Nickel (Ni): 54.0 – 60.0% (Base element). It forms the alloy's matrix, providing fundamental resistance to general corrosion and ensuring structural stability across a wide temperature range. Nickel also enhances the alloy's ability to retain other beneficial elements like molybdenum and chromium.
Chromium (Cr): 14.0 – 18.0% (Primary corrosion-resistant element). It forms a dense, adherent chromium oxide (Cr₂O₃) film on the alloy's surface, which acts as a barrier against oxidation and attack by oxidizing acids (e.g., nitric acid). This film self-heals if damaged, further maintaining corrosion resistance.
Molybdenum (Mo): 14.0 – 17.0% (Critical for pitting/crevice corrosion resistance). It increases the alloy's tolerance to chloride ions-common in seawater, brines, and chemical process streams-by suppressing the formation of localized corrosion pits and crevices. Molybdenum also boosts strength at moderate temperatures.
Tungsten (W): ≤ 3.0% (Strengthening and corrosion aid). It works with molybdenum to enhance the alloy's resistance to aggressive media and improves mechanical strength without significantly compromising ductility.
Iron (Fe): ≤ 3.0% (Controlled impurity/processing aid). Iron is added in small amounts to improve the alloy's workability during manufacturing (e.g., hot rolling, cold drawing) and to reduce overall material costs, without sacrificing corrosion performance.
Carbon (C): ≤ 0.01% (Strictly limited). Low carbon content prevents the precipitation of chromium carbides (e.g., Cr₂₃C₆) along grain boundaries during welding or heat treatment. Carbide precipitation depletes chromium near grain boundaries, creating "chromium-depleted zones" that are vulnerable to intergranular corrosion-a major failure mode in harsh chemical environments.
Trace elements (Si, Mn, P, S): ≤ 0.08% each (Minimized impurities). Silicon and manganese are controlled to avoid the formation of brittle intermetallic phases; phosphorus and sulfur are kept at ultra-low levels to prevent hot cracking during welding and to maintain the alloy's ductility.
3.What is the hardness of Hastelloy C4?
Annealed state (most common supply form):
Annealing is a heat treatment process where the alloy is heated to approximately 1150–1200°C, held for a specific time to achieve a uniform microstructure, and then rapidly cooled (quenched) in water. This process softens the alloy, improves its ductility, and optimizes its corrosion resistance-making it the standard state for most applications (e.g., pipe fabrication, equipment welding).
Brinell Hardness (HB): Typically ranges from 180 to 220 HB. The Brinell method uses a large, hard steel ball (10 mm diameter) under high load (3000 kgf), making it ideal for measuring the overall hardness of large, thick Hastelloy C4 components (e.g., heavy-walled pipes, forgings).
Rockwell Hardness (HRB): Generally falls between 80 and 90 HRB. The Rockwell B scale uses a smaller steel ball (1/16 inch diameter) and lower load (100 kgf), suited for thinner components (e.g., sheet metal, thin-walled tubes) where the Brinell indentation might be too large.
Vickers Hardness (HV): Typically ranges from 190 to 230 HV. The Vickers method uses a diamond pyramid indenter and variable loads, providing high precision for small or complex parts (e.g., welded joints, precision machined components). It is often used for quality control in manufacturing.
Work-hardened state (less common)
Work hardening occurs when the alloy is subjected to cold mechanical deformation (e.g., cold rolling, cold drawing) without subsequent annealing. This process increases hardness and strength by introducing dislocations in the alloy's crystal structure, but it reduces ductility and may slightly impact corrosion resistance (due to residual stress).
Brinell Hardness (HB): Can increase to 250–300 HB, depending on the degree of cold working.
Rockwell Hardness (HRB): May rise to 95–100 HRB or higher.
Post-weld state:
After welding, the heat-affected zone (HAZ) of Hastelloy C4 may exhibit slightly higher hardness than the annealed base metal, due to localized microstructural changes (e.g., partial grain refinement). However, this hardness increase is minimal (usually ≤ 20 HB above the base metal) because of the alloy's low carbon content, which prevents excessive carbide precipitation. Post-weld annealing is rarely required for Hastelloy C4 (unlike some other nickel alloys) but may be performed for applications requiring strict hardness uniformity.
