Welding Process Requirements For PTA Unit Titanium Heat Exchangers

The production conditions of purified terephthalic acid (PTA) are severe. The condensers in PTA units are exposed to highly corrosive media such as acetic acid and bromide ions for a long time. Titanium and titanium alloys  have become the preferred materials due to their excellent corrosion resistance and mechanical properties.

1. Titanium easily adsorbs oxygen, nitrogen and hydrogen above 400℃, forming brittle substances that reduce the plasticity and toughness of welds and even cause brittle fracture.

2. Pure titanium has low thermal conductivity, and welding tends to produce coarse grains or martensitic structures, reducing joint plasticity.

3. Frequent porosity defects: Contaminants, moisture, impure shielding gas and other factors easily cause porosity, reducing weld strength and tightness and inducing corrosion failure.

4. Susceptibility to delayed cracking: Hydrogen enrichment in the heat-affected zone combined with residual stress easily causes hydrogen-induced cold cracking.

1.Working environment:

The site shall be free of iron contamination, with qualified temperature and humidity; operators shall wear standard protective equipment and use special tools for titanium and stainless steel.

2.Material control:

Verify that base metals are free of iron contamination and welding wires match base metals; use high-purity argon and stop operation if gas pressure is insufficient.

3.Pre-welding cleaning:

Grind the groove area to metallic luster, remove contaminants with solvent, and weld within 4 hours after treatment.

4.Groove assembly:

Machine standard grooves, strictly control alignment misalignment and gap; tack welding shall comply with formal welding procedures.

1.Welding method:

Manual tungsten inert gas (TIG) welding shall be adopted, equipped with a stable DC TIG welder with high-frequency arc starting and current attenuation functions. The post-gas supply time shall be no less than 15 seconds.

2.Process parameters:

Follow the principles of low current, high welding speed and multi-layer/multi-pass welding. For Gr2 titanium, use 2.4 mm welding wire with DC electrode positive (DCEP). Strictly control current, voltage, welding speed and argon flow rate; adopt short-arc operation with arc length ≤ 3 mm, single-layer welding thickness ≤ 3 mm, and staggered layer-pass joints.

3.Argon shielding:

Fully shield areas above 400℃. Equip the welding torch with a copper trailing shield for front-side protection, and back purge the weld with argon to isolate air. Ventilate before arc starting and keep supplying gas until the weldment cools down to below 100℃ after arc extinguishing; control appropriate gas flow rate and use plastic gas hoses to avoid contamination.

4.Interpass temperature control:

Interpass temperature shall be strictly controlled below 60℃. Excessively high temperature tends to cause grain coarsening and performance degradation; copper plates can be used for auxiliary cooling.

1.Pre-welding preparation
The heat exchange tube shall extend 0.5–1.0 mm beyond the tube-sheet surface. Thoroughly remove impurities and burrs from tube ends and tube-sheet holes, and strictly control the fit gap to prevent expansion cracking.

2.Welding process
Adopt Z-step welding to reduce deformation; weld tube openings in two passes (root pass and capping pass) with staggered start and end points. Use 2.5 mm cerium tungsten electrode, welding current 80–120 A, argon flow rate 10–15 L/min. The current shall be slightly lower than that for butt welding to avoid burn-through of tube walls.

3.Post-welding expansion
Expand heat exchange tubes after welding to eliminate fit gaps and resist crevice corrosion. Control expansion force to avoid tube cracking and tube-sheet deformation.

1.Visual inspection
Qualified welds are silvery white or golden yellow. Blue, purple or gray color indicates excessive oxidation and requires grinding and rewelding. Welds shall be free of cracks, porosity and other defects, with reinforcement ≤ 1.5 mm and not lower than the base metal surface.

2. Non-destructive testing

Penetrant testing (PT): Conduct testing on all welds 24 hours after welding to detect surface cracks.

Radiographic testing (RT): Inspect key butt welds, with Grade II as the qualified standard.

Helium leak testing: Test the tightness of tube-sheet joints to prevent leakage.

3.Common defects and prevention

Oxidative discoloration: Ensure high-purity argon, improve shielding devices, and extend post-gas supply time.

Porosity: Thoroughly clean weldments and welding wires, and implement standard short-arc welding.

Cracking: Strictly control hydrogen content and interpass temperature, and arrange welding sequence reasonably.

Lack of penetration: Match current and welding speed, and reserve standard groove gap.

4.Defect repair
Mechanically remove defects before repair welding with the original process. Single defect repair shall not exceed twice, and all indicators shall be re-inspected after repair.

Ruihang Group mainly produces titanium products with the complete industry chain,including smelting,forging, straightening,rolling,surface treating,testing process. For any purchasing needs, feel free to contact us at email:Sam.Rui@bjrh-titanium.com