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embedded iron particles from the surface, if any, and the need for cleaning the formed part prior to any heat treatment or welding. Lubrication is generally not required for simple "U" bends as on a press brake. However, severe or progressive die forming operations require heavy-duty lubricants with good surface wetting characteristics and high film strength such as found in metallic soaps and chlorinated or sulfo-chlorinated oils. It is very important to remove all traces of these lubricants prior to any heat treatment or welding due to danger of the carbon pick-up and lowering of the corrosion resistance by forming complex carbides. Parts formed with zinc alloy dies should be flash pickled to prevent liquid metal embrittlement during heat treating.

A frequently raised concern is whether to anneal after forming prior to putting in service or prior to any welding. No specific guidelines can be given but from experience some general observations can be made:

• For most materials cold forming to a degree of 15% or less (maximum strain in the outer fiber) is permissible without subsequent heat treatment. However, in certain cases depending on the intended service, a full solution heat treatment may be necessary. Care should be exercised when annealing components with 5-10% reduction due to the danger of excessive irregular grain growth known as "orange peel effect." This may occur on materials that are severely bent and then annealed.

• In Ni-Mo alloys, if any welding is to be performed after forming, it is advisable to give a full solution anneal prior to welding. Prior to annealing of cold-formed heads, a shot peening operation with clean sand is necessary with alloy B-2. However, this shot peening may not be necessary with alloy B-4. However, in the author's opinion, until more data is generated, solution annealing prior to any welding of a cold-formed head or part is advisable for both alloys B-2 and B-4. The metallurgical reasons for this phenomenon is clearly explained in published studies [10, 11]. This phenomenon relates to the kinetics of formation of an embrittling body centered tetragonal P phase (Ni4Mo). Higher iron content, as in controlled chemistry B-2 and B-4 significantly retards the formation of this detrimental phase, which appears to have been responsible for the cracking behavior

In Ni-Cr-Mo alloys such as alloy C-276 and 59, the corrosion resistance does not suffer as a result of cold work. This has been proven and well documented in successful use of alloy C-276 in deep sour gas wells in the cold reduced condition. However, if any welding is to be done on these Ni-Cr-Mo alloys, which have less than 15% cold work, it can be done without any heat treatment. The superaustenitic stainless steels can be treated similar as the Ni-Cr-Mo alloys regarding this aspect.

7.12.3 Hot Forming

Hot forming generally is carried out in a temperature range between the solidus temperature and start of recrystallization temperature. For most nickel base alloys,

TABLE 7.22 Hot Working and Solution Annealing Temperature Range for Various Alloys

Alloy

Hot Working Temperature Range (°C)

Solution Annealing Temperature Range (°C)

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