Hardenability testing

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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

Much of the earlier systematic work on hardenability was done by Grossman and coworkers who developed a test involving the quenching, in a particular cooling medium, of several cylindrical bars of different diameter of the steel under consideration. Transverse sections of the different bars on which hardness measurements have been made will show directly the effect of hardenability. In Fig 1, which plots this hardness data for an SAE 3140 steel (1.1-1.4% Ni, 0.55-0.75% Cr, 0.40% C) oil-quenched from 815‹C, it is shown that the full martensitic hardness is only obtained in the smaller sections, while for larger diameter bars the hardness drops off markedly towards the center of the bar. The softer and harder regions of the section can also be clearly resolved by etching.

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 D₀. 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 D₀, 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 D₀ = 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 D₀ 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.

Fig.1: Steel Ni-0.75Cr-0.4C. Hardness data from transverse sections through water-quenched bars of increasing diameter

The Jominy and quench test

While the Grossman approach to hardenability is very reliable, other less elaborate tests have been devised to provide hardenability data. Foremost amongst these is the Jominy test, in which a standardized round bar (25.4 mm diameter, 102 mm long) is heated to the austenitizing temperature, then placed on a rig in which one end of the rod is quenched by a standard jet of water (Fig.2).

Fig.2: The Jominy test: A - specimen size; B - quenching rig

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.

Fig.3: The Jominy and quench test

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.

Fig.4: Equivalent Jominy positions and bar diameter, where the cooling rate for the bar center is the same as that for the point in the Jominy specimen.

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.