Fig 1 V-8 crankshaft Fig 2 Conventional rotational-type crankshaft hardening machine having U-shaped coil

Advanced nonrotational induction crankshaft hardening process (SHarP-C, or stationary hardening process for crankshafts) has become a proven process for V-6 crankshafts. The main features of the first machine that provided the heat treating for V-6 crankshaft journals using stationary inductors rather than the cumbersome methods of the conventional crankshaft hardening processes were discussed previously [1]. Advantages of the process include a short heating time (2 to 4 seconds), a reduction of floor space requirements (as much 80% in some cases), a four-fold increase in tooling life, a reduction of machine-life-cycle cost and better crank-performance characteristics. SHarP-C technology has progressed into a proven process for treating a family of different crankshafts. This article provides general information of the stationary induction crankshaft hardening/tempering process concentrating on the features of V-8 crankshafts (Fig. 1), and compares the main features of the nonrotational and conventional processes.

Conventional technology

Most existing induction crankshaft hardening machines require crankshaft rotation during heating. The crank is rotated about its main axis while each crankpin and main journal is heated by bringing a "U"-shaped inductor close to the pin or main bearing surface (Fig. 2). The inductor and other massive components of the induction hardening machine must travel with the orbital motion of the connecting-rod journals. The circular orbital motion of such a heavy system must be precisely maintained using a special control tracking system that provides a power variation for each heated crank feature during its rotation.

Concerns associated with conventional technology in-clude equipment maintainability, reliability, hardening pattern repeatability, substantial machine downtime and short coil life. Crankshaft manufacturers spend an average of $2 per crankshaft as a maintenance fee (based on V6-crankshaft manufacturer's practice).

Fig 3 Hardness pattern on a V-8 crankshaft journal produced using the stationary hardening process

Nonrotational hardening

SHarP-C technology was developed to address the drawbacks of conventional technology. Keeping the inductor and crankshaft stationary during heating and quenching cycles eliminates the necessity of crankshaft rotation when using U-shaped coils and the high-current contact when using encircling clamp-type coils.

Stationary heating offers simple operation, superior reliability, maintainability and cost reduction. Based on ten-year equipment life, studies show that the life-cycle cost of conventional rotational crankshaft heat-treat equipment is 3-1/3 times higher than that of SHarP-C technology.

One of the most important factors in the heat treating of crankshafts is minimizing shape and size distortion, which directly affect the amount of metal required to grind. The larger the mass of metal heated, the greater the metal's thermal expansion and, thus, greater distortion. Other factors affecting crankshaft distortion include metal properties, hardness profile, residual stresses, etc. Only a small mass of metal is heated using the short heating time of SHarP-C technology (typically in the range of 1.5 to 4 seconds compared with 7 to 12 seconds for the conventional process). This minimizes metal expansion and, therefore, minimizes size and shape distortion; distortion typically is less than 0.025 mm (0.001 in.).

The short heating time also improves the metallurgical properties of the hardened zone by reducing grain growth, decarburization and oxidation of the pin/main surface. The hardened case consists of a fine grain martensitic microstructure having a negligible amount of retained austenite and no traces of free ferrite (Fig. 3). By comparison, the conventional process produces a case having significant amounts of free ferrite; the presence of free ferrite at the pins/mains surface reduces wear resistance and some other properties.

Superior controllability of the stationary crankshaft hardening process allows modifying the hardness profile along the circumference of the pins and the mains, as well as across the width of the heat-treated journal. It also prevents localized overheating and underheating of potential "trouble spots," such as oil-hole locations.

Fig 4 Time-temperature profiles using U-shaped inductor (compare with Fig 5)

Reduction of residual heat in the crankshaft due to the short heating time is another benefit of nonrotational crankshaft hardening. Less residual heat build-up in many cases can allow cost savings and less floor space due to the elimination of special cooling systems. Figures 4, 5 and 6 compare time-temperature profiles of the SHarP-C process with conventional rotational technology. Figure 6 shows the temperature distribution along journal radius prior to quenching. Results were obtained using ADVANCE induction heating software. Surface temperature prior to quenching was assumed to be the same using both conventional and ad-vanced nonrotational technology approaches.

Fig 5 Time-temperature profile using SHarP-C technology (compare with Fig 4)

The SHarP-C process provides both hardening and tempering. However, an existing furnace/oven can be used for tempering for those preferring to use long-time, low-temperature furnace tempering instead of short time, high-temperature induction tempering.

Coils are machined from solid copper without any brazed parts, providing more rigid coils. There are fewer components involved in the coil design, which translates to higher reliability due to fewer parts that can have problems. The SHarP-C coil-to-journal air gap is larger than that required using the rotational crankshaft hardening process, which reduces stress-corrosion and stress-fatigue induction coil failures. Tooling life is increased 400% using the nonrotational process.

Fig 6 Comparison of temperature profiles along the radius of a main journal using a conventional rotational versus SHarP-C nonrotational hardening process

Accurate CNC coil shaping and use of a "quick change" pallet approach guarantees that coils are automatically aligned with respect to the crankshaft after coil replacement. No time-consuming process adjustments are required to "tweak" each coil after replacement. United construction allows quick, error-free production-ready factory installation. One user of the SHarP-C technology was able to start up the V-6 crankshaft-hardening machine using plant personnel.

There are only three crankshafts in float using the nonrotational crankshaft-hardening technology compared with 120 to 250 in float using the conventional rotational pro-cess; this results in a lower risk of rejected parts.

Energy savings

The nonrotational process technology provides energy savings due to:

  • Higher coil efficiency
  • Shorter heat time and smaller mass of metal being heated; electrical energy is not wasted to heat main/pin internal areas which do not require phase transformation changes Less energy required for the cooling/quenching station due to the nonrotational short heating time
  • High electrical efficiency (96-98%) of the SHarP-C coil for induction tempering compared with lower efficiency tempering furnaces; energy is localized in the areas of the crankshaft where it is needed using the SHarP-C coil

Studies show that in some cases, total energy cost savings can exceed $260,000 per year.

For more information: Don Loveless, Group Vice President, Research & Development, Inductoheat Group, 32251 N. Avis Dr., Madison Heights, MI 48071; tel: 248-585-9393; fax: 248-589-1062; e-mail: loveless@inductoheat.com.