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Abstract: This paper briefly describes the basic properties of Q345C steel, the welding process and the basic essentials of welding operation during on-site welding.
Keywords: Q345C on-site welding process
Hebei Dingzhou Power Plant Phase I Project 2 × 600MW unit of the main plant steel structure designed by Hebei Electric Power Design Institute, Sanmenxia Hydraulic Machinery Factory manufacturing. The steel structure of the main plant building is divided into: deoxidizing coal bunker room, heater platform, and columns A0-A of steam engine room. The main columns, beams and vertical supports of the steel frame are all made of "H" section steel, with the base material of Q345C (low alloy structural steel), and the main columns of the steel frame are connected by segmental butt joints, in which the web of "H" section steel is connected by high-strength bolts, and the connection between the wing plates is made by butt joint welding.
I. Material Introduction
1. Q345 chemical composition of the following table (%):
Element C ≤ Mn Si ≤ P ≤ S ≤ Al ≥ V Nb Ti
Content 0.2 1.0-1.6 0.55 0.035 0.035 0.015 0.02-0.15 0.015-0.06 0.02-0.2
Q345C mechanical properties are as follows (%):
Mechanical properties index Elongation (%) Test temperature 0 ℃ Tensile strength MPa Yield point MPa ≥
Value δ5 ≥ 22 J ≥ 34 σb (470-650) σs (324-259)
Where the wall thickness between 16-35mm, σs ≥ 325Mpa; wall thickness between 35-50mm, σs ≥ 295Mpa
2. Welding characteristics of Q345 steel
2.1 Calculation of Carbon Equivalent (Ceq)
Ceq=C+Mn/6+Ni/15+Cu/15+Cr/5+Mo/5+V/5
Calculation of Ceq=0.49%, greater than 0.45%, as can be seen , greater than 0.45%, it can be seen that the welding performance of Q345 steel is not very good, need to develop strict process measures in welding.
2.2 Problems that tend to occur when Q345 steel is welded
2.2.1 Hardening tendency of the heat-affected zone
Q345 steel in the cooling process of welding, the heat-affected zone is prone to the formation of quenched tissue - martensite, so that the hardness of the near-seam zone increases, and the plasticity decreases. The result is that cracks occur after welding.
2.2.2 Cold crack susceptibility
Welding cracks in Q345 steel are mainly cold cracks.
Second, the welding construction process
Bevel preparation → spot solid welding → preheating → inside mouth welding → back clearing (carbon arc gas planing) → outside mouth welding → inside mouth welding → self-inspection/specialized inspection → post-welding heat treatment → nondestructive testing (weld quality level 1 qualified)
Third, the choice of welding process parameters
Through the weldability analysis of the Q345 steel, the development of measures As follows:
1. Selection of welding materials
Due to the greater tendency of cold cracking of Q345 steel, low hydrogen welding materials should be used, taking into account the principle that the welded joints should be equal to the parent material, the selection of E5015 (J507) type electrodes.
Chemical composition of the following table (%):
Element C Mn Si S P Cr Mo V Ti
Content 0.071 1.11 0.53 0.009 0.016 0.02 0.01 0.01 0.01 0.01
Mechanical properties are shown in the following table:
Mechanical properties index σb (Mpa) σs (Mpa) ) δ5 (%) Ψ (%) AkvJ-30℃
Value 440 540 31 79 164 114 76
2. Beveling form: (according to the drawings and equipment supplied)
3. Welding method: manual electric arc welding (D).
4. Welding current: in order to avoid coarse weld organization, resulting in a decline in impact toughness, must be used in small specification welding. Specific measures are: the choice of small diameter welding rod, narrow welding channel, thin welding layer, multi-layer multi-channel welding process (welding sequence as shown in Figure I). The width of the weld channel is not greater than 3 times the electrode, the thickness of the weld layer is not greater than 5mm. the first layer to the third layer of Ф3.2 electrodes, welding current 100-130A; the fourth to the sixth layer of Ф4.0 electrodes, welding current 120-180A.
5. preheating temperature: due to the Ceq of Q345 steel>0.45%, in the welding should be carried out before the pre-heating, preheating temperature T0=100-150℃, interlayer temperature Ti≤400℃.
6. post-welding heat treatment parameters: in order to reduce the welding residual stress, reduce the hydrogen content in the weld, improve the weld metal organization and properties, the weld should be heat-treated after welding. Heat treatment temperature: 600-640 ℃, constant temperature time of 2 hours (plate thickness of 40mm), rise and fall of the temperature rate of 125 ℃ / h .
Four, on-site welding sequence:
1. pre-welding preheating
In the flange plate welding, the first flange plate preheating, constant temperature 30 minutes after the start of welding. Welding preheating, interlayer temperature, heat treatment is automatically controlled by the heat treatment temperature control cabinet, using far-infrared tracked heating furnace piece, microcomputer automatically set the curve and record the curve, thermocouple measurement of temperature. Preheating thermocouple measuring point from the edge of the bevel 15mm-20mm.
2. Welding
2.1 In order to prevent welding deformation, each column joints using a two-person symmetrical welding, welding direction from the middle of the two sides of the welding. When welding the lining (the lining is the bevel near the web), the first to the third layer must be operated using a small specification, because its welding is the main cause of welding deformation. At the end of welding one to three layers, the backside is cleaned up. In the use of carbon arc gas planing after the end of the root clearing, the weld must be mechanically polished, clean up the weld surface carburization, revealing the metal luster, to prevent the surface layer of carbonization is serious and cause cracks. Outer mouth welding should be welded once, and then finally weld the remaining part of the inner mouth.
2.2 When welding the second layer, the welding direction should be opposite to the direction of the first layer, and so on. The welding joints of each layer should be staggered by 15-20mm.
2.3 The welding current, welding speed and the number of welding layers of the two welders should be the same when welding.
2.4 Welding should start from the arc-led plate and end on the arc-receiving plate. Cut off and grind clean after welding is completed.
3. post-welding heat treatment: welded joints should be heat-treated within 12 hours after welding is completed. If the heat treatment can not be carried out in time should be taken to keep warm, slow cooling measures. In the heat treatment, two thermocouples should be used to measure the temperature, thermocouple spot welding in the welded joints on the outside and inside.
The welding temperature curve of Q345 steel is as follows
4. Welding Inspection
According to the requirements of the "Code for Construction and Acceptance of Steel Structures", the welded joints are inspected by ultrasonic flaw detection method, and the inspection ratio is 100%.
V. On-site technical management
1. Prepare detailed welding construction work instruction.
2. The whole process of controlling the welding process is the core of ensuring quality.
When welding each column joint, there should be a person to monitor the welding process, such as welders do not follow the work instructions should immediately terminate the welding. In the welding process, the heat treatment personnel should monitor the entire interlayer temperature, such as exceeding the standard should immediately notify the welder to suspend.
3. Improve the quality awareness of construction personnel is the key to implement the welding process
Before the construction, the whole staff briefing, and open the construction process card. Detailed explanation of the characteristics of the welding process and the need to strictly control the welding process on site and control points.
Six, conclusion
According to the construction of this welding process measures, in the field **** welded weld 102, by the non-destructive testing of a pass rate of 100%. After the verification of the actual construction, this welding process measures not only in the field to guide the welding of Q345 steel, but also to ensure the quality of welding.