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.

In the Grossman test, the transverse sections are metallographically examined to determine the particular bar, which has 50% martensite at its center. The diameter of this bar is then designated the critical diameter D_{0}. However, this dimension is of no absolute value in expressing the hardenability as it will obviously vary if the quenching medium is changed, e.g. from water to oil. It is therefore necessary to assess quantitatively the effectiveness of the different quenching media. This is done by determining coefficients for the severity of the quench usually referred to as H-coefficients. The value for quenching in still water is set at 1, as a standard against which to compare other modes of quenching.

Using the H-coefficients, it is possible to determine in place of D_{0}, an ideal critical diameter D_{i} which has 50% martensite at the center of the bar when the surface is cooled at an infinitely rapid rate, i.e. when H = ‡. Obviously, in these circumstances D_{0} = D_{i}, thus providing the upper reference line in a series of graphs for different values of H. In practice, H varies between about 0.2 and 5.0, so that if a quenching experiment is carried out at an H-value of, say, 0.4, and D_{0} is measured, then the graph can be used to determine D_{i}. This value will be a measure of the hardenability of given steel, which is independent of the quenching medium used.

This results in a progressive decrease in the rate of cooling along the bar from the quenched end, the effects of which are determined by hardness measurements on flats ground 4 mm deep and parallel to the bar axis (Fig. 3). A typical hardness plot for a **En 19B** steel containing 1% **Cr**, 0.25% **Mo** and 0.4% **C**, where the upper curve represents the hardness obtained with the upper limit of composition for the steel, while the lower curve is that for the composition at the lower limit. The area between the lines is referred to as a hardenability or Jominy band.

Additional data, which is useful in conjunction with these results, is the hardness of quenched steels as a function both of carbon content and of the proportion of martensite in the structure. Therefore, the hardness for 50% martensite can be easily determined for a particular carbon content and, by inspection of the Jominy test results, the depth at which 50 % martensite is achieved can be determined.

The Jominy test is now widely used to determine hardenabilities in the range D_{i} = 1-6; beyond this range the test is of limited use.

The results can be readily converted to determine the largest diameter round bar which can be fully hardened. Fig. 4 plots bar diameter against the Jominy positions at which the same cooling rates as those in the centers of the bars are obtained for a series of different quenches. Taking the ideal quench (H = ‡) the highest curve, it can be seen that 12.5 mm along the Jominy bar gives a cooling rate equivalent to that at the center of a 75 mm diameter bar. This diameter reduces to just over 50 mm for a quench in still water (H = 1). With, for example, a steel which gives 50 % martensite at 19 mm from the quenched end after still oil quenching (H = 0.3), the critical diameter D0 for a round rod will be 51 mm.

The diagram in Fig. 4 can also be used to determine the hardness at the center of a round bar of a particular steel, provided a Jominy end quench test has been carried out.

For example, if the hardness at the center of a 5 cm diameter bar, quenched in still water, is required, Fig. 3 shows that this hardness will be achieved at about 12 mm along the Jominy test specimen from the quenched end. Reference to the Jominy hardness distance plot, then gives the required hardness value. If hardness values are required for other points in round bars, e.g. surface or at half radius, suitable diagrams are available for use.

June, 2003

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