Direct Hardening Alloy Steels

1. Introduction
2. Selecting Composition for Direct Hardening Applications
3. Effects of Tempering on Direct Hardening Alloy Steels
4. Properties of Direct Hardening Alloy Steels I
5. Properties of Direct Hardening Alloy Steels II
6. Heat Treatment of Direct Hardening Alloy Steels I
7. Heat Treatment of Direct Hardening Alloy Steels II
8. Certificate of Testing
9. Certificate of Testing
10. Certificate of Testing
11. Certificate of Testing
12. Certificate of Testing
13. Certificate of Testing
14. Certificate of Testing
15. Availability and Uses of Direct Hardening Steels
16. Exercise
17. Behavior of Hardened/Tempered Alloy Steels
18. Large Diameter Alloys - Low Hardenability
19. Chromium/Boron Steel Used in a Direct Quenching Application
20. Large Forgings I
21. Large Forgings II
22. Direct Hardening Alloy Steels - Conclusion
23. Assessment
24. Summary

Direct hardening alloy steels are those that achieve their properties by quenching and tempering. Unlike carburised case hardened steels, which have very hard surface zone, direct hardening steels have high and relatively uniform hardness across the full section. However, the surface is likely to be slightly harder than the core.

Direct hardening alloy steels are heated to an austenitsing temperature (> A₃) which is selected on the basis of the carbon content of the steel. This is followed by quenching in water, oil or polymer. This choice is dictated by the property requirements, the hardenability of the steel and the ruling section size of the component. The aim is to produce sufficient martensite in the as-quenched condition as is needed to meet the specified properties. In high alloy steels and/or small sections, it may be possible to achieve a martensite microstructure in the air-cooled condition.

Direct hardening steels are usually tempered at a predetermined temperature range for sufficient time to realize the specified properties to meet design engineering requirements. These steels are characterised by an excellent combination of high strength, ductility and toughness in small and very large components. A large number of alloy steel specifications have been developed to achieve closely controlled properties in any particular ruling section.

Tensile strength levels in direct hardening steels can be realised between 750 and 2000 MPa.

The following graphic shows the Martensite microstructure in a quenched and tempered alloy steel.

The main alloying elements present in direct hardening steels are Chromium, Molybdenum and Nickel, although Vanadium, Tungsten and Cobalt may also be used. Each of these elements has its own unique effect on the hardenability, carbide type and temper resistance of the steel. To complicate matters further, there are also complex synergistic reactions between combinations of these elements, giving unique mechanical properties, which justify the extra cost of these alloy additions.