corrosion resistant steel with water, and for machinability

[hazy]

Hello,

I have a dilemna in having to choose between the best corrosion resistant steel for use with deionized water and for machinability.

I'm afraid that the answer is 316SS which is difficult to machine precisely.

I would appreciate any help you can provide.

Thanks,
Richard Hayes

 

[kenvlach]

316 SS is much-used for this purpose. Consider the trade-off: one-time machining vs. lifetime use.

Carpenter Technology sells ‘Project 70’ and ‘Project 7000’ versions of 316 and 316L for improved machinability. Use 316L if any welding is involved.

http://www.cartech.com/

For high-purity usage, it is important to have the parts passivated; electropolished + passivated is even better.

For more on electropolishing, see thread330-50008

 

[Metalguy]

Hazy,

If your stresses are low, you might consider 303 SS. Machines better than 304/316, and it doesn't sound like you need the corr. res. of 316.

 

[TEV]

Hazy:

Manufacturing 316 fluid connector and control components (read - fittings and valves) is our major business. Carpenter Proj 7000 stainless is readily machinable, as Kenvlach says. You just have to use the right tools, feeds, speeds, etc. I also agree with him about passivation, or better yet, electropolishing + passivation.
By the way Carpenter Project 7000 316 stainless is 316L grade.

I suggest you may want to look at automated orbital welding for assembly of your system, followed by passivating + EP. The smooth connections achievable by this technique will result in fewer "dead spaces" and "unsweept areas" which are sites for bacterial growth, leading to contamination. Also, choose your joining system with this in mind.

Can I help?

ps: Our ultra-high purity DI water system is constructed in PFA plastic!

 

[hazy]

Hello,

I thank you for your input into my problem. I have taken your advice and am in the process of getting information from Carpenter on their specialty 316 steels.

Aslo, I am looking into the electropolishing process. By the way, is there much of a problem with dimensional changes when electropolishing is used?

Thanks,
Richard Hayes

 

[kenvlach]

For more info and links on electropolishing of SS, see
Stainless steel finishing
thread330-50008

Usually, the material removed when electropolishing is 0.00005-0.00050 inch, and it can be controlled fairly precisely. Also note, it removes the high points (ridges), not the valleys.
From the article “Electropolishing” by Ken Hensel at

http://www.metalfinishing.com/WZ/MetalFinishing/guidebooks/MetalGB_SurfaceTreatments/000013/show/

"The amount of metal removed depends on the specific bath, temperature, current density, and the particular alloy being electropolished. Generally, on stainless steel, 0.0005 in. is removed in 1,500 amp-minutes per square foot. Current and time are two variables that can be controlled to reach the same surface finish. For example, 100 A/ft² electropolished for 5 min is 500 amp-minutes; 200 A/ft² for 2 1/2 min is 500 amp-minutes."

Note: I consider 1,500 amp-minutes per square foot as rather an upper limit, unless you are also deburring. Electropolishing should not be used in an attempt to remove deep gouges or scratches. In fact, you may need to mechanically polish first to avoid having scratches 'stand out' on the electropolished surface.