A Comprehensive Guide to Nickel-Based Alloys: Monel 401 (UNS N04401)
What is Monel 401?
Designated UNS N04401, Monel 401 (also known as "401 Alloy") is a copper-nickel alloy designed for special electrical and electronic applications. It has a very low temperature coefficient of resistance and a mid-range resistivity. Alloy 401 can be easily welded by either gas tungsten arc welding (GTAW) or resistance welding. Monel 401 is commonly supplied in wire form and is primarily used in wirewound precision resistors and bimetallic contacts. Monel 401 is a ductile copper-nickel alloy with very low resistivity and the ability to resist a wide range of corrosive conditions. Like commercially pure nickel, Monel 401 has low strength in the annealed condition and can only be cold worked to a maximum of 120 KSI.
High temperature alloy Monel 401 has a higher copper content than high temperature alloy Monel 400.
Machinability of Monel 401
Conventional machining techniques used for ferrous alloys can be used for this alloy.
Welding Monel 401
401 alloy is readily welded automatically into thin sheets. Gas tungsten arc welding is a common welding process. Resistance welding is a satisfactory method for this alloy. The usual welding methods are suitable for this alloy. A matching alloy filler metal should be used. If no matching alloy is available, the closest alloy rich in basic chemical composition (Ni, Co, Cr, Mo) should be used. All weld passes should be slightly convex. It is not necessary to use preheat. The surfaces to be welded must be clean and free of oil, paint or crayon marks. The clean area should extend at least 2 inches beyond both sides of the weld joint.
Gas tungsten arc welding: It is recommended to use a DC positive pole (electrode negative pole). Keep the arc length as short as possible and be careful to keep the hot end of the filler metal within the protective atmosphere at all times. Shielded Metal Arc Welding: Electrodes should be stored dry and if wet, should be baked at 600 F for one hour to ensure dryness. Current settings range from 0.062 amps for 50-inch thick material to 2 amps for 190/1-inch and thicker material. It is best to braid the electrode slightly because this alloy weld metal does not spread easily. Use a wire brush (manual or electric) to clean the slag. It is important to completely remove all slag before successive passes and after the final weld.
Gas Metal Arc Welding: Reverse polarity direct current should be used with the gun at 90 degrees to the joint for best results. For short-circuit transfer GMAW, typical voltages are 19-25, currents are 100-175 amps, and wire feed speeds are 225-400 inches per minute. For spray transfer GMAW, the voltage is 26 to 33, the current is 200-350 amps, and the wire feed speed is 200-500 inches per minute, depending on the filler wire diameter.
Submerged Arc Welding: The same matching filler metal as for GMAW should be used. DC current with reverse or forward polarity can be used. Convex welds are preferred.
Hot Working of Monel 401
Hot working can be performed when necessary.
Cold Working of Monel 401
Cold working can be performed using standard dies and soft die materials to obtain a better finish. Cold forming can be accomplished using standard tools, but forming with ordinary carbon tool steels is not recommended because they are prone to galling. Soft die materials (bronze, zinc alloys, etc.) minimize galling and produce a good finish, but die life is somewhat short. For long-term production, alloy tool steels (D-2, D-3) and high-speed steels (T-1, M-2, M-10) provide good results, especially when hard chrome plated to reduce galling. Tools should allow free play and radius. Heavy-duty lubricants should be used to minimize wear in all forming operations. 1 degree bends in sheet or plate are generally limited to a bend radius of 8 T for material up to 180/1 inch thick and 8 T for material over 2/1 inch thick.
Hardening Monel 401
Hardened by cold working only.
Annealing Monel 401
Monel 401 alloy should be annealed at 760°C (1400°F) for one hour to recover from cold working.
Heat Treatment Monel 401
Not heat treatable except post-cold working annealing.
Applications Nickel-Based Alloys Monel 401 (UNS N04401)
Wirewound resistors.
Bimetallic contacts.
Electrical and electronic applications.
Offshore engineering.
Chemical and hydrocarbon processing equipment.
Gasoline and fresh water tanks.
Crude oil stills.
Venting heaters.
Boiler feedwater heaters and other heat exchangers.
Bearings, pumps, shafts and fasteners.
Industrial heat exchangers.
Chlorinated solvents.
Crude oil distillation towers.
Piping systems.
Heat exchange tubes.
Fittings.
Flanges.
Valves.
