How are the process parameters for steel structure welding determined?

This process standard applies to manual arc welding work for the fabrication and installation of steel structures in general industrial and civil construction projects. Construction Preparation 2.1 Materials and main equipment: 2.1.1 Welding electrodes: Their models shall be selected in accordance with the design requirements, and they must come with quality certificates. Baking is performed before welding as required. Welding rods with flaked coating or rusted electrodes are strictly prohibited. When no specific requirements are given for the design, E43 series carbon steel structural welding electrodes are suitable for welding Q235 steel ； When welding 16Mn steel, E50 series low-alloy structural steel electrodes are recommended ； Low-hydrogen electrodes (alkaline electrodes) are recommended for welding critical structures. After baking as instructed, place it in a insulated container for easy access whenever needed. Acidic welding electrodes and alkaline welding electrodes must not be used together. 2.1.2 Arc starting plate: An arc starting plate is required when using a groove for connection; the material of the arc starting plate and the type of groove should be the same as those of the welded parts. 2.1.3 Main tools: welding machines (AC and DC), welding cable, welding tongs, mask, hammer, electrode oven, electrode insulation container, wire brush, asbestos cloth, thermometer, etc. 2.2 Operating Conditions 2.2.1 Be familiar with the drawings and conduct a welding process briefing. 2.2.2 Before welding, the validity period of the welder’s certificate should be checked to ensure that it confirms the type of welding work the welder is qualified to perform. 2.2.3 The on-site power supply shall meet the requirements for welding. 2.2.4 When the ambient temperature is below 0°C, preheating and post-heating temperatures shall be determined based on process testing. Operation Procedures 3.1 Process Flow: Preparation for work → Arc welding (flat welding, vertical welding, horizontal welding, overhead welding) → Weld inspection 3.2 Arc welding of steel structures: 3.2.1 Flat welding 3.2.1.1 Select appropriate welding parameters such as electrode diameter, welding current, welding speed, and welding arc length, which are verified through welding process tests. 3.2.1.2 Cleaning the weld joint: Before welding, check whether the groove and assembly gap meet the requirements, and whether the tack welds are secure; there should be no oil or rust around the weld seam. 3.2.1.3 Welding electrodes should be baked at the specified temperature for the specified time. Once taken out of the oven, they should be placed in a welding electrode storage container and used as needed. 3.2.1.4 Welding current: Select an appropriate welding current based on factors such as the thickness of the workpiece, the welding layer, the type and diameter of the electrode, and the welder’s skill level. 3.2.1.5 Arc initiation: The start and end points of the arc in fillet welds should be at the ends of the weld, with a distance of more than 10 mm between them. Arc initiation should not be done casually; once the arc is established, the welding rod should be immediately pulled away from the weld area, maintaining a gap of 2–4 mm between the welding rod and the component in order to sustain the arc. For butt welds and combined butt-and-angle welds, arc starting plates and lead plates are placed at both ends of the weld; the arc must be started on the arc starting plate before welding begins in the weld area. For intermediate joints, the arc should be started 15–20 mm in front of the joint, the workpiece should be preheated, then the welding rod should be moved back to the start of the weld. Only after the molten pool has reached the desired thickness can welding proceed forward. 3.2.1.6 Welding speed: Uniform welding speed is required to ensure consistent weld thickness and width; when viewed through the mask, the molten iron and slag in the weld pool should be at an equal distance from each other (2–3 mm). 3.2.1.7 Welding arc length: This is determined depending on the type of electrode; generally, the arc length should remain stable. For acidic electrodes, a length of 3–4 mm is appropriate, while for basic electrodes, 2–3 mm is suitable. 3.2.1.8 Welding angle: It is determined based on the thickness of the two welded pieces. There are two possible values for the welding angle; one is an angle of 60–75° between the welding electrode and the direction in which welding proceeds ； Secondly, there are two possibilities regarding the angle between the welding rod and the workpiece: when the thicknesses of the workpieces are equal, the angle between the welding rod and the workpiece is 45° in both cases ； When the thicknesses of the welded pieces vary, the angle between the welding rod and the side of the thicker piece should be greater than the angle between the welding rod and the side of the thinner piece. 3.2.1.9 Arc termination: At the end of each weld, after filling the crater, move the arc in the direction opposite to that of welding so that the crater is pushed into the weld bead, thereby preventing the crater from penetrating the metal. After welding is complete, the arc plate should be removed by gas cutting and polished smooth; it must not be knocked off with a hammer. 3.2.1.10 Slag removal: After the entire weld has been completed, the slag must be removed. Only after the welder has conducted a self-inspection (including checking the appearance and weld dimensions) to confirm that there are no issues can the work proceed to another location for further welding. 3.2.2 Vertical welding: The basic operational procedures are the same as those for flat welding, but the following points should be taken into account: 3.2.2.1 Under the same conditions, the welding current required is 10%～15% lower than that for flat welding. 3.2.2.2 Short-arc welding is used, with the arc length generally being 2–3 mm. 3.2.2.3 The electrode angle is determined based on the thickness of the workpiece. The thicknesses of the two welded parts are equal, and the angle between the electrodes in the horizontal direction is 45° ； When the thicknesses of the two welded pieces are different, the angle between the welding rod and the thicker piece should be greater than the angle with the thinner piece. The welding electrode should form an angle of 60° to 80° with the vertical plane, so that the arc points slightly upward toward the center of the molten pool. 3.2.2.4 Arc termination: When welding is completed, the arc pit is filled using the arc extinguishing method, and the arc is moved to the center of the molten pool to be extinguished there. It is strictly forbidden to let the arc crater fall to one side. To prevent chipping, the angle of the arc transfer electrode should be lowered so that the electrode is perpendicular to the workpiece, or a slight downward force should be applied by the arc. 3.2.3 Transverse welding: It is basically the same as flat welding; the welding current is 10%–15% lower than that used in flat welding under the same conditions, and the arc length is 2–4 mm. Regarding the angle of the welding rod, when welding horizontally it should be tilted downward at an angle of 70° to 80°, in order to prevent the slag from falling. Depending on the thickness of the two welded pieces, the angle of the welding rod can be adjusted appropriately, with the welding rod forming an angle of 70° to 90° with the direction of welding progress. 3.2.4 Upward welding: It is essentially similar to vertical and horizontal welding; the angle between the electrode and the workpiece depends on the thickness of the workpiece. The electrode should be at an angle of 70°–80° to the direction of welding, and it is advisable to use a low current and a short arc for welding. 3.3 Welding at Low Temperatures in Winter: 3.3.1 When performing arc welding in an environment where the temperature is below 0°C, in addition to complying with the regulations for welding at normal temperatures, it is necessary to adjust the welding process parameters so that the weld and the heat-affected zone cool down slowly. When the wind force exceeds level 4, wind protection measures should be taken ； Uncooled joints after welding should be kept away from ice and snow. 3.3.2 To prevent welding cracks in steel structures, preheating is required to control the interlayer temperature. When the temperature at the work site is below 0°C, process tests should be conducted to determine the appropriate preheating and post-heating temperatures. Quality Standards 4.1 Assurance Items 4.1.1 Welding materials shall meet the design requirements and relevant standard specifications; quality certificates and baking records shall be checked. 4.1.2 Welders must pass examinations, and it is necessary to check their certificates confirming compliance with the relevant welding conditions as well as the date of the assessment. 4.1.3 Welds of grades I and II must undergo flaw detection inspections and shall meet the design requirements as well as the provisions of the construction and acceptance specifications; the weld flaw detection reports shall be checked. 4.1.4 Weld surfaces of grades I and II shall be free from defects such as cracks, weld beads, burn-through, and arc pits. Grade II welds must be free from defects such as surface pores, slag inclusions, cratering, cracks, and arc scratches, while Grade I welds must be free from defects such as undercutting and incomplete welding. 4.2 Base Project 4.2.1 Weld appearance: The weld shape is uniform, with smooth transitions between weld passes as well as between the weld passes and the base metal; slag and spatter are completely removed. 4.2.2 Surface pores: Not allowed in welds of grades I and II ； For Class III welds, the allowable diameter within every 50 mm of weld length is ≤0.4t ； And there are 2 pores ≤3mm ； The pore spacing is ≤6 times the pore diameter. 4.2.3 Edge biting: Not allowed for Grade I welds. Grade II welds: the root undercut depth shall be ≤0.05t and ≤0.5mm; the continuous length shall be ≤100mm, and the total length of undercuts on both sides shall be ≤10% of the weld length. Grade III weld: root undercut depth ≤ 0.lt, and ≤ 1 mm. Note ； t is the thickness of the thinner plate at the joint. 4.3 Allowable deviation items: Weld excess height b

Selection of welding process parameters ------------ Please continue to add and refine this information. The main welding process parameters for manual arc welding include electrode diameter, welding current, arc voltage, number of welding layers, type of power supply, and polarity. 1. Electrode diameter: The selection of the electrode diameter depends mainly on factors such as the thickness of the workpiece, the type of joint, the location of the weld, and the welding layer. Under normal circumstances, the electrode diameter can be selected according to Table 6-4 based on the thickness of the welded piece, with a preference for larger-diameter electrodes. Additionally, during flat welding, the diameter can be larger ； When welding in the vertical position, the diameter of the electrode used should not exceed 5 mm ； For transverse and vertical welding, the diameter used shall not exceed 4 mm ； When performing multi-layer welding with grooves, in order to prevent defects such as lack of penetration, a welding rod with a diameter of 3.2 mm is recommended for the first layer of welds. Table 6-4 Relationship between electrode diameter and workpiece thickness (unit: mm) Workpiece thickness ≤2 3~4 5~12 >12 Electrode diameter 2 3.2 4~5 ≥15 2. Welding current Both too high and too low welding currents can affect the quality of welding; therefore, its selection should take into account factors such as the type and diameter of the electrode, the thickness of the workpiece, the joint configuration, and the position of the weld. Among these factors, the electrode diameter and the position of the weld are the most critical. In the welding of ordinary steel structures, the relationship between the welding current and the electrode diameter can be approximated using the following empirical formula: I=10d² (6-1), where I represents the welding current in amperes ； d——Electrode diameter (mm). Additionally, when welding in the vertical position, the current should be 15% to 20% lower than that used for horizontal welding ； For transverse and vertical welding, the current should be 10% to 15% lower than that for horizontal welding. 3. Arc voltage: Depending on the characteristics of the power supply, the corresponding arc voltage is determined by the welding current. Furthermore, the arc voltage is also related to the arc length. The longer the arc, the higher the arc voltage; the shorter the arc, the lower the arc voltage. It is generally required that the arc length be less than or equal to the diameter of the electrode, that is, short-arc welding. When welding with acidic electrodes, in order to preheat the area or reduce the temperature of the molten pool, the arc is sometimes stretched slightly during welding, a technique known as long-arc welding. 4. Number of welding passes: The number of welding passes should depend on the thickness of the welded part. Except for thin plates, multi-layer welding is generally used. Too few welding layers and excessive thickness of each weld layer have an adverse effect on the plasticity of the weld metal. During construction, the thickness of each weld layer should not exceed 4–5 mm. 5. Type and polarity of power supply: DC power supplies are used in important welded structures or thick, highly rigid structures due to their stable arc, minimal spatter, and excellent welding quality. In other cases, an AC welding machine should be considered first. Depending on the shape of the welding electrode and the characteristics of welding, and taking advantage of the fact that the temperature at the anode in the arc is much higher than that at the cathode, different polarities are used to weld various types of components. When using alkaline electrodes or welding thin sheets, use direct current with reverse polarity (the workpiece connected to the negative pole) ； When using acidic electrodes, direct connection is usually adopted (the workpiece is connected to the positive pole). Last edited by Lailai on 2009-3-30 18:50]