303/304 Stainless Steel, Comparison

[PeterCharles]

For use in bulk materials handling machines we have traditionally made some special bolts in 304 to resist corrosion in general (as opposed to the normal mild steel ones).

We are considering changing to 303 as our machine shop advise they can machine this quicker so reducing the cost. However, a collegue has noted that 303 is more susceptable to pitting corrosion than 304 and maybe we should stay with 304. Personally I think that there will not be a problem as other platework parts are made in 1.4003 (3CR12) and do not suffer problems.

Is the greater susceptability of 303 over 304 significant, anyone care to comment?

Bulk materials handled include wet coal, wet filter cake (delightfully vague!), steeped barley, green malt.

 

[TVP]

Personally I would never use a threaded fastener fabricated from Type 303 stainless steel. One reason is the reduced corrosion resistance. The other is the incredibly poor toughness and fatigue strength of this grade, especially in the form of a notched component. If these fasteners require anything more than finger tightening due to in-service loading then I strongly advise against Type 303.

 

[EdStainless]

I'll second TVPs concerns about toughness and fatigue. You need to talk to someone that is roll forming the threads and not machining them. That will get you smoother threads and get around the machinability issues.
Maybe you find roll threaded bolts in a high Cr ferritic grade like 18-2. These should cost less and still offer better corrosion resistance than your system.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[PeterCharles]

TVP & EdStainless, thanks for your comments.

These are in fact a purpose designed shouldered bolt so we are going to have to make them ourselves. I don't believe the corrosive conditions are that bad, but I take your point about poor toughness and fatigue strength.

Any suggestions about a suitable steel available in bar that would be easily machinable yet corrosion resistant?

 

[EdStainless]

I have watched guys roll threads in lathes one at a time.
My suggestion would be to look at duplex stainless grades. Focus on the lean alloys such as 2304, LDX2101 and AL2003. They don't machine badly, better than 304.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[TVP]

I agree with EdStainless that rolling is much preferred to cutting when it comes to threads. How many parts are we talking about? If the quantities are low, maybe you just want to use Type 304 (readily available, known performance, etc.) and politely tell the machine shop that their request has been denied.

 

[PeterCharles]

Thank you both for your assistance. Now at least I have reasons to turn down the request.
We're actually using a sub-contract maching company and I know the guy in charge of production (we worked together way back) and it was his suggestion to use 303 to reduce the cost of the item.

 

[moltenmetal]

Wow, that's pretty strong stuff- no more than finger tight etc.!

303 isn't 304, but it isn't PlayDough either! Plenty of stressed parts are made from 303 and are found to be quite serviceable- including parts with threads cut into them. It doesn't sound like a very tough corrosion service either. Are we sure we're not steering the OP away from 303 unnecessarily? There certainly would be a fabrication efficiency/cost benefit for using 303 vs other materials, especially if these small lot-order parts are being churned out of bar on a CNC machine.

 

[EdStainless]

If you are interested, LDX2101 from Outokumpu does machine better than 304. The other option would be a material like Prodec304 that has enhanced machining properties.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm