Hastelloy C4

Hastelloy C4 is a high temperature, high strength, high corrosion resistance alloy material, also known as alloy C4. It is a solid solution strengthened nickel-based alloy with excellent corrosion resistance and high temperature stability, so it is widely used in chemical, petroleum, pharmaceutical and aerospace fields.

Hastelloy C4, namely UNS N06455, has excellent thermal stability. In corrosive environments, C4 has the same corrosion resistance as C276 alloy, but performs better in strong reducing and oxidizing properties.

Hastelloy C4 Composition

Hastelloy C4 is a versatile Ni-Cr-Mo alloy with extremely high resistance to corrosion.

Hastelloy C4 Mechanical Properties

The 40% minimum elongation reflects excellent ductility — C4 can be deep-drawn, spun, and stamped without cracking, making it well-suited for expansion bellows, formed vessel heads, and complex pipe fittings.

Hastelloy C4 Physical Properties vs. Temperature

• Density: ρ=8.6g/cm³

• Melting temperature: 1335～1380℃

Hastelloy C4 Properties

High temperature stability: Hastelloy C4 alloy exhibits excellent stability in the high temperature range of 650-1040℃, which makes it an ideal material for many high temperature industrial applications.

Intergranular corrosion resistance: Through chemical composition control and heat treatment process, Hastelloy C4 alloy improves its resistance to intergranular corrosion and effectively avoids corrosion in harsh environments.

Weld heat affected zone corrosion resistance: Under proper manufacturing conditions, Hastelloy C-alloy can avoid the corrosion sensitivity of the weld heat affected zone, thereby ensuring the reliability and durability of the welded joint.

Good processing performance: Despite its high strength and hardness, C4 alloy still has good processing performance, including cold working and hot working, which makes it easy to be molded into various complex shapes and sizes.

Hastelloy C4 vs. C276 vs. C22: Which One Should You Choose?

All three are Ni-Cr-Mo alloys with outstanding corrosion resistance — but they are not interchangeable. Here is a direct comparison:

Practical decision guide:

Choose C4 when your equipment will be welded without post-weld heat treatment or when service temperatures exceed 650°C for extended periods. C4's thermal stability makes it the best choice for fabricated vessels, heat exchangers, and piping that go directly from welding to service.

Choose C276 for the broadest corrosion coverage across reducing acids, particularly when the service temperature stays below 650°C. C276 is the most widely stocked and cost-effective option for general chemical duty.

Choose C22 when the environment alternates between strongly oxidizing and strongly reducing conditions—such as flue gas desulfurization scrubbers or mixed acid pickling lines. C22's higher chromium content gives it the edge in oxidizing environments.

Hastelloy C4 Equivalent Grades

Hastelloy C4 ASTM Standard

Product	Standard	Show
Pipe SMLS & Welded	ASTM B622/B619
Sheet / Plate	ASTM B574
Bar / Rod	ASTM B574
Forging / Flange	ASTM B366

Corrosion Resistance in Detail

Hastelloy C4's corrosion performance comes from the combined effect of nickel, chromium, and molybdenum:

• Nickel provides the base corrosion resistance and keeps the alloy passive in reducing media.

• Chromium forms a stable oxide film that resists oxidizing environments.

• Molybdenum is the critical element for resistance to pitting and crevice corrosion, and dramatically improves performance in hydrochloric acid.

Performance in Key Chemical Environments

Intergranular Corrosion Resistance

When nickel alloys are heated in the sensitization range (roughly 500–800°C), carbon can migrate to grain boundaries and combine with chromium or molybdenum to form carbides. This depletes corrosion-resistant elements from the metal near the grain boundary, creating a zone vulnerable to intergranular attack.

C4's ultra-low carbon content (≤0.015%) and the absence of tungsten virtually eliminate this risk. Independent testing per ASTM G28 (Huey test in boiling 65% nitric acid) consistently shows C4 performing at corrosion rates below 1.0 mm/year even after aging at 870°C—performance that C276 cannot match under the same conditions.

This is why C4 is the preferred material for welded assemblies used in nitric acid service, pharmaceutical reactors, and nuclear fuel reprocessing equipment.

Fabrication and Welding Guidelines

Hastelloy C4 is more formable and weldable than its strength and corrosion performance might suggest. However, proper technique is essential to preserve its properties.

Cold Working

• C4 work-hardens moderately — more than austenitic stainless steels, less than high-strength duplex grades

• Deep drawing, spinning, and cold forming are all practical

• Intermediate annealing at 1065–1120°C (water quench) may be needed for multi-step forming operations

Hot Working

• Recommended hot working range: 1175°C – 900°C

• Avoid prolonged soaking at temperatures below 900°C to prevent secondary phase precipitation

• Always follow hot working with a solution anneal

Welding

• Preferred processes: GTAW (TIG), GMAW (MIG), SMAW

• Recommended filler metal: ERNiCrMo-7 (AWS classification) or equivalent C4 composition filler

• No preheat required

• Interpass temperature: ≤ 150°C

• Post-weld heat treatment: Not required for most applications — this is one of C4's key practical advantages

• Clean metal thoroughly before welding; chloride contamination from handling can cause weld-adjacent corrosion

Machining

• Machine in the annealed condition

• Use sharp carbide tooling; high-speed steel tools wear quickly

• Low cutting speeds with high feed rates

• Rigid tooling setup to minimize work hardening from cutting forces

• Flood cooling with sulfur-free, chloride-free cutting fluid

  
  

Industry Applications & Real-World Use Cases

1. Chemical Processing — Acid Production and Handling

Hastelloy C4 is a standard material of construction in plants producing or handling hydrochloric acid, sulfuric acid, and phosphoric acid. Its combination of high molybdenum and low iron content provides corrosion rates far below those of stainless steels or even C276 in many HCl-rich environments.

Typical components: Heat exchanger tubes and tube sheets, reactor vessels and agitator shafts, pump casings and impellers, pipeline piping and valves, expansion bellows in acid gas lines.

Example: A chemical plant producing wet-process phosphoric acid typically uses C4 for heat exchanger tube bundles exposed to hot, concentrated phosphoric acid with HF and HCl impurities. C4 typically achieves service lifetimes of 8–12 years in this service, compared to 2–4 years for 316L stainless steel.

2. Pharmaceutical & Fine Chemical Manufacturing

Regulatory requirements in pharmaceutical manufacturing demand materials that do not contaminate the product, resist repeated cleaning-in-place (CIP) with aggressive acid and caustic solutions, and can be welded to a smooth, crevice-free surface finish.

C4 meets all these requirements. Its low carbon and tungsten-free composition ensure that welded reactor internals, transfer piping, and vessel nozzles have the same corrosion resistance as the parent metal — no post-weld annealing required.

Typical components: Reactor vessels, distillation columns, transfer piping, condenser tubes, and fluid-handling manifolds.

3. Nuclear Fuel Reprocessing

Nuclear fuel reprocessing plants use concentrated nitric acid to dissolve spent fuel. This is one of the most demanding corrosion environments known — boiling, highly oxidizing, high concentration nitric acid with dissolved radioactive species.

C4's resistance to intergranular corrosion, particularly in boiling nitric acid, makes it one of the few alloys qualified for this service. Its performance in the ASTM G28 Huey test (boiling 65% HNO₃) is a key acceptance criterion for this application.

Typical components: Dissolution vessels, evaporator tubes, transfer piping, and off-gas condensers.

4. Flue Gas Desulfurization (FGD) Systems

FGD scrubbers in power plants and industrial boilers expose metal surfaces to hot, wet sulfuric acid with chloride contamination — an environment that corrodes most stainless steels rapidly. Hastelloy C4's resistance to chloride pitting combined with its sulfuric acid resistance makes it effective in absorber towers, spray nozzles, and recirculation pumps.

Typical components: Absorber vessel liners and internals, spray nozzles, recirculation pump components, expansion joints.

5. Expansion Bellows & Flexible Joints

C4's exceptional ductility (40% elongation minimum) and fatigue resistance make it a preferred material for expansion bellows in high-temperature, corrosive piping systems. The alloy can be formed into complex bellows geometries without cracking, and it maintains its corrosion resistance at the thin wall sections typical of bellow construction.

Industries using C4 bellows: Chemical plants, power generation, petrochemical processing, and shipboard systems.

6. Industrial Heating & Thermal Processing Equipment

At temperatures above 650°C where many other alloys begin to suffer from grain boundary precipitation and oxidation, C4 remains stable. This makes it suitable for muffle furnace components, heat treatment fixtures, and high-temperature reactor internals.

Standard Size Range

Seamless Pipe & Tube

Plate & Sheet

Bar & Rod

Pipe Fittings

All standard butt-weld fittings to ASME B16.9 and ASME B16.28:

• Elbows (90°, 45°, 180°)

• Tees (equal and reducing)

• Reducers (concentric and eccentric)

• Caps

• Stub ends

Forged fittings to ASME B16.11 (socket weld and threaded).

Product Selection Guide: Is Hastelloy C4 Right for Your Project?

Use this guide to quickly assess whether C4 is the appropriate material choice:

C4 is likely the right choice if:

Your process involves hydrochloric or phosphoric acid at any concentration

Your equipment will be welded without post-weld heat treatment.

Service temperature will exceed 650°C for extended periods.

You need nitric acid resistance combined with reducing acid capability.

Your component is a formed part (bellows, vessel head, complex fitting) requiring high ductility.

Your application is in pharmaceutical, nuclear, or food-grade service where material purity is critical

Consider C276 instead if:

You need the broadest possible coverage across mixed acid environments at moderate temperatures.

Tungsten content is acceptable for your application.

You are purchasing from standard stock and C4 lead time is a concern (C276 has broader availability).

Operating temperature stays below 500°C

Consider C22 instead if:

Your process swings between strongly oxidizing and strongly reducing conditions

You need maximum resistance to pitting in oxidizing chloride environments.

Flue gas desulfurization or similar mixed-oxidant environments are involved.

Consider 316L or 904L instead if:

The operating environment is mild to moderate (dilute acids, neutral pH, ambient temperature).

Cost is the primary constraint and the service conditions are within stainless steel's capability

Design life is shorter and material replacement is planned.

Frequently Asked Questions

Q: Can Hastelloy C4 be used in the as-welded condition without annealing?

Yes. This is one of C4's most important practical advantages. The ultra-low carbon and tungsten-free composition prevents grain boundary sensitization during welding, so most applications do not require post-weld solution annealing. This simplifies fabrication and significantly reduces cost on large welded assemblies.

Q: What is the maximum service temperature for C4?

C4 is rated for continuous service up to approximately 1040°C in oxidizing environments. For reducing or sulfur-containing high-temperature atmospheres, the practical limit is lower—consult your engineering team for specific service conditions.

Q: How does C4 compare to C276 in HCl service?

Both alloys perform very well in hydrochloric acid. C4 generally shows slightly lower corrosion rates than C276 in concentrated HCl above 50°C, partly because of its lower iron content. The difference is small under most conditions, and C276 is an acceptable alternative in the majority of HCl service applications.

Q: Is Hastelloy C4 magnetic?

No. C4 is a fully austenitic, solid-solution alloy and is non-magnetic in all standard conditions. It will not be attracted to a magnet.

Q: What certifications come with your C4 material?

All material is supplied with a full mill test report (MTR) covering chemical composition, mechanical properties, and heat treatment. Certificates reference the applicable ASTM/ASME standard. EN 10204 3.1 or 3.2 certificates are available as required. Third-party inspection by SGS, Bureau Veritas, TÜV, or others is available at the customer's request.

Q: What is the typical lead time for Hastelloy C4 pipe and fittings?

Standard sizes in seamless pipe (1/2" to 6" NPS) are typically available from stock with 2–4 weeks' delivery. Larger diameters, custom wall thicknesses, and full fitting sets are typically 6–12 weeks depending on form and quantity. Contact us with your project schedule for a precise lead time.

Hastelloy C4 Standar and Specification

Hastelloy C4 production strictly complies with ASME and ASTM industry standards. For different product forms, there are corresponding production and material standard requirements:

Plate, Sheet, and Strip: ASME SB-575 / ASTM B575

Bar and Wire: ASTM B574.pdf

Seamless Pipe and Tube: ASTM B622.PDF

Welded Pipe: ASTM B619.pdf

Forgings: ASTM B574.pdf

Available Product Forms & ASTM Standards

We supply Hastelloy C4 in all standard mill product forms, all produced and tested to the applicable ASTM/ASME standards:

All materials are supplied with full mill test reports (MTR), traceable heat and lot numbers, and chemical and mechanical certification per the applicable standard. Third-party inspection (SGS, Bureau Veritas, TÜV) is available on request.

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