Hardness and Hardenability - Part Two: A Discussion on Hardenability and Hardenability Testing
For the heat treater, the concept of hardenability is often more difficult to grasp than that of hardness. Part of the reason for this is that we seldom perform the tests that measure or predict this property in our shops. The reason why it is important to measure the hardenability of steel is to make sure that we are making the right material choice for a specific engineering application. With the supply of raw material coming from multiple worldwide sources, there is renewed emphasis on predicting how a material will respond to heat treating. Let’s learn more.
Hardenability is a material property independent of cooling rate and dependent on chemical composition and grain size. When evaluated by hardness testing, hardenability is defined as the capacity of the material under a given set of heat-treatment conditions to harden “in depth.” In other words, hardenability is concerned with the “depth of hardening” or the hardness profile obtained, not the ability to achieve a particular hardness value. When evaluated by microstructural techniques, hardenability is defined as the capacity of the steel to transform partially or completely from austenite to some percentage of martensite at a given depth when cooled under known conditions.
Effects of Alloying ElementsCertain alloying elements have a strong influence on the hardenability and response to heat treatment and ultimately on the ability of the product to perform its intended function. The main alloying elements that affect hardenability are carbon, boron, chromium, manganese, molybdenum, nickel and silicon.
Increasing the carbon content increases both the hardenability and hardness (Fig. 1) of steels by retarding the formation of pearlite and ferrite and encouraging the formation of martensite at slower cooling rates. It is more common to control hardenability, however, with other alloying elements and to use steels with carbon levels of less than about 0.4 wt%. This is due to the fact that while carbon controls the hardness of the martensite, as the carbon levels increase, the critical temperature for the formation of martensite is depressed to lower and lower temperatures, promoting the formation of retained austenite. In addition, high-carbon steels are prone to distortion and cracking during heat treatment and can be difficult to machine in the annealed condition before heat treatment.
Boron – a very potent alloying element – is extremely effective as a hardening agent and as such has a dynamic impact on hardenability. Boron is typically added to steels in the composition range of 0.0005% to 0.003%. The effect of boron is greatest at lower carbon contents, and it is typically used with low-carbon steels. It does not adversely affect formability or machinability.
Since boron has a very strong affinity for oxygen and nitrogen, failure to tie up free nitrogen during the steelmaking process results in the formation of boron nitrides that will prevent the boron from being available for hardening. Boron must be in solution to affect the hardenability of the steel. Titanium and/or aluminum are added for this purpose. It is important, therefore, that the mill carefully control the titanium/nitrogen ratio. Both titanium and aluminum tend to reduce machinability of the steel. However, the formability typically improves. Boron content in excess of 0.003% has a detrimental effect on impact strength due to grain-boundary precipitation.
Common alloying elements in steel are chromium, manganese, molybdenum, nickel, silicon and vanadium. These elements retard the phase transformation from austenite, and the complex interactions between them affect the temperatures of the phase transformation and the resultant microstructure. To assist the metallurgist and heat treater, alloy-steel compositions are often described in terms of their “carbon equivalent,” which describes the magnitude of the effect of all the elements on hardenability. Steels of the same carbon equivalent have similar hardenability. A commonly used formula for calculating the equivalent carbon content (Ec) is given in Equation 1. Finally, attention should be paid to trace-element chemistry as many tramp elements (e.g., titanium, niobium and aluminum) influence a material’s response to heat treatment.
Effect of MicrostructureAs the austenite grain size increases so too does the hardenability of the steel. The steelmaking process controls the initial grain size (fine or coarse) by the use of such additions as aluminum. During heat treatment, however, the size of the austenite grains increases with the length of time above the steel’s critical temperature and with higher temperatures. A larger (coarser) austenite grain size retards the rate of the ferrite/pearlite phase transformation. Since a substantial increase in hardenability only occurs at high austenitizing temperatures, one must be concerned about the resultant properties of the steel. Fine-grain steels promote a somewhat greater toughness and shock resistance and have less of a tendency to crack in heat treatment. Coarse-grain steels tend to harden more deeply.
Jominy and Other Hardenability TestsSeveral test methods have been developed over the years to determine the hardenability of steel. Most of us have heard of the Jominy end-quench test (Fig. 2). Another is the S-A-C test, which is often applied to steel of low hardenability. The advantage of the S-A-C test lies in the fact that the three numbers give a good visual image of the hardness distribution curve. These tests were developed as alternatives to the creation of continuous cooling transformation (CCT) diagrams.
The test sample for the S-A-C test is a cylinder with a length of 140 mm (5.5 inches) and a diameter of 25.4 mm (1 inch). After normalizing and austenitizing, the specimen is quenched into a water bath. A cylinder 25.4-mm long is cut from the test specimen, and the end faces are ground to remove any tempering effects induced by the cutting operation. Rockwell C hardness measurements are then made at four positions on the original cylinder face, and the average hardness provides the surface, or S-value. Rockwell testing is carried out along the cross section of the specimen from the surface to the center to develop a hardness profile (Fig. 3). The total area under the curve provides the area, or A-value, in units of “Rockwell-inch,” and the hardness at the center gives the C-value.