Hardenability is the ability of the steel to produce the appropriate hardness in relation to the amount of martensite when quenched for its austenitizing temperature into the appropriate quench medium. The resulting hardness will be according to its cross-sectional area and chemistry (composition).
The hardenability test is measured by heating a test piece of a very specific size, followed by heating the test bar and quenching the end face. This test is known as the Jominy End Quench Hardenability End test.
Hardenability is a term that is used to describe a given steel’s ability to harden. It does not mean “what hardness can be achieved.”
Hardenability is usually determined by the Jominy end-quench test. The method involves machining a piece of steel to specific dimensions, heating the steel to its appropriate austenitizing temperature followed by spraying a volume of water onto the end face of the bar. This will cool the face rapidly and more slowly, progressively cooling the length of the bar. The bar is marked off at 1/16th-inch intervals.
After the test is complete, the bar is hardness tested every 1/16 inch and a curve is plotted. This is the steel’s ability to respond to a specific heat-treat procedure in terms of hardness values across its cross section. In other words, it will show the depth of hardening across an equivalent diameter bar of steel. The addition of alloying elements will affect the steel’s hardenability.
The hardenability of steel is a function of the carbon content of the material, other alloying elements, and the grain size of the austenite. Austenite is a gamma phase iron and at high temperatures its atomic structure undergoes a transition from a BCC configuration to an FCC configuration.
High hardenability refers to the ability of the alloy to produce a high martensite percentage throughout the body of the material upon quenching. Hardened steels are created by rapidly quenching the material from a high temperature. This involves a rapid transition from a state of 100% austenite to a high percentage of martensite. If the steel is more than 0.15% carbon, the martensite becomes a highly strained body-centered cubic form and is supersaturated with carbon. The carbon effectively shuts down most slip planes within the microstructure, creating a very hard and brittle material. If the quenching rate is not fast enough, carbon will diffuse out of the austenitic phase. The steel then becomes pearlite, bainite, or if kept hot long enough, ferrite. None of the microstructures just stated have the same strength as martensite after tempering and are generally seen as unfavorable for most applications.
It’s important to remember that different alloys of steel contain different elemental compositions. The ratio of these elements relative to the amount of iron within the steel yield a wide variety of mechanical properties. Increasing the carbon content makes steel harder and stronger but less ductile. The predominant alloying element of stainless steels in chromium, which gives the metal its strong resistance to corrosion. Since humans have been tinkering with the composition of steel for over a millennium, the number of combinations is endless.
The hardenability of a metal alloy is the depth up to which a material is hardened after putting through a heat treatment process.The unit of hardenability is the same as of length. It is an indication of how deep into the material a certain hardness can be achieved. It should not be confused with hardness, which is a measure of a sample’s resistance to indentation or scratching. It is an important property for welding, since it is inversely proportional to weldability, that is, the ease of welding a material. When a steel work-piece is quenched, the area in contact with the water immediately cools and evens out with that of the medium. The inner depths of the material however, do not cool quite so rapidly and in work-pieces that are large, the cooling rate may be slow enough to allow the austenite to transform fully into a structure other than martensite or bainite. This results in a work-piece that does not have the same crystal structure throughout its entire structure; with a softer core and harder “shell”. The softer core is some combination of ferrite and cementite, such as pearlite. The hardenability of ferrous alloys, i.e. steels, is a function of the carbon content and other alloying elements and the grain size of the austenite. The relative importance of the various alloying elements is calculated by finding the equivalent carbon content of the material. The fluid used for quenching the material influences the cooling rate due to varying thermal conductivities and specific heats. Substances like brine and water cool much more quickly than oil or air. Additionally, if the fluid is agitated cooling occurs even more quickly. The geometry of the part also affects the cooling rate: of two samples of equal volume, the one with higher surface area will cool faster.
Hardenability of steel is the ability of the steel to achieve a hardness value at a particular depth beneath the surface. (Click here for more information on core hardness and surface hardness in fasteners). The chemical composition of a steel provides the foundation for hardenability. The iron content in steel is around 98%. Carbon (C) is the primary hardening element. Other alloying elements such as manganese (Mn), molybdenum (Mo), chromium (Cr), and nickel (Ni) are often added in small amounts to increase hardenability. The methods of heat treatment, quenching, and tempering of steel develop the material’s metallurgical structure and mechanical properties, including hardness. A measure of steel’s hardenability aids fastener engineers in selecting materials that will produce parts with the desired surface and core hardness.
Hardenability is 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. For example, a steel of a high hardenability can transform to a high fraction of martensite to depths of several millimetres under relatively slow cooling, such as an oil quench, whereas a steel of low hardenability may only form a high fraction of martensite to a depth of less than a millimetre, even under rapid cooling such as a water quench. Hardenability therefore describes the capacity of the steel to harden in depth under a given set of conditions.
Steels with high hardenability are needed for large high strength components, such as large extruder screws for injection moulding of polymers, pistons for rock breakers, mine shaft supports, aircraft undercarriages, and also for small high precision components such as die-casting moulds, drills and presses for stamping coins. High hardenability allows slower quenches to be used (e.g. oil quench), which reduces the distortion and residual stress from thermal gradients.
Steels with low hardenability may be used for smaller components, such as chisels and shears, or for surface hardened components such as gears.
The hardenability of steel is a function of the carbon content of the material, other alloying elements, and the grain size of the austenite. Austenite is a gamma phase iron and at high temperatures its atomic structure undergoes a transition from a BCC configuration to an FCC configuration.
High hardenability refers to the ability of the alloy to produce a high martensite percentage throughout the body of the material upon quenching. Hardened steels are created by rapidly quenching the material from a high temperature. This involves a rapid transition from a state of 100% austenite to a high percentage of martensite. If the steel is more than 0.15% carbon, the martensite becomes a highly strained body-centered cubic form and is supersaturated with carbon. The carbon effectively shuts down most slip planes within the microstructure, creating a very hard and brittle material. If the quenching rate is not fast enough, carbon will diffuse out of the austenitic phase. The steel then becomes pearlite, bainite, or if kept hot long enough, ferrite. None of the microstructures just stated have the same strength as martensite after tempering and are generally seen as unfavorable for most applications.