In the petrochemical industry, pressure vessels and pressure piping play a vital role. Titanium flanges, as key components for connection and sealing, are extremely common and widely used. Whether in piping design, fittings and valves, or the construction of equipment and its components, titanium flanges are indispensable components. However, the occurrence of hot cracks during titanium flange welding seriously affects weld quality and structural safety. Therefore, research on how to prevent hot cracks in titanium flange welding is of great practical significance.
According to the New Enterprise Association, with the booming petrochemical industry in recent years, the application of titanium flanges has continued to expand, but the problem of hot cracking in welding has also become increasingly prominent, becoming a key factor restricting the improvement of titanium flange application quality. The Baoji Titanium Industry Research Institute has conducted extensive testing and research on the issue of hot cracking in titanium flange welding, revealing the underlying mechanism of hot cracking and providing valuable data support for solving this problem.
I. The Effect and Control of Impurity Elements on Titanium Flange Welding
1. The Effect of S
Sulfur (S) can cause hot brittleness in titanium materials. During the forging and rolling processes of titanium flanges, the presence of sulfur (S) is highly susceptible to cracking. When titanium contains a certain amount of sulfur, it reacts with titanium at high temperatures to form low-melting-point sulfides. These sulfides precipitate at grain boundaries, weakening the bonding strength between the grains. During welding, uneven local heating generates thermal stresses that can cause these sulfides at the grain boundaries to become crack sources, leading to hot cracking. Therefore, reducing the S content in the material is a key measure to prevent hot cracking during welding.
2. Effect of Phosphorus
Phosphorus (P) increases the cold brittleness of titanium and impairs weldability. Phosphorus has a low solubility in titanium. During the cooling process, it segregates at grain boundaries, reducing their plasticity and toughness. Phosphorus also impairs titanium's cold-bending properties, making it more susceptible to cracking at grain boundaries when the weld joint is subjected to external forces. Strictly controlling the P content in the raw materials can effectively improve the weldability of titanium flanges and reduce the occurrence of hot cracking.
3. Methods for Controlling Impurity Element Content
To effectively avoid hot cracking caused by impurity elements such as S and P, high-quality raw materials should be selected and rigorously analyzed for chemical composition during the titanium flange manufacturing process. Advanced refining processes, such as vacuum melting and electroslag remelting, are used during the melting and processing to remove impurity elements from the material. Furthermore, by adding appropriate amounts of alloying elements, they can form stable compounds with impurity elements such as S and P, thereby reducing their adverse effects on titanium material properties.
II. Optimizing Welding Methods and Processes
1. Selecting the Appropriate Welding Method
Different welding methods have different characteristics and have different impacts on the occurrence of hot cracking in titanium flange welding. For example, tungsten inert gas arc welding (TIG) offers advantages such as a stable arc, high weld quality, and easily controlled heat input, making it suitable for welding thin titanium flanges. Plasma arc welding, on the other hand, offers concentrated energy, high welding speed, and a small heat-affected zone, making it particularly effective for welding thicker titanium flanges. When selecting a welding method, the most appropriate method should be selected based on a comprehensive consideration of factors such as the thickness, structural characteristics, and weld quality requirements of the titanium flange.
2. Controlling Heat Input
Heat input refers to the amount of heat input per unit length of weld, and it significantly impacts the occurrence of hot cracks. Excessive heat input can lead to weld overheating, coarsening of the grains, reduced weld plasticity and toughness, and an increased tendency to hot cracking. Therefore, during the welding process, heat input should be strictly controlled. This can be achieved by adjusting parameters such as welding current, voltage, and speed. For example, while ensuring weld quality, the welding current and voltage can be appropriately reduced and the welding speed increased to minimize weld overheating.
3. Measures to Reduce Weld Overheating
In addition to controlling heat input, other measures can be taken to reduce weld overheating. For example, preheating and post-heat treatment processes can be employed. Preheating can reduce the temperature difference between the weld and the base material, reducing weld stress and thus reducing the likelihood of hot cracking. Post-heat treatment can eliminate residual stress and improve the weld microstructure. In addition, a multi-layer, multi-pass welding method can be used. Through the welding and cooling of each weld pass, sufficient heat dissipation is achieved in the weld seam, reducing overheating.
III. Application of Rare Earth Elements in Titanium Flange Welding
1. Mechanism of Action of Rare Earth Elements
Adding rare earth elements to welding materials can inhibit the development of columnar grains and refine the grain size. Rare earth elements have unique electronic structures and chemical properties. They can segregate at grain boundaries, hindering grain growth. Furthermore, rare earth elements can form compounds with impurity elements in titanium materials, purifying the grain boundaries. By refining the grain size and purifying the grain boundaries, the mechanical properties of titanium flange welds can be significantly improved, enhancing their resistance to thermal cracking.
2. Commonly Used Rare Earth Elements and Their Addition Methods
Common rare earth elements include cerium (Ce) and lanthanum (La). In titanium flange welding, rare earth elements can be introduced into the weld joint by adding rare earth alloys to the welding wire or by using a rare earth coating. For example, an alloy wire containing a specific proportion of rare earth elements can be combined with titanium welding wire to create a rare earth-titanium welding wire. During the welding process, rare earth elements (REs) enter the weld as the welding wire melts, refining the grain size and purifying the grain boundaries.
3. Effects of Rare Earth Element Addition
Practical experience has shown that adding an appropriate amount of REs to titanium flange welding materials can effectively reduce the occurrence of hot cracking during welding. Furthermore, the addition of REs can improve the overall performance of the welded joint, including strength, toughness, and corrosion resistance. For example, one company significantly reduced the incidence of hot cracking and significantly improved the mechanical properties of the welded joint after adding 0.1% to 0.2% cerium to titanium flange welding.
Titanium flanges play a crucial role in connecting and sealing pressure vessels and pressure piping, and their welding quality directly impacts the safe operation of the equipment. Reducing the content of impurities such as S and P in the material, selecting appropriate welding methods and processes, and incorporating REs into the welding materials can effectively prevent hot cracking during titanium flange welding.
In actual production, various factors should be comprehensively considered according to the specific situation to formulate a reasonable welding process plan, ensuring the welding quality of titanium flanges and providing reliable protection for safe production in industries such as petrochemicals. In the future, with the continuous development of welding technology, more effective methods and processes for preventing thermal cracks in titanium flange welding should be further explored.
