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WELDING OF NICKEL ALLOYS

Welding of Nickel Alloys

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WELDING OF NICKEL ALLOYS

CLASSIFICATION OF NICKEL ALLOYS

Ni alloys are mainly divided into 2 groups:

1. SOLID SOLUTION ALLOYSNi-Cu alloysNi-Cr alloys Ni-Fe-Cr alloysNi-Mo alloysNi-Cr-Mo alloys

2. PRECIPITATION – HARDENING ALLOYSNi-Cu alloysNi-Cr alloysNi-Fe-Cr alloys

Solid solution nickel base alloysHastellyNHastelloySHastelloyXHaynes 230Inconel 600Inconel 601Inconel617Inconel 625

Precipitation hardenable nickel base alloys3MR 235Inconel 702 Inconel 706IN 713C In 738In 739 Inconel718Inconel 722 Inconel X-750Incoloy 901 M252Rene 41 Udimet 700Waspalloy Haynes214

Solid solution iron base alloys16-25-617-14 CUMO19-9 DLIncoloy 800HIncoloy 802

Precipitation hardenable iron base alloysA286DiscolloyHaynes 556Incoloy 903Incoloy 909

Solid solution cobalt base alloysHaynes 25Haynes

SURFACE PREPARATION

Oxides should be thoroughly removed, because they can inhibit wetting and fusion of base material.

Removal may be done by grinding, abrasive blasting, machining or pickling

Wire brushes used for cleaning should be made of austenitic stainless steel.

Tenacious surface oxides should be removed by grinding with an Aluminum oxide or Silicon carbide wheel.

WELD DESIGN

The groove angle must be large enough to permit proper manipulation of the filler metal, because Ni based alloys has lower flowability and wetting capacity than stainless steel weld metals

In GMAW process the arc do not easily get deflected from straight line, so the joint design should permit the movement of arc

U groove joints should have 30° bevel angle

Ni alloy joints have wider bevel, narrower root face and wider root opening

HEAT CONSIDERATIONS

Preheat is not required, but it is desirable to have the surface temperature at or above 16°C, to avoid moisture condensate

Maximum inter pass temperature of 93°C is recommended for corrosion resistant alloys

High heat input while welding Ni and Ni-base alloys, may result in constitutional liquidation, carbide precipitation or other metallurgical phenomenon which may lead to cracking.

The extent of the changes which takes place in the base material is determined by the heat input of the process and the interpass temperature

WELDING PROCESSES usedGTAW:

•best suited for precipitation hardening alloys because it provides excellent protection against oxidation and precipitation hardening alloys

•Ar, He or a mixture of two are used as shielding gas

•arc stability is best when W electrode is ground to a point with apex angle of 30° to 40°

•Direct current electrode negative is recommended fro both automatic and manual welding

• while welding alloys with high Al and Ti content, surface must be free from oxides to avoid lack of fusion with subsequent passes.

GMAW:

can be used for all of the solution- strengthened alloys except high Si casting alloys

Direct current electrode negative with electrode diameter of 0.9 mm, 1.2mm and 1.6mm are generally used

Ar and Ar+He are normally used for shielding, addition of 15 to 20% He increases bead width and decreases penetration.

An arc length of 6mm is considered suitable, with spray mode of metal transfer

SMAW:

•Primarily used for welding commercially pure Ni and solution strengthened Ni alloys

•Precipitation hardening alloys are not preferable for this process because the alloying elements contributing to hardening are difficult to transfer across the arc

•Minimum thickness of metal is 1.5 mm to be welded by this process

•Electrode covering should have the composition similar to base metals for which they are intended

•Direct current electrode positive is used, electrode diameter of 3.15 mm is suitable for welding in all positions

Other commonly used welding processes:

• Submerged Arc Welding

• Oxy-Acetylene Welding

• Plasma Arc Welding

• Resistance Welding

• Electron beam Welding

• Laser Beam Welding

• Soldering

• Brazing

STRAIN AGE CRACKING

Most of the precipitation-hardening Ni alloys are subject to strain age cracking

The higher the Al and Ti content the higher the greater is the possibility of cracking

If Columbium is substituted in place of Aluminum the ageing reaction is retarded , consequently the HAZ may remain sufficiently ductile during the PWHT and strain age cracking may be prevented

Alloys containing relatively large amount of Al and Ti, such as Udimet 500, have been successfully welded using overageing technique, to counter the strain age cracking