Bhadeshia A heat treatment that causes steel to harden is so much more than the meer plunging of hot metal into a fluid that is often a liquid. The initial red-hot state represents the austenitic condition and the subsequent cooling results in a variety of transformations that depend on the chemical composition of the steel. If the intention is to produce a martensitic structure, then the constituents of the steel must be such that the phase is obtained over the depth required. The Jominy test provides a measure of the ability of a steel to harden by transforming into martensite under set conditions, i. A standardised bar, This results in a progressive decrease in the rate of cooling along the bar from the quenched end.
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No Comments The Jominy end-quench test is used to measure the hardenability of a steel , which is a measure of the capacity of the steel to harden in depth under a given set of conditions. This article considers the basic concepts of hardenability and the Jominy test. So, we felt print readers needed a second look. If you want to take a look at the original, click here. Knowledge about the hardenability of steels is necessary to select the appropriate combination of alloy steel and heat treatment to minimize thermal stresses and distortion in manufacturing components of different sizes.
The Jominy end-quench test is the standard method for measuring the hardenability of steels. This describes the ability of the steel to be hardened in depth by quenching. Hardenability depends on the chemical composition of the steel and can also be affected by prior processing conditions, such as the austenitizing temperature. It not only is necessary to understand the basic information provided from the test, but also to determine how the information obtained from the Jominy test can be used to understand the effects of alloying in steels and the steel microstructure.
Hardenability Hardenability is the ability of the steel to partially or to completely transform from austenite to some fraction of martensite at a given depth below the surface when cooled under a given condition from high temperature.
A quench-and-temperheat treatment uses this phase transformation to harden steels. Tempering the martensite microstructure imparts a good combination of strength and toughness to the steel. Without tempering, martensite is hard but brittle. To select a steel for a component that will be heat treated, it is important to know its hardenability. Both alloying and microstructure affect the hardenability, allowing the correct steel and quenching rate to be selected.
Prior processing of the steel also affects the microstructure and should be considered. Hardening of steels can be understood by considering that the austenite phase of the steel can transform to either martensite Fig.
However, the martensite transformation does not involve diffusion and is essentially instantaneous. These two reactions are competitive, and martensite is obtained if the cooling rate is fast enough to avoid the slower formation of ferrite and pearlite. Transformation to another possible phase bainite can be understood in a similar way. Hardenability describes the capacity of the steel to harden in depth under a given set of conditions. For example, a steel of a high hardenability can transform to a high fraction of martensite to depths of several millimeters under relatively slow cooling, such as an oil quench.
By comparison, a steel of low hardenability may only form a high fraction of martensite to a depth of less than 1 mm, even under quite rapid cooling, such as a water quench. Steels having high hardenability are required to make large high-strength components such as large extruder screws for injection molding of polymers, pistons for rock breakers, mine-shaft supports and aircraft undercarriages and small, high-precision components such as die-casting molds, drills and presses for stamping coins.
The slower cooling rates that can be used for high-hardenability steels can reduce thermal stresses and distortion. The Jominy end-quench test is the standard method to measure the hardenability of steels. The test sample is quickly transferred to the test fixture Fig.
The cooling rate varies along the length of the sample, from very rapid at the quenched end where the water strikes the specimen to slower rates that are equivalent to air cooling at the other end.
The round specimen is then ground flat along its length on opposite sides to a depth of at least 0. Care should be taken that the grinding does not heat the sample because this can cause tempering, which can soften the steel.
Hardness is measured at intervals from the quenched end, typically at 1. The hardness decreases with distance from the quenched end. High hardness occurs where high-volume fractions of martensite develop. Measurement of hardness is commonly carried out using a Rockwell or Vickers hardness tester.
Rockwell and Vickers hardness tests deform the metal differently, and the results are affected by work hardening. The hardenability is described by a hardness curve for the steel Fig. Uses of Hardenability Values Data from the Jominy end-quench test can be used to determine whether a particular steel can be sufficiently hardened in different quenching media, for different section diameters. For example, the cooling rate at a distance of 10 mm 0.
Full transformation to martensite in the Jominy specimen at this position indicates that a mm-diameter bar can be through-hardened i.
A high hardenability is required for through-hardening of large components. This data can be presented using CCT continuous-cooling transformation diagrams, which are used to select steels to suit the component size and quenching media Fig. Slower cooling rates occur at the core of larger components, compared with the faster cooling rate at the surface. In the example in Fig. Slow quenching speeds often are selected to reduce distortion and residual stress in components.
Reference 6 contains further information on the heat treatment and properties of steels. Effects of Alloying and Microstructure The Jominy end-quench test measures the effects of microstructure, such as grain size and alloying, on the hardenability of steels. The main alloying elements that affect hardenability are carbon; a group of elements including Cr, Mn, Mo, Si and Ni; and boron. Carbon Carbon controls the hardness of the martensite; increasing carbon content increases the hardness of steels up to about 0.
At higher carbon levels, however, the critical temperature for the formation of martensite is depressed to lower temperatures. The transformation from austenite to martensite may then be incomplete when the steel is quenched to room temperature, which leads to retained austenite. This composite microstructure of martensite and austenite results in a lower steel hardness, although the hardness of the martensite phase itself is still high Fig. Carbon also increases the hardenability of steels by retarding the formation of pearlite and ferrite.
Slowing down this reaction encourages the formation of martensite at slower cooling rates. However, the effect is too small to be commonly used for control of hardenability. Furthermore, 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 using other elements and to use carbon levels of less than 0. The most commonly used elements are Cr, Mo and Mn. The retardation is due to the need for redistribution of the alloying elements during the diffusional phase transformation from austenite to ferrite and pearlite. The solubility of the elements varies between the different phases, and the interface between the new growing phase cannot move without diffusion of the slowly moving elements.
There are quite complex interactions between the different elements, which also affect the temperatures of the phase transformation and the resultant microstructure. Alloy steel compositions are, therefore, sometimes described in terms of a carbon equivalent, which describes the magnitude of the effect of all of the elements on hardenability.
Steels of the same carbon equivalent have similar hardenability. Boron Boron is a very potent alloying element, typically requiring 0. The effect of boron is independent of the amount of boron, provided a sufficient amount is added.
The effect of boron is greatest at lower carbon contents, and it is typically used with lower-carbon steels. Boron has a very strong affinity for oxygen and nitrogen, with which it forms compounds. Therefore, boron can only affect the hardenability of steels if it is in solution. Grain Size Increasing the austenite grain size increases the hardenability of steels. The nucleation of ferrite and pearlite occurs at heterogeneous sites such as the austenite grain boundaries.
This method of increasing the hardenability is rarely used because substantial increases in hardenability require large austenite grain size, which is obtained through high austenitizing temperatures. The resultant microstructure is quite coarse, with reduced toughness and ductility. However, the austenite grain size can be affected by other stages in the processing of steel.
Therefore, the hardenability of a steel also depends on the previous stages used in its production. American Society for Testing and Materials, ASTM E ASTM A Standard Hardness Conversion Tables for Metals, ASM Handbook, Vol. American Society for Metals, Steels: Microstructure and Properties, R.
K Honeycombe and H. Edward Arnold,
Understanding the jominy end quench test
No Comments The Jominy end-quench test is used to measure the hardenability of a steel , which is a measure of the capacity of the steel to harden in depth under a given set of conditions. This article considers the basic concepts of hardenability and the Jominy test. So, we felt print readers needed a second look. If you want to take a look at the original, click here. Knowledge about the hardenability of steels is necessary to select the appropriate combination of alloy steel and heat treatment to minimize thermal stresses and distortion in manufacturing components of different sizes. The Jominy end-quench test is the standard method for measuring the hardenability of steels. This describes the ability of the steel to be hardened in depth by quenching.
The Jominy End Quench Test
Depth to which a metal is hardened after being submitted to a thermal treatment Jominy test dimensioning. Used Jominy test-piece. Example Jominy results. The hardenability of a metal alloy is the depth to which a material is hardened after putting it through a heat treatment process.
Understanding the Jominy End-Quench Test