THE HARDENABILITY OF STEEL

IntroductionMaximum Hardness: Maximum hardness in steels is obtained by producing a fully martensitic

structure. This can be done by austenitizing the steel and then quenching it. During the

austenitizing treatment all of the carbides dissolve and the ferrite transforms into austenite.

Quenching this structure causes the austenite to transform via a shear mechanism into martensite.

This transformation is so fast (Martensite needles grow at close to the speed of sound.) that there is

no time to the carbon to diffuse out of the martensite grains or to form carbide phases. The

martensite, supersaturated with carbon, is very hard and also very brittle.

Carbon, being a very effective solid solution strengthening agent, essentially determines the

hardness of the martensite. Cases where a lesser degree of hardening can be attributed to the

presence of other alloying elements, but these elements tend to also make it more difficult to obtain

a fully martensitic microstructure. So while maximum hardness in a given steel is dependent on our

ability to produce a fully martensitic microstructure, the hardness of the martensite is largely

determined by its carbon content.

Hardenability: In order to form a fully martensitic structure the steel must be quenched at a rate that

is equal to or greater than a critical cooling rate. If the quench is indeed fast enough and the part is

thin then one can usually assume that this cooling rate can be achieved through the whole crosssection,

producing a fully hardened part. However, this may not be the case for thick sections

because the interior cools more slowly than the surface. But if one could modify this steel such that

critical cooling rate is lower then thick pieces can be hardened throughout and even thicker pieces

can be hardened to a considerable depth. This is of great practical importance not only in terms of

our ability to produce a fully hardened part (which will also be fully brittle) but because subsequent

tempering will be successful in producing the desired strength and ductility throughout the part. In

addition, one could use less severe quenches to avoid problems with warping and cracking.

This ability of a steel to be hardened to a specified depth is called hardenability. In general, the

hardenability of a steel is improved through alloying and all alloy additions except cobalt will

improve the hardenability of a steel. Coarse grain size and homogeneity of the austenite also

improve the hardenability. The reason this is so is not clear but is probably related to the retardation

of nucleation and growth of the ferrite, carbide and bainite phases.

Jominy End-Quench: The most direct measure of the hardenability of a steel is the “critical cooling

rate”. Hardenability is also demonstrated in cases where large part fails to fully harden. One can

measure this in terms of the depth of full hardening, the diameter of bar which will just harden to

the center and the depth where the microstructure consists of 50% martensite. A more convenient

and very widely used method of measuring hardenability is the Jominy end-quench test. (Developed

by Jominy and Boegehold in 1939, standardized in ASTM A255.) In this test a 1-inch diameter by

4-inches long bar is austenitized then quickly removed from the furnace and placed in a fixture

where a jet of water of specified temperature and pressure impinges on one end of the specimen.

Once cool, the specimen is removed, cleaned, a flat is ground along the length of the specimen and

then is hardness tested every 0.0625 to 0.25 inches from the quenched end. The result is a plot of

hardness versus distance from the quenched end. This curve is used to compare the hardenabilities

of different steels.

Ideal Diameter: The ideal diameter DI is another measure of the hardenability of steel. It is defined

as the diameter of a bar which would contain 50% martensite at its center following a quench in an

ideal medium. Clearly, the larger the ideal diameter, the higher the hardenability of the steel. The

ideal diameter of a plain carbon steel having a carbon content of 0.4% (1040 steel) and whose

ASTM grain size number is 7 is 0.215 inches. Naturally, varying the grain size or changing the

concentration of alloying elements will change the ideal diameter. An empirical method of

accounting for these effects is utilizes a series of multiplying factors:

where the base ideal diameter is a function of grain size and carbon content and the multiplying

factors fi are function of composition of element I. The ideal diameter for a 4340 steel (0.8 Cr, 1.75

Ni, 0.25 Mo) is over 6 inches.

The objective of this experiment is the measure the hardenability of several plain carbon and lowalloy

steels. The results will be used to explain the influence of alloy composition on the kinetics

of martensite formation. They will also be compared to the calculated values of the ideal diameter.Safety Considerations

This experiment involves heating several steel rods to as high as 875°C, quickly taking them out of

the furnace and loading the hot specimens into a quenching fixture and then quenching the

specimen. After this the specimens are ground flat along several sides and hardness tested along the

length of the specimen. Extreme care should be exercised during the heat treating phase of the

experiment as the temperatures are quite high and therefore pose severe burn hazards to personnel

and fire hazards to the building. Grinding will be done by technicians in the machine shop so this

will not be an issue during this experiment. Hardness testing, however, involves the use of a special

fixture and a diamond brale indentor. One should be very careful when using this fixture and the

brale indentor so that neither are damaged.

Chemical Hazards

None. No chemicals are used and the specimens are 1-inch diameter rods made from

conventional steels.

Physical Hazards

1. The potential for very serious burns exists. Temperatures approaching 900°C are

used during these experiments. At these temperatures one can easily be burned while

loading and unloading specimens from the furnaces, even if the hot specimens and

furnace are not touched. It will be important to wear heat resistant gloves and to use

long tongs. One should also take care to prepare a clear area to work, have an

emergency procedure in place in case hot specimens are dropped on the floor, etc.

It would be a good idea to rehearse the procedures for handling hot specimens.

2. Hardness testing poses very little hazard if proper testing procedures are followed.

Using the proper anvil and indentor and a clean specimen will minimize the chance

of damaging the equipment or injuring personnel.

Biohazards

None.

Radiation Hazards

None.

Protective Equipment

Recommended: The use of safety glasses is recommended during the hardness

testing phase of the experiment. The use of protective coverings for the floor and

counter tops is also recommended.

Required: safety glasses, heat resistant gloves and long tongs for the heat treatment

phases of the experiment.

Waste

Used specimens can be recycled as scrap steel.Materials

The alloys used in this experiment are standard grades of the following steels: 1045, 4140, 4340 and

8620. Several have the same carbon content but have different concentrations of the other alloying

elements. The specimens are standard Jominy specimens, 1 inch in diameter and 4 inches long with

a flat washer pressed onto one end. (This washer can be removed after quenching.) One the end

which has the washer a single letter which identifies the steel by composition has been stamped.

During the austenitizing treatment this stamp will probably be lost due to oxidation or carburization.

A more substantial marking should be used.

1. PreliminaryObtain a copy of ASTM A255 and read it.

Consult the reference books and databases to find out what the ideal diameters for the steels being

tested are.

Calculate the ideal diameters and the Jominy curves for your steels.

List the nominal compositions of each of the alloys. Mark, engrave or notch each specimen so that

they can still be identified after having spent an hour or so in the furnace.2. Prepare the Jominy quench tank

Set up the quench tank over a sink and connect a hose to the faucet. Place a specimen in the quench

tank. Open the valve on the quench tank and, using the valve on the faucet, adjust the flow of water

so that the height of the column of water is ½-inch above the bottom of the specimen. Close the

valve on the quench tank but do not adjust the valve on the faucet.3. Austenitizing Treatments

Preheat a furnace to 850°C and place the specimens in a container filled with graphite and place this

in the furnace. Allow the specimens to soak at this temperature for one hour.4. Prepare to quench the specimens

The purpose here is to prepare the work area for handling red-hot steel safely. Start by clearing a

path between the furnace and the Jominy quench tank. Next, devise a plan for dealing with

accidents such as dropping a hot specimen on the floor. Collect up gloves, tongs and safety

equipment that will be used to move the specimens to the quench tank. Decide who will remove the

specimen from the furnace and place it in the quenching fixture, who will assist in pushing the

specimen through the hole in the quenching fixture (if necessary), who will turn on the water, who

will monitor the time it took to start the quench, and who will execute the emergency procedures.

Rehearse the procedure several times using a cold specimen.5. Quench the Specimens

Clear a path between the furnace and Jominy quench tank. Quickly but carefully remove a specimen

from the furnace and place it in the quenching fixture and immediately turn on the water using the

valve on the quench tank. Continue the quench until the specimen is cool enough to handle using

bare hands. Remove the specimen from the fixture and engrave or paint an ID code on it.6. Hardness test the specimens

Clean the specimens and grind a flat surface 0.015 inches deep along four sides (90° apart) of the

specimen. This will have to be done in the machine shop.

Set up a Rockwell-type hardness tester for measuring hardness values on the C scale. Hardness test

a couple of test blocks to make sure everything is working properly. Install the Jominy hardness

testing fixture.

Take a hardness reading every 1/16 inch from the quenched end of the specimen. After the first ½

inch increase this interval to c inches and after the first full inch increase the interval to ¼ inches.

Repeat this procedure for each of the flats on the specimen and then plot each of the four sets of

results on a single graph.Analysis

1. Compare the results (maximum hardness and the hardenability curves) with published data.

2. Compare the results with the calculated Jominy curves and ideal diameters.

3. Compare the maximum hardnesses obtained for the four alloys.

4. Compare the Jominy curves to the ideal diameters.

5. Discuss the differences in the hardenabilities of the four alloys. You can use depth to obtain

a specified hardness value or the inflection point on the curves as your basis for comparison.

6. Discuss the results in terms of composition and the TTT curves.References and Further Reading

1. E.C. Bain and H.W.Paxton, Alloying Elements in Steel, ASM, Metals Park, OH, 1966.

2. Properties and Selection: Irons and Steels, Metals Handbook, volume 1, 9th edition, ASM,

Metals Park, OH, 1986.

3. Atlas of Isothermal Transformation and Cooling Transformation Diagrams, ed. H.E.Boyer,

ASM, Metals Park, OH, 1977.

The traditional route to high strength in steels is by quenching to form martensite which is subsequently reheated or tempered at an intermediate temperature, increasing the toughness of the steel without too great a loss in strength. Therefore, for the optimum development of strength, steel must be first fully converted to martensite.

To achieve this, the steel must be quenched at a rate sufficiently rapid to avoid the decomposition of austenite during cooling to such products as ferrite, pearlite and bainite. The effectiveness of the quench will depend primarily on two factors:

• the geometry of the specimen, and
• the composition of the steel.

A large diameter rod quenched in a particular medium will obviously cool more slowly than a small diameter rod given a similar treatment. Therefore, the small rod is more likely to become fully martensitic.

It has already been shown that the addition of alloying elements to a steel usually move the TTT curve to longer times, thus making it easier to pass the nose of the curve during a quenching operation, i.e. the presence of alloying elements reduces the critical rate of cooling needed to make a steel specimen fully martensite. If this critical cooling rate is not achieved a steel rod will be martensitic in the outer regions which cool faster but, in the core, the slower cooling rate will give rise to bainite, ferrite and pearlite depending on the exact circumstances.

The ability of steel to form martensite on quenching is referred to as the hardenability. This can be simply expressed for steel rods of standard size, as the distance below the surface at which there is 50% transformation to martensite after a standard quenching treatment, and is thus a measure of the depth of hardening.

Use of TTT and CCT Diagrams

TTT diagrams- TTT diagrams provide a good starting point for an examination of hardenability, but as they are statements of the kinetics of transformation of austenite carried out isothermally, they can only be a rough guide. To take one example, the effect of increasing molybdenum, Figure 1. shows the TTT diagrams for a 0.4 %C 0.2% Mo steel and steel with 0.3 %C 2 % Mo, Figure 2. The 0.2% Mo steel begins to transform in about one second at 550°C, but on increasing the molybdenum to 2% the whole C-curve is raised and the reaction substantially slowed so that the nose is above 700°C, the reaction starting after 4 minutes. The latter steel will clearly have a greatly enhanced hardenability over that of the 0.2 Mo steel.

CCT diagrams- The obvious limitations of using isothermal diagrams for situations involving a range of cooling rates through the transformation temperature range have led to efforts to develop more realistic diagrams, i.e. continuous cooling (CCT) diagrams. These diagrams record the progress of the transformation with falling temperature for a series of cooling rates. They are determined using cylindrical rods, which are subjected to different rates of cooling, and the onset of transformation is detected by dilatometry, magnetic permeability or some other physical technique. The products of the transformation, whether ferrite, pearlite or bainite, are partly determined from isothermal diagrams, and can be confirmed by metallographic examination.

The results are then plotted on a temperature/cooling time diagram, which records, for example, the time to reach the beginning of the pearlite reaction over a range of cooling rates. This series of results will give rise to an austenite-pearlite boundary on the diagram and likewise lines showing the onset of the bainite transformation can be constructed.

A schematic diagram is shown in Figure 3. in which the boundaries for ferrite, pearlite, bainite and martensite are shown for hypothetical steel. The diagram is best used by superimposing a transparent overlay sheet with the same scales and having lines representing various cooling rates drawn on it. The phases produced at a chosen cooling rate are those which the superimposed line intersects on the continuous cooling diagram. In Figure 3. two typical cooling curves are superimposed for the surface and the centre of an oil-quenched 95 mm diameter bar. In this example, it should be noted that the centre cooling curve intersects the bainite region and consequently some bainite would be expected at the core of the bar after quenching in oil.

Hardenability Testing

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
• The Jominy end quench test

Effect of Grain Size and Chemical Composition on Hardenability

The two most important variables which influence hardenability are grain size and composition.

The hardenability increases with increasing austenite grain size, because the grain boundary area is decreasing. This means that the sites for the nucleation of ferrite and pearlite are being reduced in number, with the result that these transformations are slowed down, and the hardenability is therefore increased.

Likewise, most metallic alloying elements slow down the ferrite and pearlite reactions, and so also increase hardenability. However, quantitative assessment of these effects is needed.

There are a bewildering number of steels, the compositions of which are usually complex and defined in most cases by specifications, which give ranges of concentration of the important alloying elements, together with the upper limits of impurity elements such as sulfur and phosphorus.

While alloying elements are used for various reasons, the most important is the achievement of higher strength in required shapes and sizes and often in very large sections which may be up to a meter or more in diameter in the case of large shafts and rotors. Hardenability is, therefore, of the greatest importance, and one must aim for the appropriate concentrations of alloying element needed to harden fully the section of steel under consideration. Equally, there is a little point in using too high a concentration of alloying element, i.e. more than that necessary for full hardening of the required sections.

Alloying elements are usually much more expensive than iron, and in some cases are diminishing natural resources, so there is additional reason to use them effectively in heat treatment. Carbon has a marked influence on hardenability, but its use at higher levels is limited, because of the lack of toughness which results, the greater difficulties in fabrication and, most important, increased probability of distortion and cracking during heat treatment and welding.

The most economical way of increasing the hardenability of plain carbon steel is to increase the manganese content, from 0.60 wt% to 1.40 wt%, giving a substantial improvement in hardenability. Chromium and molybdenum are also very effective, and amongst the cheaper alloying additions per unit of increased hardenabilily. Boron has a particularly large effect when it’s added to fully deoxidized low carbon steel, even in concentrations of the order of 0.001%, and would be more widely used if its distribution in steel could be more easily controlled.

Hardenabilily data now exists for a wide range of steels in the form of maximum and minimum end-quench hardenability curves, usually referred to as hardenability bands. This data is, available for very many of the steels listed in specifications such as those of the American Society of Automotive Engineers (SAE), the American Iron and Steel Institute (AISI) and the British Standards.