Experiment 2: The Jominy End - Quench Test

(1) Objective

(a) To determine properties of steel hardenability(b) To learn the techniques of heat treatment in quenching.(c) To learn the techniques of hardenability test.

(2) IntroductionHardenability is an ability of an alloy to harden with the formation of martensite

when treated with quenching. Hardenability also determines the depth and the hardness distribution that can be achieved by quenching. Hardenability is not only hardness (the ability of a material to withstand plastic deformation such as indention, abrasion and more), but it is a qualitative measurement of the rate at which force decreased with distance from the surface to the interior of specimens with a reduction in martensite.

Quenching or hardening involves heating, soaking and cooling processes of the specimen. The steel is heated to a suitable temperature , 10˚C above the higher critical point A3 for hypo- eutectoid and 10˚C above lower critical point A1 for hyper-eutectoid steel. The soaking period depends on the width of the specimen. This follows by a suitable rapid cooling process in a suitable cooling medium such as water, salt solution, air and others. The microstructure formed is martensite which is hard and brittle.

Hardening of steels can be understood by considering that on cooling from high temperature, the austenite phase of the steel can transform to either martensite or a mixture of ferrite and pearlite. A large steel specimen when quenched usually has a low hardness at the core where the microstructure formed if ferrite and pearlite. The ferrite/pearlite reaction involves diffusion, which takes time. The surface has the higher hardness with martensite microstructure. The martensite transformation does not involve diffusion and essentially is instantaneous. These two reactions are competitive, and martensite is obtained if the cooling rate is fast enough to avoid the slower formation of ferrite and pearlite. In alloyed steels, the ferrite/ pearlite reaction is further slowed down, which allows martensite to be obtained using slower cooling rates.

The differences of the hardness between the core and the surface can be explained with the steel’s continuous cooling transformation (CCT) diagram. If the cooling curve doesn’t cut the diagram, so the overall hardness will be produced.The Jominy end-quench test measures the hardenability of steel which is the ability of the steel to partially or to completely transform from austenite to martensite when cooled

from high temperature. This test also measures the effects of microstructure, such as grain size, and alloying on the hardenability of steels. The main alloying elements that affect hardenability are carbon. Carbon controls the hardness of the martensite. The increasing carbon content increases the hardness of steels. Carbon also increases the hardenability of steels by retarding the formation of pearlite and ferrite. Slowing down this reaction encourages the formation of martensite at slower cooling rates.

(3) Apparatus

(a) A steel cylinder specimen 0.3% C with 10 mm diameter and 100mm length. (Jominy)

Figure 1 : A steel cylinder specimen

(b) Furnace

Figure 2 : A furnace

(c) Quenching apparatus

Figure 3 : A Quenching Machine

(d) Filling equipment

Figure 4 : A set of filing equipment

(e) Rockwell Hardness Tester

Figure 5 : A Rockwell Hardness Tester

(4) Procedure

(a) The furnace is heated at 850⁰C. (b) The specimen was inserted inside the furnace and left for 30 minutes for the heating

process. At this temperature, the steel sample is normalized to eliminate differences in microstructure due to previous forging, and then austenitized.

(c) Then, using a thong, the specimen was quickly removed from the furnace and placed at the quenching machine.

(d) This process quenches the specimen by spraying a controlled flow of water onto the bottom of the specimen.

Figure 6 : The specimen is sprayed (quenched) with water from bottom

(e) Water was flowed constantly to the end of specimen for 30 minutes.(f) As soon the specimen cooled, the specimen was filed to remove decarburised material

and to obtain smooth surface. This is important because it would affect the reading of hardness test value.

(g) Using the Rockwell Hardness Tester, reading of each hardenability test is measured at intervals from the quenched end started at 3mm until 51mm. The interval is typically 3mm.

(5) Observation(a) While the specimen quickly removed from the furnace, the specimen was in an

extremely red-hot state. Thus, extra care and safety is needed for the handling of the specimen from the furnace to the quenching machine.

(b) In the quenching process, the specimen’s quenched end was sprayed with water, there was a scrap of carbon layer removed from the surface of the specimen.

(c) Filing is essential to remove decarburized material and to obtain smooth surface. Thus, it is difficult to tell whether the decarburized material is fully removed or not. So, we should file the specimen until we get at least 3mm width.

(6) Results(a) Reading was taken and recorded in the table below.(b) Graph of Vicker Hardness Number (HV) versus distance from the specimen’s quenched end was plotted.

Distance from end of quenched Jominy

RockwellHardness Number(HRC)

Vicker Hardness Number(HV)

3 mm 48.5 491.06 mm 22.9 253.49 mm 21.0 243.012 mm 18.7 233.015 mm 17.2 226.518 mm 19.8 237.821 mm 15.9 220.924 mm 13.2 219.427 mm 10.4 197.630 mm 9.9 195.633 mm 8.9 201.636 mm 5.0 186.739 mm 7.7 186.842 mm 5.9 179.745 mm 4.2 174.048 mm 2.8 179.351 mm 0.9 173.0

The HV can be obtained using the Hardness Conversion Table, but there are cases where the value of Rockwell Hardness Number (HRC) obtained is different from the table. Thus to obtain accurate Vicker Hardness Number (HV), interpolation was used.

Interpolation Equation:

y2= y1+( y3− y1)(x2−x1

x3−x1

)

(7) Discussion

(a) Give explanation about obtained graph including experimental error (if exist).

As the graph was plotted, it is fluctuated and not as expected. There are some decreases from 6 mm until 51 mm from the end of the specimen’s quenched end. The graph also stated that the highest Vicker Hardness Number is 491.0 which at 3mm from the specimen’s quenched end.

Graph plotation affected by other factors such as error and temperature. The common and most affecting error is by human error. The Rockwell Hardness Tester is a sensitive machine that requires smooth surface to obtain excellent result. Thus, uneven filing does not remove decarburized material and caused unsmooth surface of the specimen. As the filing is done by human effort, the result may be different depends on the filing.

Furthermore, there are also other factors such as temperature. Temperature plays the important role and for this experiment, the furnace’s temperature must be set up to 850⁰C. If the temperature is set lower than 850⁰C to heat the furnace, the specimen would not became red-hot and the steel sample is not normalized to eliminate differences in microstructure not austenitized thus, affecting its hardenability. But if it is higher than 850⁰C, the specimen might melt and the quenching will take a longer time and the graph plotted will be different.

Other error affecting this experiment is time error. If the heating process is not set to 30 minutes, the specimen way not me normalized and austinitized fully. For the quenching process which is not quenched accurately to 30 minutes, this also may cause the specimen to not fully cooled and it is not completely transform from austenite to martensite to reach its required hardness.

Figure 7 : The expected graph to be obtained

(b) Give your opinion about the relationship between specimen cooling rates and the graph form obtained.

From the graph, we could see the highest Vicker Hardness Number is 491.0 which are 3mm from the end of quenched specimen/Jominy. The shorter the distance from the quenched end of the specimen, the higher the Vicker Hardness Number. This is because the bottom of the specimen has been sprayed constantly with water for about 30 minute which allowing the specimen’s microstructure to completely transform from austenite to martensite. In this situation the bottom of the specimen experienced the fastest cooling rates.

Slower cooling rates occur at the higher part of the specimen, compared with the faster cooling rate at the bottom. The microstructure at the bottom will be transformed to martensite, but the higher part will have a bainitic structure with some martensite.

The hardness decreases with distance from the quenched end. High hardness occurs where high volume fractions of martensite develop. Lower hardness indicates transformation to bainite or ferrite/pearlite microstructures.

(c) Predict the graph form if the specimen used has two times the size of diameter from the current specimen. Explain why.

If the specimens have two times size of diameter from the current specimen, the graph form would not be perfect. The curve position might be lower than from the graph we obtained before. The sizes also play a role as well. The larger the sizes the more effort to done this process. The surface’s microstructure of the specimen will take a lower time to transform to martensite than the core of the specimen so that the core will have a bainitic structure with some martensite. So the specimen’s surface will be harder than the core. There would be modification needed in timing, cooling agent or the temperature so we could obtain the excellent graph and the hardness needed.

(8) Conclusion

Last but not least, we can conclude that lower parts of the quenched end of specimen gives the higher Vicker Hardness Number. Thus the nearer the distance of the specimen’s part to water sprays the higher the cooling rate. There are also factors affecting the hardness including time, size (diameter), cooling agent (medium), cooling rate and temperature. Errors in this factors should be avoided if able.

Data from the Jominy end-quench test can be used to determine whether particular steel can be sufficiently hardened in different quenching media, for different section diameters. For example, the cooling rate at a distance of 10mm from the quenched end of a specimen is equivalent to the cooling rate at the center of an oil-quenched 28mm diameter of a specimen. Full transformation to martensite in the Jominy specimen at this position indicates that a 28mm diameter specimen can be through hardened, that is hardened through its full thickness. Slow quenching speeds often are selected to reduce distortion and residual stress in components

A high hardenability is required for through hardening of large components. This data can be presented using CCT diagrams (continuous cooling transformation), which are used to select steels to suit the component size and quenching media.

Quenching is one of the methods to refine microstructure of steel so the properties would be repaired. It does depend on the industrial needs and the uses as well. Quenching helps in increasing the rate of hardenability of steel and contributes a lot to industrial development and technologies these days.

Also to remind us, extra care and precautions should been taken as the process of this experiment was not easy and dangerous.

(9) References

Foundations of Material Science and Engineering, Fourth Edition.Hardenability.William F. Smith, Javad Hashemi.

Dissemination of IT for the Promotion of Materials Science.(2009). The Jominy End Quench Test. University of Cambridge. Retrieved February 14, 2011 fromhttp://www.doitpoms.ac.uk/tlplib/jominy/index.php

Chemical Engineering and Materials Science. (2009).The hardenability of steel. UC Davis. Retrieved February 14, 2011 fromhttp://www.matsci.ucdavis.edu/MatSciLT/Other/Files/Hardenability.pdf

Industrial Heating. (2009). Understanding The Jominy End Quench Test.Retrieved February 14, 2011fromhttp://www.industrialheating.com/CDA/Archives/22d2fcf0ddbb7010VgnVCM100000f932a