4140?

...and pipe flanges.

This steel is also known around the forging industry as F5. It is a type of chrome molybdenum, a little more carbon than 4130 which was fairly common for motocross motorcycle frames in the '70s, ala Honda Elsinore. I have seen it forged with a hydraulic press then annealed by soaking at 1650 DegF for 8 hours, then ramped down 50 degF/hour for 16 hours. It is then no longer brittle and may be machined without chipping.

C Wilkins
 
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