316 Stainless steel property question
316 Stainless steel property question
(OP)
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
Thanks for your time





RE: 316 Stainless steel property question
RE: 316 Stainless steel property question
RE: 316 Stainless steel property question
Have they been annealed? If not they should be.
Then they should be pickled afterward.
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Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm
RE: 316 Stainless steel property question
Amunrao has experience with this. Maybe he'll chime in.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: 316 Stainless steel property question
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.
RE: 316 Stainless steel property question
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
RE: 316 Stainless steel property question
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
RE: 316 Stainless steel property question
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.
RE: 316 Stainless steel property question
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
RE: 316 Stainless steel property question
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.
RE: 316 Stainless steel property question
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
RE: 316 Stainless steel property question
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.
RE: 316 Stainless steel property question
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!!!
RE: 316 Stainless steel property question
On some 316 thick SS casting we have had to go to the quench anneal from 2100°F to fully remove the magnetism.
RE: 316 Stainless steel property question
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.
RE: 316 Stainless steel property question
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
RE: 316 Stainless steel property question
Thanks again
Regards
RE: 316 Stainless steel property question
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.