Cooling installation with liquid nitrogen for the cryogenic turning of austenitic stainless steels.

Hamat, Codruta Oana ; Popovici, Gheorghe ; Coman, Liviu 等

1. INTRODUCTION

The cooling installation with liquid nitrogen for the cryogenic turning processing (Resita, Romania variant) is an installation in modular built. The most important issue in building the installation was to bring the liquid nitrogen to the cutting area under liquid form and not as vapours, whose cooling rate is one dimension-order smaller than the liquid nitrogen.

2. AREA OF APPLICATION

The present installation is used to the cryogenic turning of austenitic stainless steels (18-8). By construction, the liquid nitrogen cooling installation can be used for the other basic procedures in metal cutting, i.e. drilling and planning.

3. CONSTRUCTIVE DESCRIPTION OF THE INSTALLATION

From the constructive viewpoint, the designed liquid nitrogen cooling installation (IRC) is made of 4 modules:

MS--the module of liquid nitrogen stocking, with the role to create a super-pressure in the Dewar-type cooling vessel and the transport of the liquid through the installation;

MA--the feeding module, assuring the transfer of the liquid nitrogen form the stocking vessel to the cutting area;

MP--the pressurisation module, with the role to locate and concentrate the liquid nitrogen on the processed part and / or cutting tool;

MR--the module for the adjustment of the liquid nitrogen flow, in accordance with the temperature imposed in the cutting area (Decker, 2004).

A modular view of the liquid nitrogen cooling installation is shown in Figure 1.

4. FUNCTIONAL DESCRIPTION OF THE INSTALLATION

The rotating adapter 19 is conceived so that the liquid nitrogen under pressure flow from the central feeding pipe into a distribution chamber, from where--through radial orifices--it must arrive at the pressurisation nozzle 20.

The orientation of the liquid jet under a certain angle compared to the tangent to the trajectory of the cutting movement (Gornic, 2007) is made with the rotating adapter 19, and the concentration of the liquid is assured through the pressurising nozzle 20.

[FIGURE 1 OMITTED]

5. RESEARCH

The efficiency of the liquid nitrogen cooling is shown by the manner it is guided through the cooling area to the cutting area. The form of the nozzle is given according to the type of procedure. When turning, circular nozzles are recommended, with the spraying holes located on the diameter.

The angle of the nozzle against the cutting trajectory and the width of the contact area are more important than the distance between the nozzle and cutting edge. (Tapalaga, 1998).

The placement angle must be between 15 and 20[degrees] towards the processed part, compared to the tangent to the cutting trajectory. The basic requirement is that the nozzle follows the processed part, and the spraying is in the sense of the part or tool rotation. Moreover, the difference between the main cutting speed (vac) and the liquid jet speed (vm) must be small (Badut, 2008):

[v.sub.c] - [v.sub.m] [right arrow] 0 (1)

Only then the liquid is "synchronously drawn" in the rotation sense to the contact area and the cooling is correct (Figure 2).

[FIGURE 2 OMITTED]

Besides, one aimed at establishing a regime of efficient cooling by determining the working pressure and the minimum nitrogen flow for a certain cooling degree.

Experimentally one found that the liquid nitrogen starts to flow to the cutting area at the pressure [p.sub.a] = 0.05 MPa (0.5 bar), but the flow is continuous only from the pressure of [p.sub.min] = 0.125 MPa (1.25 bar) and up to the pressure of [p.sub.max] = 0.35 (3.5 bar), when the turbulence effect occurs.

The recommended values of the flowing rate of the nitrogen jet, depending on the main cutting speed are given in Table 1 (Muhs, 2007).

The working pressure, depending on the flowing rate imposed for the nitrogen jet is obtained with the relation:

[p.sub.m] = 0.07 x [[rho].sub.a] x [v.sup.2.sub.m] [MPa] (2)

Where:

[[rho].sub.a] [kg / [dm.sup.3]]--the density of the liquid nitrogen;

[v.sub.m] [m/s]--the flowing rate.

The "pressure--flowing rate" diagram is shown in Figure 3. The liquid nitrogen flow (debit) is given by the relation:

[Q.sub.v] = 0.05 x [v.sub.m] x [d.sup.2.sub.0] [1/min] (3)

Where:

[v.sub.m] [m/s]--the flowing rate;

[d.sub.0] [mm]--the nozzle diameter.

The "flow--nozzle diameter" diagram is shown in Figure 4.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

6. CONCLUSION

The present installation, compared to other installations known in the field, has the following advantages:

* constructive simplicity and easy operation, as the compression of the liquid nitrogen can be done either from a cylinder, or directly from the supply network, but if the latter is the case, the air must be filtered and dried;

* the nitrogen flow (debit) can be modified within limited ranges, by the fine adjustment performed by the electro-check valve;

* the cryogenic cooling is possible in all three variants (cooling of the processed part, of the tool or of the two simultaneously), by means of the three-way distributor (handle position: 0[degrees], 45[degrees] and 90[degrees]);

* nevertheless, by construction, the installation has also a drawback, as it does not provide a perfect thermal insulation for the transfer of liquid nitrogen through copper pipes and flexible tubes for cryogenic liquids.

7. REFERENCES

Badut, M. (2008). CAD-CAM-CAE for products. Techniques and Technology, issue 3 (35), pp. 72-73, ISSN 1453 8423, Bucharest

Decker, K.-H. (2004). Maschineelemente-Aufgaben, (Machine Design), Carl Hansen, Verlag, ISBN 3-446-21525-5, Munchen

Gornic, C. (2007). What means to know how to design? Techniques and Technology, issue 5 (35), pp. 38-39, ISSN 1453 8423, Bucharest

Muhs, D. (2007). Roloff/Matek. Maschinenelemente (Machines Design), Auflage. Viewegs Fachbucher der Technik Design, ISBN 3-528-54482-1, Augsburg

Tapalaga, I. (1998). Criogenia in construcpa de masini. (Cryogenics in Machines Design), Dacia Publishing House, No. 2954-332, Cluj-Napoca

Tab. 1. Recommended values of the flowing rate at the nitrogen
jet depending of the main cutting speed

Cutting
speed
[V.sub.c] [m/min] <75 75...100

Flowing rate [V.sub.m] [m/s] 1.5-1.75 1.75-2.0

Cutting
speed
[V.sub.c] [m/min] 100...125 >125

Flowing rate [V.sub.m] [m/s] 2.0-2.25 2.25-2.5