An oxyfuel gas welding process which produces a weld simultaneously over the entire faying sui$aces. The process is used with the application of pressure and without filler metal.

The two variations of pressure gas welding are the closed joint and open joint methods. In the closed joint method, the clean faces of the parts to be joined are butted together under moderate pressure and heated by gas flames until a predetermined upsetting of the joint occurs. In the open joint method, the faces to be joined are individually heated by the gas flames to the melting temperature and then brought into contact for upsetting. Both methods are easily adapted to mechanized operation.

Pressure gas welding can be used for welding low-and high-carbon steels, low- and high-alloy steels, and several nonferrous metals and alloys. In the closed joint method, since the metal along the interface does not reach the melting point, the mode of welding is different from that of fusion welding. In general, welding takes place by the action of grain growth, diffusion, and grain coalescence along the interface under

the impetus of high temperature (about 1200°C [2200°F] for low-carbon steel) and upsetting pressure.

The welds are characterized by a smooth surfaced bulge or upset, as shown in Figure P-19, and by the general absence of cast metal at the weld line. In the open joint method, the joint faces are melted, but molten metal is squeezed from the interface to form flash when the joint is upset. These welds resemble flash welds in general appearance.

Applications

Pressure gas welding has been successfully applied to plain carbon steel, low-alloy and high-alloy steels, and to several nonferrous metals, including nickel- copper, nickel-chromium, and copper-silicon alloys. It has been very useful for joining dissimilar metals. In general, pressure gas welding has a minimum effect on the mechanical and physical properties of the base metals. Pressure gas welding has been used with low alloy and high-carbon steels for fabricating assemblies subject to high service stresses.

While pressure gas welding still has applications in the welding of railroad rails, this process has been largely superseded by flash welding. Automatic welding of pipe, a former application, is accomplished using automatic gas metal arc welding. The basic elements of the pressure gas welding assisted in the development of similar processes, such as flash and friction welding that use other sources of energy.

Equipment

Machines. The apparatus for pressure gas welding is comprised of the following:

(1) Equipment for applying upsetting force

(2) Suitable heating torches and tips designed to provide uniform and controlled heating of the weld zone

(3) Necessary indicating and measuring devices for regulating the process during welding

PRESSURE GAUGE

Pressure gauges are used to measure gas or liquid pressure. Most gauges are the Bourdon tube type, which consist of a flattened metal tube bent into a circular shape, sealed at one end and connected to the fluid to be measured at the other end. The sealed end is linked to a mechanism which moves the indicating needle. When pressure is applied to the tube it tends to straighten slightly, moving the needle upscale. The actuating mechanisms are usually mounted in a brass or steel cup or housing, which also supports the dial and cover glass. These gauges are named for the inventor, Bourdon.

Figure P-20 shows a pressure gauge with the front cover glass and dial removed. This particular one is known as an independent movement type, in which the entire mechanism is mounted independently of the case on the socket (S), which is held securely in place on the case by two holding screws (9). This arrangement protects the mechanism from damage while handling and connecting to a system.

The nomenclature of the constituent parts is as follows:

(1) Case, (2)Tube, (3) End Piece, (4) Link, (5)Sector, (6) Pinion Post, or Pinion; (7) Movement: collectively parts 5 and 6, including front and back movement plates, spacing bars and screws; (8) Socket or Connection, (9) Socket Screws.
Actuating Principle. Pressure admitted through socket (8) into tube (2) causes the tube to straighten slightly. The motion of the free end (3) is connected by link (4) to sector (5),which is engaged with pinion (6) causing it to rotate. The indicator hand, or needle, is mounted on the pinion shaft and indicates the pressure. Travel of the hand can be calibrated by adjusting the position of the lower link screw in the slotted sector arm.

Tube. The tube material, cross section dimensions, and wall thickness depend on the diameter of the gauge case and fluid pressure range to be measured. Generally, the wall thickness increases as the pressure increases. The dimensions of the tube must be such that the material is never stressed beyond its elastic limit at the maximum operating pressure. If this happened, the hand of the dial would not return to the zero position when the pressure is released. General practice is to design and calibrate a gauge for double its

maximum average working pressure. For example, a gauge intended for a maximum working pressure of 690 kPa (100 psi) would be designed and calibrated for 1380 kPa (200 psi). This design principle will avoid over-stressing a gauge in service.

Standard Equipment

The following types of gauges are normally employed with oxyfuel welding and cutting equipment and for inert gas welding and cutting:

21 MPa (3000 psi) for oxygen, nitrogen, hydrogen, argon, helium 345 kPa (50 psi) for low-pressure oxygen, argon, helium

345 kPa (50 psi) for acetylene

1380 or 2070 kPa (200 or 300 psi) for cutting oxygen

Care of Gauges

Gauges should be given the care afforded to any precision instrument.

(1) To avoid the possibility of an explosion, never permit oil to get into any oxygen apparatus.

(2) Never apply pressures to gauges suddenly. Open valves on cylinders slowly.

(3) Do not apply pressures to the full scale of the dial.

(4) Do not apply full tank pressures with the regulating screw on reducing valves which are screwed all the way in. Increase the pressure on low-pressure gauges slowly.

(5)When installing gauges to apparatus, do not attach a wrench on the pipe threads; use the square above the pipe threads.

(6) Handle gauges with care. The mechanism can be damaged or destroyed if they are bumped, jammed or allowed to fall on the floor.

(7) A gauge that is giving incorrect readings can be a hazard. Do not try to repair a defective gauge. Take it to an authorized gauge repair shop.

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