Is the ordrer important in the double tempering ? 2

[stanislasdz]

I have to perform a double tempering at T1 and T2 ( T1 > T2)on steel forging part.

Is the order important :

T1 then T2 is it equivalent to T2 then T1 ?

Many thanks

 

[metengr]

stanislasdz;
The order is important because you do not want to over temper (soften or reduce tensile strength) the component. T2 is normally below T1 for double tempering.

 

[redpicker]

This results from people who write the specifications living in their own theoretical world where heat treatment is a science, not an art. In reality, particularly with commercial heat treating operations, the variability in the material exceeds the allowable variation in the expected results, so there is a fair amount of "windage" built into the heat treater's process.

Theoritically, the first tempering temperature is chosen to provide the proper final hardness and the material and parts are designed so with the proper surface hardness, the mechanical property requirements will be met. In this scenerio, the second tempering temperature needs to be lower than the first one to prevent overtempering the material and lowering the hardness and strength of the material, just as metengr has stated.

In the real world however, with quench and temper operations the heat treater will purposely use a tempering temperature on the low side to result in hardnesses on the high side of the specification so that if the material is out of spec after tempering, it can be corrected by a re-temper operation instead of having to re-quench and temper.

Since the object of the the second tempering operation, when required, is to temper any fresh martensite that might form on cooling from the first tempering operation, it really doesn't matter if T2 happens to be above T1, it will still accomplish the goal of tempering any fresh martensite.

When I worked for a forge shop, I had to deal with customer specifications that had a requirement for double tempering on some materials. The typical wording was that the second temper had to be performed at least 50 F below the original tempering temperature. I would request from the customer an exemption that stated "the second tempering temperature shall be within 100 F of the first tempering temperature and that all mechanical property testing to be performed after the second temper". I was able to convince most customers that this was a more effective requirement and I don't think I ever had the request turned down. So, to meet this requirement, I would purposly aim approximately 50 F below what I though would be needed and base the second tempering temperature on the hardness results from the first temper. Of course, all hardnesses and tensile and impact testing (if needed) was performed after the second temper.

rp

 

[arunmrao]

T1>T2 is the accepted norm. I go along with Redpicker in his comments. Once again I repeat that samples for mechanical testing must be drawn after T2 cycle only.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[stanislasdz]

thanks arunmrao !

redpicker : great answer !!!!! ( + one STAR )