When regrinding a crankshaft, it's important to know how the original heat-treating was performed. Cast or induction-hardened cranks can generally be ground down minimum dimensions without concern. Factory nitrided or Tuftrided cranks may lose their surface hardening, requiring a trip to the heat-treater after regrinding.
Heat Treatment
While cast iron becomes work hardened during the machining process, which eliminates the need for an additional heat treatment process after final machining, steel cranks are generally too soft to allow an acceptable journal service life without some type of heat-treating to provide durability and wear resistance. OEM steel crankshafts are most commonly induction hardened, a process in which the surface is heated by a high-frequency alternating magnetic field that generates heat in the crank's surface quickly before being quenched. The ease and speed of this process makes it the favored technique of OE production. Induction hardening results in a fairly deep penetration of 0.060-0.080 inch below the surface. Since the heating and quenching is localized on the surface, and the heating and cooling is uneven across the various cross-sections, the process introduces stresses into the crankshaft. Induction hardening is a quick and economical process for high-production manufacturing, but is a less than ideal treatment for a racing crankshaft.
Two other commonly used treatments in steel-crank surface hardening are Tuftriding and nitriding. Tuftriding was a process employed by some OEM's on special high-performance cranks, primarily for the benefit of avoiding the stresses imposed by induction hardening. In Tuftriding, the crank is immersed in hot cyanide compounds, creating a tough, resistant surface that improves fatigue resistance. The hard layer in a Tufftrided crank is usually very shallow, only penetrating a few thousandths of an inch. One drawback of Tuftriding is the potential for warpage of the crank.
Nitriding is a chemical hardening process in which the part is heated in a furnace, the oxygen is vacuumed out, and a chemical gas is introduced that penetrates the entire surface. The depth of hardness is dependent upon the time the part is exposed to the gas. Typically, a nitrided crank will have a depth of hardness of about 0.010 inch. Nitriding is a low-heat process compared to Tuftriding, but it shares the advantage of avoiding the introduction of localized stress zones as in induction hardening.
An important point to consider when rebuilding an engine is the hardening process used when the crank was manufactured. Cast cranks can usually be reground without concern for any additional surface hardening. A factory-forged crank, with its deep induction-hardened journals, can also simply be cut on a crank grinder and dropped in. However, if the crank was originally Tufftrided or nitrided, a regrind will certainly go through the full surface depth of the former, and also likely that of the latter. These cranks should be heat-treated again after machining to restore the required journal hardness.
High-performance aftermarket cranks typically have beefy fillet radiuses in the corners of the journals for strength, as shown here. Stock cranks normally have a grooved undercut.
Journal Fillets
The most likely place for a catastrophic crank failure is the point where the journals meet the cheeks of the crank. Here, proper machining of the crank can actually reduce the stress concentration. Most OE cranks are made with undercut fillet radii, where the corner of the journal is actually cut away, leaving a radiused groove at this critical juncture. This technique is effective at decreasing the stress concentration, but it isn't the strongest way to do the job.
Fillet radii as found in a performance crank require chamfered bearings to clear the extra meat in the corners, such as the Federal Mogul Super Duty Alloy race bearings being used in this stroker Mopar small-block.
High-performance cranks have added material left in the corner, forming a fillet radius. The fillet makes for a stronger crank than an undercut radius, however, proper clearance in the bearings must be ensured to prevent the edge of the bearings from pinching the fillet and binding. While stock replacement bearings often don't have sufficient edge chamfer for a fillet-radius crank, bearing manufacturers offer race bearings designed to clear for most popular applications. Something to consider when building a racy engine with an upgraded crank.