4140 & 4340 Chrom-moly Steel

So what exactly is a Chrom-moly? And what is the difference between a 4340 with 4140? Why are they often refered as chrom-moly? If you heard this term and had no idea or only know them as very hard steel, read on.

Chrom-moly Steel we are talking about here are form from the process of alloying elements of Chromium and Molybdenum. Following are the percentage of elements of the 41xx and 43xx series alloy steel extracted from Pirate4x4.com writeup.

41xx - Chromium-0.50%, 0.80% OR 0.95% PLUS Molybdenum 0.25%
43xx - Nickel 1.83% PLUS Chromium 0.50% or 0.80% PLUS Molybdenum 0.25%

For example SAE 4340 , the last 2 digits indicate that the the alloy contain 0.40% carbon, the figure represent the approximate carbon content in tenths of a percent. Difference between 4140 and 4340 will be the additional nickel content. Both are often known as Chrom-moly Steel.





Element Characteristic, here are some of the few that we are interested in.
Carbon - Increases hardenability and strength
Chromium - Increases corrosion resistance, hardenability and wear resistance
Molybdenum - Deepens hardening, raises creep strength and hot-hardness, enhances corrosion resistance and increases wear resistance
Nickel - Increases strength and toughness
Vanadium - Increases hardenability
Silicon - Deoxidizes, helps electrical and magnetic properties, improves hardness and oxidation resistance

A modified version of 4340 will be the addition of Vanadium and Silicon to form one of the toughest steel also known as 300M. Internal or the star of the birfield are normally made of 300M Steel.

A chrom-moly steel had to heat treated to a desired properties to be used as axle shaft. There are basically 2 type of hardening process. Induction hardening where the heat hardening will be to a certain depth from the surface, and the axle core will remain relatively soft. There will be a residual compressive stress at the harden surface of the shaft, and these will actually improves fatigue resistance as most fatigue crack or failure are caused by imperfection (such as scuffs,knicks, burrs, rust, corrosion, grinding marks) on the surface.

Through hardening will have the entire shaft heat treated at a certain temperature and cooled at a very specific and carefully controlled rate. Through hardening of an axle shaft will makes it vastly harder but an ideal treatment would be a combination of through hardening and induction hardening.

Image for illustration purpose only, Image1 from Strange Engineering 

Read a more extensive indepth writeup on axle technology at
http://www.pirate4x4.com/tech/billavista/PR-BV60/index1a.html