Scope: Pipe, Tube and Fittings Under ASTM B363
Grade 3 titanium is the intermediate unalloyed grade of the commercially pure family and is widely selected for boiler tubing, condenser tubing and heat exchanger service where a balanced combination of strength, ductility and corrosion resistance is required. Buyers should note that ASTM B363 covers titanium welding fittings, while the pipe and tube itself is normally supplied to ASTM B338 for seamless and welded tube, ASTM B337 for welded pipe, and ASTM B861 or ASTM B862 for seamless and welded pipe. Specifying the correct product standard together with the grade avoids mismatched test requirements and documentation.
This matters in boiler and heat exchanger procurement, where a fittings specification is often copied onto a tube purchase order and then produces a certificate that does not carry the mechanical tests the designer expected.
Correct Grade Identification: UNS R50550 and Werkstoff 3.7055
Grade 3 unalloyed titanium is registered as UNS R50550 and as Werkstoff Nr. 3.7055 in the DIN numbering system. The number 3.7165, which appears frequently in Grade 3 datasheets, actually belongs to the Ti-6Al-4V alloy of Grade 5, and using it on a Grade 3 enquiry causes confusion during material verification and incoming inspection. Grade 3 sits between Grade 2 and Grade 4: it is stronger than Grade 2 because of its higher oxygen content, and more ductile than Grade 4, which makes it a frequent compromise grade for tube bundles.
| Element | Grade 3 limit, mass % | Comparison |
|---|---|---|
| Oxygen, O | 0.35 max | 0.25 max in Grade 2, 0.40 max in Grade 4 |
| Nitrogen, N | 0.05 max | same for Grade 2 |
| Carbon, C | 0.08 max | same for Grade 2 |
| Hydrogen, H | 0.015 max | same for Grade 2 |
| Iron, Fe | 0.30 max | same for Grade 2 |
| Titanium, Ti | Balance | commercially pure |
Product analysis tolerances and sampling frequency are defined by the applicable product standard, so these limits should always be read together with the specification that is ordered. Because Grade 3 is controlled by interstitial content rather than by alloying additions, an oxygen result near the upper limit raises strength and lowers ductility in a predictable way.
Mechanical Properties Used in Design
| Property | Typical annealed value |
|---|---|
| Tensile strength | 450 - 620 MPa |
| 0.2 % proof stress | 380 - 520 MPa |
| Elongation | 18 - 25 % |
| Elastic modulus | approx. 103 GPa |
| Poisson ratio | 0.34 |
| Density | 4.51 g/cm3 |
| Maximum long-term service temperature | 300 - 350 °C |
Grade 3 does not respond to precipitation hardening and cannot be strengthened by heat treatment. Strength is set by oxygen content and by the amount of cold work retained in the finished tube, so fully annealed product is the normal supply condition for boiler and heat exchanger duty. Cold work raises strength but reduces the elongation reserve that allows tubes to be rolled into a tubesheet without cracking.
Boiler and Heat Exchanger Service Conditions
Titanium resists chloride pitting, crevice corrosion and chloride stress corrosion cracking in cooling waters, which is why Grade 3 tubes are common in condensers and in seawater-cooled exchangers. The protective oxide film reforms immediately when damaged, provided that oxygen or water is present and that the environment is not strongly reducing.
Several service limits must be respected when the grade is selected:
Hydrogen pickup becomes a concern above about 75 °C in hydrogen-bearing or cathodically protected systems, because hydride formation embrittles the metal.
Hydrofluoric acid, fluoride-bearing media and hot concentrated reducing acids attack titanium rapidly and are not compatible.
Dry chlorine gas, methanol and anhydrous ammonia require specific assessment before titanium is used.
Oxidation limits long-term service to roughly 600 °C, well above the mechanical design limit of the tube.
Tube-to-tubesheet joining is done either by expansion rolling or by inert gas tungsten arc welding. Welding requires high purity argon shielding, normally 99.99 % or better, with trailing and backing protection maintained until the weld has cooled below roughly 400 °C, and the finished weld is expected to develop at least 90 % of the base metal strength while remaining ductile.
Available Sizes, Testing and Documentation
| Item | Range |
|---|---|
| Welding fittings, nominal bore | 1/2 in to 48 in |
| Fitting pressure classes | 150 to 2500 |
| Fitting pressure ratings to DIN | PN6 to PN160 |
| Tube outside diameter | 3 mm - 114 mm |
| Tube wall thickness | 0.5 mm - 10 mm |
| Pipe outside diameter | up to 610 mm |
| Pipe wall thickness | 2 mm - 120 mm |
| Length | up to 15 000 mm |
Non-destructive and destructive testing normally includes hydrostatic testing, eddy current testing, ultrasonic inspection, pneumatic underwater testing, flattening and flaring tests, plus tensile and flattening tests on the finished product. Documentation is usually issued as an EN 10204 3.1 inspection certificate that reports heat number, chemical composition and mechanical results for each lot.
Frequently Asked Questions
Q: Which standard applies to Grade 3 titanium boiler tube?
Seamless and welded tube is normally supplied to ASTM B338, welded pipe to ASTM B337, and seamless pipe to ASTM B861. ASTM B363 applies to welding fittings in the same grade and is frequently and incorrectly quoted on tube orders.
Q: What is the Werkstoff number for Grade 3 titanium?
Grade 3 unalloyed titanium is Werkstoff Nr. 3.7055 and UNS R50550. The number 3.7165 that sometimes appears on Grade 3 certificates belongs to Ti-6Al-4V, which is Grade 5.
Q: Is Grade 3 stronger than Grade 2?
Yes. Grade 3 permits up to 0.35 % oxygen against 0.25 % for Grade 2, so annealed Grade 3 tube is stronger and slightly less ductile than Grade 2 tube of the same size.
Q: Can Grade 3 tube be used in seawater condensers?
Yes. It is resistant to chloride pitting and stress corrosion cracking in seawater and is widely used in condensers and desalination plant, provided that service temperature and hydrogen pickup risk are assessed.
Q: How should Grade 3 titanium be welded?
Inert gas tungsten arc welding under high purity argon with full trailing and backing protection. Oxide colour on the finished weld indicates contamination and is normally a cause for rejection and rework.
Q: Does Grade 3 titanium require post-weld heat treatment?
For most heat exchanger and piping fabrication, no heat treatment is needed after welding. Stress relief may be specified for heavy sections or for service in corrosive media where residual stress would increase cracking risk.
