Application of titanium steel composite plate in chimney and welding mechanism and method

1. Application of titanium steel composite plate in chimney

Since the flue gas after wet desulfurization of thermal power plant flue gas is in a low heat and wet weight state during the emission process, and contains untreated sulfide molecules, it is very corrosive to the chimney steel inner tube, so titanium steel composite plate with high corrosion resistance and good plastic toughness began to be used in the chimney inner tube.

The titanium steel composite plate is made of carbon steel as the base layer and industrial pure titanium as the composite layer by explosion, rolling and other methods. The base layer of the titanium steel composite plate mainly meets the strength and rigidity requirements of the welding structure design, while the composite layer meets the requirements of special properties such as corrosion resistance, which saves a lot of precious metal materials such as titanium, saves energy and reduces engineering costs.

2. Welding mechanism of titanium steel composite plate

The base layer and the composite layer of the titanium steel composite plate are very different in chemical composition, metallographic structure, physical properties, etc., which greatly increase the difficulty of welding. Therefore, when welding the titanium steel composite plate, the base layer and the composite layer are welded separately. Steel has good welding performance. Considering the characteristics of titanium such as preventing overheating, on-site working environment and titanium’s fear of pollution, how to control the welding heat input during welding, how to prevent the composite titanium plate from overheating and oxidation due to the welding of the base steel plate, and how to strengthen the protection of the titanium composite layer during welding are the difficulties in titanium steel composite plate welding.

(1) Titanium at room temperature is a silver-gray metal with a high melting point and low density, but it is very tough and corrosion-resistant. As the temperature rises, the affinity with oxygen, hydrogen, nitrogen and other substances in the air continues to increase, so the molten pool in a high temperature state is easily contaminated by impurities such as gas, so it is very important to protect the molten pool during titanium welding.

(2) When the oxygen and nitrogen content in the titanium weld is high, the weld or heat-affected zone becomes brittle, and cracks will appear under greater stress; at the same time, if there are cracks and interlayers in the welding wire and the base material, there may be a large number of harmful impurities in these cracks and interlayers, which will also cause thermal cracks.

(3) Porosity is the most common welding quality defect when welding titanium plates. The welding pores of titanium plates mainly appear in the middle of the weld and the edge of the fusion line.

2. Welding method of titanium steel plate

(1) Cleaning before welding is a very important work link. Before welding, the weld joint, the surface of the base groove and the oil, paint, rust, water stains and other dirt inside and outside the groove should be carefully cleaned, and polished until the metal luster is exposed;) Titanium filler bars and titanium cover plates need to be cleaned with pickling liquid. After pickling, they must be rinsed with clean water and then dried.

(2) The composite layer is welded by semi-automatic consumable argon arc welding, and continuous welding with smaller heat input and slower speed is used as much as possible; the base layer is primed by semi-automatic consumable argon arc welding, and carbon dioxide gas shielded semi-automatic welding is layered for multiple times for filling and covering.

(3) Spot welding and welding of the base steel plate are carried out first. After passing the quality inspection, spot welding and welding of titanium filler bars and titanium composite layers, and titanium cover plates and titanium composite layers are carried out.

(4) After welding, the iron ion content in the area on the outside of the titanium cladding that may be contaminated by iron ions needs to be checked.