Titanium Grade 2

What is Titanium Grade 2?

Titanium Grade 2 is an unalloyed commercially pure titanium designated UNS R50400. It is supplied in the annealed condition to ASTM B348 for bars and billets and ASTM B265 for plate, sheet and strip. It combines moderate strength with formability, weldability and resistance to most chloride environments that limit stainless steel.

The specification sets a minimum tensile strength of 345 N/mm squared. The specified 0.2 percent yield strength is 275 to 450 N/mm squared, with a minimum elongation of 20 percent. The upper yield limit controls interstitial content because excess interstitial content reduces the expected formability and weldability of the commercially pure grade.

Titanium Grade 2 has a density of about 4.5 grams per cubic centimetre, which is a little over half the density of steel. Its corrosion resistance depends on a passive oxide film. The film reforms immediately after damage when oxygen is present.

Laxcon Steels lists Titanium Grade 2 in its grades reference as a titanium grade. The same steel is written Ti Grade 2, Ti Gr 2, CP titanium and R50400.

UNS R50400. ASTM B348 / B265. Titanium is the balance, determined by difference. Other elements 0.1 maximum each, 0.4 maximum total.

What is the chemical composition of Titanium Grade 2?

Composition limits in weight percent are carbon 0.08 maximum, oxygen 0.25 maximum, nitrogen 0.03 maximum, hydrogen 0.015 maximum and iron 0.3 maximum.

Mechanical properties of Titanium Grade 2

Titanium Grade 2 is purchased in the annealed condition, which defines its specified properties. Commercially pure titanium has no hardening heat treatment. Cold work can increase strength above the annealed level, but titanium has high springback and tends to gall against forming tools.

The elastic modulus of titanium is roughly half that of steel. A Grade 2 component with the same section therefore deflects about twice as much under the same load. Substitution for steel on strength alone can satisfy a stress calculation while failing the stiffness requirement.

Interstitial content and commercially pure grades

Grades 1, 2, 3 and 4 are commercially pure titanium grades distinguished by interstitial content, principally oxygen, and by iron content. Their compositional limits establish a progressive strength range.

Interstitial and iron limits in weight percent for the commercially pure grades, from the Laxcon Steels grade reference. All values are maximums; titanium is the balance.

Oxygen dissolved in the hexagonal alpha lattice provides strong strengthening. An increase of a tenth of a percent in oxygen changes strength more than an increase of a whole percent in a substitutional alloying element. Higher oxygen also reduces ductility and fracture toughness. Grade 1 is consequently used for deep drawing, Grade 2 for general purposes, and Grade 4 as the strongest and least formable of the four. Grade 23 is the extra-low interstitial version of alloyed Grade 5. Its reduced oxygen content provides greater toughness for implant and cryogenic use.

Corrosion behaviour of Titanium Grade 2

The corrosion resistance of titanium results from a thin, adherent and self-repairing oxide film. The film reforms almost immediately after scratching or erosion when oxygen or water is available. Titanium therefore resists flowing seawater, brines, hypochlorite, wet chlorine, oxidising acids such as nitric acid, and most chloride process streams. Corrosion rates in these environments permit design for service life rather than corrosion allowance. Uses include seawater heat exchangers, desalination plant, condenser tubing and chemical process equipment.

Corrosion limitations occur when the oxide film cannot form or remain stable. Hydrofluoric acid and acidic fluoride solutions dissolve the film and rapidly attack titanium at low concentrations. No reliable inhibitor applies. Dry chlorine gas attacks titanium because water is absent and cannot maintain the oxide film. Chlorine handling systems therefore specify a minimum moisture content. Strongly reducing acids, including hot concentrated hydrochloric and sulphuric acids, also attack titanium unless oxidising species are present. Crevice corrosion can occur in hot chloride solutions above roughly 70 to 80 degrees Celsius, depending on chloride concentration and pH. Alloyed corrosion grades are used under these conditions.

Grade 7 consists of Grade 2 with 0.12 to 0.25 percent palladium. Grade 16 uses 0.04 to 0.08 percent palladium for the same purpose. Palladium raises the surface potential and maintains passivity under reducing conditions. Grade 12 instead contains 0.6 to 0.9 percent nickel and 0.2 to 0.4 percent molybdenum for resistance to crevice corrosion in hot brines.

Welding and machining Titanium Grade 2

Titanium Grade 2 is readily weldable but requires control of atmospheric exposure. Above about 500 degrees Celsius, titanium absorbs oxygen, nitrogen and hydrogen from air. The absorbed gases permanently embrittle the weld, and no post-weld treatment removes them. Welding requires inert gas shielding of the arc, root, weld and heat-affected zone until the metal cools below the reaction temperature. The process uses a trailing shield, purged backing, and clean, degreased and oxide-free preparation. Gas contamination produces straw, blue, grey or powdery white discolouration. Contaminated metal must be cut out rather than reworked.

Machining practice reflects the low thermal conductivity and high-temperature chemical activity of titanium. Heat remains in the cutting zone rather than leaving in the chip. Suitable practice uses low surface speeds, generous feeds, sharp positive-rake tools, rigid setups and flood coolant. Fine chips and dust can burn, so swarf handling is a fire safety requirement.

Applications of Titanium Grade 2

Applications include seawater and brine heat exchangers, condenser and desalination tubing, chemical process vessels, piping, pumps and valve components. Other uses include electroplating and anodising racks and baskets, chlor-alkali plant, pulp and paper bleaching equipment, offshore and marine hardware, and cryogenic components. Medical and dental parts use the grade where biocompatibility and lack of magnetic response are relevant.

Where Titanium Grade 2 is not the right choice

Titanium Grade 2 is unsuitable where load rather than environment determines material selection. Its specified minimum tensile strength is 345 N/mm squared, and its modulus is about half that of steel. A Grade 2 part designed for stiffness may therefore require a section much heavier than its density advantage suggests. Grade 5 provides roughly two and a half times the specified tensile minimum when strength relative to weight is the main requirement.

Titanium Grade 2 is unsuitable for untreated sliding or threaded contact. Titanium galls severely against itself and most other metals. Its oxide film is thin, and the underlying metal is adhesive. A threaded joint can seize during its first assembly. Anti-seize compound, dissimilar-metal mating faces, anodising or a hard coating are required.

Titanium Grade 2 is unsuitable for hydrofluoric acid or fluoride-bearing solutions at any concentration, dry chlorine, hot concentrated reducing acids, and anhydrous or oxygen-free conditions in which a scratch cannot repassivate. Grade 7, Grade 16 or Grade 12 provide alternatives for hot chloride crevices above roughly 70 to 80 degrees Celsius.

Titanium Grade 2 is not a high-temperature material. Commercially pure titanium loses strength rapidly as temperature rises. It also begins to absorb oxygen from air well below temperatures at which a nickel alloy remains serviceable. Hot service therefore requires a nickel-chromium alloy rather than titanium.

Other titanium grades