Relationship between grain size and magnetism

[EngineerDave]

They say you learn something new everyday.

Today someone brought in a magnetic iron material. They stated that the grain size affects the magnetic properties.

I did some quick searches on this (found a girls presentation that was incomplete on this!), but was wondering if any of you knew off the top of your head what the general relationship is between grain size and magnetism in magnetic irons if there is any!

 

[hacksaw]

Yes and no. There are so many factors involved that a simple answer is not possible. In some cases extremely small grain-size helps, in other cases large grain size helps. If it is rolled sheet, or contains Si or Molly, or Ni, etc, then other factors come into play. The physical form of your specimen also plays a role as does its degree of heat treatment(and how it was heat treated), machining,etc.

Ultimately the answer depends on the intended use of the material.

How's that for a straight answer...

 

[EngineerDave]

Thanks I knew it probably wasn't something that has a direct correlation.

At the company we also used to have a device for measuring magnetic field strength but it is no longer available. This would at least give some data.

 

[mcguire]

Finer grain size makes a ferrous material harder magneticly. This is because the domains can move easily under changing magnetic fields.

 

[hacksaw]

At the risk of hijacking the original inquiry:
for generally soft materials, the relationship between grain size and magnetic softness is not monotonic and is characterized by a critical grain size for which hardness(magnetic) is a maximum. Below that size (more or less) the material can be softer. The actual result is a function of many factors.

With an unknown sample of iron, about all you can say is that if it is heated to redness and allowed to cool slowly,it will be soft more-or-less, unless it is rod shaped and magnetized during cooling.

Iron is such an extremely complex material a simple answer is not possible: the amount of graphite and its form, the minor allowing components, the deoxidizer used, purity, etc. have significant roles.

Extremely pure iron, especially single crystal iron, cut along the proper crystallographic planes, in the shape of a square window can be very soft if properly heat treated.

 

[hacksaw]

as a post script,

Mcguire you are correct, for soft iron where the grain sizes and other pinning distances are fairly sizeable to begin with. In that case magnetic softness correlates with increasing grain size.