Construction-grade titanium selection resolves around four decision gates: required tensile/yield strength, service environment (seawater, chlorides, sour service), fabrication route (forming, welding, machining), and life-cycle cost against steel or nickel-alloy alternatives [S3].
The reference dataset comes from commercial mill tables: JIS H 4600 Class 1 to Class 4 and ASTM Grade 1 to Grade 4 cover unalloyed grades with tensile strength from 270 to 750 N/mm²; JIS Class 60 / ASTM Grade 5 sets the Ti-6Al-4V bar at ≥895 MPa UTS, 0.2% yield ≥825 MPa, ≥10% elongation [S2].
Commercially pure grades: where CP-Ti is the right call
Commercially pure (CP) titanium is cast, formed, joined, and machined more easily than alloyed grades, and should be selected wherever its mechanical envelope covers the design load [S3]. Across JIS Class 1 to 4 / ASTM Grade 1 to 4 / DIN 3.7025 to 3.7065, UTS climbs stepwise: 270-410, 340-510, 480-620, 550-750 N/mm², with 0.2% yield rising in parallel from ≥165 to ≥485 N/mm² [S2].
For construction, CP Grade 2 (UTS 340-510 N/mm², yield ≥215 N/mm², elongation ≥23%, hardness ≥110 HB) is the workhorse for architectural cladding, chemical-plant ductwork, and saltwater piping where corrosion resistance matters more than absolute strength [S2]. Where the wall sees higher stress but still must remain weldable, CP Grade 4 (UTS 550-750 N/mm², yield ≥485 N/mm², elongation ≥15%) closes most of the gap to Grade 5 without giving up the formability advantage [S2].
Ti-6Al-4V and the α+β alloy class
This grade is the default for load-bearing structural members in marine and aerospace construction where specific strength (strength-to-density) drives the design trade.
For service in deep-sea pressure hulls, the titanium-alloy class shifts to near-α and extra-low interstitial α alloys, replacing traditional hull steel on the metric of specific strength plus seawater corrosion resistance; the engineering literature documents Soviet Alfa/Mike/Seira class all-titanium hull submarines as the reference installations [S1]. Within this class, Ti-6Al-4V ELI (ASTM F136, AMS 4930) trades a small slice of ultimate strength (UTS ≥860 MPa) for better fracture toughness, which is the property of interest for welded pressure vessels subject to low-oxygen, high-hydrostatic-pressure service [S2].
Near-α and intermediate alloys for moderate-temperature service
Where moderate temperature capability matters, Ti-3Al-2.5V (JIS Class 61, ASTM Grade 9, UTS ≥620 MPa, yield ≥485 MPa, ≥15% elongation) is the structural tubing alloy, used in hydraulic lines and bracketing where weldability plus density savings offset cost [S2]. Grade 9 sits between CP titanium and Grade 5 on strength and on fabricability, making it the natural pick for constructional assemblies that need to be formed and welded on site.
For higher-temperature envelopes, β-rich and near-β alloys (Daido DAT55G, DAT56M, solution-treated, UTS 770-1100 N/mm²) deliver elevated-temperature strength; the trade is reduced weldability and higher buy cost, so they are usually reserved for hot-section brackets or fasteners rather than primary hull structure [S2].
Selection criteria comparison: CP-Ti vs Grade 5 vs Grade 9 vs near-α hull alloy
For a 1 mm sheet, 1 m² reference panel, density sits at 4.51 g/cm³ for titanium versus 7.85 g/cm³ for carbon steel and 4.43 g/cm³ for the aluminum-alloy 5xxx/6xxx series used in light construction; that mass differential is the upstream justification for a titanium buy [S2].
On the corrosion axis, the deep-sea reference case shows that the reduced oxygen content of deep seawater accelerates passivation breakdown and cracking tendency, and the engineering recommendation is to specify near-α and ELI α alloys rather than CP titanium for cyclic pressure-hull service [S1].
On the fabrication axis, CP grades bend on a 2T inner radius (Classes 1, 2) or 3T (Classes 3, 4) and weld with standard inert-gas procedures, whereas Grade 5 and the near-α hull alloys require tighter interpass temperature control and post-weld stress relief; this shifts the field labour cost and is the main reason CP Grade 2 dominates architectural and chemical-construction bidding despite its lower strength [S2].
Standards and data to pin to a datasheet
The four standards that anchor a mill cert in this material class are JIS H 4600 (Class numbering), ASTM B265 / B348 (Grade numbering for sheet, bar, and billet), AMS 4928 / 4930 / 4914 (aerospace material specs for Ti-6Al-4V bar, ELI bar, and Ti-6Al-4V plate), and DIN 3.7025-3.7164 (European designation by Werkstoff number) [S2].
For load-bearing construction in chloride-rich environments, NACE MR0175 is the governing sour-service gate for any titanium component exposed to H₂S, while ASME BPVC Section VIII governs pressure-vessel design using titanium. The reference mill data above is sufficient to draw the UTS/yield comparison but does not in itself certify fitness for service; the construction engineer should pull the current revision of the cited standard at the time of procurement, as Daido's datasheet explicitly directs [S2].
Operating-temperature envelope and failure modes
Titanium alloy strength degrades with temperature along a polynomial that is now well-characterised for the 20-600 °C range and parameterised against aluminium and molybdenum equivalents, so a Grade 5 component rated for 900 MPa at 20 °C will not retain the same margin at 400 °C; the curve is the first check before any elevated-temperature specification [S7].
The two failure modes to design against are adiabatic-shear-induced serrated-chip formation during high-speed machining (more sensitive in Ti-6Al-4V than in CP titanium TA2 at matched cutting conditions) and hydrogen-assisted cracking in sour or cathodically charged service; the first drives the choice of cutting parameters and tool coating, the second drives material selection and cathodic-protection design [S6].
Cost gating and where titanium stops being justified
The selection rule documented in the foundational materials-engineering text is that titanium's premium price is offset either by mass savings (its 4.51 g/cm³ density versus 7.85 g/cm³ for alloy-steel) or by extended service life in corrosive media, and where neither lever applies the choice should fall back on a lower-cost alloy system [S3]. For construction projects this means CP Grade 2 for corrosive-service ductwork and cladding, Grade 9 for formed structural tubing, Grade 5 / Grade 23 ELI for high-stress marine members, and near-α hull alloy for deep-sea pressure structures; each step up the strength ladder must be justified by either a load or a corrosion requirement, not by habit [S2][S3].
Reference decisions worth tracking: mill test certs citing JIS H 4600, ASTM B265/B348, and AMS 4928 on every heat; a documented welding-procedure-specification (WPS) for the chosen grade; and for any subsea or sour-service component, an NACE MR0175 compliance line. Material selection for high-pressure hydraulic lines is the natural adjacent reading for engineers sizing Grade 9 tubing, and selection rules for sour-service alloy steel covers the corrosion-gate logic that titanium is often used to bypass in the first place.