The success of the of induction hardening is closely related to using an optimal quench process, which depends on design considerations including quenchant cooling rate remediation mechanisms, material effects, and spray and immersion quench systems.

Induction hardening is accomplished by the use of a suitable quenchant, which provides the required cooling rate to achieve the desired hardness. The selection of a specific quenchant depends on such factors as material, shape, crack-sensitivity and hardness pattern. The basic elements of the quench process were discussed in Part 1 of this article (see November 2005 IH). This article discusses the factors to be considered in designing a quench system.

Induction heating process

Two primary ways to apply heat inductively to a component are single-shot (static) and progressive scanning (shooting). In progressive scanning, the part is progressively moved through the inductor during the heating process versus static heating with single-shot. Typically, the component is rotated during heating and quenching cycle in both processes. Recently developed nonrotating systems are not discussed here [1].

It is important to understand the induction heating system design because it impacts quenching system design. Induction quenching systems are classified as direct or separate [2]. In a direct system, it is possible to heat and cool concurrently. The quenching system is built into the inductor by constructing a spray quench ring with quench holes facing the rotating part placed around a multiturn coil. The quench ring may be concentric with (Fig. 1), below or beside the coil [3,4]. Figure 2 illustrates cross section of a single turn scan coil to show the quench and coil cooling chambers. This system is often preferred for in-line production processes. In a separate system, the component is quenched into an agitated bath containing the desired quenchant, which is not an integral part of the induction heating system. Other coil and quench ring arrangements are summarized in Fig. 3 [4,5].

Material considerations

Several important material effects must be taken into consideration in the design of an induction heat treating system including part size and shape, whether through or surface hardening is performed, whether single-shot or scanning heating and hardenability [4].

Part size and geometry. Complex shaped parts containing holes, sharp corners, grooves, undercuts and diameter changes may lead to cracking and hardness distribution problems [6,7]. In addition, cooling rate for thin cross sections are very different than those for thicker cross sections, which require variations in the inductor design and/or quench blocks [7].

Through hardening and surface hardening. Cooling rates vary with the position in the cross section for a given material (Fig. 4); data show that the cooling rate increases from the core to the surface. At a specific section size, depending on the hardenability of the steel, the formation of upper-transformation products leads to a softer core than the surface. Heating time is shorter in surface hardening than in through hardening, and the core temperature does not increase significantly. During quenching, the cooler core provides an additional cooling potential relative to through-hardening [8].

Single shot and scanning heating [9]. Quench-ring configuration relative to the coil depends on the induction heating design (Figs. 1 and 2). In designing a system, it is necessary to determine whether it will be possible to quench a part and still provide enough residual heat to eliminate stress relieving or drawing. For example, by using time quenching, it is possible to use a separate immersion quench to control the amount of residual heat content in a single-shot process. Similarly, it may be possible to design a horizontal heating process for longer parts such as shafts to provide a spray quench with the desired residual heat content that is impossible to achieve with a vertical heating design. (Either process may be used with shorter parts such as track pins.)

Steel hardenability [6]. Steel selection is application-dependent and is determined by the final operational requirements of the component, as well as hardness requirement and cost. Medium carbon steels (AISI 1038 to 1055) are among the most common, and are used for transmission shafts, gears, camshafts, etc. High-carbon steels (AISI 1060 to 1080) may be used, but they suffer from low ductility, poor machinability and relatively high cost. Cracking propensity increases with increasing carbon content, and sulfur and phosphorous in carbon steel should be minimized. Alloy steels, such as AISI 4140 and 4340, are also induction hardened but are prone to cracking during rapid cooling such as encountered with spray quenching. Although oil, oil-in-water emulsions and other quenchants may be used in certain situations, polymer quenchants currently are most often used. The user should consult with the equipment and quenchant supplier for recommended use conditions, but generally relatively low concentration (about 4-7 %) polymer quenchant solutions are used for medium carbon steels and about 10% polymer quenchant solutions are used for high-carbon and alloy steels. Bath temperatures of 30°C (86°F) are often recommended. Figure 5 provides a correlation of steel hardenability and depth of hardening for high-pressure water spray quenching [10]. The higher cooling rates provided by water spray quenching provided higher cooling rates and greater compressive stresses relative to a water immersion quench and correspondingly greater fatigue strength. As a result of this study, a less expensive carbon steel was used to replace a more expensive alloy steel.

Separate immersion quench systems

Immersion quenching may, in some cases, be preferred over spray quenching due to its greater uniformity of heat transfer during quenching. Nonuniform quenching may lead to cracking at the ends of shafts and sides of lobes (such as a camshaft) when they are spray quenched. Immersion quenching can avoid this problem. Cracking occurs in nonuniform quenching not only because of increased thermal stresses, but also due to increased transformational stresses. This is a result of different times required for transformation to occur due to the variation in fluid flow around the critical regions of the component.

One of the most common problems with immersion quenching is an undersized reservoir. It is important that the reservoir be sufficiently large to allow any foam head present to dissipate before the quenchant is pumped back into the recirculation line [11] that provides quench agitation. If the reservoir is too small, a mixture of foam and quenchant will be used to quench the part, which leads to increased distortion and cracking. It is important to keep the quenchant clean and free from contamination by metal chips (which can plug holes that a part might contain), cutting fluids and forging lubricants, hydraulic oils, etc. [9]. Therefore, immersion quenching systems require the use of adequate filtration.

Adequate heat exchanger sizing is also crucial because a maximum temperature variation of ±1-2°C is not uncommon. In addition, it is often recommended that the reservoir volume be at least 5 to 8 times the volume rate of flow. For example, a flow rate of 10 gal/min (0.63 liter/s) would correspond to a 50-80 gal (189-303 liter) reservoir [9,11]. In some cases, quench severity of polymer quenchants can be varied on a single concentration by varying the agitation and delaying the immersion of the component after heating.

Open and submerged spray systems

Spray quench processes may involve an open or a submerged spray. Major factors involved in the selection and design of spray quenching systems are the material being hardened, area to be quenched and the quenchant. For example, when using a single-shot inductor design, it is desirable to quench from two sides with the quench holes facing the part using a 0.1875 to 0.25 in. (~5 to 6 mm) hole spacing arrangement with the holes in a staggered pattern. In addition to the area being quenched, the orifice size depends on the cross section size of the component as shown in the following table [1].

See "Recommended component diameter and orifice hole size"

Generally, it is preferable for the orifice holes to be placed as close to the surface being quenched as possible.

Total flow rate into the inlet depends on the inlet pipe diameter as shown in Fig. 6 and the following table [12].

See "Relationship between hole cross section and flow rate"

Figure 7 can be used to determine the total flow required by multiplying the flow rate/hole times the total number of holes in the spray ring for a given hole size and pressure [13]. Generally, the total orifice hole area should be a minimum of 5-10% of the area being quenched [12]. Figure 8 shows the relationship between fluid flow, pressure and ratio of the total flow rate through the inlet and outlet orifices [13]. The total flow rate through the inlet must be equal or greater than the total flow rate through the outlet orifices at a given constant pressure. Ideally, the ratio of the orifice area to the surface area of the lines feeding quench ring will be at least 1:1 and not greater than 2:1 [12]. In general, higher flow pressures are required for submerged sprays than open sprays as shown in the following table [11].

See "Pressure and orifice size for spray quench systems"

When the inlet area is less than the outlet area, or when the orifices are large, baffles (small plates with or without holes) may be used to provide more uniform flow between the inlet and outlet orifices. Factors that affect the decision to use baffling include position of the inlet relative to the orifices, ratio of inlet to outlet area, orifice size and the use of low flow pressures [2].

The total number of rows of orifices depends on the required cooling rate to obtain the desired depth of hardening. For straight shafts, a single row of orifices may be adequate. For scanning processes, the optimum orifice angle to the surface is 30 degrees unless the component geometry dictates a different angle. If more than one row is necessary for optimal heat removal, all of the fluid flow from the orifices should impact the surface at the same angle [2].

For a more complicated shape such as a gear, in addition to ensuring uniform orifice fluid flow over the surface to be quenched by controlling orifice spacing and fluid delivery parameters, it may also be necessary to design a quench ring to deliver optimal flow. For example, it is well known that a relatively stable vapor film may be formed in the gear root area. In such cases, the vapor film can be ruptured by directing the spray stream into the root area. Furthermore, it may also be necessary to omit orifices over the gear tips to prevent cracking. An alternative approach could be to vary the size of the orifice to vary the pressure of the fluid stream.

Soft spotting

Uniform flow impingement is a critically important design parameter. Typically, heating and quench uniformity is enhanced by the rotating the component during the entire induction heat treating process. It is important that the system be designed so the part is centered during the process to prevent nonuniform surface temperature, depth of hardening and cooling rate, which can result in soft spotting, illustrated in the examples below.

Soft spotting occurs when steam or vapor from the heated surface is prevented. Figure 9 shows an example of soft spotting evident as quench spots on the part surface after quenching [9].

Figure 10 illustrates how a spray pattern may be affected when inappropriate inductor design is used or when the shaft being quenched deviates from ideal cylindrical geometry due to the presence of splines, keyways and other similar features [10]. When a vertically mounted shaft is progressively scanned through the inductor ring, the quenchant flow along the dashed line in Figure 10 results in nonuniform flow impingement on the shaft surface due to a wobble effect, which is due to an impeded exit of steam or vapor from the shaft surface [2,9]. Wobble can be caused by distortion of the shaft or if it is not being centered in the inductor during rotation. One possible effect is the so-called barber pole effect (Fig.11), where a spiral pattern of surface hardening on the shaft is observed [9]. The pitch of the spiral pattern is related to the rotational speed and traverse.



For more information: Dale Poteet is president, Innovative Metallurgical Technology Inc., Emerald Fields Ct., Hartland, WI 53029; tel: 262-538-1744; fax: 262-538-1766; e-mail: dp1942@aol.com; Internet: www.innmettech.com; George Totten, e-mail: getotten@aol.com; L.C.F. Canale, e-mail: lfcanale@sc.usp.br.


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