [23].

Fig. 14: Typical Fe2B-layer produced by plasma boronizing in an argon-hydrogen-trimethylborate

gas mixture at 1000ƒC for 16 hours at 1 mbar [28].

plasma. Figure 14 shows a typical Fe2B-layer produced by plasma-boronizing in an argon-hydrogen-trimethylborate gas mixture at 1000°C for 16 hours at 1 mbar [28].

More lately, the also harmless precursor triethylboran B(C2H5)3 proved to be an excellent boron source as well [29]. Thus, it can be expected that these plasma- boronizing processes using precursors will soon develop to fully fledged industrial processes.

5. Innovative Equipment for Thermo-Chemical Diffusion ProcessesIn the past, thermo-chemical diffusion processes were carried out in batch furnaces (pit, bell, 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]. Figure 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 like 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 (6 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 metallic components.

6. ConclusionsThe examples mentioned above of carburising, carbonitriding, ferritic nitrocarburising and boronizing 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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