Nonrotating Induction Heat Treating Meets Industry Needs
Heat treating camshafts and crankshafts, two common engine components, is an important part of the engine manufacturing process. Current processes used to heat treat crankshafts and camshafts have various process-related shortcomings that can lead to operational inefficiencies including excessive cycle times, high tooling costs, excessive energy use, difficulty in controlling the product, shortened locator tooling and inductor life, and difficulties in maintaining process capabilities.
Many engine manufacturers around the world are considering converting from conventional carburizing, neutral hardening and traditional induction hardening to the newer nonrotational induction hardening. One automotive company and several small engine manufacturers already have completed the necessary process development and production validations to implement the new technology. The technology is being supplied by several induction heating equipment manufacturers under respective trade names, including Welduction's SCPL, (semi circle phase locked) technology.
Nonrotational induction hardening technology dramatically improves induction heat treating of crankshafts and camshafts in three interrelated areas: process, machinery and product. The SCPL system simplifies induction hardening of parts by reducing equipment cost, minimizing floor space, easing equipment installation, and increasing tooling and coil life by up to a factor of ten. Moreover, equipment using this technology is significantly smaller and requires a lower set-up skill level.
The nonrotating process is suitable for selective heating small diameters of irregular surfaces adjacent to, or confined within, larger diameters-areas that normally would require the use of a clamshell or probe-type inductor. This makes the process suitable for induction hardening and tempering crankshafts and camshafts made of medium carbon steel, which is less expensive compared with carburizing lower carbon alloy steels.
Other advantages include less lateral growth and lower total indicator runout (TIR) distortion by putting less energy into the part with no axial force as the part is being treated. The process also is designed so the induction heating system will not crack around oil holes as the inductor coil is selectively profiled.
With respect to part material, the induction process provides a smaller grain size for better wear resistance. The part spends less time at austentizing temperatures and at a fixed lower surface temperature because the system can heat the part in 1?to 1?the time typically required with conventional systems.
Figure eight-shaped coils often used with conventional induction heat-treating equipment for crankshafts and camshafts pose a number of difficulties. Their copper-to-steel ratio, an important factor in heat-treating efficiency, is about 20 to 30%. Tight air gaps between the part being processed and the inductor coil produce arcing as the coil unintentionally touches the part, which can lead to frequent and costly coil repairs.
The figure-eight, or side-approaching, inductor encircles less than 50% of the part. Because only a small portion of the part face intended for heat treatment is heated simultaneously, the part must be rotated and heated for a longer time. Rotation also requires that the machine have complex hydraulically counterbalanced heat stations. Energy lost during heating from radiation, convection and conduction is compounded with the longer heat time and from scanning parts radially, instead of a full coverage single shot. Typical "heat-on" time in automotive applications such as cams or cranks used in passenger vehicles currently is two to five times longer than optimal, and is five to ten times longer than optimal for cams and cranks for a large engine.
This nominal five times longer "heat on" time is cumulative, as it applies to each feature on the cam or crank. A ten second per feature excessive heat time, including the extra quench time needed, can translate to a 100 second overall penalty, depending on the total number of features. Parts generally should be able to be heated in three to ten seconds (depending on carbon content and microstructure).
Conventional induction heat treating processes are hard on perishable tooling, which must be replaced approximately every 5,000 to 20,000 hits. The short tooling life using traditional induction heat treating often results in tooling cost alone in the range of $1-3 per part. Each time tooling is changed, the machine must undergo a costly and time-consuming validation. Reduction of induction tooling or furnace fixturing is the leading justification for using the nonrotating process.
Because SCPL is noncontact process, tooling can last significantly longer (ten times current tooling life), leading to increased equipment uptime. A higher capability index results from less tooling changeover from repair and replacement coils. Heat times are significantly faster with less energy being put into the parts as shown in Table 1.
A proven alignment system is required on most applications to ensure fast and repeatable positioning of the halves of the Windowpane(r) bus used in the process to each other and also of the entire coil/bus assembly with the particular crank feature being induction hardened.
To summarize, counter-balanced heat stations are no longer required, inductor coils have a dramatically longer life due to the non-contact process, and no rotation of the part is required.
Current machinery issues
Equipment currently used to induction heat treat cranks and cams adds to the inefficiency of the process. These inefficiencies include tight air gaps in the machinery, the need to change power level with radial orientation to avoid cracking around the oil holes and unpredictable degradation of machine tooling touching the part.
Currently used crankshaft induction hardening equipment is expensive ($1.5-3.5 million for a system capable of processing 60 to 110 parts per hour, for example), and due to high capital equipment cost, induction tempering is not feasible using traditional equipment. The equipment also requires considerable floor space.
By comparison, the nonrotating machine offers advantages of using heavy-duty machined copper inductors with proven slot quench (not intricate brazed copper tubing), using low cost, simple tooling (no floating heat stations or overhung load), having a smaller footprint and offering a more ergonomic design. A conventional machine tool design is used and is compatible with standard gantry part handling units, and the quench system is more accessible for easy cleaning.
The machine has fewer moving components and does not use any moving components during the critical heating interval. The more efficient and lower cost machinery lends itself to induction tempering, eliminating the need for a separate temper furnace. An SCPL machine requires only 40% of the floor space as a conventional machine; 20 ft wide by 12 ft deep by 7 ft high compared with conventional machine dimensions of 15 ft by 40 ft by 15 ft.
Product issues related to traditional processes
"Heat on" time using a traditional induction heat treating process typically is two to five times longer than that for nonrotating process, while the large-engine process operates in the range of five to ten minutes more than required. Excessive energy, which can lead to cracking, must also be put into the part, due to the inefficiency of heating only a small area of a given outer diameter (small ratio of copper to steel) and required rotation. Surface condition also is compromised due to excessive formation of scale and unacceptable grain size, resulting from the long excessive temperatures and heat time needed with the current processes. A fine grain size is desirable because it improves wear resistance.
Along with cost and time savings, the nonrotating process technology allows heat treat patterns to be profiled to eliminate problems with cracking around the oil holes. Fillet hardening, bearing face only, split pins and common pins can all be heat treated to required specifications. The inductor coil can be profiled as needed to handle fillets and variance in adjacent mass.
Distortion, or total indicator runout, is a problem that requires many parts to be straightened, a non-value added manufacturing step, which also takes up valuable floor space. Cost of labor and capital equipment for the straightening operation are substantial. Parts are distorted from the compound effect of excess heat time and axial load on the part from the overhead mounted isolation transformer. Lateral growth also is a problem for cranks that require fillet hardening while being processed using current technology.
Reducing in TIR
TIR is a major concern of camshaft and crankshaft manufacturers. Current processes dictate that nearly 100% of the parts undergo post-heat treat straightening to fall within a TIR specification. Nonrotating induction heat treat technology lends itself to far less dimensional distortion than current processes for various reasons.
The nonrotating process is compatible with steady rest rollers on servo slides (lateral axis) to control the part to theoretical center during the heating and quenching. These rollers are hydraulically operated (both open and closed) with a three-point contact to theoretical center.
The radial profile of the coil compensates for variance in adjacent mass and oil holes. Because the process is nonrotating, the inductor coil can be selectively profiled to optimize the process. In other words, there will not be any overheating around the oil holes or underheating in areas where there is an increase in adjacent mass.
There is less effective energy and heat put into the part through the more efficient process. About one half of the energy is required per part feature, resulting in less distortion. A ten-second heat on time at 250 kW (2,500 kW s) is predicted, as opposed to the current 25 s heat time with 200 kW (5,000 kW s). Less heat into the part translates to less dimensional growth and distortion from the release of preexisting residual stresses from prior cold working operations.
Control of case pattern depth and location is greater. This is accomplished with/or near a one-to-one ratio or 100% coverage of copper to steel, as opposed to 25 to 30% with the conventional process.
The noncontact process has no radial forces from an overhead mounted hydraulically counterbalanced transformer. The inductor coil and related tooling do not touch the part.