DESIGNING OF ADVANCED AND ORIGINAL AUSTEMPERING PROCESSES BASED ON THERMAL SCIENCE AND ENGINEERING PHYSICS APPROACHES

In the paper, a small concentration of inverse solubility polymers in water and other liquid media is recommended to eliminate film boiling by means of reducing initial heat flux density. Quenching steel parts and tools in a small concentration of water solutions under pressure allows performing austempering process just using cold liquids. Its essence consists in coinciding martensite start temperature MS with the average temperature of self-regulated thermal process during nucleate boiling mode and further immediate transferring steel parts for tempering at the temperature which exceeds value MS. The new technology increases the service life of austempered workpieces by more than two times, saves alloy elements, is suitable for larger metallic components, improves environmental conditions, since instead of melted salts and alkali, plain water and water salt solutions can be used. The new austempering process can be used in forging shops to obtain super-strengthened materials in order to switch from alloy steel to plain carbon steels. And it can be also widely used for obtaining nano-bainitic structure in plain carbon steels resulting in saving alloy elements and improving mechanical characteristics of materials.


Introduction
When quenching steel parts in water or water solutions, local film boiling often can take place resulting in a big distortion and crack formation during hardening. Local film boiling is observed when initial heat flux density in q is comparable with the first critical heat flux density cr1 q . When the initial heat flux density exceeds the critical heat flux density cr1 q , full film boiling takes place. When the initial heat flux density is less than the critical heat flux density cr1 q , there is no film boiling. To optimize quenching processes correctly, one should always compare the initial heat flux density with the first critical heat flux density cr1 q . There is a big problem when calculating the initial heat flux density. The matter is that according to the classical parabolic heat conductivity equation initial heat flux density, as known, is infinity. It means that the full film boiling always should be present during immersion of heated to high temperature steel parts into cold liquid. In reality, initial heat flux density is a finite value which can be calculated solving the hyperbolic heat conductivity equation. Initial heat flux density can be significantly reduced if steel part is quenched in cold polymeric water solutions of inverse solubility. In the paper, a new austempering process is proposed which is performed by quenching steel parts in low concentration polymeric water solutions of inverse solubility under pressure until nucleate boiling is finished. In this case, film boiling is completely prevented. After that, steel parts go to immediate tempering. This core technology of the paper is discussed step by step below.

Initial heat flux densities evaluation
To make possible initial heat flux density calculation, let's start calculating initial heat flux when the developed transient nucleate boiling process begins. Before it, cold liquid layer around the steel part is heated to boiling point and during that time a surface of steel parts cools drastically since the heat capacity of liquid, for example water, is significantly larger as compared with a steel. Assume that regular thermal process of Kondratjev [1,2] establishes at the moment of establishing developed transient nucleate boiling process. To be more specific, let's consider an arbitrary shape of a body, for example, very long parallelepiped with a section shown on Fig. 1 covered by a polymeric layer of thickness δ (Fig. 1). Taking into account existence of polymeric layer, the author [3] proposed an equation for calculating initial heat flux density for any forms of steel parts which is covered with a thin layer of thickness δ. It can be written as:

Materials Sciences
where in q is the initial heat flux density in W/m 2 ; 1 k is the coefficient depending on the form of a steel part; λ is the thermal conductivity of steel in W/mK; coat λ is the thermal conductivity of the coating (polymeric layer); Kn is the dimensionless Kondratjev number; δ is the thickness of the polymeric layer in m; l is the radius or half of the thickness of the plate in m; rg T is the average temperature at the moment of establishing a regular thermal process; S T is the saturation temperature; S is the surface in m 2 ; V is the volume in m 3 ; K is the Kondratjev form factor coefficient in m 2 ( Table 1).
Using equations (1) and (2), engineers can predict local and full film boiling which are not desirable. Some suitable data for calculating initial heat flux densities are provided in Table 1,   Table 2 and Table 3.  12 % water solution of CaCl2 at 20 o C 12 [6] As seen from Table 4, initial heat flux density for cylinders of 20 mm is equal to 3.56 MW/m 2 . Critical heat flux density for water at 20 o C is 5.5 MW/m 2 , i. e. in cr1 qq < . It means that any kind of film boiling is absent. However, for cylindrical specimen 6 mm in diameter, film boiling takes place since in cr1 qq . Film boiling can be eliminated for small specimens if water contains a small amount of inverse solubility polymer which creates a thin insulating layer that reduces initial heat flux. It is known that thermal conductivity of polymers is very low. For example, the thermal conductivity of polystyrene is 0.14 W/mK and thermal conductivity of polytetrafluoroethylene is 0.34 W/mK. Assume that the thermal conductivity of inverse solubility polymer is 0.22 W/mK. Then taking into account that data and using equation (1), one can calculate the initial heat flux density for a small cylinder if the thickness of the layer is known. Assume that it is 0.1 mm. Then using Eq. (2), we have In this case, the initial heat flux density in cr1 qq << ( Table 2, Table 4). It means that there is no possibility for the film boiling. Note that during quenching of steel probe in vegetable oils in many cases film boiling is absent completely. It is due to creation insulating layer at the surface of tested probe caused by oxidation of the vegetable oil. It turned out that inverse solubility polymers can be used to eliminate completely film boiling and by that way intensify quenching processes which decrease significantly distortion and increase the strength of materials. It is a new approach based on engineering physics and thermal science achievements.

Quenching under pressure
Pressure increases the first critical heat flux density and the average temperature of self-regulated thermal process (Fig. 2, Fig. 3) [6,8,9].

Reports on research projects
(2016), «EUREKA: Physics and Engineering» Number 2 Table 6 Convective HTC S versus pressure ( MPa) and temperature of water As one can see from Fig. 3, the surface temperature of cylindrical specimen drops within 1.5 sec almost to boiling point of water (Table 5) and then maintains at that level 9 seconds until convection starts. The higher the convective heat transfer coefficient (HTC) is, the shorter of self-regulated thermal process is and quicker convection starts. Convective HTCs are provided in Table 6. However, during nucleate boiling process surface temperature reduces very slowly ( Table 7). Instead of that, the intensity of the transient nucleate boiling process is very high. HTCs during nucleate boiling reach values 200 KW/m 2 K and more (Fig. 4).  (4) Here nb τ is the duration of the transient nucleate boiling process in seconds; Ω is characteristic of a quenchant when the austenitizing temperature is fixed at 850 o C; F k is coefficient responsible for a geometrical form of a steel part; D is the size (thickness or diameter) in m; a is the thermal diffusivity of a material in m 2 /s [13].
It should be underlined that real heat transfer coefficients (HTCs) provided in Fig. 4  . Here α is HTC; q i s t h e h e a t fl u x d e n s it y ; sf T is the surface temperature; S T is the saturation temperature; m T is the medium temperature. Boiling process is a function of overheat sf S TT − and doesn't depend significantly on underheat Sm TT − . That is why, temperature fields during nucleate boiling processes should be calculated using real HTCs.

Austempering process performed in low concentration of polymers under pressure
Taking into account the absence of film boiling in cold water polymeric solutions of low concentration and possibility of increasing the average temperature of the self − regulated thermal process by adjusting pressure, one can perform the austempering process using cold liquids. The procedure is as follows: 1. A water polymeric solution of low concentration (up to 2 %) is prepared which is used as a quenchant in the apparatus where cooling from high temperature (850 o C) takes place.
2. A sample made of AISI steel W1 with martensite start temperature 200 o C, for example cylinder of 20 mm in diameter, is immersed into 2 % water polymeric solution (2 %) of inverse solubility under pressure 1 MPa and is kept in solution for 10 seconds (Fig. 3 and use Eq. (4)).
3. The sample is quickly taken out of solution and put immediately into furnace for tempering at 200 o C. Duration of tempering is 2 hours.
The technology requires special installation similar to vacuum furnaces. However, for high carbon steels and irons with the martensite start temperature of 100 o C, conventional technological lines can be used for performing austempering at normal atmospheric pressure.

Discussion
Self − regulated thermal process, discovered by the author, was used for developing intensive technologies called IQ-2 and intensive austempering processes [14][15][16][17][18][19][20]. The new technologies were developed and patented in Ukraine and are used in US [7,21]. Especially, excellent results are obtained in the heat treating industry for rolls made of Ductile Irons [16,21,22]. The proposed technologies are improved by creating an insulating layer on the surface of quenched steel parts that decreases initial heat flux density. It eliminates local and full film boiling completely during hardening.
Mathematicians Prof. Buikis, Prof. Guseynov and their colleagues paid serious attention to solving hyperbolic heat conductivity equations with the appropriate boundary conditions responsible for transient nucleate boiling processes [23][24][25][26][27]. Now, engineers and metallurgists are looking forward to obtaining practical data, based on the hyperbolic equation, to be easily used for recipes development for different configuration of steel parts during quenching. Especially, initial heat flux densities are important since they allow to predict possible local film boiling. Local film boiling is the main reason for big distortion. Bearing industry spends a lot of money (millions) to reduce distortion of bearing rings. Proposed engineering physics approach, discussed in the paper, can help minimize distortion of different steel parts after quenching. Developed new austempering processes can reduce radically distortion of steel parts after hardening and increase significantly strength of materials [28].

Reports on research projects
(2016), «EUREKA: Physics and Engineering» Number 2 6. Conclusions 1. For the first time, it is recommended to use small concentrations of inverse solubility polymers in water and other liquid media to eliminate any kind of film boiling by means of reducing initial heat flux density.
2. It is proposed to quench steel parts in small-concentration water solutions under pressure where the advanced austempering process can be performed. The new technology increases the service life of austempered steel parts, saves alloy elements, is suitable for larger steel parts, improves environmental conditions since instead of melted salts and alkali plain water and water salt solutions can be used.
3. The new austempering process can be used in forging shops to obtain super-strengthened materials in order to switch from alloy steel to plain carbon steels. And it can be also widely used for obtaining nano -bainitic structure in plain carbon steels resulting in saving alloy elements and improvement of mechanical characteristics [28].