Hardenability of Steels
The extent of martensite formation, including hardness and depth of formation, is known Key Words: Hardness, Hardenability, Jominy test, Quench, Steel. The main alloying elements which affect hardenability are carbon, boron and a Increasing the carbon content increases the hardness of steels up to about. The traditional route to high strength in steels is by quenching to form martensite that is The ability of steel to form martensite on quenching is referred to as the hardenability. Relation between cooling curves for the surface and core of an oil-quenched 95 mm diameter The Grossman test; The Jominy end quench test .
Increasing the carbon content increases the hardness of steels up to about 0. At higher carbon levels, the formation of martensite is depressed to lower temperatures and the transformation from austenite to martensite may be incomplete, leading to retained austenite. This composite microstructure of martensite and austenite gives a lower hardness to the steel, although the microhardness of the martensite phase itself is still high.
However, the effect is too small be be commonly used for control of hardenability.
Jominy End Quench Test
High carbon steels are prone to distortion and cracking during heat treatment, and can be difficult to machine in the annealed condition before heat treatment. It is more common to control hardenability with other elements, and to use carbon levels of less than 0.
Boron Boron is a very potent alloying element, typically requiring 0. The effect of boron is also independent of the amount of boron, provided sufficient is added, and the effect of boron is greatest at lower carbon contents.Hardenability – Steel – Snippet from ‘Steel Metallurgy’
It is typically used with lower carbon steels. Boron has a very strong affinity for oxygen and nitrogen, with which it forms compounds. Boron can therefore only affect the hardenability of steels if it is in solution.
This requires the addition of "gettering" elements such as aluminium and titanium to react preferentially with the oxygen and nitrogen in the steel.
Chromium, molybdenum, manganese, silicon, nickel, vanadium The elements Cr, Mo, Mn, Si, Ni and V all retard the phase transformation from austenite to ferrite and pearlite. The most commonly used elements are Cr, Mo and Mn.
DoITPoMS - TLP Library The Jominy End Quench Test - Uses of Jominy data 2
This series of results will give rise to an austenite-pearlite boundary on the diagram and likewise lines showing the onset of the bainite transformation can be constructed.
A schematic diagram is shown in Figure 3. The diagram is best used by superimposing a transparent overlay sheet with the same scales and having lines representing various cooling rates drawn on it. The phases produced at a chosen cooling rate are those which the superimposed line intersects on the continuous cooling diagram.
In this example, it should be noted that the centre cooling curve intersects the bainite region and consequently some bainite would be expected at the core of the bar after quenching in oil. TTT diagram of a molybdenum steel 0. Relation between cooling curves for the surface and core of an oil-quenched 95 mm diameter bar and the microstructure Hardenability Testing The rate at which austenite decomposes to form ferrite, pearlite and bainite is dependent on the composition of the steel, as well as on other factors such as the austenite grain size, and the degree of homogeneity in the distribution of the alloying elements.
It is extremely difficult to predict hardenability entirely on basic principles, and reliance is placed on one of several practical tests, which allow the hardenability of any steel to be readily determined: The Grossman test The Jominy end quench test Effect of Grain Size and Chemical Composition on Hardenability The two most important variables which influence hardenability are grain size and composition.
The hardenability increases with increasing austenite grain size, because the grain boundary area is decreasing. This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased. Likewise, most metallic alloying elements slow down the ferrite and pearlite reactions, and so also increase hardenability.
However, quantitative assessment of these effects is needed. There are a bewildering number of steels, the compositions of which are usually complex and defined in most cases by specifications, which give ranges of concentration of the important alloying elements, together with the upper limits of impurity elements such as sulfur and phosphorus.
While alloying elements are used for various reasons, the most important is the achievement of higher strength in required shapes and sizes and often in very large sections which may be up to a meter or more in diameter in the case of large shafts and rotors. Hardenability is, therefore, of the greatest importance, and one must aim for the appropriate concentrations of alloying element needed to harden fully the section of steel under consideration.
Equally, there is a little point in using too high a concentration of alloying element, i.
Alloying elements are usually much more expensive than iron, and in some cases are diminishing natural resources, so there is additional reason to use them effectively in heat treatment. Carbon has a marked influence on hardenability, but its use at higher levels is limited, because of the lack of toughness which results, the greater difficulties in fabrication and, most important, increased probability of distortion and cracking during heat treatment and welding.
The Jominy End Quench Test, ASTM A
The most economical way of increasing the hardenability of plain carbon steel is to increase the manganese content, from 0. Chromium and molybdenum are also very effective, and amongst the cheaper alloying additions per unit of increased hardenabilily.
Hardenabilily data now exists for a wide range of steels in the form of maximum and minimum end-quench hardenability curves, usually referred to as hardenability bands.
Feb Finding heat treatment diagrams in the Total Materia database Heat treatment diagrams are available for a huge number of materials in the Total Materia database.