SS 316L or SS 316Ti for freeze dryers 3

[quark]

Hi Guys!

We are in the process of finalizing a lyophilizer for drying of our medicinal product. The equipment is subjected to extremes in temperature and pressure. During sterilization cycle, the pressure is about 1.2 bar and the temperature is 121⁰C and during drying, the pressure is 0.001 mBar and the temperature is -60⁰C.

One manufacturer offers SS316L and other SS316Ti. SS316L is well known across the pharma industry and no problems with it. The manufacturer who is supplying SS 316Ti lyophilizer is a reputed and so does the other. Still, leg pulling seems to be obvious.

As SS316Ti contains 0.05% more carbon(this equipment has to undergo lot of welding), will this effect the life of the equipment? What are the other disadvantages you guys foresee? What advantages I have if I go for SS316Ti?

Thanks in advance,

 

[Metalguy]

Depends on the thickness. If your parts are fairly thin, say less than 1/4" thick, use 316L. The cooling rate during welding will easily be fast enough to avoid the "dreaded" sensitization, which might not affact your dryer anyway.

 

[mcguire]

Neither material will sensitize, but the 316Ti will have MUCH better corrosion resistance in the welds. You're looking at an advantage of 20C in critical pitting temperature for the 316Ti at the welds. The titanium is really advantageous in this regard.

 

[quark]

Metalguy and Mcguire!

Thanks for the input. I did a lot of search regarding this during these 6 days but I couldn't get anything other than what you people indicated(perhaps my search tags are very weak). Though this is sufficient to impress my boss, I just like to know a little bit extra to my knowledge.

One of the webpages says "sensitization can be minimized either by reducing carbon content or by changing constituents of base metal". Can you guys, please, tell me by what mechanism Ti reduces sensitization though carbon content is high? Any links, will also help.

Thanks once again.

Regards,

 

[Metalguy]

Very briefly, Ti will form a carbide (with C, of course) at higher temps. than Cr will. Therefore during HAZ cooling much of the C becomes TiC rather than Cr carbides, leaving the Cr "free" to maintain the protective Cr oxides.

 

[78parag]

When Ti or any carbide forming element like Nb or V is added to stainless steel it is termed as stabilised stainless steel.Due to higher affinity towards carbon than chromium, they avoid the formation of Chromium carbide and in turn avoid the deplation(reduction) of Chromium in free form.This retains the corrosion resistance of the stainless steel.

 

[quark]

Thank you all. Is there anything else that I should know?

Regards,

 

[mcguire]

From a corrosion resistance point of view, the surface finish and sulfur content are important. You want bright anneal or electropolished. You don't want abrasively polished. If you get 316L make sure steel sulfur content is <0.003%. With 316Ti it doesn't matter.

 

[quark]

OK, got it. Internals are electropolished to 0.6 micrometers.

Regards,