Scope of ASTM B338 and the Position of Grade 1
ASTM B338 is the specification covering seamless and welded titanium and titanium alloy tube for condensers, evaporators and heat exchangers. Grade 1, UNS R50250, is commercially pure titanium and the softest and most formable of the pure grades. Its combination of corrosion resistance, weldability and cold formability makes it the first choice for heat transfer service in seawater, chemical and food processing plants. The same material is designated TA1 in GB/T 3620.1 and is supplied as tube under GB/T 3624 and GB/T 3625.
Grade 1 is normally specified for lower strength duties where maximum formability and weldability matter, such as expanded tube to tubesheet joints, coiled tube, and tube that must be bent to a tight radius in the bundle. Where higher strength is needed at the same corrosion level, Grade 2 is specified; the additional strength comes from a higher oxygen allowance.
Why Cold Rolling and Cold Drawing Are Used
Titanium heat exchanger tube is produced by cold working rather than by hot finishing alone, for three reasons. Cold rolling and cold drawing give tight dimensional control of outside diameter and wall thickness, which is essential for tube bundles and for reliable expanded or welded tube to tubesheet joints. They produce a smooth, defect free bore and outside surface that resists fouling and localised corrosion. And they allow the mechanical properties to be set by a final anneal after the required amount of cold work.
Seamless tube is produced from extruded or pierced hollows and reduced by pilgering or plug drawing with intermediate annealing and pickling. Welded tube is formed from strip, longitudinally welded under inert gas, then cold worked and annealed. Both routes can meet ASTM B338 Grade 1 requirements, and the customer should state which type is wanted, because pressure ratings, eddy current test procedures and cost differ between them.
Chemical and Mechanical Requirements
| Element, ASTM B338 Grade 1 | Maximum, percent by mass |
|---|---|
| Iron | 0.20 |
| Carbon | 0.08 |
| Nitrogen | 0.03 |
| Hydrogen | 0.015 |
| Oxygen | 0.18 |
| Other elements, each | 0.10 |
| Other elements, total | 0.40 |
| Property, annealed | Minimum requirement |
|---|---|
| Tensile strength | 240 MPa |
| Yield strength, 0.2 percent offset | 170 MPa |
| Elongation in 50 mm | 24 percent |
| Hardness, typical | About 120 HBW |
| Density | 4.51 g/cm3 |
Properties That Matter in Heat Exchanger Service
Corrosion resistance: Gr1 forms a stable, self-healing titanium dioxide film. It resists seawater, chlorides, oxidising acids and most organic media, in conditions where stainless steels suffer pitting and stress corrosion cracking.
Low density: at 4.51 g/cm3 the tube weighs about 57 percent of an equivalent steel tube, which reduces support loads in the exchanger and simplifies handling during retubing.
Thin wall capability: because the material is strong and corrosion is a surface phenomenon rather than a thickness allowance problem, wall thicknesses of 0.5 to 1.2 mm are common in condensers, improving heat transfer and reducing cost.
Clean surfaces: a smooth, pickled or bright annealed surface limits scale and biological fouling so that the heat transfer coefficient is maintained between cleaning cycles.
Thermal performance: thermal conductivity of about 17 W/(m·K) is lower than copper alloys, so wall thickness is kept thin and the dominant resistance remains the film coefficient of the fluid.
Delivery, Testing and Fabrication Notes
Gr1 tube is normally supplied in the annealed condition, either pickled and descaled or bright annealed in vacuum or inert gas, with plain or beveled ends and in straight lengths or coiled form. Standard testing covers chemical analysis, tensile testing, flattening and flange or flare testing, hydrostatic or pneumatic pressure testing and eddy current examination of the tube wall and weld. A mill test certificate to EN 10204 3.1 is issued with the heat number, and positive material identification is available for critical orders.
During installation the tube ends are expanded into the tubesheet, sometimes with a seal weld, so the delivered hardness and ovality directly affect joint quality. Tube that has been excessively cold worked without a full anneal will not expand uniformly, so the delivered condition and the temper should be stated on the purchase order alongside the ASTM B338 grade.
Frequently Asked Questions
Q: What does ASTM B338 Grade 1 cover?
A: Seamless and welded titanium tube for condensers and heat exchangers, made from Grade 1 commercially pure titanium with a minimum tensile strength of 240 MPa and a minimum yield strength of 170 MPa in the annealed condition.
Q: Why is Grade 1 chosen rather than Grade 2 for heat exchangers?
A: Grade 1 is softer and more formable, which helps expansion into the tubesheet and tight radius bending; Grade 2 is chosen where higher strength and pressure rating are required.
Q: What is the difference between seamless and welded Gr1 tube?
A: Seamless tube is made from pierced or extruded hollows and reduced by drawing or pilgering; welded tube is formed from strip and welded along its length, then cold worked and annealed. Both are permitted under ASTM B338.
Q: Which tests are performed on titanium heat exchanger tube?
A: Chemical analysis, tensile testing, flattening or flare testing, hydrostatic or pneumatic pressure testing and eddy current examination, reported on a mill test certificate.
Q: Can Gr1 tube be used with seawater cooling?
A: Yes. Titanium tube is the preferred material for seawater cooled condensers and desalination plant because it resists chloride attack that would pit copper alloys and stainless steels.
