One of the primary concerns with thermal processing is the formation of alpha-case. Alpha-case forms on the surface of titanium alloy when parts get above 1000 F (538 C) in the presence of oxygen. One problem with alpha-case is that it is a crack-starter for subsequent fatigue failures. So if alpha-case is present, we always remove it by either machine milling or by chemical-milling. Removal of alpha-case can cause a second problem; that of maintaining proper part thickness.

The heaviest alpha-case that I have measured was 0.013 in. (0.33 mm) thick from an air-electric furnace.

Gas furnaces usually heat parts with the products of combustion, meaning that most of the oxygen was burned up creating the flames. This oxygen-lean atmosphere around the part works quite well to minimize the formation of alpha-case. My records show that parts heated in a gas furnace pick up alpha-case that usually measures from 0.0002 0.0020 in. (0.005 0.05 mm) thick on the part surface. There are exceptions where readings are higher.

The thickness of alpha-case is determined 1) by laboratory testing from a coupon of material that matches the part in thickness and accompanies the part throughout the thermal process or 2) by examination of a small piece of material removed from the part (provided there is enough extra material on the part to provide a test sample). Even though I have seen heavy amounts of alpha-case from gas furnace processing, most parts do not acquire more than 0.0008 in. (0.015 mm) of alpha-case.

To get even...