New liquid induction heat treatment and thermochemical processing technologies offer the capability to rapidly create a wide range of metal structure and properties modifications on various parts and components.

General view of LINCARB-1 production equipment prototype

Heat treatment and thermochemical processing of metal components is a vital part of many machine, equipment and tool manufacturing processes. SPI has developed new thermal processing techniques including liquid induction carburizing [LINCARB(tm)] and some other liquid induction thermochemical processes [LINTERPROCESS(tm)] and liquid induction heat treatment [LINHEAT(tm)] technologies, which have excellent possibilities to produce stronger, more durable products cost effectively. The simple, inexpensive processes offer the capability to rapidly create a wide range of metal structure and properties modifications on various parts and components.

Fig 1 LINCARB and various subsequent LINHEAT treatments are used to process Pyrowear Alloy X-53: (I) liquid induction carburizing; (II) intermediate cooling to a temperature below Ac1 in the inductor; (III) intermediate quenching in sprayer; (IV) liquid induction annealing (LINANNEAL); (V) liquid induction hardening (LINHARD); (VI) final quench in the sprayer.

Key characteristics of the new liquid induction technologies are the use of induction heating in a liquid medium to achieve the desired surface properties on a part. An inductive heater and the part are immersed in a liquid active medium (LAM) or liquid protective medium (LPM), and the selected surface of the part is rapidly heated to the required temperature using precisely controlled induction heating. The part can be stationary, rotating and/or moving through the inductor.

Dissociation of the liquid medium at the surface of the part produces a gas phase containing a large amount of atomized protective or highly active alloying element(s), such as carbon, for example. These elements either function as a surface protective coating or are absorbed and diffused into the surface to the desired depth, which is time dependent. The liquid medium also insulates and protects the part being treated from oxidation and contamination from the surrounding environment. The coatings can increase the durability and service life of the treated part.

LAM is used in the LINTERPROCESS to alloy the part surface with specific metallic or nonmetallic elements, and is used in LINCARB as a source of carbon to saturate the metal surface. LPM is used in LINHEAT as an effective quenchant and to provide a protective environment. Liquid medium consumption during all liquid induction processing is negligible, because only a very thin surrounding film on the part surface is heated, vaporized and dissociated, and subsequently participates in the thermochemical reaction and serves as a blanket, significantly decreasing processing time and energy costs.

The surface layer and core properties of parts can be adjusted to meet various application requirements, such as producing a low friction surface or wear and corrosion resistant surfaces, simply by varying the inductor design, active media and processing regimes. The process is tightly controlled, thus ensuring high quality of each treated part, as well as eliminating the harmful influence of heat and gas emissions on the environment.

Fig 2 LINCARB-1 production equipment prototype used to test LINTERPROCESS and LINHEAT technologies and approve the design of production equipment for various parts and applications: (1) power supply; (2) machine for metal product processing (I-ventilation chamber, II-docking zone, III-working zone, IV-auxiliary tank); (3) control system; (4) ventilation system; (5) light probe; (6) part-handling device; (7) control computer; (8) general equipment control desk; (9) printer; (10) water line for power supply; (11) observation window; (12) water line for inductor; (13) connection between power supply and inductor; (14) power supply control desk.

Treatment possibilities

The LINTERPROCESS produces both localized and overall protective diffusion surface layers on various metals and alloys. Coating characteristics depend on the characteristics of metal surface being treated, the liquid active medium and process parameters. Individual processes are identified based on the (L)iquid active medium, (IN)duction heating and alloying element(s), such as boron, carbon, nitrogen, aluminum, etc. For example, LINBOR corresponds to boronizing, LINCARB to carburizing, LINNITRO to nitriding, LINCARBONIT to carbonitriding, etc. Diffusion coatings created on carbon or low-alloy steels are reliable, cost-effective substitutions for special, expensive steels use in a majority of cases.

Various parameters can be used in the LINHEAT process. Heating rates depend on the high-frequency power supply, temperature-control system, LAM and the part to be treated. All structures created by induction carburizing can be subsequently heat treated (if necessary) using LINHEAT in the same equipment. Both processes can be performed using the same active medium and inductor without removing of the part.

Possible heat treatments include liquid induction annealing (LINANNEAL), liquid induction hardening (LINHARD), liquid induction double hardening (LINHARD-D) and liquid induction tempering (LINTEMP). Tempering can be done at low (200C, or 390F), medium (400C, or 750F) and high (600C, or 1110F) temperatures. Figure 1 shows examples of various treatment processes.

The processes are environmentally friendly and cost effective, reducing energy, labor, space, materials and maintenance costs. The technology offers alternative heat treatment and thermochemical processing methods to existing furnace technologies.

Fig 3 LINCARB-1 automatic control system: (a) general view; (b) computer touch screen with processing data sheet; (c) display of entire processing cycle; and (d) printing of the process documentation

Equipment characteristics

Design of production equipment depends on part characteristics and the protective coating to be formed. Figure 2 shows the initial production equipment prototype LINCARB-1. The four main components are a high-frequency power source, a machine for metal product processing, a programmable system to control all equipment parameters and auxiliary systems.

The high-frequency power supply includes the necessary apparatus to measure, control and support electrical parameters; induction coils, which have to be designed for each application; and a special safe connection between power supply and inductor. The induction coil is a critical component, which essentially defines the quality and efficiency of the induction heating. Its design depends on the workpiece geometry, heating pattern, cycling time, power and frequency.

LINCARB-1 has ancillary devices, sensors and instruments to provide precise, effective and repeatable treatment of the parts with predictable results. An innovative automatic programmable, computerized system (Fig. 3) provides both manual and automatic registration, control, and regulation of treatment parameters such as processing temperature and time of all procedures, position, rotation, and vertical movement of the part, LAM temperature and level, along with the sprayer, water and compressed air systems working parameters.The computerized system includes touch-screen control and registers all actions of the equipment and closed-loop temperature control system. The principal scheme of the closed-loop temperature control system is represented in Fig. 4.

Fig 4 Schematic of LINCARB-1 closed -loop temperature control system: (1) power supply; (2) inductor; (3) part to be treated; (4) sensor; (5) IR instrument; and (6) central computer

The system precisely controls metal surface temperature (submerged) in the liquid active medium during processing. The power supply (1) sends the necessary energy to the inductor (2), which has to heat the part (3) to the selected processing temperature. A sensor (4) measures the part surface temperature and sends the corresponding signal to the IR instrument (5). The instrument analyzes this signal, compares its value with processing temperature set point and sends signal to the power supply, which controls the heating power.

Also included in the system are various part-handling devices for chucking, rotation and linear and/or vertical motion with corresponding loading and unloading devices. Holding tanks contain liquid active media with devices for agitation, circulation, cooling, filtration and emergency systems. A sprayer is used to increase quench coolant velocity, which improves part surface quenching. Several auxiliary systems including water, compressed air and ventilation provide the complete package for a stand-alone system.

The equipment can easily be installed in a continuous production line or cell requiring minimum space and maintenance and a single operator, and it is environmentally friendly. The process is cost competitive and can treat suitable parts without any preparation, producing repeatable, reproducible results.

Fig 5 Schematic of scanning LINCARB (a), and shaft ready for treatment (b): (1) part to be treated; (2) handling device; (3) sensor of IR temperature control system; (4) inductor; and (5) sprayer

Production of diffusion surface layers

A wide variety of overall and localized diffusion protective coatings having good wear and corrosion resistant properties can be produced. Various types of stable and metastable structures can be created using the LINTERPROCESS, and can be improved further via LINHEAT using the same equipment in seconds or minutes. Surface layer form, thickness, chemical composition, structure and properties can be adjusted to suit the particular application. Examples of some applications are discussed below.

Fig 6 AISI 1045 carbon steel bars treated using scanning LINCARB have about a 4-mm deep effective case with a hardness of more than 50 HRC and a maximum hardness of 67 HRC

Treatment of shafts, axles, rolls, and similar products. Figure 5 shows the setup for a scanning LINCARB treatment of AISI 1045 carbon steel cylindrical shafts, and corresponding results are shown in Fig. 6. Various carbon and alloy steels, scanning velocities and processing temperatures are used to produce different structures and properties. Typical applications for these parts include railway transport, civil and military vehicles, tractors and farm machinery, etc.

Fig 7 AISI 1026 cold-rolled carbon steel DOMERW tube (a), macrostructure (b), microstructure (c) and microhardness profiles (d) after scanning LINCARB treatment

Treatment of pipes, tubes, bushings, and similar products. Hollow parts having a cylindrical or similar form can be scan-processed. Figure 7 shows the results of LINCARB treated pipe, and Fig. 8 shows the influence of pulling velocity of the part through an inductor on the type of surface layer structure and effective case thickness. Hard wear- and corrosion-resistant coatings can be produced on both external and internal surfaces. The most important applications for this technology are drill pipe, line pipe, bearings, and various bushings, liners, etc.

Fig 8 Influence of pulling velocity through the inductor on effective case depth (HRC>50) of 1.125 in. OD and 0.120 in. wall thickness AISI 1026 cold-rolled carbon steel DOMERW (ASTM A513T5) after LINCARB treatment: (a) OD surface; (b) ID surface

Corrosion-resistant coatings on carbon and low-alloy steels

Fighting corrosion in various applications requires the use of special protective coatings and/or the use of expensive corrosion-resistant materials, such as stainless steels and titanium, to increase component service life. SPI has developed and tested the use of LINCARB for the rapid, simultaneous production of high-quality (stainless) diffusion coatings on external and internal (in bores) surfaces together with a strong, durable core on inexpensive carbon and low-alloy steels. Optimum coating thickness and chemical composition is achieved via diffusion of carbon only in seconds to minutes. The core and case properties of plain carbon steels, powder-metallurgy products, special alloys and superalloys can be adjusted simultaneously. For example, SPI has produced a stable high-carbon corrosion-resistant austenitic diffusion coating on the surface of a part having a high strength (load carrying capacity) core (Fig. 9). This significantly reduces materials cost in the shipbuilding industry, for instance.

Fig 9 Stainless surface with high carbon content and high-strength core (I), created simultaneously from the base structure (at 100_) by scanning LINCARB (II) and corresponding 500-g load microindentation hardness profiles; red line = base metal surface, blue line = treated surface


There currently are no similar technologies to SPI's liquid induction heat treatment and thermochemical processing technologies. The new processing methods represent a significant breakthrough in material science, metallurgy and heat treatment, and compared with existing technologies, offer advantages such as 25-35% reduction of energy consumption, elimination of such harmful gases as CO2 and CO and 50-70% reduction of heat emission, 50-75% reduction in labor costs, 3-5 times reduction in manufacturing space, 1.5-3 times reduction in material costs, 2-4 times reduction in equipment cost, 5-10 times reduction in equipment maintenance and repair and the ability to be incorporated into production lines and cells. The ability to easily create high-performance surfaces on parts made of inexpensive steels can provide significant savings in material costs. Various types of licensing agreements for the new technologies and equipment are available from SPI.


The author thanks Drs. Julie Christodolou and George Yoder (U.S. Navy) and Douglas R. Harry and Vincent D. Schaper (Office of Naval Research) for their help in transforming the new technologies into real practice; Charles Hoffman and Edward Auger (Naval Air System Command, Propulsion Power Engineering) for making it possible to use the new technologies to produce helicopter main drive system parts; Dr. Robert Mullins, Charles Kilmain, Jerry Nanni, Frank Minden and Jene Alexander for active participation of Bell Helicopter Textron Inc. (BHTI) in this work; James Schuppe (Induction Systems Inc.) for manufacturing equipment; and colleagues Dr. Arif Azimov, Dr. Igor Komarovskiy, Dr. Harold Margolin, (Brooklyn Polytechnic University), and Fred Heinzelman and Jack Adelman for their invaluable input. IH