Patent GB-1083269-A (GB)
Title: A process of, and apparatus for, forming a hollow part
IPC: B21D26/041
Filed: 1964-12-23
Abstract: <PICT:1083269/C6-C7/1> A method of making hollow rotor blade spars for helicopters &c., comprises heating a workpiece in a die and forming it by supplying fluid under pressure to its interior.  In an embodiment, the electrically-heated die comprises ceramic sections (59, Figs. 8 and 9) held by bolts (61) in casings 12, 14 that may be swung apart, mechanically or by hydraulic cylinders 25, 32.  The casings are locked in the closed position, (Fig. 2) by clamps 39 turned about a pivot 54 mechanically or by hydraulic cylinders 49. After positioning the clamp, further operation of cylinders 49 lock pivoted cams 45 between shoulders 43 on the clamps and shoulders 41 on casing 14.  Inert gas and cooling fluid may be supplied to the die sections from pipes (149, 150, Fig. 8) at intervals along the die. Passages (71, 72) connect the pipes to axial ducts (73, 78) having lateral passages (74, 75, 79, 80) leading to axial ducts (63, 65, 67, 69) which are connected to further axial ducts (64, 66, 68, 70).  The ducts (63-70) contain heating coils and are connected to the die chamber by passages (83-90).  The ducts (63-70) are also interconnected by lateral passages (97, 98, 101, 102, Fig. 9) adjacent other passages (95, 96, 99, 100) leading to exhaust passages (91-94).  The workpiece is supported in the die by a tubular support (114, Fig. 12) welded to one end of the work and by a tubular support (112) engaging screwthreads in the other end of the work, the supports passing through the die-end plants 19, 20.  Operation. After the work (111) has been positioned in the die, valves (159,160 Fig. 14) in the inlet system, and other valves in the exhaust system are closed while the ports (83-90 and 103-110) are connected to a vacuum pump (155).  The interior of the work (111) is also connected to the pump.  Nitrogen is then forced at about 100 p.s.i. through ports (83-90) into the die chamber, the exhaust valves remaining closed.  The heating coils (137) are now supplied with current to raise the temperature of the steel workpiece to about 1500 DEG F. in about 30 minutes.  During the following ten minutes, in which the steel transforms to an austenitic structure, the work-temperature falls to about 1430 DEG F. The temperature is measured by thermocouples (133) and accorded at a console, the thermocouples being pressed against the work by springs.  After the work has been maintained at a temperature greater than 1430 DEG F. for about nine minutes, nitrogen is supplied to the interior of the work at about 1000 p.s.i. for about one minute to complete the forming operation. The heating coils are now de-energized and the interior and exterior of the work vented. The nitrogen supply to the interior and exterior of the work is continued to effect cooling to about 500 DEG F. within about twenty seconds and transformation to a martensitic state.  The exhaust valves are now closed, but nitrogen is still supplied through the ports (83, 90).  To temper the work, the heating coils are again energized, raising the work-temperature to about 1000 DEG F. in about thirty minutes, this temperature being maintained for about five minutes and followed by rapid cooling. A perforated diffuser (143) is mounted in the work so that if nitrogen is used as a coolant, this is diffused effectively and passes through the perforations as a gas.