A Comprehensive Guide to Nickel-based Alloy: Monel 401 (UNS N04401)

What is Monel 401?

Monel 401 (also known as “401 alloy”) is named UNS N04401, a copper nickel alloy designed for special electrical and electronic applications. It has a very low temperature coefficient of resistance and a medium range of resistivity. Alloy 401 can be easily welded by gas tungsten arc welding (GTAW) or resistance welding. Monel 401 is usually supplied in the form of wire, mainly for wound precision resistors and bimetallic contacts. Monel 401 is a ductile Copper Nickel alloy with a very low coefficient of electrical resistivity and resistance to a wide variety of corrosive conditions. Like with commercially pure Nickel, Monel 401 is low in strength in the annealed condition and can only be cold worked to a maximum of 120 KSI.

Super alloy Monel 401 has high contents of copper than that of super alloy Monel 400.

Equivalent Grades of Monel 401 (UNS N04401)

STANDARD	WERKSTOFF NR.	UNS	AFNOR	EN	JIS	BS	GOST
Monel 401	1.7471	N04401	NU-30M	NiCu30Fe	NW 4401	NA 13	МНЖМц 28-2,5-1,5

Chemical Composition of Monel 401 (UNS N04401)

C	Si	Mn	S	Ni	Co	Cu	Fe
≤0.10	≤0.25	≤2.25	≤0.015	40.0~45.0	≤0.25	Bal	≤0.75

Physical Properties of Monel 401 (UNS N04401)

Density	Thermal conductivity	Thermal coefficient of Resistance	Electrical Resistivity	Specific heat capacity
g/cm3	W/m. °C	ppm/°C	µΩ·m	（J/kg/℃ ）
8.91	21	±20	0.498	414

Mechanical Properties of Monel 401 (UNS N04401)

Tensile Strength, min.		Yield Strength, min.		Elongation	Hardness
Mpa	ksi	Mpa	ksi	%	HRB
441	64	134	19.5	51	43

Specifications of Monel 401 (UNS N04401)

Rod, Bar, Wire, Forgings	ASTM B163/ASME SB163, DIN 17471
Plate, Sheet, Strip	ASTM B127/ASME SB127, DIN 17750
Pipe, Tube	ASTM B163/ASME SB163, ASTM B725/ASME SB725, ASTM B775/ASME SB775, ASTM B751/ASME SB751, DIN17751
Others	ASTM B366/ASME SB366, DIN 17743

The machinability of Monel 401

• Traditional machining techniques used on ferroalloys can be used for this alloy.

Welding of Monel 401

Alloy 401 is easy to weld into thin sheets automatically. Gas tungsten arc welding is a common welding process. Resistance welding is a satisfactory method for this alloy. The commonly used welding methods work well with this alloy. Matching alloy filler metal should be used. If matching alloy is not available then the nearest alloy richer in the essential chemistry (Ni, Co, Cr, Mo) should be used. All weld beads should be slightly convex. It is not necessary to use preheating. Surfaces to be welded must be clean and free from oil, paint or crayon marking. The cleaned area should extend at least 2″ beyond either side of a welded joint.

• Gas-Tungsten Arc Welding: DC straight polarity (electrode negative) is recommended. Keep as short an arc length as possible and use care to keep the hot end of filler metal always within the protective atmosphere. Shielded Metal-Arc Welding: Electrodes should be kept in dry storage and if moisture has been picked up the electrodes should be baked at 600 F for one hour to insure dryness. Current settings vary from 50 amps for material 0.062″ thick up to 190 amps for material of 1/2″ and thicker. It is best to weave the the electrode slightly as this alloy weld metal does not tend to spread. Cleaning of slag is done with a wire brush (hand or powered). Complete removal of all slag is very important before successive weld passes and also after final welding.
• Gas Metal-Arc Welding: Reverse-polarity DC should be used and best results are obtained with the welding gun at 90 degrees to the joint. For Short-Circuiting-Transfer GMAW a typical voltage is 19-25 with a current of 100-175 amps and a wire feed of 225-400 inches per minute. For Spray-Transfer GMAW voltage of 26 to 33 and current in the range of 200-350 amps with wire feed rate of 200-500 inches per minute, depending on filler wire diameter.
• Submerged-Arc Welding: Matching filler metal, the same as for GMAW, should be used. DC current with either reverse or straight polarity may be used. Convex weld beads are preferred.

Hot working of Monel 401

• Hot working can be carried out if necessary.

Cold working of Monel 401

Cold working can be carried out with standard tooling and soft mold materials for better finish. Cold forming may be done using standard tooling although plain carbon tool steels are not recommended for forming as they tend to produce galling. Soft die materials (bronze, zinc alloys, etc.) minimize galling and produce good finishes, but die life is somewhat short. For long production runs the alloy tool steels ( D-2, D-3) and high-speed steels (T-1, M-2, M-10) give good results especially if hard chromium plated to reduce galling. Tooling should be such as to allow for liberal clearances and radii. Heavy duty lubricants should be used to minimize galling in all forming operations. Bending of sheet or plate through 180 degrees is generally limited to a bend radius of 1 T for material up to 1/8″ thick and 2 T for material thicker than 1/8″.

Hardening of Monel 401

•  Hardens by cold working only.

Annealing of Monel 401

• Monel 401 alloy should be annealed at 760 ° C (1400 ° f) for one hour to recover from cold working.

Heat Treatment of Monel 401

• Not heat treatable except for annealing after cold working.

Application areas of nickel-based alloy monel 401 (UNS N04401)

• Wire wound resistors.
• Bimetal contacts.
• Electric and electronic applications.
• Marine engineering.
• Chemical and hydrocarbon processing equipment.
• Gasoline and freshwater tanks.
• Crude petroleum stills.
• De-aerating heaters.
• Boiler feed water heaters and other heat exchangers.
• Bearings, pumps, shafts and fasteners.
• Industrial heat exchangers.
• Chlorinated solvents.
• Crude oil distillation towers.
• Piping system.
• Heat exchange tubes.
• Pipe fittings.
• Flanges.
• Valves.

Variety specifications and supply status of Nickel-based super alloy: Monel 401 (UNS N04401)

Variety classification:

Yaang Pipe Industry can produce various specifications of Monel 401 seamless pipe, Monel 401 steel plate, Monel 401 round bar, Monel 401 forgings, Monel 401 flange, Monel 401 pipe fittings, Monel 401 welded pipe, Monel 401 steel strip, Monel 401 wire and supporting welding materials. 

Delivery status:

• Seamless pipe: solid solution + acid white, length can be set;
• Plate: solid solution, pickling, trimming;
• Welded pipe: solid solution acid white + RT% flaw detection;
• Forging: annealing + car polish; Bars are forged and rolled, surface polished or car polished;
• Strips are delivered after cold rolling, solid solution soft state, and deoxidized;
• Wire rods are finely ground in solid solution pickled disk or straight strips, solid solution straight strips Delivery in light state.

Effect of Heat Treatment Temperature on the Structure and Properties of Monel 401 Alloy Pipe

This article introduces the production and annealing test methods of Monel 401 alloy steel pipes, analyzes the test results of cold rolled Monel 401 alloy steel pipes and focuses on the influence of different annealing temperatures on the metallographic structure and mechanical properties of the alloy steel pipes.
Monel alloy is a type of nickel-based alloy made by adding elements such as Cu, Fe, Mn, etc., to the metal nickel matrix. Ni, as a matrix, has compatibility with various metal elements, which can improve thermal stability and manufacturing performance and enhance corrosion resistance in neutral-reducing and alkaline media. Cu can improve resistance to acidic environments such as sulfuric acid and hydrofluoric acid. Many studies have shown that Monel alloys have excellent corrosion resistance, oxidation resistance, and high strength and are widely used in industries such as metallurgy and petrochemicals.
Monel 401 has excellent strength plasticity and good pressure processing performance in both cold and hot states. It is a corrosion-resistant alloy with large dosage, wide use, and excellent comprehensive performance. This alloy has excellent corrosion resistance in HF and F2 media and is one of the few important materials that can withstand hydrofluoric acid. An important feature of this alloy is that it generally does not produce stress corrosion cracks and has good cutting performance. It is a type of Monel alloy with a high-strength single-phase solid solution structure similar to Monel400’s nickel based alloy. The corrosion resistance of metals is fundamentally achieved by hindering the migration of charged particles through the discharge process of hydrogen ions. The addition of 30% Cu minimizes the corrosion rate of Ni in halide ion media such as F and Cl, while the higher the Cu content, the better the resistance to sulfuric acid and phosphoric acid. The addition of 55% Cu in Monel 401 improves the corrosion resistance of the alloy in reducing acids.

1. Monel 401 production method and annealing test method

The material studied is the Monel 401 forging billet provided by a certain factory, which is extruded and formed by a 6300t horizontal extruder and then cold rolled. The specification of the cold rolled alloy steel pipe is Φ 177.8mm × 8.05mm; the chemical composition of Monel 401 provided by a certain factory is shown in Table 1, requiring tensile strength ≥ 420MPa, yield strength ≥ 150MPa, and elongation ≥ 35%.
Table.1 Chemical composition (mass fraction) % of Monel 401

Element	C	S	Ni	Co	Mn	Fe	Si
Actual value	0.048	0.006	41.69	0.1	1.75	0.45	0.12
Standard value	≤0.10	≤0.015	40.0-45.0	≤0.25	≤2.25	≤0.75	≤0.25

Note: The rest are Cu elements.
Cut a 220mm section of cold-rolled alloy steel pipe and divide it into 4 pieces vertically. Conduct annealing tests in a muffle furnace at annealing temperatures of 550600650 and 700 ℃, with annealing holding time of 120 minutes. After annealing, the sample is air-cooled. Observing the microstructure of alloy steel pipes in the cold rolled state and at different annealing temperatures using an optical metallographic microscope, conducting tensile tests at room temperature on a universal material testing machine according to ASTM E8/E8M-2016a “Metallic Materials Tensile Testing Method”, and testing the Rockwell hardness according to ASTM E18-2015 “Metallic Materials Rockwell Hardness Testing Method Standard”.

2. Test results of Monel 401 steel pipes in cold rolled and annealed states

2.1 Test results of cold rolled alloy steel pipes
Observing the microstructure of Monel 401 cold rolled alloy steel pipe using a metallographic microscope, the metallographic structure of the cold rolled alloy steel pipe is shown in Figure 1. From Figure 1, it can be seen that after cold rolling, the grains and twins are broken and deformed, increasing crystal storage energy and providing a driving force for the nucleation and growth of new grains. The microstructure of cold-rolled alloy steel pipes is fibrous, with obvious deformation flow lines. The fibrous deformation structure gives the alloy steel pipes a good strengthening effect.

Figure.1 Metallographic Structure of Cold Rolled Alloy Pipe
After the alloy steel pipe is rolled and deformed, the number of defects in the crystal increases, dislocations increase, and internal stress increases, resulting in the strengthening of the alloy. At this point, its tensile strength is 517MPa, yield strength is 479MPa, elongation is 21.5%, tensile strength and yield strength are relatively high, and the difference is small. The yield strength ratio is 0.926, indicating a high yield strength ratio, Low elongation, severe work hardening of alloy steel pipes, less prone to plastic deformation, and strong resistance to deformation.
2.2 Test Results of Annealed Alloy Pipes at Different Temperatures
2.2.1 Effect of annealing temperature on metallographic structure
Observe the microstructure of Monel 401 alloy steel pipe at different annealing temperatures, and the metallographic structure of the alloy steel pipe at different annealing temperatures is shown in Figure 2. From Figure 2, it can be seen that after annealing at 550 ℃, the fibrous structure of Monel 401 after cold rolling deformation did not disappear, and there was little change compared to the cold rolling state. After annealing at 600 ℃, the fibrous structure decreased but remained significant. When the temperature rises to 650 ℃, most of the deformation streamlines disappear, the fiber structure significantly decreases, the grains continue to nucleate and grow, and the deformed grains undergo recrystallization. When the annealing temperature rises to 700 ℃, the deformation streamline disappears, the grains are completely recrystallized, and the microstructure is twinned α Single phase recrystallized grains.

Figure.2 Metallographic Structure of Alloy Pipes at Different Annealing Temperatures
2.2.2 Effect of annealing temperature on the tensile properties of alloy steel pipes
The tensile properties of alloy steel pipes under different annealing temperatures are shown in Table 2. The tensile strength and yield strength of annealed alloy steel pipes are significantly lower than those of cold-rolled ones, and the elongation is significantly higher than those of cold-rolled ones. The effect of annealing temperature on the tensile properties of alloy steel pipes is shown in Figure 3. From Figure 3, it can be seen that the tensile strength, yield strength, and elongation remain basically unchanged at an annealing temperature of 550-600 ℃. At an annealing temperature of 600-700 ℃, the tensile strength slowly decreases, while the yield strength rapidly decreases in a straight line, and the elongation rapidly increases in a straight line. When the annealing temperature is 650 ℃ and 700 ℃, the performance meets user needs. At 700 ℃, the yield strength is only 8MPa higher than the required value, which poses a certain risk. When the annealing temperature is 650 ℃, the overall performance of the Monel 401 alloy steel pipe is good, and the final annealing treatment is carried out at this temperature.
Table.2 Tensile Properties of Alloy Pipes at Different Annealing Temperatures

Temperature/°C	Tensile strength Rm/MPa	Yield strength Rp0.2/MPa	Elongation A/%
550	470	352	31
600	468	348	32.5
650	449	251	40.5
700	445	158	50

Figure.3 Effect of annealing temperature on the tensile properties of alloy steel pipes
2.2.3 Effect of annealing temperature on hardness
Monel 401 undergoes work hardening during the cold rolling process, with a high hardness value of 33.2HRC. It requires annealing and recrystallization treatment to eliminate work hardening, reduce the hardness of the alloy steel pipe, and eliminate internal stress. The effect of annealing temperature on the hardness of alloy steel pipes is shown in Figure 4. As shown in Figure 4, annealing temperature has a significant impact on hardness. At 550 ℃, the hardness value decreases to 79HRB, and the alloy undergoes significant recrystallization softening. When the temperature is raised to 600 ℃, there is no significant change in hardness. Continuing to heat up, it shows a linear decrease at 600-700 ℃. The recrystallization softening effect at high annealing temperatures is more significant than that at low annealing temperatures. As the annealing temperature increases, the hardness value also decreases.

Figure.4 Effect of annealing temperature on the hardness of alloy steel pipes

3. Conclusion

• (1) The cold rolled microstructure of Monel 401 alloy steel pipe is fibrous. As the annealing temperature increases, the fibrous structure gradually decreases. At 700 ℃, complete recrystallization occurs, and the fibrous structure disappears.
• (2) Monel 401 cold rolled alloy steel pipe has a tensile strength of 517MPa, a yield strength of 479MPa, an elongation of 21.5%, a yield ratio of 0.926, a hardness of 33.2HRC, and strong resistance to deformation. At an annealing temperature of 550-600 ℃, the tensile strength, yield strength, hardness, and elongation remain basically unchanged. At an annealing temperature of 600-700 ℃, the tensile strength slowly decreases. In contrast, the yield strength and hardness rapidly decrease in a straight line, and the elongation rapidly increases in a straight line.
• (3) When the annealing temperature is 650 ℃, the comprehensive performance of the alloy steel pipe is good, meeting user requirements.

Author: Pang Yusi