The current study focuses on an industrial hardening process where the steel parts are austempered in a molten salt bath. The aim of the study was to compose a predictive tool for more qualified process adjustment and control in a hardening plant. The process approach considers specific 3.0 to 4.0 mm thick strip steel products and steel grades with carbon content in the range from 0.27 to 0.62 wt%.
Austempering in salt produces high hardness and ductility of steel as the final microstructure is bainitic. Salt bath cooling is not so severe as cooling in oil, polymers, or water and has an advantage of a uniform cooling rate. There are several approaches to determining hardenability of steels and interrelating it to the quenching process to predict the final hardness of the parts. The best known are the Jominy hardenability test and the Grossman hardenability number where hardness is presented as the function of the specimen geometry. To determine the quenchant properties the Grossman number, nickel ball, hot wire, cooling curve, and quench factor methods are used. These methods are guiding tools to the engineer for choosing the steel grade and the hardening process. The latest approach is quench factor analysis, which interrelates the cooling curve of the quenchant and material hardenability data with a single number and allows prediction of the properties of the hardened material. The imperfection of the quench factor is limited availability of mathematical constants for different steel grades to make calculations. As the cooling process and phase transformations are nonlinear, the computation accuracy does not seem to satisfy the heat treaters to utilize it in the hardening plant. The types of heat treatment equipment in use have different characteristics, which are related to the technological process used and the design of the machine. The available engineering diagrams for steels and processes are useful to have a general view of the process but still each workshop has to specify the approach to be used in its particular case.
1. Totten, G. E. Steel Heat Treatment Handbook, 2nd ed. CRC Press, Boca Raton, 2006.
2. Bhadeshia, H. K. D. H. Martensite and bainite in steels: transformation mechanism & mechanical properties. J. Phys. IV, 1997, 07(5), 367–376.
http://dx.doi.org/10.1051/jp4:1997558
3. Abbaszadeh, K., Saghafian, H., and Kheirandish, S. Effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC steel. J. Mater. Sci. Technol., 2012, 28, 336–342.
http://dx.doi.org/10.1016/S1005-0302(12)60065-6
4. Fuchs, A. Application of Microstructural Texture Parameters to Diffusional and Displacive Transformation Products. PhD Thesis. University of Birmingham, 2005.
5. Fielding, L. C. D. The bainite controversy. Mater. Sci. Technol., 2013, 29, 383–399.
http://dx.doi.org/10.1179/1743284712Y.0000000157
6. Trzaska, J., Jagiełło, A., and Dobrzański, L. A. The calculation of CCT diagrams for engineering steels. Arch. Mater. Sci. Eng., 2009, 39, 13–20.
7. Wever, F. and Rose, A. Atlas zur Wärmebehandlung der Stähle. Part I. Verlag Stahleisen, Düsseldorf, 1961.
8. Atkins, M. and Met, B. Atlas of Continuous Transformation Diagrams for Engineering Steels. ASM, Ohio, 1980.
9. Smoljan, B., Smokvina Hanza, S., Tomašić, N., and Iljkić, D. Computer simulation of microstructure transformation in heat treatment processes. Journal of Achievements in Materials and Manufacturing Engineering, 2007, 24, 275–282.
10. EVS EN ISO 642. Hardenability Test by End Quenching, 1999.
11. Liedtke, D. Wärmebehandlung von Stahl - Härten, Anlassen, Vergüten, Bainitisieren. Stahl-Zentrum, Düsseldorf, 2005.
12. Kirkaldy, J. S., Thomson, B. A., and Baganis, E. A. Hardenability Concepts with Applications to Steel (Kirkaldy, J. S. and Doane, D. V., eds). AIME, Warrendale, PA, 1978, p. 82.
13. Kirkaldy, J. S. and Venugopolan, D. Phase Transformations in Ferrous Alloys (Marder, A. R. and Goldstein, J. I., eds). AIME, Warrendale, PA, 1984, p. 125.
14. Saunders, N., Guo, Z., Li, X., Midownik, A. P., and Schille, J. P. The calculation of TTT and CCT diagrams for general steels. Internal Report. Sente Software Ltd., U.K., 2004.
15. Pacheco, P. M. C. L., de Souza, L. F. G., Savi, M. A., de Oliveira, W. P., and Silva, E. D. Modeling of quenching process in steel cylinders. Mechanics of Solids in Brazil, 2007, 445–458.
16. Kobasko, N. I., Aronov, M. A., Kobessho, M., Hasegawa, M., Ichitani, K., and Dobryvechir, V. V. Critical heat flux densities and Grossmann factor as characteristics of cooling capacity of quenchants. In Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology. WSEAS Press, 2012, 94–99.
17. Trzaska, J., Sitek, W., and Dobrzański, L. A. Selection method of steel grade with required hardenability. Journal of Achievements in Materials and Manufacturing Engineering, 2006, 17(1–2), 289–292.
18. Trzaska, J. and Dobrzański, L. A. Application of neural networks for selection of steel with the assumed hardness after cooling from the austenitising temperature. Journal of Achievements in Materials and Manufacturing Engineering, 2006, 16(2), 145–150.
19. Totten, G. E., Webster, G. M., Bates, C. E., Han, S. W., and Kang, S. H. Limitations of the use of Grossman quench severity factors. In Heat Treat: Proceedings of the 17th Heat Treating Society Conference Including the 1st International Induction Heat Treating Symposium (Milam, D., Poteet, D. A., Pfaffmann, G. D., Rudnev, V., Muehlbauer, A., and Albert, W. B., eds). ASM International, Materials Park, OH, 1998, 411–422.
20. Evancho, J. W. and Staley, J. T. Kinetics of precipitation in aluminum alloys during continuous cooling. Metall. Trans., 1974, 5, 43–47.
21. Felde, I. Report on IFHTSE liquid quenchant database project. International Heat Treatment and Surface Engineering, 2014, 8(1), 2–7.
http://dx.doi.org/10.1179/1749514813Z.000000000103
22. Liščić, B. and Filetin, T. Measurement of quenching intensity, calculation of heat transfer coefficient and global database of liquid quenchants. Mater. Eng., 2012, 19(2), 52–63.
23. Rose, A., Peter, W., Strassburg, W., and Rademacher, L. Atlas zur Wärmebehandlung der Stähle. Part 2. Verlag Stahleisen, Düsseldorf, 1961.
