Hastelloy C-4 vs C-276: Thermal Stability Advantage for As-Welded Chemical Process Equipment

What Are Hastelloy C-4 and C-276, and How Do Their Compositions Differ?

C-4 and C-276 share nearly identical chromium and molybdenum content (16% each), but C-276 adds 4% tungsten and carries more iron and cobalt, while C-4 adds a small titanium addition and holds iron to a stricter 3% maximum - differences that look minor on paper but govern how each alloy's grain boundaries behave when heated during welding.

Source: Haynes International nominal composition data sheets for HASTELLOY C-4 and C-276 alloys.

Both alloys are ultra-low-carbon (0.01% max.) and ultra-low-silicon (0.08% max.) nickel-chromium-molybdenum alloys - the combination that first made C-276 famous in the 1960s as the material that let engineers weld a Ni-Cr-Mo alloy without triggering the intergranular corrosion problems that had plagued earlier, higher-carbon nickel alloys. C-4 was developed afterward specifically to go one step further: by removing tungsten and tightening iron content, Haynes International engineered an alloy that is, in its own words, "the most microstructurally stable of the widely used nickel-chromium-molybdenum materials."

Why Does Thermal Stability Matter for As-Welded Chemical Process Equipment?

Welding heats the metal next to the weld - the heat-affected zone (HAZ) - into a temperature range where chromium, molybdenum, and tungsten can precipitate out of solid solution as second-phase particles along grain boundaries, locally depleting the surrounding metal of the very elements that give the alloy its corrosion resistance and creating a corrosion-prone path exactly where the equipment is most likely to be in tension.

This process - commonly called sensitization when it happens in austenitic stainless steels, and more broadly known as grain-boundary or intermetallic precipitation in nickel alloys - does not require a mistake. Every multi-pass weld, every thick-section repair, and every field weld made without a furnace nearby exposes some volume of the HAZ to the danger-zone temperature range for some length of time.

The question that actually matters for fabricators and asset owners is not whether that exposure happens, but how much time at temperature it takes before precipitation reaches a damaging depth. That is exactly what time-temperature-sensitization (TTS) data measures, and it is the single most important technical fact separating C-4 from C-276 in as-welded service.

How Do C-4 and C-276 Compare in Time-Temperature-Sensitization Behavior?

C-4 must be held at its most critical temperature (825°C / 1,517°F) for a full 2 hours before grain-boundary attack reaches a damaging 0.05 mm depth in the ASTM G28A test; C-276 reaches the same damage after only 3 minutes at temperatures between 925°C and 1,050°C (1,697-1,922°F) - a difference of roughly 40x in the time available before harm occurs.

Source: Haynes International time-temperature-sensitization (T-T-S) chart, HASTELLOY C-4 alloy data sheet, tested in the ASTM G28A boiling ferric sulfate/sulfuric acid solution.

Haynes International states this trade-off directly: "during welding, heat input is important in the case of C-276 alloy, but of little consequence in the case of C-4 alloy." In practical terms, C-276's TTS "nose" - the fastest point on its precipitation curve - sits at just 3 minutes, a window that a single weld pass on a moderately thick section can pass through during normal air cooling.

C-4's nose sits at 2 hours, a duration that ordinary arc-welding heat input essentially cannot reach without a deliberate, sustained furnace exposure. That is why C-4 is frequently specified for thick-section vessels, multi-pass field repairs, and any job where a welder's heat input and interpass temperature cannot be measured and enforced pass-by-pass.

Which Alloy Provides Better Localized Corrosion Resistance - In the As-Supplied Condition?

In the solution-annealed, unwelded condition, C276 actually outperforms C-4 on the standard pitting and crevice benchmarks - its tungsten addition raises its critical pitting temperature to 150°C versus C-4's 100°C - which is precisely the alloying strategy that trades away C-4's thermal-stability advantage.

Source: Haynes International critical pitting/crevice temperature data (acidified 6% FeCl3, ASTM G48) and stress-corrosion cracking data (boiling 45% MgCl2, ASTM G36), HASTELLOY C-4 and C-276 alloy data sheets.

The same pattern holds in Haynes' more severe "Green Death" pitting test (11.5% H2SO4 + 1.2% HCl + 1% FeCl3 + 1% CuCl2): C-4 first shows pitting at 85°C, while C-276 resists pitting all the way to the boiling point of the solution. This is the article's central trade-off: tungsten measurably improves C-276's resistance to chloride-driven pitting and crevice attack in properly solution-annealed material - but it is also the alloying addition most responsible for C-276's much faster secondary-phase precipitation kinetics, and therefore its narrower welding-heat-input tolerance. Neither alloy is simply "better"; each optimizes a different property at the other's expense.

How Does Welding Affect Corrosion Resistance in Practice?

Haynes International publishes a dedicated weld-metal-versus-wrought-base-metal corrosion comparison specifically for C-276, showing weld metal running close to, but measurably higher than, wrought base metal in key acids - while no equivalent table exists for C-4, consistent with a chemistry that does not sensitize under normal welding heat input and therefore needs no separate weld-metal qualification.

Source: Haynes International all-weld-metal (multi-pass GMAW cruciform) vs. wrought base-metal corrosion data, HASTELLOY C-276 alloy data sheet.

The gap between C-276's weld metal and wrought base metal is modest at these moderate temperatures - typically 2-4x, not catastrophic - but it is real, and it is the reason welding procedure specifications (WPS) for C-276 pressure equipment routinely mandate maximum interpass temperature limits, controlled heat input, and multi-pass stringer-bead technique on thick sections. For C-4, Haynes' fabrication guidance imposes no comparable heat-input discipline specifically to protect corrosion resistance, because the alloy's chemistry does not give heat input a meaningful lever to pull.

How Do C-4 and C-276 Compare in Bulk Acid Resistance - HCl and Sulfuric Acid?

Haynes International describes C-4 as having "virtually the same corrosion resistance" as C-276, and that holds well in hydrochloric acid - but published boiling sulfuric acid data shows C-276 with meaningfully lower corrosion rates, roughly half of C-4's, across the 10-30% concentration range.

Source: Haynes International hydrochloric and sulfuric acid corrosion tables, HASTELLOY C-4 and C-276 alloy data sheets.

This is a useful, data-backed correction to the common shorthand that C-4 is "just a more weldable C-276." For dilute-to-moderate boiling sulfuric acid duty specifically, C-276 is the better-performing alloy in the properly solution-annealed or correctly welded condition. C-4's real advantage is not raw acid resistance - it is the guarantee that the alloy's resistance will not degrade in the HAZ regardless of how the equipment was welded.

Neither alloy is intended for strongly oxidizing acid service: Haynes does not publish a nitric acid table for C-4 at all, and C-276's own nitric acid data shows corrosion rates climbing sharply at higher concentrations and temperature (18.42 mm/y at 60% boiling, 22.12 mm/y at 65% boiling) - both alloys should be ruled out for boiling, concentrated nitric acid duty. Where oxidizing or mixed nitric-acid service is the driver, higher-chromium alloys such as G-30, G-35, or C-22 are the more appropriate specification.

How Do Mechanical Properties and Fabrication Characteristics Compare?

C-4 and C-276 share essentially the same strength class and both are supplied mill-annealed with no post-weld heat treatment required for corrosion performance, but C-4 anneals and hot-forges at a slightly lower temperature and is not available as forgings, while C-276 is.

Source: Haynes International tensile, hardness, and fabrication data, HASTELLOY C-4 and C-276 alloy data sheets.

Which ASTM, ASME, and Industry Standards Govern C-4 and C-276?

C-4 and C-276 share nearly parallel ASTM/ASME product-form specifications for sheet, bar, and pipe, but they diverge on two points worth flagging in a procurement specification: C-276 carries a dedicated forgings specification and NACE MR0175/ISO 15156 sour-service qualification, while C-4's published specification table lists neither.

Source: Haynes International "Specifications and Codes" tables, HASTELLOY C-4 and C-276 alloy data sheets.

Which Alloy Should You Specify for As-Welded Chemical Process Equipment?

Specify C-4 wherever weld heat input cannot be tightly measured and controlled - thick-section field welds, multi-pass repairs, and shops without qualified low-heat-input procedures; specify C-276 where forgings are needed, where shop welding procedures are qualified and heat-input-controlled, or where its higher critical pitting temperature and lower boiling-sulfuric-acid corrosion rate deliver a measurable service-life benefit.

Choose C-4 for: Thick-section vessels and piping fabricated with multiple weld passes, where cumulative HAZ heat exposure is hard to bound.

Choose C-4 for: Field repair welding where interpass temperature and heat input cannot be instrumented and enforced the way they can in a controlled shop.

Choose C-4 for: Equipment that may see incidental service exposure in the 650-1050°C range (e.g., near furnaces or during upset conditions), where resistance to in-service sensitization matters as much as weld sensitization.

Choose C-276 for: Forged components - flanges, fittings, valve bodies - since C-276 carries an ASTM B564 forgings specification and C-4 does not.

Choose C-276 for: Shop-fabricated equipment with a qualified, low-heat-input welding procedure specification (WPS) and controlled interpass temperature, where C-276's sensitization risk is well managed.

Choose C-276 for: Boiling, dilute-to-moderate (10-30%) sulfuric acid service and severe chloride-pitting environments, where its measurably lower corrosion rate and higher CPT/CCT translate into longer service life.

Choose C-276 for: Sour-service (NACE MR0175/ISO 15156) applications, where C-276 carries the explicit qualification and C-4 does not.

In both cases, the underlying engineering principle is the same one Haynes International states directly for C-4: microstructural stability determines whether heat input is a variable you must manage or one you can largely ignore. Where welding conditions are well controlled, that variable matters less, and C-276's broader property set and wider availability often make it the more economical, more corrosion-resistant choice. Where welding conditions cannot be tightly controlled, C-4's thermal stability removes the single biggest source of as-welded corrosion risk.

Frequently Asked Questions

Q: What is the main difference between Hastelloy C-4 and C-276?

A: C-276 (UNS N10276) adds 4% tungsten and slightly more iron than C-4 (UNS N06455), which gives C-276 higher pitting and crevice resistance in the solution-annealed condition but makes it far more prone to fast grain-boundary precipitation when heated during welding. C-4 removes tungsten and is, in Haynes International's own words, the most microstructurally stable of the widely used nickel-chromium-molybdenum alloys.

Q: Why is C-4 considered better for as-welded equipment than C-276?

A: Because C-4 needs 2 hours at its most critical temperature (825°C) before grain-boundary attack reaches a damaging depth, while C-276 reaches the same damage in just 3 minutes between 925-1050°C. That roughly 40x difference means weld heat input control is critical for C-276 but of little consequence for C-4.

Q: Does C-4 or C-276 have better corrosion resistance overall?

A: Neither is categorically better. C-276 has a higher critical pitting temperature (150°C vs. 100°C for C-4) and lower corrosion rates in boiling dilute sulfuric acid, but only if welding heat input is controlled. C-4 delivers essentially the same as-welded corrosion resistance as its wrought, unwelded state, regardless of weld procedure.

Q: Do C-4 and C-276 require post-weld heat treatment?

A: Neither alloy requires post-weld heat treatment for corrosion resistance under normal welding conditions; both are supplied mill-annealed. However, C-276 fabrication procedures typically specify controlled heat input and interpass temperature limits to avoid sensitization on thick sections, a precaution C-4 does not require.

Q: Can Hastelloy C-4 be used where forgings are required?

A: Not according to Haynes International's published specification table, which lists no forgings specification for C-4. C-276 carries ASTM/ASME B564/SB564 for forgings, making it the appropriate choice for forged fittings, flanges, and valve bodies.

Q: Are C-4 and C-276 suitable for nitric acid service?

A: No. Haynes International does not publish nitric acid corrosion data for C-4, and published C-276 data shows corrosion rates rising sharply in concentrated, boiling nitric acid (over 18 mm/y at 60% boiling). For oxidizing or mixed nitric-acid duty, higher-chromium alloys such as G-30, G-35, or C-22 are better suited.

Q: What welding processes and filler metals are used for C-4 and C-276?

A: Both alloys are well suited to GTAW, GMAW, and SMAW. C-4 uses matching ERNiCrMo-7 (bare wire) and ENiCrMo-7 (coated electrode) filler metals; C-276 uses ERNiCrMo-4 and ENiCrMo-4.