One old engineer's thoughts on magnet testing of steels-
The heart of the issue is the crystal structure of the steel. Heat steel above
about 1300 degF (higher for the low carbon steels) and the structure turns to
what we refer to as "Austenitic". Steel with this structure is non-magnetic.
This applies to all steels, 300 & 400 stainless, low carbon (like 1020, 4130,
4140 chrome moly, 4340, 8620, etc.) tool steels, high carbon steels (0.9-1.0%
carbon for blades, files, springs), "PH" stainless steels.
Steel that has other crystal structure forms like "Ferritic" or "Martensitic"
are magnetic.
The key here is what you do to the steel after you heat it up. For most
steels slow cooling, like cooling down in a furnace or covered with sand
allows the soft magnetic Ferritic crystal form to develop. Fast cooling
(quenching) forms hard magnetic Martensite. More carbon gives more complete
Martensite formation and on and on------til you can fill the bed of a pickup
with books on metallurgy. So what about Stainless Steel?
Well, when there is more than about 6-7% nickel in the stainless steel Ferrite
and Martensite crystals cannot form no matter how fast you cool the steel. It
all stays as corrosion resistant (and nonmagnetic) Austenite crystals. This
is why you have the 300 series family of 8%-16% nickel stainless steels.
There is only one way to make 300 series stainless steel magnetic and increase
it's strength and hardness. Cold working!! Heat treating doesn't help here.
That's why stainless springs and lockwashers are slightly magnetic. 400
series and the PH types (like 17-4ph) don't have enough nickel to stay
austenitic as they cool. They are magnetic. Fortunately they aren't very
common in the scrap piles (or small machine shop stock racks) most of us pick
our material out of. And note-most young machinists today don't know as much
about metallurgy as their forbears did.
Ed Weldon
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