1. Introduction

Hardenability is the capacity of a material to be hardened by heat treatment (quenching). Hardenability of steels can be measured using the Jominy end test. The Jominy end test testifies the incidence of the composition of the alloy and heat treatment procedures for manufacturing purposes.

2. Abstract

This is a report on the Jominy end quench test. The jominy test is carried out; the regions of cooling are recorded and shown in a graph, as tested by the Rockwell C hardness test machine. Performing the Jominy test it is possible to know the hardenability of steel.

3. Experimental Technique

Fig 1 (Steel test piece) [3]

The test piece is a high carbon steel cylinder of 100mm in length and 25.4mm in diameter (Fig 1), with atypical analysis shown in Table 1. The steel is normalized to eliminate differences in microstructure due to previous forging, and then it is austenitised. It is heated to 850°C in a furnace for around 30 minutes until austenitised. The steel is then transferred from the furnace and carefully place into the quenching apparatus (Fig 2). Where it is held vertically and sprayed with a controlled flow of water (Fig 3A) at room temperature onto one end of the sample. The end of the steel that is in direct contact with the water will cool rapidly the rest of the bar will cool gradually, the furthest away from the water will cool the slowest, simulating the effect of quenching a larger steel component in water. Because the cooling rate decreases further from the quenched end, you can measure the effects of a wide range of cooling rates from vary rapid at the quenched end to air cooled at the far end.

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Table 1: Typical chemical composition [8]

Once the steel has completely cooled, it is ground flat along its length to a depth of .38mm to remove decarburized material. Along this flat section of the steel, measured intervals are marked out (Fig 3B). The steel is then taken to the hardness test machine from which the recordings are made. The Rockwell hardness tester is used for measuring hardness. It consists of a platform where the steel is placed, which can be adjusted in height so that the indenter can make contact with the surface of the cylinder without causing false readings. A lever is used for starting the machine, which exerts a force into the steel until the measurement is shown.

Fig 2 (Quench apparatus) Fig 3A, water jet Fig3B, Indentations along flat

[3] [7]

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Carbon 0.85 – 1.05 %Manganese 1.00 – 1.40 %Tungsten 0.40 – 0.60 %Chromium 0.40 – 0.60 %Vanadium 0.15 – 0.30%

3.1. The Rockwell Hardness Test

The Rockwell hardness test involves indenting the test specimen with a diamond cone or hardened steel ball indenter. An initial indent is made using an initial preliminary test force, F0, of either 98.07 N or 29.42 N with this being applied for no longer than 3 seconds. The force is then increased to a secondary level, F1, for a duration of 1 – 8 seconds, with the total force, F, being the sum of these. Once the required time period for the application of F1 has been attained this load is removed, although the preliminary load is maintained. Removal of the secondary load allows a partial recovery to occur and the depth of the penetration is reduced compared to that for the total load F, but is greater than that for the preliminary load F0. The permanent increase in the depth of penetration from application and removal of the secondary load is used to calculate the Rockwell hardness number, HR. The Rockwell hardness scales range from A to V for different types of material with each range having a specified type and/or size of indenter and preliminary and secondary force. The advantages of the Rockwell test are the direct readout of the result from a scale and rapid testing time, although disadvantages include the many non‐related

scales (see BS ENISO 6508 parts 1‐3 for further information).[8]

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4. Jominy Test Results

4.1. Rockwell C Hardness V Distance

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Distance from Quenching End (mm)

Rock

wel

l C

Distance From

Quenching End (mm) HRC (avg)

1.5 623 545 407 419 39

11 3813 3815 3720 3525 3235 2340 2645 24

4.2. Results Discussion

After plotting our results into the graph, it can clearly be seen that there is a pattern. The indentation closest to the quench end has the highest Rockwell number or is the hardest area. As the indentations get further from the quenched end the Rockwell number starts to decrease. Therefore the test piece is hardest at the quenched end and gradually starts to become softer towards the opposite end of the piece. Once the test piece is place in the quench apparatus, the bottom end is in direct contact with the water, therefore this end is cooled rapidly, almost in an instant. The rest of the piece is gradually cooled, the furthest away from the quench end taking the longest to cool and hence the softest. This can be related to the results in the graph.

4.2.1. Microstructure

As the test piece is transferred from the furnace to the quench apparatus it is in the austenite phase (Fig 4). The quench end is then rapidly cooled by the water jet, which, if rapid enough will form martensite (Fig 5).

Martensite is formed when austenitised steels are rapidly cooled to a relatively low temperature. The martinsite transformation occurs if the cooling rate is rapid enough to avoid carbon diffusion. It is almost instantaneous and there is no time for the carbon to diffuse out of the martensite grains.

From the results on the graph, the first two indentations at 1.5mm and 3mm which are closest to the quench end are in the martensite phase. The opposite end, which is furthest away from the water jet, is ferrite/pearlite (Fig 6). The space in between is a mixture of both. The proportions of the phases at any position depend on the cooling rate, with more martensite formed where the cooling rate is fastest. Ferrite and pearlite are formed where the cooling rate is slower.

Ferrite/pearlite phase and martensite phase are competitive, if cooled fast enough martensite is formed as the cooling gets slower ferrite/pearlite is formed. Unlike martensite, ferrite/pearlite involves carbon diffusion, which takes time.

Fig 4 (austenite)

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Fig 5 (martensite) Fig 6 (Ferrite/pearlite)

Microstructures observed in the Jominy end-quench test of a 0.4wt% carbon steel: (Fig 2) untempered martensite; (Fig 3) ferrite and pearlite. Pearlite, the darker

constituent, is a eutectoid mixture of ferrite and iron carbide. [5]

4.2.2. Microstructures of Steel: [9]

1. Austenite – the fcc phase of Fe that can have up to 2% C in interstitial solid solution.

2. Pearlite – the eutectoid microstructure of ferrite and cementite. There is coarse pearlite and fine pearlite. (These are relative terms as all eutectoid microstructures are typically fine grained.)

3. Bainite – another eutectoid microstructure of ferrite and cementite. It has a different grain morphology than Pearlite. There is upper bainite and lower bainite that differs in the grain morphology as well. As you would expect, lower bainite is a finer grained material.

4. Martensite – a metastable (non-equilibrium) single phase, supersaturated, interstitial, solid solution of C in Fe. It can be bct or bcc depending on the amount of C. It is the result of a diffusion less (time independent, instantaneous) transformation. (These types of transformations are sometimes called martensitic transformations, even when there is no martensite involved.) Martensite is extremely hard and extremely brittle. Think of a glass hammer. It is not really a practical material. It needs to be made more ductile in order to be able to use it. The microstructure of martensite will depend on the amount of C. Lathe martensite results with <0.6% C. It is long, thin grains called laths. Plate or lenticulor martensite is needle like or plate like grains. In general martensite is acicular or needle like grains which is why it is so brittle.

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5. Heat treatment of steel

Steel’s versatility is due to its response to thermal treatment. Although most steel products are used in the as-rolled or un-heat-treated condition, thermal treatment greatly increases the number of properties that can be obtained, because at certain “critical temperatures” iron changes from one type of crystal structure to another. This structural change is spontaneous and reversible and can be made to occur by simply changing the temperature of the metal.

The transformation in crystal structure changes over a range of temperatures, determined by lower and upper critical range. When heated, most carbon and low–alloy steels have a critical temperature range between 850°C and 900°C. If heated above this temperature, but below the melting point, the steel has a crystalline structure called austenite, in which the carbon and alloying elements are dissolved in a solid solution. Below this critical range, the crystal structure changes to a phase called ferrite, which is capable of maintaining only a very small percentage of carbon in solid solution. The remaining carbon exists in the form of carbides, which are compounds of carbon and iron and certain of the other alloying elements. Depending primarily on cooling rate, the carbides may be present as thin plates alternating with the ferrite (pearlite); as spheroidal globular particles at ferrite grain boundaries or dispersed throughout the ferrite; or as a uniform distribution of extremely fine particles throughout a “ferrite-like” phase, which has an acicular (needle like) appearance, named martensite. In some of the highly alloyed stainless steels the addition of certain elements stabilizes the austenite structure so that it persists even at very low temperatures (austenitic grades). Other alloying elements can prevent the formation of austenite entirely up to the melting point (ferrite grades)

[6]

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Today’s demands of high quality products have made heat treatment an importantprocess for most engineering applications. Heat treatment involves the property improvement of metals by changing their microstructure. Heat treatmentcan be done to accomplish a number of outcomes.

• Diffuse carbon and alloying elements

• Soften the material

• Improve machine-ability

• Harden the material

• Increase toughness

• Increase wear resistance

• Stress relieve

Steel heat treatments involve one or more series of operations in which the metal is heated to specific temperature, and then cooled under specified conditions to develop a required structures and properties.

5.1 Annealing

Annealing is a process in which a material is heated to a suitable temperature, then held for a period of time and then cooled at a certain rate. This process is used to soften the material and its can also be used to produce desired changes in the microstructure.

The purposes of such changes include improvement of machinability, facilitation of cold work, improvement of mechanical or electrical properties, and/or increase in Stability of dimensions

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There are different stages of the annealing process:

5.1.1. Recovery

Residual stress and internal strain energy is relieved by dislocations moving into lower energy configurations

Strength decreases slightly and ductility increases slightly

5.1.2. Re-crystallization

Formation of new strain-free, low energy, low dislocation density grains. Temperature at which re-crystallization just reaches completion in one hour.

Usually this is about 1/3 – ½ the melting temperature. The temperature of re-crystallization depends on time, purity and initial grain

size.

5.1.3. Grain Growth

Once the re-crystallization has occurred and the structure is made of new crystal grains, the grains will continue to grow. The grain boundaries will move and the larger grains will grow at the expense of the smaller ones.

[1]

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6. Quenching

Quenching is the rapid cooling of the work piece with an air, gas, liquid, or solid medium.

Hardness related to the Carbon content of the steel. Hardening of steel requires a change in structure from the body-centred cubic structure to the face-centred cubic structure found in the Austenitic region. The steel is heated to Austenitic region. When rapidly quenched, the Martensite is formed. This is a very strong and brittle structure. When slowly quenched Austenite and Pearlite are formed which is a partly hard and partly soft structure. When the cooling rate is extremely slow then it would be mostly Pearlite which is very soft.

Hardenability, which is a measure of the depth of full hardness achieved, is related to the type and amount of alloying elements. Different alloys, which have the same amount of Carbon content, will achieve the same amount of maximum hardness; however, the depth of full hardness will vary with the different alloys. The reason to alloy steels is not to increase their strength, but increase their hardenability — the ease with which full hardness can be achieved throughout the material. Usually when steel is quenched, most of the cooling happens at the surface, as does the hardening. This propagates into the depth of the material. Alloying helps in the hardening and by determining the right alloy one can achieve the desired properties for the particular application. Such alloying also helps in reducing the need for a rapid quench cooling, which will eliminate distortions and potential cracking. In addition, sections can be hardened to the core. Quenching is the act of rapidly cooling the austenite form to harden the steel.

6.1 Quench Media [4]

6.1.1. Water Quench

Quenching can be done by cooling the steel in water. The water adjacent to the hot steel vaporizes, and there is no direct contact of the water with the steel. This slows down cooling until the bubbles break and allow water contact with the steel. As the water contacts and boils, a great amount of heat is removed from the steel. With good agitation, bubbles can be prevented from sticking to the steel, and prevent soft spots. Water is a good rapid quenching medium, provided good agitation is done. However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking.

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6.1.2. Salt Water Quench

Salt water will cool the steel more rapidly than plain water because the bubbles are broken easily and allow for rapid cooling of the part. But, salt water is even more corrosive than plain water, and therefore must be rinsed off immediately.

6.1.3. Oil Quench

Where a slower cooling rate is needed, oil is used. Since oil has a very high boiling point, the transition from start of Martensite formation to the finish is slow and this reduces the likelihood of cracking.

6.1.4. Polymer Quench

Polymer quenches will produce a cooling rate in between water and oil. The cooling rate can be altered by varying the components in the mixture, as these are composed of water and some glycol polymers. Polymer quenches are capable of producing repeatable results with less corrosion than water. But, these repeatable results are possible only with constant monitoring of the chemistry.

6.1.5. Cryogenic Quench

Cryogenics or deep freezing is done to make sure there is no retained Austenite during quenching. The amount of Martensite formed at quenching is a function of the lowest temperature encountered. At any given temperature of quenching there is a certain amount of Martensite and the balance is untransformed Austenite.

7. Tempering

In heat-treatment, reheating hardened steel or hardened cast iron to a giventemperature below the eutectoid temperature will decrease hardness and increase toughness. The process also is sometimes applied to normalized steel. In nonferrous alloys and in some ferrous alloys (steels that cannot be hardened by heat-treatment), the hardness and strength produced by mechanical or thermal treatment, or both, and characterized

8. Conclusion

The Jominy test describes the ability of the steel to be hardened in depth by quenching. The hardenability depends on the alloy composition of the steel, and can also be affected by processing, such as the austenite temperature. Knowledge of the hardenability of steels is necessary in order to select the appropriate combination of alloy and heat treatment for components of different size, to minimise thermal stresses and distortion.

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9. References

1. www.ndt-ed.org/.../Graphics/Recovery.gif

2. www. huyett .com/E handbook .pdf

3. http://www.doitpoms.ac.uk/tlplib/jominy/printall.php

4. http://www.efunda.com/processes/heat_treat/hardening/direct.cfm

5. http://www.industrialheating.com/CDA/Archives/ 22d2fcf0ddbb7010VgnVCM100000f932a8c0

6. http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/ FeC.gif

7. commons.wikimedia.org

8. 2009 coarse notes

9. fog.ccsf.cc.ca.us/~wkaufmyn/.../Chap10_Kinetics-HeatTreatment.doc

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