Nickel Alloy Welding Guide: Process, Filler Metal & Defect Prevention

Why Nickel Alloys Are Different to Weld

Nickel alloys have roughly 3x higher electrical resistance than carbon steel and lower thermal conductivity. This means: the same amperage produces a smaller, hotter weld pool. Get this wrong and you'll spend more time fixing defects than making progress.

The three rules of nickel alloy welding:

• Low heat input — use pulsed GTAW, keep travel speed 2-3x faster than you would for carbon steel
• Clean everything — sulfur, phosphorus, lead, and zinc from any source cause cracking. No grease, no oil, no shop air
• Never substitute filler metals — one wrong wire choice can destroy corrosion performance

Welding Processes Compared

Process	Code	Best For	Shielding Gas	Speed
Gas Tungsten Arc Welding	GTAW (TIG)	All nickel alloys, critical welds, thin sections	Argon 100%	Slow-Med
Gas Metal Arc Welding	GMAW (MIG/Spray)	Thick sections >6mm, high deposition	Argon + 2% CO₂	Fast
Shielded Metal Arc Welding	SMAW (Stick)	Field repairs, positions 3 & 4	N/A (flux)	Medium
Pulsed GMAW	GMAW-P	All positions, heat-sensitive alloys	Argon + 5% He	Fast

GTAW is the default choice for pump shafts, turbine components, and any pressure-containing welds. The cold electrode (no filler contact) gives precise arc control. For offshore piping with limited access, pulsed GMAW with ERNiCrMo-4 on C276 is common.

Filler Metal Selection Guide

Inconel 625 — ERNiCrMo-3

Inconel 625 welds straightforwardly with ERNiCrMo-3 (AWS A5.14, UNS N06625). This filler delivers weld metal with minimum 110 ksi yield strength and excellent corrosion resistance matching the base metal. For high-temperature service above 1000°F (540°C), ERNiCrMo-3 outperforms base metal because the weld doesn't overage the same way.

Application	Filler	Gas	Notes
Inconel 625 to Inconel 625	ERNiCrMo-3	Ar 100%	Standard procedure
Inconel 625 to 316L SS	ERNiCrMo-3 or ER309L	Ar 100%	309L as buttering; ERNiCrMo-3 on top
Inconel 625 to Carbon Steel	ERNiCrMo-3 butter + fill	Ar 100%	Nickel-alloy buttering layer required
Inconel 718 to Inconel 718	ERNiCrMo-3 or ER718	Ar 100%	ERNiCrMo-3 for general; ER718 for age-hardened

Hastelloy C276 — ERNiCrMo-4

C276 is the most crack-sensitive of the common nickel alloys. The key variables: heat input must be low and interpass temperature must stay below 100°C. Use short arc length, high travel speed. ERNiCrMo-4 contains 2% W which maintains the weld metal's resistance to hydrochloric acid and wet chloride.

Monel 400 — ERNiCu-7

Monel 400 is one of the easiest nickel alloys to weld — but only if you use ERNiCu-7. This filler has 65% Ni, 30% Cu and produces weld metal that matches base metal's seawater corrosion resistance. Do not use stainless steel fillers like 308L or 316L — they'll create galvanic cells in seawater and fail.

316L Stainless to Nickel Alloy Transition

Welding 316L to Inconel 625: use ER309L as a buttering layer (3-4mm) on the 316L side, then fill with ERNiCrMo-3. Buttering prevents carbon migration from the stainless into the fusion zone and gives a more ductile transition.

Heat Input & Interpass Temperature Control

Alloy	Max Interpass Temp	Typical Heat Input	Pulsed GTAW Benefit
Inconel 625	150°C (300°F)	0.8-1.5 kJ/mm	High — reduces heat buildup
Hastelloy C276	100°C (212°F)	0.6-1.2 kJ/mm	Critical — lowest heat possible
Monel 400	120°C (250°F)	0.7-1.3 kJ/mm	Moderate
Incoloy 825	150°C (300°F)	0.8-1.5 kJ/mm	High

Common Defects & How to Prevent Them

Porosity

Nickel alloys are highly susceptible to porosity from gas entrapment. Root cause is almost always inadequate shielding — check gas flow rate (15-20 CFH for GTAW torch), torch angle (70-80°), and whether there's wind or drafts. For out-of-position welds, use a trailing shield attachment. Nickel weld porosity is 100% preventable with proper gas coverage.

Microfissuring (Ductility Dip Cracking)

Small cracks (0.1-0.5mm) that appear in the weld or HAZ during cooling or shortly after. Caused by: excessive heat input, impure base metal (S > 0.005%), and inadequate restraint. Solution: reduce heat input by 20-30%, verify base metal heat number, use ERNiCrMo-4 with lowest possible travel speed for C276.

Lack of Fusion

Nickel alloys have a "sluggish" weld pool compared to steel — the pool doesn't "wet out" as easily. The fix: tilt the torch 5-10° ahead of perpendicular, use smaller diameter electrodes (2.4mm instead of 3.2mm), and reduce travel speed slightly. On 6G pipe welds, this is where skilled vs. average welders separate.

Post-Weld Treatment

One of the advantages of nickel alloys: most don't require post-weld heat treatment (PWHT). Inconel 625 and Hastelloy C276 are used in the solution-annealed condition; welding doesn't sensitize them the way 300-series stainless can be sensitized.

• Inconel 625: No PWHT required for most applications. Solution anneal only if the weld is to be age-hardened (rare for static equipment).
• Hastelloy C276: No PWHT. If stress relief is required by code, specify 600-650°C with specific time — verify with your metallurgist.
• Monel 400: Stress relief at 425°C (800°F) for heavy-wall vessels (>25mm). Verify no embrittlement risk for your service.

What This Means for Your Project

Nickel alloy welding is solvable — it just requires matching the right filler metal, controlling heat precisely, and maintaining shielding discipline. The most common failures we see in incoming pump shafts from other suppliers: wrong filler metal, excessive porosity, and lack of fusion at root. All three are preventable with proper WPS and qualified welders.

If you're specifying weld overlay or fabricating clad steel, specify the overlay process (GTAW or SAW), dilution control method, and NDE requirements upfront. The difference between 15% dilution and 30% dilution in a C276 overlay on carbon steel is the difference between 3mm and 6mm required overlay thickness.