7-22. NICKEL AND MONEL WELDING
a. General. Nickel is a hard, malleable, ductile metal. Nickel and its alloys are commonly used when corrosion resistance is required. Nickel and nickel alloys such as Monel can, in general, be welded by metal-arc and gas welding methods. Some nickel alloys are more difficult to weld due to different compositions. The operator should make trial welds with reverse polarity at several current values and select the one best suited for the work. Generally, the oxyacetylene welding methods are preferred for smaller plates. However, small plates can be welded by the metal-arc and carbon-arc processes, and large plates are most satisfactorily joined, especially if the plate is nickel clad steel.
When welding, the nickel alloys can be treated much in the same manner as austenitic stainless steels with a few exceptions. These exceptions are:
(1) The nickel alloys will acquire a surface or coating which melts at a temperature approximately 1000°F (538°C) above the melting point of the base metal.
(2) The nickel alloys are susceptible to embrittlement at welding temperatures by lead, sulfur, phosphorus, and some low-temperature metals and alloys.
(3) Weld penetration is less than expected with other metals.
When compensation is made for these three factors, the welding procedures used for the nickel alloys can he the same as those used for stainless steel. This is because the melting point, the coefficient of thermal expansion, and the thermal conductivity are similar to austenitic stainless steel.
It is necessary that each of these precautions be considered. The surface oxide should be completely removed from the joint area by grinding, abrasive blasting, machining, or by chemical means. When chemical etches are used, they must be completely removed by rinsing prior to welding. The oxide which melts at temperatures above the melting point of the base metal may enter the weld as a foreign material, or impurity, and will greatly reduce the strength and ductility of the weld. The problem of embrittlement at welding temperatures also means that the weld surface must be absolutely clean. Paints, crayon markings, grease, oil, machining lubricants, and cutting oils may all contain the ingredients which will cause embrittlement. They must be completely removed for the weld area to avoid embrittlement. It is necessary to increase the opening of groove angles and to provide adequate root openings when full-penetration welds are used. The bevel or groove angles should be increased to approximately 40 percent over those used for carbon steel.
b. Joint Design. Butt joints are preferred but corner and lap joints can be effectively welded. Beveling is not required on plates 1/16 to 1/8 in. (1.6 to 3.2 mm) thick. With thicker materials, a bevel angle of 35 to 37-1/2 degrees should be made. When welding lap joints, the weld should be made entirely with nickel electrodes if water or air tightness is required.
c. Welding Techniques.
(1) Clean all surfaces to be welded either mechanically by machine, sand-blasting, grinding, or with abrasive cloth; or chemically by pickling.
(2) Plates having U or V joints should be assembled, and if nickel clad steel, should be tacked on the steel side to prevent warping and distortion. After it is determined that the joint is even and flat, complete the weld on the steel side. Chip out and clean the nickel side and weld. If the base metal on both sides is nickel, clean out the groove on the unwelded side prior to beginning the weld on that side.
(3) If desired, the nickel side may be completed first. However, the steel side must be tacked and thoroughly cleaned and beveled (or gouged) down to the root of the nickel weld prior to welding.
(4) Lap and corner joints are successfully welded by depositing a bead of nickel metal into the root and then weaving successive beads over the root weld.
(5) The arc drawn for nickel or nickel alloy welding should be slightly shorter than that used in normal metal-arc welding. A 1/16 to 1/8 in. (1.6 to 3.2 mm) arc is a necessity.
(6) Any position weld can be accomplished that can be satisfactorily welded by normal metal-arc welding of steel.
d. Welding Methods.
(1) Almost all the welding processes can be used for welding the nickel alloys. In addition, they can be joined by brazing and soldering.
(2) Welding nickel alloys. The most popular processes for welding nickel alloys are the shielded metal arc welding process, the gas tungsten arc welding process, and the gas metal arc welding process. Process selection depends on the normal factors. When shielded metal arc welding is used the procedures are essentially the same as those used for stainless steel welding.
The welding procedure schedule for using gas tungsten arc welding (TIG) is shown by table 7-29. The welding procedure schedule for gas metal arc welding (MIG) is shown by table 7-30. The procedure information set forth on these tables will provide starting points for developing the welding procedures.
(3) No postweld heat treatment is required to maintain or restore corrosion resistance of the nickel alloys. Heat treatment is required for precipitating hardening alloys. Stress relief may be required to meet certain specifications to avoid stress corrosion cracking in applications involving hydrofluoric acid vapors or caustic solutions.No tags for this post.