ELIMINATION OF GALVANIC COPPER PLATING PROCESS USED IN HARDENING OF CONVENTIONALLY CARBURIZED GEAR WHEELS

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1 International Journal of Automotive Technology, Vol. 11, No. 0, pp (2010) DOI /s Copyright 2010 KSAE /2010/ ELIMINATION OF GALVANIC COPPER PLATING PROCESS USED IN HARDENING OF CONVENTIONALLY CARBURIZED GEAR WHEELS Z. GAWRONó SKI 1), A. MALASINó SKI 2) and J. SAWICKI 1)* 1) Institute of Material Science and Engineering, Technical University of Lodz, Lodz, Stefanowskiego 1/15, Poland 2) Pratt&Whitney Kalisz Sp. z o.o Kalisz, Elektryczna 4, Poland (Received 30 April 2008; Revised 24 August 2009) ABSTRACT Recent developments in the aerospace and automotive industries have significantly affected the progress of modern manufacturing technologies, including the heat treatment of gear wheels. This view has been expressed in the works of Gräfen and Edenhofer (1999), Herring and Houghton (1995), Preisser et al. (1998) and Sugiyama et al. (1999). For ecological and economic reasons, however, traditional treatments are still in use. Additionally, the implementation of a new process in the aerospace industry is very difficult due to the safety precautions that are involved in this kind of production. In order to protect the surfaces of components from disadvantageous structural changes related to the hardening process (oxidation, decarburization and carburizing) galvanic copper plating is widely used even though the process is known to be harmful to the environment. On the other hand, as pointed out by Dawes and Cooksey (1965), it is commonly known that the most effective protection of a batch against these undesirable effects is a protective atmosphere applied during the heating. Therefore, the development of a fully controlled and repeatable process of gear wheel heat treatment under a protective atmosphere will reduce the global emission of toxic substances originating from galvanic copper plating and cooper stripping processes, while at the same time providing more effective protection of the parts. KEY WORDS : copper plating, heat treatment, protective atmosphere, hardening, gas carburizing, gear wheels 1. INTRODUCTION Gears constitute essential design components of most machines and equipment. As a result of their mating, gear teeth are subjected to temporary and permanent bending, abrasive wear and surface pressure, as they constantly face changes in the direction and magnitude of the applied forces. The above working conditions require that the gear material be characterized by proper impact strength and plasticity, fatigue strength and gear surface hardness. In order to meet these requirements, it is necessary to carefully select both the grade of steel and the type of heat treatment. Gears made of steel are usually subjected to the following heat treatment procedures: annealing or normalizing, hardening and tempering, surface hardening, nitriding, carburizing or cyaniding and hardening. Due to mass production and as a result of high quality requirements, gas carburizing and hardening are the most frequently used processes. During the hardening process, *Corresponding author. jacek.sawicki@p.lodz.pl different phenomena (such as decarburizing, stray carburizing and oxidation) often take place on the component surfaces. These phenomena are responsible for the detrimental impact of the furnace atmosphere. The following procedures are therefore used to protect the component surface: technological material allowance, overall coating with special protective pastes, heating in salt baths, heating in furnaces with a protective atmosphere, covering the surface with a galvanic copper layer. Of currently run processes, PRATT & WHITNEY KALISZ meets the strict requirements (in particular, those of decarburization and oxidation of the surface layer) of hardened gear wheels by covering the surface with a galvanic copper layer, assisted by additional application of a protective paste. Despite the protective procedures used, an increase in the oxidation value to a moderate level of mm was observed (see Fig. 1), with the maximum acceptable level of intergranual oxidation being mm. Additionally, superficial decarburization (Fig. 2) takes place, with hardness dropping to a moderate level of 80 HRA, whereas the level of hardness following the entire process (carburizing and hardening) should be no lower than 81 HRA. 1

2 2 Z. GAWRONó SKI, A. MALASINó SKI and J. SAWICKI Figure 1. Intergranular oxidation versus successive number of batches showing an elevated value for intergranular oxidation. Figure 4. Typical view of a part with stray carbon in a noncarburized area. The marked areas with dark spots show unacceptable indications. surface tempering NDT method as dark spots indicating an increase in surface carbon content (see Fig. 4). Oxidation and superficial decarburization are often a result of mechanical damage to the copper layer, weak adherence of the copper to the base, too thin a layer of copper. Figure 2. Plot showing the drop in superficial hardness of the top layer of a gear after conventional carburizing and hardening processes (PD pitch diameter, RL root lowest diameter). The decrease in the hardness reflects structural changes as assessed by non-destructive testing (NDT) carried out via the surface tempering method. In figure 3, which shows the results of the testing, we can observe whitish areas (indications) resulting from lower surface carbon contents. In addition to oxidation and decarburization of the surface layer, stray carburizing in the noncarburized areas is another frequently encountered effect. This effect is visible with the At PRATT-WHITNEY, the majority (over 90%) of quality non-conformances are due to mechanical scratches of the copper layer, with the rest being made up of stray carburizing due to inappropriate application of copper plating. The damage to the copper layer often occurs because of mechanical scratching during milling, slotting, polishing and washing, damage inflicted during transportation between operations, the porosity of copper (Fig. 5), buildup of copper (Fig. 6) or its brightening (Fig. 7), resulting from an inappropriate copper plating run. Figure 3. Typical view of a part with decarburized surfaces. Surfaces with whitish indications are marked. Figure 5. Example of a copper layer with pinholes due to inappropriate copper plating process guiding.

3 ELIMINATION OF GALVANIC COPPER PLATING PROCESS USED IN HARDENING 3 Figure 6. Example of a copper layer with grain-sized areas due to improper cleaning preceding the process of copper plating. Figure 7. Comparison of a brightened part with high potential for lowered thickness of the copper plate (on the left side) and a part without brightening (on the right side). 2. IMPROVEMENT OF HEAT TREATMENT AFTER CARBURIZING OF GEAR WHEELS It is widely known that effective protection (against oxidation, carburizing or decarburization) of a batch during heating and soaking may be provided by protective atmospheres, which can be applied to all kinds of heat treatments (carburizing, hardening, annealing and tempering). Assuming we run a fully controlled thermal process, we can expect predictable and, more importantly, repeatable results from this treatment. As a consequence, attempts have been made to eliminate the copper plating step and to replace it by introducing a protective atmosphere to the gear hardening step. Before the tests began, some organizational changes had been made in the Hardening Shop. The equipment used for hardening had to be modernized, with the authorization of the manufacturer. In the case of the ENE 10 generators, which generate the atmosphere in the furnaces, the following approved modifications were introduced: a procedure for registering the regeneration of generators was introduced (preceded by serial control pressure with the help of U-tube manometers), a special installation of atmosphere control to draw the gas sample was installed. The above changes were made in all generators, allowing for sampling of the gas and continuous control of the composition of the manufacturing atmosphere with a mobile gas analyzer 3 IR PGA Similar systems were installed in the hardening furnaces. Additionally, the current composition of propane used to produce the furnace atmosphere was controlled (based on certificates showing the chemical composition of the gas provided by the supplier). In each furnace used for carburizing (batch furnaces of type CASEMASTER AFS with integral quenching oil tank), the following changes were made: a system for continuous control of the pressure, consisting of a U-tube manometer, ball valve and tubular installation, was installed in the heating chamber of the furnace, a tube with a moveable rod allowing for collection of samples (which are periodically chemically or gravimetrically analyzed) was installed on the back wall of the heating chamber of each furnace; as a result, it was possible to determine the surface oxidation of a sample inside the heating chamber without influencing the ambient atmosphere (see Fig. 8), a procedure for saturation of the furnace with the desired atmosphere was introduced following each down-time longer than 24 h, with no manufacturing being possible during this time, a procedure for registration of each furnace chamber cleaning was implemented, optimum settings of generators, depending on the kind of processing, were experimentally established for carburizing of parts with surfaces pre-finished, carburizing of grinded details, hardening (hardening depending on the steel grade), Figure 8. Sampling set-up.

4 4 Z. GAWRONó SKI, A. MALASINó SKI and J. SAWICKI Figure 9. System for transport chamber blowthrough. depending on the progress of a process run, the system uses a variable setting of the dew point of the generator feeding the furnace, a system for blowthrough (with manufacturing atmosphere) of a leakproof chamber (casing) of a reloading chain integrated with the heating chamber of the furnace was introduced; this assembly prevents moisture accumulation in the chamber of the chain from influencing the heating chamber (see Fig. 9), a Lean Manufacturing philosophy was implemented in the heat treatment shop, changes in the construction of the explosion-proof hatches and upper burner were introduced due to the better atmosphere control, an additional gas feed-in system, consisting of separate gas lines, an electromagnetic (usually closed) valve and a cut-off valve (see Fig. 10), was introduced in order to have better atmosphere control, a procedure for periodic inspection of the oxygen probe controlling the carbon potential in the atmosphere and consisting of the measurements of response time and resistance was implemented, carbon potential controller settings were adjusted based on experiments. As a result of the changes introduced, we gained the following advantages: Figure 11. Lower values (on the order of mm) of intergranular oxidation after carburizing following the introduction of the protective atmosphere. Figure 12. Plot showing practically no drop in the superficial hardness of the top layer of a gear following the introduction of the protective atmosphere. (PD pitch diameter, RL root lowest diameter). the level of oxidation after the carburizing and hardening steps equals mm (see Fig. 11), with the admissible value being mm, a constant neutral atmosphere is present in CASE- MASTER furnaces during the hardening of samples (witness samples made of AMS 6265 material) in a continuous process. This solution entirely protects gear wheels against decarburization of their surface (Fig. 12). The consistently good results of the above tests inspired us to modify the manufacturing process for selected parts (particularly those that are subjected to grinding or turning operations after heat treatment) in such a way that the application of copper plating before hardening (of both witness samples and pieces) is eliminated. An example of the differences between the previously used and current heat treatments is shown in Table CONCLUSION Figure 10. Additional gas feed-in system. By applying the above changes, one should anticipate a

5 ELIMINATION OF GALVANIC COPPER PLATING PROCESS USED IN HARDENING 5 Table 1. Comparison of a sequence of operations for the current versus the revised heat treatment procedures a planet gear. Currently used process of heat treatment No. Operation Modified process of heat treatment No. Operation 35 Hardening 35 Hardening 45 Tempering 45 Tempering 90 Copper plating * 90 Copper plating * 120 Copper plating** 120 Copper plating** 123 Chemical cleaning 123 Chemical cleaning 135 Carburizing 135 Carburizing 145 Copper plating of samples 150 Heat treatment of samples 150 Heat treatment of samples 160 Copper strip 165 Cleaning 170 Copper plating*** 175 Hardening 175 Hardening 185 Cold treatment 185 Cold treatment 190 Low tempering 190 Low tempering 195 Copper strip 195 Copper strip 210 Inspection 210 Inspection Protection of surfaces: *surface of faces- other protected with masking stoppers. **surfaces of hole- toothing protected with mastic gum ***all surfaces protected decrease in the number of deviated and scrap components (made at the stage of heat treatment), as well as the following additional advantages: a shortening of the entire lead time for gear production by removal of certain operations (copper stripping after carburizing, copper plating before hardening and transportation time), which results in faster delivery to the customer. a reduction in the production costs of gears due to implementation of the modifications listed, as well as an improvement in gear produceability due to elimination of difficult logistic operations such as transportation to the plating shop (usually localized in a different building) and omission of copper plating and copper stripping. a unification of hardening procedures for gears protected with allowances and those, for which the second copper plating operation has been removed and the hardening operation directly follows the carburizing process. This may result in batch matching of different part numbers as well as lead time shortening. benefits resulting from the elimination of additional galvanic solutions that are hazardous to the environment used in copper plating and copper stripping processes. What is most important above all is the safety and health of operators. REFERENCES Dawes, C. and Cooksey, R. J. (1965). Surface treatment of engineering components. Metal Heat Treatment Conf., Birmingham, Gräfen, W. and Edenhofer, B. (1999). Acetylene lowpressure carburizing - a novel and superior carburizing technology. Heat Treatment of Metals, 4, Herring, D. H. and Houghton, R. L. (1995). The influence of process variables on vacuum carburizing. Proc. Sec. Intern. Conf., Carburizing and Nitriding with Atmospheres, Cleveland, Preisser, F., Seemann, W. and Zenker, R. (1998). Vacuum carburizing with high pressure gas quenching. The Process. Proc. 1 Int. Automotive Heat Treating Conf., Puerto st Vallarta, Mexico, Sugiyama, M., Ishikawa, K. and Iwata, H. (1999). Vacuum carburizing with acetylene. Advenced Materiale & Processes, 4,