Welding methods of alloy structural steel for the valve industry – Technical specification for low temperature steel castings for valves
Strength steel, also known as high strength steel, has a yield strength of no less than 1290MPa and a tensile strength of no less than 440MPa. According to the yield point and heat treatment state, strength steel can be divided into hot rolled normalizing steel, low carbon tempered steel and medium carbon tempered steel. Hot rolled normalizing steel is a kind of non-heat treatment strengthened steel, which is generally supplied in hot rolled or normalizing state. It mainly relies on mass dissolution strengthening, increasing the relative amount of pearlite, refining grain and precipitation strengthening to ensure the strength. Low carbon tempered steel is dependent on quenching, high temperature tempering heat treatment process (tempered treatment) to strengthen the mass alloy structural steel…
Welding methods for alloy structural steels
(1) Classification of alloy structural steels
Alloy structural steel is a kind of steel with some alloying elements added on the basis of ordinary carbon steel to meet the requirements of various work strips and properties. Alloy structural steels for welding are generally divided into the following two categories.
1 Steel for strength
Strength steel, also known as high strength steel, has a yield strength of no less than 1290MPa and a tensile strength of no less than 440MPa. According to the yield point and heat treatment state, strength steel can be divided into hot rolled normalizing steel, low carbon tempered steel and medium carbon tempered steel. Hot rolled normalizing steel is a kind of non-heat treatment strengthened steel, which is generally supplied in hot rolled or normalizing state. It mainly relies on mass dissolution strengthening, increasing the relative amount of pearlite, refining grain and precipitation strengthening to ensure the strength. Low carbon tempered steel is a mass alloy structural steel strengthened by quenching and high temperature tempering heat treatment process (tempered treatment). Its carbon content is generally wc0.25%, and it has the characteristics of high strength, good plastic toughness, and can be welded directly in the state of tempered. The carbon content of medium carbon tempered steel is 0.3% higher than wc, and the yield strength can reach more than 880MPa. After quenching and tempering treatment, it has high strength and hardness, but low toughness, so the weldability is poor.
2. Special steel
According to the use of environmental conditions or performance requirements can be divided into pearlite heat resistant steel, low alloy corrosion resistant steel and low temperature steel three.
Pearlite heat resistant steel wc≤5%, chromium and aluminum based hypoeutectoid steel. It has good thermal strength and stability. Its special point is that it still has a certain strength and oxidation resistance at the temperature of up to 500~600℃. It is mainly used to manufacture high-temperature components in thermal power equipment and petrochemical equipment. Low alloy corrosion resistant steels include aluminum-bearing corrosion resistant steels used for petrochemical equipment and phosphorus-bearing and copper-bearing corrosion resistant steels used for seawater or atmospheric corrosion resistant steels. In addition to satisfying the comprehensive mechanical properties, this kind of steel also has corrosion resistance in the corresponding medium. It is generally used in hot rolled or normalizing state, is a non-heat treatment of strengthened steel. Low temperature steel sheet should be used in -40~196℃ low temperature equipment and structural parts, the main requirement of low temperature toughness, the strength is not high. It is usually divided into nickel-free steel and nickel-containing steel, generally used in normalizing or normalizing fire state, belongs to the non-heat treatment of strengthened steel.
3. Weldability analysis of high strength steel
The main problems of weldability of high strength steel are: crystallization crack, liquefaction crack, cold crack, reheat crack and heat affected zone performance change
(1) Crystal crack The crystal crack in the weld is formed in the late welding solidification period because the eutectic with low melting point forms liquid film at the grain boundary and cracks along the grain boundary under the action of tensile stress. Its production is related to the content of impurities (such as sulfur, phosphorus, carbon, etc.) in the weld. These impurities are the elements that promote crystallization cracks and should be strictly controlled. Manganese has a desulphurization effect, which can improve the crack resistance of the weld.
(2) Heat affected zone of liquefied crack welding Liquefaction crack is caused by the local melting of low melting eutectic near the metal grain boundary in the multi-layer welding under tensile stress due to the thermal cycling of welding.
4 Welding process of high strength steel
The welding process includes the selection of welding methods and welding materials, the determination of welding specifications, the formulation of heat treatment workers and the formulation of welding assembly and welding sequence. Reasonable welding process is of great significance to ensure product quality, improve efficiency and reduce cost.
(1) Hot rolling and the welding process of normal steel Hot rolling normal steel has good weldability, only when the welding process is not right will appear joint performance problems.
Hot rolled and normal steel is suitable for various welding methods, mainly according to the thickness of the material, product structure, weld position and specific conditions under the application. Usually, welding can be done by arc welding, arc welding, carbon dioxide gas shielded welding and electroslag welding. In order to avoid embrittlement in overheated area, small heat input should be selected. Small heat input and preheating measures can be used to control interlayer temperature to prevent cracks when welding steel with large thickness and base metal alloy elements.
The purpose of choosing welding materials is two: one is to avoid all kinds of defects in the weld, the other is to match the mechanical properties of the base metal. Due to the particularity of weld crystallization, its chemical composition is usually different from that of base metal.
When using electrode arc welding, you can choose the electrode whose strength level corresponds to the base metal, that is, according to the b of the base metal to choose. The hot rolled steel with low welding strength and little crack tendency can choose the calcium electrode with good process performance or the low hydrogen electrode. For high strength steel, low hydrogen electrode should be selected.
Low temperature steel castings for valves This standard is applicable to valves, flanges and other castings under pressure used at low temperature from -254℃ to -29℃. All castings shall be heat-treated according to the design and chemical composition of the material. In order to make thick-wall castings conform to the required mechanical properties, it is usually required to quench the steel castings of the cable body. Before normalizing or quenching, it is permissible to cool the casting directly below the temperature range of the phase transition after casting and solidification. When the method of *** casting surface defect will produce high temperature, the casting should be preheated to at least the minimum temperature specified in Table 4 before implementation.
The scope of
This standard specifies the technical requirements, test methods, inspection rules and marks for low-temperature steel castings for valves (hereinafter referred to as “castings”).
This standard is applicable to valves, flanges and other castings under pressure used at low temperatures from -254℃ to -29℃.
Normative reference document
The terms in the following documents become terms of this Standard by reference to this Standard. For dated citations, all subsequent amendments (excluding errata) or amendments are not applicable to this Standard, however, parties to agreements under this Standard are encouraged to explore the use of versions of these documents. For undated references, their versions are applicable to this standard.
GB/T222-2006 steel for chemical analysis – Sample sampling method and permissible deviation of finished product chemical composition
GB/T 223(all parts) Methods for chemical analysis of iron, steel and alloys
GB/T 228-2002 Metallic materials — Tensile test at room temperature (ISO 6892:1998 (E), MOD)
GB/T 229-1994 Metal Charpy notch impact test method (eqv TSG 148:1983)
Dimensional tolerances and Machining allowances for Castings (eqv ISO 8062:1994)
GB/T 9452-2003 Heat treatment furnace — determination of effective heating zone
Cast carbon steel parts for general engineering purposes (neq ISO 3755:1991)
GB/T 12224-2005 steel valves General requirements
GB/T 12230–2005 stainless steel castings for general valves — Technical specifications
General principles for welding quality assurance (>
GB/T 13927 General valve pressure test (GB/T 13927– 1992.neq ISO 5208:1382)
GB/T15169-2003 Steel melting welding welders skill assessment (ISO/DIS 9606-1:2002)
JB/T 6439 Valve compression cast steel magnetic particle inspection
Radiographic examination of compression cast steel parts of JB/T 6440 Valve
JB/T 6902 valve cast steel – test method for liquid penetration
JB/T 7927 valve steel castings appearance quality requirements
ASTM A3S1/A3S1M Austenite and austenite for pressure parts. Specification for ferritic (biphase) steel castings
ASTM A352/A352M Specification for Ferritic and Martensitic Steel Castings for Parts Under Low Temperature Compression
Material grade and service temperature
The material grade and service temperature of the casting are shown in Table 1.
Table 1 Casting material grade and service temperature
Chemical composition and mechanical properties
The chemical composition of castings shall conform to the requirements in Table 2.
Table 2 Chemical composition of castings (mass fraction)
Post time: Nov-24-2022