Study on prediction of mechanical properties of large ring-shaped forging depending on cooling rate

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Large ring-shaped forgings manufactured by ring rolling, as heavy as 10 tons, are greatly affected by cooling. In the present study, controlled cooling was performed to improve the mechanical properties of large ring-shaped forgings.

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International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 03, March 2019, pp. 69-79. Article ID: IJMET_10_03_007 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=3 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed STUDY ON PREDICTION OF MECHANICAL PROPERTIES OF LARGE RING-SHAPED FORGING DEPENDING ON COOLING RATE J. H. Kang Department of Aeromechanical Engineering, Jungwon University, South Korea Y.C. Kwon Korea Conformity laboratory, South Korea ABSTRACT Large ring-shaped forgings manufactured by ring rolling, as heavy as 10 tons, are greatly affected by cooling. In the present study, controlled cooling was performed to improve the mechanical properties of large ring-shaped forgings. To quantify cooling rate, thermocouples were used to measure the cooling rate and the microstructures of the products were observed during still air cooling, fan cooling, mist control cooling, and water quenching. The temperature distribution measured in the four cooling methods was used to calculate the heat transfer coefficient in each cooling method by the inverse method. The mechanical properties were tested with specimens obtained from the test block for each cooling method, and continuous cooling transformation (CCT) curves were obtained by using measured microstructure contents. The mechanical properties of the regions corresponding to the regions of the specimens were calculated on the basis of the CCT curves and the heat transfer coefficients. The experimental values and the analytical values of the strength of the products manufactured by each cooling method were compared to verify that the mechanical properties at each region of the products depending on the cooling methods may be predicted Keywords: Mechanical Property prediction, Large sized forging, cooling heat transfer coefficient, Inverse method, CCT Curve. Cite this Article: J. H. Kang and Y.C. Kwon, Study on Prediction of Mechanical Properties of Large Ring-Shaped Forging Depending on Cooling Rate, International Journal of Mechanical Engineering and Technology, 10(3), 2019, pp. 69-79. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=3 1. INTRODUCTION Large forgings show a mass effect due to the presence of a significant difference in the cooling rate between the surface contacting a coolant and the core. Since larger forging http://www.iaeme.com/IJMET/index.asp 69 editor@iaeme.com
J. H. Kang and Y.C. Kwon products require longer cooling time due to the larger volume, their mechanical properties are relatively poor in comparison with those of smaller forging products cooled by an identical method. Therefore, it is difficult to use a simple normalizing process to satisfy the mechanical properties required of large forgings due to the mass effect. Hence, micro-alloys are added or the cooling rate is controlled to obtain the mechanical properties required of low carbon steel- based forging products.[1] Mechanical properties of steel materials, such as large forgings, are greatly affected by the rate of cooling following heat treatment.[2] Since the cooling rate is determined by the magnitude of the heat transfer coefficient between a coolant and the surface of the cooled product, many studies have been conducted to quantify the cooling capacity depending on cooling methods and coolants. With the development of an analytical method based on the finite element method, the inverse method has often been applied to calculate the heat transfer coefficients between coolants and materials using the empirical temperature distribution. Chen et al. proposed conjugate gradient inverse method is suitable for application to a rapid-heating or cooling process comparing measured temperature and analysis results.[3] Kim and Oh developed an inverse heat transfer formulation using two-dimensional finite element method and calculated heat transfer coefficients with the measured temperature.[4] Buczek and Telejko determined the values of the local heat transfer coefficient by inverse method for different quenching conditions.[5] Sugianto et al. insisted zone-based heat transfer coefficient should be applied to predict precise temperature distribution.[6] To calculated heat transfer coefficient considering phase transformation, steel probe was developed and validated with the cooling tests of various alloy steel. [7] Buczek and Telejko tried to quantify the heat transfer coefficient using 2 kinds of oils and various working conditions.[8] Pola et.al performed spray quenching tests for Ф840 heavy forging and compared numerical simulation and experimental tests.[9] Zbigniew et al. applied nonlinear shape function on inverse analysis to reduce the variation between finite element analysis and measurements.[10] Since the heat transfer coefficients in a thermal treatment process or TMCP(a thermomechanical controlled processing) determine the mechanical properties of steel products, the correlations among cooling rate, mechanical properties, and metal microstructure have been important topics of experimental and analytical studies. The relationship between Microstructure and Mechanical properties was investigated for various forging steel after hot working and concluded that mechanical properties are influenced by the microstructure due to cooling rate and hot working temperature.[11-14] Zaky et al. performed different cooling test after hot forging to improve mechanical properties of micro-alloyed steel. Air cooling after water quenching to 600℃ satisfied the target values and microstructure composition.[15] Wu et al. investigated microstructural evolution depending cooling rate after annealing process for cold rolled 1045 steel and verified the grain size of the rings becomes smaller and the lamellar spacing of pearlite decreases as the annealing cooling rate increases, resulting in a stronger and tougher material.[16] And the studies on the mechanical properties improvement are performed using TMCP process on heavy plate and on low carbon bainitic steel.[17-18] The mechanical properties of large ring-shaped forgings manufactured using carbon steel for general structural purposes need to be improved by means of grain size strengthening by refining crystal grains though normalizing. While forging products are cooled by still air cooling or fan cooling after normalizing thermal treatment, large forgings are cooled at a very slow rate due to the mass effect, at a cooling rate even slower than that of the annealing process for small products.[19] For this reason, the mechanical properties required of large http://www.iaeme.com/IJMET/index.asp 70 editor@iaeme.com
Study on Prediction of Mechanical Properties of Large Ring-Shaped Forging Depending on Cooling Rate forgings are difficult to obtain only by still air cooling or fan cooling. The present study was conducted to quantify the effect of cooling rates after normalizing on the mechanical properties of large forgings. Experiments were performed with structural carbon steel of different cooling methods, such as still air cooling, fan cooling, mist control cooling, and water quenching to calculate the heat transfer coefficients by inverse method. And the tensile strength, microstructure contents and grain size were measured with specimens obtained from ring rolled forgings. The mechanical properties and grain size can be analyzed by Deform 3D and Jmatpro software using calculated heat transfer coefficient and CCT curve. In this study, the CCT curve was obtained by iterative method comparing measured strength and grain size and calculated ones for different cooling method. The mechanical properties calculated by obtained CCT curves and F.E analysis were compared with the experimental results to verify the accuracy of analytical method. 2. COOLING TEST Fig. 1 shows the shapes of the specimens obtained to measure the temperature of large forging products made of carbon steel for general structural purposes. The material of the specimens was EN10025 S355NL; the specimens were obtained from ring-shaped forgings formed by a hot ring rolling process. To insert thermocouples for the measurement of temperature profiles at different positions, 12.5 mm (location „a‟) and 130 mm (location „b‟) deep holes were made. The temperature was measured using K-type thermocouples, which were sealed with 1/4 PT tap to prevent an inflow of the coolant into the materials in the cooling process. The weight of the ring forging product before the sampling of the specimens was 12.7 tons. The weight of the specimen block obtained from the ring forgings was 465 kg. (a) Ring rolled forging product (b) Cooling specimen extracted from forging Figure 1 Temperature measurement block of the forged ring Four specimens were obtained from a ring forging and then processed to measure the temperature profiles depending on the cooling conditions in different cooling methods, including still air cooling, fan cooling, mist control cooling, and water quenching, following a normalizing process, as the temperature was lowered from 900℃ to room temperature. The process specimen block was charged into a heat treatment furnace, and cooling tests were performed under different conditions. For fan cooling, a large cooling fan was used after normalizing heat treatment. For mist control cooling, the specimen was extracted from the furnace and moved to a cooling die; then, a cycle including three minutes spraying and three minutes still air cooling was performed eight times to transfer the inside thermal energy to the surface quickly and uniformly. Subsequently, still air cooling was performed. Water quenching was performed for two hours by dipping the specimen block into a cooling bath at 40℃. Fig. 2 shows the methods of fan cooling and mist control cooling. http://www.iaeme.com/IJMET/index.asp 71 editor@iaeme.com
J. H. Kang and Y.C. Kwon (a) Spraying in control cooling (b) Spraying in control cooling(spray off) Figure 2 cooling and mist control cooling Fig. 3 shows the temperature distribution in different regions of the specimen obtained by still air cooling, fan cooling, mist control cooling, and water quenching. As Fig. 3 shows, the cooling rate increased in the order of still air cooling, fan cooling, mist control cooling, and water quenching. While the cooling rate of mist control cooling, where mist cooling and still air cooling were performed alternately, was faster than that of fan cooling down to 300℃, the cooling rate in the still air cooling stage of mist control cooling below 300℃ was slower than that of fan cooling. Figure 3 Temperature distribution of the specimen measured by cooling tests. 3. TEST OF MECHACNICAL PROPERTIES OF SPECIMENS The mechanical properties of the specimens obtained from the cooling test block were compared among the cooling methods by performing a tensile test and taking pictures of the microstructures. The tensile tests were performed and the microstructure images were obtained with respect to three specimens obtained at 1/2", which was the EN10025 specimen sampling position, and the average values from the three specimens were tabulated. The test specimens were prepared according to the ISO 862 specifications. Table 1 shows the yield strength, tensile strength, elongation rate, and reduction of area obtained from the tensile tests of the specimens cooled using the different cooling methods. As was predicted, the yield strength of the specimens increased from 296 MPa to 304 MPa, 321.3 MPa, and 349.4 MPa as the cooling rate increased. The tensile strength of the specimens also increased from 508.6 http://www.iaeme.com/IJMET/index.asp 72 editor@iaeme.com
Study on Prediction of Mechanical Properties of Large Ring-Shaped Forging Depending on Cooling Rate MPa to 515 MPa, 542.5 MPa, and 563.2 MPa as the cooling rate increased. On the contrary, the elongation rate and the reduction of area decreased as the cooling rate increased, as shown in Table 1. Table 2 shows the room temperature impact toughness of the specimens obtained from the position of 1/2" from the material surface and the grain size measured from the microstructure images. The impact toughness and the ASTM number for the grain size increased as the cooling rate increased. Table 1 Mechanical properties depending on cooling methods Tensile Test Method Yield Tensile Remark Elong- Reduction of Strength Strength ation [%] Area [%] [MPa] [MPa] Still air Cooling 296.0 508.6 33.2 74.6 Forced air cooling 304.0 515.0 32.7 75.7 Mist control cooling 321.3 542.5 31.8 75.9 Water quenching 349.4 563.2 31.2 78.0 Table 2 Impact toughness and grain size Charpy Impact Test [J] Grain Size Method 1 2 3 Ave. [ASTM] Still air Cooling 51.9 70.8 54.9 59.2 6.6 Forced air cooling 45.9 70.7 71.2 62.6 6.9 Mist control cooling 69.7 113.7 90.1 91.1 7.6 Water quenching 151.0 125.2 154.1 143.4 8.8 Table 3 Microstructure contents depending on the cooling methods Ferrite Contents Pearlite Contents Cooling Method Remark [%] [%] Still air Cooling 63.8 36.2 Forced air cooling 57.0 43.0 Mist control cooling 55.3 44.7 Water quenching 50.2 49.8 Fig. 4 shows optical microscope images of the microstructure of the specimens obtained from the position 1/2" from the surface depending on the cooling methods. Table 3 shows the microstructure contents of individual specimens measured using the microstructure images. The results show that the ferrite content decreased from 63.8% to 57%, 55.3%, and 50.2% as the cooling rate increased. Neither martensite nor bainite microstructure was found even in the water quenching specimens, because the cooling rate was slow due to the heavy weight of the specimens and because the material of the specimens was low carbon steel. http://www.iaeme.com/IJMET/index.asp 73 editor@iaeme.com
J. H. Kang and Y.C. Kwon (a) Still air cooling (G6.6) (b) Forced air cooling (G6.9) (c) Mist control cooling (G7.6) (d) Water quenching (G8.8) Figure 4 Microstructure and ASTM grain size by the cooling methods. 4. FINITE ELEMENT ANALYSIS 4.1. Analysis of Heat Transfer Coefficients by Inverse Method Cooling of products after heat treatment is quantified by convective heat transfer coefficients between coolants, such as air, mist, and water, and the product surface. Convective heat transfer coefficients, which are dependent on medium type, cooling methods, and coolant type, should be obtained with respect to the individual cooling method. Since the specimens were not located at symmetrical positions, as shown in Fig. 1, the Inverse Heat Module of Deform 3D, a three-dimensional shape analysis software program, was used to calculate the cooling heat transfer coefficients by inverse method for the four cooling methods. JmatPro, a thermo-physical calculation software program, was used to calculate the physical property values needed for the calculation of the heat transfer coefficient of the material by inputting the chemical compositions of the EN10025 S355NL material. The chemical composition is shown in table.4. The calculated thermal capacity and conductivity are shown in Fig. 5. Table 4 Chemical Composition of S355NL steel (Wt%) C SI MN P S CU TOT_AL 0.17 0.320 1.31 0.018 0.003 0.006 0.026 http://www.iaeme.com/IJMET/index.asp 74 editor@iaeme.com
Study on Prediction of Mechanical Properties of Large Ring-Shaped Forging Depending on Cooling Rate Figure 5 Thermo-mechanical properties of EN10025 S355NL Figure 6 Mesh system for the calculation of the heat transfer coefficient The cooling heat transfer coefficients in each of the cooling methods were calculated by using Deform 3D Inverse Heat and inputting the empirical temperature curves to the positions shown in Fig. 1(a) and 1(b). The temperature input for the mist control cooling reflected the cycle, including processes of three-minute cooling and three-minute still air cooling. To calculate the heat transfer coefficients in the mist control cooling by the inverse method, the maximum temperatures following the still air cooling stages were connected and put into the software, as shown in the "Control(a)_Inverse" curve of Fig. 3. Fig. 6 shows the mesh system for the calculation of the heat transfer coefficients by the inverse method. The heat transfer coefficients were calculated by the inverse method at a temperature interval of 50℃. Fig. 7 shows the plots of the heat transfer coefficients calculated for each temperature interval. Figure 7 Heat transfer coefficients calculated by the inverse method http://www.iaeme.com/IJMET/index.asp 75 editor@iaeme.com
J. H. Kang and Y.C. Kwon (a) Temperature comparison of air cooling and control cooling (b) Temperature comparison of fan cooling and water quenching Figure 8 Comparison of empirical results and analytical results The heat transfer coefficients calculated by the inverse method may be used to calculate the temperature distribution of the specimen block by the finite element method. Fig. 8 compares the empirical results and the finite element method results obtained at the measurement positions of the cooling tests. The trends of the temperature profile shown in Fig. 8 were consistent with the empirical results and the finite element method results. However, the cooling rate near the surface was slightly faster in the analytic results, while the cooling rate near the core was slight slower in the analytical results. This difference may be due to cooling retardation caused by the micro-porosities contained in large forging products 4.2. Analysis of Microstructure and Strength Depending on Cooling Rates CCT curves are needed to calculate the microstructure contents and the strength depending on the cooling rates. The CCT curves were obtained in the present study not only by considering the chemical compositions but also by modifying the Austenite grain size and the pearlite, ferrite and bainite contents so that the microstructure contents measured from the specimens obtained under different cooling conditions could be approximated as closely as possible with the microstructure contents calculated by Jmatpro. Modification was performed to minimize the differences in the microstructure contents and the mechanical property values between the calculated results and the empirical results. Fig. 9 shows the CCT curves obtained by the method described above, and the cooling rates. Figure 9 Calculated CCT Curve and cooling curves The CCT curves and the cooling curves of the specimens shown in Fig. 9 may be used to predict the microstructure after cooling. Since all the curves in Fig. 9 pass through the http://www.iaeme.com/IJMET/index.asp 76 editor@iaeme.com
Study on Prediction of Mechanical Properties of Large Ring-Shaped Forging Depending on Cooling Rate transformation point between ferrite and pearlite, only the ferrite and pearlite matrix structure is found, as shown in Fig. 4. The complex profile cooling module of Jmatpro was used to calculate the microstructure contents and the strength on the basis of the temperature profiles and the CCT curves of the individual cooling methods. Table 5 and Fig. 10 compare the yield strength, tensile strength, and microstructure contents calculation with the corresponding empirical results for each of the cooling methods. Fig. 10 shows that the yield strength and the tensile strength increased more in the results obtained using software for the CCT curves shown in Fig. 9 than in the empirically measured results. The empirical yield strength was lower than the calculated yield strength in air cooling. However, the calculated values were higher than the empirical values in fan cooling and faster cooling methods, and the difference between the calculated values and the empirical values increased as the cooling rate increased. The same trend was found in the tensile strength values. The difference may be because the yield strength calculation by the Jmatpro software considered both the Hall-Petch Equation (Eq. (1)) and solid solution hardening. σy = (1-x)σyf + xσyp (1) where x denotes the fraction of pearlite, and σyf and σyp are the yield strength of ferrite and pearlite, respectively. Table 5 Mechanical property and micro structure calculation Yield Tensile Ferrite Pearlite ASTM Method Results Strength strength Contents Contents No. [MPa] [MPa] [%] [%] Experiment 292 475 63.8 36.2 6.6 Air cooling Calculation 281.1 439.7 74.9 25.1 Difference - 10.9 -35.3 11.1 -11.1 Experiment 304 503 62.9 37.1 6.9 Fan cooling Calculation 308.5 476.8 67.6 32.4 Difference 4.5 -26.2 4.7 -4.7 Experiment 339 542.5 55.3 44.7 7.6 Mist cooling Calculation 357.6 541.8 57.2 42.7 Difference 18.6 -0.7 1.9 -2 Experiment 366 563.2 50.2 49.8 8.8 Water quenching Calculation 396.2 591.3 52.8 47.1 Difference 30.2 28.1 2.6 -2.7 Figure 10 Comparison of empirical results and analytical results. http://www.iaeme.com/IJMET/index.asp 77 editor@iaeme.com
J. H. Kang and Y.C. Kwon In other words, the calculated yield strength values by the software for Equation (1) were greater than the yield strength values measured from the actual specimens, and thus the resulting strength values were greater. 5. CONCLUSION The present study was conducted to predict the mechanical properties and the microstructural changes of large forgings depending on cooling methods. The following conclusions can be drawn from the present study. 1) Still air cooling, fan cooling, mist control cooling, and water quenching were performed with a cooling test block obtained from a large forging product. The temperature was measured at a depth of 1/2" and at the core. The measured temperature distributions were used to calculate by the inverse method the cooling heat transfer coefficients for each of the cooling methods. It was verified that the calculated cooling heat transfer coefficients may be used to calculate temperature profiles that approximate the empirical values. 2) Specimens were obtained from the cooling block to measure the tensile strength, yield strength, and microstructure contents at a depth of 1/2". The pearlite content and the tensile strength were found to increase as the cooling rate increased. The temperature profile at a depth of 1/2" calculated by the finite element analysis was used to calculate the mechanical properties and the microstructure contents by using a commercially available software program. CCT curves were generated to minimize the difference between the analytical results and the empirical results. 3) The trends of the tensile strength, yield strength, and microstructure contents in each of the cooling methods were identical in the empirical results and the analytical results. However, as the cooling rate increased, the tensile strength and the yield strength were found to increase at a faster rate in the analytical results than in the empirical results. However, with regard to the microstructure contents, the measured pearlite content was higher than the calculated content. This difference may be because the yield decreased due to microstructural defects remaining in a large forging product. 4) The present study showed that the temperature profile in different parts of large forging products, depending on cooling methods, may be predicted by finite element analysis; it was also found that the mechanical properties may be reliably predicted by using CCT curves. The results of the present study suggest a method of quantitatively predicting mechanical properties of large forging products depending on cooling methods despite changes in weight and dimensions. ACKNOWLEGEMENT This work was supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173030024670). REFERENCES [1] G. Krauss, “Steels: Heat Treatment and Processing Principles”, ASM International, Ohio, pp.133-142, 1990. [2] P.M. Unterweiser, and W.H. Cubberly, “Atlas of continuous cooling transformation diagrams for engineering Steels”, American Society for Metals, Ohio, pp.10-16, 1980. http://www.iaeme.com/IJMET/index.asp 78 editor@iaeme.com
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