Wrought superalloys can often be formed using techniques similar to stainless steels, although forming is more difficult. Like stainless steels, superalloys work harden rapidly during forming. The work hardening rate usually increases with alloy complexity, i.e. the precipitation hardening alloys have higher work hardening rates than the solid solution strengthened alloys.22 The cobalt based alloys require greater forces than the nickel or iron based alloys. Alloys that contain substantial amounts of molybdenum or tungsten are also harder to form. Forming presses are the same as those used for forming steel; however, because of their higher strength, more power is needed, usually 50-100% more power.
In cold forming operations, the rate of work hardening in relation to the amount of cold deformation achieved determines the frequency of intermediate anneals. For example, some alloys, such as A-286, Nimonic 75 and Hastelloy X, can be reduced as much as 90% before annealing, while others, such as René 41 and Udimet 500,
can only be reduced 35-40% before an intermediate anneal is required.24 The rate of work hardening for four superalloys is compared to type 302 stainless in Fig. 6.15. The iron-nickel base alloy A-286 work hardens similar to type 302, while the others work harden at higher rates, with the cobalt base alloy S-8l6 displaying the highest work hardening rate.
Since the window for hot forming is narrow (1700-2300° F), cold forming with intermediate anneals is preferred for thin sheets. To preserve a fine grain structure during cold forming operations, a critical amount of cold work (8-10%) must be achieved prior to intermediate anneals. Annealing material with less than sufficient amounts of cold working can result in abnormal grain growth, which can cause undesirable orange peel effects during subsequent forming, and also adversely affects fatigue strength. Cold working can accelerate the aging process in precipitation hardened alloys; in other words, overaging can occur at otherwise normal aging temperatures.24 The aging temperature, or time, may have to be adjusted in such situations. If annealing causes distortion, the part can be formed to within 10% of its final shape, annealed and then give a light forming operation to bring it into tolerance.
The annealed condition is usually soft enough to permit mild forming. If the annealed alloy is not soft enough for the forming operation, a higher temperature solution annealing treatment can be used that will remove all of the effects of cold work, but will dissolve the age hardening and other secondary phases. Some grain size control is sacrificed, but if cooling from the solution annealing temperature is very rapid, the age hardening elements will be retained in solution. Several process anneals may be required in severe forming, but the high temperature solution anneal need not be repeated. Annealing should be performed at a temperature that produces optimal ductility for the specific metal. Typical minimum bend diameters for annealed material are in the range of 1-2 X thickness.
Superalloys often exhibit galling between the workpiece and tool. The oxide film formed due to chromium addition does not help in reducing galling, since it is easily fractured during forming. Therefore, steel dies, punches and mandrels are often chrome plated to minimize adherence. Cast iron tools are often used for low production runs while steel is used for longer runs. Areas which experience high wear rates are often locally hardened. Lubrication is required for drawing, stretch forming or spinning. Although lubrication is seldom needed for the press brake forming of V-bends, its use will improve the quality. Mild forming operations (<10% reduction) can be conducted with unpigmented mineral oils and greases. For more severe forming, lubricants include chlorinated, sulfochlorinated, or sulfurized oils or waxes. After forming, it is important to thoroughly clean the material before heating it to elevated temperatures. Due to the tendency to rapidly work harden and gall, forming operations are usually performed at relatively slow speeds.
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