Thread Content
Material of the valve body 1. Some basic knowledge about the common materials used for valves: 1.1. Steel and iron: The main difference between steel and iron lies in their carbon content. According to metallurgical standards, materials with a carbon content of ≥2% are considered cast iron, while those with a carbon content of <2% are considered steel. Of course, the steel used for our valves does not have such a high C content. 1.2. Alloy steels – Alloy steels are produced by adding alloying elements such as Cr, Ni, Mo, Cu, N, W, V, Ti, Nb, etc., to Fe. Low-alloy steel: Alloy steels with 10% alloy element content, such as Cr13; 18-8 stainless steel, etc. 1.3. Alloys and high-alloy materials: Alloys are metal materials in which other alloying elements are added to a base metal; for example, zinc is added to Cu to form brass, and tin is added to Cu to form bronze, thereby creating metal materials with improved properties. High-alloy: refers to alloys in which the total content of alloying elements is 50% or greater. 1.4. Introduction to Several Common Heat Treatment Methods for Valve Steels ⑴. Annealing: A heat treatment process in which the steel is heated above its critical point or recrystallization temperature; after holding at that temperature, it is cooled at a rate slower than that in still air. ⑵ Quenching: A heat treatment process in which a metal is heated to a specified temperature and held there for a certain period of time before being cooled rapidly. ⑶ Tempering: A heat treatment process in which steel that has been quenched or deformed through cold working is heated to a selected temperature (below the steel’s lower critical point or recrystallization temperature) and held there for a sufficient period of time, in order to eliminate the residual stresses resulting from quenching or cold working, and to achieve a more stable microstructure as well as the desired overall mechanical properties. ⑷ Solution treatment: The alloy is heated to an appropriate temperature and held there for a sufficient length of time, allowing certain components within the alloy to dissolve into the matrix and form a homogeneous solid solution. The alloy is then rapidly cooled, causing the dissolved components to remain in the matrix as a supersaturated solid solution. This process improves the ductility and toughness of the alloy, while also preparing it for further precipitation hardening treatments. This type of treatment is known as “solution treatment”. 1.5. Introduction to several common microstructural phases of steels used for valves: (1) Austenite: It is a solid solution with a face-centered cubic structure formed by iron and other elements; it generally refers to a interstitial solid solution of C and other elements in iron. It was named in honor of its inventor, Austen. Such as 304; 316, etc., are austenitic stainless steels. ⑵.Ferrite: A solid solution with a body-centered cubic structure formed from iron and other elements; it generally refers to the interstitial solid solutions of carbon and other elements in α-iron. ⑶Pearlite: a layered microstructure in which ferrite lamellae and cementite lamellae are arranged alternately; it is the direct product of the eutectoid reaction of supercooled austenite. It gets its name from the mother-of-pearl-like luster that samples with this structure exhibit after polishing and etching. WCB has a pearlitic structure. ⑷Martensite: It is a metastable phase formed by the transformation of austenite through a diffusion-free phase transition. It is actually a gap solid solution of C in iron, with a body-centered tetragonal crystal structure; under a microscope it appears bamboo-leaf shaped. It was named in honor of the metallurgist Martens. Cr5Mo has a martensitic structure. ⑸.Separation hardening: A hardening effect that occurs under certain conditions as a result of the precipitation of another phase from a supersaturated solid solution. Since strength also increases with an increase in hardness, it is also known as precipitation strengthening. 17-4PH is a precipitation-hardening stainless steel, etc. ⑹. Duplex stainless steel: A type of corrosion-resistant stainless steel that is precipitation-hardening; it is a high-strength corrosion-resistant stainless steel containing 35%~40% austenite within a ferritic matrix. The yield strength of duplex stainless corrosion-resistant steel is approximately twice that of 19Cr-9Ni austenitic stainless steel, and it possesses high hardness as well as good plasticity and impact toughness. It is particularly suitable for use in conditions of both abrasion and erosion, along with corrosion, at working temperatures of ≤316°C. 1.6. Several commonly used nickel-based alloys for valves: Corrosion-resistant cast nickel-based alloy valves are primarily made from materials specified in ASTM A494, the Standard for Cast Nickel and Nickel Alloys, such as cast Monel alloy (M35-1), cast nickel alloy (CZ-100), Inconel alloy (CY-40), and Hastelloy B alloys: N-12MV, N-7M. Hastelloy C alloys include CW-12MW, CW-7M, CW-6MC, and CW-2M. (1) Monel alloy: Monel alloy refers to a nickel-copper alloy with nominal compositions of 70% Ni and 30% Cu. Material code: Monel alloy as a whole is MM ； Material code for Monel alloy internal component valves: when the housing is made of carbon steel, the valve material grade is C/M; when the housing is CF8, the valve material code is P/M; when the housing is CF8M, the valve material grade is R/M. (2) Cast nickel alloy: The chemical composition of the cast nickel alloy (CZ-100) is 95% Ni and 1.00% C; it has no corresponding rolled product. (3) Inconel and Incoloy alloys: Inconel and Incoloy are Ni-Cr-Fe-based alloys among nickel-based alloys; those with a lower iron content are referred to as Inconel alloys, while those with a higher iron content are called Incoloy alloys. Incoloy contains up to 45% Fe, and therefore it is essentially an Fe-Ni-Cr alloy among nickel-based alloys. Among the current corrosion-resistant nickel-based alloys, their use in valve manufacturing makes them an important alloy second only to Monel alloy. (4) Hastelloy: Hastelloy is a trade name that refers to a series of alloy grades. The ones primarily used in corrosion-resistant valves are Hastelloy B and Hastelloy C. Hastelloy B is a Ni-Mo-Fe-based alloy; Hastelloy C is a Ni-Mo-Cr-based alloy. For Hastelloy B, the casting alloy grades are N-12MV (referenced as N-12M-1 in some sources) and N-7M (referenced as N-12M-2 in some sources) according to the American ASTM A494 standard, while the rolled alloy grade is UNSN10665 under the ASTM B335 standard. Hastelloy B is resistant to corrosion in hydrochloric acid at various concentrations, as well as to non-oxygenous salts and acids. For corrosion-resistant valves made of Hastelloy B, it is advisable to choose the low-carbon grade of Hastelloy B (N-7M) considering its corrosion resistance and resistance to intergranular corrosion. The cast alloy grades for Hastelloy C are CW-12MW (referenced as CW-12M-1 in some sources) and CW-7M (referenced as CW-12M-2 in some sources), as well as Hastelloy C-276, whose cast alloy grade is CW-6MC. Hastelloy C-4 has a cast alloy grade of CW-2M. The corresponding rolled product grades for the cast Hastelloy alloys CW-7M, CW-12MW, CW-6MC, and CW-2M are: UNS N10001, UNS N10003, UNS N06625, UNS N10276, and UNS N06455 respectively, according to the UNS system.