The Automotive Racing Products (ARP) staff are pretty darn “geeky” when it comes to the science of working with varied metals to give them certain desired strengths or properties. Here are some key terminology nuggets from ARP that influence the design and manufacturing of their product lines.
The importance of a metal’s grain size comes from the ways different alloys are cooled and form a grain pattern. For example, grains in cast metals are quite large. Grains can be refined (made smaller) by first cold working and then by recrystallizing at high temperature.
Smaller grain size equals stronger metals. Alloy steels, like chrome-moly, do not need any cold work to do this – reheat treatment will refine the grain size. All ARP bolts and studs are of a fine-grain – usually eight or finer on the ASTM scale, with 10 being the finest.
Metal Toughness vs. Brittleness
Recognizing the laws of nature concerning steel, as the strength goes up, the toughness decreases. At too high a strength, the metal tends to be brittle. And threads accentuate the brittleness. Tool steel, which can be heat-treated to 350,000 psi, would be a disaster as a bolt because of the threads.
Modulus of Elasticity
Metals are like a spring – if you double load on them, they will also stretch to a doubled factor. This metallurgy is important in connecting rod bolts because we are measuring the load by measuring the stretch.
The modulus of elasticity of all alloy steels is the same – 30,000,000 psi. Whether it is heat-treated or not, this modulus is true – whether it is a 100,000 psi strength level or 300,000 psi.
What is Chrome?
Chrome is the metal chromium best known for chrome plating. It is also used as an alloy addition to iron to form stainless steel. Stainless steel must contain at least 12-percent chromium. These low-chromium steels can still show rust on the surface. Using 18-percent chromium will make stainless steel more rust-resistant. In fastener applications, stainless exposed to oxygen at temperatures above 1200 degrees Fahrenheit will cause the chromium to join the oxygen and leave the surface depleted in chromium.
Differences Between 4130 and 8740 Chrome Moly
Both metals have chrome-moly (most alloy steels have moly) and similar chemistry. A 4130 fastener has only .3-percent carbon and can’t be hardened as high as 8740, which has .4-percent carbon. Also, 8740 has about .45-percent nickel, and 4130 has none. The chromium content of 4130 is slightly higher, .95-percent instead of .55-percent. However, 8740 is generally considered to have somewhat better toughness due to the nickel.
Common Fastener Failures
The most common cause of a connecting rod and wheel stud failure is too little induced load (stretch) during installation. This induced load allows the alternating load to impose a cyclic loading on the bolt. Overtightening a fastener is another cause of failure because the induced stress is too close to the yield point.
When you decide upon various fasteners for performance and racing applications, knowing the more common terms can help you make the proper decisions.
The ARP fasteners catalog is not only filled with pages of various fasteners comprised of different metals, but it can also become a classroom-style textbook for learning exactly why different metal alloys are used for varied fasteners.