Advanced carburizing in muffle type furnaces e-light

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Advanced carburizing in muffle type furnaces e-light

  1. 1. Advanced Carburizing in Muffle-type FurnacesThe so-called "advanced carburizing processes" were developed in the early 90s in Europe and in themid-90s, the automobile industry has invested in production line furnaces based on vacuum carburizingassociated with high pressure gas quenching for gears and shafts. As the vacuum carburizing processwas not protected by international patents, many furnaces manufacturers worldwide have proposed thistechnique on the market. At this period of time, most of users had the feeling that this new process wouldreplace gas carburizing in the very near future.After almost 10 years of experience, the market for vacuum carburizing furnaces undergoes a seriousdrop 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 productivityAlthough the overall carburizing duration is reduced by vacuum carburizing since the saturation andcarbon enrichment phases are processed at the maximum theoretical speeds, the productivity in termsof tons/hour with respect to the furnace e volume is limited. This is mainly due to required spacingbetween the parts for the carburizing but more specifically to the fact that vacuum carburizing is most ofthe 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 limitedproductivity is major disadvantage.II. Flexibility and hardening mediaDespite intense technical development, the application of the vacuum carburizing is mainly restricted tohigh 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 industrialproduction. Although the steel manufacturers have increase their efforts in developing new high alloyedsteels to optimize the use of gas quenching at reasonable pressures (5 to 8 bars) [1], the scattering ofthe results (dispersion) is still very important and the expected reduced distortion with respect to oil is stillnot reliable.III. Mechanical propertiesThe most expected benefit of the vacuum carburizing was a significant increase of the mechanicalproperties, namely the fatigue resistance and the resilience resistance. This increase was expectedsince the vacuum carburizing inhibits the formation of the intergranular oxidation at the surface of theparts compared to conventional gas carburizing. Recently published reports [2], [3] have shown suchwas not the case, and that in most of the cases, the measured mechanical resistance properties werelower (20 - 30%) for the vacuum carburized and gas quenched parts that for the conventionalcarbonitrided and oil quenched parts [2] for 16MnCr5 and 27MnCr5 type of steels. The main reason forthis 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 themeasured properties (fatigue, hardness) had a much broader dispersion for the vacuum carburizedparts, leading to poor statistical CAM/CPK results. www.soloswiss.com
  2. 2. Alternatives to vacuum carburizingSince 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 newtechniques to overcome both the problems related to conventional gas carburizing and vacuumcarburizing.One of the current trends is to develop new steels for high temperature carburizing application, in orderto reduce the carburizing and diffusion duration [4]. Such future steels would have a stabilized graingrowth at elevated temperatures, but their use in vacuum carburizing would be limited sincemeasurements [4] have shown an important drop in nitrogen content (> 50%) in the steel close to thesurface (0 - 0,2 mm); elevated temperatures will also increase the evaporation and migration of alloyelements mentioned before.Another trend is to develop new processes and furnaces design to overcome the difficulties faced withvacuum carburizing furnaces. The goal is to obtain a carburizing process which inhibits the formation ofintergranular oxides network without modifying the alloy elements dispersion (no Mn evaporation) evenat high temperatures, and a furnace design which allows to harden (quench) in any media to get the bestpossible quality, productivity and mechanical properties. Figure 1 : Schematic representation of a gas-solid reaction with respective kinetics resistancesIn the recent years, SOLO Switzerland has developed and patented a process to meet suchrequirements:- 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 www.soloswiss.com
  3. 3. SOLO ECOCARB process descriptionThe general flow rate of carbon in a conventional carburizing process is given by the combination ofthree 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 whichgoes 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 equilibriumPci : carbon potential at the interfaceCs : carbon concentration at the surface in the partCo : initial carbon content of the part (core carbon content)R1,2,3 : individual resistance for the respective 3 mechanismsIn the case of carburizing with CO/H2 gas mixtures, the resistance R3 is much smaller the resistances R1or 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 theother 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 andspeeds, 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. www.soloswiss.com
  4. 4. The ECOCARB process takes place in a tight retort or bell-type furnace, equipped with a metallic muffleto inhibit soaking effects and ensure a perfect inertia of the furnace with regard to the atmosphere. Thefurnace has to be equipped with a very efficient convection system (designed turbine to enable constantflow with variable resistance (∆P), defectors, etc. (figure 2). This design enables a temperature accuracyof =/- 2,5 C, together with perfect gas convection and agitation (radial design)to allow fast purging andvery homogeneous distribution of the treatment gas. Figure 2 : Schematic representation of a SOLO bell-type carburizing furnace showing the radial designThe principle of the process is basically identical to the so called "vacuum carburizing" but the majordifference is that it does not require vacuum. It can be decomposed into 4 major steps (figure 3). Figure 3 : Representation of the ECOCARB process according to 4 major steps (phases I, II, III and IV).I. The heating up phaseThe heating up takes place under pure nitrogen up to the enrichment temperature. This presents theadvantage to have a fast and homogeneous heating up duration and to avoid any oxidation of thesurface which could influence the process.During this step, Φ =0, and Cs = Co. www.soloswiss.com
  5. 5. II. The enrichment phaseWhen 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 thefurnace. Once the saturation is obtained (typically within a few minutes Cs = C saturation) the requiredamount of carbon is provided according to different methods: additional hydrocarbon injection pulsesseparated 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 perfectcontrol of the flow conditions over an oxygen probe and/or a CO/CO2 infrared equipment. For this lastcase, no intergranular will issue since the change in atmosphere from hydrocarbon to controlled highcarbon potentials is very fast due to the metallic muffle and Cs remains almost at Cs = C saturation, so nooxidation may occur. 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] /III. The diffusion phaseThe diffusion takes place once all the carbon has been put into the parts. The diffusion takes place atΦ =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 tocontrol the amount of the rest-austenite in the superficial structure. This can be achieved by adapting anaccurate NH3 flow rate and allows to set up the austenite content from a few percent to some 35% on thesurface without increasing the carbon concentration at the surface to high values (see figure 5). www.soloswiss.com
  6. 6. IV. The final phaseDuring the final phase, the temperature is reduced in a controlled manner using heat exchangers toreach 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 aredirectly transferred from the furnace into the quench tank(s) with no vestibules (see figure …). As aresult, the microstructure shows perfect carburizing profile, with no carbides and no superficial orintergranular oxidation (figure 6) 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)Figure 6 : c)a) Macrostructure of an ECOCARB carburized gearb) Surface of the parts carburized with ECOCARB process without acid attack showing no intergranular oxides at the surface and c) typical structure without oxidation www.soloswiss.com
  7. 7. Practical examples: gear partsThe ECOCARB process does not require specific equipment with respect to conventional gascarburizing process, provided the furnace has a metallic muffle to change rapidly the atmosphere, so allECOCARB equipped furnaces can also run:- gas carburizing or carbonitriding- austenitisation under controlled or neutral atmosphere- annealing- temperingThe modular design of the SOLO bell-type furnaces (figure 7) allows any quenching media so thequenching occurs in hot oil (>130 C) in high pressure gases or in hot baths > 220°C. a) b)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 carburizingb) Hardness and carbon profilesFigure 8 shows a typical recorded data file for an ECOCARB process with enrichment techniqueaccording to IIc. It can be seen that the advantage of this process is to provide a reliable file for theprocess control in accordance with ISO 9000 requirements to ensure reproducibility. The results are infull accordance to the calculated values. www.soloswiss.com
  8. 8. Major obtained resultsThe overall treatment time at 940 C (type low carbon MnCr or NiCr steels) for a carburizing depth of 1,2mm including heating up is approx 6 hours depending on the load weight and on the specific surface ofthe parts.The surface hardness are all located at 62 +/- 1 HRC after oil quenching within one load (6 samples) andperfectly reproducible.The very accurate control of the carbon at the surface, together with controlled additional nitrogen profileenables to adapt the level of residual austenite at the surface to the required value (figure 9). a) b) c)Figure 9 :Microstructures showing different rest-austenite at the surface of the steels obtained by setting thenitrogen enrichment to the required values a) 25%, b) 35% and c) > 50%.Figure 10:Example of gear loads treated with EcocarbFigure 10 show typical gear parts treated with the SOLO ECOCARB process. The gross weight variesfrom 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 themicro-probe tests do not point out any significant variation of the Mn and Cr contents at the surface ofthe steel. www.soloswiss.com
  9. 9. As a result, the expected benefits concerning the improved mechanical properties can be verified bymeasuring the residual stressed in the carburized layer:It can be seen on figure 11 that the ECOCARB process creates the expected compressive stresses atthe 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)The fatigue resistance has been significantly increased compared to classical gas carburizing processformerly 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 residualcompressive stresses at the surface.The perspective of special alloys for high temperature carburizing applications (above 1000 C) will alsoincrease the interest for the ECOCARB process, since no deterioration of the surface quality can beexpected.————————————————————————————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 www.soloswiss.com

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