Question 1: What exactly is UNS N10675, and how does it differ from its predecessor, Alloy B-2?
Answer:
UNS N10675, or Alloy B-3, is a nickel-molybdenum alloy with a nominal composition of 65% nickel and 28.5% molybdenum. It is the evolutionary successor to the older Alloy B-2 (UNS N10665). While both alloys were designed to resist reducing acids, the key differentiator lies in their thermal stability.
Alloy B-2 had a significant metallurgical flaw: when exposed to intermediate temperatures (especially around 1200-1300°F / 650-700°C) during welding or heat treatment, it would rapidly precipitate a deleterious intermetallic phase known as Ni₄Mo. This precipitation made the alloy extremely brittle and susceptible to stress-corrosion cracking, posing major fabrication challenges.
UNS N10675 was specifically engineered to solve this problem. Through precise adjustments in its chemistry-including a controlled shift in molybdenum content and the addition of minor elements-it dramatically slows down the formation of harmful phases. As the Time-Temperature-Transformation (TTT) diagram from Haynes International shows, while B-2 can become brittle in minutes, B-3 can withstand these intermediate temperatures for several hours without significant loss of ductility . This superior thermal stability makes UNS N10675 plate much more forgiving to fabricate, weld, and place into service without the risk of unexpected brittle failure.
Question 2: What governing specification applies to UNS N10675 plate, and what are its key mechanical and chemical requirements?
Answer:
The procurement, testing, and delivery of UNS N10675 plate, sheet, and strip are governed by the ASTM standard B333 / ASME SB333
. This specification outlines the chemical composition limits and the mechanical properties that the material must meet.
Here are the critical requirements as defined by ASTM B333 for UNS N10675 plate in the solution-annealed condition:
Chemical Composition (Key Elements): The strict control of elements is what gives the alloy its performance.
Nickel (Ni): 65.0% minimum (balance of the alloy).
Molybdenum (Mo): 27.0 - 32.0% - This high level provides the primary resistance to reducing acids.
Chromium (Cr): 1.0 - 3.0% - Unlike C-type alloys, chromium is kept low to optimize performance in reducing environments.
Iron (Fe): 1.0 - 3.0%.
Carbon (C): 0.01% max - The ultra-low carbon content minimizes the risk of carbide precipitation and intergranular corrosion during welding.
Mechanical Properties: For plate up to 2.5 inches (63.5 mm) thick, ASTM B333 requires the following minimums in the annealed condition:
Tensile Strength: 110,000 psi (760 MPa)
Yield Strength (0.2% offset): 51,000 psi (350 MPa)
Elongation: 40% in 2 inches
These properties ensure that the plate has sufficient strength and, crucially, high ductility for forming operations.
Question 3: In what specific corrosive environments does UNS N10675 plate excel, and where should its use be avoided?
Answer:
UNS N10675 plate is the premier material for handling non-oxidizing (reducing) acids. Its high molybdenum content allows it to form protective films in environments where hydrogen is cathodically evolved.
Ideal Applications:
Hydrochloric Acid (HCl): It offers exceptional resistance to HCl across a wide range of concentrations and temperatures, all the way up to the boiling point.
Sulfuric Acid (H₂SO₄): It performs very well in dilute to mid-range concentrations of sulfuric acid, particularly at elevated temperatures.
Phosphoric Acid (H₃PO₄): It is highly resistant to pure phosphoric acid.
Other Reducing Media: It is also excellent for handling acetic acid, formic acid, and hydrobromic acid.
Critical Limitations (Areas of Avoidance):
The alloy's performance is highly dependent on the environment's purity. It must be avoided in the presence of oxidizing agents.
Oxidizing Salts: The alloy should never be used in services containing ferric (Fe³⁺) or cupric (Cu²⁺) salts. These oxidizing species will rapidly destroy the passive layer and cause catastrophic, accelerated corrosion.
Oxidizing Acids: It is not suitable for strong oxidizing acids like nitric acid (HNO₃).
Presence of Oxygen or Other Oxidizers: Even in hydrochloric acid service, if the acid is contaminated with air or other oxidizers, the corrosion rate can increase significantly.
Question 4: What are the critical best practices for welding UNS N10675 plate to maintain its corrosion resistance?
Answer:
Welding UNS N10675 is a routine but critical operation. The goal is to create a weld joint that retains the same corrosion-resistant properties as the base plate. Following these best practices is essential:
Cleanliness is Paramount: The plate surface and weld joint must be meticulously cleaned. Any grease, oil, paint, or even dirt can introduce carbon or other contaminants that lead to cracking or porosity. Use clean, stainless steel wire brushes dedicated only to nickel alloys.
Low Heat Input: To prevent overheating the base metal and to minimize the heat-affected zone (HAZ), use a low heat input welding process. Gas Tungsten Arc Welding (GTAW/TIG) is the preferred method, especially for thinner plate sections. Control interpass temperatures to allow the material to cool between weld passes.
Use Matching Filler Metal: The welding filler metal must be matched to the base alloy. The appropriate filler metal is designated ERNiMo-10 (for TIG or MIG) or ENiMo-10 (for stick electrode).
Excellent Shielding Gas Coverage: The molten weld pool and the hot, solidified weld metal must be protected from atmospheric oxygen. This requires adequate shielding gas flow on the torch and, crucially, back-purging with an inert gas (like argon) on the underside of the joint. This prevents oxidation (sugaring), which would compromise corrosion resistance.
No Post-Weld Heat Treatment (PWHT) Required (Typically): One of the major advantages of UNS N10675 is that, due to its superior thermal stability, it does not typically require a post-weld solution annealing treatment to restore corrosion resistance or ductility, provided the welding was performed correctly. This is a significant cost and time savings compared to less stable alloys.
Question 5: What are the typical applications and available product forms for this alloy?
Answer:
UNS N10675 is used wherever equipment must withstand highly corrosive reducing chemicals. The plate form is particularly prevalent in the fabrication of vessels and large-scale equipment.
Common Applications:
Reaction Vessels: Used in the chemical and pharmaceutical industries for manufacturing and handling products involving hydrochloric or sulfuric acid
Columns and Towers: For distillation, scrubbing, or separation processes involving aggressive media.
Heat Exchangers: Plate-type or shell-and-tube heat exchangers where the corrosive medium is on one side. Tube sheets are a classic application for alloy B-3 plate
Flue Gas Desulfurization (FGD) Systems: In specific sections of FGD equipment where chlorides concentrate under reducing conditions
Piping Systems and Linings: Used as solid plate for piping or as a lining material for less expensive carbon steel vessels to provide corrosion resistance
Product Forms:
While our focus is on plate, UNS N10675 is widely available in various forms to meet all fabrication needs:
Plate, Sheet, and Strip: As governed by ASTM B333
Bar, Rod, and Billet: For manufacturing fittings, flanges, and fasteners (ASTM B335)
Pipe and Tube: Both seamless and welded (ASTM B619, B622, B626)
Forgings: For complex shaped components like flanges and valves (ASTM B564)
