Low-Distortion Heat Treatment of Thin Parts
Rotating symmetrical parts having small-to-medium cross-sectional thickness are required in the manufacture of automatic transmissions and clutches. The part material is selected based on the forming procedures that will be used, the material's hardening properties and dimensional stability and the heat-treating process. Use of high-carbon steel makes it very difficult or impractical to use efficient mass-production fabrication processes such as stamping and forming. Therefore, it makes good sense to use inexpensive, formable steels, such as unalloyed low-carbon steels, in these applications. Benefits from using these materials include lower tooling and raw-material costs and improvement in the microstructure, durability and fatigue life of parts. However, unalloyed, low-carbon steel parts produced using punching and stamping require additional processing to enhance their durability (high wear resistance), stiffness and fatigue life, as well as to reduce stress fractures. At Daimler-Chrysler Corp.'s Toledo (Ohio) machining facility, a carbonitriding system including an integrated quench press-hardening machine installed by Aichelin Heat Treatment Systems is used to heat treat thin automotive torque-converter parts.
Daimler-Chrysler's torque-converter application requires heat treatment of a piston (Fig.1) and spring retainer ring (Fig. 2). After heat treatment, the retainer, springs and piston are assembled and joined using rivets. Thus, the assembly process requires dimensionally stable parts and controlled tolerances, such as rivet hole sizes, hole alignment and flatness for both parts. For the torque converter to perform properly, the accuracy of the inner diameter and the flatness and the taper of the lockup surface for the piston are essential and critical. In addition, the surfaces of the parts are required to have sufficient hardness and good wear and corrosion resistance. Metallurgical properties, heat-treating requirements and critical dimensions for the torque-converter parts are listed in Table 1.
The carbonitriding process was selected over alternative processes including ion carburizing, ion nitriding and salt-bath nitriding to obtain high surface hardness and wear resistance on heat-treated torque-converter parts. Carbonitriding primarily is used to achieve shallow case depths.
In the carbonitriding process, parts are heat treated at a temperature of approximately 850C (1560F) in an atmosphere containing carburizing gases and ammonia (NH3). The carbonitriding process can be operated at lower temperatures than a typical carburizing process because the Fe-C-N crystal transformation temperature is lower in the ternary system than that of a binary Fe-C system. Carbon and nitrogen diffuse from the surface into the metal and form a hardenable layer. The hardenability of the unalloyed steel is improved by adding ammonia, although carbon is the primary determining element for hardness. Additionally, wear resistance and tempering stability of the part surface is increased.
The NH3 content generally is selected to be between 2 and 5% of the furnace atmosphere, but must be adapted to the specific requirements of the particular material and shape of the parts to be heat treated. Very high ammonia concentrations leads to increased retained austenite in the surface layer. As in all carbonitriding applications, parts need to be thoroughly cleaned of all surface contaminants before heat treatment. Only clean surfaces ensure uniform carburizing and nitriding, thus providing uniform hardness and dimensional results.
In this particular heat-treating line, three parts are automatically loaded from a storage magazine onto a tray, which subsequently is fed by a manipulator arm into the rotary hearth furnace. To minimize the introduction of oxygen into the furnace, the entry doors for the furnace are designed as small as possible. Prior to entry, the door area is protected by a flame curtain before the entryway is engaged. A single tray containing three green parts is charged, and a single tray containing three carbonitrided parts is removed from the furnace within a cycle time of 35 seconds.
The furnace atmosphere is formed by the addition of endothermic gas having a controlled dew point of less than 3C (38F). Methane (CH4) carburizing gas is introduced in sufficient quantity to maintain a carbon potential of 0.8-1.0%. Endothermic gas is combined with up to 8% ammonia.
Due to the difference in part geometry, the furnace temperature is set at 900C (1650F) for the pistons and 830C (1525F) for the retainers. Furnace residence (cycle) time is set at 45 minutes. Within the 45-minute period, parts are heated to the prescribed temperatures and the transfer of carbon and nitrogen occurs via their respective transfer and diffusion mechanisms.
Moving parts from the furnace to the quench press needs to be achieved in the shortest possible time to avoid oxidation and cooling effects. In this application, the time interval was determined to be 5 seconds or less. The exiting heat-treated parts (fixtured in groups of three) are secured by the loading/removal arm and immediately loaded into an adjacent quench press machine to achieve low distortion and dimensional accuracy during quenching. The polymer quench medium is maintained at a 15% concentration, and the quench bath temperature is controlled at 50C (120F) using an external cooling device. Quench press forces up to 50 kN (11,240 lbf) are used.
After the quench press operation, a material handling device flips, drains and rearranges in six/row configuration and transfers the pistons. The parts are flipped to drain the residual quench medium and minimize product waste. Afterward, it also is possible to remove parts before the washing and tempering processes for the purpose of inspection, quality check and storage.
The parts are conveyed through a parts washer by means of a mesh belt. In the parts washer, parts are completely cleaned of adhering quench medium and other solid surface contaminants. To achieve optimum cleaning, the parts washer consists of a wash and rinse zone, and the washing solution concentration level is monitored and maintained at 5% cleaner solution in the washer and less than 0.5% washing solution (defined as carryover) within the rinse zone. Hot washing solution (80C, or 180F) is directed toward the parts via nozzle jets from the top and the bottom of the spray zones. In the final stage, parts are dried via air jets before leaving the parts washer.
The piston is tempered at a temperature of 175C (350F) for 50 minutes. At the outlet of the tempering furnace, an in-line cooling station cools the parts to ambient temperature. Finally, the heat treated parts are transferred by means of a roller transport system to an assembly line.
The heat-treating plant consists of two production lines. Figure 3 illustrates the heat-treating line for the pistons, which includes a tempering furnace. Tempering is not required for the very thin cross section retainer parts. The parts are washed after the quench press, and are sent to the next step in the manufacturing process.
Both furnace lines have a capacity of 300 parts per hour for parts that range from 100 to 280 mm (4 to 11 in.) in diameter and weigh less than 2.3 kg (5 lb). Attention to uniformity in design characteristics was a prime consideration to provide for production flexibility. A flexible platform having common components linked by a data highway was designed for both lines. This makes it possible to change over from one production part to another on the same heat-treating line by means of a rapid exchange device for the quench system and selection of the appropriate recipe.
Protective gas is generated using two endothermic gas generators, each having a capacity of 60 m3/h (2,120 ft3/h). During typical operating conditions, one endothermic generator can supply the protective atmosphere for both lines. The gas flow from one generator can be controlled between 20 and 60 m3/h (705 and 2,120 ft3/h). The second gas generator typically is on standby mode and can be used for supply at any time.
The rotary hearth furnace (Fig. 4) has an effective diameter of approx. 3.5 m (11.5 ft) and can be loaded with 76 trays maximum. Recuperative burners housed within ten radiant tubes are used to heat the furnace, which contains two heating zones. Each burner is operated via pulsed burner control using a pulsed frequency proportional to the load. Temperature uniformity at operating conditions in the entire furnace chamber is better than I5C (9F). The concentration of the process gas is controlled nearly constant over the furnace volume by means of a ventilator.
An electromechanical device operates the furnace door and its movements are initiated using the signals from the manipulator arm. To avoid oxygen entering the furnace chamber, a flame curtain covering the door area is ignited before and during the interval that the furnace door is open. The hearth is driven using a speed-controlled electric drive; rotational movement is transferred to the hearth by means of a gear and a gear rim. An absolute-position controller maintains and controls the hearth position.
The manipulator arm discharges the parts out of the furnace and shifts them by a flip and push mechanism into the quench press. The essential components to achieve low-distortion quenching are the quench press and its tooling. Three quench tools are arranged in parallel on the quench press (Fig. 5). The tools consist of an upper and lower plate, the shapes of which are adapted to the part. Tooling geometry may differ from the part dimensions on the blueprint to achieve part print tolerances. The plates secure the parts at critical locations during the quenching process. Consequently, distortion due to uncontrolled "shrinking" during the hardening process is minimized. The quench medium cools the surface of the part during the press process through small bore holes in the tool plates. Tooling design, such as where to hold the parts and where to allow movement, is based on experience. The quench press is equipped such that it can automatically discharge scrap parts that are out of tolerance due to the heat-treating or quenching processes. The quench press also is capable of running one, two and three parts at a time.
The plant was designed for fully automated operation. All major components including rotary hearth furnace, endothermic gas generator, hardening quench press (including magazine and manipulator), parts washer and tempering furnace are equipped with separate control loops. All signals necessary for the overall control of the line are sent to the central operator panel (Fig. 6) via a data bus system. The operator supervises all functions of the heat treating lines from the centralized location. The process control system is based on the Data Highway and Modbus System from Allen Bradley. The data-acquisition system enables personnel to have an on-line survey of all plant data, part quality records, trends about the most important parameters, failure analysis, on-line documentation and operating and maintenance instructions.
To achieve targeted metallurgical parameters, additional steps were necessary to optimize part quality. This especially was true given the required critical tolerances for piston flatness and tapers. By adapting the shape of the stamping tool in the prefabrication stage, subsequent deviations in dimensional tolerances were accounted for in the tool design. Intensive detailed optimization and development of the quench tools, careful prewashing of the parts and optimal control of furnace parameters enabled bringing the dimensional variance of the taper within the outer ring into an acceptable tolerance.
The selected steel grades are very sensitive to oxide formation at high temperatures, and removal of sticking scale particles (iron oxides) required an upgrade of the parts washer during trial operation. Implementation of additional nozzles, a stronger pump and an increase in temperature of the washing solution achieved a significant increase in the cleaning effect. As torque converters require absolute scale-free part surfaces, the parts had to be descaled using acid cleaning. Several tests were performed to change to an integrated cleaning system. A vibratory abrasive system turned out to be the best fit and is currently being used.
For more information: Dr. Peter Schobesberger is vice president, Aichelin Heat Treatment Systems Inc., 37584 Amrhein, Livonia, MI 48150; tel: 734-953-1303 ext. 13; fax: 734-953-0980; email: firstname.lastname@example.org