Above is a link to a video of a pressure vessel we just completed the fabricating. This vessel has Hastelloy for all of it’s wetted surfaces. It has a diameter of ~83″ with a length of ~191″. It weighs 14,000 lbs.
Hastelloy is a trade name for a family of high-performance, corrosion-resistant nickel-based alloys. These alloys often contain significant amounts of molybdenum and chromium, and sometimes other elements like iron, cobalt, copper, manganese, silicon, tungsten, aluminum, titanium, phosphorus, and sulfur. The specific composition varies depending on the Hastelloy grade (e.g., C-22, C-276, B-3, G-30), each tailored for particular corrosive environments.
Key properties of Hastelloy alloys include:
- Exceptional corrosion resistance: They are highly resistant to various forms of corrosion, including pitting, crevice corrosion, stress corrosion cracking, and general uniform corrosion, even in aggressive chemical environments (strong acids, chlorides, oxidizing and reducing media).
- High-temperature strength and stability: Many Hastelloy grades retain their mechanical properties at elevated temperatures and resist oxidation.
- Good fabricability: They can be forged, hot-upset, impact extruded, deep-drawn, spun, press-formed, and punched, although they tend to work-harden.
- Excellent weldability: Generally considered weldable by most common methods, although specific procedures are crucial and it does involve a number of difficulties.
Difficulties in Welding Hastelloy:
While Hastelloy is generally weldable, it presents certain challenges that require careful attention to achieve high-quality welds:
- Sluggish Weld Puddle and Shallow Penetration: Nickel-based alloys like Hastelloy tend to have a more viscous (sluggish) molten weld pool compared to stainless steels or carbon steels. This makes the puddle less fluid and can hinder proper flow and wetting of the joint edges, potentially leading to incomplete fusion. They also typically exhibit shallower penetration characteristics.
- Hot Cracking Sensitivity: Hastelloy alloys can be susceptible to hot cracking (solidification cracking) during welding if proper procedures and filler metals are not used. Factors contributing to this include:
- Improper heat input: High heat input can widen the heat-affected zone (HAZ) and increase the risk of cracking. Low to moderate heat input is generally recommended.
- Inadequate joint design: Joint designs that restrict thermal expansion and contraction can increase stress and the likelihood of cracking. Wider groove angles and root gaps may be necessary.
- Improper filler metal selection: Using a filler metal with an unsuitable composition can increase the risk of cracking. Matching or slightly over-alloyed filler metals are usually preferred.
- Oxide Formation: Hastelloy alloys readily form tenacious oxide films on the weld bead surface at high temperatures. These oxides must be completely removed between passes (typically by grinding or wire brushing with stainless steel brushes dedicated to nickel alloys) to prevent inclusions and ensure sound multi-pass welds.
- Cleanliness is Critical: Contamination from grease, oil, lead, sulfur, phosphorus, and other low-melting-point elements can lead to severe cracking problems. Thorough cleaning of the welding surfaces and adjacent areas with appropriate solvents (e.g., isopropyl alcohol) before welding is essential. Dedicated cleaning tools should be used to avoid cross-contamination.
- Shielding Gas Requirements: Adequate shielding gas coverage (usually argon) is crucial to protect the weld pool and the hot HAZ from oxidation and atmospheric contamination, which can lead to porosity and other defects. For multi-pass welds, superior gas coverage, including trailing shields for GTAW, might be necessary to protect the cooling weld bead.
- Inter-pass Temperature Control: Maintaining a low inter-pass temperature is often recommended to minimize heat buildup and reduce the risk of hot cracking and other issues. Allowing the weldment to cool between passes until it is warm to the touch is a common practice.
- Dissimilar Metal Welding: Welding Hastelloy to dissimilar metals (e.g., stainless steel, carbon steel) can introduce additional challenges due to differences in melting points, thermal expansion coefficients, and chemical compatibility. Special filler metals and techniques (like buttering) may be required, and the corrosion performance of such welds should be carefully evaluated.
- Appearance of Welds: Hastelloy welds may have a different visual appearance compared to stainless steel welds, often exhibiting oxide islands on the weld bead. This is normal and permitted by some standards but can be misleading if not understood.
In summary, welding Hastelloy requires skilled welders who understand the specific characteristics of these alloys and adhere to carefully developed welding procedures (WPS) that address issues like sluggishness, hot cracking sensitivity, oxide formation, and the need for stringent cleanliness.