Did we significantly alter the strength of this part?

Part is a thin-walled (approx 1/8" avg) 4130 casting, finished machined and "heat treated" - to what extent, I do not know at this time - supplier is investigating - probably an "average" heat treat. The part after we received it was painted with a bake-on system that required it to be cycled as follows: 350 F 30 min, 30 min air cool, 500 F 30 min, air cool. It was re-painted again later and has gone through the same cycle again. So to total: 2 30 min 350 F cycles and 2 500 F 30 min cycles. Other much smaller steel parts of various types were also hanging in this oven and I observed varying color changes from dark blue to straw/gold on unpainted areas of these. The main larger part I am discussing here is totally coated flat black with the paint system and I cannot tell surface coloration on it. Question: did this temp cycling temper and toughen or anneal and weaken. There could be a safety issue here for the handler if the part is significantly weakened. I feel we are OK from reading other docs (such as pg 7 of

http://www.carreramini.com/TensileTestNotes-II.pdf

). Would like to know general consensus. Any feedback will be greatly appreciated.

 

[unclesyd]

As long as you heat cycles are below the tempering temperature you are OK if they are heated in an oven where there is no exposed elements that would subject the parts to much higher temperature than indicated on temp control.

We have thousands of 4140 parts that are tempered at 900F-1000F that have undergone thousands of temperature cycles of 70F to 600F to 70F to 900F and back again. The cycles times vary from hours to days and some even years with no problems.

 

Thanks for the reply, unclesyd. From online sources, such as

http://www.suppliersonline.com/research/property/metals/19.asp

, I see that the "average" tempering temperature for 4130 is 750 F to 1050 F. So, we never exceeded 70% of the lowest recommended number.

 

[TVP]

The answer to your question is no, you did not alter the strength of your part so long as the tempering temperature was in the range of 750-1050 F.

 

[EdStainless]

If you need to prove this to someone take before and after hardness readings. This temperature should have no negitive impact on the parts.

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Plymouth Tube

 

Thanks to all for the replies. I think the consensus is that the part is good to go!