Intensive Quenching of Steel Parts: Equipment and Method

Transcription

1 Intensive Quenching of Steel Parts: Equipment and Method N. KOBASKO, M. ARONOV, J. POWELL, J.VANAS IQ Technologies, Inc., Akron, USA Euclid Heat Treating Co, Euclid, USA Abstract: - The paper discusses an intensive quenching (IQ) method and IQ equipment. It is shown that film boiling is absent and a main mode of heat transfer is nucleate boiling during intensive quenching of large steel parts in electrolytes (water salt and alkaline solutions). In such condition during immersion of steel parts into water solutions, the heat transfer coefficient is extremely high and the part surface temperature drops to a quenchant saturation temperature within 1 or 2 seconds. It is recommended that when quenching intensively low and medium carbon steels the steel chemical composition should be tailored to provide an optimal quenched layer after complete cooling. Due to providing the optimal quenched layer, very high residual surface compressive stresses form. Since the martensite finish temperature is about 100 o C for low carbon and medium carbon steels, significant cooling rate within the martensite range is achieved. Due to this fact, the superstrengthening of a material in the part surface layers is observed. Both the high residual surface compressive stresses and the superstrengthening of material increase service life of steel parts. It is shown that water salt solution Na 2 (NO 3 ) of optimal concentration prevents corrosion of the parts being quenched and IQ equipment. Key Words: - Intensive quenching, Nucleate boiling, Optimal quenched layer, Service life, Corrosion, Environment. 1 Introduction To develop correctly intensive quenching process for steel parts, it is necessary to take into account a dependence of the martensite start temperature and martensite finish temperature on carbon content in steel (see Fig. 1). Fig. 1 Martensite start temperature M S and martensite finish temperature M F versus carbon content in steel. As known, a self-regulated thermal process takes place during quenching of steel parts. The selfregulated thermal process is characterized by a very slow rate of the surface temperature change. During this process, for example, surface temperature changes from 110 o C to 104 o C and core temperature changes from 850 o C to 200 o C [1]. During quenching of high carbon steel in water, the transformation of austenite into martensite can be delayed if the martensite start temperature is below 100 o C. The martensite transformation in this case starts when natural or forced convection begins. During natural convection, the cooling rate within the martensite range is very low. Because of this neither the superstrengthening effect nor high surface compressive stresses are observed [1]. The forced boiling didn t impact the phase transformation. The final result is like after conventional cooling in oil. In the contrary, when quenching low carbon steels, having martensite finish temperature about 100 o C, very intensive quenching will be produced within the martensite range since during cooling in water salt solution film boiling is absent and M F is above saturation temperature. Let s consider these processes in ISSN: ISBN:

2 more detail on the basis of inverse heat conduction problem solution. Table 1 Time required for the surface of steel spheres of different sizes to cool to different temperatures when quenched from 875 o C in 5% water solution of NaOH at 20 o C and moving at m/s [2]. Size, Time, inches seconds (m) 4.75 (06) Average 7.15 (0.1816) Average 700 o C 600 o C 500 o C 400 o C 300 o C 250 o C 200 o C 150 o C (0.2858) Average Table 2 Thermal conductivity of supercooled austenite versus temperature T, ºC W λ, mk W λ, mk Table 3 Thermal diffusivity a of supercooled austenite versus temperature T, ºC m a 10 6, s a 10 6, 2 m s Note: λ and a at 500 С mean average values for the range of 100 С С (analogously for other temperatures). Data provided in Table 1, Table 2, and Table 3 are used at solving inverse problem. ISSN: ISBN:

3 2 Analyzing of quenching processes on the basis of inverse heat conduction problem solution To solve an inverse problem and to get accurate results of calculations, very accurate experimental data are needed. Previously we use our experimental data, however in this paper we will use accurate experiments of French [2]. Many authors checked these data and they are worthy to analyze (see Table 1). Table 1 provides surface temperature versus time for steel spheres, which were quenched in 5% water sodium hydroxide solutions. The steel spheres were heated to 875 o C and quenched in the solutions at 20 o C. Software IQLab was used for solving inverse heat conduction problem (IP). The IQLab software comprises the latest achievements in computational science including Tikhonov method [3, 4]. Tables 1, 2, and 3 provide the surface temperature and thermal properties of steel, which were used to solve inverse problem (IP). To restore surface temperature during a self regulated thermal process after 150 o C, equations (1), (2), and (3) were used [5] which allow calculating a duration of the self regulated thermal process and overheats at the beginning and at the end of the process [6]: where ϑ = Tsf TS is overheat; m = 10/3; β is between 3 and After a reconstruction of the surface temperature (see Fig. 2), the core temperature was calculated using the boundary conditions of the first kind. Having these data, heat flux densities were calculated by solving inverse heat conduction problem. Results of calculations are presented in Fig. 3. It should be noted that the boiling process in water starts if the heat flux density is within MW/m 2 and overheat is about 4 o C. That is true when the part surface is heated from the room temperature to the saturation temperature and over. During quenching these values could be less. ϑi K τ = Ω + b ln, (1) ϑii a where b=3.21; 1 2λ( ϑ0 ϑi ) ϑ I = ; (2) β R ϑ II [ ( + β )] 0.3 = conv ϑii ϑuh (3) These equations were obtained analytically assuming that the heat transfer coefficient is a function of the heat flux density, i.e. 0.7 nb = βq (4) or m m 1 nb = β ϑ, (5) Fig. 2 Surface and core temperature versus time during transient nucleate boiling when quenching spheres mm in diameter in 5% water sodium hydroxide solution: a) is for β = 3; b) is for β = It was noticed that the duration of transient nucleate boiling changes from 60 s to 68 s when β changes from 3 to ISSN: ISBN:

4 Heat flux densities during quenching of the sphere of mm in diameter are presented in Fig. 3. at 20 o C. It means that no film boiling will take place under these conditions. Experiments of French support this fact (see Table 1). Now let s see how heat transfer coefficients during quenching are changing. It should be noted that there are two kinds of heat transfer coefficients: a real heat transfer coefficient, which is calculated from equation (6) q nb = (6) T T sf S and an effective heat transfer coefficient, which is calculated from equation (7). q ef =. (7) T T sf m Historically heat treaters are using in the practice equation (7). Fig. 3 Heat flux densities versus time during quenching of the steel sphere in 5% water sodium hydroxide solution at 20 o C: a) is the heat flux when β = 3; b) is heat flux when β = Note that values β = 3 and β = 7.36 were used only for overheat calculations and reconstruction of the surface temperature, but not for equation (4) or (5). We think that during quenching these values significantly differ from empirical data, which were received for boiling at heating small elements in liquid to produce boiling processes. Such difference is explained by the existence of shock boiling and by an extremely high gradient of the temperature. In Fig. 4, the heat flux density is provided as a function of the part surface temperature. As we can see from Fig. 4, the heat flux is equal to zero at the beginning of the quench. It increases further to maximum value of about 13 MW/m 2, which is less than the first critical heat flux density for 5% water sodium hydroxide solution Fig. 4 Heat flux densities versus surface temperature during quenching the steel sphere (120,65 mm in diameter) in 5% water sodium hydroxide solution at 20 o C: a) is heat flux when β = 3; : b) is heat flux when β = ISSN: ISBN:

5 Fig. 5 and Fig. 6 present heat transfer coefficients versus time and surface temperature for sphere quenched in 5% water sodium hydroxide solution at 20 o C. Fig. 5 Real and effective heat transfer coefficients versus time for steel sphere mm in diameter quenched in 5% water sodium hydroxide solution at 20 o C from 875 o C. and there is a moment of time when they reach maximum value and then decrease very slowly with time (see Fig. 5). 3 Optimal quenched equipment to provide it layer and An optimal quenched layer provides an optimal residual stress distribution after intensive quenching: high compressive residual stresses at the surface and small residual stresses at the core of steel parts. Such distribution of residual stresses is achieved if ratio (8) is fulfilled, i.e. DI Dopt = const., (8) 0.5 abτ DI = M Ω + lnθ, (9) where DI is an ideal critical diameter; a is an average value of thermal diffusivity of steel; b is a constant which has the following numb ers for cylinders and spheres: and 39.48; τ M is a cooling time from the initial temperature to the martensite start temperature which provides 50% martensite at the core (see Fig. 7); Ω = for a cylinder and Ω = for a sphere; θ is dimensionless temperature; the constant in Eq.(8) varies between 0.3 and 0.5; Dopt is a real diameter of the steel part. Fig. 6 Real and effective heat transfer coefficients versus surface temperature for steel sphere in diameter quenched in 5% water sodium hydroxide solution at 20 o C from 875 o C: a) are real and effective heat transfer coefficients for β = 3; b) are heat transfer coefficients for β = As we can see from Fig. 6, heat transfer coefficients increase with the decrease of the wall temperature Fig. 7 CCT diagram of AISI 1045 steel to evaluate cooling time τ M ISSN: ISBN:

6 To provide an optimal quenched layer in steel patrs after quenching and the effect of superstrengthening, three companies in the State of Ohio built production equipment which is shown in Fig cm 91cm 182cm ( ) integral quench atmosphere furnace equipped with an IQ water quench tank of 41.6 m 3 (11,000 gallons). The furnace is equipped with an intermediate chamber connecting the heating chamber with the IQ tank to ensure that quenchant vapor does not contaminate the furnace atmosphere. As a quenchant the water solution of Na 2 (NO 3 ) is used. The described above method of quenching and equipment make steel parts cheaper and stronger and environment cleaner. Conclusions 1. It has been established that the heat transfer coefficients during transient nucleate boiling can achieve a value of 200,000 W/(m 2 K) and greater. 2. During transient nucleate boiling, the surface temperature changes insignificantly and maintains at a level of the saturation temperature. 3. Method of quenching for low and medium carbon steels is proposed which provides high residual compressive stresses at the surface of steel parts due to the optimal quenched layer and very intensive quenching within the martensite range. 4. Equipment for intensive quenching of steel parts has been manufactured and implemented into production. Fig. 8 Equipment for intensive quenching of steel parts: Fig. 8 a) shows a production IQ system for batch quenching consisting of a 6,000-gallon stand-alone IQ water tank equipped with four props and a 36 x36 x48 atmosphere furnace. The IQ system shown in Fig. 8b is capable of quenching loads of up to 3,000 lb. Fig. 8b presents a production IQ system for batch quenching consisting of a 1,900-gallon standalone IQ water tank equipped with one prop and Ø23 x23 atmosphere pit furnace and load transfer mechanism. The IQ system is capable of quenching the load of up to 800 lb. Fig.8c presents a picture of the production References: [1] N.I.Kobasko, Steel superstrengthening phenomenon, JAI, Vol. 2, No. 1, [2] H.J. French, The Quenching of Steels, Amer. Society Treat [3] 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), [4] V.V.Dobryvechir, N.I.Kobasko, E.N.Zotov, W.S.Morhuniuk, Yu.S.Sergeyev, Software IQLab, ITL, Kyiv, Ukraine, [5] 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, 1992, pp [6] Kobasko, N.I., Self- regulated thermal processes during quenching of steels in liquid media, IJMMP, Vol.1, No 1, 2005, pp ISSN: ISBN: