Incoloy
Incoloy is a trademarked family of austenitic nickel-iron-chromium superalloys developed by Special Metals Corporation, engineered for exceptional high-temperature strength, corrosion resistance, and oxidation resistance in extreme environments.[1] These alloys typically contain 30-45% nickel, 19-23% chromium, and significant iron content, along with additions like molybdenum, copper, and titanium to enhance specific properties such as pitting resistance and fabricability.[1] Known for their versatility, Incoloy alloys maintain structural integrity and mechanical performance in aggressive conditions, including sour gas, acidic media, and temperatures up to 1100°C (2012°F), distinguishing them from standard stainless steels.[1] The development of Incoloy alloys began in the late 1940s with the introduction of alloy 800 by the International Nickel Company (Inco), now part of Special Metals, to address the growing demand for materials in heat-treating and chemical processing equipment.[1] Subsequent innovations expanded the family, including alloy 825 in 1952 for broader corrosion resistance in sulfuric acid environments and alloy 925 in 1982 for high-strength applications in oilfield equipment.[1] More recent additions, such as alloys 945 and 945X introduced in 2008-2009, incorporate age-hardening mechanisms to achieve yield strengths exceeding 860 MPa (125 ksi) while resisting sulfide stress cracking in sour oil and gas service.[1] Key variants in the Incoloy lineup include 800/800H/800HT for thermal processing and nuclear applications, 020 and 25-6HN for wet corrosion resistance in chemical plants, 909 for low thermal expansion in aerospace components, and 27-7MO as a super-austenitic grade with 7% molybdenum for seawater handling.[1] These alloys are widely employed in industries such as power generation (e.g., superheater tubes), oil and gas (e.g., downhole tubing and valves), aerospace (e.g., exhaust systems), and petrochemical processing (e.g., reaction vessels and piping).[1] Their fabricability allows for welding, forging, and machining similar to austenitic stainless steels, ensuring broad industrial adoption despite higher costs compared to conventional alloys.[1]Introduction and History
Definition and Characteristics
Incoloy is a trademarked family of nickel-iron-chromium superalloys engineered for superior high-temperature strength and exceptional corrosion resistance in aggressive environments, such as those involving oxidation, sulfidation, and aqueous media. These alloys maintain structural integrity and fabricability under demanding conditions, making them suitable for applications requiring durability in chemical processing and thermal systems. The trademark originated with the International Nickel Company (Inco) in 1952 and is now owned by Special Metals Corporation.[2][3] A key distinguishing feature of Incoloy alloys compared to Inconel is their higher iron content, which reduces the nickel proportion to enhance cost-effectiveness while preserving robust performance. This composition results in an austenitic crystal structure that provides excellent thermal stability and resistance to stress-corrosion cracking. Incoloy alloys are often classified as super-austenitic stainless steels due to their enhanced resistance to pitting, crevice corrosion, and general degradation in harsh settings.[2] Compositions vary across the family, but many include significant nickel (typically 25-50%), iron as a major component, and chromium (often 19-25% in corrosion-focused alloys), with strategic additions of elements such as molybdenum, copper, and titanium to optimize specific properties like resistance to reducing or oxidizing acids. For instance, alloys like Incoloy 800 and 825 exemplify this family by balancing these elements for versatile high-performance use.[2][4]Development and Evolution
The Incoloy family of alloys was initially developed in the early 1950s by the International Nickel Company (Inco) to provide corrosion-resistant materials suitable for demanding applications in chemical processing and high-temperature environments, where traditional materials like stainless steels fell short in performance.[5][6] Inco registered the Incoloy trademark in 1952 and secured early patents for these nickel-iron-chromium-based superalloys, emphasizing their enhanced resistance to oxidation and corrosive media compared to earlier nickel alloys.[7][8] The alloys evolved significantly through the 1960s and 1970s, with key variants addressing specialized needs; for instance, Incoloy alloy 800, introduced in the 1950s but widely adopted in the 1960s, was optimized for nuclear reactor components requiring high-temperature strength and resistance to carburization.[9][10] Similarly, Incoloy alloy 825, introduced in 1952, offered superior resistance to acids, such as sulfuric and phosphoric, in chemical processing equipment.[11][12][13] Ownership of the Incoloy alloys transferred from Inco to Special Metals Corporation following the 1998 acquisition of Inco Alloys International, enabling continued innovation into the 1990s and beyond, including the development of Incoloy alloy 945X for high-strength applications in sour oil and gas wells.[14][15] This evolution was driven by post-World War II industrial expansion in petrochemical refining and aerospace, where the need for materials outperforming stainless steels in aggressive, high-temperature conditions spurred advancements in alloy design.[16] Special Metals further supported these developments through technical publications, such as the 2000 corrosion resistance handbook detailing Incoloy performance in aqueous and high-temperature corrosive environments.[17]Properties
Mechanical and Physical Properties
Incoloy alloys, a family of nickel-iron-chromium superalloys, exhibit a range of mechanical properties that provide high strength and ductility suitable for demanding environments. At room temperature, typical tensile strength for annealed Incoloy alloys such as 800 and 825 ranges from 550 to 800 MPa, with yield strength between 200 and 450 MPa and elongation of 30-60%, demonstrating good ductility.[9][12] For high-temperature variants like Incoloy 800H, room-temperature tensile strength is approximately 780 MPa, yield strength 540 MPa, and elongation 22%.[18] These properties vary by alloy form (e.g., plate, bar, tubing) and processing condition, with cold-worked forms showing higher strength but reduced elongation.[12] At elevated temperatures, Incoloy alloys maintain significant strength. For instance, Incoloy 800 annealed material reaches tensile strengths up to 820 MPa at 540°C and 455 MPa at 760°C, though yield strength decreases to 307 MPa at the latter temperature.[9] Incoloy 800H and 800HT are optimized for creep-rupture performance, offering rupture strengths of 121 MPa at 650°C and 50 MPa at 760°C for 100,000-hour exposure, far exceeding standard Incoloy 800 in prolonged high-heat service up to 1000°C.[18] Hardness typically falls in the range of 130-200 Brinell (equivalent to Rockwell B 80-95), depending on heat treatment and alloy variant.[9][18] Fatigue resistance and impact toughness align with ASTM specifications for nickel alloys, with Incoloy 825 showing excellent low-temperature impact strength down to cryogenic levels.[12] Compared to carbon steels, Incoloy alloys provide superior strength retention and ductility in high-temperature scenarios, where carbon steels soften rapidly above 500°C.[18] Physical properties of Incoloy alloys support their use in thermal cycling applications. Density is consistently around 7.94-8.14 g/cm³ across variants like 800, 825, and 800H.[9][12][18] The melting range spans 1357-1400°C, enabling robust high-temperature processing.[9][12][18] Thermal conductivity increases with temperature, from 11-13 W/m·K at 20-25°C to 19-32 W/m·K at 540-600°C for Incoloy 800 and 825.[9][12] The coefficient of thermal expansion is 13-18 × 10⁻⁶/°C over 20-600°C, with values around 14 × 10⁻⁶/°C for Incoloy 800H in this range.[9][12][18] Modulus of elasticity starts at 196 GPa at room temperature and drops to 157-162 GPa at 600°C.[12][18] The following table summarizes representative room-temperature mechanical properties for select annealed Incoloy alloys:| Alloy | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (BHN) |
|---|---|---|---|---|
| 800 | 551 | 250 | 60 | 140 |
| 825 | 690-772 | 324-441 | 36-45 | N/A |
| 800H | 780 | 540 | 22 | 130 |
Corrosion and Oxidation Resistance
Incoloy alloys exhibit superior resistance to pitting, crevice corrosion, and stress corrosion cracking, particularly in chloride-rich environments and acidic media such as sulfuric and phosphoric acids. This performance stems from their composition, which includes high nickel content that mitigates chloride-induced stress corrosion cracking, while molybdenum enhances protection against localized pitting and crevice attacks in reducing conditions. For instance, Incoloy alloy 825 demonstrates excellent resistance to these forms of degradation in chloride-containing solutions and acids up to moderate concentrations and temperatures.[12][19] The alloys' pitting resistance is quantified by a Pitting Resistance Equivalent Number (PREN) typically ranging from 30 to 40, depending on the variant, which correlates with strong performance in aggressive chloride media. Critical pitting temperatures exceed 50°C in 3% NaCl solutions for many Incoloy grades, indicating robust localized corrosion resistance under standard testing conditions like ASTM G48. Additionally, Incoloy alloys perform well in freshwater, seawater, and alkaline solutions, with minimal degradation due to the formation of stable passive films; for example, alloy 27-7Mo shows excellent seawater compatibility without significant pitting or cracking. Compliance with NACE MR0175 standards for sour service further validates their suitability in hydrogen sulfide-containing environments.[20][21][22] At elevated temperatures, Incoloy alloys provide oxidation resistance up to approximately 1100°C through the formation of a protective chromium oxide layer on the surface, which acts as a barrier against further oxygen ingress. This layer, primarily Cr₂O₃, is stable in oxidative atmospheres and contributes to resistance against carburization and nitridation in high-temperature gases, as seen in alloys like Incoloy 800, where titanium additions form stable nitrides to prevent embrittlement. Carburization resistance is particularly notable in petrochemical applications, where the alloy maintains integrity in carbon-rich environments up to 1000°C.[23][9][24] The underlying mechanisms for this corrosion and oxidation resistance rely on the stable austenitic microstructure, stabilized by nickel, which ensures ductility and prevents phase transformations that could compromise protective films. Molybdenum and copper additions promote the development of dense, adherent passive layers in aqueous media, repassivating localized breaches and inhibiting propagation of pits or crevices. Unlike some alloys, Incoloy variants require no post-weld heat treatment to preserve these corrosion properties, as welding does not significantly alter the passive film integrity when proper techniques are employed.[5][12]Alloys and Compositions
Major Alloy Variants
Incoloy alloys encompass a family of nickel-iron-chromium superalloys engineered for enhanced performance in corrosive and high-temperature environments, with major variants tailored for specific industrial demands. These variants differ primarily in their optimization for thermal stability, acid resistance, or low thermal expansion, evolving from foundational alloys like 800 series to advanced formulations such as 945X for ultra-high strength applications.[2] The Incoloy 800 series, including alloys 800 (UNS N08800), 800H (UNS N08810), and 800HT (UNS N08811), is designed for high-temperature oxidation and carburization resistance in petrochemical and heat-treating processes. Alloy 800 serves as the base with general elevated-temperature service, while 800H and 800HT offer improved creep and rupture strength through controlled carbon content and annealing, meeting ASTM specifications such as B408 for rods and B409 for plates. Applications include furnace components like radiant tubes, muffles, and petrochemical furnace tubing, where they maintain structural integrity up to 1100°C.[18] Incoloy 825 (UNS N08825) provides enhanced resistance to acids, including sulfuric, phosphoric, and nitric, making it suitable for chemical processing and oilfield equipment. Its nickel, molybdenum, copper, and chromium composition ensures stability in both reducing and oxidizing conditions, with titanium addition for intergranular corrosion resistance, compliant with ASTM B425 for rods and B424 for sheets. Key uses encompass acid production vessels, pollution control scrubbers, and sour gas recovery systems in oil and gas operations.[12] Incoloy 020 (UNS N08020) and 028 (UNS N08028) are specialized for sulfuric acid and chloride environments, targeting pulp and paper as well as fertilizer industries. Alloy 020 excels in phosphoric and nitric acid settings, used in mixing tanks, heat exchangers, and process piping per ASTM B462 for forgings and B463 for plates, while 028 offers broader resistance to oxidizing and reducing media for similar chemical processing equipment. Incoloy 25-6HN (UNS N08367) is a super-austenitic grade with 6% molybdenum and nitrogen additions for superior pitting and crevice corrosion resistance in seawater and chemical environments, suitable for heat exchangers and piping in offshore and chemical processing per ASTM B462.[25][2][26] Incoloy 330 (UNS N08330) and DS (W. Nr. 1.4862) prioritize thermal stability for furnace components and heat exchangers. Alloy 330 delivers oxidation resistance up to 1095°C in heat-treating furnaces and chemical process equipment, whereas DS, originally for conveyor belts, resists carburization in heat-treatment applications like retorts and trays. Incoloy 27-7MO (UNS S31277) is a super-austenitic grade with 7% molybdenum for exceptional resistance to localized corrosion in seawater and acidic media, used in desalination plants and chemical processing equipment per ASTM B690.[27][28][21] Advanced variants include Incoloy 907 (UNS N19907) and 908 (UNS N09908) for low thermal expansion in aerospace turbine components and space applications, and 909 (UNS N19909) with similar properties for precision instruments and cryogenic seals requiring dimensional stability. Incoloy 925 (UNS N09925) provides age-hardenable high strength for oilfield tubing and valves, and 945 (UNS N09945) with 945X (UNS N09946) for ultra-high strength in sour oil and gas service. Additionally, MA956 employs oxide dispersion strengthening for superior oxidation resistance in space reactor components and high-temperature aerospace exhaust systems. These represent the progression to specialized, high-performance alloys beyond basic corrosion resistance.[1][29][30][31]Chemical Compositions and Designations
Incoloy alloys are a family of nickel-iron-chromium superalloys with tailored elemental compositions that provide enhanced corrosion resistance in harsh environments, such as those involving acids and high temperatures.[2] The specific percentages of nickel, chromium, iron, and alloying elements like molybdenum, copper, and titanium are precisely controlled to meet industry standards, ensuring consistent performance across applications.[9] One of the foundational variants, Incoloy 800 (UNS N08800), features a composition dominated by iron with significant nickel and chromium content for balanced oxidation and aqueous corrosion resistance.[9] Its limiting chemical composition is as follows:| Element | Percentage Range |
|---|---|
| Nickel (Ni) | 30.0–35.0 |
| Chromium (Cr) | 19.0–23.0 |
| Iron (Fe) | ≥39.5 |
| Carbon (C) | ≤0.10 |
| Manganese (Mn) | ≤1.50 |
| Sulfur (S) | ≤0.015 |
| Silicon (Si) | ≤1.0 |
| Copper (Cu) | ≤0.75 |
| Aluminum (Al) | 0.15–0.60 |
| Titanium (Ti) | 0.15–0.60 |
| Element | Percentage Range |
|---|---|
| Nickel (Ni) | 38.0–46.0 |
| Chromium (Cr) | 19.5–23.5 |
| Iron (Fe) | ≥22.0 |
| Molybdenum (Mo) | 2.5–3.5 |
| Copper (Cu) | 1.5–3.0 |
| Titanium (Ti) | 0.6–1.2 |
| Carbon (C) | ≤0.05 |
| Manganese (Mn) | ≤1.0 |
| Sulfur (S) | ≤0.03 |
| Silicon (Si) | ≤0.5 |
| Aluminum (Al) | ≤0.2 |
| Element | Percentage Range |
|---|---|
| Nickel (Ni) | 32.0–38.0 |
| Chromium (Cr) | 19.0–21.0 |
| Iron (Fe) | Balance |
| Copper (Cu) | 3.0–4.0 |
| Molybdenum (Mo) | 2.0–3.0 |
| Niobium + Tantalum (Nb + Ta) | 8 × C min – 1.00 |
| Carbon (C) | ≤0.07 |
| Manganese (Mn) | ≤2.0 |
| Phosphorus (P) | ≤0.045 |
| Sulfur (S) | ≤0.035 |
| Silicon (Si) | ≤1.0 |
| Element | Percentage Range |
|---|---|
| Nickel (Ni) | 30.0–34.0 |
| Chromium (Cr) | 26.0–28.0 |
| Iron (Fe) | Balance |
| Molybdenum (Mo) | 3.0–4.0 |
| Copper (Cu) | 0.6–1.4 |
| Carbon (C) | ≤0.030 |
| Manganese (Mn) | ≤2.50 |
| Phosphorus (P) | ≤0.030 |
| Sulfur (S) | ≤0.030 |
| Silicon (Si) | ≤1.00 |
| Element | Percentage Range |
|---|---|
| Nickel (Ni) | 34.0–37.0 |
| Chromium (Cr) | 17.0–20.0 |
| Iron (Fe) | Balance |
| Silicon (Si) | 0.75–1.50 |
| Carbon (C) | ≤0.08 |
| Manganese (Mn) | ≤2.0 |
| Phosphorus (P) | ≤0.030 |
| Sulfur (S) | ≤0.030 |