Why some Inconel is magnetic and others are not 2

[MATE45]

So Im fairly new here, and have a question with regards to why some inconel is magnetic and some others are not. Im assuming right now that it has to relate to the elements in the material. Inconel 600 series a Ni-Cr is magnetic, where a 718 series is Ni-Fe is non-magnetic.Thanks in advance.

 

[EdStainless]

First, 600 is nonmagnetic. If you think that you have a piece that is, then it is marked wrong.

Second, the tradename "Inconel" is a marketing device, it does not mean anything metallurgically. The alloys that are marketed under that general name are all high in Ni content, but very different in alloy elements and properties.

the name is used to distinguish Ni based alloys from Fe based alloys (called Incoloy). Both were originally trademarks of the International Nickel Company, later know as INCO, and now held by Special Metals, a division of PCP.

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[metengr]

If you have some time, try the web site below. One of the best in terms of technical information on various Fe-Ni-Cr alloys.

http://www.specialmetals.com/

 

[MATE45]

Going from the ASM Metals Handbook ninth edition I have, Inconel 600 has magnetic permability of 1.010 @ field strength of 15.9 KA/m with a currie temp @ -124 C. If it has a currie temprature how is not magnetic? I am aware that Inconel is a "marketing name" and that these are high Ni content superalloys. My initial assumption was that Cr in one type of the alloy vs Fe type in the other alloy would make a magnetic difference. Maybe i should rephrase the question to what makes a high Nickel alloy mangnetic and what makes one not magnetic.

 

[TVP]

MATE45,

I think EdStainless was trying to indicate that Alloy 600 is not ferromagnetic at ambient/room temperature and above. Unless your application is intended for cryogenic applications, then neither Alloy 600 nor Alloy 718 should exhibit magnetism. If you review the datasheets from Special Metals or one of the other producers of these alloys, they should reference the Curie point/temperature. I am not expert enough in metallurgy of nickel-based alloys to address how alloy content affects magnetism, but I would assume for a given microstructure (single-phase solid solution, multi-phase precipitated structure, etc.) the higher the Fe content the higher the Curie temperature.

 

[lgearhart]

MATE45: OK, the Curie temp is minus 192 F, so that means that Inconel 600 sitting in liquid nitrogen will likely attract a magnet. Same with Inconel 718, since Special Metals says it has a Curie temp of minus 170 F.

You'll also note the permeability you mentioned was 1.010, hardly over 1. That's at room temperature, and means that you get an increase in an applied magnetic field of 1%, not really enough to be ferromagnetic.

 

[MATE45]

I guess what i was looking for in an answer was more along the lines of maybe crystal structure or a explaination of how 2 materials rather quite similar could have different magnetic properties. But like anything else in metallurgy there is always the exception.

although this is off topic it still some what relates. The difference in magnetic properties is due to the structure of the material at room temperature. The structure is due to (in most cases) the chemistry and thermal processing of the steel.

Nickel is added to stainless steel to stabilize the austenitic phase. Most 300 series stainless steels contain sufficient amounts of nickel so the structure is austenitic at room temperatures.

300 series stainless steel (also know as Austenitic stainless steel) has "higher" nickel contents and consists of a single phase, (austenite) which is non-magnetic.

Most 400 stainless steels have "lower" nickel contents and have a martensitic or ferritic structure (both of which are magnetic).

Does the above apply as well to high nickel alloys. Is it possible that 2 pieces of 718 Alloy based on how it was processed could create/ destroy magnetisim.

 

[EdStainless]

Well it is related to crystal structure and electron spin alingemnt. There are some alloys that when you age them become slightly magnetic.
Most of these high Ni alloys will not become magnetic under any room temperature. If they do it usually means serious trouble, such as carborization.

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[strokersix]

But isn't there also a work induced magnetisim in austenitic stainless? Does this effect occur in the original poster's alloy question?

 

[MATE45]

Here is the response i got back from special metals:

INCONEL alloys should not exhibit magnetic properties ... they are non-magnetic. The only time I have known of INCONEL alloys to be magnetic is after long exposure in a severely carburizing atmosphere such that all the surface chromium is combined with carbon and effectively removed from the metallic matrix. But under normal conditions, all INCONEL alloys should be non-magnetic. If you have a product that is supposed to an INCONEL product and it is magnetic, I would suggest that you have it analyzed to verify its identity.

 

[EdStainless]

In these high Ni alloys you can cold work them considerably and not get any strain induced transformation. Even high tensile spring wire in these grades only has a perm of 1.02 or so.
The phase stability in cold work is related to their high temp phase stability.

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[JamesCKelly]

The Creator made three pure metals to be magnetic at room temperature. One is iron, which we all know, the second is nickel (older Canadian coins are very high Ni, and are magnetic. The current production is lower in Ni, and non-magnetic at room temperature). The third alloy is cobalt.
If one starts with pure nickel, and progressively alloys it with more and more chromium, it goes from being magnetic at room temperature, to being non-magnetic.
The same is true of cobalt. Common high temperature alloys based on cobalt, e.g. Haynes 188 & 556, L-605, have sufficient Cr in them to be non-magnetic at room temperature.
The most common reason for a piece of Inconel alloy 600 (UNS N06600, nominal 76%Ni 15.5%Cr bal Fe) to be magnetic is, as has been said, that it has been carburized. Carburization forms chromium carbides, removing Cr from the matrix. Once enough Cr is occupied being a carbide, the Ni matrix becomes low enough in Cr to be magnetic at room temperature.
Hot corrosion from various alkali metal salts also can make Inconel 600 magnetic. In this case the Cr selectively oxidizes and not only becomes a chromium oxide, or alkali metal chromate, it may be physically removed from the chunk of metal in question.
The same thing happens to the common heat resistant alloy RA330 (UNS N08830), mominal 35%Ni 19%Cr 1.2%Si 1.5%Mn balance iron. It has long been practice amongst heat treaters to check their alloy fixturing with a pocket magnet. If the metal is strongly magnetic, it cannot be weld repaired, for one reason or another. Carburization makes the metal too brittle to weld without base metal cracking. Hot salt corrosion physically penetrates the metal with salt & makes weld repair both unpleasant and ineffective. If magnetic, just sell the fixture as scrap & buy a new one.
I never did catch why you cared that your 600 alloy was magnetic, or what it had been used for?

 

[MATE45]

I was just using Inconel 600 as an example after flipping through the ASM handbooks and comparing it to what we use (Inconel 718). I personally think this has become a good discussion and very informative. I was trying to find some explanation on the Cr carbide being pulled out and carburization making it magnetic, but your thoughts James would make sence. Take a magnetic material, alloy it to the point that is not magnetic, then having the alloy removed during a processing application (carburization) making the material magnetic again.

 

[ccccc]

We had several pumps ready for transport. The material was Monel Alloy 400 for all pumps. (63 Nickel, 28 Copper) At final inspection several pumps were magnetic and several were not. So the whole lot was put on hold. The reason for this behavior was that some were standing in the sun and some were not. The Curie temperature of this material is 70 degrees F. At that temperature the magnetism switches just like that. Problem was solved and all pumps released.

 

[EdStainless]

cccc, Cast Monel? I can see why they were put on hold. A bit too much Fe and a poor anneal and you can have real problems.

Mate45, There are a number of the Ni based alloys that become slightly magnetic after aging because of the slight change in matrix chemistry and the associated shift in CP.
Most common alloying elements lower the CP of Fe and Ni. The only exceptions that I know of are Mn in Ni and V in Fe, these raise the CP.

There are a lot of common alloys and compounds that we think of as nonferromagnetic that at common cold temperatures are ferromagnetic. I have seen this mess up more than one application.

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[mshimko]

Nickel alloys are "variable" (I forget the technical term....), and may be weakly magnetic under some but not all conditions.

I had the following experience about 20 years ago when I was a construction superintendent at a shipyard:

The ship's Damage Control Assistent would go around with a small magnet to ensure ferrous fasteners were not used in the firemain system which had CuNi piping, bronze valve, and should have monel 400 NiCu fasteners.

He found his magnet had attraction to some of the fasteners, and got very upset, while I was amused. I convinced the yard to remove a few of these fasteners with the DCA standing there, I handed the fasteners to the DCA and had him re-check them with his magnet. There was no NO attraction. I had the fasteners reinstalled and retorqued, and they were now weakly attracted to the magnet.

Real story, and a very good example of stress affecting magnetic properties.