Here at Power Automedia we like the nitty-gritty tech, the engineering we gearheads can all geek-out over. ARP bolts are some of the industry leaders in keeping the high-stress components of your engine together under the punishment it sees on a daily basis. The raw materials used to produce a structural part like a bolt are integral to the mechanical properties of the end product. Steel is alloyed with far more elements than just the required iron and carbon, and the molecular structure is a variable condition that heat treating and other factors will influence.

ARP strives to produce the strongest, most reliable, and consistent hardware to meet your racing demands. At the root of its engineering task is a understanding of fundamental metallurgy. In ARP’s technical bulletins the company puts this academic subject in layman’s terms, providing its customers and curious wrenchers an insight to their manufacturing and engineering. Metallurgy is a science that addresses the atomic, elemental, structural, and application of metals, and alloys (metals combined with other elements).

We’d like to share some of the key considerations ARP discusses in its educational bulletin. By better understanding some of these metallurgical concepts, and how they relate to hardware, we hope you will be more selective in your purchases, and consider the importance of quality hardware in high-stress applications.

Grain Structure

The first fundamental to understand in metallurgy is that metals have a directional grain, very much like wood. This grain structure is the result of the molecular arrangement, and any subprocesses the material may have undergone. According to ARP, “Metals freeze from the liquid state during melting from many origins (called allotropic) and each one of these origins grows until it bumps into another during freezing. Each of these is a grain…”

The grain structure of a metal determines many factors relating to it’s utility. Brittleness, ductility, hardness, and other considerations are related to grain size and direction. “Grain size is very important for mechanical properties. High temperature creep properties are enhanced by large grains but good toughness and fatigue require fine grain size-the finer the better. All ARP bolts and studs are fine grain — usually ASTM 8 or finer. With 10 being the finest.” reports ARP.

Modulus Of Elasticity

Ever wonder why you need to upgrade connecting-rod bolts, cylinder head studs, or other internal engine hardware? As you demand more horsepower and torque out of an engine it is typically accompanied by higher cylinder pressures, higher RPM running, and greater loads. All these contributing factors can stretch hardware.

ARP defines this metallurgical measurement explaining, “the modulus of elasticity of all alloy steels is exactly the same — 30,000,000 psi … Metals are like a spring — put a load on them and they will stretch. Double the load and they will stretch double. This is important in connecting rod bolts because by measuring the stretch we really are measuring the load.”

Steel Phases And Heat Treating

As we mentioned in the beginning, steels exist in a few different atomic arrangements depending on their condition. If you have hear the words, annealed, normalized, hardened etc. you are somewhat familiar with the basics of these phases. Heat treatment is a broad term for the process of heating and cooling metals, while carefully controlling their phase.

Face Centered Cubic structure.

This can be done by processes such as annealing in vermiculite, normalizing in air, quenching in water or oil, and a number of other techniques.

It should be noted that different alloys react differently to each of these processes, some steels are air hardening, others require the aggressive cooling of a water quench.

ARP talks about some of the phases they see in its manufacturing process, “Alloy steels which are bcc (base center cube), at room temperature, become fcc (face centered cubic) at temperatures above 1400 degrees F… Management of this phase is extremely critical and ARP maintains a complete in-house heat-treatment facility. It’s the only way we can assure material integrity.”

Base Cube Center Structure.

When referring to BCC and FCC, ARP is talking about the arrangement of carbon and iron atoms as they make a steel molecule.

There are many more concepts in metallurgy that affect the manufacturing of racing hardware, but we hope these fundamental will get you on the path to discover them for yourself. ARP has a wealth of knowledge and experience in material engineering when it comes to something as deceptively complex as a nut or bolt. If you would like more information about the materials and processes ARP uses, give them a call or check out its technical bulletins.