4140 is usually quenched from ~1,525 Deg. F into oil. Complicated shapes usually fracture if quenched in water. Holding time at temp. is fairly short-something around 15 min. per inch of thickness.
You should be aware that different heats of 4140 can behave quite differently, depending on the actual composition--especially the Mn, Mo, and Cr.
Thanks Metalguy for the information. I have one more question concerning this process. Since I would like to achieve a hardness of around 42-46RC, do I need to anneal the metal back in the quenching process?
The as-quenched hardness depends on the section size. For a 0.5 inch diameter bar oil quenched from 1550 F, the surface hardness will be ~ 57 HRC, and the center will be ~ 55 HRC. For a 2 inch diameter bar, the surface hardness will be 49 HRC and the center will be 38 HRC. Use a tempering temperature of ~ 370-400 C (700-750 F) if the section size is small and the as-quenched hardness is similar to the numbers above.
There are several points concerning safety for both you and your parts. Quenching in oil is inherently hazardeous for several reasons, flash fires and the expulsion of oil from the quench vat. The oil should be a "quench oil" and preferebily agitated and warmed. This does two things, prevents the oil from locally getting too hot and lessens the severity of the quench of your part. If your part can trap oil be very careful to prevent expulsion of hot oil by vaporized oil. The mass of the part being quenched will determine the amount of fumes and flash you will produce. Watch doing any oil quenching inside without proper ventalation.
BE SAFE!
