Receptacles – High-pressure-gas tank
Reexamination Certificate
1997-07-29
2002-12-10
Young, Lee (Department: 3727)
Receptacles
High-pressure-gas tank
C220S608000, C072S056000
Reexamination Certificate
active
06491182
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns pressure vessels, for example, high pressure gas cylinders.
2. Discussion of Prior Art
Such pressure vessels are currently manufactured in aluminium, steel and composite materials. These vessels need to have excellent fracture and fatigue properties. Repeated cycling of pressure inside the vessel causes the vessel to flex, and flexing encourages propagation of any cracks that may appear at the metal surface. Fatigue crack initiation and growth in such vessels occurs at those points where pressure cycling causes maximum flexing (change in strain). This invention concerns treatment of pressure vessels to improve their resistance to fatigue and prevention of premature burst failure.
An established method for improving the fatigue resistance of tubes and cylinders is known as autofrettage. This involves applying a pressure within the bore of the cylinder or tube sufficient to plastically deform the metal at the inner surface. The technique produces compressive residual stresses near the bore, and thus enhances the fatigue resistance of the tube or cylinder subjected to cyclic internal pressure loading. The technique has been applied to continuous lengths of thick walled tubing for at least 70 years.
Autofrettage has also been applied to pressure vessels known as full wrap cylinders, whereby generally a complete thin-walled metal e.g. aluminium inner liner is put into compression. This invention is not concerned with full wrap cylinders of that kind.
U.S. Pat. No. 3,438,113 describes the application of autofrettage to metallic pressure vessels, with the object of increasing the permissible internal pressure loading of the vessel. The invention involves performing autofrettage with the vessel at elevated temperature.
SUMMARY OF THE INVENTION
Fatigue failure of pressure vessels such as high pressure gas cylinders particularly those with flat bottoms normally occurs, not in the cylindrical wall, but at or adjacent the closed end of the vessel. This invention arises from the idea that the autofrettage technique might be used to improve the fatigue performance of such closed-end vessels.
In one aspect the invention provides a method of treating a pressure vessel of aluminium or an Al alloy, having a cylindrical side wall and a closed end and having, when at service pressure, at least one region of peak stress located at an internal or external surface of or adjacent the said closed end,
which method comprises subjecting the inside of the vessel to autofrettage by applying a pressure sufficient to plastically deform the said at least one region, said plastic deformation being confined to less than 25% of the wall thickness,
whereby the treated pressure vessel has the property that, when at elevated pressure, each region of peak stress is located away from any internal or external surface at a distance less than 25% of the wall thickness from said internal or external surface.
A region of peak stress is defined as one where the local stress decreases in all directions with increasing distance from the region.
An effect of this treatment is to reduce the absolute value of the peak stress (when the cylinder is under any pressure above atmospheric and less than the autofrettage pressure) in the region of stress raisers (discussed below), and to move the position of peak stress away from a surface of the vessel. Thus in another aspect the invention provides a pressure vessel of aluminium or an Al alloy having an axis, a cylindrical side wall and a closed end joined to the side wall at a knuckle, and having the property that, when at elevated pressure, a region of peak stress is located, within the material of the vessel away from any internal or external surface at a distance less than 25% of the wall thickness from said internal or external surface, in the knuckle and/or axially of the vessel in the closed end. Preferably the said region of peak stress is located within the material of the vessel at least 0.5 mm away from any internal or external surface.
Surface flaws are tears, pits, creases and are typically up to 1-200 &mgr;m deep. If regions of peak stress coincide with these surface flaws, they tend to propagate. Moving regions of peak stress at leas 0.55 mm into the interior of the material of the vessel should reduce or avoid this problem.
Autofrettage is normally performed at ambient temperature. At temperatures substantially above ambient, the creep properties of aluminium become more pronounced, and this progressively reduces the beneficial effects of autofrettage.
The vessel may be of any aluminium (including alloys where aluminium is the major component) material that can be formed into an appropriate shape and provide sufficient properties such as mechanical strength, toughness and fatigue and corrosion resistance. Among aluminium alloys, those of the 2000, 5000, 6000 and 7000 Series have been used to make pressure vessels and are preferred for this invention. The vessel is preferably formed by extrusion.
Although hot extrusion according to the invention is possible, cold or warm extrusion is preferred as being a lower cost procedure. Cold or warm extrusion may also give rise to an extrudate having a better combination of strength and toughness properties. The preferred technique is backward extrusion. This technique involves the use of a recess, generally cylindrical, with parallel side walls, and a ram to enter the recess, dimensioned to leave a gap between itself and the side walls equal to the desired thickness of theextrudate. An extrusion billet is positioned in the recess. The ram is driven into the billet and effects extrusion of the desired hollow body in a backwards direction. The forward motion of the ram stops at a distance from the bottom of the recess equal to the desired thickness of the base of the extruded hollow body. Extrusion speed, the speed with which the extrudate exits from the recess, is not critical but is typically in the range 50-500 cm/min. Lubrication can substantially reduce the extrusion pressure required.
The initial extrudate is cup-shaped, with a base, parallel side walls and an open top. The top is squared off and heated, typically induction heated to 350-450° C., prior to the formation of a neck by swaging or spinning. The resulting hollow bodyis solution heat treated, quenched, generally into col water, and finally aged.
The requirements of backward extrusion place constraints on the shape of the closed end of the resulting vessel, particularly the base and a knuckle by which the base is joined to the cylindrical side wall. Other production techniques may place other constraints on the geometry of the vessel.
The inventors have performed finite element analysis which shows tht the major stress raisers in such hollow bodies are located in two places: on the inside of the vessel at the knuckle where the base joins the side wall; and on the outside of the vessel at the centre of the base. The relative values of these stress raisers may depend on the cylinder wall and base thicknesses, the dimensions particularly the diameter of the vessel, and the particular base geometry chosen, especially the internal base radius of the knuckle. The method of the invention involves applying a pressure within the vessel sufficient to cause plastic deformation of the metal at one or both of these regions. The applied pressure must obviously not be so great as to burst the vessel, and is preferably less than that required to cause plastic deformation of metal throughout the thickness of the base or knuckle. The applied pressure may be such as not significantly to plastically deform the side wall of the vessel. Alternatively, any plastic deformation of metal in the side wall should be confined to a region at or adjacent the inner surface thereof, e.g. less than 25% and preferably less than 10% of the wall thickness.
The effectiveness of autofrettage in improving fatigue performance does depend on the design of the closed end of the pressure vessel. Thus for example pressure vessels wi
Gao Yi
Hepples Warren
Holroyd Nigel John Henry
Eloshway Niki M.
Luxfer Group Limited
Nixon & Vanderhye P.C.
Young Lee
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