316 Stainless steel property question 8

[ca4suskyhawk]

Hi all, my company produces explosion proof boxes for hazardous areas. Recently, a customer asked for 4 boxes to be casted in 316 stainless steel. We just received them today but the boxes are attracted to magnet and seems to be a bit rusty too. My question is whether this is normal? I've always thought that 316 SS cannot be magnetised and wouldn't rust. If I use a magnet on 316 SS rods, they will not be attracted. Why?

Thanks for your time

 

[Metalguy]

The magnet is probably being attracted to the small amount of ferrite that is present in most 304/316 etc. castings and weldments. It usually reverts to non-mag. austenite during mechanical working.

 

[stanweld]

As Metalguy stated 316 Castings are chemically formulated to provide a mixed microstructure of ferrite (ferromagnetic)and austenite (non ferromagnetic)to preclude hot cracking and enhance castability, while wrought 316 SS products are generally formulated to be 100% austenite. Ferrite content of 316 castings are often in the range of 10% to 15%, which is permanent in the casting. General corrosion resistance of the casting will not be equivalent to that of properly solution treated wrought 316 SS product. It is assumed that your castings have been solution treated.

 

[EdStainless]

as Stan said, or it may just be a surface effect.
Have they been annealed? If not they should be.
Then they should be pickled afterward.


= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[mcguire]

This ferromagnetism probably won't go away with annealing since fairly long range diffusion of nickel is required to transform ferrite to austenite. That would mean hours at very high temperatures, say over 1200C. You're kind of stuck with the ferrite. It shouldn't be too inferior to the austenite in corrosion resistance, but since this alloy doesn't have nitrogen as a true duplex would, the ferrite is the leaner phase from a PREN point of view.
Amunrao has experience with this. Maybe he'll chime in.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[arunmrao]

Mcguire! you tempted me into coming in. How were the castings shot blast? If steel shots were used or if the iron dust from the fettling shop contaminate the surface. (Probable cause for rust appearing on the surface)

Perhaps pickling the castings might improve the situation. What is the carbon in this grade? Also was it processed as 316 or CF8M. There is a difference in Cr and Ni limits in these alloys.

 

[ca4suskyhawk]

Hi guys, the following is what I managed to obtain regarding the properties of the castings.

C Mn Si S P Ni Cr
0.05 1.01 1.05 0.011 0.025 10.8 18.8

Mo Cu Nb Ti
2.4 0.09 0.070 0.020

(Weight percentage by optical emission spectrometry)

Regards

 

[EdStainless]

Not a bad chem. Good Mo and Cr. Typical for a casting hte Si is high and Mn low. FN should be 18-21. Higher than I like, but again, not bad for a casting.

I still suspect surface contamination. A little oxide, a little residue from mold binders, some Fe from the shot blast. A pickle will remove it all.

The ferrite is a 'leaner' phase in terms of corrosion resistance. In this material the ferrite composition will be similar to a 409 or 410 ferritic stainless. This whouldnt be a problem in general exposure. This is a serious issue in exposure to acidic environments.

In the future, getting this alloy with some N would help. If the N was 0.05% the FN would be 7-8. This level could be removen in a reasnable anneal cycle. For any of these grades there is no benifit to annealing at temps above 2150F. Grain growth is bad enough there. But when annealing temp is your friend. Roughly speaking the following are equivalent: 30min at 2150F; 1.25hr at 2100F; 3hr at 2050F; and 8hr at 2000F.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[Metalguy]

"The ferrite is a 'leaner' phase in terms of corrosion resistance."

This statement depends on the local corrosive environment. In strong oxidizing conditions (such as created by iron-oxidizing bacteria, etc) the Cr-rich ferrite phase is far more resistant than the Ni-rich austenite.

I have been involved with at least one case where the aust. was dissolved, leaving the ferrite untouched.

 

[EdStainless]

In a base chemistry like this the ferrite is pretty weak. I could see it maybe holding up better in a caustic environment. But at low pH the lack of Mo will be its downfall.
I have seen many failures in slightly acidic conditions due to high ferrite levels.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[Metalguy]

Ed,
What you may be seeing is the loss of corrosion resistance in the austenite caused by high ferrite levels. All that ferrite removes the oxidation-resistant Cr from the aust. Mo doesn't help against oxid. conditions. It really depends on just what the local environ. is--oxid. or red.

 

[arcstrike]

ka4huskyhawk,

In your manufacturing process, do you pickle the finished boxes and then fresh air passivate?

316L contains upto about 68% Fe and I calculate yours to contain approx 65%. If the in house QC did not wrap the boxes in polythene after P & P and subsequently they have been exposed to moisture of some sort then surface discolouration could result.

"Rouging" of stainless steel is the result of the formation of iron oxide, hydroxide or carbonate either from external sources or from a degredation of the passive layer.

Color variation is a result of the oxide/hydroxide/
carbonate type and variations in the water of hydration associated with the molecule. These colors range from orange to red to black.

Bright red streaks on the surface of stainless steel after it has been exposed to an electrolyte usually are the result of iron contamination from dragging carbon steel over the surface, from welding carbon steel to the stainless steel, from iron contaminated grinding wheels, steel wire brushes or from blast abbrasive contaminated with fe particles.

In untreated water the discoloration may be the result of oxidation of ferrous bicarbonate in the water forming a brownish red deposit. This oxidation may be from added chlorine or dissolved oxygen.

As far as the magnetism problem goes, will this be detrimental to the castings service? if not it is not really a problem, If so and the boxes are small enough, you could pass them through a DC coil a few times to reduce / eliminate the residual magnetism.

hope this helps,

K. Madden,
Tech Support Eng,
kevin.madden@oceanteam.com.mx

 

[Metalguy]

Arcstrike,
Magnetism isn't his concern-it's the fact that ferrite is magnetic, and the amount of Fe in the alloy isn't a factor in this case-it's the phases which are present that count.

Also, an AC coil is used to demag., not DC.

 

[arcstrike]

Metalguy,

Yes you are quite correct it was my mistake, an AC coil is widely used for demagnetisation, a simple mistake.

Someone else in the thread had stated that he probably had 10 - 15% Fe, that was my point,

Therefore 65% Fe (in same phase) as opposed to 15% Fe in any alloy = stronger likelihood of magnetic permeability!!

You stated that "the magnet was probably being attracted to the SMALL amount of ferrite in most 304/316 etc"
Go to the stock room and put a magnet on some 316L Sch 10 linepipe - does it stick? I think not.

now perform a butt-weld in the same pipe using all DC-, does the magnet stick now?? please explain.

you also state that "Mo does not help against oxidisation conditions" if that is so, why is upto 3% Mo added to 316L, wehat purpose does it serve???

Q.E.D

P.S Why not put your name & company to your posts, don`t be shy!!!

 

[unclesyd]

I don't think you will remove the magnetism until you raise the temperature above 1500°F hold for a few minutes and let it cool.
On some 316 thick SS casting we have had to go to the quench anneal from 2100°F to fully remove the magnetism.

 

[Metalguy]

Arcstrike,
Fe in its austenitic phase is non-mag, while in its delta ferrite phase it is. They can both be 100% Fe-that is not the factor.

Wrought 304/316 et.al. usually has no delta ferrite, while most castings and weld metal deposits do. THAT is what is attracting the magnet. Elementary.

Mo is added primarily for resistance to chloride pitting. It is the Cr that gives oxidation resistance. 316 has 2-3 % Mo, while 317 has 3-4%. When the Mo reaches 6% the stainless resists sea water pretty well.

 

[EdStainless]

The important fact with the current problem is will the boxes not rust. Surface cleanliness is the biggest factor, any free iron on the surface and they will bleed. Anneal and pickle will get the best possible performance out of the material. How good that will be is another issue.

Don't confuse magnetic with magnetized. Two different issues. If the material is ferromagnetic then a magnet will be attracted to it. As has been said already, in a nominally austenitic alloy (non-magnetic) this is because of the delta ferrite (ferromagnetic) in the structure.
If you have pipe out on your rack I'll bet that a magnet will stick to the welds. Pipe is a commodity and most of it is under-annealed. Some of it may also be manetized, take a paperclip and see. If they Eddy Current tested it it may still be magnetized. This is a real pain if you are going to machine the material. In some thick material there are a lot of things other than ferrite in the structure and some of them require serious annealing to remove.

The key to the corrosion resistance of stainless is the existance of the passive oxide film and the resistance of the film to breakdown. Any composition or structure that causes non-uniformity in the passive layer is bad. Some areas will be weaker than others. When the weak areas breakdown there may be enough galvanic difference to start driving pitting/crevice corrosion. The critical issue in stainless corrosion isn't general attack, it is pitting resistance, and that is almost always related to low pH and chlorides.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[ca4suskyhawk]

Wow, thanks for everybodies help in my clarification. I had learnt much more about SS316 than I already know. Ok, one of the main concerns when I asked this question is that the customer may decide to try a magnetic test on the boxes to see the composition, so right now it will fail the test. At least I have explanation for them if they asked. Now the most important concern of mine is that since it will corrode as I had seen it myself, what will be the extent of it since it is used in a highly corrosive environment and it had to resist up to 10 bar of internal pressure should an ignition occur. Worst case scenario will be around 30 bar if it manages to suck in external air and fuel and blew them all out. Anyway, these boxes are Ex d 'Flameproof" boxes. There's just too much info to digest, I'll read thru' all the post slowly and surely you guys would have answered my questions and satisfy my concerns.

Thanks again

Regards

 

[stanweld]

ca4suskyhawk,
While the "magnet test" cannot verify chemistry, it can be used to verify minimum/maximum ferrite content when specifying measurement with a Magna/Severn Gauge. It would appear that you need to provide an appropriate purchase specification for these castings. To assure corrosion resistance, specify that the castings be supplied in the solution annealed condition and that they be pickled and passivated. You may also wish to specify a maximum ferrite content of 10% as measured by the gauges listed above or by "Fisherscope" from Fisher Technology or equivalent. Finally you should specify that the castings be supported by Materials Test Reports to a recognized standard like ASTM A-351 Grade CF8M or equivalent.