Scope of ASME SB348 and the Commercially Pure Titanium Family
ASME SB348 is the ASME Boiler and Pressure Vessel Code adoption of the specification for titanium and titanium alloy bars and billets, and its technical content is identical to ASTM B348. Material certified to this specification is accepted for pressure-retaining construction, including vessels, heat exchangers, columns, nozzles and piping systems.
The designations compared in this guide are all unalloyed commercially pure (CP) titanium. They share one corrosion mechanism and differ mainly in strength, which is governed by interstitial oxygen and iron content.
| Designation | UNS number | Strength level | Distinguishing characteristic |
|---|---|---|---|
| GR1 (Grade 1) | R50250 | Lowest | Highest ductility, formability and impact toughness |
| GR2 (Grade 2) | R50400 | Intermediate | Best all-round balance, the industry workhorse |
| CP2 | R50400 | Intermediate | Legacy designation equivalent to modern GR2 |
| GR4 (Grade 4) | R50700 | Highest | Strongest unalloyed grade, lower ductility |
Interstitial chemistry is what separates the grades. The table below lists the maximum levels normally specified for bar product.
| Grade | Oxygen, max % | Iron, max % | Carbon, max % | Nitrogen, max % |
|---|---|---|---|---|
| GR1 | 0.18 | 0.20 | 0.10 | 0.03 |
| GR2 | 0.25 | 0.30 | 0.10 | 0.03 |
| GR4 | 0.40 | 0.50 | 0.08 | 0.05 |
Round bar to SB348 is a semi-finished product used to machine flanges, valve stems, pump shafts, fasteners, tube sheets and similar pressure-system components.
Do GR1, GR2 and GR4 Really Differ in Corrosion Resistance?
All unalloyed grades rely on the same protective mechanism: a thin, adherent, self-healing titanium dioxide film that forms spontaneously in air or water. Because that film is chemically the same on every grade, general corrosion rates in most industrial media are effectively identical.
Practical differences appear only where mechanical condition interacts with the environment:
Erosion-corrosion: in high-velocity streams such as pump impellers and inlet nozzles, the greater hardness of GR4 gives it a marginal advantage over the softer GR1 and GR2.
Crevice corrosion: in tight gaps under gaskets and deposits in hot chloride solutions, all unalloyed grades can be attacked. The higher ductility of GR1 can slightly reduce crevice tightness, but for severe crevice duty a palladium-modified grade such as GR7 (UNS R52400) is the more reliable choice.
Fabrication-induced vulnerability: welding and forming leave residual stress, and GR4 develops higher residual stress for the same strain. Titanium remains highly resistant to stress corrosion cracking, yet heavily stressed components in aggressive service deserve a stress-relief review.
The design conclusion is straightforward: for common chemical services such as seawater, chlorates and nitrates, GR1, GR2 and GR4 perform alike, so the grade is normally selected on mechanical and fabrication grounds rather than on chemical resistance.
Grade Selection for Pressure Vessels and Heat Exchangers
Select GR1 when maximum formability is needed, for deep-drawn heads or tight U-bends in exchanger tubes, when the component is lightly stressed, or for linings and baffles in low-pressure service.
Select GR2 as the default for process piping, exchanger shells and tube sheets, vessel shells and nozzles in chloride, seawater and oxidizing-acid service. It combines adequate strength for most pressure-retaining designs with good weldability and formability.
Select GR4 when the corrosion profile of pure titanium must be retained but GR2 would demand an excessively thick, heavy wall. It is a cost-effective bridge between GR2 and alloyed titanium such as Ti-6Al-4V (Grade 5).
Allowable stress values for each grade across the design temperature range are listed in ASME BPVC Section II, Part D, while the design rules sit in Section VIII. Those documents, not a supplier catalogue, govern the final wall thickness calculation.
Fabrication Practice for Code Work
Gas tungsten arc welding is the standard process, and all three grades weld readily without post-weld cracking. The decisive requirement is shielding:
primary shielding with high-purity argon or helium from the torch;
a trailing shield that floods the cooling bead with inert gas;
back purging of the root side with argon to prevent underside oxidation.
Filler metal usually matches the base grade, for example ERTi-2 for GR2. Using one strength grade lower than the base metal, ERTi-2 for GR4 for instance, is common practice to maximise weld ductility, and the choice must be written into the qualified welding procedure specification.
Cold forming is straightforward for all grades. GR1 tolerates the most severe deformation, while GR4 needs higher forces and shows more springback. Hot forming is carried out in the 425 to 650 C (800 to 1200 F) range in a slightly oxidizing or inert atmosphere to prevent hydrogen pickup, which embrittles titanium. Welding procedures must be qualified to ASME BPVC Section IX.
Documentation, Marking and Traceability
A certificate of compliance states conformity with ASME SB348 and the ordered grade, but it is the mill test report that matters for code work. The report must carry:
the heat number, giving traceability to the original melt;
actual chemical analysis for the elements controlled by the specification;
actual tensile, yield and elongation results from samples of the same heat and condition;
any supplementary test results called for on the purchase order.
Identification is transferred to the bar itself by low-stress stamping or durable tags showing the manufacturer, the specification, the grade, the heat number and the size. The vessel fabricator reviews the reports before release to production, and the authorised inspector audits the records as part of the stamp holder quality system.
Frequently Asked Questions
Q: Are GR2 and CP2 the same material?
Yes. CP2 is an older designation corresponding to modern Grade 2 (UNS R50400), and current mill certificates normally print GR2.
Q: Which CP titanium grade has the best corrosion resistance?
Uniform corrosion resistance is essentially the same for GR1, GR2 and GR4 because all rely on the same titanium dioxide film. Differences appear in erosion, crevice and fabrication-stress situations rather than in uniform attack.
Q: Can GR4 replace GR2 to reduce wall thickness?
Yes. Where a GR2 design produces an overweight wall, GR4 permits a thinner section at the same corrosion allowance, provided the allowable stress values in ASME BPVC Section II, Part D support the change at the design temperature.
Q: Is preheat or post-weld heat treatment required for CP titanium?
Preheat is not used. Post-weld heat treatment is rarely required for unalloyed grades, though where residual stress is a concern in aggressive service a stress-relief cycle in an inert or slightly oxidizing atmosphere may be specified.
Q: What filler metal suits GR4 bar welds?
ERTi-2 filler is frequently selected for GR4 to maximise joint ductility. The choice must be qualified in the welding procedure specification to ASME BPVC Section IX.
Q: When is GR1 chosen instead of GR2?
For severe cold forming such as deep-drawn heads and tight tube bends, and for linings or low-pressure baffles where maximum ductility matters more than strength.
