Hardenability Bands

Hardenability Bands

Because of differences in chemical composition between different heats of the same grade of steel, so-called hardenability bands have been developed using the Jominy end-quench test. According to American designation, the hardenability band for each steel grade is marked bythe letter H following the composition code. Figure 1 shows such a hardenability band for 1340 H steel. The upper curve of the band represents the maximum hardness values, corre-sponding to the upper composition limits of the main elements, and the lower curve representsthe minimum hardness values, corresponding to the lower limit of the composition ranges. Hardenability bands are useful for both the steel supplier and the customer. Today the majority of steels are purchased according to hardenability bands. Suppliers guarantee that 93 or 95% of all mill heats made to chemical specification will also be within the hardenability band. The H bands were derived from end-quench data from a large number of heats of aspecified composition range by excluding the upper and lower 3.5% of the data points. Steels may be purchased either to specified composition ranges or to hardenability limits defined by H bands. In the latter case, the suffix H is added to the conventional grade designation, for example 4140 H, and a wider composition range is allowed. The difference in hardenability between an H steel and the same steel made to chemical specifications is illustrated in Figure 1. These differences are not the same for all grades.
High-volume production of hardened critical parts should have close tolerance of the depth of hardening. The customer may require, at additional cost, only those heats of a steel grade that satisfy, for example, the upper third of the hardenability band. As shown in Figure 2, the SAE recommended specifications are: means-different ways of specifications.

Symbols:
H - Symbol indicating a steel with normal hardenability band (100% range).
HH - Symbol indicating a steel with restricted hardenability band with respect to the upper limiting curve.
HL - Symbol indicating a steel with restricted hardenability band with respect to the lower limiting curve.

Hardenablity band

Figure 1: Hardenablity band for SAE 1340 H steel.

Hardenability limits

Figure 2: Different ways of specifying hardenability limits according to SAE.

Reference: George E. Totten, Steel Heat Treatment: Metallurgy and Technologies, CRC Press, 2007, pp. 237-238.