Removing magnetism from 304 Stainless Steel
A discussion started in 2007 & continuing through 2017(2007)
I need to know how to remove any magnetism from 304 Stainless Steel. I've machined a 304 stainless steel disk that is 6" in diameter. I put it into a lathe and removed a 3" hole in the center and drilled four 1/2" holes on the solid ring that was remaining. This was for a vacuum system with cryogenic feeds. Once I powered up the electromagnet in the lab to 0.8 Tesla (the Earth's magnetic field is about 0.00005 Tesla) the part was very strongly attracted to the magnet.
Here is the issue, I need to create a hole in a 304 stainless steel disk. This disk will be the vacuum seal for the sample mount in a cryogenic chamber that can produce a field of 10 Tesla. The field is created with a superconducting electromagnet. If there is anything magnetic with in 1.5 meters of the magnetic it will quench (very bad).
So how do I remove the magnetism from machined/welded 304 stainless steel? I know a lot of articles on here say that 304 is barely magnetic when machined, but I'm working with very high fields, and yes machines stainless is very magnetic.
Student - Mad, Wisconsin
? Why are you concerned about removing the magnetism? Wouldn't it be better to change material? If your steel is magnetic, will it not magnetize again in service under such high fields?Guillermo Marrufo
Monterrey, NL, Mexico
Q. It is standard to use 304 Stainless Steel when developing high vacuum system, I need to go to 10^-7 Torr. Flanges and other machined parts(304 SS) can be purchased with out being attracted by a magnetic field. I want to know the process to remove magnetism. I've read that 304 SS may need to be heat treated to turn BCC into FCC. If this is what's done then what are the details. 304 SS is machinable and initially non-magnetic.Dustin Kreft [returning]
- Madison, Wisconsin
A. Dustin, when you machine the 304 stainless steel, you can work harden it. The change in structure that the material experiences can make it ferromagnetic, such that it'll be unsuitable for your applications. I think you're on the right track trying to anneal it.
We generally use 1950 F for about 20 minutes of soak time. We also figure it takes about an hour for a one inch thick part to heat up, maybe two hours for a two inch thick section to heat up, etc. So the general, rule of thumb is to heat it for one hour per inch of thickness.
Cooling is another question. We commonly use a water quench, and then take steps to deal with the distortion. The reason you want to cool fast is that 304 MIGHT be susceptible to sensitization, wherein the carbon in the stainless steel combines with chromium, and this takes place primarily at the grain boundaries. With the grain boundaries depleted of chromium, the corrosion resistance is decreased, and you can get significant corrosive attack of a sensitized part, since the corrosion is concentrated along the grains. Cooling fast through the sensitization range keeps the carbon and chromium separate.
If you use 304L, you won't have that problem, since it has low carbon to start with.
1950 °F is fairly hot, so if you heat your part to that temperature, you'll get scaling and discoloration if you do it in air. Vacuum of 10 to the minus 3 Torr works well, as do protective atmospheres of argon, hydrogen, and others.
If you send your parts to a heat treater, I'd recommend using one that will certify the work in accordance with AMS 2759/4, a specification used in the aerospace industry for heat treating of austenitic stainless steel.
September 3, 2008
I'm doing machining of SS 304 grade materials and some parts are magnetic after machining and some are not. I would like to know whether It will stick to magnet after machining or not. Could you please clarify my doubt?
Product Engineer - Coimbatore, India
September , 2008
A. Hi, Shanmugam. Sorry but I don't know how to further clarify this :-)
You have proven to yourself through experience that machining can render SS 304 parts slightly magnetic, and you are reading here that it can do so as well. The practical answer is that SS 304 is not strongly magnetic, but machining and cold working can make it somewhat magnetic. If you need a quantitative relationship between degree of magnetism vs. type & degree of machining, sorry, it's beyond my knowledge. Good luck.
Ted Mooney, P.E.
Pine Beach, New Jersey
May 14, 2012
I just tried two ss 316L plates for welding, but after the welding the material shows magnetism (only for welded space) what is the reason? How will I prevent it? Is it possible or not? Welding rods used are ss 316L.
- Doha, Qatar
January 8, 2013
Welding consumables for 316L materials are designed to produce a small amount of ferrite in the deposited weld to avoid weld hot cracking, this is why the weld shows some magnetism i.e. the deposited weld metal does not contain a 100% austenite phase. It is normal to check the ferrite level of completed 316L welds to ensure the ferrite % is in the range 3% to 8% in order to give adequate protection from hot cracking and required properties at low temperature.
October 12, 2017
Q. I'm a forger in India and I have a little question to ask. I had forged 304L round bar and made a sheet ring from it.
But now I'm a little tense: the material is lightly catching magnet and I don't know how to remove magnetism from it.
Can you please help me out of it
October 13, 2017
A. Hi Mahendra!
As our partner Lee Gearhart says, you can heat treat AISI 304 steel to regain its austenite structure. This parameters work (1950 °F = 1065 °C, 20 minutes, then quench in water), and you can rotofinish later with some brighteners to reach something aesthethically "correct" finish for SS304.
Be aware that this WILL lower the part hardness (for SS304 i think final value will be like 80 HRB) and could modify the part dimensions, so please make some tests before treating the whole batch.
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina
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