What happens when a steel gets tempered?

[sara120]

My first post at this forum.

This maybe very simple question for some of you but I really wanted to know what happens to the material in my case steel, when it gets tempered at high temperatures (e.g 800 C)?

- What happens to the inner material structure?
- How does the variation in the structure affect the resistivity and consequently the conductivity?

Thank you all for your help

 

[stanweld]

Assuming carbon steel, the microstructure will depend on the carbon content and whether it is fully autenitized (annealed) or intracritically annealed (partially austenitized) and the cooling rate. More info needed.

 

[WSALDUA]

Do you mean "tempered" or "heat treated" ? Tempering is done to relieve internal material stress that develope during quenching, after heat treatment. Usually, tempering is done at a low enough temperature so that no phase changes occur. The ASM handbook is a great resource for cooling curves that definines final material microstructure based upon annealing temperatures and cooling rates.

 

[sara120]

Thank you both for your responses.

@ stanweld : Yes - E.g. 6%C Steel at temperature of 800 C.

@ WSALDUA : Thanks - Yes i mean heat treated when i say tempered.

What element in Steel structure hugely affects its resistivity and hence its conductivity?

 

[arunmrao]

sara120 , correct me if I am wrong 6% carbon steel?? If you consider transformer steel,we have grain size and grain orientation to be considered to minimize core losses.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[sara120]

Arunmrao, Thank you for your reply.

To tell you the truth, I don't have much background on metallurgy and materials, as I am from an Electronic background.

So since you are from materials background, do you know what happens to the grain size and the grain orientation of the transformer steel, when they are heat treated?
in other words, what change occur to the grain size and grain orientation of these materials, making them change their properties ( properties such as resistivity, conductivity and permeability?)

Thank you

 

[lkrzyzan]

I suspect that the electrical properties would be similiar to that of a varistor when considering grain size and orientation. The orientation of grains will be dependent on temperature gradients and the overall grain size on the preexisting grain size and cooling rate. Any other processing steps? Rolling? This would decrease grain size; sufficient heat treatment would increase grain size.

As for elemental factors it is highly dependent on the steel in question. I suspect that some graphite will form as an inclusion depending on the cooling rate. Here orientation is important as graphite is isotropic when it comes to electrical properties. Also do you mean 6% silicon? Electrical steel is high in silicon. Higher silicon tending to increase resistivity.

hope this helps some