ASTM B338 Grade 2 Titanium Tubing

Characteristics & Applications

The ASTM B338 Grade 2 titanium tubing refers to the seamless or welded tubing made from commercially pure (CP) titanium Grade 2, which is an unalloyed titanium with UNS designation R50400. It has minimum titanium content ranging from about 98.925% to 99.5% in weight percent. ASTM B338 Gr.2 titanium tubes are primarily used for the construction of surface condensers, evaporators, and heat exchangers. These applications generally require excellent corrosion resistance, as well as high ductility for fabrication but relatively low strength in service. These titanium tubes may be further processed to helically-coiled tubing, U-type tubing, serpentine tubing, concentric-coiled tubing, as well as other specially shaped tubing.

The automatic TIG welding production line for the manufacture of ASTM B338 Gr.2 titanium tubing; courtesy of Baoti.

Commercially pure titanium has very good thermal conductivity, which is roughly 50% higher than for stainless steel. Moreover, experience has shown that even water speeds of 10 m*s^–1 do not cause any erosion corrosion, cavitation, or impingement attack in the titanium tubes. Titanium tubes made of ASTM B338 Grade 2 are particularly used for heat exchangers in which the cooling medium is seawater, brackish water, and polluted water. Thin-walled titanium tubing for condensers can often be used with zero corrosion allowance. Worldwide, welded or seamless titanium tubes made of ASTM B338 Grade 2 are widely used in steam turbine power plants, refineries, chemical plants, air conditioning systems, multi-stage flash distillation, desalination and vapor compression plants, offshore platforms, surface ships and submarines, as well as for swimming pool heat pumps.

Chemical Composition Requirements

The main elements in unalloyed titanium grade 2 are iron and interstitial elements such as carbon, oxygen, nitrogen, and hydrogen. The oxygen content determines the strength. In fact, it is the oxygen content that differentiates grade 2 from grade 1 and grade 3. Despite of the strengthening effect, carbon & nitrogen are viewed as impurities as hydrogen. Hydrogen may cause embrittlement of the unalloyed titanium.

Mechanical Properties

Crystal Structure

Similar to commercially pure titanium Grade 1 and Grade 3, Grade 2 titanium of ASTM B338 at room temperature has an alpha (hexagonal close-packed) crystal structure. It transforms to a beta (body-centered cubic) structure at a temperature of about 885°C [1625°F]. This transformation temperature can be raised or lowered depending on the type and amount of impurities or alloying additions. The addition of alloying elements also divides the single temperature for equilibrium transformation into two temperatures–the alpha transus, below which the alloy is all-alpha, and the beta transus, above which the alloy is all-beta. Between these temperatures, both alpha and beta are present. The typical α transus temperature and β transus temperature for titanium Grade 2 are 890°C [1635°F] and 913°C [1675°F], respectively.

Material, Manufacture & Heat Treatment

ASTM B338 Grade 2 titanium welded tubes shall be made from annealed, flat-rolled sheets or strips by an automatic arc welding (TIG) process. The main production procedures include: titanium sheet de-coiling ➡ shear butt welding ➡ cleaning ➡ tubular forming ➡ TIG welding ➡ eddy current testing ➡ pre-sizing ➡ annealing ➡ precise reducing sizing & straightening ➡ eddy current test ➡ diameter gauging ➡ marking ➡ cutting ➡ ultrasonic testing ➡ hydraulic testing ➡ final inspection ➡ packaging. Welded tubing shall be heat treated by at least a stress relief after forming and welding. Use of filler material is not permitted.

The hot extrusion mill for the fabrication of titanium seamless tubing.

ASTM B338 Grade 2 titanium seamless tubes shall be made from hollow billet by the hot extrusion process or cross rolling & piercing process, with subsequent multi-pass cold rolling process. The seamless tube shall be produced with a continuous periphery in all stages of manufacturing operations. The manufacture of titanium seamless tubing from scratch mainly includes: titanium sponge vacuum arc melting for ingot ➡  blooming to get the hollow billet ➡ cleaning ➡ cutting & feeding ➡ hot extrusion, or cross rolling + piercing ➡ degreasing ➡ drying ➡ cutting ➡ on-line annealing & straightening ➡ pickling ➡ multi-pass cold rolling ➡ degreasing ➡ drying ➡ vacuum annealing ➡ straightening ➡ cutting ➡ pickling ➡ final inspection ➡ marking & packaging.

The 12m vacuum annealing furnace for titanium seamless tubing; courtesy of Hongbao Group.

Shijiazhuang Metalsin supplies titanium tubes through several prominent manufacturers in China including Baosteel, Baoti, Western Titanium, Pangang Group, and Hongbao Group. Specifications: outside diameter from 2 mm to 200 mm and wall thickness from 0.2 mm to 30 mm.

Inspection & Testing

Tensile test, flattening test, flaring test, eddy current test, ultrasonic test, hydrostatic test, dimensional inspection and visual examination.