Difference Between Q&T and N&T? 1

[tc7]

What is the difference and advantages between "normalized and tempered" or "quenched and tempered" ?

This is in regard to ASTM A757 B2N and B2Q; both versions are indicated to have the same tensile strength and the same Charpy value. So why would you choose one or the other method to arrive at the same values?
Thanks

 

[metengr]

Normalized and tempered (N&T) is a thermal heat treatment operation where a component is removed at the austenitizing temperature and is force cooled in air, and subsequently tempered. Quenching refers to the use of a liquid medium to assist in rapid cooling, versus air cooling, because of component thickness limitations to achieve the same mechanical properties as N&T. Typically, for liquid quenching, this has to be agreed upon between the vendor and purchaser.

 

[tc7]

I would have expected that the different cooling rates would have resulted in different grain structures (our martensite/bainite discussion comes to mind) and that even though tempering can provide equivalent tensile strengths from the cooled state, the toughnesses would vary.

 

[metengr]

tc7;
My response to your OP was framed in the context of how ASME Codes and Standards endorses ASTM material specifications with certain provisions related to heat treatment.

Look at it this way, some materials have limited hardenability because of chemical composition. In some cases, a N&T heat treatment may not achieve thru thickness properties. As an alternative to rapid air cool, a liquid medium for quenching (more aggressive heat removal) can be used to achieve minimum thru thickness properties. Yes, you may have mixed structures with Q&T, but as long as tempering is performed and you have achieved minimum strength, ductility requirements, and minimum notch toughness (if specified) using an alternative Q&T to achieve the same means as N&T what difference does this make?

 

[davefitz]

Often , air cooling only cools from the outside of the componet toward the ID; there is little effective cooling of the ID sruface in that case. This is acceptable up to a max wall thickness of ZZZ inch thick. For thicknesses over ZZZ inch thick, air cooling of only one side may cuase excessive ID to OD temperature differentials , and the resulting thermal stress may exceed yield at hightmeps and lead to residual stresses inside the wall.

To obtain cooling of both ID and OD simulatineously , and thus reduce the magnitude of the imposed thermal stresses, once can use Q+T. For a typical ferritic material , the cutoff point is about 3" thick wall; below 3", N+T ( air cooled from one side) is OK, but a Q+T would be required for thicker walls. However, for very thick walls on material that is forming brittle martensite, someone needs to evaluate whether the residual thermal stress would lead to cracking.