Hardenability
The term hardenability refers e.g to changes in hardness which arise during hardening when cooling out of the austenite area. The term "hardening capacity" descibes the hardest surface achievable by quenching. This comes about when there is practically 100% martensite in the microstructure. This hardness value depends on the carbon content of the steel or, more precisely, on the C-content which is disolved in austenite (figure 14) . A differentiation must be made to so-called hardness penetrability. This is what is meant when the term "hardenability" is used in everyday usage. It describes the changes in hardness thoughout the whole cross section of the workpiece. Among other factors, these changes largely depend on the alloy content, the size of austenite grains and the conditions under which the steel was hardened.

These statements suggest that the time-temperature diagram for continuous cooling along with the cooling curves (figures 8-12) give a comprehensive description of the hardenability of steels. In practice, a less complex process is generally used where only the hardness values are calculated and the calculation of the microstructural composition is not considered. With this method, it is assumed that in most cases, a microstructure of 50% martensite can be seen as the minimum value for hardness penetrabilitiy.

Figure 14 (To enlarge, please click on the picture in question )
Dependence of hardness on carbon content for microstructures with various martensite contents (according to Gerber and Wyss)



The Jominy (DIN EN ISO 642) face quenching test where a specimen of 100 mm in length and 25mm in diameter is quenched from the face with a water jet is used to test hardness. (figure 15)

After quenching, the hardness is measured in relation to the distance from the face along the casing which is partly polished at the sides and the result of this is shown in a diagram. Due to the uniform testing conditions, there is only slight scattering of results with this process. Figure 16 shows hardness curves for face quenching (Jominy curves) for various alloyed steels with approx. 0.3% C, marking out the influence of the various chemical compositions on the course of hardening.

Figure 15 (To enlarge, please click on the picture in question )
Face quenching test according to Jominy



Figure 16
Medium curves from face quenching hardenability tests on a high alloy, low alloy and non-alloy steel with about 0.3% carbon



Since their C-contents are almost the same, they all have the same hardening capacity; the curves begin at the left of the picture with the same hardness. Batch infuences such as chemical fluctuations as they occur within the acceptable analysis range of a steel, are characterized by a scattering range for hardening capacity.

From the results of the face quenching hardness test for a steel, it is possible to make predictions regarding the course of hardening in other bodies with a regular shape and size made of the same materials since the distance from the quenched face of a face quenched specimen can be matched to a particular cooling speed in each case.. This matching applies to all common case-hardening and quench and temper steels as long as these steel groups have largely similar physical properties - heat conductibiity and heat expansion. Since, for a given quality of steel, the same cooling speeds from austenitising temperature always lead to certain transformation processes and certain microstructural developments and hardness values, it must be possible to match the hardness values from the face quenching hardness curve to the same hardness values at points with an equivalent cooling speed on other bodies. Hence it is possible to match the hardness values calculated on the specimen used in the face quenching hardness test to points with the same cooling speeds in round rods and to calculate the course of hardening in these cross-sections.

For practical use, diagrams for hardening in water and in oil have been developed to show the cooling speed at individual points in the cross-section of round rods in relation to the diameter. These diagrams take the form of transparent cover-sheets for the curves showing hardening capacity, the scattering range and face quenching.. It is therefore possible to determine the hardness of any point in the cross section of a hardened round rod from the face quenching curve and to give the course of hardening for the cross section.

Figure 17 (To enlarge, please click on the picture in question )
Relation between the distance from the quenched face and
zones with the same cooling speeds for hardening with water (left) and with oil (right).


Example applications:
The course of hardening is to be determined throughout the cross-section of a round rod of 100mmØ after hardening with water using the scattering range for the hardening capacity of a quench and temper steel 34 Cr 4.

The cover sheet for hardening with water is laid over the scatteing range for hardening capacity of 34 Cr 4. The points where the 100 mm horizontal line crosses the 4 curves of the cover sheet are ascertained (Points 1-4 in figure 18b). The vertical lines running through these crossing points each cut through the scatter range at 2 points - the lower limit curve in points 1´ to 4´ and the upper in 1" to 4". These points correspond to the following hardness values:

Edge 1'...49 HRC 2'...33 HRC 3'...25 HRC Core 4'...20 HRC
Edge 1"...57 HRC 2"...48 HRC 3"...41 HRC Core 4"...37 HRC

This gives us the distribution of hardness presented in figure 18a through the cross-section Ø 100 mm for the hardness range of the quality of steel concerned (in the current example 34 Cr 4). Since the curve showing the hardening capacity of a melt has to lie within the scatter range of the quality, the corresponding curve showing the course of hardening must also lie within the two limit curves of figure 18a.

2. Determining the maximum dimension of round rod for the steel 34 Cr 4 at which full hardening is achieved when hardening with water.

For the C-content of a melt with 0. 34% carbon, diagram 14 shows the individual hardness which corresponds to a proportion of 50% martensite in the microstructure. In our example this isl 40 HRC. The diagram for hardening in water (figure 17, left) is transferred to the diagram for the scattering range of the hardening capacity of the steel concerned.

Figure 18a and 18b (to enlarge klick the diagram in question)



In 40 HRC, the scatter range is crossed at two points, 1 and 2. The vertical lines through these points cross the cooling curve for the core in points 1´ and 2´. They correspond to a round bar diameter of 40 to 85 mm (points 1" and 2" in figure 19),whereby these points give the limit values of this scatter range. For a melt where the hardenability is at the lower limit, there is therfore a maximum rod diameter of 40mm Ø. For higher harderining capacity it is a max.of 85mm Ø.