Advanced gas carburizing technology for the automotive industry

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1 ISSN X VULKAN-VERLAG ESSEN HEAT PROCESSING INTERNATIONAL MAGAZINE FOR INDUSTRIAL FURNACES HEAT TREATMENT PLANTS EQUIPMENT Advanced gas carburizing technology for the automotive industry Dr.-Ing. Herwig Altena, Dr. Peter Schobesberger, Ing. Franz Schrank, Aichelin Ges.m.b.H., Mödling (Austria) Published in HEAT PROCESSING 3/2006 Vulkan-Verlag GmbH, Essen (Germany) Editor: Dipl.-Ing. Stephan Schalm, Tel. +49 (0) 201/ , [email protected]

2 Advanced gas carburizing technology for the automotive industry Dr.-Ing. Herwig Altena Aichelin Ges.m.b.H., Mödling (Austria) Conventional gas carburizing has become indispensable in the automotive industry. Increased demands in cost effectiveness, reduced gas consumption and reduced distortion caused the development of a new generation of gas carburizing furnaces. Two examples of modern furnace design will be presented. The first part deals with a high pressure gas module for gas carburizing furnaces, which can be adapted both on new gas carburizing plants and on existing plants as a renewing equipment. Due to the improved ability, advanced high pressure chambers allow full substitution of oil devices for case hardening of gear parts. The gas causes reduced distortion reduced grinding costs and saving of post-washing. As an alternative to pusher furnaces a new generation of ring hearth furnaces caused growing interest in the automotive market. The new design allows significant time savings in combination with low gas and energy consumption. Some furnace designs and applications will be presented. A comparison of ring hearth furnaces and modern pusher-type furnaces under similar heat treatment conditions was carried out, showing the advantages and disadvantages of both concepts. Tel.: 0043 / [email protected] Dr. Peter Schobesberger Aichelin Ges.m.b.H., Mödling (Austria) Tel.: 0043 / [email protected] Ing. Franz Schrank Aichelin Ges.m.b.H., Mödling (Austria) Tel.: 0043 / [email protected] Introduction Great efforts were made during the last 20 years to improve the process technology and furnace design of conventional gas carburizing plants. Due to that the carburizing with endothermic gas, nitrogen/methanol or natural gas/air is a proven technology that has gained a firm place in the field of heat treatment [1]. Continuous pusher furnaces have become indispensable in many sectors of the car and truck industry. Increased demands in cost effectiveness led to the development of a new generation of plants, showing an improved product quality in combination with highly reproducible heat treatment results. The target for the development was a reduction of gas consumption, minimization of intergranular surface oxidation, shortening of process time, reduced distortion and high quality level of the parts. The following two examples will show how advanced furnace designs help to fulfil the requirements and finally cut heat treating costs. Pusher furnace with gas module The execution of gas carburizing furnaces differs according to the require- Run-in sluice Lifting platform Fig. 1: Run-in sluice with bottom loading (scheme) Ignition burner ments, such as case hardening depth (CHD) or throughput capacity, but the over-all furnace design usually shows the same characteristic components. Run-in sluice For minimizing parts distortion a preheating furnace is applied, which can be used up to 500 C and allows a pre-oxidation of the load. For minimizing gas consumption and for reduction of the intergranular surface oxidation the loading of the heating chamber is executed by lifting the load into the run-in sluice from below (Fig. 1). To keep the oxygen contamination of the atmosphere as low as possible, an ignition burner is mounted beneath the run-in opening, which is started shortly before the sluice is opened. Gas module Improved ability of high pressure chambers allows to substitute oil devices for case hardening of gear parts for the automo- HEAT PROCESSING (4) ISSUE

3 Pusher furnace N 2 2-supply Intermediate chamber Storage tank 10 m³ Gas 25 bar Gas balloon compressor 60 m³ Fig. 3: Nitrogen recovery unit High pressure gas chamber Carburizing plant Quenching chamber approx. 20 bar Vacuum pump Fig. 2: Gas carburizing furnace with high pressure gas module (scheme) Suction tank 6-10 m³ Exhaust tive industry. The advantages of high pressure gas, such as reduced distortion or saving of the post-washing operation, had been achievable only in combination with low pressure carburizing technology [2, 3]. A high pressure gas module for gas carburizing furnaces was thus developed, which allows combining the advantages of gas with gas carburizing furnaces. However, for this combination some problems needed to be solved: mixing of process gas and gas due to load transport gas recovery: it had to be assured that the amount of process gas (CO + H 2 ) stays below the safety limit of 5 % (explosion limit). diluting of process gas with gas (N 2 ) in the hardening zone formation of bainite at the parts surface in correlation with material, parts geometry and ability. The execution of the gas module helped to solve these problems. Fig. 2 shows a scheme of the gas module, which is connected with the furnace via a small intermediate chamber. After opening the insulating door, the hot load is quickly transported to the chamber. The hot load heats up the nitrogen, causing a nitrogen stream against the loading direction. Since a mixing of process gas and nitrogen cannot be avoided completely, the chamber is quickly evacuated via a suction vessel after closing the vacuum-tight, sealed door, followed by flooding with up to 20 bar N 2 gas (Fig. 3). This feature even allows a permanent recovery of process gas without reaching a concentration of 1 % CO in the gas. The gas module was successfully implemented both in new plants and as a substitution of an existing oil quench tank. Fig. 4 shows a doubletrack pusher furnace with high pressure module (in the rear). Improvement of distortion values due to gas The following example shows the influence of the media on the distortion of the toothing of the flank (fhß-value) of sensitive gear shafts (Fig. 5). Fig. 6 shows the flank line after oil ; in Fig. 7 the flank line after gas can be seen in comparison. Fig. 4: Double track gas carburizing plant with gas module (overview) Fig. 5: Full load of gear shafts 188 HEAT PROCESSING (4) ISSUE

4 Fig. 6: Flank line of a gear shaft after hot oil costs. So, despite the higher investment costs of the gas module it could be proven as the more economic solution with a return on investment in less than two years. Ring hearth furnace design The improved design combines the advantages of a rotary hearth furnace and a pusher furnace. The furnace is equipped with separated heating, carburizing and diffusion zones. Therefore different carbon levels and temperature profiles can be processed nearly independently in the different chambers. The measured shape was typical for the chosen media and could be reproduced in rather narrow tolerances. A certain difference in the flank line before and after heat treatment can be compensated by soft machining before treatment, as long as the flank line shows a continuous, convex shape. The wave-like shape after oil (Fig. 6) could not be compensated before Fig. 7: Flank line of a gear shaft after gas treatment and therefore grinding after oil was always required. In contrast the distortion of the flank line after gas was found highly reproducible, showing a continuous convex shape (Fig. 7). Therefore the whole post grinding operation could be saved. Additionally the case depth requirement could be reduced, which saved further heat treatment time and Plant design Depending on the requirements two different designs can be used: single parts loading, carburizing and direct hardening via press quench for sensitive parts like camshafts, crown wheels or synchronizing slide gears or batch loading, carburizing and oil. This type is mainly used for transmission gear wheels or shafts as an alternative for pusher furnaces. Furthermore gas in a pressurized chamber can be executed. Carburizing with subsequent press Due to changes in dimension and distortion, sensitive gear parts like synchronizing gears, crown wheels or camshafts have to be press quenched. Depending on the throughput capacity, these parts are usually carburized in chamber furnaces or small continuous furnaces and gas cooled, followed by reheating in a Fig. 8: Ring hearth furnace with press (scheme) Fig. 9: Ring hearth furnace HEAT PROCESSING (4) ISSUE

5 rotary furnace and press. The new ring hearth concept allows carburizing and direct hardening with press quench in a single step because of the separated zones of the furnace, which can be used at different temperatures and C-levels without interaction (Fig. 8). Fig. 11: Ring hearth furnace with gas module (scheme) In comparison with the conventional process (single hardening) up to 30 % of the process time, 30 % of energy and 50 % of personnel costs can be saved. No additional charging trays for the carburizing furnace are needed either. The over all cost saving potential is between 20 and 25 % in comparison with a single hardening process. Fig. 9 shows a typical ring hearth furnace. Carburizing of full loads with oil As an alternative to pusher furnaces full loads can be carburized and subsequently quenched in big ring hearth furnaces, too. The load is pushed directly onto the ring hearth, which rotates all loads without any further push or pull through the whole furnace. Loading and unloading is done at the same position (sluice). Fig. 10 shows a typical furnace layout. The advantages of this concept are reduced strain and longer life time of the trays due to the rotation of the loads in the furnace, savings of basic trays and motor drives of the furnace and a significant shortening of the process time. The reason for this was to be investigated. Usually the ring hearth furnace is combined with an oil device which is similar to the oil of pusher furnaces. For further improvement of the distortion values of the load, oil can be substituted by gas (Fig. 11). Comparison of pusher and ring hearth furnaces Experiences with the ring hearth furnace design indicated a significant reduction of process time in comparison with pusher furnaces. However, this information was qualitatively and did not compare equal throughput capacities and heat treatment conditions. Therefore a double-track pusher and a double-load ring hearth furnace with comparable throughput capacity were chosen for a comparison of process time and heat treat results. Fig. 10: Ring hearth furnace with oil (scheme) Test conditions The test run was carried out with one batch of gear wheels, material 20MoCr4 (Ù SAE 4118 H). Furthermore small bars of the same material were added to each load for measuring the carbon profile. All process parameters were exactly measured during the test run with reference probes, only the heating-up of the parts could be measured semi-quantitatively via optical pyrometer. 190 HEAT PROCESSING (4) ISSUE

6 Table 1: Process time and carburizing depth of a pusher furnace and a ring hearth furnace A carburizing depth of 0.80 mm at 0.35 %C was choosen as a target. The measurement of the carbon profiles allowed the comparison of the carburization. In addition, the case depth of the gear parts was measured and evaluated, taking into consideration the influence of. Results Table 1 shows the comparison of process time, archieved case depth at 0.35 %C and the difference between measured and calculated case depth. The target of 0.8 % carburizing depth at 0.35 %C was perfectly achieved in both furnaces. However, the over all process time in the pusher furnace was 22 % longer, which was in correlation with the experiences of some customers. The measured process parameters allowed an exact comparison of the achieved carbon profile and the calculation via FOCOS-off-line diffusion program, showing a 0.06 mm higher carburizing depth in the ring hearth and a 0.09 mm lower carburizing depth in the pusher furnace. Discussion The reason for that significant difference in carburizing speed had to be found. It has to be mentioned that the calculation was carried out with measured and verified process parameters. First of all, differences between measured heating times and real values had been evaluated. Therefore the heating times were calculated via computer simulation, too. It could be found that differences between real parts temperature and optical measurement might influence the over-all case depth only in the range of approx to 0.03 mm. The main difference between the two furnace types was found in the improved gas transport ability between the loads in the ring hearth furnace, since the loads are not pushed together, leaving some space in between. Due to that, a better gas exchange and a higher carbon transfer coefficient can be reached in the ring hearth furnace. Furthermore the complete separation of heating, carburizing and diffusion chamber allowed a better separation of different temperatures and C-levels. Due to the size of the carburizing zone and the limited number of measuring positions in the pusher furnace this difference was not easy to be verified in the test run. Economical considerations A comparison of pusher furnaces and ring hearth furnaces for carburizing and oil of full loads shows advantages and disadvantages for both designs: The pusher furnace in a two- or threetrack design is very flexible, since two different case depths can be applied simultaneously. Furthermore, very high throughput capacities ( 1500 kg/h, depending on the desired case depth) can be achieved. The ring hearth furnace allows less strain and higher lifetime of the trays due to rotation of the loads, therefore savings of basic trays and furthermore a significant reduction of process time. On the other hand, this design is less flexible and has a limited throughput capacity of kg/h. An economical comparison shows that the investment cost of both furnace designs with similar throughput capacity are comparable. Therefore the over-all heat treatment costs do not differ that much. However, the ring hearth allows some cost savings due to shorter process time, less energy consumption and a reduced amount of charging trays. Conclusion Two examples of modern furnace design demonstrate that modern gas carburizing furnaces can fulfil high requirements for improved parts quality and economy. The use of a high pressure gas module in combination with a pusher furnace enabled reduction of the distortion of the toothing of gear shafts significantly, so the grinding operation could be saved. This justified the higher investment costs with a return on investment of less than two years. Secondly, an improved ring hearth furnace design was presented. The ring hearth can be used both for carburizing with subsequent press of sensitive parts and carburizing with subsequent oil or gas for gear wheels and shafts. Both concepts allow some cost savings due to significant shortening of the process cycle, less energy consumption, less space requirement and savings of charging trays. On the other hand, pusher furnaces are more flexible if different case depth is required and they are preferably used for high throughput capacities. Literature [1] D. Liedtke: Proc.of the Int.Conf., Zürich, CH, , P [2] H. Altena, F. Schrank: Härterei-Techn. Mitteilungen 57(2002)4, P [3] K. Löser, G. Schmitt: Heat Processing (2) Issue 3/2004, P HEAT PROCESSING (4) ISSUE