One of the major benefits of the high-frequency induction carburizing process, known as InduCarb, is that there are no restrictions in form, shape, dimensions, or material of the part to be carburized[1,2]. The part to be treated, together with an inductive heater, are immersed in a liquid active medium containing carbon, and its surface is heated using high-frequency current. High concentration and precise limitation of energy only in the surface layer results in very rapid surface heating to any high temperature up to onset of melting, while the core temperature and properties remain unchanged.
During the process, the liquid active medium decomposes at the surface of the part being treated and a large amount of atomized, highly active carbon is released and is available for absorption and further diffusion into metal. Surface layers having optimum carbon contents and thicknesses ranging from the micron range to several millimeters can be obtained within a cycle time of seconds to minutes.
The mechanical and physical properties can be adjusted for various applications by varying the inductor design, active medium and processing regime. The process can be made continuous, and, together with only the local heating, is highly cost effective and environmentally friendly because no noxious emissions are produced. Because the carburizing process can be tightly controlled, the desired properties can be reliably reproduced, thus assuring high quality, reliability and longevity for each treated part with a corresponding decrease in life-cycle costs. The processing method and equipment are simple, which save capital expenditures, energy, labor, space, materials and maintenance costs[1].
Applications for this technology discovered through significant research and development include:
- A substitute for batch and inline furnace carburizing to improve the part quality and significantly decrease production costs
- A processing method for the restoration of the carbon content in any machine part
- A treatment for producing antigalling and antigripping properties on the surfaces of friction and conjugate pairs
- A substitute for flame, welding, and induction surfacing of fast-wearing parts by hard alloys
- A substitute for existing methods of steel anticorrosion protection
- Substitution of carbon steels for expensive high-alloy steels and special alloys
- Creating multilayered diffusion coatings having various properties
- Improving the surface properties of various special alloys based on metals such as titanium, nickel, cobalt, etc.
- Creating graphite-containing surface layers on friction pairs
- Creating heat-resistant diffusion coatings for use in applications at elevated temperatures
These possibilities have attracted the attention of many potential users worldwide.
Induction carburizing equipment and technology
The basis of the induction carburizing process, various schemes of equipment for this technology, some results of research and development, and other possible applications have been presented in several publications[1-4]. The process is carried out in special patented equipment, which is an adjunct to the existing equipment for high-frequency heating.
Specific design parameters for induction carburizing depend on peculiarities of the parts to be treated and the protective coating to be formed. The form and thickness of the protective diffusion layer depend on peculiarities of induction coil design, frequency and power rating of the power supply, time and rate of heating. Chemical composition, structure, and properties of the surface layer depend on the composition and properties of the original alloy, active medium, temperature, time, heating rate and cooling conditions after treatment.
An example installation for induction carburizing of small parts is shown in Fig. 1. In general, the installation consists of eight main systems:
- High-frequency power supply
- Different buses, leads, sprayers, and coils, the designs of which depend on the particular form and shape of the part to be treated
- Part handling equipment for chucking, rotation, linear and vertical motion with correspondent loading and unloading devices
- Control desk for equipment having push-button manual and automatic control
- Temperature sensing and control system, which can include the control of the complete process cycle; a computer program also can be designed to control processing regimes and all acting parts of equipment such as active media system, ventilation, and emergency devices
- Active media storage tank having recirculating, cooling and emergency systems
- Ventilation system
- System to control the part's surface form, dimensions and roughness
The main component of the installation is a bath containing liquid active medium. The part to be treated together with handling devices, heaters and all their connections from correspondent power supply, and some other installation components are immersed into the liquid medium. The equipment requires minimum space and maintenance, a single operator, and produces no harmful effluents. The technique, which can operate either in a continuous or batch process, is highly cost effective and affordable.
Control systems
The characteristics of the new technology have inspired the development of a dedicated control system to accurately measure and control part temperature in an application previously unexplored. A closed-loop control system for induction carburizing technology has six main components: (1) A sensor, which detects the temperature of the part surface being treated; (2) A compatible temperature instrument that converts a temperature value into an electrical signal; (3) A temperature recorder, which helps in optimization and supporting of the technological parameters in production conditions; (4) A temperature feedback controller, which measures the value of the control variable and compares the electric signal delivered by the sensor with the setpoint as a feedback value. If any deviation exists, the process variable is calculated from the deviation and the corresponding signal is sent to the power supply (5), which delivers the appropriate corrective power to the heater (6).
It should be noted that for accurate control of the induction carburizing process, it is necessary to properly match the characteristics of all equipment components to eliminate the possibilities of dead band, droop and overshoot. Despite the use of very short cycle times, high heating rates and high temperatures in induction carburizing technology, a special closed-loop control system can provide reliable and repeatable processing of parts as shown on the temperature charts in Fig. 2. The desired properties can be reliably reproduced, thus assuring high quality of each treated part.
Processing conditions
The main treatment parameters of induction carburizing are heating rate, processing temperature, and time. These parameters have a significant influence on the structure, thickness, chemical composition and carbon profile of the part surface layer. Superior quality of the surface layer structure can be obtained at various processing regimes, but for the greatest effectiveness, the highest temperature and heating rates and the shortest possible processing time are preferable. It is necessary to use different processing regimes for different parts, materials and active media.
Heating rates ranging from 1200 to 1500C/sec (2160 to 2700F/sec) for 1 second of total heating time to 20 to 25C/sec (35 to 45F/ sec) for 1 minute of total heating time are acceptable. A processing temperature up to the melting point can be readily attained. Steel carburizing temperature intervals from 950C (1740F), which is the maximum furnace carburizing temperature, to 1350C (2460F) and even 1450C (2640F) can be used. Higher carburizing temperatures create higher active atmosphere carbon potential to produce higher absorption rates, increase the carbon concentration gradient, and decrease the processing time to minutes or seconds. Various, well-known cold liquid active media (LAM) of various chemical compositions, physical conditions and active element contents (potential) can be used in various applications.
The two main types of induction carburizing used for iron-base alloys are divided by processing temperature; that is, low- and high-temperature processing. Low-temperature processing occurs well below any possibility of melting of the carburized surface layer, which can contain any carbon content. This treatment temperature cannot be higher than 1150C (2100F) or lower than Ac1 line (727C/1340F). The cycle time for this process can be unlimited and depends on surface layer thickness, chemical composition and profile of alloying element(s) that must be obtained in a given machine part. High-temperature processing is conducted above the eutectic temperature, which would cause melting of surface layer. This processing temperature is limited from slightly below 1150C (2100F) to 1540C (2800F). The cycle time must be strictly controlled and limited. In general, processing time usually is in intervals from 1 to 60 sec depending on surface layer thickness.
Protective-coating microstructures after treatment
The distinctive attributes associated with high-frequency induction carburizing makes it possible to quickly produce any structure as represented in the Fe-C phase diagram. The carbon content in surface layers can be varied in wide limits from such optimal carbon concentrations as 0.78 to 0.85% to high concentrations such as 2 to 5% or higher. Characteristics of the part being treated, such as its service conditions, protective diffusion coating thickness, and part surface roughness, determine possibilities and principal ways of using of this technology.
After induction carburizing, the microstructure of the surface layers can be pure cementite, hypereutectic (carbides and ledeburite), eutectic (ledeburite), hypoeutectic (ledeburite and austenite or ledeburite and products of decomposed austenite), pure austenite, and/or products of austenite transformation. Very stable high-carbon austenite and mixtures of this austenite and carbides also can be produced. The properties of these structures can be varied from very hard to very soft, from friction and wear resistant to corrosion, fretting, erosion and cavitation resistant.
This technology also can be used to improve surface layer properties of different special alloys and superalloys created on a metal base such as iron, titanium, nickel, cobalt, etc., or a metal containing sufficient amounts of carbide forming elements such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, boron, beryllium, etc. In these cases, the surface layer microstructure will contain the corresponding carbides, carbon solid solution, and other components, whose particular properties depend on the type and chemical composition of the alloys.
Applications
One application of induction carburizing is the production of a pearlitic surface layer (Fig. 3a) on various machine parts requiring a hard protective case on a softer, tougher elastic core. It is possible to create any useful thickness, chemical composition, structure and properties of the pearlitic surface layers. A small surface layer thickness (less than 0.25 mm, or 0.01 in.) can be used for carbon restoration of the intermediate products after conventional heat treatment and for small parts. A thickness of over 0.25 mm can be used for all other parts.
It is also possible to produce a carburized surface layer thickness that does not exceed 0.5 to 0.6 mm (0.02 to 0.024 in.) for the majority of machine parts and for completely finished parts. This possibility significantly decreases the machining of carburized parts, especially grinding, because the maximum wear of most carburized surfaces does not usually exceed 0.2 to 0.3 mm (0.008 to 0.01 in.).
There are countless numbers of machine parts that can be successfully treated by this kind of processing. Induction carburizing can be used to create both pearlite with globular and/or laminate carbides (Fig. 3b and 3c) depending on the necessary surface layer properties. All of these structures must be hardened and tempered using a separate treatment. The significant advantage of induction carburizing is that all types of reheat treatment can be produced using the same active medium and inductor that was previously used for carburizing, (for individual, small and medium types of production); that is, without removing the part from the equipment. For large scale or mass production, separate inductors can be used for reheat treatment in the same manner.
Another application of induction carburizing is the ability to create a martensitic (or austenitic) surface layer in a single cycle; that is, without separate reheat treatment (Fig. 4a). These surface layers can differ in thickness, chemical composition, structure and properties as needed, depending on the treatment and service conditions of the parts. For superhard, wear-resistant surface layers, a mixture of martensite and globular carbides can be created (Fig. 4b), and for hard surface layers, a pure martensitic structure can be created. For a hard, and simultaneously anticorrosive and wear-resistant surface layer, a mixture of austenite and globular carbides can be created. For corrosion resistive surface layers, a pure austenite can be obtained.
Another application is the possibility of creating multilayered surfaces (Fig. 5a, b, c and d). This kind of treatment can be used for special applications such as self-sharpening blades and cutting members. The thickness, hardness, and/or other properties of the different layers can be varied depending on the service conditions of the specific parts. It is not possible to achieve these results using conventional carburizing furnace technology.
Induction carburizing also is used to create unusual surface layers having high carbon content. These diffusion coatings consist of ledeburite structures or graphite particles. Ledeburite structures (Fig. 6a and b) have structural properties very similar to those of high-alloy materials, which are now widely used in harsh, abrasive conditions such as agricultural machinery, mining, oil and gas equipment, excavators and road machinery. In these applications, very expensive and complicated flame, welding, and induction surfacing technologies currently are used. Structures that contain graphite particles can be used for self-lubricating parts in friction pairs and in many other applications. Life-cycle costs can be reduced by creating ledeburite surface layers on parts that now have short life spans.
Other applications include the creation of the special anti-galling coatings (Fig. 7) for machine parts that work as friction and conjugate pairs, or parts of threaded, detachable, grinding-in, breaking-in, slip, telescopic, etc. joints.
For more information: Saveliy Gugel is president, Sanova-Polytech Inc., 109-10 Park Lane South, Suite B3, Richmond Hill, NY 11418; tel:/fax: 718-847-1157
