Transient Nucleate Boiling as a Law of Nature and a Basis for Designing of IQ Technologies

Transcription

1 Transient Nucleate Boiling as a Law of Nature and a Basis for Designing of IQ Technologies NIKOLAI KOBASKO Intensive Technologies Ltd, Kyiv, Ukraine and IQ Technologies Inc, Akron, USA Abstract: - In the paper duration of transient nucleate boiling is discussed. It is established that the duration of transient nucleate boiling is directly proportional to square of thickness of a body and inverse proportional to thermal diffusivity of a material, depends on configuration of a body, liquid properties and its velocity. It is shown that the transient nucleate boiling is followed by amazing regularities. The surface temperature of steel part during nucleate boiling is maintained at the level of boiling (saturation) point. The discovered regularity extends the opportunity of high temperature thermomechanical treatment (HTMT) and low temperature thermomechanical treatment (LTMT) for designing of high strength materials. In this case alloying elements are saved. Environment is improved due to plain water which is used as a quenchent. Key Words: - Transient nucleate boiling, Duration, High strengthening, Carbon steels, Material savings, Environment, Improvement. 1 Introduction As a rule, during quenching of steel parts one can consider three stages of cooling: film boiling, transient nucleate boiling and ection. In this case cooling time can be evaluated as follows: = + +, (1) fb where is cooling time from temperature (8 o C 9 o C) to room temperature in seconds; fb is time of film boiling; is time of transient nucleate boiling; is time of ective heat transfer process. In the last three decades the new intensive quenching methods were developed and successfully applied into the practice. In is supposed that during these processes film boiling is completely prevented and only transient nucleate boiling and the forced ection are utilized. These technologies were called IQ-, and IQ-3 processes [1-5]. The IQ- process is a three-step quench method that is usually used for batch quenching of parts. First, the parts are intensively cooled in an IQ water tank. The key element of the IQ- technique is to fully eliminate film boiling at the very beginning of cooling when the heat flux from the part surface is high. A high rate of the water agitation, low water temperature, and small additives of salt to the water, all of these work in concert to eliminate or minimize the duration of film boiling. The nucleate boiling starts immediately on the part surface after the part is put into the quench media. As known, a very high heat transfer coefficient on the part surface characterizes the nucleate boiling mode of heat transfer. High surface compressive stresses develop on the part surface by the end of the nucleate boiling. After the nucleate boiling stage of quenching is completed the parts are taken from the quench bath to continue cooling in the air of the first stage of cooling, i.e. duration of transient nucleate. Then the parts are cooled again intensively either in the same quench tank or by any other means, for example, by water jets, to complete the final phase transformations in the part core. In this third stage of the intensive quenching process, the heat transfer from the part is by uniform ection cooling through the quenched shell. A key element of the IQ- method is to determine the duration of cooling process [1-4]. The IQ-3 method is a one-step quenching technique and it is the most intensive quenching process. The part is quenched either in a high-velocity water flow or by water jet ISSN: ISBN:

2 impingement. The ection heat ransfer coefficient on the part surface is so high that the boiling process, both film boiling and nucleate boiling, are completely eliminated from the very beginning of the quench. The IQ-3 is characterized by direct ection cooling. The intensive water cooling of the part is interrupted at a time when the surface compressive stresses are at their maximum value and the hardened layer is at its optimum depth. The part core is still very hot after the interruption of the quench. The phase transformations in the part core complete in the air by uniform conduction cooling through the cold shell. Duration of transient nucleate boiling and self regulated thermal process and their regularities The transient nucleate boiling and self regulated thermal processes were investigated since 1968 and their results were published in [1-4]. Duration of self regulated thermal process differs insignificantly from the time of transient nucleate boiling (within.5 1 second). The notion of self- regulated thermal process was proposed in 1968 [1] and it means that wall temperature of steel part is kept at the level of saturation point varying insignificanly during all process of transient nucleate boiling. The equation for determining the duration of transient nucleate boiling (self-regulated thermal process) was firstly received by generalization of experimental data [1, ] and then derived from the analytical equation and has the form [4]: D = Ωk F kw, () a where value Ω depends on initial temperature of a steel part and condition of cooling. For initial temperatures 85 o C it can be within (see Table 1). Koefficient k F depends on configuration of steel part (se Table ). For plateshaped forms k F =.113; for cylinder shaped form k F =.43; for ball shaped forms k F =.53; kw is dimensionless coefficient which depends on liquid flow velocity. For motionless liquid k W = 1. For high flow velocity of liquid which prevents nucleate boiling k =. That is W why for different condition we have k W 1. D is thickness of the body: diameter of cylinder, sphere or thickness of the plate; a is thermal diffusivity of a material. The purpose of presentation is to explain what self - regulated thermal process is and how it is used successfully in the practice to manufacture very high strength materials. The duration of transient nucleate boiling () can be evaluated analytically from generalized equations (3), (4), (5), and (6) which were checked and widely discussed during last three decades in scientific and technical literature [3, 6]: I K =.4k + b ln, (3) II a 1 λ( I ) I = ; β R 1.3 II = [ α ( II + uh )] ; (5) β 1 3 ' gβ uh α = λ, (6) av.3 where b=3.1; k =1, and 3 for plate shaped forms, cylinder-shaped forms, and ball- shaped form correspondingly; is duration of transient nucleate boiling (self-regulated thermal process) in seconds; K is Kondratjev form factor (m ); a is thermal diffusivity of steel (m /s); λ is heat conductivity of steel (W/mK); = T TS ; I = TI TS ; II = TII TS ; uh = T S T m ; T is initial temperature; T I is initial temperature of the surface at the beginning of self-regulated thermal process; T II is temperature of the surface at the end of self-regulated thermal process; α is ective heat transfer coefficient (W/m K); β = 7.36 [3, 7]. To calculate ective heat transfer coefficient (6), the next properties of water at o W C were used: λ =.597 is heat mk m conductivity of water; g = 9.81 is s ISSN: ISBN:

3 1 gravitational acceleration; β ' = is K volumetric expansion; = T T = K is uh S m 8 underheat; T m is bath temperature; 8 m a = is thermal diffusivity; s m ν = is kinematic viscosity. s By substituting these data into Eq. (6), we obtain α = 76 W/(m K). Average heat transfer coefficient between o C and 1 o C is α = 138 W(m K). As we can see from Eq. (), the duration of transient nucleate boiling is directly proportional to square of thickness of a body and inverse proportional to thermal diffusivity of a material, depends on configuration of a body, liquid properties and its velocity. The transient nucleate boiling (self regulated thermal process) is followed by amazing regularities: the surface temperature of steel part during nucleate boiling is maintained at the level of boiling (saturation) point (see Eq. (7)). In the same time the bath temperature doesn.t changed significantly and is almost equel to room temperature. Using established regularities, as was already mentioned, the new intensive quenching (IQ) technologies were developed: IQ-: IQ-3 [3, 5]. Table 1 Coefficients Ω depends on initial temperature and properties of quenchants at o C Quenchant Ω Water, o C % CaCl % NaOH % NaNO Note: Initial temperature is fixed at 85 o C; For 6 8% NaNO 3 water solution duration of transient nucleate boiling for cylindrical specimen ( mm), according to Eq. () and Table 1, is: (,m) m = 3 = / s 1.1s. k F Table Coefficients for bodies of different shapes Shape of a k Equation F body Plate.113 π Cylinder ν Sphere π Round plate.33 1 with hight Z 4ν +π and dia D = nz; n = 1 n = ν +π n = ν + 5π Finite.33 1 cylinder with 4ν +π dia D and Z = nd; n = 1 n = ν +π n = ν +π Notes: ν =. 448 and is a root of the first kind of Bessel function; π = , Z is hight of a cylinder or round plate. Example of simpolified calculations is presented below. Using Eq. (4) and Eq. (5), for cylindrical probe 1.5 mm in diameter made of AISI 34 steel, which was quenched from 85 o C in technical water at o C we can find: I =.136 (75 I ) = o C,.65 II = [ 138( + 8) ] = II o C. Here λ = W/(m K) is heat conductivity of AISI 34 steel, R =.65 is probe radius in (m), = T TS = 85 1 = 75. And finaly, duration of transient nucleate boiling can be found from Eq. (3), i.e. ISSN: ISBN:

4 m = ln m / s, = 4.97s where T sf is average surface temperature of a body; T S is saturation temperature; ξ = T sf T S is average wall overheat. Fig. CCT diagram for AISI W1 steel (a) and impact of the pressure on self regulated thermal process (b) which show that martensite start temperature M S and T sf during self regulated themal process are comparable and transformation can be delayed: I.1 MPa; II -.4 MPa; III.7 MPa. Overheat at the beginning of self regulated thermal process can be evaluated using Eq. (4), i.e..3 1 (775 I ) I = = 1. C Overheat at the end of self regulated thermal process can be evaluated using Eq. (5), i.e. 1.3 II = [ 1 (775 + II )] = 4,3 C Then duration of self regulated thermal process is calculated from Eq. (3): Fig. 1 Temperature at the core (1) and at the surface () of spheres versus time in the process of their quenching in 5% NaOH water solutions at C: (a) is sphere 6.35 mm; (b) is sphere 1.7 mm;(c) is sphere 38.1 mm [8]. The character of surface temperature changing during self- regulated thermal process is shown in Fig, 1 (see curve ). Such trend can be written as: T sf = TS + ξ const, (7) ln = = = 6 Here K R.4s. = π = m is Kondratjev form factor for the sphere of 38.1 mm in diameter; m a = s is thermall diffusivity of steel. 3 Thermomechanical heat treatment of steel The scheme of high temperature (HTMT) and low temperature (LTMT) thermomechanical heat treatment is shown in Fig.3 [3, 9 11]. To make the process of low temperature thermomechanical heat treatment possible, it is required to supercool austenite to 4 5 o C where it should be ISSN: ISBN:

5 deformed to certain degree. Transformation during this time should be delayed and supercooled austenite should be stable. For this purpose, as a rule, high alloy steels are used which allow such procedure. In this paper is developed an idea that combination of intensive quenching and thermomechanical treatment (HTMT and LTMT) can provide very high mechanical properties for plain carbon steels. In Table 3 are shown mechanical properties of AISI 14 steel subjected only to HTMT process. As we can see from Table 3, the mechanical properties are increased significantly, especially after low tempering at o C. After combining HTMT and LTMT with the intensive quenching process, the strength of a material and its plastic properties will increase significantly further. Fig. 3 The scheme of high and low temperature thermomechanical heat treatment The process of low temperature thermomechanical heat treatment requires delay of tranformation austenite into martensite during quenching. Especially that is important when applying intensive quenching. The matter is that during intensive quenching the temperature at the surface of steel part drops repidly to saturation temperature while temperature at the core remains very high. Immediately at the surface brittle martensitic layer is formed which makes the process of mechanical deformation not allowable (see Fig. 4 a)). Fig. 4 Scheme of incorrect (a) and correct (b) low temperature thermomechanical heat treatment To design the process of low temperature thermomechanical treatment allowable, one should delay transfrmation austenite into martensite during intensive quench (see Fig. 4 b)). It can be done by using regularities of self regulated thermal process [1-4]. As is already known, delay of transformation austenite into martensite can be fulfilled by using appropriate pressure or water salt solutions [3] or just use high carbon steels which have martensite start temperature about 1 o C or below. At present, low temperature thermalmechanical treatment was used for high alloy steels where supercooled austenite remains stable at low cooling rate to 4-5 o C. Unfortunately, it can not be done for low alloy and plain carbon steels. That is why the low temperature thermomechanical heat treatment is not widely used in the practice. Table 3 Mechanical properties of steel AISI 14 after HTMT and entional quenching process [1, 11] T-ring R m, R p, A, % Z, % a n, MPa MPa J/cm C C 6 C The matter consists in stong dependence (Eq. (8)) of heat transfer density q from the wall overheat which can be written as: q = β Δ m T m, (8) ISSN: ISBN:

6 where, according to Dhir [1], the value m can be found between 3 and 4, i.e.: 3 m 4 ; β depends on condition of cooling and changes from 3 to ΔT is wall overheat and is evaluated as Δ T = T sf T S. It means that very small changes in wall overheat Δ T will lead to very big changes in heat flux densities. To support stated above, the inverse problem was solved using very accurate experiments of French which are provided in [8]. These experimental data were achieved by accurately arranged thermocouples, as shown in Fig. 5. For solving inverse problem software IQLab was used [13, 14]. Results of calculations (heat flux densities and real heat transfer coefficients) are presented in Fig. 6 and Fig.7. At the beginning heat flux densities and heat transfer coefficients are really extremely high and then decrease rapidly. It should be noted that initial heat flux density during quenching in water alkaline solution of 5% is much higher than the first critical heat flux for the same solution which is provided in Table 4. Instead of very high heat flux density the film boiling was absent. Obviously, shock boiling increases significantly the first critical heat flux density which is comparable with the initial heat flux density. This phenomenon was discussed in the paper [15]. Table 4 The first critical heat flux density for water and water salts and alkaline solutions at o C. Quenchant at o C Critical heat flux densities, q cr1, in MW/m 13 1% NaCl water solution 5% NaOH water 15 solution 1% NaOH water solution Water Ref. [3] presents average (effective) values of heat transfer coefficients for water solutions of NaCl and NaOH versus concentrations. The experiments were carried out with cylindrical samples of 1 mm and 1 mm in dameter, made of stainless steel Kh18N9T (AISI 34). By these experiments was found out that the average heat transfer coefficient reaches maximal values at concentration of 1-1 % NaOH in water, and then it goes down. There are various explanations of the character of the established dependence [3, 15]. Fig. 5 The scheme which shows how thermocouples were arranged and accurately flattened to the wall of spheres and polished by French [8] Optimal concentrations (see Table 4) of water salt solutions have the direct relation to their direct use in industrial conditions as quenchants. Such quenchants are used for the intensification of heat transfer and preventing of corrosion of quenched steel parts. These quenchants based on water solutions of salts should meet the abovementioned requirements. Fig. 6 Heat flux density versus time during quenching spheres of 6.35 mm and 1.7 mm in 5% water alkaline solution ISSN: ISBN:

7 Fig. 7 Real heat transfer coefficient versus time for sphere quenched in 5% water alkcaline solution at o C B i V = const and Kn = constant. This fact needs explanation. To understand correctly this claim, let s consider temperature fields which were received during quenching of cylindrical specimens in water salt solutions (see Fig. 9). Cylidrical specimens and 4 mm in diameter were made from AISI 34 steel and quenched from 85 o C in water salt solution at o C. For both specimens surface temperature drops immediately almost to saturation temperature and then regular thermal proces is established, as is shown in Fig.9a and Fig. 9 b). During transient nucleate boiling gradient of temperature depends on size of specimen and is inverse proportional to its diameter. Using IQLab software [14], the Kondratjev numbers were calculated for ballshaped and cylindrical - shaped specimens (see Fig. 1 and Fig. 11). As we can see from Fig.1, for real heat transfer coefficients Kondratjev number changes from 1 to.9 and for effective heat transfer coefficients Kn changes from.9 to approximately.3. For cylindrical specimens, 3, and 4 mm in diameter average effective Kondratjev number K n =. 5 (see Fig. 11). Fig. 8 Double electrical layer according to Frenkel [16] a) b) Fig. 9 Temperature distribution versus time during quenching of cylinders in water salt solution at o C: a) is for cylinder mm, b) for cylinder 4 mm In many publications [1-3] is stated that during quenching of cylidrical specimens (1 6 mm in diameter) in water salt and alcaline solutions average generalized Biot number and average Kondratjev number Kn are constant, i.e.: Fig. 1 Kondratjev numbers Kn versus time recalculated from the French experiments which were made by quenching sphere 38.1 mm in cold water with small amount of salts: Kn boil real value; Kn is effective value. Note that real heat transfer coefficient is evaluated as q α = and effective as T sf T s q α =. T sf T s ISSN: ISBN:

8 In the heat treating industry the most popular are effective heat transfer coefficients. They provide good results only for core temperature predicting. Fig. 11 Kondratjev number Kn versus Fourier number Fo for cylinders, 3, and 4 mm quenched in 5% water alkaline solution Substituting this result of calculation into generalized equation for cooling time evaluating [17] (see Eq. (9) and (1)), we obtain equation (11) similar to equation (), i.e.: kbi v Fov Kn = + lnθ (9) Biv or kbiv T Tm K = + ln. (1) BiV T Tm akn 85 D = ln a or D = 3.7k F kw. (11) a Here 85 Ω =.33 + ln = 3.7, 1 D =. K =, Kn 3.13 K F = = k D.43D. Note that Bi =. 87 when Kn =.5. That is V why for cylinders BiV =. 33. For Bi k W + V our particular case is equal to 1, i.e k = 1. 4 Discussions Many specialists, working in the field of heat and mass transfer, cannot believe that during quenching film boiling can be eliminated. However, there are several evidences proving that film boiling is completely absent. They are: Prior to boill cold liquid should be heated to saturation temperature and during this time wall temperature of steel part drops significantly because specific heat of water and water salt solutions is rather high [3]. During quenching in water and warer salt solutions, a double electrical layer is established between wall and liquid which demolishes the film boiling [3, 16]. Ιnvestigations showed that initial heat flux density, moving from wall to liquid, at the very beginning of immersion is a finite value, which in many cases is less than the first critical heat flux density [17, 18, and 19]. Modern investigations provided by CFD modeling show that during quenching in agitated water, temperature of boundary layer remains sometimes below saturation temperature [17]. Discovered shock boiling, probably, increases critical heat flux density and wall superheat [15]. Investigations performed on the basis of the noise control systems provide evedences that film boiling during quenching in water salt solutions of optimal consentration is absent [3, 15]. These facts have been discussed by heat treaters since 1968 working in the heat treating industry [1, 3, and ] and continue attracting specialists. Water salts solutions have been used which prevent corrosion and effectively eliminate film boiling to intensify the processes of quenching. And self-regulated thermal process is utilized to manufacture high strength materials using just plain carbon steels. W ISSN: ISBN:

9 5 Conclusions 1. Duration of tramsient nucleate boiling (self regulated thermal process) is directly proportional to square of the thickness of a body and inverse proportional to thermal diffusivity of a material, depends on the corfiguration of the body, its initial temperature, velocity of a quenchant and thermal properties of the cooling system.. On the basis of discovered regularities high strength materials can be manufactured by means of use high temperature and low temperature thermomechanical treatment combined with the intensive quenching. 3. New methods of quenching save alloying elements and improve environment since plain water is used as a quenchant. References: [1] N.I.Kobasko, Self regulated thermal processes, In a book Heat and Mass Transfer Vol. 8, Minsk, Nauka i Tekhnika, 1968, pp [] Kobasko, N.I., Self- regulated thermal processes during quenching of steels in liquid media, International Journal of Microstructure and Materials Properties (IJMMP), Vol.1, No 1, 5, pp [3] Kobasko, N.I., Steel Quenching in Liquid Media under Pressure, Naukova Dumka, Kiev, 198, 6 p. [4] N.I.Kobasko, What are Duration of Non Stationary Nucleate Boiling and Thermal Equilibrium during Quenching of Steel Parts, Proceedings of the 6 th International Conference on Heat Transfer, Thermal Engineering and Environment (THE 8), Rhodos, Greece, August, 8, pp [5] N.I.Kobasko, M.A.Aronov, J.A.Powell, and G.E.Totten, One more discussion What is Intensive Quenching Process, Journal of ASTM International (JAI), Vol. 6, No. 1, 9, Paper ID JAI1179. [6] N. I. Kobasko, H. I. Zhovnir. Analytical Method of Solving Thermal Problems for Steel Quenching, Visnyk of Academy of Sciences of Ukranian SSR, (No.1), 1979, p [7] V.I.Tolubinskiy, Heat Transfer at Boiling, Kyiv, Naukova Dumka, 198, 316p. [8] H.J. French, The Quenching of Steels, Amer. Society Treat [9] N.I.Kobasko, Steel superstrengthening phenomenon, JAI, Vol., No. 1, 5, Paper ID JAI 184. [1] N.Kobasko, The steel superstrengthening phenomenon, part, International Journal of Microstructure and Materials Properties (IJMMP), Vol. 3, No. 4/5, 8, pp [11] M.L.Bernshtein, Thermomechanical Treatment of Metals and Alloys (in Russian: Termomekhanicheskaya obrabotka metallov i splavov), Moscow, Metallurgy, Vol.1, 1968, 586p. [1].K, Dhir, Boiling heat transfer, Annu. Rev. Fluid Mech., Vol. 3, 1998, pp.: [13] A.N.Tikhonov, V.B.Glasko, Application of Regularization Method in Non-Linear Problems, Jour. of Comp.Math. and Math.Physics, Vol. 5 (No. 3), [14] V.V.Dobryvechir, N.I.Kobasko, E.N.Zotov, W.S.Morhuniuk, Yu.S.Sergeyev, Software IQLab, ITL, Kyiv, Ukraine, [15] N.I.Kobasko, A.A.Moskalenko, G.E.Totten, G.M. Webster, Journal of Materials Engineering and Performance, 1997, 6 (1), p [16]. Ya. I.Frenkel, Kinetic Theory of Liquids, Selected Works (in Russian), Vol. 3, AN USSR, Moscow - Leningrad, [17] P. Krukovskyi, N. Kobasko, and D. Yurchenko, Generalized Equation for Cooling Time Evaluation and Its Verification by CFD Analysis, JAI, Vol. 6, No. 5, 9, Paper ID JAI1176. [18] Sh.E.Guseynov, N.I.Kobasko, On One Nonlinear Mathematical Model for Intensive Steel Quenching and its Analytical Solution in Closed Form, Proc. of the 5th WSEAS Int. Conf. on Heat and Mass Transfer (HMT'8), Acapulco, Mexico, January 5-7, 8. [19] N.I. Kobasko, Sh.E.Guseynov, Initial heat flux densities and duration of non-stationary nucleate boiling during quenching, Proc. of the 5th WSEAS Int. Conf. on Heat and Mass Transfer (HMT'8), Acapulco, Mexico, January 5-7, 8, pp [] N.I.Kobasko, Intensive Steel Quenching Methods, in a Handbook Theory and Technology of Quenching, Liscic, B., Tensi, H.M., and Luty, W., (Eds.), Springer-Verlag, New York, 199, pp ISSN: ISBN: