High performance racing gears require steel that can withstand high forces for long periods of time and that has exceptional wear resistance. Unfortunately, these properties are diametrically opposed, because when steel is made harder to prevent wear it will in turn become brittle and less able to absorb high forces, and vice versa. There are, however, techniques available to manipulate steel after it has been formed to achieve the best of both worlds - a hard exterior with a tough inner core.
Sourcing steel for generic automotive gears is easy because there are many grades readily available that, if heat treated correctly, will produce an acceptable product. All it takes is a forge and a helical gear grinding machine to manufacture gears capable of lasting a lifetime in a street driven vehicle. These types of gears can be made cheaply in the U.S. and even more inexpensively when imported, allowing for even greater profit margins.
Selecting the proper steel formula for race gears is much more difficult and requires starting with the end results in mind and working backwards. Every process that Crown Race Gears go through during production, including machining, heat treatment, cryogenic treatment, shot peening, and REM ISF finishing, requires certain characteristics in the steel to achieve the desired improvements. This process is extremely difficult because certain elements in steel that are ideal for one process may also be highly undesirable for another process. Furthermore, each process will alter the steel in ways that then later affect subsequent treatments.
Finding the perfect steel alloy for Crown Race Gears was no easy task. It took our team over a year working with domestic and international gear manufacture’s, world class metallurgists, a renowned gear grinding specialist, and top industry experts to provide the extensive experimental analysis and real world testing data needed to identify the optimal alloy.
Steel is an alloy made up of iron and carbon with numerous other elements added to change specific properties and characteristics of the metal. While improving hardness and toughness is the primary goal in developing a steel alloy, some elements are added to improve grain structure and machinability, tie up impurities, or offset the negative side effects of other elements. The table below lists twenty one of these elements along with brief explanations of their effects on steel.
Iron is the base element of steel and is mined from the earth’s crust as iron ore. When mined, iron ore contains many unwanted impurities that need to be removed to obtain pure iron. Refining iron ore is done by heating it in a furnace to 900 Deg. (C) and exposing it to specific substances that bind with the impurities to create a more refined form called wrought iron. Another similar heating process is then used to further refine the iron into pig iron, which is pure enough to be combined with carbon to create steel.
Carbon is the only element required to be combined with iron to classify the material as steel. Carbon is the core alloying element of steel and is the basis for heat treatments used to alter the hardness and toughness of the material. In particular, hardness, which is responsible for wear resistance, is the direct result of adding carbon to form an ultra-hard micro-structure called martensite. The proportion of carbon in steel is used to classify it as low-carbon steel (0.05% to 0.4%), medium-carbon steel (0.3% to 0.6%), and high-carbon steel (0.7% to 2.5%).
Manganese is a very important alloying agent in steel and is added to improve the hardness penetration of steel during heat treatment by slowing down the quenching speed. Steel with the proper manganese content can be quenched in oil instead of water, which makes it less susceptible to cracking during the temperature change shock. Additional benefits of manganese include increasing the rate of carbon penetration during carburizing, improving machinability, reducing brittleness from sulfur, and improving surface finish.
Chromium is another highly beneficial element in steel alloys because it increases hardness penetration (similar to manganese), improves corrosion resistance, promotes grain growth, and improves strength at higher temperatures. Chromium also gives steel the ability to resist softening during tempering, however, it does require higher tempering temperatures or longer tempering times. Finally, chromium forms a variety of carbides that along with vanadium, molybdenum, and other elements provides excellent wear and abrasion resistance.
Silicon, used in conjunction with other alloy elements, increases toughness and hardness of steel, but to a lesser degree than manganese. Additionally, it provides a slight resistance to tempering and is a principal deoxidizer that prevents defects, decays, and damage. Silicon is a naturally occurring element in iron ore and is the first major element removed during the refining process, where it eliminates gasses as it is burned away. After refining, a small amount (about .015% to .035%) of silicon is retained after the melt-down to protect against re-oxidation.
Nickel, when combined with chromium and vanadium or molybdenum, promotes high toughness in steel, especially at lower temperatures. It also improves the hardenability of steel and helps reduce distortion and cracking during the quenching phase of heat treatment. Because nickel, manganese, and chromium are all used primarily to help improve and balance the hardness and toughness of steel, they can be used in various combinations to achieve the desired results based on their slightly different effects on other elements and the heat treating process.
Molybdenum primarily improves hardenability in steel; however, it also slows softening at high temperatures, which is why it is often paired with chromium to reduce its tendency towards temper embrittlement during heat treatment. Molybdenum steels do tend to cause slight surface decarburization during heat treatment so protective measures need to be considered when adding it to an alloy. Steel containing molybdenum is also often abbreviated as "moly" as in chrome-moly (chromium-molybdenum), which is commonly used for roll cages in race vehicles.
Vanadium is added to steel in order to better control grain growth by slowing it down during heat treatment and tempering, to improve overall wear resistance and strength. Compared to other elements in this list, vanadium's reaction in steel is far more complex and requires an extremely precise and controlled heat treatment process. When treated correctly, steel containing vanadium will be harder and stronger, with a finer and more consistent grain structure than that of steel not containing vanadium.
Tungsten is added to steel so that it will combine with free carbides (carbon) during the heat treatment process, which will ultimately improve wear resistance with little or no loss of toughness. Tungsten is best known for its uses as tungsten-carbide (a material that is 50% tungsten and 50% carbon), where it is found in high speed cutting tools and armor piercing bullets due to its extreme hardness. With external hardness and internal toughness being key properties of high performance gears, it is easy to see why tungsten is such an important element to have in the alloy.
Copper helps improve corrosion resistance in steel alloys; however, it can have a detrimental effect to the surface quality and hot-working behavior, so it is generally only used in very small amounts. Because copper does not interact with carbon, it will dissolve or precipitate in the ferrite (iron) instead, where it produces a slight hardening effect that is similar in scale to that of nickel. While copper can improve hardness, it is never added to steel for the purpose of improving hardenability due to its negative side effects when used in the quantity necessary to achieve those results.
Niobium, previously named columbium, is a steel additive that promotes a fine grain structure and improves the mechanical properties of steel by slightly increasing strength and toughness while maintaining ductility. It does slow the tempering process a bit, decreasing hardenability (by forming extra stable carbides) and reducing the amount of carbon that can be dissolved into austenite during heat treating, which is why niobium is only added in trace amounts. Niobium is often added to construction grade steel because it reduces the overall weight of the alloy, but that benefit is too insignificant to be a factor in gears.
Boron increases the hardenability of steel and does so without sacrificing ductility, which makes it an excellent partner to Niobium. It reacts negatively with oxygen and nitrogen (both in the air and in the steel) causing it to becomes useless, so it must therefore be added extremely carefully and only after the steel has been completely deoxidized. Only a very small amount of boron is needed to get the intended results, with typical quantities ranging from 0.0005 to 0.003 percent of the alloy by weight.
Titanium is added to protect boron by tying up any remaining oxygen or nitrogen that would prohibit it from improving the hardenability of the steel. It forms stable bonds with oxygen, carbon, nitrogen, and sulfur, so it is often used to fix these elements and lessen their harmful effects. Titanium is also an effective grain refiner in steel and, as such, tends to act similar to aluminum and niobium. During heat treatment, titanium does require a higher austenitizing temperature to prevent it from negatively affecting hardenability and offsetting the gains made protecting boron.
Sulfur is often regarded as an impurity in steel but, it is critical for improving machinability. It promotes proper chip formation during gear tooth cutting and allows the steel to be machined more accurately, preventing defects and failures. The goal with sulfur is simply to retain the bare minimum amount needed for the gear to be cut and machined without creating excessive heat or causing excessive tool wear or failure. Because it is all but insoluble in steel, sulfur has no effect on the heat treatment process.
Phosphorus slightly increases strength and hardness in steel; however, that comes at the cost of increased grain size, reduced ductility, and lower toughness, so it is normally limited to below 0.05% in low carbon steel. While it does help improve corrosion resistance and machinability, like sulfur, it is typically considered an impurity in steel. Phosphorus is a naturally occurring element in iron ore and is removed to acceptable levels early on in the refining process.
Aluminum is primarily used as a deoxidizer, although it also has strong grain refining and nitride forming properties in steel that promote high toughness. It bonds with any oxygen and nitrogen left over after refining to prevent them from disrupting other elements such as boron, used to improve hardenability. Aluminum does have some negative side effects on steel but, they are only exposed when the material is held above 800°F (426°C) for extended periods of time, and thus they can be ignored in the production of high performance gears.
Cobalt is a strong ferrite strengthener that promotes and improves the bond between iron and carbon, which make up the core structure of steel. This is because cobalt is highly soluble in iron and can form a homogeneous crystalline structure with it and still maintain the original ferrite structure, even up to very high temperatures. While cobalt is most often associated with ultra-high strength steels, high speed tool steels, rare earth magnets, and super alloys, its ability to improve hardness makes it a valuable part of low carbon alloys as well.
Tin is considered an impurity in steel and, while it is often applied to an end product as a protective coating, it is highly undesirable in an alloy mixture. Even in small quantities, tin will negatively impact ductility and, like phosphorus, it can make steels susceptible to temper embrittlement when chromium, nickel or manganese are present. Tin is a very difficult element to remove once it has dissolved in iron, and while it cannot be avoided entirely, every effort must be taken to prevent it from entering the steelmaking process and to keep its content to a bare minimum.
Lead is added to steel alloys for the sole purpose of improving machinability. During machining, the lead melts (at a microscopic level) and acts as a lubricating agent. It is not technically an alloying element because it does not bond with the steel, but rather disperses itself throughout the material as inclusions, usually on the tail end of manganese sulphide. Unlike sulfur, which also improves machinability, lead has no apparent effect on the yield strength, tensile strength, impact strength, or fatigue strength of steel. Only a small amount of lead, about 0.010% or less, is needed to get the desired benefits.
Selenium, like lead and sulfur, is used to improve the machinability of steel. While relatively expensive, adding selenium to improve machinability means that less sulfur and lead need to be used, so their negative side effects can be reduced. Selenium does not affect hardening or tempering reactions during heat treatment and has almost no effect on the mechanical properties of steel. In higher concentrations, selenium begins to impact nitrogen absorption and, when added in equal parts to sulphur, it can alter the shape of sulfide inclusions, which is why it is normally kept below 0.005%.
Zirconium forms stable compounds with oxygen, sulfur, and nitrogen so it is commonly added to steel as a way of neutralizing the negative effects of those elements. In addition to being an effective deoxidizer, zirconium helps promote grain growth, prevents strain aging, and improves ductility and impact strength to a small degree. During heat treatment, zirconium does help improve hardenability; however, it is rarely ever added for that purpose as other elements provide those benefits more effectively and economically.
Even with these simplified descriptions, it should be evident that metallurgy is an extremely complex science which is critical to the development of professional grade racing gears. While it took over a year and extensive resources to develop, the steel alloy used by Crown Race Gears is perfectly suited for being forged, machined, heat treated, and REM ISF finished into the strongest, toughest, and most efficient racing gears possible.