Gr12 Titanium Forgings per ASTM B381: Beta Segregation and Cooling Control
Apr 03, 2024
Grade 12 as a Forging Alloy
Grade 12 (UNS R53400, Ti-0.3Mo-0.8Ni) is a weldable near-alpha alloy that fills a specific gap in titanium forging practice. It offers useful strength to about 300 C, strong resistance to chlorides and mildly reducing media, and a cost far below palladium modified grades. Forgings in this grade are produced to ASTM B381, where the alloy appears as Grade F-12, with chemistry and properties aligned to the wrought limits of ASTM B348 and ASTM B338.
Unalloyed titanium melts at about 1668 C, and the alloying additions in Grade 12 depress the melting range only slightly, so a figure of about 1600 C is an approximation rather than a melt point. The beta transus, at which the hexagonal alpha phase transforms to body centred cubic beta, lies near 875 C, a little below the 882 C of unalloyed titanium because molybdenum and nickel are both beta stabilisers. Every forging decision, from billet selection to finishing temperature and cooling rate, is judged against that transus.
Defects That Originate in Melting: Beta Segregation and Beta Fleck
Most forging rejects in titanium are inherited from the ingot rather than created at the press, and two related defects dominate.
Beta segregation is a region enriched or depleted in beta stabilising elements relative to the nominal composition. Molybdenum and nickel segregate during solidification, and multiple vacuum arc remelting passes are used to break up and homogenise those channels.
Beta fleck is a localised beta rich area that survives forging as a small elongated region with a different structure and hardness. It is difficult to detect by chemistry alone and is usually found by ultrasonic inspection or by destructive sectioning after etching.
Both reduce fatigue life and ductility, and both are controlled at the melt shop by raw material quality, electrode build, melt rate and the number of melting passes. A forge shop can detect and segregate them; it cannot remove them. Forging stock should therefore be purchased with ultrasonic inspection to an agreed acceptance level, and heat and ingot traceability to each forging is a real quality control tool rather than paperwork.
Forging Temperature Windows and the Transus
The choice between alpha-beta forging and beta forging sets the character of the finished part.
| Practice | Working temperature | Resulting structure | Effect on properties |
|---|---|---|---|
| Alpha-beta forging | Below about 875 C, typically 800-870 C | Equiaxed or bimodal alpha in a transformed beta matrix | Better ductility, fatigue and notch toughness |
| Beta forging | Above about 875 C, typically 900-1050 C | Coarse prior beta grains with acicular alpha on cooling | Higher creep resistance, lower ductility and fatigue resistance |
| Excessive beta working | Well above the transus with a long soak | Very coarse grains, possible uncontrolled beta grain growth | Brittle response and poor ultrasonic transmission |
Two further temperature rules matter. Overheating at any stage above the transus creates permanent grain growth that no later alpha-beta working or heat treatment can reverse, so furnace control and thermocouple calibration are critical. Below about 700 C the alloy becomes much less forgeable, and surface cracking or internal shear can follow from a single cold press stroke.
Deformation, Cooling Rate and Final Structure
Deformation converts cast structure into wrought structure and closes porosity, but only when the strain is applied hot enough and distributed deeply enough. Surface strain with a cold centre leaves internal defects untouched, so practical controls include sufficient reduction per pass, suitable anvil geometry and, in open die work, repeated rotation of the workpiece to distribute strain through the section.
Cooling rate after the last forging step controls the alpha morphology and therefore the balance of strength and ductility.
Air cooling produces finer acicular alpha with higher strength and some loss of ductility.
Furnace cooling or another controlled slow cool produces coarser alpha with lower strength and improved ductility.
Section thickness matters, because a heavy forging cools slowly at its core and quickly at its surface. One part can therefore contain more than one microstructure and should be characterised at several locations.
Heating Atmosphere, Hydrogen and Post-Forge Processing
Titanium reacts with air above roughly 400 C, forming an oxygen enriched alpha case and, in the presence of moisture or hydrocarbons, absorbing hydrogen. At forging temperatures the reaction is fast.
Heat in a furnace with a controlled atmosphere, or at minimum with a slightly reducing, moisture free and sulphur free flame, and verify the atmosphere with periodic coupon checks.
Remove the alpha case by machining or by pickling in a nitric-hydrofluoric bath to the practice of ASTM B600 before final dimensional finishing.
Keep hydrogen below the 0.015% maximum allowed for Grade 12, because excess hydrogen forms brittle hydrides and lowers impact toughness.
Ultrasonic inspection to an agreed procedure such as ASTM E2375 is normally specified for critical forgings, together with dimensional checks and metallographic verification of the required structure.
The finished forging should arrive with a documented route stating forging temperatures, cooling practice, heat treatment and inspection results, because identical geometry can be produced with very different properties depending on those choices.
FAQ
Q: What is beta fleck and why does it matter?
It is a localised beta rich region left from segregation in the ingot. It differs in structure and hardness from the surrounding metal, acts as a fatigue initiation site, and can only be controlled at the melting stage.
Q: Can a coarse forged structure be corrected by heat treatment?
Not where grain growth is involved. Coarse prior beta grains produced by overheating cannot be refined by later heat treatment; only further hot working below the transus refines them.
Q: Which standard covers Grade 12 forgings?
ASTM B381 and ASME SB381, in which the alloy appears as Grade F-12, supported by material specifications such as ASTM B348 and the descaling practice of ASTM B600.
Q: Should alpha-beta or beta forging be used?
For general structural, valve and pump components where ductility and fatigue resistance matter, alpha-beta forging below the transus is preferred. Beta forging is used where higher temperature strength is needed and is accepted with reduced ductility.
Q: How does hydrogen enter a forging?
From moist furnace atmospheres, contaminated fuel, lubricants and pickling baths. It must be controlled because hydrides embrittle the alloy, so hydrogen is checked on the finished part rather than only on the incoming billet.
