Determination of Specific Hardenability of Shallow-Hardening Steels

A TAPER test specimen having a taper of 1 in. in 5 in. of length, ¼ in. in diameter at one end and 1¼ in. in diameter at the other end, is proposed as a test for measuring the specific hardenability of shallow-hardening steels. The hardened taper test specimens are split and Rockwell hardnesses obtained along the central axis of test specimen, or the central section may be etched lightly to bring out the relative colors of case and core. This taper test specimen has been correlated with the Jominy-Boegehold “L” bar. This enables the rate of cooling at any point along the central axis of the taper test specimen to be expressed in terms of deg F per sec at 1300 F.

This taper test specimen has also been correlated with the work of Grossmann and his associates concerning critical bar diameters and severity of quench. It is shown experimentally that this degree of taper is small enough so that the taper test specimen behaves within experimental error like a series of round bars whose diameters are given by twice the perpendicular distance from the surface of the taper test specimen to any point along the center axis.

A correlation is shown between the Shepherd disc hardenability Nos. 10 to 16 inclusive and the critical cooling velocity in deg. F per sec at 1300 F (as determined from the cooling rates published for the Jominy-Boegehold Type “L” bar), and critical bar diameters at a severity of quench of H = 4.5, or the “ideal critical bar diameters” for H = ∞.