Heat-treating technology provides a smoother V6.

New induction heating equipment facilitates crankshaft prduction

Because of the high ignition pressure on a V6 engine's split-pin crankshaft, the component has to be both strong and inflexible. To achieve these qualities, pin bearing diameters must be proportionately larger than those of conventional crankshafts. Although V8 engines utilize an optimal 90-deg cylinder angle, resulting in a smooth running engine, the typical V6 utilizes a 60-deg angle to achieve minimal vibration.

For fabrication reasons, however, Mercedes opted for a 90-deg cylinder angle on its new V6 engine. For this design to achieve vibration characteristics similar to those of their V8 engine, the V6 incorporates a compensation shaft with identical revolutions; it counter rotates to minimize vibration. In addition, the adjacent pin bearings are offset by 30-deg to achieve a 90-deg ignition-timing interval. New heat-treating equipment was developed to accommodate the 30-deg offset design while providing "fillet" hardening at all pin and main bearing positions. Distortion minimization was also a key requirement.

Crankshafts with split-pin designs have been used for many years. Until now, surface hardening these crankshafts was done predominantly by classic nitriding and, in a few instances, by induction heat treatment.

The induction hardening of adjacent pin bearing surfaces of the earlier V6 split-pin crankshafts for trucks--with separating collar and no 30-deg offset--is possible with two inductors having individually synchronized medium frequency power supplies. The inductors are lowered onto the bearing surfaces, and the crank is rotated while the bearings are heated and quenched either in sequence or simultaneously.

Quenching is done with an integral spray designed specifically to control the onset of the martensite transformation relative to the circumferential position of the crankshaft. In some instances, one bearing may be cooled while the adjacent split bearing is heated. This is done to minimize heat soak into the collar and to lower the susceptibility to quench crack formation.

Power pulsing technology may also be employed to further reduce distortion and produce a more consistent case depth. This process was developed by the end of the 1980s and fully automated hardening systems have been in production since 1989.

The split-pin crankshaft for Mercedes passenger cars (utilizing the 30-deg offset pair without the intervening collar) required simultaneous hardening of both bearing surfaces. The hardness and metallurgical structures in the gusset area were critical and resulted in improved fatigue life. This heat-treat process required a significant advancement in inductor design to ensure that a precise case-hardened zone was formed.

The process parameters developed also had to be consistent with those used in the previous hardening cycles of Mercedes' conventional crankshafts.

Precise control of the martensite start position, "power on location," power pulses, and hardening sequence was achieved while utilizing a non-hindering expansion chuck during heating and quenching. The bottom line is minimal distortion with no straightening required and improved metallurgical and fatigue properties.

The transformer/inductor assemblies are positioned on the bearing locations with just enough contact pressure to provide a precise dynamic coupling with the bearing as it rotates. All of the earlier practical process experience to achieve optimal case depths and minimize distortion is utilized. Appropriate quality measures--such as monitoring, collecting, and evaluating process parameters--are also employed.

Current cycle time of the fully automated hardening machine at Mercedes is 43 sec. The process achieves a rotational deformation of 0.2mm on short-stroke crankshafts and 0.3mm on long-stroke crankshafts.

These comparatively low distortion values are credited to the part-specific process parameters and the use of micro-alloy steels that replaced the manufactured alloys.

Inductors are the key feature of the newly developed processing technique. The two active heating coils in each inductor are independent of each other. In addition, they can both be wider than the bearing width due to the absence of the collar. This ensures complete pattern overlap.

Several components developed during this project have been patented or have patents pending. These relate particularly to the newly developed inductor and also to the near frictionless length-expansion allowance of the crankshaft during the hardening process.