Objectives:
- Distinguish between hardness and hardenability.
- Understand the concept of mass effect and ruling section.
- Perform Jominy test as a method to indicate hardenability.
Introduction:
One of the most important properties of materials is Hardness; which describes the resistance of a material to a permanent indentation. In steels, the value of hardness depends on many factors such as the carbon content, alloying elements and the cooling rate during hardening process. The Maximum hardness in steels is obtained by producing a fully martensitic structure. This can be done by austenitizing the steel and then quenching it. During the austenitizing treatment all of the carbides dissolve and the ferrite transforms into austenite. Quenching this structure causes the austenite to transform into Martensite. This transformation is so fast that there is no time to the carbon to diffuse out of the Martensite grains or to form carbide phases. This Martensite is very hard and also very brittle. Another important property of materials is Hardenability which describes the ability of material to be hardened in large depth, or the ability of a steel to partially or completely transform from austenite to some fraction of martensite at a given depth below the surface, when cooled under a given condition (the maximum depth of Martensite). When a thick steel component is quenched from its hardening temperature, it will take longer time to the inner core of the component to cool than for the surface layers that are in contact with quenching medium. This leads to a variation in hardness across the section of the steel component, and this mass variation in hardness is referred to as “mass effect”. This mass effect in plain carbon steel may be remedied to some extent by adding alloying elements to the steel, such as nickel, chromium, molybdenum, etc. these alloying elements make it possible to get a martensitic structure throughout the section of the steel component, even by oil-quenching. This is one of the most important functions of alloying, but the lack of uniformity of structure and hardness in steels, can seriously affect the other mechanical properties. For this reason it becomes necessary to specify the maximum diameter of the bar of “ruling section”.
The hardenability of materials can be studied by a particular test called Jominy Test; in which a heated specimen of steel (austenite) is dropped into position in a frame, and quenched by spraying a pre-set standard jet of water against its lower end. The standard specimen cools very rapidly at the quenched end and progressively less rapidly towards the opposite end. When cold, a flat is grounded along the side of the specimen and its hardness is measured every 3mm form the quenched end.
* Continue reading the full report from the attached PDF below, pages 39-46
Hardenability and Jominy Test.pdf
Jominy Test Results.xlsx .