BeginnerTopic 21 of 100Materials & Chemical
Hardenability
Ability of steel to form martensite to depth; tested by Jominy test.
In depth
Hardenability is the depth to which a steel can harden on quenching, not its maximum hardness. Alloying elements (Mn, Cr, Mo, Ni) increase it. Jominy end-quench test gives hardness vs distance; H-bands specify steels. Section size and quench severity matter.
Key points
- Ideal critical diameter D_I
- Carbon sets max hardness
- Through-hardening of large sections
Example4140 steel hardens deeply while 1045 hardens only near the surface.
What is Hardenability?
Ability of steel to form martensite to depth; tested by Jominy test.
Why is it important to understand hardenability?
Hardenability tells you how deep a steel can be hardened, so it decides whether a large part will be hard at its core.
What is the Jominy test?
An end-quench test that cools a bar from one end, and the hardness along its length shows how the steel hardens at different cooling rates.
What is martensite?
A very hard, supersaturated phase formed by rapid cooling of steel, which gives high hardness but needs tempering.
How is Hardenability used in practice?
Typical applications include choosing a steel for a large shaft that must be hard to the centre, checking the hardness profile of a quenched bar after the Jominy test and specifying a grade for a gear where case depth matters.
What should I learn before Hardenability?
Study "TTT and CCT diagrams" first (topic 20 of 100 in Materials & Chemical). It covers the groundwork this topic builds on.
What are the key points of Hardenability?
Ideal critical diameter D_I. Carbon sets max hardness. Through-hardening of large sections.
Can you give an example of Hardenability?
4140 steel hardens deeply while 1045 hardens only near the surface.
What comes after Hardenability?
The next topic is "Cast irons", which builds directly on this one.
How difficult is Hardenability?
It is classed as Beginner in Materials & Chemical (topic 21 of 100). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.