Can someone help me out here...
I have a roller to quote making for a customer where they specified "4140 Normalized & Tempered 275-315 Bhn"
I often also see "4140 Quenched & Tempered"
So, what is the low-down on "Normalized & Tempered" vs. "Quenched & Tempered"? Especially if the final Bhn is the same range?
Generally speaking, for the same section thickness and tempering temperature quenched and tempered steel will be both stronger and tougher than normalized and tempered steel. How much better depends on the section size, which you did not specify. Before you consider changing the heat treatment you should always ask your customer for their permission.
For the same Brinell hardness (Bhn, or more properly HB) normalized and tempered steel will have lower yield strength than quenched and tempered steel. Hardness correlates well with tensile strength, so at the same hardness the two heat treatments will produce similar tensile strengths.
Normalized steel is air cooled rather than being quenched in oil or water.
As-quenched steel typically will have a uniform microstructure consisting solely of martensite. When it is tempered a uniform distribution of carbides will precipitate. If the diameter of a oil- quenched round bar of 4140 steel is greater than about 50 mm then the microstructure will begin to contain a mixture of bainite and martensite.
Normalized 4140 steel probably will have a non-uniform (mixed) microstructure. If the diameter of a round bar is greater than about 3 mm, then the microstructure will begin to contain a mixture of bainite and martensite. As section thickness increases more bainite will form and hardness will decrease. If the diameter of a round bar is greater than about 50mm then the microstructure will have patches of very soft ferrite and bainite. The presence of ferrite reduces the yield strength and hardness. For slightly thicker sections the microstructure will also contain pearlite, and for even thicker sections the microstructure will consist solely of ferrite and pearlite.
Pittsburgh Pete
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