Heat Treater The Choice of Steels for Case Hardening

The Choice of Steels for Case Hardening

The low carbon steels which are suitable for case hardening may be sub-divided further into plain carbon steels and low alloy steels.

A steel is considered to be an alloy grade when it contains more than 1.65% manganese or 0.6% silicon or 0.6% copper, or if nickel, chromium or molybdenum are added to the levels found in constructional steels.

A plain carbon steel is one which falls outside this category.

Full hardening occurs when a steel, heated to it's hardening temperature is cooled at a rate which exceeds a critical value. For plain carbon steels this value is approx. 500oF/sec. With the addition of alloying elements to levels typical of an 8620 steel. this cooling rate may be as slow as 100oF/sec and if the levels are
increased to those of a 4320 steel the cooling may be as low as 25oF/sec - in both cases achieving full hardness.

Cooling rate depends upon the size of the part being hardened, on the severity of the quench (water is faster than oil but is likely to cause distortion) and on the degree of agitation in the quench.

It is also important to note that on thicker sections, cooling rates will be higher at the edges than towards the centre.

All these factors must be considered when selecting a suitable steel for case hardening. For thin sections (up to ½” thick) where the only requirement is a thin hard case - use 1008 or 1015, or for ease of machining a leaded grade C12L14.
For thicker sections or parts which require some increase in their core strength use 1018 or 1020.

For a more reliably consistant case where oil is used as a quenchant, for less distortion and for even greater core strength use 8620.

For greatest hardenability and highest core strength in the largest sections use 4320 or 3312. These are the 3% nickel alloys which combine strength with toughness (resistance to shock loading)

In all these examples the carbon is kept low so that high levels of hardness in the core and associated lack of toughness are avoided.