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Is your steel frame non-magnetic?(13 posts)

Is your steel frame non-magnetic?Bruno S
Apr 21, 2001 1:28 PM
The best stainless steels are non-magnetic. Has anyone tried to stick a magnet to an expensive steel frame to check if it is really made with the good stuff?
re: Is your steel frame non-magnetic?ha ha
Apr 21, 2001 1:45 PM
Bruno says, "I read this on cycling forum, and now I'm running to roadbikereview to see if it's true... is it?"
It was me that posted it in cyclingforum...Bruno S
Apr 21, 2001 3:01 PM
Yesterday I replied to a post about non-magentic steel in cycling forum. After reading about someone conplaining that a magnet would not stick to its frame, I looked in a book about materials that I used in college. It turns out that good stainless steels are non-magnetic. Now I am running a survey here, the forum that I like, to see if in general good frames are made with austenitic stainsteel. Maybe it is something known through out the industry but I hadn't heard of this before.
their is magnetic and no-magnetic stainless but,mondo mike
Apr 21, 2001 3:36 PM
that doesn't mean that the no-magnetic is better. the 300 series
is no-magnetic, the 400 series is magnetic. the only big differance
is that the magnetic stainless rusts and the non-magnetic doesn't.

yes there are other things, like some series can be hardned to a
higher rockwell than others. there are even special typs of stainless,
like the 445c series that is used for core rods in nukler reactors.

i think most high end bikes are made out of vandium, or out of a
spring steel. very tuff, but has give to it. like valve springs in
your car.

what was the frame in question? are you sure its not aluminum or
carbon. could be one of them old raleigh's with carbon, aluminum,
or ti tubes painted.

peace
their is magnetic and no-magnetic stainless but,Bruno S
Apr 21, 2001 3:44 PM
The frame was made with columbus tubing.
uhhh.... not quiteDrD
Apr 21, 2001 7:29 PM
The tendency for a stainless to show a surface rust is a function of the chromium content - the vast majority of stainlesses in use today don't generally flirt with the 12% or so required to render the material "stainless" (sort of a misnomer - you have sufficient Cr to form a protective chomium oxide layer, much like the aluminum oxide layer on aluminum - thereby preventing iron oxidation (i.e., the formation of a rust stain))

Whether or not a stainless is magnetic is determined by it's crystal structure - ferritic stainlesses (body centered cubic structure), such as the 400 series, are magnetic. Austenitic stainlesses, on the other hand, are non-magnetic, and have a face centered cubic structure - this is achieved through large additions of austenite stabilizers, such as nickel. As a rule of thumb, austenitics will be more ductile than ferritics.

You are correct that there are many different types of stainless steels.

Most high end bikes are not made out of vanadium (though the steel alloy may contain some as a minor alloying addition) - nor are they made from a spring steel. The steel alloys used for bikes have a fairly similar modulus (yield strength varies, though) - not sure how "tuff, but has give to it" translates into actual material properties.

For more info - pick up a decent metallurgy text - Steels: heat treatment and processing principles (by Krauss) is a good place to start...
drd , don't know nothing!steelman
Apr 22, 2001 1:20 PM
i think, mr know it all, if you look at a colnago
master x lite. you will find that it is made out of
" chromemolybdenum-vandium " so i guess you don't
know it all! columbus dt 15v tubing.
my old colnago was made out of a vanadium alloymondo mike
Apr 22, 2001 1:31 PM
just didn't remember the chromemolybdenum part. i use 17-4
prehard to make axles for my rear hubs on my azonic ds-1 that
i race on the bmx track as a 2 speed.
Oh, please...DrD
Apr 22, 2001 7:44 PM
Ok mr "steelman", it's pretty clear you basically have no idea what you are talking about, but are at least capable of restating what is written on the product label (which is good - everyone needs at least one skill) - a metallurgist, however, you are not.

Chromoly or chromemolybdenum steels are slang terms referring to iron based materials with small additions of molybdenum (0.2 to 0.5% or so) and small additions of chromium (0.3 to 1.25% or so) - other additions often include carbon, nickel and manganese, in similar concentrations - the balance (that would be ~90%+) is iron (typical alloys called chromoly are things like 4130 or 4340). High strength steels which include vanadium (your "chromemolybdenum-vanadium") generally have less than half a percent of the addition - they are still ~90+% iron.

I'm guessing you basically have no idea why the alloys are designed that way, so here's a bit of information for you about vanadium - Vanadium is in general added to high strength steels for two reasons - the first is as a carbide stabilizer, and the second is to refine the prior-austenite grain structure (the latter being a result of the former) - resulting in an improvement in strength and toughness (in the resistance to crack propogation sense of the word) - so the vanadium bearing alloys will (if tempered properly) be stronger and have a higher plain strain fracture toughness. They are, however, IRON alloys, not vanadium alloys (you generally use the majority material to determine what sort of alloy it is - esp. when there is 180 to 190x more of that material present than the alloying addition...)
Pardon?DrD
Apr 21, 2001 7:35 PM
Please explain why the "best" stainless steels are non-magnetic... in terms of choosing alloys for a bike application, one could argue that many of the "best" stainless steels would be ferritic in nature - such as precipitation hardening alloys like PH17-4 (used in pedal axles by Speedplay, as an example). Ferritics are magnetic.

Steel frames are generally not stainless steel, anyway - Reynolds 853 isn't stainless, nor are many of the common columbus or deda alloys...
Pardon?Bruno S
Apr 21, 2001 8:25 PM
I'm looking at this from a point of view of a text book, not from experience.

According to my engineering materials book:

"Austenitic steels have a number of advantages over ferritic. They are
tougher and more ductile. They can be formed more easily by stretching and deep drawing and have better creep properties as well. The only draw back is that they are more difficult to machine."

To make a bike frame with today's complex shapes (triple butted, curved chainstays, etc) an austenitic steel would seem ideal. A stainless steel would also seem better than a non-stainless because it will not rust.

So, if you have a frame that is non-magnetic you know its made from a stainless with properties that are good for a bike frame and it will not rust. If your frame is magnetic it could also be good or it may not.

Also, I think that that the requirements for a pedal axel material are very different to those of the tubes of the frame. Some materials maybe good for both applications but the fact that one is good for one application dosen't mean it will be good in the other.

BTW, I ride aluminium!
Yield strength...DrD
Apr 22, 2001 7:17 PM
You are certainly correct about the processing issues with the two alloy classes - however, there is more to it than that.

The elastic properties of a ferritic and austenitic are fairly close overall (i.e., same youngs modulus - the slope of the stress/strain curve prior to plastic, or permanent, deformation) - so up until a sufficient stress is reached to permanently deform the material, the austenitic looks great. (this is deformation sort of like bending a clothes hanger at stresses such that it springs back into shape)

Unfortunately, the main "problem" with a material like an austenitic stainless is that it has a considerably lower yield strength than it's ferritic counterparts (the exact difference is a function of alloy and temper, but figure a factor of 2, on average) - as a result, the austenitic material will bend (or dent, in the case of a tube) under a lower stress than the stronger ferritic. (this is bending the clothes hanger and having it take a permanent bend - in this case, it will take alot more stress to bend the ferritic than the austenitic)

So what does this mean for a bike frame - you will need more material to get the job done - in other words, thicker tube walls - and a much heavier frame. That's the whole reason why everyone is chasing after these very high strength steels - you can do more with less material - you get a frame which is just as strong, but weighs alot less.

So - from a material properties point of view, the austenitic is a poor choice as a frame material.

In any event, stainless steel bikes are few and far between - I believe Rhygin made some a while back, but nothing comes to mind along the lines of new framesets...

The pedal axle material is actually very nice to work with it - because it is rather easy to deform and machine in the un-tempered state - once you have it in more or less final form, you can heat treat it to precipitation harden the material - so you work it when it's soft, then make it hard when it's done (there are other issues, such as deformation during heat treatment, precipitation during welding, etc. which, while certainly surmountable, make it less cost effective to use such a material for a bicycle tubeset)
Yield strength...mondo mike
Apr 23, 2001 5:41 AM
being in the tool and gage feld for 10 years, i think DrD
knows what he is talking about. i have done some heattreat
in my time, but spend more time on cnc and wire edm.

peace