Ramble on about welding deformation of channel bars

Ramble on about welding deformation of channel bars

During welding, the high temperature molten iron shrinks during solidification, leading to the deformation of channel bars, which has been a headache for all steel structure practitioners. Therefore, when the production of general channel bars is completed, their deformation should be measured. If it exceeds the allowable tolerance range, rectification work should be done [1](Photo 1). In serious cases, even channel bars can only be scrapped. Therefore, the control of deformation, in the factory’s manufacturing quality control, occupies a very important channel bar.

To control deformation, the basic principle is to reduce the internal stress, or to give enough beam, so that the deformation can be acceptable or controlled. The methods of controlling welding deformation can be roughly divided into the following:

1. Review welding procedures. When serious post-weld deformation is foreseen or has occurred, the first thing to do is always to review the welding procedure. For example, modifying the joint form, whether it is fully permeable or semi-permeable welding, and changing unilateral grooving to bilateral grooving, although it may increase the cost, is often a very effective way to reduce deformation. At the same time, bilateral grooving, and can increase the plastic deformation ability, resistance to fatigue benefits.
2. The unit welding length of the heat into the reduction. For example, reduce current, speed up moving speed, use smaller diameter electrode or thinner wire.
3. Reduce the internal stress generated. For example, increase the preheating temperature, or reduce the cooling rate after welding (Photo 2), or adopt the post-heating method, or change a continuous welding to a piecewise welding method to complete. In addition, the interpass temperature is controlled so that the temperature is below a certain temperature before the next weld. This method can not only reduce thermal stress, but also reduce the problem of cold cracking because hydrogen can escape [2].
4. Balance the stress of welding. The left and right sides of the cross welding, every welding one to two on the exchange of the method.
5. The channel bars to be bunched to prevent deformation. For example, temporary supports can be added (Photo 3), and fixtures can be used for small channel bars.
6. Reduce the specification of welding material. In some small stress position, you can change to a lower strength of the electrode or welding line to weld. Low strength welding material has better deformation and extension ability, so it can reduce the stress during shrinkage.

The above methods of reducing deformation may be used in one or more combinations, as the case may be. But in many cases, the deformation is slightly large, and it may be purely the result of workers not applying welding according to the welding procedure book. For example, failure to comply with preheating regulations is a common problem in many plants. As long as the control of quality control is strengthened, sometimes there can be some improvement.

The deformation of member is also different due to the thickness of steel sheet. The ability of resistance to deformation of thin steel sheet member is low, and the slightly larger stress may lead to the overall distortion deformation. Therefore, when the structure is safe, the welding pass amount of thin-plate welding should be reduced as much as possible, or jump welding should be used to reduce deformation.

Photo 1 Component deformation after welding

Photo 2 Welding back cover fire blanket reduces cooling rate

Photo 3 Add temporary support

Author: Liu Zeshan

Source: China Stainless Steel Plate Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Reference

• 1. Liu Zeshan (104). “Rambling on thermal correction of steel channel bars”. Technician Bulletin, No. 985.

• 2. Liu Zeshan (102). “A brief introduction to some regulations on hydrogen welding”. Technician Bulletin, No. 866.

• 3. Liu Zeshan (107). “Steel structure construction”.