Forming of nickel alloys

Nickel alloys in the annealed condition can generally be processed using all conventional cold-working methods. However, their formability varies significantly from one alloy to another—some grades are highly ductile and easy to form, while others require considerably greater forming forces. This article outlines the key factors affecting the cold working of nickel alloys and discusses the lubricants best suited for these operations.

General characteristics

Work hardening – Cold working increases the strength and hardness of nickel alloys while reducing their ductility. The rate of work hardening depends on the alloy's chemical composition. Nickel-chromium, nickel-chromium-cobalt, and nickel-iron-chromium alloys harden more rapidly than commercially pure nickel and nickel-copper alloys. As work hardening progresses, additional forming operations become increasingly difficult.

Springback – Nickel alloys have a modulus of elasticity similar to that of steel. As a result, they tend to partially recover their original shape after deformation, making it necessary to account for springback when designing forming operations and tooling.

Heat treatment condition – Most cold-forming operations are performed on material in the annealed condition, as it provides the highest ductility. However, certain softer nickel alloys are intentionally supplied in a partially hardened condition to improve their performance during cutting, blanking, and the manufacture of fasteners.

Galling and cold welding – Nickel alloys have a tendency to adhere to tooling and mating metal surfaces during forming. This increases friction, accelerates tool wear, and can make forming operations more difficult. The problem is particularly common in alloys with a low chromium content, since chromium forms a protective oxide layer that helps reduce surface adhesion.

Lubricants for cold working nickel alloys

Choosing the right lubricant is essential for minimizing friction, reducing tool wear, and producing high-quality formed parts.

Light mineral oils and water-based lubricants provide only limited lubricating performance and are suitable for light-duty forming operations.

Phosphate coatings are not recommended because they do not form effective conversion coatings on nickel-based alloys.

Conventional petroleum-based lubricants are rarely used for cold working nickel alloys.

Copper coatings provide excellent lubrication under severe forming conditions. However, because they are expensive to apply and remove, metallic coatings are generally reserved for demanding cold-forming operations where they can be completely scrubbed from the finished component afterward.

Lubricants containing lead carbonate, zinc oxide, or other compounds of low-melting-point metals should be avoided if the finished component will operate at elevated temperatures. Residual deposits can remain on the alloy surface and impair its high-temperature performance. Where a solid filler is required, inert materials such as talc are a suitable alternative.

Lubricants containing sulfur- or chlorine-based additives, including molybdenum disulfide, may be used provided that the workpiece can be thoroughly cleaned after forming. Even trace residues of sulfur or chlorine compounds can lead to pitting and should not be left on the finished component.

Tooling Materials

In general, the same tools used for cold forming austenitic stainless steels can also be used for cold forming nickel alloys. However, because of their mechanical properties, forming nickel alloys require 30–50% more power than comparable parts made from low-carbon steel. This higher forming load also results in increased tool wear.

Parts formed using zinc dies should be briefly pickled in diluted nitric acid to remove any traces of zinc that may have transferred from the die surface onto the component during forming. Zinc contamination can cause embrittlement of nickel alloys during subsequent heat treatment or when the components are exposed to elevated temperatures in service. For the same reason, components formed using brass or bronze dies should also be pickled if the dies leave a bronze-colored surface.

Cutting and Punching Nickel Alloys

The required heat treatment condition for parts that will be sheared, blanked, or punched depends on the grade and thickness.

• Thin strips of soft alloys (such as Nickel 200) should be supplied in a work-hardened condition.
• Higher-strength alloys (such as Alloy 600) generally give the best results when blanked in a skin-hardened condition.
• Precipitation-hardened alloys (such as Alloy X-750) should be blanked in the annealed condition.

Lubrication is not normally required for shearing operations, but it should be used during blanking and punching.

• Thickness below 3 mm - a light mineral oil.
• Thickness above 3 mm - a heavier sulfurized oil.

The recommended clearance between the punch and die is:

• 3–5% of material thickness for thin sheet (up to 3 mm thick)
• 5–10% of material thickness for materials thicker than 3 mm

The clearance between the punch and the stripper plate should be kept as small as practical.

Shearing - the shearing equipment should be capable of cutting low-carbon steel that is 50% thicker than the nickel alloy material being processed. For example, to cut a 6 mm thick Alloy 400 (UNS N04400) plate, the shears should be rated for cutting 9 mm thick low-carbon steel.

Punching - the minimum hole diameter should generally be equal to or greater than the material thickness. The exact ratio depends on the alloy, material thickness, and heat treatment condition.

For Alloy 200, Monel 400, and Alloy 600, the recommended minimum hole diameters are:

• Material thickness 0.5–0.9 mm: 1.5 times the thickness
• Material thickness 0.9–1.8 mm: 1.3 times the thickness
• Material thickness 1.8–3.6 mm: 1.1 times the thickness
• Material thickness above 3.6 mm: equal to the thickness

The shear strength of nickel alloys is approximately 65% of their tensile strength.

When shearing nickel alloys, the load on guillotine shears is typically around 113–131% of the load required for cutting comparable low-carbon steel components.

Deep Drawing of Nickel Alloys

For most simple shapes, standard dies and tooling designed for drawing steel or copper alloys can be used. However, for complex components requiring high dimensional accuracy, minor tooling adjustments may be necessary. These typically include increasing the working clearances and enlarging the die radius.

Larger working clearances help prevent wrinkling and are required because nickel alloys generally have higher tensile strength than cold-drawing steels. For cylindrical drawn parts, the typical clearance between the punch and die is 120–125% of the sheet thickness.

The die radius must be large enough to allow smooth metal flow. If the radius is too small, the material may gall between the punch and die. Recommended minimum punch radii are therefore:

• Ferritic stainless steel: 6 times the sheet thickness;
• Austenitic stainless steel: 4 times the sheet thickness;
• Thin nickel alloy sheets: 5–12 times the sheet thickness.

Spinning of Nickel Alloys

Manual spinning is generally not recommended, except for very soft alloys such as Nickel 200 and Monel 400. The maximum hardness and sheet thickness suitable for manual spinning are:

• Nickel 200: 64 HRB and 1.57 mm;
• Nickel 201: 55 HRB and 1.98 mm;
• Monel 400: 68 HRB and 1.27 mm;
• Alloy 600: 80 HRB and 0.94 mm;
• Alloy 722: 94 HRB and 0.94 mm;
• Alloy X-750: 94 HRB and 0.94 mm;
• Alloy 801: 88 HRB and 0.94 mm.

Heavy lubricants are recommended, such as beeswax or tallow.

Selecting the correct tooling material is essential for successful spinning. The preferred material for spinning mandrels is highly polished hard alloy bronze. For spinning rollers, hardened tool steels—especially chrome-plated grades—are recommended because they reduce adhesion between the workpiece and the tool. Mandrels and rollers made from standard brass or carbon steel, which are commonly used for softer materials, are not suitable for processing nickel-based alloys.

Bending of Nickel Alloy Tubes

Nickel alloy tubes can be bent and coiled using conventional methods. Stress-relieved tubes generally require bending radii approximately 25–50% larger than annealed tubes of the same size.

Typical bending limits for nickel alloy tubes are as follows:

• Press bending of an unfilled tube: minimum bending radius: 6D; maximum bending angle: 120°
• Roll bending of a filled tube: minimum bending radius: 4D; maximum bending angle: 360°
• Mandrel bending: minimum bending radius: 2D; maximum bending angle: 180°
• Rotary draw bending of an unfilled tube: minimum bending radius: 3D; maximum bending angle: 180°
• Rotary draw bending of a filled tube: minimum bending radius: 2D; maximum bending angle: 180°

where D represents the outside diameter of the tube.

Unfilled tube bending limits the achievable bend radius and is generally suitable only for tubes with wall thickness greater than 7% of the outside diameter.

Mandrel bending and filled-tube bending allow smaller bend radii. The best results are achieved using hard bronze mandrels, which reduce the risk of galling, together with suitable lubricants. For severe bending operations, lubricants containing chlorine compounds are recommended. Sand is commonly used as a filler material.

Press bending does not always provide the best results for fully annealed tubes. Low-hardness alloys, such as Nickel 200 and Monel 400, are easier to press-bend in the stress-relieved condition. Nickel-chromium alloys, however, should generally be bent in the annealed condition.

Hot bending is usually not recommended, as cold bending typically produces better results. If hot bending is required, it should only be performed with the tube filled. The sand filler must be clean and free from sulfur compounds, as sulfur can cause cracking during bending. Sulfur removal requires heating the sand to 1150°C in an oxidizing atmosphere.

Bending of Nickel Alloy Plates, Strips, and Sheets

Nickel alloy plates, strips, and sheets can be bent using conventional methods. The required bend radius depends on the alloy type and material thickness.

For a 180° bend, the minimum recommended bend radii are:

• Alloy 200
  • Sheet and strip, 0.3–6.4 mm: 1t
  • Plate, 4.5–6.4 mm: 2t
• Alloy 400
  • Sheet and strip, 0.3–2.8 mm: 1t
  • Sheet and strip, 2.8–6.4 mm: 2t
• Alloy 600
  • Sheet and strip, 0.3–6.4 mm: 1t
  • Plate, 4.7–6.4 mm: 2t
• Alloy 625
  • Sheet and strip, 0.3–6.4 mm: 2t
  • Plate 4.7–6.4 mm: 2t
• Alloy 718
  • Sheet and strip, 0.3–1.24 mm: 1t
  • Sheet and strip, 1.24–6.35 mm: 2t
• Alloy X-750
  • Sheet and strip, 0.3–1.24 mm: 1t
  • Sheet and strip, 1.24–6.35 mm: 2t
• Alloy 800
  • Sheet and strip, 0.3–6.4 mm: 1t
  • Plate 4.7–6.4 mm: 2t
• Alloy 825
  • Sheet and strip, 0.3–6.4 mm: 2t
  • Plate 4.7–6.4 mm: 2t

where t is the material thickness.

Tube Expansion

Nickel alloy tubes can be expanded into tube sheets of shell-and-tube heat exchangers using standard expansion methods.

The tube sheet holes should be only slightly larger than the outside diameter of the tubes:

• For tubes with an outside diameter below 38 mm, the hole diameter should be 0.10–0.20 mm larger than the tube diameter.
• For tubes above 38 mm, the hole diameter should be 0.20–0.25 mm larger than the tube diameter.

The tube sheet should have a higher hardness than the tubes being expanded. Therefore, tube sheets are typically supplied in the rolled or forged condition, while the tubes are usually supplied in the annealed condition.

Bars Bending

Nickel alloy bars are typically cold worked in the annealed condition. The bending of bars is carried out using principles similar to those applied when bending filled tubes.

Coiling nickel alloy bars into springs is usually performed at elevated temperatures, particularly for precipitation-hardened alloys. However, in some applications, bars must be coiled in the cold-worked (strain-hardened) condition. This significantly increases the minimum achievable coiling radius.

Heading and Extrusion of Nickel Alloys

Nickel alloys can be subjected to cold extrusion and cold heading. Heading is usually performed in combination with extrusion. Due to their high strength and tendency to gall, nickel alloys require lower processing speeds (the heading speed should typically be around 10–15 m/min) and more durable tooling, such as tools made from SW18 high-speed steel.

Straightening

Nickel alloys can be straightened using conventional methods. However, they require approximately 50% more power than low-carbon steel. For example, a machine capable of straightening 20 mm diameter low-carbon steel bars would only be suitable for straightening nickel and nickel-copper alloy bars with diameters of approximately 10 mm.

A common issue during straightening is the formation of spiral scoring. This can be minimized by using suitable lubricants and processing the material in the annealed condition. Straightening dies may be made from bronze or cast iron. Cast iron dies should be used when no pickling operation is performed after straightening and contamination of the product with bronze particles cannot be tolerated.

Cold Working of High-Temperature Parts

Excessive cold working can reduce the high-temperature strength of nickel-chromium alloys, which are often used in elevated-temperature applications. If the amount of cold work exceeds 10% and the component is intended to operate above 650°C, heat treatment should be carried out after forming.

Cold Working of Heat-Resistant and Precipitation-Hardened Alloys

Most heat-resistant alloys are formed by cold working because their hot-working temperature range is very limited. In some cases, several forming operations are required, with intermediate annealing treatments.

Grain Growth Control - for certain heat-resistant alloys, achieving the desired grain size requires a significant amount of cold deformation. For example, Alloy X subjected to only 5% cold reduction followed by heat treatment may experience excessive grain growth, which can result in an “orange peel” surface effect. This effect is avoided when the cold reduction is increased to 10%.

Alloying Elements impact on cold working behaviour of nickel alloys:

• Boron (e.g., Alloy 41): concentrations above 0.03% increase the risk of cracking, while lower levels help prevent carbide precipitation at grain boundaries.
• Silicon: levels above 0.3%, particularly above 0.6%, increase the risk of cracking during cold drawing.
• Carbon: contents above 0.15% reduce ductility and increase susceptibility to cracking.
• Molybdenum and tungsten (e.g., Alloy 230): increase mechanical strength, making the alloy more difficult to form and increasing the required forming force.
• Aluminium and titanium, which promote the formation of the γ′ strengthening phase (e.g., Alloy 80A), also increase alloy strength and make forming more demanding.

Precipitation-hardened alloys may be susceptible to cracking if they are precipitation hardened (aged) immediately after plastic deformation. To avoid this problem, the component should be annealed before aging. Some precipitation-hardened alloys, such as Alloy 41, have two possible annealing temperature ranges. Annealing at the higher temperature (1175°C for Alloy 41) improves ductility and formability but decreases weldability. Annealing at the lower temperature (1070°C for Alloy 41) reduces formability by approximately 10–20%, but allows welding without the risk of cracking.