Hastelloy C-4: The Most Microstructurally Stable Ni-Cr-Mo Alloy for Chemical Processing

1. Introduction: What Makes Hastelloy C-4 Different?

Hastelloy C-4 (UNS N06455 / W.Nr. 2.4610) occupies a unique position in the Hastelloy C-series family. While C-276 is the "workhorse" general-purpose alloy and C-22 excels in oxidizing acids, C-4 was specifically engineered to solve one critical problem: high-temperature microstructural instability leading to embrittlement.

The key innovation in C-4 is what it removes: no tungsten. The tungsten in C-276 provides excellent corrosion resistance but promotes the formation of sigma (σ) and mu (μ) intermetallic phases when the alloy is exposed to temperatures between 650–1090°C for extended periods. These phases cause severe embrittlement, loss of ductility, and reduced corrosion resistance. By eliminating tungsten, reducing carbon to ≤0.015%, lowering silicon to ≤0.08% and iron to ≤3.0%, and adding titanium (0.70–1.20%) as a carbide stabilizer, C-4 achieves exceptional thermal stability without sacrificing the essential Ni-Cr-Mo corrosion resistance.

This makes C-4 the premier choice for applications where the material will experience prolonged high-temperature exposure, thermal cycling, or where post-weld heat treatment is impractical. It is the only C-series alloy that can be welded and placed directly into high-temperature service without risk of heat-affected zone (HAZ) degradation.

2. Chemical Composition

The chemical composition of Hastelloy C-4 is carefully controlled to maximize thermal stability while maintaining excellent corrosion resistance. The table below presents the composition per ASTM B574/B575 specifications.

Element	Weight % (Range)	Typical %	Role in the Alloy
Nickel (Ni)	Balance	~65.0	Austenitic matrix, stress corrosion cracking resistance, high-temperature toughness
Chromium (Cr)	14.0 – 18.0	16.0	Oxidizing media resistance, passivation, high-temperature oxidation protection
Molybdenum (Mo)	14.0 – 17.0	15.5	Reducing acid resistance (HCl, H²SO&sup4;), pitting & crevice corrosion resistance
Iron (Fe)	≤ 3.0	~1.5	Strictly controlled to enhance thermal stability and corrosion purity
Carbon (C)	≤ 0.015	~0.005	Ultra-low carbon prevents Cr²³C&sup6; precipitation at grain boundaries
Silicon (Si)	≤ 0.08	~0.03	Extremely low silicon improves welding quality and reduces intermetallic phase risk
Manganese (Mn)	≤ 1.0	~0.3	Deoxidation during melting
Cobalt (Co)	≤ 2.0	~0.5	Controlled to minimize cost and avoid high-temperature brittle phases
Titanium (Ti)	0.70 – 1.20	~0.9	Signature element: fixes carbon as stable TiC, preventing intergranular Cr depletion
Phosphorus (P)	≤ 0.040	~0.010	Impurity control
Sulfur (S)	≤ 0.030	~0.005	Impurity control for hot workability
Tungsten (W)	—	None	Deliberately excluded — eliminates sigma/mu phase embrittlement

Key compositional insight: The absence of tungsten combined with titanium stabilization is what separates C-4 from every other C-series alloy. Tungsten, while beneficial for corrosion resistance, is the primary driver of sigma and mu phase formation in the 650–1090°C range. By replacing W's corrosion contribution with carefully balanced Cr+Mo and using Ti to immobilize carbon, C-4 achieves equivalent corrosion resistance without the thermal stability penalty.

3. Physical Properties

Property	Value	Units
Density	8.64	g/cm³
Melting Range	1330 – 1380	°C
Melting Range	2425 – 2515	°F
Thermal Expansion Coefficient (20–100°C)	11.2 × 10−6	/°C
Thermal Expansion Coefficient (20–200°C)	11.8 × 10−6	/°C
Thermal Expansion Coefficient (20–400°C)	12.5 × 10−6	/°C
Thermal Conductivity (at 20°C)	10.1	W/(m·K)
Thermal Conductivity (at 100°C)	10.9	W/(m·K)
Thermal Conductivity (at 200°C)	12.2	W/(m·K)
Thermal Conductivity (at 400°C)	14.5	W/(m·K)
Elastic Modulus (at 20°C)	211	GPa
Elastic Modulus (at 200°C)	204	GPa
Elastic Modulus (at 400°C)	193	GPa
Electrical Resistivity (at 20°C)	1.25	μΩ·m
Specific Heat Capacity (at 20°C)	408	J/(kg·K)
Crystal Structure	Face-Centered Cubic (FCC) — austenitic

4. Mechanical Properties

4.1 Room Temperature Tensile Properties (Solution-Annealed)

Property	Typical Value	ASTM B574 Min. Requirement
Tensile Strength (Rm)	690 – 790 MPa (100 – 115 ksi)	≥ 690 MPa (100 ksi)
Yield Strength (Rp0.2)	280 – 350 MPa (40 – 51 ksi)	≥ 280 MPa (40 ksi)
Elongation (A)	40 – 50%	≥ 40%
Reduction of Area	50 – 65%	—
Hardness	180 – 220 HB	—
Impact Toughness (ISO-V, RT)	≥ 120 J/cm²	—
Impact Toughness (ISO-V, −196°C)	≥ 120 J/cm²	—

4.2 Elevated Temperature Tensile Properties

The following data illustrates C-4's retention of mechanical properties at elevated temperatures. The gradual strength decrease and corresponding ductility increase are characteristic of a stable austenitic structure free from embrittling phase transformations.

Temperature (°C)	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)
Room Temperature	730	310	45
200	650	270	45
400	590	240	45
600	520	220	48
800	380	190	55

Key observation: Even at 800°C, C-4 retains over 50% of its room-temperature tensile strength with increased ductility (55% elongation vs 45% at RT). This is a direct result of the alloy's tungsten-free, titanium-stabilized design — no sigma or mu phases form to embrittle the microstructure.

5. High-Temperature Performance & Thermal Stability

The defining characteristic of Hastelloy C-4 is its unmatched microstructural stability in the 650–1040°C temperature range. This is the critical differentiator between C-4 and all other C-series alloys.

5.1 The Sigma/Mu Phase Problem

In nickel-based superalloys containing tungsten and/or high levels of molybdenum, prolonged exposure to 650–1090°C causes precipitation of topologically close-packed (TCP) phases — primarily sigma (σ) and mu (μ). These intermetallic compounds are hard, brittle, and form at grain boundaries. Their effects are catastrophic:

• Severe embrittlement: Room-temperature ductility can drop from 40%+ to less than 5% after just 100 hours at 870°C
• Loss of corrosion resistance: Chromium and molybdenum are consumed from the matrix to form sigma/mu phases, depleting the surrounding areas of corrosion-resistant elements
• Weld HAZ degradation: The heat-affected zone of welds, which passes through the critical temperature range during cooling, becomes a preferential site for intergranular attack

5.2 How C-4 Solves It

C-4's formulation directly addresses each root cause:

• No tungsten: Tungsten is the most potent sigma/mu promoter in Ni-Cr-Mo alloys. Eliminating it removes the primary driver of TCP phase formation
• Ultra-low carbon (≤0.015%): Prevents M²³C&sup6; chromium carbide precipitation at grain boundaries during thermal exposure
• Titanium stabilization (0.70–1.20% Ti): Titanium preferentially combines with residual carbon to form stable TiC particles. This "locks up" carbon so it cannot form chromium carbides, even at the critical 600–800°C sensitization range
• Low silicon and iron: Both Si and Fe can promote intermetallic phase formation; keeping them at minimum levels further enhances stability

5.3 Oxidation Resistance

With 14–18% chromium, C-4 forms a protective chromium oxide (Cr²O³) scale at elevated temperatures. It can be used in oxidizing atmospheres up to approximately 1000°C (1832°F) for continuous service. However, for the most demanding oxidation resistance above 1000°C, alloys with aluminum additions (such as Haynes 214 or Alloy 602CA) are recommended.

5.4 Cryogenic Performance

Hastelloy C-4 retains its austenitic FCC structure down to cryogenic temperatures. Charpy V-notch impact toughness at −196°C (−320°F) remains above 120 J/cm², making it suitable for LNG, liquid oxygen, and other cryogenic process applications where corrosion resistance is also required.

6. Corrosion Resistance

Despite its tungsten-free design, C-4 delivers corrosion resistance essentially equivalent to C-276 across a wide range of chemical environments. The balanced 16Cr-15.5Mo composition provides comprehensive protection in both oxidizing and reducing media.

6.1 Reducing Acid Environments

• Hydrochloric acid (HCl): Excellent resistance at all concentrations up to moderate temperatures. The high molybdenum content provides protection in reducing HCl where chromium-based passive films are ineffective.
• Sulfuric acid (H²SO&sup4;): Good resistance in dilute to medium concentrations. For concentrated hot sulfuric acid, silicon-containing alloys (such as Hastelloy D-205) may be preferred.
• Phosphoric acid (H³PO&sup4;): Excellent resistance, including wet-process phosphoric acid containing fluoride and chloride impurities — a key application in fertilizer production.
• Organic acids: Outstanding resistance to formic acid, acetic acid, and other organic acids even at elevated temperatures.

6.2 Oxidizing Media

• Nitric acid (HNO³): Good resistance due to chromium content. For very aggressive nitric acid service, higher-chromium alloys such as C-22 (22% Cr) may offer additional margin.
• Chlorine and hypochlorite: Resists wet chlorine gas, sodium hypochlorite, and chlorine dioxide solutions commonly encountered in bleach plants and water treatment.
• Ferric and cupric chlorides: Excellent pitting and crevice corrosion resistance in these aggressive oxidizing chloride environments.

6.3 Localized Corrosion

• Pitting Resistance Equivalent (PREN): PREN = %Cr + 3.3 × %Mo ≈ 16 + 3.3 × 15.5 ≈ 67 — significantly higher than super-duplex stainless steels (PREN 40–50) and comparable to other C-series alloys.
• Crevice corrosion: Excellent resistance in chloride-containing environments, making C-4 suitable for gasketed flanges, tube-to-tubesheet joints, and under-deposit conditions.
• Stress corrosion cracking (SCC): The high nickel content (>60%) provides virtual immunity to chloride-induced SCC — a critical advantage over austenitic stainless steels in hot chloride service.

6.4 Intergranular Corrosion — The C-4 Advantage

This is where C-4 truly excels. The combination of ultra-low carbon and titanium stabilization means that even after prolonged exposure to the 600–800°C sensitization range (e.g., during welding or high-temperature service), the alloy does not develop chromium-depleted zones at grain boundaries. The standard ASTM A262 Practice C (Huey test) and Practice E (Strauss test) show negligible intergranular attack on C-4 in both the solution-annealed and sensitized conditions. This performance is maintained even in the as-welded condition without post-weld heat treatment — a capability that C-276 cannot match.

7. Welding & Fabrication

Hastelloy C-4 exhibits excellent weldability and fabricability, with one crucial advantage: no post-weld heat treatment is required to restore corrosion resistance in the weld zone.

7.1 Welding Processes

All common welding processes are suitable:

• GTAW (TIG): Preferred for root passes, thin sections, and high-quality joints. Use DCEN polarity with argon or argon-helium shielding gas.
• GMAW (MIG): Suitable for thicker sections. Use pulsed-spray transfer mode for best results.
• SMAW (Stick): Acceptable for field welding and repair. Use ENiCrMo-7 coated electrodes.
• Plasma Arc Welding (PAW): Excellent for automated welding of tube-to-tubesheet joints in heat exchangers.
• Submerged Arc Welding (SAW): For heavy-section fabrication with appropriate flux.

7.2 Filler Metals

ERNiCrMo-7 (AWS A5.14) bare wire for GTAW/GMAW/PAW and ENiCrMo-7 (AWS A5.11) covered electrodes for SMAW are the recommended matching filler metals. These fillers are specifically formulated to match C-4's tungsten-free composition and maintain the same thermal stability characteristics in the weld metal.

7.3 Welding Procedure Recommendations

• Pre-weld condition: Material must be in the solution-annealed condition. Remove all scale, oil, grease, and marking ink from the weld area (minimum 25 mm on each side).
• Interpass temperature: Keep below 150°C (300°F) to minimize heat input and prevent excessive grain growth.
• Heat input: Use low to moderate heat input (typically 0.5–1.5 kJ/mm) to maintain fine grain structure in the HAZ.
• Post-weld cleaning: Remove surface oxides and heat tint by pickling (nitric-hydrofluoric acid mixture) or mechanical brushing with a stainless steel wire brush. No post-weld solution annealing is required for corrosion resistance — this is a major operational cost savings over C-276.

7.4 Cold and Hot Forming

• Hot working: 900–1150°C (1650–2100°F). Finish forging above 900°C. Cool rapidly after hot working. Solution anneal after hot working to restore optimum properties.
• Cold working: C-4 has excellent cold formability due to its high ductility (40–50% elongation). It work-hardens at a rate similar to austenitic stainless steels. For severe cold forming (bend radius < 2T), intermediate annealing may be required.
• Solution annealing: 1065–1150°C (1950–2100°F) followed by rapid cooling (water quench or rapid air cool for thin sections). Hold time: typically 30 minutes per 25 mm of thickness.

8. Applications by Industry

8.1 Chemical Processing

• High-temperature reactors and pressure vessels for mixed acid processes where thermal cycling occurs during batch operations
• Distillation columns and trays handling chlorinated organics at elevated temperatures
• Shell-and-tube heat exchangers with C-4 tubes and tubesheets for acid recovery systems
• Acid evaporation and concentration equipment where the alloy sees both high temperatures and aggressive chemistry
• Pharmaceutical reaction vessels requiring both corrosion resistance and product purity (no metal ion contamination)

8.2 Flue Gas Desulfurization (FGD)

• Scrubber vessels and internals exposed to hot, wet, chloride-laden flue gas
• Outlet ducting and dampers experiencing thermal cycling and acid condensation
• Mist eliminators and spray headers in limestone/gypsum FGD systems
• Waste incineration flue gas treatment where HCl, SOx, and heavy metal chlorides are present at elevated temperatures

8.3 Petroleum Refining & Petrochemical

• Crude distillation unit overhead condensers where HCl from chloride hydrolysis causes severe corrosion
• Hydrodesulfurization (HDS) and hydrocracking reactor internals exposed to H²S, H², and naphthenic acids at 300–500°C
• Catalytic reforming unit heat exchangers with chloride stress corrosion cracking risk
• Sour water stripper components handling H²S, NH³, and chlorides

8.4 Nuclear & Energy

• Nuclear fuel reprocessing equipment exposed to nitric acid with fission product contamination
• Geothermal energy extraction handling hot, acidic, chloride-rich brine with dissolved H²S and CO²
• Waste-to-energy plants with aggressive flue gas condensates

8.5 Other Industries

• Pulp & paper: Bleach plant washers, chlorine dioxide towers, and filtrate systems
• Seawater desalination: Multi-stage flash (MSF) and multi-effect distillation (MED) evaporator components
• Fine chemicals: High-purity organic synthesis equipment where metal contamination must be avoided
• Electronics: Wet etching and cleaning equipment for semiconductor manufacturing

9. Product Forms & Availability

Hangbo Alloy Group supplies Hastelloy C-4 in the following product forms, all manufactured to applicable ASTM/ASME specifications:

Product Form	Applicable Standard	Typical Size Range
Round Bars / Rods	ASTM B574 / ASME SB574	Diameter 6 – 320 mm
Flat Bars	ASTM B574	Thickness 5 – 80 mm, Width 20 – 200 mm
Hexagonal Bars	ASTM B574	Across flats 8 – 65 mm
Plates (Hot-Rolled)	ASTM B575 / ASME SB575	Thickness 3.0 – 100 mm
Sheets (Cold-Rolled)	ASTM B575	Thickness 0.4 – 3.0 mm
Strips / Coils	ASTM B575	Thickness 0.1 – 2.0 mm
Seamless Tubes / Pipes	ASTM B622 / ASME SB622	OD 6 – 650 mm, Wall 1 – 32 mm
Welded Tubes / Pipes	ASTM B619 / B626 / B775	OD 12 – 1200 mm, Wall 0.5 – 12 mm
Butt-Weld Fittings	ASTM B366	Elbows, Tees, Reducers, Caps, Stub Ends (1/2" – 24" NPS)
Forgings	ASTM B564 / ASME SB564	Flanges, Valve Bodies, Custom Shapes (up to 2000 kg)
Welding Wire	AWS A5.14 ERNiCrMo-7	Diameter 0.8 – 4.0 mm (spools, coils, cut lengths)
Welding Electrodes	AWS A5.11 ENiCrMo-7	Diameter 2.5 – 5.0 mm

All products are supplied in the solution-annealed and pickled/passivated condition, with EN 10204 Type 3.1 material certificates including chemical analysis, mechanical test results, and NDT reports as applicable. Full material traceability from melt to finished product is maintained.

10. Applicable Standards & Specifications

Standard	Description
ASTM B574	Standard Specification for Nickel-Chromium-Molybdenum Alloy Rod and Bar (UNS N06455)
ASTM B575	Standard Specification for Low-Carbon Nickel-Chromium-Molybdenum Alloy Plate, Sheet, and Strip
ASTM B619	Standard Specification for Welded Nickel-Chromium-Molybdenum Alloy Pipe
ASTM B622	Standard Specification for Seamless Nickel-Chromium-Molybdenum Alloy Pipe and Tube
ASTM B626	Standard Specification for Welded Nickel-Chromium-Molybdenum Alloy Tube
ASTM B366	Standard Specification for Factory-Made Wrought Nickel Alloy Fittings
ASTM B564	Standard Specification for Nickel Alloy Forgings
ASTM B775	General Requirements for Nickel Alloy Welded Pipe
ASME SB-574 / SB-575 / SB-622	ASME Boiler & Pressure Vessel Code equivalents (Section II, Part B)
DIN 17744 / 17750 / 17752	German standards for NiMo16Cr16Ti (W.Nr. 2.4610)
VdTÜV 502	German pressure vessel material approval for 2.4610
AWS A5.14 ERNiCrMo-7	Filler metal specification for GTAW/GMAW/PAW
AWS A5.11 ENiCrMo-7	Covered electrode specification for SMAW
GB/T NS335	Chinese national standard equivalent grade
NACE MR0175 / ISO 15156	Sour service qualification (Level VI/VII for H²S-containing environments)

11. Hastelloy C-Series Selection Guide

Alloy	UNS	Key Feature	Best Application	Thermal Stability
C-4	N06455	No W, Ti-stabilized, ultra-low C/Si/Fe	High-temp long-term service, thermal cycling, weld-heavy fabrication	Excellent (best in class)
C-276	N10276	General-purpose with W for broad corrosion	Mixed acid environments, FGD, chemical processing	Poor (embrittles 650–1090°C)
C-22	N06022	Higher Cr (20–22.5%) for oxidizing acids	Nitric acid, pickling, pollution control	Moderate
C-2000	N06200	Cu addition for oxidizing/reducing balance	Multi-purpose, varying media composition	Moderate
B-3	N10675	High Mo (27–32%), optimized for HCl	Pure HCl service at all concentrations/temperatures	Good (but limited oxidizing media resistance)

Selection rule of thumb: If your application involves prolonged exposure above 600°C, thermal cycling, or requires as-welded corrosion resistance without PWHT → choose C-4. If your application is primarily at ambient to moderate temperatures and you need the broadest chemical resistance → C-276 or C-2000 may be more cost-effective.

12. Frequently Asked Questions (FAQ)