Method of Brazing Tungsten Carbide

This invention is concerned with a brazing alloy for brazing tungsten carbide.

A brazing alloy in accordance with this invention has the following composition, in weight percent: 5-30 gold; 15-35 palladium; 10-30 nickel; 20-48 copper; 5-25 manganese. It has a solidus temperature above about 950.degree. C. and a liquidus temperature above about 1018.degree. C. The alloy also has superior resistance to high temperature creep.

U.S. Pat. No. 4,527,998 discloses a brazing alloy having a composition, in weight percent, of 18-39.5 gold, 2.5-10.5 palladium, 3.5-14.5 nickel, 7.5-9 manganese, balance copper, and which is used for brazing carbides. However, the alloys disclosed therein do not have adequate resistance to high temperature creep.

Following are examples of tungsten carbide parts that were brazed to other parts in accordance with this invention. In these examples, compositions are expressed in weight percent.

A 6% cobalt content cemented tungsten carbide part is brazed to a similar part using 4 mil alloy with a composition of 25% Au, 15% Pd, 18% Ni, 31% Cu, 11% Mn at 1075.degree. C. by 10 minutes hold under 10.sup.-5 torr vacuum.

The metallograph of the polished brazed section shows excellent dense brazed layer. The SEM (Scanning Electron Metallograph) at 5000.times.shows the penetration of the filler metal into the cemented tungsten carbide and altering the joint mechanical property. The brazed material was subjected to 20,000 lb force or about 60,000 psi shear force without failure.

Six percent cobalt cemented tungsten carbide, 1/2".times.1/2".times.1/4" was brazed to 4340 steel, 1/2".times.1/2.times.3" long using 4 mil foil with a composition, 15% Au, 20% Pd, 13% Ni, 41% Cu, 11% Mn. The assembly was brazed at 1075.degree. C. by 10 minutes hold under 10.sup.-5 torr vacuum. The brazed assembly was subjected to 20,000 pound force or about 70,000 psi shear without failure. At 20,000 pound force, the 4340 steel distorted without the tungsten carbide being sheared off. The polished section showed that about 2 mil of filler metal layer remained along the cemented carbide steel interface, with most of the alloy Penetrating the 4340 steel. The alloy also showed that it attached the cemented tungsten carbide along the grain boundary and penetrated about 25 to 30 micrometers (1.2 mils).

An alloy with a composition of 5% Au, 30% Pd, 12% Ni, 40% Cu, 13% Mn was cast and rolled to 2 mil foil.

A 2 mil foil was placed between two cemented tungsten carbide parts, each 1/2".times.1/2".times.1/4", and vacuum brazed at 1075.degree. C. by 10 minutes hold under 10.sup.-5 torr. An excellent strong braze was obtained that did not fail under about 60,000 psi shear loading.

A 4 mil foil of 25% Au, 15% Pd, 18% Ni, 31% Cu, 11% Mn alloy was placed between 1".times.1".times.1/8" cemented tungsten carbide and 1".times.1".times.1/8" molybdenum, and also between molybdenum and tool steel 1".times.1".times.1/2". The assembly was brazed at 1075.degree. C. by 5 minutes hold under 10.sup.-5 torr vacuum. The brazed assembly was cooled to 500.degree. C. under 2.degree. C./minute cooling rate, and then furnace cooled. The tungsten carbide did not crack.

The low thermal expansion cemented tungsten carbide of above size brazed to high thermal expansion steel will generally crack on cooling, but by placing an interlayer material with similar thermal expansion material with much lower yield strength resulted in reducing the thermally induced stress at room temperature.

Using a 4 mil thick alloy of 25% Au, 15% Pd, 18% Ni, 31% Cu, 11% Mn, a cemented tungsten carbide is brazed to 4340 steel at 1075.degree. C. by 10 minutes hold under 10.sup.-5 torr vacuum. A 20,000 pound force or about 60,000 psi shear was applied against the brazed joint without failure.