Just as diamonds are created by pressure and time, a forged crank is the result of heat and pressure, and a bit of precision machining. A forged crank offers vastly increased grain structure and strength as opposed to a casting. Shown here is a Lunati bigblock Chevy stroker, precision-machine-finished to an incredible degree of accuracy. Ever wonder exactly what's involved in making a forged crankshaft? Well, we did, so we looked into the process. This was more fun than a trip to Disneyland accompanied by a flock of Playboy bunnies.
THE FORGING PROCESS
Individual crankshaft manufacturers often employ their own proprietary formulas but, generally speaking, a steel ingot is heated in an oven to about 2,200 degrees F (at which point the steel very formable-not yet a liquid, but very easy to move around and shape). The ingot is then placed in a forging die and squeezed into the approximate shape of the desired crank profile. This "squeezing" is performed either by a hammering or pressing process. Either way, the goal is to compress the alloy mix not only to precisely fill the die, but also to increase the strength of the alloy by compacting the molecules and aligning and strengthening the grain structure. Basically, the size of the ingot is much larger than the volume required in the die (generally, a crank maker will start with an ingot that weighs about twice as much as the desired final product). During the forging/compacting process, the excess material is forced out of the die at its mating lines. This excess is later sheared off in a trimming die. The trimmed, rough-shape forging is then quenched and tempered. Heat-treating is best done before machining since the heat-treating/tempering process can deform the crank shape by as much as 0.060". Methods vary, but this might involve quenching the crank in a glycol solution. The crankshaft is then machined to its final shape. Once machining is completed, the crank is stress-relieved to remove any stresses that might have been induced by the machining process. This may be done in an oven, heating the crank to within 400-600 degrees F. Both the heating and cool-down times are carefully controlled. The crank is then final-machined in terms of surface finish. This step involves achieving the desired surface hardness. By the way, when a forging is made using a hammer press, according to Pacific Forge's John Callies, the hammer hits the die multiple times to obtain full die closure, with the hammer hitting at an impact pressure of 240,000 psi each time.
PARTING LINES ON FORGINGS?
Upon examining a forged crankshaft, you may notice what appears to be a broad "parting line" that may make some folks wonder if the crank is cast or forged. This "parting line" sometimes seen on forged cranks is merely evidence of the excess steel that was forced out of the die during the forging process. This excess steel is sheared off of the crank after forging (performed in a "trimming die"), with dress-up trimming done during crankshaft machining. In some cases, this area (where near-liquid steel has been forced out of the die during hot-hammering or pressing) is machined completely away, with no trace of the trim line, while in other cases the trim area may not have been machined as closely, leaving a slight telltale sign of the trim area (if this evidence exists, this does not indicate any problem whatsoever and is merely cosmetic). In other words, even forged cranks may feature a slight evidence area of the die mating path. This slight trace of the trim area is normal and does not present a problem.
Here are some broad industry guidelines regarding the materials used to make a steel forging. While the following information applies in a generic sense, keep in mind that some crankshaft makers have developed their own unique and proprietary alloy formulas. We've included basic data on both 4340 and 4310 steel, but the crank makers we spoke with use 4340 steel in all of their forgings.
4340 carbon steel
0.40 percent carbon steel, with carbon as the prime alloying ingredient. Nickel, chromium and molybdenum affect the cooling rate. 4340 is a heat-treatable, low-alloy steel containing nickel, chromium and molybdenum. It is known for its toughness and capability of developing high strength in a heat-treated condition while retaining good fatigue strength. Machining is best done in the annealed or normalized and tempered condition. In the high-strength conditions of 200 ksi or greater, machinability is only from 25-10 percent that of the alloy in the annealed condition. 4340 has good ductility in the annealed condition and most forming operations are carried out in that condition. It can be bent or formed by spinning or pressing in the annealed state. Heat-treatment for strengthening is done at 1,525 degrees F, followed by an oil quench. For high strength, the alloy should be first normalized at 1,650 degrees F prior to heat treatment. Forging may be done in the range of 1,800-2,250 degrees F. 4340 has very good cold-forming capability so hot-working should not be needed. Hot-working in any but the annealed condition can affect the strength level. A full anneal may be done at 1,550 degrees F, followed by controlled furnace-cooling at a rate not faster than 50 degrees F per hour, down to 600 degrees F. From 600 degrees F, it may be air cooled. 4340 steel is considered a through-hardening steel. Large sections sizes can be heat-treated to high strength.
Tensile strength (yield) 162,000 psi
Elongation at break is 17.1 percent Carbon 0.430 percent Chromium 0.700-0.900 percent Iron 96 percent Manganese 0.700 percent Molybdenum 0.200-0.300 percent Nickel 1.83 percent Phosphorous less than 0.0350 percent Silicon 0.230 percent Sulfur less than 0.0400 percent
Crower uses 4340 and EN30B (an alloy that is often used for billet cranks) for its forgings. The forgings are compacted in a hammer process, with all manufacturing handled in the United States. The units are heat-treated for core hardness to around 36 Rockwell. After machining and finish-grinding, Crower nitridee for surface hardness to a depth of approximately 0.002-0.003". Crower's Kerry Novack noted that the company doesn't like to go deeper in surface hardness because it doesn't want to create potential brittle points. Scat employs a press technology instead of a traditional hammering approach. According to Tom Lieb, this press-forging is done in three progressive stages with the material essentially pushed into shape as the nose and tail is formed to create front and rear main bearing areas. As the center bearing areas are formed, the pressing action creates a wedge of material that is pushed into the counterweight areas of the die, etc. Lieb noted that this new technology press-forging is handled in China using the latest German press equipment. As with hammering, excess material oozes out from the die, which is then trimmed off. After tempering and machining, surface-hardening is handled via nitriding. "Hardening metal is like baking a cake," noted Lieb. "Ingredients include a lot of different things such as vanadium, manganese, nickel, etc. The heat-treatment process is designed around the different elements in the steel mix, and how they react to each other. With chromoly, we heat to about 2,600 degrees F, followed by a dip in a vat of heated glycol. As the metal is control-shocked, all of the elements hold hands and establish a grain structure. We use short-cycle nitriding to create an extremely hard surface that's about 0.002-0.004" deep. You can, of course, case-garden deeper, but that can create brittle fillet areas. If during the service life of the crank you need to regrind the journals, simply get it re-nitrided after grinding." Bryant Racing produces billet and forged cranks. "Ninety percent of what we do are billet cranks," noted Bryant Racing's Joe Squires, "but we do offer a few forged units as well. Basically, we buy raw forgings from Ford and GM (the same forgings previously used in NASCAR). We heat treat them, finish them and nitride to 0.005" deep. Our billet cranks are made from American Timken 4340 steel made to order for us, using our own recipes, through-hardened, cryo-stablized, stress-relieved and plasma-ion-nitrided." Lunati's Matt Sadler noted that the company uses 4340 steel and ion nitride surface-hardening to a depth of 0.010-0.012". Callies employs 4340 steel in its forgings and offers its "Perma-Tough" heat-treatment process that "changes the micro-structure of the steel" and penetrates deep into the crankshaft. Callies claims that "even after a 0.030" regrind, these shafts can go directly back into service with complete confidence." Bullet Racing Cams' John Partridge told us that the company heat treats its cranks after forging to a range of 32-34 Rockwell. After final-machining, its ion nitride surface hardening creates a 62-65 Rockwell surface that's about 0.012-0.020" deep. It uses both U.S. and overseas forging plants. K-1 Technologies, a division of Carrillo, offers both forged and billet cranks, all made from 4340 steel and nitrided for improved bearing life.