Steel structure processing and fabrication process (introductory knowledge)

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1) Mechanical properties of steel: The mechanical properties of structural steel include yield strength, tensile strength, elongation, and low-temperature impact toughness. These parameters shall meet the requirements of the Code for Design of Steel Structures, except that low-temperature impact toughness is only required to be tested when the structure may operate in a low-temperature environment. The determination of the mechanical properties of steel must comply with the \"Regulations on Sampling for Testing the Mechanical and Process Properties of Steel\" (GB2975-82). (2) Chemical composition of steel: It is related to factors such as the workability, toughness, and durability of steel. The main factors include the carbon content, the content of alloying elements, and the allowable limits for impurity elements such as sulfur and phosphorus, all of which must meet the requirements of the standard (GB222-84). (3) Processability: Processability mainly includes weldability and machinability. Weldability is related to carbon content or carbon equivalent (in low-alloy steel), and can be determined through weldability tests. The workability is determined through a cold bending test. Compliant with (GB232-88) standards. (4) Geometric dimension tolerances: The deviation of the external dimensions of steel materials (steel plates, section steels, round bars, steel pipes) from their theoretical dimensions must be within acceptable limits. The allowable deviation values can be referred to in the standards GB709-88, GB706-88, GB787-88, GB978-88, GB707-88, GB816-87, etc. (5) Surface defects of steel: The surface of the steel shall be free from bubbles, scabs, pulls, cracks, folds, inclusions, and embedded scale. These defects must be removed, and after removal, the depth of the depression in that area shall not exceed the negative deviation value of the steel thickness. Furthermore, when the surface of the steel contains defects such as rust, pitting, or scratches, their depth shall not exceed 1/2 of the negative deviation value of the steel’s thickness. (6) Mechanical cutting: Cutting is carried out using mechanical force (shearing, sawing, grinding); the corresponding machines include plate shears, sawing machines, grinding wheels, etc., and are suitable for straight-line cutting of steel plates or profiles with a thickness of 12–16 mm or less. (7) Oxy-acetylene cutting: Metal is heated and melted using flames such as oxygen-acetylene, propane, or liquefied petroleum gas, and the molten metal is blown away with compressed air, thereby separating the metal. This method is suitable for cutting curves and multiple sections at once. ) (8) Plasma cutting: Cutting is achieved using a plasma arc; it is suitable for cutting materials with high melting points such as stainless steel. (9) Hot forming: It refers to the processing of steel by heating it to a certain temperature first. This method is suitable for shaping, bending, and correcting workpieces that cannot be done at room temperature. The termination temperature for hot working shall not be lower than 700°C. When the heating temperature is between 200 and 300°C, the steel becomes brittle at low temperatures; hammering and bending are strictly prohibited. Steel with a carbon content outside the range of low-carbon steel generally cannot be hot-worked. (10) Cold forming: It is carried out at room temperature. The material undergoes the desired permanent deformation due to external forces exceeding its yield strength, or certain parts of the material separate from it as required when the external forces exceed its ultimate strength. Cold working always tends to make the material harder and more brittle; therefore, heat treatment can be used to restore the steel to its normal state or to remove the parts at the edges that have become severely hardened. Cold working of carbon steel is not allowed when the ambient temperature is below -16°C. Low-alloy steel shall not be processed below -12°C. (11) Bending processing: A manufacturing method that, in accordance with design requirements, uses processing equipment and appropriate tooling to bend sheets or steel profiles into a specific shape. Cold bending is suitable for thin sheets and small steel parts ； Hot bending is suitable for thicker plates as well as more complex components and sections, with a hot bending temperature of 950–1100°C. (12) Sheet bending: A method that involves stretching the outer fibers of a flat steel sheet and shortening the inner fibers under external force to induce bending deformation. Coiling is done by a coiler. Depending on the temperature of the material, it is further divided into cold-rolled and hot-rolled sheets. Coiled plates are mainly used for welding circular tube columns, pipes, air tanks, etc. (13) Folding: The process of bending the edges of steel structural components at a certain angle or into a specific shape is called folding. Bending is generally used for thin plate components. Folding is usually done using a folding machine, along with appropriate molds. (14) Molding: Molding is a method of shaping steel using molds in a pressuring device. Specific methods include blank forming, stamping forming, bending, rolling, drawing, rolling, etc. (15) Edge chipping: Edge chipping is the process of removing excess metal from the edge by striking the chipping tool, thereby creating a bevel. Edge shoveling can be done manually or with pneumatic power, with pneumatic methods using pneumatic shovels. The precision of edge shaving is low; it is generally used for processing grooves with modest requirements and in small quantities. (16) Edge filing: During edge filing, the workpiece is pressed firmly, and the filing tool moves back and forth along the edge to be processed, thereby creating a bevel. Edge trimming can be done on straight or beveled edges. (17) Edge milling: Edge milling is similar to planing, except that the tool holder and planing tool in the feed box of the planer are replaced by disc milling cutters; thus, the milling cutter moves linearly along the edge while also rotating, resulting in higher processing efficiency. (18) Carbon arc gas gouging: A carbon rod is used as an electrode; an arc is generated between it and the metal to be gouged, heating the metal to a molten state, after which compressed air is used to blow away the molten metal. It has high work efficiency. (19) Site sample method: Actual component samples are placed on the assembly platform at a 1:1 scale, and then the components are assembled based on the position of the parts on these samples. (20) Molding replication method: First, a single-sided structure is assembled using the sample method and spot welding is applied; then it is turned over to serve as a replication mold, on which another single-sided structure is assembled. Applicable to truss structures with symmetric cross-sections. (21) Vertical assembly: Depending on the characteristics of the component and the stable positioning of its parts, assembly from top to bottom or from bottom to top is chosen; this method is suitable for structures that can be placed stably and are not very tall. (22) Horizontal installation: The components are assembled in a horizontal position. Used for slender members with a small cross-section but large length. (23) Membrane assembly method: Positioning the parts of a component using a membrane to assemble them in their proper positions. Used to manufacture products in large batches with high precision. (24) Arc welding: The process of fusing metals using the heat generated by the arc between the electrode and the workpiece is called arc welding. Arc welding is divided into manual welding, automatic welding, semi-automatic welding, and gas shielded welding. (25) Resistance welding: Electric current is passed through two welded pieces in contact; the resistance is highest at the point of contact, and the flow of current generates high temperatures that cause the material to become semi-melted, allowing it to be fused together under applied pressure. It is generally used for welding round steel bars or spot-welding steel plates. (26) Electroslag welding: Welding is carried out by melting metal using the resistive heat generated by an electric current passing through the slag. Such as the penetration welding of stiffeners at the upper and lower flanges of the connecting beam inside the column when it is used for a rigid connection between box columns and beams. (27) Manual welding: Arc welding that is performed entirely by hand; it has low productivity, quality depends on the welder’s skills, and stability is poor, but it is flexible and suitable for shorter, complex welds or welding in the field. (28) Submerged arc automatic welding: During welding, the arc is buried beneath a powdered flux; the flux is automatically applied mechanically while the welding wire is fed forward. Suitable for longer welds. Good weld quality and high efficiency. (29) Gas shielded welding: It uses CO2 or argon to shield the arc and protect the molten metal; both the wire feed and the gas supply are automated, requiring only manual movement of the welding torch. It belongs to the semi-automatic welding category, offering good welding quality and high efficiency. (30) Shot blasting: In shot blasting, steel shots with a particle size of 0.8–2.0 mm are drawn in through the center of the impeller of the blasting machine and then ejected at high speed from the tip of the impeller toward the surface of the steel structures that need rust prevention treatment, thereby achieving mechanical rust removal. This method offers high efficiency in rust removal, low costs, and minimal pollution.