CHAPTER 1 n THERMAL AND METALLURGICAL BASICS OF DESIGN OF HIGH-STRENGTH STEELS 23[64] Gayal, J., Latest Developments on Steel Front, Science Tribune,January 28, 1999, http://www.tribuneindia.com/1999/99jan28/science.htm.[65] Evo Tech Pvt Ltd., Thermomechanical Treatment for ReinforcementBars Opening Up New Vistas, http://rajmarkan.tripod.com.[66] U.S. Patent No. 6,458,226, Process for the ThermomechanicalTreatment of Steel, filed on July 20, 1999, published on July 20,2002.[67] Nazarenko, V. R., Yankovskii, V. F., Dolginskaya, M. A., andYakovenko, M. P. Damascus Steel: Myths and Reality, Metal Scienceand Heat Treatment, Vol. 34, No. 6, 1992, pp. 402–410.[68] Kobasko, N. I., Quenching Apparatus and Method for HardeningSteel Parts, U.S. Patent No. 6,364,974 B1, April 2, 2002.[69] Kobasko, N., Aronov, M., Powell, J., and Totten, G., One MoreDiscussion: “What Is Intensive Quenching Process?”, Journal ofASTM International, Vol. 6, No. 1, 2009.[70] Vergana-Hernandez, H. J., and Hernandez-Morales, B., A NovelProbe Design to Study Wetting Kinematics During Forced ConvectiveQuenching, Experimental Thermal and Fluid Science,Vol. 33, No. 5, 2009, pp. 797–807.[71] Liščic, B., Heat Transfer Control During Quenching, Materialsand Manufacturing Processes, Vol. 24, 2009, pp. 879–886.
2MNL64-EB/NOV. 2010Transient Nucleate Boiling andSelf-Regulated Thermal ProcessesN. I. Kobasko 12.1 INTRODUCTIONThe investigation of transient nucleate boiling is of great importancesince knowledge of the laws of nucleate boiling processesare used to control phase transformations in metals. It is especiallyrelated to martensite transformations where the martensitestart temperature M S is comparable to the boilingtemperature of the boundary liquid layer. Since the self-regulatedthermal process has been introduced, which is the mainpart of transient nucleate boiling where wall temperaturechanges very slowly and is supported at approximately thesame level up to the start single-phase convection, it makessense to use this discovery to control phase transformation insteel. It is necessary to develop a generalized equation for determinationof the duration of transient nucleate boiling process.Note that pressure can shift boiling temperature to themartensite start temperature M S , which may be used to controlmartensite phase formation. This is possible when theself-regulated thermal process is established. However, in practice,it is always difficult to determine the duration of transientnucleate boiling. This chapter describes a generalized equationfor the determination of the time of transient nucleate boilingand the self-regulated thermal process, which do not differ significantlyfrom each other. Calculation results obtained usingthe generalized equation agree well with the experimental data.The generalized equation has been used to create intensivesteel quenching technologies designated as IQ-1, IQ-2,and IQ-3 (see Chapter 10) and for proving the steel superstrengtheningphenomenon (see Chapter 9). This chapterstarts with the survey of nucleate boiling parameters and finisheswith the practical application of the generalized equationfor intensive technologies development.2.2 BUBBLE PARAMETERS AND DYNAMICSDuring intensive steel quenching, nucleate boiling and onephaseconvection prevail, and therefore it is important tostudy their regularities. When IQ-2 technology is implemented,nucleate boiling prevails. With IQ-3 technology, theone-phase convection prevails, and there is neither film boilingnor nucleate boiling. The regularities of nucleate boilingprocesses are discussed in [1–7].The inner characteristics (bubble dynamics) of the boilingprocess will be considered first. During the boiling process,the bubble departure diameter d 0 on a heated metal surface atatmospheric pressure is about 2.5 mm. When the pressureincreases, value of d 0 decreases. Some parameters—bubbledeparture diameter, bubble release frequency, and vapor bubblegrowth rate—are presented in Tables 1 and 2.When austenitized steels are immersed into a coldquenching bath, at the time of immersion, a boundary liquidboiling layer is formed. The boundary liquid layer is heatedto the saturation (boiling) temperature, and at the same timethe surface is intensively cooled. Then the liquid at theboundary layer starts to boil and a certain heat flux densityis reached, which depends on the shape and size of the partto be quenched and the thermal conductivity of the material.Depending on the heat flux density, film boiling may occuror may be absent. If there is no film boiling, transient nucleateboiling is established where the heat flux density isreduced exponentially. It is important to determine theeffect of heat flux density variation on the inner characteristicsof boiling, in particular, multiplication of d 0 f. This multiplication(d 0 f) is called vapor bubble growth rate anddesignated by W 00 ; that is,W 00 ¼ d 0 f :Tolubinsky [1] showed that the effect of heat flux densityvaried by four or five times with the average value of W 00 .The average vapor bubble growth rate W 00 is affected byWpressure. Fig. 1 shows the relationship of 00versus pres-W 000:1sure. The value of W0:1 00 is vapor bubble growth rate at normalpressure; W 00is the vapor bubble growth rate at a higher Wpressure. As the pressure increases, the ratio of 00W:0:100abruptly decreases (see Fig. 1).It is of practical interest to understand the effect ofaqueous salt solution concentrations on the inner characteristicsof the nucleate boiling process, including the bubbledispatch diameter, dispatch frequency, and their multiplication,the steam bubble growth rate W 00 . Experimental resultsobtained with boiling solutions of sodium chloride (NaCl)and sodium carbonate (Na 2 CO 3 ) at normal pressure are presentedin Table 3.Table 3 shows that in the case of boiling of highconcentrationsolutions of sodium chloride and sodiumcarbonate, the vapor bubble growth rates are the same asfor water. Vapor bubble growth rate for boiling aqueoussolutionsisdeterminedbypropertiesofthesolvent(water)and its vapor pressure and is only very weakly affected byproperties of the dissolved substances, viscosity, andPrandtl number of the solution [1]. Therefore, the value ofW 00 for boiling aqueous solutions can be considered to beequal to W 00 for boiling water.Unfortunately, there are no data for optimal concentrationsof aqueous solutions (10–12 %) where the mostð1Þ1 IQ Technologies, Inc., Akron, Ohio, and Intensive Technologies Ltd., Kyiv, Ukraine24Copyright © 2010 by ASTM Internationalwww.astm.org
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