410 Stainless, why not UTS=160ksi if HeatTreat per AMS 2759/5? 2

[realJedi]

I'm using 410 CRES in a design, and I want to use it in a heat treated condition Rc35 approximately, which equates to about 160ksi ultimate tensile strength. To acheive this condition, 410 is tempered at 540 deg. celsius after it has been austenitized.

Here's my question: I want to define the heat treat process using AMS 2759/5C, however it for some reason does not recomend using 410 tempered at 540 deg. celsius. Why is this? The specification warns to avoid tempering 410 CRES in the 371-538 deg celsius range. Why again? I have many material specs from vendors of 410 CRES at a 540 temper, which suggests to me that it is commonly used at this temper. If 410 shouldn't be tempered at 540*C, why are all these specs available for 410 CRES at 540*C? I am wondering if anyone knows why the 371-538 deg celsius range should be avoided.
Thanks!

 

[mcguire]

The reason is to avoid loss of corrosion resistance. It's the carbide precipitation that occurs with 3xx series stainless. In as-quenched martensite the carbon is supersaturated and quenched-into solution. Tempering at too high a temperature permits the diffusion-controlled precipitation of Cr23C6 with attendant loss of chromium from the matrix and local loss of corrosion resistance.

 

[unclesyd]

Tempering 410 or any martensitic SS in the 371°C - 538°C range causes a condition called "Temper Embrittlement". The metal gets extremely brittle if tempered in this temperature range. If tempered in this range ductility can be restored by tempering above 538°C and rapidly cooled to below 371°C.

There is quite a lot metallurgy involved so come back if you have any specific questions

 

[mcguire]

Unclesyd is also correct on the temper embrittlement which comes from precipitation on prior austenitic grain boundaries. Molybdenum relieves this somewhat, which is one of the enhancements to basic 410 grade which have been made over the years.

 

[israelkk]

According MIL-H-6875H page 18 you can temper AISI 410 at a tempering tempearture of 500F to receive 180ksi minimum tensile strength.

 

[realJedi]

israelkk,

I should clarify, the reason I need a 160ksi strength is I plan to threadroll this part, and it needs to stay below Rc40 to facilitate the manufacturing process.

Thanks you all for your replies.