Know Your Metal (Pt. 3): High-Nickel Alloys – The Divas of Durability
Previously in the Series…
We began this series with carbon steel, a reliable and forgiving material that’s still widely used. In August, we explored stainless steel, where cleanliness and heat input control are key to preventing costly rework. This month, we focus on high-nickel alloys — materials engineered for harsh environments, where strength and corrosion resistance are critical, and margin for error is low.
The High-Nickel Group
High-nickel alloys are designed for severe service: aggressive chemical environments, high temperatures, and demanding mechanical stresses. They’re widely used in pressure vessels, heat exchangers, tanks and process piping in industries like chemical processing, power generation, and oil and gas.
Common high-nickel alloys include:
- Inconel® series: 600, 625, 718, 800, 825
- Hastelloy® series: C-276, C-22, C-2000
- Monel®: 400, K-500
- Alloy 20 (austenitic stainless with high nickel content)
- Nickel 200 / 201 (commercially pure nickel)
Quick Spotlight on C-276 (UNS N10276)
Hastelloy® C-276, officially recognized as UNS N10276, is one of the most widely used corrosion-resistant nickel alloys in the industry. Known for its outstanding resistance to strong oxidizers, chlorides, and acidic environments (including wet chlorine gas and hot contaminated mineral acids), C-276 is a go-to material for many chemical processing components.
The Strengths
- Exceptional corrosion resistance: These alloys perform well in harsh chemical and marine environments where traditional stainless steels fail.
- High-temperature performance: Inconel 625 and C-276 maintain mechanical strength and corrosion resistance in elevated service temperature conditions.
- Reliability in critical service: Proper fabrication and inspection protocols can deliver long-lasting service life in extremely aggressive conditions.
The Challenges
- Tight heat input control required: Excessive heat input and/or high interpass temperature control during welding can lead to hot cracking or metallurgical degradation.
- High distortion risk: These alloys are sensitive to thermal expansion and require careful sequencing and restraint.
- Weld bead technique: Minimize the use of the weave technique. Instead, utilize stringer beads with minimal manipulation instead of wide weave beads.
- Filler metal compatibility: Filler selection must consider both chemistry and service conditions. ERNiCrMo-3 (commonly associated with Inconel 625) is often used across a range of nickel and stainless base metals, but it’s not always the correct choice. This is a critical detail and one left to the true experts.
- Expensive to get wrong: High-nickel alloys are costly to purchase, fabricate, and repair. Mistakes carry real and significant financial impact.
Ward Wisdom: Real Lessons from the Field (as told by our customers)
- C-276 Heat Input Misstep
- A customer-spec’d nozzle made from UNS N10276 was welded using excessive heat input, which altered the corrosion resistance of the weld area known as sensitization. Post-weld testing showed reduced performance in hydrochloric acid exposure. The solution required a revised WPS with lower interpass temperature and tighter stringer bead control.
- Overlay Distortion (ERNiCrMo-3 on CS)
- During a full-overlay weld on a carbon steel shell, the buildup using ERNiCrMo-3 led to unexpected warping and flange misalignment. The repair required extensive machining and highlights the importance of managing bead size, travel speed, and opposing weld sequences during overlay work.
- Cracking in Monel Tubes
- A Monel 400 exchanger bundle failed helium leak testing due to cracking at the tube end seal welds. Improper gas shielding and inconsistent purge flow created the issue. Remember, gas flow is an essential variable and more is not necessarily better here. It served as a reminder that even simple filler-free autogenous welds demand full process discipline.
Best Practices
- Control heat input and interpass temperature: Use stringer beads, verify interpass with a temp stick or calibrated thermometer, and avoid over-welding.
- Purge properly: Use high-purity argon and flow-controlled backing gas systems. Monitor purge flow continuously during welding. Pay attention to the WPS and note the allowed flow rates.
- Confirm filler compatibility: Check chemistry, corrosion environment, and mechanical properties before choosing a filler. Don’t assume that 625 (ERNiCrMo-3) is always the correct solution…
- Sample testing: Use chemical exposure samples and mock-ups when welding overlays or multi-pass joints on high-value components.
Looking Ahead…
In Part 4: Duplex Stainless – Strong but Stubborn, we’ll dive into the complex world of duplex alloys, where ferrite control, interpass management, and proper cooling rates all determine whether a weld holds or fails under stress.
