41xx steel is a family of SAE steel grades, as specified by the Society of Automotive Engineers (SAE). Alloying elements include chromium and molybdenum, and as a result these materials are often informally referred to as chromoly steel (common variant stylings include chrome-moly, cro-moly, CrMo, CRMO, CR-MOLY, and similar).
AISI or SAE 4140 grade is a low-alloy steel containing chromium and molybdenum as strengthening agents. Its chemical composition is seen to the right:
AISI/SAE 4140 high strength steel plate and tube is a versatile alloy with good atmospheric corrosion resistance and reasonable strength up to around 600o F (315o C.) It shows good overall combinations of strength, toughness, wear resistance and fatigue strength.
Although AISI/SAE 4140 high strength steel is chromium steel and often referred to as "chromoly steel", it does not contain as much chromium as stainless steel. Therefore the corrosion resistant is improved compared to standard steel but not to the same degree as stainless steels. The purpose of the molybdenum is to add uniformity of hardness and strength to this alloy. 4140 alloy responds readily to heat treatment and is relatively easy to machine in the heat treated condition.
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