Question

A cylindrical piece of steel 44 mm (13/4 in.) in diameter is to be austenitized and quenched such that a microstructure consisting of at least 50% martensite will be produced throughout the entire piece. Of the alloys 4340, 4140, 8640, 5140, and 1040, which qualify if the quenching medium is (a) moderately agitated oil

Answer 1

Steels 4340, 4140, and 8640 qualify to produce at least 50% martensite throughout a 44 mm diameter piece when quenched in moderately agitated oil. Steels 5140 and 1040 may not meet the requirements due to lower alloy content and hardenability.

Step-by-step explanation:

The correct answer is that 4340, 4140, and 8640 steels may qualify to produce at least 50% martensite throughout the piece when quenched in moderately agitated oil. These alloys contain elements such as manganese, chromium, nickel, molybdenum, and carbon, which are known to increase hardenability – the ability of steel to become hardened throughout upon quenching. Based on their chemical composition and transformation characteristics, these steels are capable of forming martensite at the specified diameter when subject to a thermal treatment involving austenitization followed by quenching.

Alloys like 5140 and 1040, with lower alloy content and hardenability, might not form the desired amount of martensite throughout the entire cross-section of a 44 mm diameter when using the same quenching medium. Hardenability is crucial for ensuring that the steel doesn't remain too brittle due to untransformed phases or doesn't have insufficient strength due to a lack of martensite.