Out-of-range ultimate tensile strength (UTS) of a cold-drawn 304 tube.

[Laboratorio TRF]

Hello everybody,
I work in a company that produces seamless tubes through cold-drawing method. Sometimes for stainless steel tubes (TP304 and 316) of low wall thickness (1.75mm and below) after solution heat treatment at 1050°C in a continuous furnace, the ultimate tensile strength of tensile test is higher than maximum allowable in EN 10216-5. This does not happen for "heavier" tubes. I have tried heat-treatment with lower passing speed through the furnace, but could not resolve the problem.
can someone give me an explanation for this?

 

[EdStainless]

1. Your temperature is too low. 1100C is more typical for this work.
2. Your speeds are still too high. I would bet that your speed is based on weight, and for very light tubes you are not allowing enough heat up time.
Your furnace speed needs to account for heat up as well as soak.

But raising temp is easier. 6 min at 1100C is roughly the same as 1050C for 30 min.

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P.E. Metallurgy, consulting work welcomed

 

[Laboratorio TRF]

Thanks Ed for sharing your knowledge.
Yes our speed is based on the wall thickness and external diameter of the tubes, and of course the furnace potential (which is a constant).
We can change the passage velocity and number of tubes passing the furnace at each time. I tried very little number of the same tubes (to reduce the flow of input material, ie. to increase the heating power of the furnace) which did not respond well.
The temperature is based on the EN 10216-5 and ASTM A213-1016 and yields to acceptable results for tubes of higher wall thickness.
I am not sure if the furnace can support 1100°C. I will look into that.

 

[EdStainless]

Is it roller hearth or a belt furnace?
Either way as long as you load a fairly constant width the only variable is wall thickness (and alloy).
The difference is with a belt furnace you have to account for the weight of the belt (as a function of speed).
We used a two-factor calculation, one based on wall for heat up time, and then a fixed hold portion.
We also found that after draw the response was affected by the amount of cold work.
Tubes with more worked recovered faster than tubes with less cold work, but in both cases, there was a very definite delay in the anneal response.
I was spoiled, our furnaces could handle 1225C, we needed it for Ni alloys.

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P.E. Metallurgy, consulting work welcomed

 

[Laboratorio TRF]

It is a roller furnace, so the belt factor does not exist.
I will do a study on the riduction percent of heavy light tubes to see what changes in the tensile strength results.
Thanks again