The Foundation: Extracting Pure Titanium
Before any pipe can be made, titanium must be extracted from its ores, primarily rutile and ilmenite. This is achieved through the Kroll process, a complex and energy-intensive procedure that produces the foundational material for all titanium products.
- Chlorination:The ore is first reacted with chlorine gas and coke at high temperatures to produce titanium tetrachloride (TiCl4), a volatile liquid. This step effectively separates titanium from other impurities.
- Distillation:The TiCl4 is purified through fractional distillation, removing contaminants like vanadium and iron chlorides to achieve a high-purity liquid.
- Reduction:The purified TiCl4 is then reduced in an inert argon atmosphere using molten magnesium. This highly controlled reaction produces porous, high-purity titanium “sponge” and magnesium chloride (MgCl2) as a byproduct.
- Purification:The final step involves vacuum distillation to remove the residual MgCl2 and magnesium, leaving behind the titanium sponge. This sponge, typically 99.5% pure, is the raw material for every mill product that follows, including the ingots used for pipe manufacturing.
The Manufacturing of Seamless Titanium Pipe
The seamless pipe process is a journey of hot-working and deformation, transforming a solid mass into a hollow tube without any welds.
- Melting and Ingot Production: The titanium sponge is first mixed with alloying elements (like aluminum and vanadium for Grade 5) and melted in a Vacuum Arc Remelting (VAR) or Electron Beam Melting (EBM) furnace. This process is performed under a vacuum to prevent contamination and ensure a homogeneous chemistry. The result is a large, solid, cylindrical ingot. At Daxun Alloys Co., Ltd., we utilize multiple VAR melts to guarantee the highest possible purity and consistency in our ingots.
- Forging: The large ingot is too coarse for direct piercing. It is heated and forged in large presses into a smaller, more robust billet. This forging process refines the internal grain structure, making it suitable for the subsequent high-stress operations.
- Piercing: The forged billet is heated again, this time to a specific temperature within the beta phase field (typically 850-950°C). It is then fed into a piercing mill, where two skewed rollers rotate and draw the billet over a central piercer, forming a thick-walled hollow shell known as a “bloom.”
- Rolling (Pilger Milling): The bloom is placed over a long mandrel and fed into a Pilger mill. This mill uses contoured rolls that intermittently compress and advance the tube, significantly reducing its wall thickness and increasing its length. This cold-working process further enhances the mechanical properties of the material.
- Finishing and Inspection: The pipe is annealed to relieve stress, straightened, and cut to length. The final, critical step is 100% ultrasonic testing (UT) in accordance with standards like ASTM B861. This inspection validates the integrity of the entire pipe body, ensuring it is free from internal flaws.
The Manufacturing of Welded Titanium Pipe
The welded pipe process is one of precision assembly. It takes a flat-rolled titanium product and forms it into a tube, joining the seam with a high-integrity weld.
- Melting and Slab Casting: The process begins identically with VAR or EBM melting to produce a pure titanium ingot. However, instead of being forged into a billet, this ingot is cast into a flat slab.
- Hot and Cold Rolling: The slab is heated and hot-rolled into a thick plate, which is then cold-rolled to the final required thickness for the pipe. This process results in a high-quality titanium coil or sheet with a uniform wall thickness and excellent surface finish. This flat-rolled product is the direct raw material for welded pipe.
- Forming: The titanium coil is fed through a series of forming rolls that gradually bend it into a circular shape. The edges of the strip are brought together in precise alignment, creating a continuous seam ready for welding. The control of this edge alignment, or “squeeze,” is a key operational parameter.
- Welding: The seam is welded in a chamber flooded with high-purity argon gas to shield the molten titanium from atmospheric contamination. A critical operational requirement is the use of backing gas; the inside of the pipe must also be purged with argon to protect the root of the weld from contamination. The primary methods are TIG (Tungsten Inert Gas) welding for high quality or Laser welding for speed and a minimal heat-affected zone.
- Sizing and Inspection: After welding, the tube passes through a sizing stand to finalize its outer diameter. Quality control is concentrated on the weld seam itself. Eddy Current Testing (ET) is used to detect surface-breaking defects, while Ultrasonic Testing (UT) is employed to find sub-surface flaws within the weld volume. At Daxun Alloys Co., Ltd., our welding procedures strictly define gas purity, edge preparation, and inspection sensitivity to ensure the weld seam is as reliable as the parent material.
