effect of quenching media, specimen size and shape on … › d269 ›...

Emirates Journal for Engineering Research, 19 (2), 33-39 (2014) (Regular Paper)

33

EFFECT OF QUENCHING MEDIA, SPECIMEN SIZE AND SHAPE ON THE HARDENABILITY OF AISI 4140 STEEL

Ali RafaAltaweel1 , Majid Tolouei-Rad2

1School of Engineering / Edith Cowan University /270 / Joondalup/

Western Australia / 6027/ Email: [email protected] 2School of Engineering / Edith Cowan University /270 / Joondalup/

Western Australia / 6027/ Email: [email protected]

(Received October 2013 and Accepted April 2014)

الغرض الرئيسي من هذه الدراسه يتمثل في تحديد مدى تأثير بعض وسائل التبريد ، حجم وشكل العينة على تباين تم قياس هذا التباين في عمق التصلب لعينات ذات حجم وشكل مختلف من السطح إلى . 4140التصلب للصلب

اء مع جل العينات، وأقل عمق تصلب أثناء إستناد للنتائج ، اتضح أن أعلى عمق تصلب صاحب استخدام الم. المرآزوبالتالي . استخدام الهواء الجوي آوسيل للتبريد، بينما شهد التصلب تحسنًا ملحوظًا أثناء استخدام الهواء المضغوط

فإن الفهم الجيد للعوامل الرئيسية التي تؤثر على التصلب ، والتي تعتبر أحد أهم الخواص الميكانيكية للصلب، يمكن .ساهم بشكل آبير في اختيار نوع الصلب المناسب في مختلف التطبيقات الهندسية أن ي

The purpose of this study is to investigate the influence of different quenching media, size and shape of the specimen on the hardened depth of AISI 4140 steel. The paper demonstrates how these parameters can affect the hardness from the surface to the core of the samples. This study represents the relationship between hardened depth and geometry. Findings reveal the fact that high hardening effect was obtained in water-quenched samples by the virtue of the martensitic structure and lower hardened depth achieved in the air-quenched samples. Significant improvement achieved by increasing the velocity of air when quenched by compressed air. It was also concluded that the hardness of the quenched samples at certain depths can be estimated on the basis of heat transfer equations.

1. INTRODUCTION Hardenability is a property of steel which determines the distribution and depth of hardness induced by quenching treatment. From a metallurgist's point of view, hardenability can loosely be defined as the ability of steel alloy to develop a desired hardness from the surface to the core by forming a martensite under specified quenching conditions [1]. There are a number of factors influencing the hardenability of steel alloys. These include [2]:

• Shape and size (geometry) of the workpiece • Quenching conditions (cooling rate of quenching

medium); and • Chemical composition of the steel (carbon and

alloy content).

Thus, a piece of steel requires a rapid cooling rate in order to achieve high hardening depth because of the formation of a large amount of martensite. In the hardenability tests the cooling media, temperature, and thermal effects between the environment and the sample all play very important roles.

The cooling rate; therefore, is a fundamental factor in carrying out of all heat treatments. For hardening steels, a rapid cooling rate is used which results in non-equilibrium transformations of solid phases. In practice, quenching process is usually conducted by immersing the steel parts in a medium which extracts the heat form the surface of the part to quenching medium which could be:

• Gas: air, other gases like H2, Ar, N2, and He; • Immersion quench using liquid (water, oil, etc.); • Spray quenching: Mixed (gas + liquid).

1.1 Quenching Quenching treatment is performed in order to produce a desired microstructure (i.e., effective hardening depth) in the sample. This thermodynamical process needs to be performed negligible deformation of the sample [3]. In general, the quenching is relatively complex process and can be considered as a heat transfer problem. In quenching process cooling starts from the surface of the sample, and hence, sample’s surface to volume ratio is a key factor in assessing its heat transfer behaviour. In addition, the surface heat transfer coefficient is not only proportional with the

Ali RafaAltaweel, Majid Tolouei-Rad

34 Emirates Journal for Engineering Research, Vol. 19, No.2, 2014

surface temperature, but also with the changes in conditions of quenching media and the form of the sample surface [4].

1.2 Continuous Cooling Transformation of AISI 4140 Steel

Continuous cooling transformation (CCT) is a plot of temperature against time logarithm for a given steel alloy that has a particular chemical composition. CCT diagrams are widely used in heat treatments of steels and specially the decomposition of austenite phase, which is most often described by using this diagram [5].For a given steel alloy, the CCT diagram includes the critical cooling rate required for producing martensite as demonstrated in the Figure 1. The critical cooling rate for producing a martensitic microstructure is exactly where the starting of pearlite transformation is just missed [6]. In fact, avoiding the nose of pearlitic structure means the resulting microstructure will be totally martensite. Therefore, the microstructure for a particular steel alloy can be predicted after cooling for some time by using CCT diagrams. Figure 1 shows CCT diagram of AISI 4140 steel.

Figure 1. The CCT diagram of AISI 4140 steel [5].

2. PROCEDURE

2.1 Sample Preparation A number of AISI 4140 steel specimens with different geometries were produced by machining processes. Table 1 gives the chemical composition of AISI 4140 steel employed in the study.

Table 1 Chemical composition of AISI 4140 steel

(wt%) C SI Mn NI Cr Mo S P

0.4 0.2 0.8 - 1.0 0.2 0.025 0.025

Three sets of samples were produced. Each set included two round solid bars of 25 mm and 40 mm and to a length of 100 mm; and two solid square bars having dimensions of 13×13×100 mm3 and 30×30×100 mm3.

2.2 Heat Treatment First, all specimens were heated-up to austenitizing temperature in an electric furnace. The specimens were held at 860°C for sufficient time (approximately 30 minutes) to ensure uniformity of temperature throughout the entire volume to achieve a homogeneous structure of austenite. This was followed by the quenching treatment and each group of samples were quenched in different quenching mediums (water, dry air, and compressed air).Then the Rockwell C hardness tester was used to measure the hardness at various positions along the radius of selected cross section from the surface to the core of the specimen, as illustrated in Figure2.

Figure 2. The hardness evaluation of the round

sample.

In addition, to studying the variation of the microstructure in different media, segments of the specimens (30 mm from one end) were first cut, polished and then etched with a concentrated solution of 5 % Nitral (mixture of 3ml Nitric acid and 97ml Ethyl alcohol). The resulted microstructures were then investigated at these positions using an optical microscope at a magnification of 1000 X.

3. RESULTS AND DISCUSSION In this study, the hardenability curves were related to the shape and size of specimens made of AISI 4140 steel after quenching in different media.

3.1 Hardness Curves and Microstructures 3.1.1 Smaller Square Samples

Figure 3. Hardness values along the cross-section for

the small square specimens.

Effect of Quenching Media, Specimen Size and Shape on the Hardenability of AISI 4140 Steel

Emirates Journal for Engineering Research, Vol. 19, No.2, 2014 35

Figure 4. An optical metallographic examination of small square samples quenched in (a) water, (b) air, and (c) compressed air.

Figure 3 shows the hardness curves of smaller square samples which are heated and quenched in different media (water, dry-air, and compressed-air). Since the cooling rate is afunction of quenching media, the cooling rate was extremely rapid for water-quench and therefore a martensitic microstructure was produced. In this process, the diffusion time for carbon atoms was not sufficient to form pearlite and/or bainite, particularly for smaller specimen size [7].Thus, nearly 100% martensite was formed throughout the interior of the water-quenched specimen. In fact, this microstructure is in good agreement with CCT diagram of AISI 4140.However, during the air quenching process, the cooling rate was quite slow allowing some of the carbon atoms to diffuse at higher rates, and consequently a pearlitic microstructure is achieved which is relatively soft.

In contrast to dry air quenching, dissipating heat from the specimen surface to the quenching medium significantly improved for the compressed-air quenched samples. In other words, the effectiveness of quenching was substantially enhanced by increasing the air velocity (Forced convection) which in general led to a relatively higher hardness. The microstructure produced in this quenching process was bainite (see Figure 4(c)), which results in achieving higher hardness values.

3.1.2 Larger Square Samples

The hardness curves obtained in the larger square specimens are illustrated in Figure 5.

With this sample size, no remarkable difference was observed in the hardness curves of the water-quenched sample. However, for the air-quenched

samples a relatively larger drop rate in the hardness values from surface to core was noticed. For the compressed-air quenched sample the effectiveness of quenching medium was diminished as the specimen size increased, and as a result, hardness values were decreased at a higher rate

Figure 5. Hardness values along the cross-section for

large square specimens quenched in different quenching mediums.

The microstructural examination revealed that martensite was the dominant phase in the microstructure of the water-quenched sample, whereas bainite was the dominant microstructure found in the compressed air-quenched sample, and only pearlite was observed in the sample that was quenched in dry air. The metallographic examination of the samples by means of optical microscopy is shown in Figure 6.



Figure 6. Optical micrographs of specimens quenched in (a) water, (b) air, (c) compressed air.

3.1.3 Smaller Round Samples

The three hardness curves for smaller round specimens quenched in different quenching media are illustrated in Figure 7. The same analysis, used for the square samples, has been applied to round samples.

Figure 7. Hardness variations throughout cross

section for the smaller round specimens.

According to Figure 7the water-quenched sample represents the highest hardness values recorded. This is due to the water-quenching severity, which has the ability to remove much heat in a reasonably short time, and consequently a martensitic microstructure was dominant under this condition. As expected the hardness values of the compressed-air sample were much lower compared to those values obtained in the water-quenched sample which is due to a lower cooling rate. For the air-quenched sample, the characteristic of dry-air as a coolant is almost constant and, in turn, no remarkable improvement of quenching rate was observed in the round specimen. Therefore, the rate of heat dissipation throughout the interior of the cross section was very slow compared to the heat dissipation by water or compressed-air. With less heat transfer, austenite to pearlite phase transformations occurred along the specimen cross-section. Figure 8 shows optical microstructures of smaller round specimens.

Figure 8. Optical pictures of smaller round specimens quenched in (a) water, (b) air, and (c) compressed air



3.1.4 Larger Round Samples

The hardenability curves for specimens with larger round are shown in Figure 9.

Figure 9. Hardness variations throughout the cross

section for the large cylindrical specimens

Likewise, for larger round samples and in the absence of any other parameters except specimen size, the hardness values depend on the type quenching medium. In water-quenching the decrease of hardness from the surface to the specimen core was very small due to the rapid cooling rate. However, in the specimen that was quenched by compressed-air, the hardness value at the core was substantially lower than the hardness value at the surface. This variation was due to the agitation of air applied on the surface of specimen, and thus, the rapid cooling rate was established close to the surface (heat transfer by forced convection) resulting in a high hardness value on the surface.

For the air-quenched sample, the heat dissipated very slowly from the sample surface to the quenching media (i.e., free convection). By considering this matter and referring to CCT diagram of the AISI 4140 steel, the austenite-to-pearlite transformation should develop in the entire cross section of the specimens shown in Figure 10.

3.2 Effect of Specimen Shape As the heat energy is removed from the surface of the sample, the cooling rate depends upon the surface-to-volume ratio. By increasing this ratio, the associated cooling rate will be higher and, consequently, a higher hardness in the entire sample can be achieved. The surface-to-volume ratio of specimens with edges and irregular corners is bigger than regular samples and therefore they can be hardened rapidly by quenching [8]. However, it should be noted that presence of irregular corners and sharp edges may

result in formation of cracks and warpage of the specimen in the heat treatment process.

3.3 Quenching Heat Transfer Analysis Quenching analysis plays an important role in order to fully understand the effect of cooling rate on the hardening depth. The quenching process can be treated as a heat transfer problem and in turn the convection heat transfer coefficient depends on the density, viscosity, and velocity of the cooling fluid. The amount of heat that can be removed from the surface of a given sample geometry by air during the convection heat transfer process is implicitly related to Reynolds number (Re):

(1) where the density of air is given by ρ, V is the cooling medium velocity, L is the specimen length, and μ is the dynamic viscosity of the air (kg/m · s) [9].

In this study, when the sample was left in a free convection environment (small Re), it took about 35 minutes to cool down to room temperature, while it took only 7 minutes during the forced convection (large Re). However, the cooling time was just about 2 minutes when the specimen was quenched in water. For this size of samples the lumped system analysis was used since the interior temperature of the samples remains uniform at any time during a heat transfer process.

Typical temperature histories of the specimens during free and forced convection cooling (air) at different velocities are plotted in Figure 11. The diversity of the plots indicates that the cooling curve depends on the flow rate of cooling fluid and hence the convection heat transfers mode.

The energy balance between the sample and medium can be expressed as:

Change in internal energy of the specimen =

Net heat flow from the specimen to the

medium

((2)

where Specific heat at constant pressur (kJ/kg · K)

= convection heat transfer coefficient (W/m2)

As= the sample's surface area (m2)



Figure 10. Optical images of specimens quenched in (a) water, (b) air, and (c) compressed air

Figure 11. Cooling curve of the sample during free

and forced convection

The negative sign in Equation (2) indicates that the internal energy of the specimen consensually decreases. By rearranging the Equation (2):

(3)

, since (final temperature) is constant. The integration of Equation (3) can be done with initial Temperature and Temperature T at time (t) are integration limits, and consequently:

(4)

Or,

(5)

1/ (6)

b is a positive quantity whose dimension is (time)-1. Equation (5) is related to all the parameters that are required to calculate the temperature T(t) of the specimen at particular time (t) or, alternatively, the time (t) required for the specimen to reach a particular temperature. Also, the hot sample will reach the medium temperature (equilibrium state) in a short time with large value of b. Once the temperature T(t) at time t is obtained from Equation (3), the rate of convection heat transfer

between the hot body and the surrounding medium at that time can be calculated from Newton’s law of cooling as:

(7)

Also, the total amount of heat transfer between the body and the surrounding medium which is the change in the energy content of the hot sample over the interval t= 0 to t. That is,

(8)

Therefore, based on these equations and the martensite formation temperature (Ms), formation temperature of bainite and pearlite from CCT diagram of AISI 4140 steel, the amount of heat transfer from the sample surface, the temperature and the time can be estimated for a particular hardness value along the cross section with considerable accuracy as shown in Figure 12. These equations would be valid once the convection heat transfer coefficient between the surface of the sample and the quenching medium is known.

Figure 12. Relationship between hardness and heat dissipation from the specimen for three different

quenching media over some time interval.



4. CONCLUSIONS The aim of this study was to demonstrate the influence of different quenching mediums as well as sample size and shape on the hardened depth of the AISI 4140 steel.

Based on the results of the investigation conducted the following conclusions were drawn:

• The cooling rate of the samples largely depends on the type of quenching medium and specimen shape and size;

• Changing the condition of quenching medium (pressure, velocity, temperature) can increase the amount of heat dissipation during the quenching process;

• Agitation of quenching media can significantly increase the peak heat flux during the quenching treatment of the sample;

• Microstructure examination of the samplesis in accordance with the prediction made by the CCT diagram of AISI 4140;

• Martensite is the predominant microstructure in water-quenched specimens, whereas respectively bainite and pearlite are predominant microstructures for compressed-air and dry air quenching mediums;

• The changes of microstructure are confirmed by hardness variation across the cross section of the specimen;

• Based on the Newton’s law of cooling and first law of thermodynamics, equations were developed for estimating hardness values of samples when using different quenching mediums.

REFERENCES

1. Daniel, H. H. (2008). Hardness and Hardenability: Part Two: A Discussion on Hardenability and Hardenability Testing. Industrial Heating, 75(7), 18.

2. Grum, J., Božič, S., & Zupančič, M. (2001). Influence of quenching process parameters on residual stresses in steel. Journal of materials processing technology, 114(1), 57-70.

3. Callister, W. D., & Rethwisch, D. G. (2007). Materials science and engineering: an introduction.

4. Çakir, M., & Özsoy, A. (2011). Investigation of the correlation between thermal properties and hardenability of Jominy bars quenched with air–water mixture for AISI 1050 steel. Materials & Design, 32(5), 3099-3105.

5. Kakhki, M. E., Kermanpur, A., &Golozar, M. A. (2009). Numerical simulation of continuous cooling of a low alloy steel to predict microstructure and hardness. Modelling and Simulation in Materials Science and Engineering, 17, 045007.

6. Bailey, N. S., Tan, W., & Shin, Y. C. (2009). Predictive modelling and experimental results for residual stresses in laser hardening of AISI 4140 steel by a high power diode laser. Surface & Coatings Technology, 203(14), 2003-2012.

7. Sinha, A. K. (2003). Physical metallurgy handbook (Vol. 8): McGraw-Hill New York.

8. Smith, W. F., & Hashemi, J. (2006). Foundations of materials science and engineering: Mcgraw-Hill Publishing.

9. Askeland, D. R., Fulay, P. P., & Wright, W. J. (2011). The Science and Engineering of Materials. Stamford, CT: Cengage Learning.

effect of quenching media, specimen size and shape on … › d269 ›...

Documents