Which Factors Determine the Properties of Nickel
Commercially pure nickel is supplied as wrought rod, bar, plate, sheet, strip and pipe in two standard grades: UNS N02200 with a carbon maximum of 0.15 % and UNS N02201 with a carbon maximum of 0.02 %. Both are specified with a minimum nickel plus cobalt content of 99.0 % in product standards such as ASTM B160 for rod and bar, ASTM B162 for plate, sheet and strip, and ASTM B161 for seamless pipe. Five factors control the properties finally delivered: chemical purity, service temperature, alloying and residual elements, fabrication route, and microstructure.
Purity and Residual Elements
Nickel is purchased on a nickel plus cobalt minimum, but the residuals that matter most in practice are carbon, sulfur, oxygen, iron and copper. Carbon controls high temperature behaviour: in the high-carbon grade it precipitates as graphite along grain boundaries above roughly 315 °C, and the low-carbon grade is specified for service above that point. This is why construction codes limit the high-carbon grade to about 315 °C while the low-carbon grade is accepted for higher temperature chemical process service.
Sulfur causes the greatest difficulty in hot working and welding. Free sulfur at grain boundaries embrittles nickel at elevated temperature, so producers keep sulfur low and make a deliberate manganese addition that ties the sulfur up as manganese sulfide. Oxygen forms nickel oxide inclusions that reduce ductility and act as crack initiation sites. Copper is a controlled residual rather than an addition in the pure grades, and iron is limited to 0.40 % maximum.
| Element | UNS N02200 max (%) | UNS N02201 max (%) |
|---|---|---|
| Ni + Co | 99.0 min | 99.0 min |
| C | 0.15 | 0.02 |
| Mn | 0.35 | 0.35 |
| Fe | 0.40 | 0.40 |
| S | 0.010 | 0.010 |
| Si | 0.35 | 0.35 |
| Cu | 0.25 | 0.25 |
Temperature Dependence and Service Limits
Nickel retains useful strength to elevated temperature, but its physical properties change progressively with temperature. The thermal conductivity of wrought product is approximately 90 W/(m.K) at room temperature and falls as temperature rises, while the coefficient of thermal expansion is about 13 x 10-6 per K. The Curie temperature of commercially pure nickel is 358 °C, above which the material becomes paramagnetic, and the melting range is approximately 1435 to 1446 °C, which sets the upper bound for any hot working operation.
Design codes apply a ceiling of about 315 °C to the high-carbon grade, and use of the low-carbon grade above that temperature is normal in chemical process equipment. Where the material will be welded and then operated hot, the low-carbon grade also removes the risk of graphitisation in the heat affected zone.
How the Fabrication Route Changes the Result
Rolling, forging and casting produce different grain structures and different residual stress states in the same nominal material. Hot rolling and forging refine the cast structure and align inclusions, while extrusion is used for tube and complex sections. Cold rolling and cold drawing raise tensile strength and hardness while reducing elongation, and the residual stress that accompanies cold work affects dimensional stability during later machining, which matters for machined components held to close tolerance.
Annealing after cold work is carried out in the range of roughly 705 to 925 °C, with the schedule chosen to give full recrystallisation without excessive grain growth. Annealed product is the normal delivery condition for plate and bar, and mechanical properties are certified in that condition: annealed wrought nickel of the 200 grade typically shows a tensile strength of about 460 MPa, a 0.2 % proof strength of about 150 MPa and an elongation of about 45 %.
Microstructure and Grain Size Control
Microstructure determines the mechanical, physical and chemical response of the finished part. Grain size is measured by comparison with reference charts under ASTM E112 and is specified where fatigue life or forming behaviour matters: finer grains raise yield strength and improve toughness, while very coarse grains reduce both. Heat treatment and thermomechanical processing are the levers used to hold grain size inside the specified band.
Mechanical testing follows ASTM E8/E8M for tensile properties and ASTM E18 for Rockwell hardness, and results are reported per heat. Where the material is destined for corrosive service, surface condition forms part of the specification as well, because scale, embedded iron and residual oxide behave as initiation sites in aggressive media and are removed before delivery.
Practical Selection Summary
Selection normally reduces to four decisions: choose the low-carbon grade where service temperature exceeds about 315 °C or where welding is followed by hot operation; specify annealed condition unless cold worked strength is required; state the grain size requirement when fatigue or deep forming is involved; and confirm the applicable product standard for the supplied form, since plate, bar and pipe are covered by different specifications with different tolerances and test requirements.
Frequently Asked Questions
Q: What is the difference between UNS N02200 and UNS N02201?
A: The carbon maximum: 0.15 % for N02200 and 0.02 % for N02201. The low-carbon grade is chosen for service above about 315 °C, where the higher carbon content would risk graphitisation at grain boundaries.
Q: Which elements most affect the hot workability of nickel?
A: Sulfur and oxygen. Sulfur segregates to grain boundaries and causes hot shortness, so it is held to a low maximum and controlled with manganese, while oxygen forms oxide inclusions that reduce ductility.
Q: Does cold working increase the strength of nickel?
A: Yes. Cold rolling and cold drawing raise tensile strength and hardness while lowering elongation. Annealing in the range of roughly 705 to 925 °C restores ductility and relieves residual stress.
Q: How is the grain size of nickel specified and verified?
A: Grain size is specified as an ASTM E112 size number and verified by comparison with reference charts on a polished and etched specimen taken from the product.
Q: What delivery condition is normal for nickel plate and bar?
A: Annealed. Mechanical properties are certified in the annealed condition, typically about 460 MPa tensile strength, 150 MPa proof strength and 45 % elongation for the wrought 200 grade.