Layman question: Nomalized .. tempered

[Flareman]

I need to use steel able to resist sour gas attack and I am familiar with the idea of normalizing the (Carbon) steel grain size and controlling the material and weld hardness to minimize the risk of intergranular attack.

"Tempering" was recently thrown around in a discussion and I'm a little confused. Doesn't this harden the surface again or am I off-base?



David

 

[Carburize]

Tempering is the term used to describe the heat treatment carried out after normalizing or quenching. On carbon and low alloy steels it is typically carried out in the range 900 - 1200F depending on the alloy, required final hardness etc and typically for about 1-hour per inch of section thickness. Some codes specify minimum tempering temperatures which can be used on material for specific kinds of service.

 

[metengr]

To add slightly to the above post by carburize, tempering is performed to reduce hardness and improve toughness from quenching or normalizing.

 

[tomwalz]

Tempering
1. In heat-treatment, reheating hardened steel or hardened cast iron to a given temperature below the eutectoid temperature to decrease hardness and increase toughness. The process also is sometimes applied to normalized steel. 2. In nonferrous alloys and in some ferrous alloys (steels that cannot be hardened by heat-treatment), the hardness and strength produced by mechanical or thermal treatment, or both, and characterized by a certain structure, mechanical properties or reduction in area during cold working.

Austempering
Heat-treatment for ferrous alloys in which a part is quenched from the austenitizing temperature at a rate fast enough to avoid formation of ferrite or pearlite, and then held at the appropriate transformation temperature to achieve the desired characteristics. Austempering at lower temperatures (240° C to 270° C) produces a part with maximum strength, while austempering at higher temperatures (360° C to 380° C) yields high ductility and toughness.

Martempering
Hardening procedure in which an austenitized ferrous workpiece is quenched in a medium whose temperature is maintained substantially at the MS temperature (temperature at which martensite starts to form from austenite) of the workpiece. It is held in the medium until its temperature is uniform throughout-but not long enough to permit bainite to form-and then cooled in air. The treatment is frequently followed by tempering. 2. When the process is applied to carburized (casehardened) material, the controlling MS temperature is that of the case. This variation of the process is frequently called marquenching.



Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessor.com

 

[Flareman]

Gentlemen
Thank you for your various contributions, which have cleared things up for me quite well.
One more question .. just for educational purposes. If I'm a manufacturer (which I'm not) is there a time/temperature scale to help me determine the final material hardness or is it suck-it-and-see?

David

 

[Carburize]

Depends on the alloy - there are tables in handbooks such as ASM Vol 4 "Heat Treating" showing generally what hardness specific alloys have after tempering at various temperatures.