Advanced Carburizing in Muffle-type Furnaces

The so-called "advanced carburizing processes" were developed in the early 90's in Europe and in the mid-90's, the automobile industry has invested in production line furnaces based on vacuum carburizing associated with high pressure gas quenching for gears and shafts. As the vacuum carburizing process was not protected by international patents, many furnaces manufacturers worldwide have proposed this technique on the market. At this period of time, most of users had the feeling that this new process would replace gas carburizing in the very near future. After almost 10 years of experience, the market for vacuum carburizing furnaces undergoes a serious drop since the recent 4 years and the investment is now limited to specific applications and special parts. Three main reasons explain the lack of interest for the vacuum carburizing industrial applications: I. Costs and productivity Although the overall carburizing duration is reduced by vacuum carburizing since the saturation and carbon enrichment phases are processed at the maximum theoretical speeds, the productivity in terms of tons/hour with respect to the furnace e volume is limited. This is mainly due to required spacing between the parts for the carburizing but more specifically to the fact that vacuum carburizing is most of the time associated with high pressure gas quenching which limits the loading density.

As the investment cost is much higher than that of a gas carburizing furnace it is obvious that the limited productivity is major disadvantage. II. Flexibility and hardening media Despite intense technical development, the application of the vacuum carburizing is mainly restricted to high pressure gas quenching, up to 15-20 bars. Industrial oil quenching applications are rare and hot oil (> 130 °C) or salt quench applications (> 200 °C) have not been successfully applied for industrial production. Although the steel manufacturers have increase their efforts in developing new high alloyed steels to optimize the use of gas quenching at reasonable pressures (5 to 8 bars) [1], the scattering of the results (dispersion) is still very important and the expected reduced distortion with respect to oil is still not reliable. III. Mechanical properties The most expected benefit of the vacuum carburizing was a significant increase of the mechanical properties, namely the fatigue resistance and the resilience resistance. This increase was expected since the vacuum carburizing inhibits the formation of the intergranular oxidation at the surface of the parts compared to conventional gas carburizing. Recently published reports [2], [3] have shown such was not the case, and that in most of the cases, the measured mechanical resistance properties were lower (20 - 30%) for the vacuum carburized and gas quenched parts that for the conventional carbonitrided and oil quenched parts [2] for 16MnCr5 and 27MnCr5 type of steels. The main reason for this lack of mechanical properties is explained by the evaporation and the migration of alloy element (namely Mn) at the surface of the steel in vacuum. Furthermore, the results clearly show that the measured properties (fatigue, hardness) had a much broader dispersion for the vacuum carburized parts, leading to poor statistical CAM/CPK results.

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Alternatives to vacuum carburizing Since the vacuum carburizing did not process did not meet the expected advantages in terms of quality, productivity and mechanical properties, furnaces manufacturers and steel producers are developing new techniques to overcome both the problems related to conventional gas carburizing and vacuum carburizing. One of the current trends is to develop new steels for high temperature carburizing application, in order to reduce the carburizing and diffusion duration [4]. Such future steels would have a stabilized grain growth at elevated temperatures, but their use in vacuum carburizing would be limited since measurements [4] have shown an important drop in nitrogen content (> 50%) in the steel close to the surface (0 - 0,2 mm); elevated temperatures will also increase the evaporation and migration of alloy elements mentioned before. Another trend is to develop new processes and furnaces design to overcome the difficulties faced with vacuum carburizing furnaces. The goal is to obtain a carburizing process which inhibits the formation of intergranular oxides network without modifying the alloy elements dispersion (no Mn evaporation) even at high temperatures, and a furnace design which allows to harden (quench) in any media to get the best possible quality, productivity and mechanical properties.

Figure 1 :

Schematic representation of a gas-solid reaction with respective kinetics resistances

In the recent years, SOLO Switzerland has developed and patented a process to meet such requirements: - inhibit intergranular oxidation at the surface - use of normal hot wall furnaces (no vacuum, inhibit Mn evaporation) - no loading density limitation - enable maximum flexibility for mechanical requirements (controlled rest-austenite from 1-2 % to over

30%) - quench and harden in any media including high gas pressure, hot oil or hot baths

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SOLO ECOCARB process description

The general flow rate of carbon in a conventional carburizing process is given by the combination of three flows (figure 1):

Φ = Φ1 = Φ2 = Φ3 (1)

Whereas, Φ1 is the flow density into the gas towards the surface of the part, Φ2 is the flow density which goes along with the chemical reaction of oxygen desorbtion at the surface of the steel (O adsorbed + H2 = H2O)

and Φ3 is the flow density which transport the carbon into the steel by diffusion.

The equation (1) can also be written:

Φ = Φ1 = Φ2 = Φ3 = (Pc - Pci)/ R1 = (Pci - Cs)/R2 = (Cs - Co)/R3 (2)

Whereas : Pc : carbon potential in the gas in equilibrium Pci : carbon potential at the interface Cs : carbon concentration at the surface in the part Co : initial carbon content of the part (core carbon content) R1,2,3 : individual resistance for the respective 3 mechanisms

In the case of carburizing with CO/H2 gas mixtures, the resistance R3 is much smaller the resistances R1 or R2, so the reaction normally writes:

Φ = (Pc - Cs)/R = h (Pc - Cs) whereas R= R1 + R2 (3)

Therefore, should R2 << R1 is the kinetics controlled contorted by the transport in the gas phase, on the other hand if R1 << R2 the kinetics is controlled by the kinetics of the chemical reaction on the interface.

In practice, such a situation is unfortunately never true for the following reasons (see figure 4): - the Cs value varies slowly during the carburizing process, never reaching the set value of Pc - the transport coefficient h is not constant during the cycle - the formation of an oxide layer at the metal/gas interface modifies the conditions and makes it more

difficult for the carbon to diffuse into the steel. It shall be notified that this oxide layer takes place even during the early stages of the carburizing process when the carbon content is still very low close to Co.

In other words, the carbon transfer coefficient does not take place at constant concentrations and speeds, leading to a difficult accurate control of the real situation. To get rid of these perturbation effects, the process shall meet following requirements: - the Cs concentration shall be fixed and controlled in order to have a clear picture of the flow density - in the carbon enrichment phase, the surface concentration Cs shall reach the value of the saturated

austenite in order to enables the maximal carbon flow theoretical speed.

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The ECOCARB process takes place in a tight retort or bell-type furnace, equipped with a metallic muffle to inhibit soaking effects and ensure a perfect inertia of the furnace with regard to the atmosphere. The furnace has to be equipped with a very efficient convection system (designed turbine to enable constant

flow with variable resistance (∆P), defectors, etc. (figure 2). This design enables a temperature accuracy

of =/- 2,5 C, together with perfect gas convection and agitation (radial design）to allow fast purging and

very homogeneous distribution of the treatment gas.

The principle of the process is basically identical to the so called "vacuum carburizing" but the major difference is that it does not require vacuum. It can be decomposed into 4 major steps (figure 3).

I. The heating up phase The heating up takes place under pure nitrogen up to the enrichment temperature. This presents the advantage to have a fast and homogeneous heating up duration and to avoid any oxidation of the surface which could influence the process.

During this step, Φ =0, and Cs = Co.

Figure 2 :

Schematic representation of a SOLO bell-type carburizing furnace showing the radial design

Figure 3 :

Representation of the ECOCARB process according to 4 major steps (phases I, II, III and IV).

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II. The enrichment phase

When the enrichment temperature T is reached, the conditions are set to get Cs = C saturation, so Φ = max. This very rapid saturation of the austenite at the surface is obtained by injection of an hydrocarbon in the

furnace. Once the saturation is obtained (typically within a few minutes Cs = C saturation) the required amount of carbon is provided according to different methods: additional hydrocarbon injection pulses separated by nitrogen purging phases, b) controlled hydrocarbon flow rate with time (see figure 4) or c)

by setting a conventional carbon potential Pc = C saturation which presents the advantage to have a perfect control of the flow conditions over an oxygen probe and/or a CO/CO2 infrared equipment. For this last case, no intergranular will issue since the change in atmosphere from hydrocarbon to controlled high

carbon potentials is very fast due to the metallic muffle and Cs remains almost at Cs = C saturation, so no oxidation may occur.

III. The diffusion phase

The diffusion takes place once all the carbon has been put into the parts. The diffusion takes place at

Φ =0, and Cs = variable, until the required carburizing depth and final carbon surface Cs final is obtained.

During the diffusion phase, it is possible to add a nitrogen profile to the carbon in order for instance to control the amount of the rest-austenite in the superficial structure. This can be achieved by adapting an accurate NH3 flow rate and allows to set up the austenite content from a few percent to some 35% on the surface without increasing the carbon concentration at the surface to high values (see figure 5).

Figure 4 : a) Calculated and measured weight

increase using a controlled hydro-carbon flow rate adapted to the weight increase (enrichment b-type at 950 C);

b) Carbon profiles after the enrichment

phase and after the diffusion phase [5] /

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IV. The final phase During the final phase, the temperature is reduced in a controlled manner using heat exchangers to reach the quenching temperature. Then, the parts may be immerged in any possible quenching media according to the specifications (distortion, hardness, etc.). Note that in the SOLO bell-type furnaces, the transfer duration is reduced to zero since the parts are directly transferred from the furnace into the quench tank(s) with no vestibules (see figure …). As a result, the microstructure shows perfect carburizing profile, with no carbides and no superficial or intergranular oxidation (figure 6)

Figure 6 : a) Macrostructure of an ECOCARB carburized gear b) Surface of the parts carburized with ECOCARB process without acid attack showing no

intergranular oxides at the surface and c) typical structure without oxidation

Figure 5 :

a) Weight evolution using a final nitrogen enrichment after diffusion and

b) Respective carbon and nitrogen

profiles in the steel [5]

a) b) c)

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Practical examples: gear parts The ECOCARB process does not require specific equipment with respect to conventional gas carburizing process, provided the furnace has a metallic muffle to change rapidly the atmosphere, so all ECOCARB equipped furnaces can also run:

- gas carburizing or carbonitriding - austenitisation under controlled or neutral atmosphere - annealing - tempering The modular design of the SOLO bell-type furnaces (figure 7) allows any quenching media so the quenching occurs in hot oil (>130 C) in high pressure gases or in hot baths > 220°C.

Figure 7 :

a) Schematic example of a SOLO Profitherm bell-type installation with different quenching media; b) The modular design allows any combinations for direct quenching with no vestibule.

Figure 8 :

a) Typical data record for a computer controlled ECOCARB carburizing b) Hardness and carbon profiles

Figure 8 shows a typical recorded data file for an ECOCARB process with enrichment technique according to IIc. It can be seen that the advantage of this process is to provide a reliable file for the process control in accordance with ISO 9000 requirements to ensure reproducibility. The results are in full accordance to the calculated values.

a) b)

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Major obtained results The overall treatment time at 940 C (type low carbon MnCr or NiCr steels) for a carburizing depth of 1,2 mm including heating up is approx 6 hours depending on the load weight and on the specific surface of the parts.

The surface hardness are all located at 62 +/- 1 HRC after oil quenching within one load (6 samples) and perfectly reproducible. The very accurate control of the carbon at the surface, together with controlled additional nitrogen profile enables to adapt the level of residual austenite at the surface to the required value (figure 9).

Figure 9 : Microstructures showing different rest-austenite at the surface of the steels obtained by setting the nitrogen enrichment to the required values a) 25%, b) 35% and c) > 50%.

Figure 10: Example of gear loads treated with Ecocarb Figure 10 show typical gear parts treated with the SOLO ECOCARB process. The gross weight varies from 350 Kg to 700 Kg and the typical requirements are carburizing depths 0,8 - 1,0 mm; 1,0 - 1,2 mm. The microstructures show no formation of oxide layer and no intergranular oxidation network and the micro-probe tests do not point out any significant variation of the Mn and Cr contents at the surface of the steel.

a) b) c)

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The fatigue resistance has been significantly increased compared to classical gas carburizing process formerly used in pusher type furnaces. The effective useful torque could be raised by more than 35% with increased fatigue and resilience resistances. The amount of rest-austenite will transform under high stresses in use, increasing the residual compressive stresses at the surface. The perspective of special alloys for high temperature carburizing applications (above 1000 C) will also increase the interest for the ECOCARB process, since no deterioration of the surface quality can be expected.

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Bibliography: [1] B. Maisant, D. Forest, C. Pichard, Ascometal, SVW/ASTT, 3 - 4 April 2003 Zurich, Conference Proceedings,

p.47 [2] Fernand Da Costa, Renault, European Congress, ATTT/AWT/ASTT-SVW/VWT, 18 - 19 March 2004,

Strasbourg, France, Conference Proceedings [3] B. Clausen, F. Hoffmann, P. Mayr, SVW/ASTT, 3 - 4 April 2003 Zurich, Conference Proceedings, p.159 [4] Frank Hippenstiel, Walter Grimm, SVW/ASTT, 3 - 4 April 2003 Zurich, Conference Proceedings, p.59 [5] D. Zimmermann, Haerterei Technische Mitteilungen, 47, 1992/1, p.3

As a result, the expected benefits concerning the improved mechanical properties can be verified by measuring the residual stressed in the carburized layer: It can be seen on figure 11 that the ECOCARB process creates the expected compressive stresses at the surface although conventional gas carburizing generates tensile stresses on approx 20 microns. Such compressive stresses will inhibit surface micro-cracks to propagate under fatigue solicitations.

Figure 11 :

Measured residual stresses at the surface of gas carburizing and ECOCARB carburizing gears (treatment at T 940 C, depth 1,1 mm)

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