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

1. Application of Titanium Steel Composite Plates in Chimneys

The flue gas emitted from thermal power plants undergoes wet desulfurization, resulting in low heat and a moist state. This gas contains untreated sulfide molecules, making it highly corrosive to the inner steel tubes of chimneys. To combat this, titanium steel composite plates, known for their high corrosion resistance and excellent plastic toughness, have been increasingly used for inner tubes of chimneys.

A titanium steel composite plate consists of carbon steel as the base layer, with industrial pure titanium as the composite layer, created through explosion bonding and rolling processes. The base layer provides the necessary strength and rigidity for the welded structure, while the composite layer delivers specialized properties such as corrosion resistance. This composite approach not only conserves valuable titanium resources but also reduces energy consumption and lowers project costs.

2. Welding Mechanism of Titanium Steel Composite Plates

The chemical composition, metallographic structure, and physical properties of the base and composite layers of titanium steel composite plates vary significantly, complicating the welding process. Consequently, the welding of these plates is carried out separately for both layers. Steel exhibits excellent welding performance. However, key challenges include managing overheating, ensuring proper working conditions, controlling heat input during welding, and protecting the titanium composite layer from oxidation and overheating due to the welding of the base steel plate.

(1) Titanium is a silver-gray metal with a high melting point and low density. It is exceptionally tough and corrosion-resistant. As the temperature rises, its affinity for oxygen, hydrogen, and nitrogen increases, making the molten pool vulnerable to contamination by gases. Therefore, protecting the molten pool during titanium welding is critical.

(2) High concentrations of oxygen and nitrogen in titanium welds can render the weld or heat-affected zone brittle, leading to cracks under stress. Additionally, impurities may be trapped in cracks and interlayers of the welding wire and base material, further exacerbating thermal cracking.

(3) Porosity is a common defect encountered when welding titanium plates. The welding pores typically form in the weld's center and along the fusion line edges.

3. Welding Methods for Titanium Steel Plates

(1) Proper cleaning before welding is crucial. All grease, paint, rust, and moisture on the weld joint and base groove surfaces must be thoroughly removed and polished until the metal shines. Additionally, titanium filler rods and cover plates should be cleaned using a pickling solution, followed by rinsing and drying.

(2) For the composite layer, semi-automatic consumable argon arc welding is employed, prioritizing continuous welding that minimizes heat input and uses slower speeds. The base layer is initially welded, followed by carbon dioxide gas shielded semi-automatic welding layered multiple times for filling and covering.

(3) Spot welding of the base steel plate is performed first. Upon passing quality inspections, further spot welding is conducted on titanium filler rods and composite layers.

(4) After welding, it's essential to check for iron ion contamination in areas outside the titanium cladding that may have been exposed to iron ions.