From a young age, I've been exposed to engines by my father. As an accomplished mechanic, he retains many friends to this day who regularly seek his assistance or recommendations for their drag-racing builds. He's been taking me to the track for longer than I can remember, and has always found a way to include me in his projects. He guided me work through my first few engine projects, which included a 100cc ATV and a 3.5hp lawn mower engine which we used on a mini bike. My interest in engines never subsided, so naturally the projects only got bigger, more elaborate, and of course, faster.
Most internal combustion engines have a rotating assembly, which comprises the crankshaft, connecting rods, and pistons. Rotating assemblies for production engines are typically made of cast iron. Iron can be cast relatively cheaply and is perfectly suitable for making any reasonable amount of power, which makes it a natural choice for OEMs. But where’s the fun in making reasonable power?
That’s where forged steel comes in. Once an engine has very high compression, high boost, lots of nitrous, or a high rpm, cast iron can start to fail, and this is catastrophic for the engine, destroying most internals and the block itself. Forged steel is used because it can take the beating that racers like to give it.
While forging itself extends back thousands of years, forging steel seemed to originate at about 400 b.c. In India, metalworkers found the optimal combination of wrought iron and charcoal that, when melted in a furnace, combined to form steel. Because of its superior properties to iron, this opened up a global market, and the Indian steel found its way to Syria, Spain, and Italy, among other nations. There it would be forged primarily into tools and weapons, now much stronger than they ever could have been. This combination of charcoal and wrought iron was later discovered by the Japanese. With this they were able to forge swords, katana, that were far lighter and stronger than those of Medieval Europe. It wasn't until the 18th century, in England, that this process was further developed. A man by the name of Benjamin Huntsman began making steel by a slightly different method. First, rather than combining the iron with charcoal, he combined it with roasted coal. Also, instead of heating the materials in a crucible, he heated them over a bed of coals. This resulted in a much more uniform product. In the next century England became a major producer of high quality steel. In 1856, a British engineer named Henry Bessemer discovered that blowing air at molten "pig iron" would blow impurities from the metal. It would leave behind pure iron, which was much easier to use in creation of steel. He invented a special egg-shaped oven to mass produce steel using what he had learned. However, due to differences in phosphorous concentration, only brittle steel could be produced. This issue was not understood for about 20 years, when a man by the name of Sidney Gilchrist Thomas changed the clay-based lining in the oven with a lime-based lining, which allowed for the production of much better steel. This process soon found its way to the United States and around the world, and it is still very similar to modern methods.
For its application to engines, forged steel has been a premium choice for internal components since the beginning. In fact, it's not even known when the first wrought steel components were used. The strongest rotating assembly components are still made from forged steel, which remains stronger than the more modern powder forging process. The process for shaping the components involves repeatedly slamming the two piece mold on either side of the metal blank with great heat and pressure to harden it. The pieces then undergo extensive treatment and balancing. This process has not experienced significant changes since it was first done for engines.
The atomic structure of forged steel can vary. Seeing as it is pressed or hammered, its grain structure can be modified. It is possible to have one piece of forged steel with multiple atomic crystal configurations within it. Generally, it has more of a face-centered cubic structure when heated and a body-centered cubic structure at room temperature. However, this is an oversimplification as there are more factors that influence this, such as the speed at which the metal is cooled down at and the local concentration of carbon.
One very important property of forged steel is that it is anisotropic. This means that it does not have equal strength in all directions. In forging, a grain direction is created within the steel. This direction will exhibit the greatest strength after forging has stopped. For instance, if you were to test the tensile strength in the direction of the grain, it is likely to be higher than the tensile strength orthogonal to the grain direction.
Another important property of forged steel is its great strength. Even compared to regular, cast steel, it has high strength, durability, and hardness. This is particularly important for automotive applications, as they tend to heat and beat these components regularly. If they were not as durable it is likely that they would fragment or distort their shape. Either of these cases would greatly damage your engine.
A final property of forged steel that is very important for automotive applications is uniformity. Because the process for forging steel is so tedious, there are many steps that can be measured and recorded. This allows for very similar repetition. This is crucial for automotive applications because all components have to be carefully balanced to prevent and gyration forces or harmonics from occurring, which would greatly damage the driving performance of a vehicle.
My Understanding of Forged Steel
Through studying materials science, I gained a much better understanding of the variations that can be made in the production of steel among other metals. This had direct and considerable impacts on many different fundamental properties of the metal, such as hardness, strength, and ductility. These are important factors for determining what carbon concentration and what microstructure of steel may be most appropriate for a given application. The most significant sections for my understanding of forged steel were chapters 6 and 9, as they dealt with the general trends due to processing factors and precise impacts on steel subject to heat, respectively. In class, I wish I had learned more about scenarios in which the relative brittleness or ductility of a given material was necessary for something to function. For example, a system that had frequent impact such that only a very ductile steel could be used in construction, or one where its extreme hardness allowed for some particular feat.
References
https://www.steelavailable.com/en/what-is-steel-forging/
https://www.popularmechanics.com/technology/infrastructure/a20722505/history-of-steel/
https://www.forging.org/uploaded/content/members/field_document/A_Comparison_of_Manufacturing_Technologies_in_the_Connecting_Rod_Industry.pdf