What Alpha Case Is and Why It Matters
Titanium is highly reactive with oxygen at forging temperature. During heating above roughly 600 °C a titanium billet picks up two surface layers: an outer oxide scale, and beneath it an oxygen-enriched, alpha-stabilised zone known as alpha case. That layer is hard, brittle and not representative of the parent metal, so it must be removed before the forging goes into service.
The practical consequence is economic as much as metallurgical. The thicker the alpha case, the more material has to be removed by machining or blasting, and the lower the metal yield from each billet. Controlling surface oxidation is therefore one of the most direct levers available for improving material utilisation and reducing the finished cost of forged parts in alloys such as ASTM B348 Grade 2 and Grade 5 (Ti-6Al-4V).
Controlling Oxygen Uptake During Heating
The depth of contamination is governed mainly by two variables: the peak temperature reached and the time the billet spends at that temperature. For Ti-6Al-4V the allotropic (beta transus) temperature lies close to 995 °C, and the risk of a deep contaminated layer rises sharply once the metal is held above that level for long periods.
Trial work on titanium alloy extrusion billets, heating polished billets to 950-980 °C in an electric furnace before forging, compared four variables and found that each of them measurably affected the physical and mechanical properties of the finished parts:
a pre-oxidation treatment applied before high-temperature heating;
glass enamel protective coatings on the billet surface;
the heating medium and the heating method used; and
the surface treatment applied after die forging.
Pre-Oxidation: A Smooth, Removable Scale
Billets heated without any pre-treatment develop a fish-scale appearance, with an uneven oxide surface that is difficult to strip cleanly. Billets that receive a controlled pre-oxidation treatment come out of the furnace with a smooth surface and a scale that detaches far more readily. Two further benefits follow from this:
The condition of the surface layer, both oxide and absorbed gas, has a significant effect on the mechanical properties of hot-forged blanks, particularly ductility. Sandblasting directly after forging measurably improves ductility by removing that layer.
A glass enamel coating applied to a pre-oxidised billet improves surface ductility further, and the enamel is very easy to strip from the pre-oxidised surface, which keeps the finishing operation short.
Where the heating cycle is long, or where the forging geometry demands extended furnace time, a glass enamel coating acts as a diffusion barrier between the hot metal and the furnace atmosphere. Because the coating is applied before heating and stripped afterwards, it adds one process step but removes far more cost downstream: less scale, less alpha case to machine away, and a sounder surface on the finished forging.
Heating Practice: Fluidized Bed and Conventional Furnaces
When titanium billets are heated in a conventional electric furnace, exceeding the beta transus does not by itself produce a deep contaminated layer, provided the holding time above that temperature stays within about one hour. Under those conditions the oxygen-enriched layer remains shallow and ordinary sandblasting removes it without difficulty. Shop-floor control therefore means minimising both temperature overshoot and total time at temperature.
Fluidized-bed heating deserves particular attention in titanium forging. In a fluidized bed of loose particulate material, heat exchange between the particles, the fluidising gas and the billet surface is intense. The effective heat-transfer coefficient is on the order of one order of magnitude higher than in a forced-convection furnace, and it is comparable to the heat transfer achieved in a molten salt bath. Because the billet reaches forging temperature much faster, its total exposure at high temperature, and therefore the depth of oxidation and alpha case, is reduced. The result is a cleaner surface, better metal yield and lower finishing cost.
| Heating method | Heat transfer and effect on oxidation |
|---|---|
| Fluidized bed | Highest effective heat-transfer coefficient, about an order of magnitude above a forced-convection furnace and comparable to molten salt; shortest time at temperature, least alpha case |
| Conventional electric furnace | Slower heat transfer; contamination stays shallow when the hold above the beta transus is kept within about one hour, and is then removed by sandblasting |
Post-Forging Surface Treatment
After die forging, the contaminated layer has to be removed completely; partial removal simply leaves brittle patches that behave as crack initiation sites in service. The sequence used in practice combines mechanical and, where necessary, chemical methods:
Abrasive blasting to strip scale and restore surface ductility
Chemical pickling in a nitric-hydrofluoric acid solution for a uniform, scale-free surface
Verification of surface quality, with machining of any residual alpha case before final inspection
Recommended process sequence.
Polish the billet surface to remove prior contamination
Pre-oxidise the billet to generate a smooth, removable scale
Apply a glass enamel protective coating where extended heating or complex geometry demands it
Heat in a fluidized bed, or in a controlled electric furnace keeping the hold above the beta transus under about one hour
Forge, then sandblast to restore surface ductility and remove residual scale
Pickle or machine as required, then verify the surface before final inspection
FAQ
Q: What is alpha case on a titanium forging?
It is a hard, brittle, oxygen-enriched surface layer that forms when titanium is heated above roughly 600 °C in air, sitting beneath the outer oxide scale.
Q: Why must alpha case be removed?
Alpha case has low ductility and poor fatigue performance compared with the parent metal, so leaving it in place introduces a brittle surface that can initiate cracking in service.
Q: What does pre-oxidation of the billet achieve?
It produces a smooth, uniform and easily removable scale, gives a better surface after forging, and helps avoid the fish-scale surface that untreated billets develop.
Q: Does exceeding the beta transus always cause deep contamination?
Not by itself. In a conventional electric furnace, a contaminated layer stays shallow provided the hold above the beta transus is kept within about one hour, and sandblasting then removes it.
Q: Why is fluidized-bed heating preferred for titanium billets?
Because its heat-transfer coefficient is about an order of magnitude higher than a forced-convection furnace and comparable to molten salt, so the billet heats faster and spends less time at high temperature.
Q: How is alpha case removed after forging?
By abrasive blasting, by chemical pickling in a nitric-hydrofluoric solution, and where required by machining off any residual layer before final inspection.
