Through observation, it is not difficult to find that all thick-walled steel pipes, hot-expanded pipes, etc. are produced with strip steel as the production material. The pipes obtained by thick-walled welding on high-frequency welding equipment are called thick-walled steel pipes. According to different uses, there are different post-production processes (roughly divided into scaffolding pipes, fluid pipes, wire casings, bracket pipes, guardrail pipes, etc.). Thick-walled welded pipe specification GB/T3091-2008 and low-pressure fluid welded pipes are a kind of thick-walled welded pipes, usually transported by water and gas. After welding, one more water pressure test is performed than ordinary welded pipes. Therefore, the price of low-pressure fluid pipes is usually higher than that of ordinary thick-walled welded pipes.
The inspection standards for thick-walled steel pipes are mainly as follows:
1. Thick-walled steel pipes should be submitted for inspection in batches, and the batching rules should comply with the rules of the corresponding product specifications.
2. The inspection items, sampling quantity, sampling location, and test method of thick-walled steel pipes shall be by the provisions of the corresponding product standards. With the consent of the buyer, hot-rolled seamless thick-walled steel pipes can be sampled in batches according to the rolling roots.
3. When a certain item of the test results of thick-walled steel pipes does not meet the requirements of the product standards, the unqualified ones shall be selected, and double the number of samples shall be randomly taken from the same batch of thick-walled steel pipes for re-inspection of the unqualified items.
If the re-inspection results are unqualified, the batch of thick-walled steel pipes shall not be delivered.
The following inspection items are not allowed to be re-inspected when the initial inspection fails.
4. For thick-walled steel pipes with unqualified re-inspection results, the supplier may submit them for inspection one by one; or re-heat treat them and submit them for inspection in a new batch.
5. If there are no special provisions in the product standards, the chemical composition of thick-walled steel pipes shall be inspected according to the smelting composition.
6. The inspection and testing of thick-walled steel pipes shall be carried out by the supplier’s technical supervision department.
7. The supplier has the right to ensure that the delivered thick-walled steel pipes meet the relevant product specifications. The purchaser has the right to inspect and check according to the relevant product specifications.
In addition, we also need to understand some issues regarding the welding control of thick-walled steel pipes:
1. Control of welding temperature of thick-walled steel pipes: The welding temperature is affected by the high-frequency eddy current thermal power. According to formula (2), the high-frequency eddy current thermal power is affected by the current frequency, and the eddy current thermal power is proportional to the square of the current excitation frequency; and the current excitation frequency is affected by the excitation voltage, current, capacitance, and inductance. The formula for the excitation frequency is: f=1/[2π(CL)1/2]…(1)
Where: f-excitation frequency (Hz); C-capacitance in the excitation circuit (F), capacitance = charge/voltage; L-inductance in the excitation circuit, inductance = magnetic flux/current. From the above formula, it can be seen that the excitation frequency is inversely proportional to the square root of the capacitance and inductance in the excitation circuit, or directly proportional to the square root of the voltage and current. The excitation frequency can be changed by simply changing the capacitance, inductance, or voltage and current in the circuit, thereby achieving the purpose of controlling the welding temperature. For low-carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the requirements of 3~5mm penetration of the pipe wall thickness. In addition, the welding temperature can also be achieved by adjusting the welding speed.
When the input heat is insufficient, the edge of the heated weld cannot reach the welding temperature, and the metal structure remains solid, resulting in unfused or incomplete penetration; when the input heat is insufficient, the edge of the heated weld exceeds the welding temperature, resulting in overburning or molten droplets, causing the weld to form a molten hole.
2. Control of weld gap of thick-walled steel pipe: The strip steel is fed into the welded pipe unit, and after multiple roller rolling, the strip steel is gradually rolled up to form a round tube blank with an open gap. The pressing amount of the kneading roller is adjusted to control the weld gap to 1~3mm, and the two ends of the weld are flush. If the gap is too large, the proximity effect is reduced, the eddy current heat is insufficient, and the weld intercrystalline bonding is poor, resulting in unfused or cracked. If the gap is too small, the proximity effect is increased, the welding heat is too high, and the weld is burned; or the weld may form a deep pit after kneading and rolling, affecting the weld surface.
Post time: Dec-30-2024