Any Metallurgists out there?

[rjcoles]

I have just had a new rudder heel fitting fabricated in 316 Stainless and it looks excellent. The machining is good and the welds have good penetration, but, being a cautious sole I checked it with a magnet. The body of the fitting is non-magnetic but the area of the welds was, I have checked with the welder and he has said that he used 316 sticks.
What is going on, and will the welds still be there in a few years time?

 

[electrosys]

Welding 316 with 316 or 316L rods usually results in a magnetic zone in the weld area. Nothing to worry about.
If it's important to maintain a low magnetic permeability - say, in a piece of scientific equipment - then 310 rods or wire should be used.

 

[vyv_cox]

Welding 316 with 316 or 316L rods usually results in a magnetic zone in the weld area. Nothing to worry about.
If it's important to maintain a low magnetic permeability - say, in a piece of scientific equipment - then 310 rods or wire should be used.

This isn't an area I'm familiar with but my reading suggests that the amounts of ferrite created in the weld and HAZ are small. I also read that fairly sophisticated equipment would be needed to detect them, e.g. a Severn Gage, and that a magnet would not be attracted to them. Do you know different?

 

[rjcoles]

Thanks for the info, I was worried that the welder had used a ferritic stick and that the welds would drop off when exposed to warm seawater (I eventually hope to go south).

 

[YELLOW BOAT]

I suppose the man used 316L/1.4404 ( L for low carbon) and not 316/1.4401 ( which has higher carbon) because otherwise you get exactly the phenomenon you noticed. For welding you must always use 316L!
I have another question.
Have the welds been pickled and passivated after the welding?
If not , yuo should have it done as soon as possible because without that the welds are proine to corrosion!
By the way, 316L is in fact not totally resistant against seawater , especially not in warm seawater like the Med!
A better choice would have been duplex ( 1.4462 or 1.4463) which has an excllent Chloride(salt) pitting and crevice corrosion resistancy.
Its a bit more difficult to get and you need a special electrode for welding but its much better.
Nowadays propeller shafts are also often made from duplex to avoid the pitting sometimes seen on stainless shafts.
If yuo want to now everything about corrosion resistancy of stainless steel, have a look at:
http://www.smt.sandvik.com/sandvik/...D170514454C55319C12577A0003C4DFA?OpenDocument
But that's only for real material freaks!
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[electrosys]

This isn't an area I'm familiar with but my reading suggests that the amounts of ferrite created in the weld and HAZ are small. I also read that fairly sophisticated equipment would be needed to detect them, e.g. a Severn Gage, and that a magnet would not be attracted to them. Do you know different?

I'm not a metallurgist (although I've got various degrees including one in chemistry), but I'm a keen amateur welder with a host of fabricated s/s fittings behind me. The behaviour of s/s never ceases to amaze me - I have found, for example, that 316 can be made magnetic by cold working, as well as welding. I have found that in practice 304 is 'harder' than 316, although the spec sheets say the opposite.

In looking for info, I find that the most informative (for my purposes) sites are those where people are real-world 'hands-on' types, rather than those of manufacturers or academics.

A few examples:
http://cr4.globalspec.com/thread/21001/Welding-Stainless-Steel-S310-and-Magnetic-Permeability
http://www.eng-tips.com/viewthread.cfm?qid=117830
http://www.eng-tips.com/viewthread.cfm?qid=214631&page=8
http://www.finishing.com/112/70.shtml
http://www.ukwelder.com/forum/index.php?showtopic=7319

But I'm not saying that the info there is in any way conclusive ... !

Good luck.

 

[electrosys]

On further reflection, the cause may have been the welding of 316 base metal with 316 or 316L rods.

If 316L base metal had been used, together with 316L rods, then very little magnetic attraction should have resulted.

But - magnetism is unrelated to corrosion resistance - so again, no need to worry.

 

[vyv_cox]

I suppose the man used 316L/1.4404 ( L for low carbon) and not 316/1.4401 ( which has higher carbon) because otherwise you get exactly the phenomenon you noticed. For welding you must always use 316L!
I have another question.
Have the welds been pickled and passivated after the welding?
If not , yuo should have it done as soon as possible because without that the welds are proine to corrosion!
By the way, 316L is in fact not totally resistant against seawater , especially not in warm seawater like the Med!
A better choice would have been duplex ( 1.4462 or 1.4463) which has an excllent Chloride(salt) pitting and crevice corrosion resistancy.
Its a bit more difficult to get and you need a special electrode for welding but its much better.
Nowadays propeller shafts are also often made from duplex to avoid the pitting sometimes seen on stainless shafts.
If yuo want to now everything about corrosion resistancy of stainless steel, have a look at:
http://www.smt.sandvik.com/sandvik/...D170514454C55319C12577A0003C4DFA?OpenDocument
But that's only for real material freaks!
Ad

There's a certain amount of misinformation in this post. The reason for using 316L is to prevent migration of chromium away from the heat affected zone, which happens when 316 is used. This phenomenon is known as sensitisation. In corrosive conditions the area of reduced chromium content will corrode preferentially. However, this has nothing to do with magnetism, which is dependent upon the nickel content.

Duplex stainless steel might well be better, but in fully aerated conditions 316 will be perfectly OK. Avoid crevices by bedding the component on a decent mastic, including around any nuts and bolts, and you will not have a problem.

316L has exactly the same corrosion resistance as 316, the only difference between them being a very small amount of carbon.

My 316 shaft is eight years old, all of that spent in the Mediterranean. It is not pitted at all. Its predecessor was around 17 years old and was also not pitted. I have seen a couple of 316 shafts that suffered crevice corrosion under the cutless bearing but these were exceptional cases.