Electric heating – Metal heating – Of cylinders
Reexamination Certificate
1999-09-16
2001-04-24
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
Of cylinders
Reexamination Certificate
active
06222148
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of machine building, particularly to a method for producing a multilayer thin-walled bellows of stainless steel to be welded with fittings and intended for operation under the extreme conditions.
BACKGROUND OF THE INVENTION
Multilayer thin-walled bellows are widely used in different branches of engineering, for example in the aircraft building, engine building, and oil industry, when it is necessary to secure the movable jointing of pipelines for compensating their relative displacement.
Stainless steel is the most acceptable material for producing such bellows, because it secures their operation under the conditions of high temperature and pressure, corrosive media and vibration.
A method is known for producing a multilayer thin-walled bellows of stainless steel, including manufacturing round billets by their multiple drawing through matrices using punches with a diameter variation, packing the round billets of given diameter into a multilayer bank, its corrugation into a bellows with subsequent operations of surface deforming and heat treatment—subrecrystallization annealing at a temperature of 680±10° C. (the USSR Inventor's Certificate N
o
1292870, B21D15/00, 1987).
The operation of drawing each billet before packing into the bank allows to increase wall strength, and the heat treatment after the corrugation allows relieving of residual stress in the metal. However, the billet drawing is a rather labour consuming operation. It reduces abruptly the steel ductility and worsens its structure. This fact may cause the appearance of cracks in the bellows during the corrugation, and thus decrease its serviceability under the extreme operating conditions. Besides, the absence of tightness test for external and internal bellows layers after the corrugation may lead to its destruction during the operation.
Another method is known also for producing a multilayer thin-walled bellows of stainless steel, that includes producing thin round billets, rolled up of the sheets and lap or butt welded, their corrugating using a press with a bellows forming and its tightness test by immersing into the water (K. N. Burtsev “Metal Bellows”, Mashgiz, 1963, pp. 8-11).
The above described method is less labour consuming as compared with the previous one and allows to keep the chemical composition and the structure of the initial material during the production process. However, the round billet corrugation just after their manufacturing by welding the sheets may cause the formation of cracks both in the welds and in the steel because of their low ductility and strength. Besides, the bellows tightness testing by immersing into the water is more labour consuming and not easily producible.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method for producing a multilayer thin-walled welded bellows of stainless steel with improved operating characteristics.
From the engineering point of view, the present invention results in the operation of bellows, produced by this method, without destruction under extreme conditions during a long time period at a temperature of up to 400° C., and the growth of finished product yield due to the tightness testing of the internal and external layers of the bellows.
The above object is achieved by a method for producing a multilayer thin-walled bellows of stainless steel, comprising manufacturing round billets by welding sheets of stainless steel, packing the round billets into a multilayer bank, corrugating the bank with the bellows formation and its tightness testing. According to the invention the manufacturing round billets is made of preliminary cut sheets of stainless steel of given dimensions by their electric arc pulsed gas-shielded welding, the packed multilayer bank is welded from two sides over the end faces and subjected to a heat treatment by its heating in the shielding medium up to the temperature of 1000-1130° C. with holding at this temperature during 20-45 minutes and subsequent cooling.
A pulsed argon-arc welding may be used as the electric arc pulsed gas-shielded welding.
An air medium with the rarefaction of 1·10
−2
-1·10
−3
mm of the mercury column may be used as the shielding medium during the heating.
The multilayer bellows tightness test may be made by pumping an inert gas of high pressure between layers of the multilayer bellows and checking for possible loss of tightness from the side of internal and external surfaces of the multilayer bellows.
A gaseous mixture containing helium may be used as the inert gas, and the tightness test may be made by a helium leak detector.
The growth of ductility for the multilayer bank material is achieved by its heat treatment before corrugating, and sufficient strength is secured by obtaining the welds after the pulsed welding, the strength of which is equal to the strength of the main material.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, the method is realized in the following way.
The stainless steel sheets are cut to have given dimensions (thickness, width and length). Then they are rolled up into round billets and welded by electric arc pulsed welding. An electric arc pulsed gas-shielded welding may be used as such.
Depending on the required diameter of each billet, a corresponding number of welds is obtained. While using the pulsed argon-arc welding, the welds are obtained with strength equal to the strength of the main material.
From seven to twelve round billets are manufactured for one bellows in such a way. The number of round billets for the bellows depends on the pressure of operating environment during its use. The manufactured round billets are packed into a multilayer bank, the bank is welded from two sides over its end faces and placed into a vacuum furnace, in which it is heat-treated. The bank is heated in the furnace up to the temperature of 1000-1130° C. and held at this temperature during 20-45 minutes depending on the billet dimensions and the thickness of its walls. An air medium with the rarefaction of 1·10
−2
-1·10
−3
mm of the mercury column is used as the shielding medium, but an inert gas, argon for example, may be used also. The cooling is made in the furnace too.
The choice of the heat treatment modes is stipulated by the necessity of obtaining a uniform structure in steel under the indicated temperature; this allows increasing its ductility and preventing the steel component burning fast during the process of holding.
The heating up to the temperature below 1000° C. does not secure the obtaining of a uniform material structure. The heating up to the temperature above 1130° C. causes the grain growth and the loss of material ductility, correspondingly.
The holding period of more than 45 minutes may promote the appearance of separate strengthening phases along the grain boundaries; this will lead to the reduction of material ductility. The holding period of less than 20 minutes will not secure the required uniformity of the material structure.
After the heat treatment, the multilayer bank is subjected to corrugating using a press with the corrugations forming as a result. Then, the corrugations of external and internal layers of the bellows walls are tested for interlayer tightness. It is a pressure test by feeding an inert gas containing helium into the internal space of the bellows. Then it is tested for leakage from the external side and from the side of internal space, correspondingly. Interlayer leakage is tested by a helium leak detector. Any noticeable defect in the metal is detected using helium.
The presence of operation on testing the tightness of the bellows layers allows preventing its destruction during the operation.
The bellows produced by the above-presented method is welded to fittings and subjected to the hydrostatic strength test.
REFERENCES:
patent: 3405228 (1968-10-01), Polizzano
patent: 3873799 (1975-03-01), Scheffler et al.
patent: 1292870 (1987-02-01), None
Burtsev, K.N., “METAL BELLOWS,” Mashgiz, State Scientific
Avsenjuk Tatiana Mikhailovna
Bobkov Vladimir Ilich
Chvanov Vladimir Konstantinovich
Derkach Gennady Grigorievich
Katorgin Boris Ivanovich
Oykrutoe Aktsioneroe Obschestvo “Nauchno-Proizvodstvennoe Obiedi
Seed IP Law Group
Shaw Clifford C.
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