Vacuum Carburization of High-Performance Automotive PM Parts
Automotive gears, particularly highly stressed engine and transmission helical gears, traditionally are machined from wrought bar stock and subsequently heat treated to develop the required surface hardness and tooth bending fatigue durability. Conventional powder metallurgy components, given their high residual porosity, cannot meet stringent bending and surface fatigue requirements, and, therefore, generally have not been considered for use in such high performance applications.
However, since 1994, innovations and advances in PM materials and net-shape processing at Stackpole Ltd., have been well proven in high volume production on complex geometry parts, and have demonstrated the cost advantages of PM . A combination of several key technologies, such as appropriate alloying, net-shape forming, high-temperature sintering, densification and vacuum carburizing, is used to raise the performance level of PM steels to the required level.
The development of high strength PM components having steel-like microstructures, and the role of vacuum carburization in this process is discussed. A recent development in the use of acetylene low pressure carburizing (ALPC) also is described. This process, originally developed in Japan, and also practiced in Europe by Ipsen International , is gaining widespread use because of its processing advantages over conventional carburizing gases such as propane.
Powder metallurgy processing using conventional press and sinter methods typically produces part densities of about 7.0 g/cm3 (0.252 lb/in.3). Which is about 90% that of wrought steels. Consequently, PM cannot be considered for automotive transmission gears as fatigue and surface durability are inferior to those of fully dense steels. Residual pores are defect sites and serve as stress raisers, reducing dynamic properties and stiffness.
In operation, transmission gears are subjected to high fatigue stresses, which arise from high hertzian contact pressures (with sliding) and high tooth bending loads. There are very few automotive gear applications for which PM, at 7.0 g/cm3, has adequate bending and surface durability properties. However, by enhancing the density of PM using a variety of recently developed methods, and subsequent vacuum carburization, the appropriate properties may be produced for a wide range of gearing applications.
At one end of the durability requirement range are relatively lightly loaded engine balancer gears. Tooth flank densification and hardening to 50 HRC is adequate for tooth surface durability. Speed gears, on the other hand, are subject to more severe bending and surface stresses. Tooth root and flank densification and vacuum carburization is recommended for this application.
Transmission sprockets having spur gear tooth forms require good tooth flank and journal bearing surface endurance properties; the latter should be equivalent to AISI 52100 carburized bearing grade steel. These parts are routinely vacuum carburized in hot-wall, continuous Ipsen-Abar through-feed furnaces at a rate of 20,000 pieces per day. Furnace loads of 390 kg (175 lb) are typical.
Durability Enhancement in PM
The technologies used to enhance PM part performance are briefly summarized below. A more detailed discussion of these technologies is given in .
New high-temperature sintered alloys have been designed to provide adequate strength and hardenabilty for use in gearing applications. For example, sprockets and balancer gears are made of a lean FeMnCrMoC grade, having a total alloy content of only 1.5%, but having excellent hardenability to produce good core hardness on quenching after carburizing. Also, specific compositions have been developed for transmission gears, tailored to application requirements [4-6].
Core density and surface density have a strong effect on fatigue performance. In addition, core density also influences tooth deflection behavior because elastic modulus (E) is strongly dependent on density :
E = (1 - P)3.4 c 206 GPa
where P = volume fraction of porosity.
In considering density, a critical concept with regard to gearing applications is the ability to selectively densify only those regions of the component that are subjected to high stresses. Selective surface densification to a depth of between 0.2 and 0.3 mm (0.008 and 0.01 in.) specifically is designed for particular gearing applications depending on functional contact and bending operating stress requirements.
Bending and contact stresses are at a maximum value either at the surface or within subsurface layers. FEA (finite element analysis) models of gear contact stresses show that the densified surface layer may contain these stresses. In general, contact stresses are higher than bending stresses, but are localized at the surface. Bending stresses penetrate deeper and, therefore, core density, in addition to surface density, becomes important in bending fatigue. Figure 1 shows the effect of density on bending fatigue of PM steels compared with hardened AISI 8620 .
An inherent advantage of the PM process is the ability to produce very complex shapes by means of net-shape compaction. Shape capability, coupled with low cost and steel-like performance, provides engineers with greater freedom in designing transmission gearing. For example, helical gears and clutch gears are conventionally machined in separate operations, and may require a subsequent joining process. However, it is possible to compact two combined gears to net shape at lower cost and without loss in properties using the appropriate materials, shaping and densification processes.
Vacuum carburizing PM parts
Vacuum carburization produces the optimum metallurgical microstructures for durability, and also minimizes part distortion, further enhancing the attributes of PM discussed above. Shape and size control are important economic considerations in process viability. Vacuum carburizing enhances the advantage of PM net-shape forming, as tight tolerances are maintained. Tooth leads and profiles are well controlled given predictable process characteristics.
Figure 3 shows a cross section and density profile of a selectively densified helical gear flank. The heat-treated surface hardness typically is >700 HV, with a case hardness of 500 HV at 350 Km (0.013 in.).
Stackpole has been very successful in using propane carrier gas (C3H8) to produce parts having high durability in high volumes. Ethylene (C2H4) also can be used as a carbon source. Both of these gases dissociate rapidly above 600C (1110F), into carbon, hydrogen and methane.
Although propane produces parts having good metallurgical structures and properties, soot and tar formation can present operating problems if allowed. The carburizing power of methane at pressures below 200 mbar is negligible. Therefore, relatively high pressures and an excess of propane are used to produce the required surface and case hardness and carbon levels.
Propane dissociates into a single free carbon atom, hydrogen and methane (CH4). Process controls ensure all parts of the load are adequately carburized to achieve surface carbon levels of 0.8%. Figure 4 shows a typical well-carburized, transformed microstructure. The structure consists of fine, high-carbon martensite with about 20% retained austenite.
Process controls (whether propane or acetylene) ensure all parts of the load are adequately carburized to achieve required surface carbon levels and a surface hardness greater than 700 HV.
Acetylene Low-Pressure Carburizing
Results of research in Europe  has shown that acetylene low-pressure carburizing (ALPC) is a highly effective alternative to using propane. Acetylene (C2H2), upon dissociation, has greater carburizing power, as each molecule dissociates into two free carbon atoms and one hydrogen molecule. Moreover, its tendency to dissociate only in contact with metallic surfaces makes for uniform carburizing of dense loads. Acetylene has excellent "throwing power" and is able to reach deep recesses and blind areas. Stackpole currently uses ALPC to process sprockets and gears. Figure 5 shows a typical load of sprockets ready for carburization.
Bearing tests have shown that endurance levels are equivalent to those for parts carburized using propane. Moreover, soot formation is minimal. It is of a loose, particulate nature, and is easily removed. A further benefit of ALPC is that cycle times can be reduced, depending on the product being run. Figure 6 shows an ALPC carburizing zone, showing only slight soot build up.
Densification processes can significantly enhance the fatigue durability of PM materials. Wrought-steel performance levels are achieved at lower cost, and the technology is applicable to a broad range of automotive transmission and engine gearing applications. A key factor in advanced PM processing is the application of vacuum carburizing to develop the necessary surface and case properties. Acetylene low-pressure carburization is now being applied in high volume production of sprockets and gears.
For more information: Dr. Rohith Shivanath is director, Metallurgy, Stackpole Ltd., Automotive Gear Div., 2430 Royal Windsor Dr., Mississauga, Ontario L5K 1J7 Canada; tel: 905-822-6015, ext. 2253; fax: 905-855-7363; e-mail: firstname.lastname@example.org