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.
The Rockwell test measures the depth of an indentation produced by a load on the indenter as well. With this method, however, there is a preliminary test force that is applied to the metal, which breaks through the surface. The pretest force is held for a specified time, then the depth of indentation is measured. After that is done, a major load is added to the preload. The force is held again for a determined amount of time, allowing for elastic recovery. When the major load is released, the final depth of indentation is measured. The hardness value is derived from the difference in the baseline and final depth of measurement. That number is then converted to a hardness number. Just like with the Brinell test, the deeper the indent, the softer the material. Because the Rockwell test indenter is a diamond tip, the tested area is a finite point in the steel, which may provide a misleading hardness reading. The Brinell test covers a broader area and is more representative of the plate hardness.
Hardness
Now that we know how to determine hardness, let’s dive a little deeper. We know it’s a characteristic of the actual metal, but what does it mean? To put it simply, hardness is the resistance of the steel to penetration. There are different types of steel, for example, AR400 OR AR500, that are produced to a specific hardness. At Clifton, for steel to be considered AR400, the plate must measure between 360 and 440 BHN. AR500 steel plate measures between 477 and 550 BHN. You can find more information on other grades of steel on our products pages.
Hardenability
While hardness is a material property, hardenability describes the ability for material to be hardened by thermal treatment. To put this one simply, it talks about potential. When a piece of steel goes through thermal treatment, it’s called quenching and tempering. Tempering is heating the plate to a high temperature, and quenching is rapidly cooling the hot plate through a medium such as water, oil, or something else. When the steel is quenched, the outside of the plate is cooled rapidly. Depending on the thickness of the plate, the inner depths of the material may not cool as quickly. If it cools too slowly, the piece could have a softer core and a harder “shell”. Hardenability refers to the ability of the steel to be hardened by that process.
Hardenability of steel is the ability of the steel to achieve a hardness value at a particular depth beneath the surface. 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.
Why does austenite grain size increase hardenability?
Austenitization is very important in the quenching process. The formation of martensite depends a lot on the austenite phase as explained in the article, “Effect of austenitization temperature in Steel”.
At the start of austenitization, the structure contains a fine and large quantity of austenite grains. These large numbers of austenite grains have large grain boundaries per unit area which increases the chances of heterogenous nucleation giving rise to Pearlitic formation.
So, giving more soaking time on austenitization temperature increases in austenite grain size. This increase in austenite grain size lowers the chances of Pearlitic formation and raises the hardenability of steel.
Although an increase in austenite grain size gives better hardenability this process is not recommended for practical use, because higher grain sizes have their demerits like lower impact strength, lower ductility, and higher chances of quench cracking.
How does carbon content affect hardenability?
Practically, carbon steels have lower hardenability. In plain carbon steel, there is no hindrance to diffusion of atoms and the normal quenching method can not achieve CCR. This is why manganese and silicon are added to shift the TTT curve to the right side and delay the diffusion transformation giving easier martensitic formation.
If we consider only the carbon percentage effect on the hardenability of steel, hardenability first increases as we move up to 0.8% and then it decreases from 0.8% to 2%.
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