1. Q&A: Defining Characteristics of UNS N02200 Round Bar
Q: Our company frequently procures "Nickel Round Bar" for various chemical processing applications. We want to standardize our purchasing to UNS specifications. What are the defining chemical and physical characteristics of Nickel 200 Round Bar (UNS N02200) that distinguish it from other nickel-based materials?
A: Standardizing to UNS N02200 is an excellent practice for ensuring material consistency. Nickel 200 Round Bar is defined by its high purity and specific set of physical and mechanical properties.
Chemical Characteristics (Per ASTM B160):
The defining characteristic is its minimum 99.6% Nickel (plus Cobalt) content. The allowable impurities are strictly controlled:
Carbon: 0.15% max
Copper: 0.25% max
Iron: 0.40% max
Manganese: 0.35% max
Silicon: 0.35% max
Sulfur: 0.01% max
This high purity is what gives Nickel 200 its unique corrosion resistance, particularly in caustic environments. The low impurity levels, especially sulfur, are critical for maintaining workability and preventing issues like hot shortness during welding.
Physical Characteristics:
Density: 0.321 lb/in³ (8.89 g/cm³)
Melting Range: 2615-2635°F (1435-1446°C)
Magnetic Properties: Ferromagnetic at room temperature. It retains its magnetic permeability up to its Curie temperature of approximately 680°F (360°C).
Thermal Conductivity: Approximately 40-50 W/(m·K) at room temperature, which is relatively high compared to many alloyed materials.
Electrical Resistivity: Low, around 9.5 µΩ·cm at room temperature.
Mechanical Characteristics (Annealed Condition):
Tensile Strength: 55,000 – 75,000 psi (380 – 520 MPa)
Yield Strength (0.2% offset): 15,000 – 30,000 psi (105 – 205 MPa)
Elongation: 40% – 55% (indicating excellent ductility)
The combination of high purity, magnetic properties, good thermal conductivity, and excellent ductility makes UNS N02200 round bar suitable not only for chemical service but also for electronic, aerospace, and marine components where these specific characteristics are required.
2. Q&A: Room Temperature Caustic Service with Nickel 200
Q: We are designing a series of agitator shafts for large tanks storing 50% caustic soda at ambient temperatures. Our structural calculations show that Nickel 200 round bar has sufficient strength, but we are concerned about long-term corrosion performance. What corrosion rates can we realistically expect, and is there any risk of environmental cracking?
A: For your application-50% caustic soda at ambient temperatures-Nickel 200 round bar is an outstanding, time-tested choice. You can expect exceptional long-term performance.
Expected Corrosion Rates:
In 50% caustic soda at room temperature (20-25°C), the corrosion rate of Nickel 200 is virtually negligible, typically less than 0.002 mm per year (0.08 mpy) . This essentially means no measurable loss of wall thickness over the lifespan of the equipment. This performance is far superior to stainless steels, which can suffer from pitting or stress corrosion cracking in chloride-contaminated caustic, and even to many other nickel alloys.
Risk of Environmental Cracking:
This is the key advantage of Nickel 200. It is not susceptible to caustic stress corrosion cracking (caustic embrittlement) at room temperature. Caustic embrittlement in other materials typically requires a combination of high temperature, high concentration, and tensile stress. At ambient temperatures, even under high stress, Nickel 200 remains immune.
Important Considerations:
Iron Contamination: One practical concern is surface contamination. If the surface of the Nickel 200 shaft becomes contaminated with iron (e.g., from carbon steel tooling or grinding debris), that iron can rust in the caustic environment, creating unsightly and potentially pitting-inducing conditions. You should specify that the round bar be supplied with a clean, oxide-free surface and, after fabrication, consider a light pickling or passivation treatment to remove any embedded iron.
Aeration: While Nickel 200 performs well in aerated caustic, de-aeration (removing oxygen) can actually slightly improve its already excellent corrosion resistance.
For your ambient-temperature caustic agitator shafts, UNS N02200 round bar will provide decades of reliable, crack-free service with negligible metal loss.
3. Q&A: Fabrication of Threaded Components from Nickel 200
Q: We need to produce a large quantity of threaded studs and nuts from Nickel 200 round bar for securing lined tanks. We are concerned about galling during thread cutting and during final assembly. What best practices do you recommend for machining threads and preventing seizure during nut-and-bolt installation?
A: Your concern about galling is well-founded. Nickel 200, like many other nickel alloys and stainless steels, is prone to galling during threading operations and when threaded components are assembled. This is due to its ductility and tendency to adhere to itself under pressure and friction. Here are best practices to address this:
For Thread Cutting (Machining Phase):
Sharp Tooling: Use sharp, high-positive-rake tooling to cut cleanly rather than push the material. Dull tools generate friction and heat, exacerbating work-hardening and galling.
Aggressive Feeds: Maintain a consistent, moderately aggressive feed rate. A feed that is too light will cause the tool to rub, work-harden the surface, and make subsequent cuts difficult.
Ample Coolant: Use a high-quality sulfur-free cutting oil or a heavy-duty water-soluble coolant with extreme pressure (EP) additives at a rich concentration. This provides lubrication and flushes away chips.
Thread Forming (Alternative): For nuts, consider thread forming (cold forming) instead of cutting. This displaces the material rather than removing it, resulting in a work-hardened, smoother thread surface that is more resistant to galling during assembly.
For Assembly (Preventing In-Service Galling):
Lubrication is Mandatory: Never assemble dry. Use a high-quality anti-seize compound specifically formulated for nickel alloys or stainless steel. These compounds contain solid lubricants like molybdenum disulfide (Moly) or graphite that create a barrier between the thread surfaces.
Controlled Torque: Use a calibrated torque wrench and follow specified torque values. Over-torquing increases contact pressure and friction, dramatically increasing the risk of galling.
Surface Treatment (Optional): For critical applications, consider specifying a surface treatment. Silver plating is an excellent, though expensive, solution for preventing galling on nickel alloy threads. Electropolishing can also help by smoothing the micro-surface.
Clean Threads: Ensure threads are free of machining chips, dirt, and debris before assembly.
By combining careful machining practices with mandatory lubrication during assembly, you can successfully produce and install Nickel 200 threaded components without seizure.
4. Q&A: Sourcing Large-Diameter Nickel 200 Bar for Shafts
Q: We are sourcing material for a 6-inch diameter mixer shaft in a sodium chlorate crystallizer. The environment is highly oxidizing due to chlorate and chloride ions. Is Nickel 200 round bar suitable for this application, and what manufacturing process (forged vs. rolled bar) should we specify for this large diameter?
A: This application presents two separate challenges: material suitability for the environment and the correct product form for a large-diameter shaft.
Material Suitability:
This is the critical question. Sodium chlorate (NaClO₃) is a strong oxidizer. Nickel 200 has limited resistance to highly oxidizing environments. In the presence of chlorates and chlorides, the protective oxide film on nickel can break down, leading to accelerated corrosion, pitting, and possibly stress corrosion cracking. Nickel 200 is generally not recommended for strongly oxidizing chloride or chlorate environments.
A more suitable material for your application would be a nickel-chromium-molybdenum alloy such as Alloy 625 (UNS N06625) or Alloy C-276 (UNS N10276) . These alloys have significant chromium and molybdenum additions that provide resistance to both oxidizing and reducing conditions. We strongly recommend corrosion testing or consulting with a materials engineer before proceeding with Nickel 200 in a sodium chlorate crystallizer.
Product Form for Large Diameter (Assuming Material is Suitable):
If the environment were suitable, for a 6-inch diameter shaft, you have options:
Hot-Rolled Bar: Possible, but large-diameter hot-rolled bar can have a coarse, less uniform grain structure and may contain more internal stress.
Forged Bar: This is the preferred product form for critical rotating equipment like mixer shafts. Forging refines the grain structure, closes internal porosity, and provides directional strength. The forging process also produces a more uniform, defect-free cross-section.
Specification: You should specify "Nickel 200 Forged Round Bar" to ASTM B160. Ensure the forging reduction ratio is adequate (typically 3:1 or 4:1 minimum) to guarantee sound centerline properties.
Conclusion: Please re-evaluate the material choice for the sodium chlorate environment. If Nickel 200 is deemed unsuitable and you switch to Alloy 625 or C-276, the recommendation for a forged bar product form remains the same for a critical 6-inch shaft.
5. Q&A: Heat Treatment for Stress Relief of Machined Nickel 200 Components
Q: We have rough-machined a series of complex valve components from Nickel 200 round bar. We are concerned about dimensional stability during final finishing and in service due to residual stresses from the machining process. Can we stress relieve Nickel 200, and what is the recommended time-temperature cycle?
A: Yes, you can and should stress relieve your rough-machined Nickel 200 components to ensure dimensional stability. However, you must be extremely vigilant about temperature control to avoid damaging the material.
Why Stress Relieve?
Machining removes material unevenly, releasing the inherent residual stresses in the bar stock in a non-uniform way. This can cause parts to distort during final machining or, worse, during service. Stress relieving thermally relaxes these stresses, stabilizing the component.
The Recommended Process:
Temperature Range: For Nickel 200, stress relieving is performed between 480°C and 650°C (900°F – 1200°F) .
Soak Time: Hold at temperature for one hour per inch of cross-section (e.g., a 2-inch thick component requires a 2-hour soak).
Cooling: After soaking, cool slowly in the furnace or in still air. Avoid forced air cooling or quenching, as this can introduce new thermal stresses.
The Critical Warning: Do Not Exceed 650°C (1200°F)
This is absolutely vital. If the stress-relieving temperature approaches or exceeds 700°C (1300°F) , you risk a phenomenon called graphitization. At these elevated temperatures, the carbon in Nickel 200 (max 0.15%) precipitates out at the grain boundaries as graphite. This renders the material brittle and completely ruins its mechanical properties. The component could fail catastrophically under load.
