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J. Heat Treat. (1990) 8:147-155 �9 1990 Springer-Verlag New York Inc.

Technical Note: Induction Heat Treating Processes for Near-Net

Shape Applications I

George F. Bobart

Abstract. An overview of manufacturing processes and heat treatments and their use in near-net shape part processing applications is given. Recent advances in process/power interfacing and their beneficial effects are covered as well.

Introduction

In typical metalworking operations, products start with raw materials and move through a number of processing stages as they are transformed into finished parts. The initial processing generally involves melting or softening the material so that it can be cast, forged, or rolled into the size and shape to meet end product requirements. Once it is shaped to near- finished dimensions, heat treating processes are often required to improve mechanical properties such as hardness, strength, and toughness in the finished product. Some heat treating processes, such as hardening, tempering, annealing, and normalizing may be required while the material is worked in continuous form; others after it has been cut or formed into near-finished dimensions of the end product. However, regardless of where it is applied, savings can normally be obtained by increasing the speed and/or reducing the number of processing stages to meet the application performance requirements of the finished part. To provide the required mechanical properties, while maintaining dimensional stability and surface conditions, induction heat treating is often required to provide fast, controllable, and selective near-net shape part process.

Presented at the 12th Heat Treating Conference/Exhibition, Indianapolis.

The author is a consultant with Pillar Industries, Inc., Men- omenee Falls, WI 53051.

Near-Net Shape Processing

The performance characteristics and the overall eco- nomics generally dictate whether the end product is:

1. cast 2. formed 3. pressed/sintered

Let's walk through each of these manufacturing processes and identify where and how precision heating/ forming techniques can be used to obtain near-net shape parts with fewer operations.

Cast Products

In the case of foundry products, most of the casting is performed using conventional sand molding processes. However, investment casting using solid or shell mold techniques is widely used for precision and specialty types of components, such as those required by the aerospace industry. Here, save induction melting is widely used as the process of choice to provide the control and flexibility requirements. Likewise, in ferrous and nonferrous primary mill operations, continuous casting to near-net shape products has rapidly expanded to replace older ingot casting techniques. In the case of ferrous products, electromagnetic induction stirring (EMS) is being used in the mold, as well as after the mold, to minimize surface and subsurface (slag and pin holes) defects in billets, blooms, and slabs. This eliminates the need

G.F. Bobart �9 Induction Heat Treat in Near-Net Operations

for subsequent reduction operations to overcome the presence of these casting objections. It, therefore, permits direct casting (rather than rolling) to near- net shapes (such as structural members) and to near- net dimensions (such as thin slabs) close to final product requirements. When continuous casting aluminum products, a basic surface quality problem exists that requires removing about i in. of the outer rough and porous surface area. However, this operation can be eliminated by incorporating electromagnetic induction casting (EMC) equipment that levitates the molten aluminum so the surface can be water chilled before making contact with the reciprocating mold. This precision casting process is being used exten- sively and can only be provided by a well regulated induction system to develop and maintain the levi- tating levels of the molten metal as it is vertically cast and solidified into blooms and slabs.

Forming

Most of the ferrous hot metalworking processes (forging, extruding, upsetting, etc.) only form the part to rough dimensions and, therefore, require further finishing processes such as flash removal, ma- chining, and grinding. However, warm (rather than hot) forging that is performed at lower processing temperatures of 1000-1500~ F can produce closer tolerance parts that require very little postprocessing operation. This has certain advantages over both hot and cold forging processes; however, it requires much more precision heating and forging equipment. In these applications, induction heating with special power/temperature controls and atmosphere protec- tion is generally needed to provide the near-net shape processing requirements.

Powdered Metals

A relatively new method of producing near-net shape products is via use of powdered metals. Here, the material is first compacted, using high pressures (30 tons/sq in.) to densify the basic powder, binder, and lubricants. Hot isostatic pressing (HIP) is a special process that provides high densification, lower temperature processing, improved microstructure (fine grain), and near-net shape capability. However, it typically requires close temperature control of the product and densification dies to ensure uniform compression of all powdered material. In these cases, the dies can be preheated via resistance or induction susceptor techniques, while the product is.uniformly heated in a separate temperature/atmosphere con-

trolled resistance or induction furnace. Once the parts are densified and sintered to near-net shape dimensions, they can then be directly induction heat treated as may be required for end product requirements.

Heat Treatment

Regardless of the materials and manufacturing processes to develop the final products, some basic considerations apply regarding how induction heat treating is performed to maintain the near-net shape properties in the part. In most applications, both heating and cooling parameters affect distortion and surface conditions and, therefore, are critical to maintain near-net shape requirements. As a result, various types of restraint features and protective at- mospheres may be required in each phase of the heat treating process.

Induction heating is ideally suited to surface hardening and tempering applications, since heat can be readily generated in the part to the required depth, while maintaining unheated and, therefore, strong rigid mechanical properties in the deeper core areas of the product. Here, as shown in Table 1, frequency selection of the induction power source for hardening ferrous materials is the key factor in generating the desired case depth:

Table 1

Hardened Depth Frequency Inches Hz

<~ 450000 1 3 ~ - ~ 100000 ~-~ 10000 ~-~ 30oo ~-~ 1000 h-through 200 Through hardening 60

In addition to frequency selection, power density can be used to expand the range of hardened depth for any operating frequency. The use of higher frequency, as well as higher power density heating (like 15-20 kW/sq in.) and fast quenching action, eliminates heating the core area of the workpiece. This minimizes distortion and often negates the need for external restraint features. While the chart as shown identifies the frequency selection for a hardening operation, a lower frequency is often used for the com- panion tempering process. The lower frequency and/ or a lower power density with a longer heat time is often necessary to ensure that the tempering process is effective over the full hardened depth.

148 ~ J. Heat Treating, Vol. 8, No. 2, 1990


While lower frequencies are needed for through- hardening applications, caution is required that current cancellation is avoided to insure good heating efficiency. A frequency selection chart for typical through-heating applications is given in Table 2.

Although some applications require through- hardening, most are better suited for surface hardening and tempering processes. The surface processes not only provide the hardness and compressive strength characteristics where they are needed, but retain the desirable properties in the core to minimize distortion and/or prevent cracking under certain load conditions. As a result, two types of case or

Table 2. Ferrous Materials

Cross Section, Magnetic Nonmagnetic Nonferrous In. Hz Hz Hz

Over 7 60 60 60 4-7 60 200 60 3-4 200 600 200 2-3 600 1000 600 1-2 1000 3000 1000

0.5-1 3000 10000 3000

Fig. 1. Scan hardening process.

surface hardening processes are widely used for in- line and discrete part processing applications.

Scan hardening and tempering, such as that shown in Figure 1, is often used to cover short-order processing of a wide range of part length and process variables. Here the coil as shown can be scanned along the part length while the power and scan speed are changed to compensate for part dimensional changes or process requirements. In a similar man- ner, the same process results can be obtained by scanning the workpiece and holding the inductor in a fixed position. Since only the surface area is heated to full austenitizing temperature, distortion is often within part processing requirements, even without the use of restraint features. However, on longer shafts, or those where greater hardened depth (as a percentage of the product cross section) is required, restraint features can often be added to maintain distortion within product limits. As shown in Figure 2, restraint rolls and/or chucks can be applied on machined areas along the length and at each end of flanged axles, shafts, trunnions, etc. In these cases, it is easier to scan the coil, and using encoder positioning sensors, lower the restraint rolls or release the chuck jaws when the coil passes the restraint area. If the part is short, such as with gears and bearings, stationary heating is often employed where neither the coil nor the part move during the heating cycle. The part is then quenched in position or rapidly moved to a submerged quench area. However, in most cases, particularly where only case hardening is involved, dimensional stability is good, without any need for restraint during either the heating or cooling cycle.

Single-short hardening and tempering, such as that shown in Figure 3, is often used to cover long-run operations. Since the coil is designed for a specific part, this method can maintain a uniform hardened case, even where basic steps in cross-sectional areas of the part are involved. In most cases, the coil travels along the length of the part rather than around its

Fig. 2. Low distort ion restraint diagram.

J. Heat Treating, Vol. 8, No. 2, 1990 ~ 149

G . F . Bobart �9 I n d u c t i o n Heat Trea t in N e a r - N e t O p e r a t i o n s

Fig. 3. Single-shot hardening process.

circumference. The part in Figure 3 is a 14 in. shaft for front wheel drive automotive applications and required near-net shape hardening with nil part distortion. As a result, a 600 kW-10 kHz power source with only a 4 sec heat time was employed and restraint rolls were provided to support the part during the heating/quenching cycle. However, in production, process straightness requirements were obtained even without using the restraint features.

Many of the scanning and some of the stationary and single-shot hardening and tempering applications are performed with the part in a vertical ori- entation. However, some of the scanning processes, particularly those with long part lengths and/or restraint features, are performed with the part in the horizontal position. However, whether processed vertically or horizontally, power source control and frequency selection, hardened depth, heat time, and quenching action have more effect on distortion and, therefore, near-net shape processing results.

Selective, rather than uniform case hardening can provide large bending stresses in the part, due to dimensional growth/contraction as the part is austenitized and then transformed into a harder pearlitic microstructure. Such is the case in Figure 4 where only the crown portion of railroad rails are hardened. Here, the rail head is heated to austenitizing temperature and then air quenched to provide the necessary bainite microstructure. Tempering is auto- matic due to the residual heat in the rail. However, to produce a rail that required nil straightening after heat treating, it was necessary to prebend (and, therefore, prestress) the 39 ft rail with the ends bowed downward about 3 ft. Using special power control, sensing, and hydraulic locating fixtures, the inductor and quench ring were adjusted in elevation and ori- entation (while the prebowed rail was scanned) to

maintain a constant clearance to the rail head and thus a hardened crown over the length of the rail.

Short cycle tempering and annealing are common processes where induction is used to dramatically reduce the heat treat time from that required in a fossil fired or electric radiant furnace. Here, a typical cycle for furnace batch annealing is 15 hr; whereas, with induction, it is the area of 25 sec. There is a logarithmic time/temperature relationship that permits the extremely short induction time cycles to yield very similar physical properties as a long heat time batch type furnace. This can generally be calculated with reasonable accuracy; however, the overall heating, soaking, and cooling cycles may need refinement to obtain the needed properties without further processing. Such is the case in Figure 5, where short cycle annealing of grade 1006 steel is accomplished by soaking at the recrystallization temperature for several seconds, aging at 800-950~ F for about 15 sec, and finally quenching to room temperature. This typically provides annealed tubing (that will not age harden) at speeds of 150 ft/min with a 60 ft line length.

Oxidation of the surface during a heat treating process is far less of a problem with induction as opposed to furnace heating, due to the short time at the elevated temperatures. However, in certain cases, discoloration would be detrimental, such as on a bright annealing line for copper or stainless steel tubing. Likewise, any scale that forms on a bearing or seal surface area would be detrimental and require a subsequent grinding operation. In these cases, a protective atmosphere can readily be provided and maintained during the induction heating and cooling cycles, thus eliminating the basic problem or any need for subsequent processing operations.

Fig. 4. Selective crown hardening of railroad rails.



Temp ~

1400

1200

I000

800

600

400

200

SHORT CYCLE ANNEALING

A - H e a t t o r e e r y s t a l l ~ z a t i o n t e m p . /

C - A l r c o o l t o 9 5 0 ~ F .

D - Age from 950 t o 850~F.-

E - Water quench E \

\ I i i i i 5 I0 15 20 25

Time In Seconds

Fig. 5. Typical heat/cooling curve for Grade 1006 steel-- short cycle annealing.

P r o c e s s / P o w e r I n t e r f a c e

In near-net shape heat treat processing, proper control and interfacing of the induction heating power supply with the process parameters is a key application requirement. While energy (kW.sec) into the workpiece is the key process heating consideration, the way the power, frequency, and time cycle are controlled by the power source will vary for different application requirements.

There are two basic types of solid state power sources; a voltage fed variable frequency and a current fed load resonant type. Most of the heat treat processes will use these types of power sources with frequencies up to about 100 kHz and use an (RF) vacuum tube oscillator if over 100 kHz in operating frequency.

The voltage fed units, such as the Pillar MARK 6 power source, typically utilize diode rectification, and, therefore, control the output power level by shifting the frequency of operation off the load resonant condition. In high Q lod circuits (such as nonferrous loads), the frequency shift needed to go from low to full power is very slight. However, in low Q circuits (such as close coupled magnetic loads), the frequency will be shifted further to provide the same power level change.

The current fed units, such as the Pillar MARK 7 and 8 power sources, typically utilize thyristor rectification and, therefore, control the output power by phasing back the thyristor rectifiers and holding a constant load resonant operating frequency. There-

fore, the operating frequency is not affected by power level but only by any changes in the reflected load on the resonant frequency of the inductor coil and load matching components.

Both current and voltage fed power sources can be properly applied to most applications; however, the type of regulation required may be different, particularly when critical applications, closed-loop control, or near-net shape processes are involved. For example, power regulation is often employed for applications such as static or single-shot hardening. Here, full power can easily be maintained with either type of power source but load matching requirements may be different. Likewise, power regulation is well suited for uniform loads to maintain a preset power level for any given production rate. However, if the load is dynamic and can vary, such as where spacing can occur between parts being processed, power regulation would typically overheat adjacent parts. Here, voltage regulation would often be the choice for control, as it would allow the power to adjust upward or downward to match the varying load conditions.

At times, sensitive near-net shape processes, such as electromagnetic casting (EMC), require control of the magnetic field associated with the inductor and thus, regulation of the coil current. A typical circuit uses a stiff regulator to maintain _ .05% coil current (rather than a typical _+.5%) on a current fed Pillar MARK 7-W power source. Since this utilizes a load resonant circuit, it phases back the thyristor rectifiers to accurately maintain the coil current for this sensitive levitation casting process.

In addition to basic process control and power regulation techniques, there are methods of super- imposing other control signals to cover special application requirements. For example, on a typical annealing application, a 140 kW-100 kHz solid state RF generator was used to heat tool steel to 1330 ~ F for in-line drawing of wire. However, the line speed varied from 20-120 ft/min and, therefore, a square root function generator was needed to convert a speed signal from a tachometer generator to automatically increase the power proportion to line speed. In addition, a temperature signal from an Ircon radiation sensor was superimposed on the power controller to hold the wire at 1330 -+ 10~ over the full speed range. Another sensitive application involves semi- conductor processing using a 125 kW RF vacuum tube oscillator. Here, the RF oscillator heats a graph- ite susceptor for epitaxial growth of integrated circuits on silicon wafers. Due to the extreme requirements for precise temperature control, a special three mode PID temperature control signal is superim-

J. Heat Treating, Vol. 8, No. 2, 1990 ~ 151


posed on the thyristor power setting to hold the susceptor temperature at 1200 _+ 1 ~ C.

I n - L i n e A p p l i c a t i o n s

In-line heat treat processes are ideally suited for high volume production operations. However, since line speeds of several hundred ft/min are typically required, near-net shape processing often requires larger power sources and longer line lengths with special material handling and process/power controls. A good example is shown in Figure 6 where 18 tons/hr of 2~ to 8~ in. pipe is hardened and tempered for oil country applications. Here, skewed-roll drives were used to feed and rotate the pipe throug h the heating, quenching, and tempering line to obtain final heat treat properties and mill straightness standards. Spe- cial processing controls were required to regulate the 8000 kW of power equipment, time at temperature, and cooling parameters for the various product sizes to meet distortion levels of + ~ in. over a 40 ft pipe

Fig. 6. In-line hardening/tempering of oil country pipe.

length with minimum in-line straightening requirements.

While it is common for the tube and pipe manu- facturers to process and ship the product (after final heat treatment) in reels or long lengths, end users often cut and form the material into discrete parts for final finishing and assembly applications. Such is the case, as shown in Figure 7, where strut rods for shock absorbers are manufactured by a major automotive company. Here, the 12 in. long product of varying diameters has been accurately machined to final part tolerances and required hardening and tempering without distortion or structural changes. The in-line quad head system shown in Figure 7 provides the flexibility to handle part lengths from 10 in. to 30 ft using special preloaded roll drives with a skew angle that rotates the part at a rate of 4.8 turns per linear inch of part travel. In this way, the product can be austenitized, quenched, and tempered without distortion at a rate of 12 ft/min within a 7 ft line length. In this case, an 80 kW RF generator was used to heat the strut rod to 1750~ F in 0.4 seconds to confine the case depth to 0.020 in. and it was then quenched and properly in-line tempered with a 50 kW-3 kHz power source. Special part positioning and temperature sensors with high speed precision controls were incorporated in the system to cycle/ regulate the power and line speed for this near-net shape processing application.

Certain in-line applications dictate no oxidation or discoloration of the product in its final processing stages. Such is the case in the production of stainless steel tubing, particularly where it is required for applications where dirt or scale on the inner diameter could be detrimental to end user requirements. Shown in Figure 8 is the heating and entry cooling portion of a line provided for bright annealing of stainless steel tubing used in the petrochemical and aerospace

Quad head machine and part flow diagram Support Base Skewed Temperature hlduchon

Main Frame ' b - ~ S k e w Head Assembly Adlustable~j__Chuck Rollers Sensor / Heohng Ceil

l ii Out feed Conveyor 1'

. . o~ ood . H I I M~176176 Fou. F,oder \ j j /ondR' C~176176

__/~Mechine Operating Control ~ H20 Quench ~ F,nder Part Present Fiber Optics

Tempering Section 4 Hardening Sechon

Fig. 7. In-line hardening/tempering of shock absorber components.

152 �9 J. Heat Treating, Vol. 8, No. 2, 1990


Fig. 8. In-line bright tube annealing. Fig. 9. Restraint hardening system for long shafts.

industries. Here, l in-�89 in. diameter tubing is heated to 1600 ~ F at rates of 250 ft/min using a 500 k W - 1 0 kHz preheat and a 125 kW RF postheat induction power source. In this installation, exogas was in- serted in the center of the 10 ft heating area and the protective atmosphere was maintained via sealed connections throughout the heating zone as well as throughout the 225 ft double-walled chamber that cooled the product to 600 ~ F. For other product applications, such as with copper and titanium tubing, different line arrangements using a protective atmosphere via a sealed direct quenching arrangement, can dramatically shorten the cooling portion of the bright annealing line.

Fig. 10. Narrow zone annealing.

D i s c r e t e P a r t P r o c e s s i n g

After the product has been cut, formed, and machined to near-net shape requirements; hardening, tempering, and stress relieving are often required before final assembly in the finished product. Here , the applications often require selective precision heating and handling equipment to maintain dimensional stability and prior processing performance characteristics. When the parts are relatively short and mechanically stable (like gears), lift rotate fixtures are often used as the handling equipment to process the part. When the part is longer, vertical or horizontal scanners are generally used. This type of handling equipment processes the part between end locating centers or chucks and at times, requires restraint features to maintain part straightness over its length, particularly where bearing areas are included. Such is the case in Figure 9 on a 500 k W - 10 kHz low distortion shaft hardening system. Here , an inductor scans shafts that are up to 8 in. in di-

ameter and 9 ft in length to provide case depths of 0.150 in. to RC 50 hardness levels. The system uses restraint roll assemblies that maintain contact at five points along the shaft length (retracting only to per- mit inductor passage), thereby maintaining average straightness of the preprocessed part.

In other applications, special techniques may be required to selectively heat for final processing requirements without altering the properties in adjacent heat treated areas. Such is the case in Figure 10 where narrow zone annealing was required by an outboard motor manufacturer. Here , a worm gear had been inertia welded to a lower drive shaft assembly and the annealed zone had to be confined to the circumferential weld area. In this case, a dual frequency 3/10 kHz system was used along with an energy monitor on the heat cycle to prevent softening of an adjacent hardened area on the drive shaft.

At times, special processes are required to provide distortion-free hardening of gears while producing a microstructure that contains optimum properties for

J. Heat Treating, Vol. 8, No. 2, 1990 �9 153

G . F . Bob ar t �9 I n d u c t i o n Heat Treat in N e a r - N e t O p e r a t i o n s

touch end use applications. Such is the case in Figure 11 that provides an ideal contour hardened pattern using a unique dual frequency process developed by Contour Hardening, Inc. in their Indianapolis facil- ity. Here, the gear is first preheated for less than 10 sec with a 3 kHz low frequency solid state power source. It is then flash heated in a very high density postheat inductor for 0.32 sec with a 450/600 kW Pillar RF vacuum tube oscillator. The gear is then rapidly submerge quenched to obtain the hardened pattern with a uniform 0.040 in. case depth to RC 56 hardness throughout each gear tooth. These sys- tems utilize precision handling and process control equipment as shown in Figure 12. They incorporate a real time on-line computer to control all machine and process functions. This includes an energy monitor to integrate power and time (kw.sec) and statis- tical quality control (SQC) of each part. This provides the heating accuracy to maintain dimensional stability within _ 0.0004 in. for a typical production gear. While the average automotive gear degrades two classes (typically from AGMA class 10 to 8) during a carburize hardening process, this selective

induction hardening system retains the initial class 10 or 11 standards on the heat treated gear. This process has been used to selectively harden over 350,000 production gears and several complete sys- tems for processing large diameter gears are being supplied by Contour Hardening to a major offshore gear manufacturer.

An alternative process utilizes powdered metal to produce near-net shape gears for select application areas. Here, sintered gears can be induction heated for applications that require hardened teeth with nil depth in the root. Such a pattern is shown in Figure 13 on an installation obtained using a 40 kW RF vacuum tube oscillator to heat the gear in 6 sec to 1600~ The hardened depth of 0.18 in. to Rockwell 77 (15N scale) was obtained from the tip of the tooth using sintered powdered metal 0.8% carbon material. Using a power density of 20 kW/sq in. to obtain the shallow pattern and a 14% polymer quenchant, good dimensional stability was retained in the near net shape part.

Fig. 11. Dual frequency gear hardening.

Fig. 13. Powdered metal gear hardening.

Fig. 12. Special dual frequency gear hardening system. Fig. 14. Torque converter hardening under protective atmosphere.


G.F. Bobar t �9 Induction Heat Treat in Near-Net Operations

At times, final heat treat processing must be performed under an atmosphere to eliminate any oxidation or scale formation on the finished part. A good example is shown in Figure 14 where a major automotive manufacturer required scale-free hardening of the bearing surface area of a torque converter hub assembly. Since the bearing seal was part of a special car warranty program, the bearing area was scan hardened using a 80 kW RF generator while the heating and quenching was performed under a sealed protective atmosphere.

Another application involved annealing beryllium bellows so they can be formed into pressure sensors for an automotive transmission application. As shown in Figure 15, this new installation heats the 9 in. long hollow tubes to 1600 ~ F in a horizontal end-to-end feeder using a Pillar 15 kW RF generator. However, since beryllium readily oxidizes, the heating had to be performed under a protective atmosphere. In addition, special power/temperature controls were incorporated in the process since the forming was required within 100 ~ F of the melting temperature of the beryllium tube. The various near-net shape part processing stages are shown in Figure 16 (left--un-

Fig. 15. Special beryllium tube annealing system.

Fig. 16. Near-net shape bellows part processing.

treated; center--annealed tube; and right--final formed part) for producing the bellows component.

Summary

This paper covered induction heat treating associated with a variety of near-net shape part processing applications. It included the metallurgical, equipment, and process control considerations associated with both in-line and discrete part processing. It identified where and how induction heating is being applied, and the associated process parameters and interface power requirements to provide the mechanical properties, surface conditions, and dimensional stability in the finished parts.

Many new near-net shape processes have surfaced and expanded in scope in the 1980s due to the ben- efits they provide to manufacturing operations to make things better, faster, and less expensive. Likewise, many advancements have been made in solid state power sources, power controls, and diagnostics in this same time frame to expand induction heating's capabilities to meet the near-net shape processing requirements.

Received March 15, 1990.

J. Heat Treating, Vol. 8, No. 2, 1990 * 155

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