js
Flashlight Enthusiast
So, I've just read some of the sections in the Donachie book on Heat Treatment. The short of it is this: heat treatment is very difficult for the reasons Don already mentioned: it must be done in an inert atmosphere to avoid oxygen and nitrogen infiltration and consequent embrittlement and degradation.
(The beta transus of Ti-6-4 is 1000C +/- 20 C) (!)
Moreover, it only increase the hardness and strength by a pretty small margin in the case of Ti-6-4. As received from the supplier, grade 5 Ti-6-4 has a yield strength of 141ksi. With further heat treatment, you can increase this to 153ksi. However this will be at the expense of ductility. And, as I just implied, grade 5 Ti-6-4 is already annealed. In fact, all the stock titanium has to be straightened by what is essentially annealing:
Straightening, Sizing, and Flattening. Straightening, sizing, and flattening of titanium alloys are often necessary to meet dimensional requirements because it can be difficult to prevent distortion of close-tolerance thin sections during annealing. Because titanium alloys have excessive springback, the straightening of bar to close tolerances and the flattening of sheet present major problems for titanium producers and fabricators. Straightening, sizing, and flattening can be conducted independently of other related processes or can be combined with annealing (or stress relief) by use of appropriate fixtures.
Unlike aluminum alloys, titanium alloys are not easily straightened when cold, as explained previously. (See the section "Forming" in Chapter 5.) Because of springback and resistance to straightening at room temperature, it is necessary to employ elevated-temperature forming. Therefore, titanium alloys are straightened primarily by creep straightening processes.
Creep straightening uses the concept that at annealing temperatures, many titanium alloys have low creep resistance. The creep resistance can be sufficiently low enough to permit the alloys to be straightened during annealing. With proper fixturing and, in some instances, with judicious weighting, sheet metal fabrications and thin complex forgings have been straightened with satisfactory results. Again, uniform cooling to below 315 °C (600 °F) after straightening can improve results.
Creep flattening consists of heating titanium sheet between two clean, flat sheets of steel in a furnace containing an oxidizing or inert atmosphere. Various jigs and processing techniques have been proposed for annealing titanium in a manner that yields a flat product. Creep flattening and vacuum creep flattening are two such techniques. Vacuum creep flattening is used to produce stress-free flat plate for subsequent machining. The plate is placed on a large, flat, ceramic bed that has integral electric heating elements. Insulation is placed on top of the plate, and a plastic sheet is sealed to the frame. The bed is slowly heated to the annealing temperature while a vacuum is pulled under the plastic. Atmospheric pressure is used to creep flatten the plate.
*****
So I'm guessing that there is no way that heat treatment wouldn't be stupidly expensive, and ultimately more or less just cosmetic for our purposes. Although I am definitely curious to hear what the response to Don's question is!
(The beta transus of Ti-6-4 is 1000C +/- 20 C) (!)
Moreover, it only increase the hardness and strength by a pretty small margin in the case of Ti-6-4. As received from the supplier, grade 5 Ti-6-4 has a yield strength of 141ksi. With further heat treatment, you can increase this to 153ksi. However this will be at the expense of ductility. And, as I just implied, grade 5 Ti-6-4 is already annealed. In fact, all the stock titanium has to be straightened by what is essentially annealing:
Straightening, Sizing, and Flattening. Straightening, sizing, and flattening of titanium alloys are often necessary to meet dimensional requirements because it can be difficult to prevent distortion of close-tolerance thin sections during annealing. Because titanium alloys have excessive springback, the straightening of bar to close tolerances and the flattening of sheet present major problems for titanium producers and fabricators. Straightening, sizing, and flattening can be conducted independently of other related processes or can be combined with annealing (or stress relief) by use of appropriate fixtures.
Unlike aluminum alloys, titanium alloys are not easily straightened when cold, as explained previously. (See the section "Forming" in Chapter 5.) Because of springback and resistance to straightening at room temperature, it is necessary to employ elevated-temperature forming. Therefore, titanium alloys are straightened primarily by creep straightening processes.
Creep straightening uses the concept that at annealing temperatures, many titanium alloys have low creep resistance. The creep resistance can be sufficiently low enough to permit the alloys to be straightened during annealing. With proper fixturing and, in some instances, with judicious weighting, sheet metal fabrications and thin complex forgings have been straightened with satisfactory results. Again, uniform cooling to below 315 °C (600 °F) after straightening can improve results.
Creep flattening consists of heating titanium sheet between two clean, flat sheets of steel in a furnace containing an oxidizing or inert atmosphere. Various jigs and processing techniques have been proposed for annealing titanium in a manner that yields a flat product. Creep flattening and vacuum creep flattening are two such techniques. Vacuum creep flattening is used to produce stress-free flat plate for subsequent machining. The plate is placed on a large, flat, ceramic bed that has integral electric heating elements. Insulation is placed on top of the plate, and a plastic sheet is sealed to the frame. The bed is slowly heated to the annealing temperature while a vacuum is pulled under the plastic. Atmospheric pressure is used to creep flatten the plate.
*****
So I'm guessing that there is no way that heat treatment wouldn't be stupidly expensive, and ultimately more or less just cosmetic for our purposes. Although I am definitely curious to hear what the response to Don's question is!
