Hastelloy Welding: Process Selection, Consumables, and Precautions

Category: Welding Metallurgy  |  Reading time: ~20 min

Hastelloy is the trade name for a family of nickel-molybdenum and nickel-chromium-molybdenum superalloys developed by Haynes International. These alloys are specified in chemical processing plants, pharmaceutical reactors, flue gas desulphurisation (FGD) systems, nuclear waste handling, and offshore oil and gas equipment precisely because they resist corrosive environments that rapidly degrade carbon steel, stainless steel, and even titanium. Hastelloy C-276, for instance, is one of the most widely used materials of construction for vessels and piping handling wet chlorine, chlorinated solvents, hydrofluoric acid, and reducing acids.

Welding Hastelloy is not inherently difficult — the alloys are not prone to hydrogen-assisted cold cracking and generally do not require preheat — but they demand strict procedural discipline. Contamination with low-melting-point elements, excessive heat input, inadequate shielding, and incorrect filler metal selection are the four failure modes that account for the majority of defective Hastelloy welds encountered in the field. This guide addresses each of these systematically, covering all common grades, processes, consumable specifications, parameter ranges, and the inspection requirements applicable under ASME and AWS codes.

1. Hastelloy Grades — Composition and Corrosion Properties

The Hastelloy family encompasses more than a dozen commercial grades. For pressure equipment and piping in the chemical process industry, four grades account for the vast majority of fabricated components. Understanding the composition and intended service of each grade is essential before selecting consumables and designing a welding procedure.

1.1 Hastelloy C-276 (UNS N10276)

C-276 is the workhorse of the Hastelloy family. Its composition of nominally 57% Ni, 15.5% Cr, 16% Mo, 3.75% W, and 5% Fe delivers outstanding resistance to both oxidising and reducing environments, pitting, crevice corrosion, and stress-corrosion cracking. C-276 is the standard choice for chemical reactors, heat exchangers, and piping handling sulphuric acid, hydrochloric acid, phosphoric acid, wet chlorine gas, and mixed acid systems. It is also widely used in FGD scrubbers, pharmaceutical manufacturing, and pollution control equipment.

1.2 Hastelloy C-22 (UNS N06022)

C-22 has higher chromium (22%) and lower molybdenum (13%) than C-276, plus a small tungsten addition, giving it superior resistance to pitting and crevice corrosion in oxidising chloride environments. It was specifically developed to address the localised corrosion susceptibility of C-276 in certain highly oxidising conditions. C-22 is the alloy of choice for nuclear waste handling canisters (the Yucca Mountain repository specifications are written around C-22), pharmaceutical API synthesis vessels, and bleaching equipment in pulp and paper mills.

1.3 Hastelloy B-2 (UNS N10665)

B-2 is a nickel-molybdenum alloy with ~69% Ni, 28% Mo, and minimal chromium. The very high molybdenum content gives it exceptional resistance to hydrochloric acid at all concentrations and temperatures, as well as to sulphuric, phosphoric, and acetic acids in their reducing forms. It has essentially no resistance to oxidising environments or oxidising acid mixtures (even small amounts of ferric or cupric ions dramatically accelerate corrosion). B-2 has particular welding challenges discussed in Section 2.4.

1.4 Hastelloy X (UNS N06002)

Hastelloy X contains ~47% Ni, 22% Cr, 9% Mo, and 18% Fe. It was designed primarily for high-temperature applications (up to ~1200°C) where oxidation resistance and strength at elevated temperature are required: gas turbine combustion chambers, petrochemical furnace components, and industrial furnace muffles. Unlike the C-series alloys, X is not optimised for aqueous corrosion resistance; its higher iron content makes it less resistant to reducing acids.

2. Weldability of Hastelloy Alloys

Hastelloy alloys are generally considered to have good weldability relative to many other nickel superalloys. They are not age-hardenable in the way that Waspaloy or IN-718 are, and they do not suffer from hydrogen-assisted cold cracking. However, three metallurgical challenges are specific to these alloys and must be managed during welding procedure development.

2.1 Hot cracking (solidification and liquation cracking)

Hastelloy alloys solidify over a wide temperature range, and the weld pool is susceptible to solidification cracking if low-melting-point impurities (sulphur, phosphorus, silicon, boron) segregate to the solidification front. The high alloy content produces a sluggish, viscous weld pool that does not degas or reject impurities as effectively as carbon steel. Liquation cracking in the HAZ — where grain boundary films melt at temperatures below the solidus — is also possible if the base material or filler contains elevated sulphur or if contamination is introduced before welding.

The metallurgical remedy is compositional: Hastelloy filler metals are produced with very low sulphur, phosphorus, and silicon contents. The fabrication remedy is rigorous cleanliness, addressed in Section 6.

2.2 Sensitisation and secondary phase precipitation

When Hastelloy C-series alloys are held in the temperature range approximately 600–1000°C, chromium- and molybdenum-rich carbides (M₆C, M₂₃C₆) and intermetallic phases (sigma, mu, P-phase) precipitate at grain boundaries. This depletes the adjacent matrix of Cr and Mo, reducing corrosion resistance in the sensitised zone. The HAZ of a Hastelloy weld necessarily passes through this sensitisation range on both heating and cooling. The goal of welding procedure development is to minimise the time spent in this range — achieved through heat input control, interpass temperature limits, and rapid cooling.

2.3 Grain coarsening in the HAZ

Hastelloy alloys have a face-centred cubic (FCC) austenitic structure with no solid-state phase transformation. Unlike steel, there is no transformation-based mechanism for grain refinement on cooling. Grains therefore grow monotonically during the weld thermal cycle in proportion to peak temperature and time at temperature. Coarse-grained HAZ microstructures reduce toughness and increase susceptibility to liquation cracking in subsequent passes. Heat input limits minimise the grain-coarsened zone width.

2.4 Special considerations for Hastelloy B-2

B-2 presents additional challenges that set it apart from the C-series. Its very high molybdenum content makes it particularly prone to segregation of molybdenum-rich phases during solidification, producing a heterogeneous as-welded microstructure. B-2 welds are also susceptible to ordered B2 phase formation if cooled slowly through ~700–900°C, causing severe embrittlement. Rapid cooling after welding is therefore essential. The interpass temperature limit for B-2 is lower than for C-series (generally 93°C / 200°F maximum). Solution annealing of finished B-2 weldments is strongly recommended.

3. Welding Process Selection

Several fusion welding processes are used for Hastelloy fabrication. The selection depends on component size, thickness, position, joint geometry, and the quality requirements of the applicable code. Each process has distinct advantages and limitations for nickel alloy work.

4. Filler Metals and Consumable Specifications

Hastelloy filler metals are covered by AWS A5.14 (bare wire, GTAW/GMAW) and AWS A5.11 (covered electrodes, SMAW). Selecting the correct filler metal involves matching the base metal composition, the service environment, and in some cases applying a deliberate overmatching strategy for particularly aggressive corrosive service.

4.1 AWS A5.14 bare wire specifications

4.2 AWS A5.11 covered electrode specifications (SMAW)

4.3 Electrode conditioning and storage

Hastelloy SMAW electrodes are supplied in hermetically sealed containers. Once opened, they must be stored in a heated oven at 120–150°C and conditioned (rebaked) at 260–315°C for 1 hour before use. Electrodes exposed to ambient air for more than 4 hours should be reconditioned or discarded. Moisture absorbed by the flux coating introduces hydrogen into the weld pool, increasing the risk of porosity and potentially contributing to embrittlement in stress-corrosion-cracking-sensitive environments.

5. Welding Parameters and Joint Preparation

5.1 Joint design

Hastelloy weld pools are significantly more viscous than carbon or stainless steel pools due to the high alloy content. This reduces fluidity and makes it harder to wet out the fusion faces, increasing the risk of lack of fusion at the groove walls. Joint angles are therefore typically wider than for equivalent steel joints: included angles of 60–70° for V-grooves (versus 60° for steel) and 75–80° for GTAW multi-pass butt joints in pipe. Root openings of 2.5–3.5 mm are standard for backed butt welds; root faces are kept narrow (0.5–1.5 mm) to ensure complete root fusion with the low heat input GTAW root pass.

5.2 Typical GTAW parameters

5.3 Electrode (tungsten) selection for GTAW

Thoriated tungsten (EWTh-2, 2% ThO₂) or ceriated tungsten (EWCe-2, 2% CeO₂) electrodes are used for GTAW of Hastelloy on DCEN. Electrode diameter is selected based on current range: 1.6mm for <100A, 2.4mm for 100–200A, 3.2mm for >200A. The electrode tip should be prepared to a sharp taper (30–45° included angle) with a small flat tip (approximately 0.5–1mm diameter). Electrodes contaminated by contact with the filler wire or weld pool must be re-ground immediately using a grinding wheel dedicated to nickel alloys.

5.4 Bead technique

String beads (stringer beads) are strongly preferred over weaving for Hastelloy. Wide weave beads increase the heat input per unit length and extend the time the HAZ spends in the sensitisation temperature range. Maximum bead width should be limited to approximately 3× the electrode or wire diameter. Each bead should be deposited relatively fast to keep heat input low, and the torch should be moved with a steady, consistent travel speed rather than the slow oscillation used in structural steel work.

6. Critical Precautions

6.1 Surface cleanliness — the most important precaution

Contamination of Hastelloy joint surfaces with low-melting-point elements is the single most common cause of weld cracking in the field. The following contaminants are particularly dangerous:

The required pre-weld cleaning procedure is:

1. Degrease all joint surfaces and adjacent metal (at least 50mm each side of the joint) with a clean, lint-free cloth and an approved solvent (acetone or isopropyl alcohol). Use fresh solvent and clean cloths — do not recycle contaminated solvent.
2. Wire brush with a stainless steel brush that is dedicated exclusively to nickel alloys. Wire brushes used on carbon steel or stainless steel introduce iron contamination.
3. Inspect for any residual paint markings, adhesive labels, or protective coatings. Remove completely before welding.
4. Weld immediately after cleaning. If more than 2 hours elapse, reclean.
5. Ensure galvanised or zinc-coated items (including temporary supports, clamps, and fixtures) do not contact Hastelloy weld areas during or after heating.

6.2 Back-purging for pipe and vessel work

Hastelloy weld roots exposed to the atmosphere during GTAW develop heavy oxidation (“sugaring”) that severely damages corrosion resistance by depleting chromium at the weld surface. Back-purging with high-purity argon (99.999%) during root pass welding is mandatory for all pipe and vessel root passes. The back-purge system must maintain a positive argon flow throughout root welding, and the purge gas must be allowed to fully displace atmospheric oxygen before the arc is struck. The residual oxygen level inside the purge cavity should be <100 ppm, ideally <20 ppm, before welding begins. Oxygen monitors (weld purge monitors) are used to verify this in critical applications.

6.3 Heat input control

Maximum heat input should be specified in the WPS and must not be exceeded in production. For Hastelloy C-series alloys, maximum heat input limits typically fall in the range 1.5–2.5 kJ/mm for wall thicknesses above 10mm, with tighter limits of 0.5–1.5 kJ/mm for thin sections (<6mm). The heat input formula:

where k = 0.6 for GTAW, 0.8 for GMAW, 0.8 for SMAW. Welding engineers should specify and document actual voltage, current (measured at the arc, not the power source display), and travel speed for each pass during PQR.

6.4 Interpass temperature

The 177°C (350°F) maximum interpass temperature limit is non-negotiable for Hastelloy C-series in corrosion service. Temperature must be measured by contact pyrometer or calibrated thermocouple immediately adjacent to the joint, not by estimated time between passes. If the interpass temperature is exceeded, the weld must cool before the next pass is deposited. For Hastelloy B-2, reduce the maximum interpass temperature to 93°C (200°F).

6.5 Avoiding arc strikes outside the weld groove

Stray arc strikes on Hastelloy base metal create localised hardened, sensitised zones that are potential initiation sites for stress-corrosion cracking. All arc strikes outside the fusion zone must be reported, and the affected area must be examined by PT (dye penetrant testing). If cracking is found, the affected material must be cut out and replaced. WPS documentation and welder qualification records should specifically prohibit arc strikes outside the weld preparation.

6.6 Grinding and mechanical preparation

All grinding discs, cutting discs, and abrasive media used on Hastelloy must be iron-free and chloride-free, and dedicated to nickel alloy use. Alumina (Al₂O₃) grinding wheels are preferred. Silicon carbide can be used but carbide contamination of the surface is possible. Zirconia abrasives are acceptable. Interpass cleaning between passes should be done with a dedicated stainless steel wire brush or by light grinding; remove all slag (for SMAW), spatter, and discolouration before depositing the next pass.

7. Post-Weld Heat Treatment

PWHT requirements for Hastelloy diverge significantly from the approach used for carbon and low-alloy steels, and misapplication of standard steel PWHT procedures to Hastelloy weldments is a serious fabrication error.

7.1 Standard stress-relief PWHT — do not apply

The temperature range used for standard carbon steel stress-relief PWHT (600–720°C) falls directly within the sensitisation range for Hastelloy C-series alloys. Holding Hastelloy weldments at these temperatures — even for short periods — will cause extensive carbide and intermetallic precipitation at grain boundaries, severely degrading corrosion resistance throughout the weld and HAZ. This is not recoverable without a subsequent solution anneal. Standard PWHT must never be applied to Hastelloy welds.

7.2 Solution annealing — the correct treatment

Where post-weld thermal treatment is required to dissolve sensitisation products, reduce residual stresses, or restore full corrosion resistance, solution annealing is the correct approach. Parameters by grade:

Solution annealing must be performed in a furnace with accurate temperature control and uniform atmosphere. The heating and cooling must be sufficiently rapid to avoid extended residence in the sensitisation range on both heating and cooling. For large fabrications where water quenching is impractical, forced-air cooling using compressed air jets must achieve a cooling rate of at least 55°C/min through the range 1000–550°C.

8. Dissimilar Metal Welding

In chemical plant fabrication, Hastelloy components frequently connect to adjacent piping and structures fabricated from carbon steel, 300-series stainless steel, or other nickel alloys. Dissimilar metal joints require careful filler metal selection and WPS development.

8.1 Hastelloy to austenitic stainless steel (304, 316, 321, 347)

ERNiCrMo-4 (C-276 composition) or ERNiCrMo-3 (Alloy 625 composition) are the standard filler metals for Hastelloy-to-austenitic stainless joints. Both provide a compositional bridge that accommodates the dilution from both base metals and produces a weld deposit with adequate corrosion resistance in most environments. ERNiCrMo-3 (Alloy 625) is often preferred in mixed service where the stainless steel side is the primary structural alloy and the Hastelloy is the corrosion-resistant liner.

8.2 Hastelloy to carbon or low-alloy steel

Direct welding of Hastelloy to carbon steel using Hastelloy filler is technically possible but problematic due to the large difference in thermal expansion coefficient (α_Hastelloy ≈ 11.2 × 10⁻⁶/°C; α_C-steel ≈ 12 × 10⁻⁶/°C) and the potential for carbon migration from the steel into the nickel alloy weld zone at service temperatures. The standard approach is to apply a nickel alloy butter layer (ENiCrMo-4 or ENiCrFe-3 / Inconel 182) to the prepared carbon steel face, PWHT the buttered assembly if required for the carbon steel component, and then complete the joint to Hastelloy using ERNiCrMo-4 with no further PWHT.

8.3 Hastelloy to duplex or super-duplex stainless steel

ERNiCrMo-10 (C-22 filler) or ERNiCrMo-4 are used depending on the duplex grade and service environment. Super-duplex (2507) to Hastelloy C-276 joints are sometimes specified in subsea and offshore applications. These joints require careful procedure qualification because the high nitrogen content of the duplex base metal can diffuse into the Hastelloy weld deposit, affecting its corrosion resistance. Interpass temperature limits must satisfy the Hastelloy requirement (≤177°C) and not the higher interpass temperatures sometimes applied to duplex welding.

9. Inspection and Testing Requirements

9.1 Visual inspection

All completed Hastelloy welds must be visually inspected per the applicable code. Pay particular attention to: weld surface discolouration (golden-brown or blue tint is acceptable; black or grey heavy oxidation indicates inadequate shielding); arc strikes outside the weld groove; surface porosity; crater cracks; and underfill. Any discolouration darker than light straw colour on the root side indicates inadequate back-purging and must be investigated — the affected run should be removed and re-welded with improved purge gas coverage.

9.2 Liquid penetrant testing (PT)

PT is the primary surface NDT method for Hastelloy weld examination. Water-washable or solvent-removable penetrant systems are used. All Hastelloy welds in pressure equipment service should receive 100% PT of the final weld surface and all interpass surfaces after each pass (particularly after root and first fill passes). PT is more sensitive than MT for the surface-breaking cracks relevant to Hastelloy (hot cracks, liquation cracks), and MT is not applicable to the paramagnetic Hastelloy alloys. PT developer must be removed completely before service exposure in corrosive environments.

9.3 Radiographic testing (RT) and ultrasonic testing (UT)

RT is widely used for butt welds in Hastelloy piping and vessels per ASME Section V and VIII Div. 1. The high alloy content of Hastelloy produces a weld with good radiographic contrast. PAUT (phased array UT) is increasingly specified as a supplement or replacement for RT in heavy-wall vessels where RT geometry or access is limiting. For PAUT of Hastelloy, note that the alloy’s coarser grain structure in the weld and HAZ produces higher acoustic backscatter than carbon steel — calibration blocks from the same grade of Hastelloy must be used for reference reflector sensitivity setting.

9.4 Corrosion testing

For critical chemical plant applications, corrosion testing of weld procedure qualification test coupons is specified in addition to mechanical tests. The most commonly referenced methods are:

• ASTM G28 Method A (boiling ferric sulphate-50% sulphuric acid test) — primarily applicable to C-series Hastelloy alloys to detect HAZ sensitisation by grain boundary attack.
• ASTM G28 Method B (boiling 23% sulphuric acid-1.2% HCl-1% FeCl₃-1% CuCl₂) — broader corrosion attack test applicable to both C-series and B-series alloys.
• ASTM A262 Practice E (boiling copper-copper sulphate-16% sulphuric acid) — sometimes specified for Hastelloy alloys that are used in environments similar to sensitised austenitic stainless applications.

9.5 Hardness testing

Vickers hardness surveys (HV10) are performed across the weld cross-section as part of PQR. Hastelloy C-276 in the as-welded condition typically exhibits hardness in the range 170–260 HV. There is no mandatory maximum hardness limit for Hastelloy under ASME IX or EN ISO 15614-1 for most applications, but values exceeding 300 HV should be investigated as they may indicate contamination, excessive work hardening, or unexpected phase formation. For sour service applications (ISO 15156), the 250 HV maximum limit applies across the entire HAZ.

10. Applicable Codes and Standards

11. Recommended Reading

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12. Frequently Asked Questions

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