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430FR "Solenoid Quality" stainless steel rod is specifically formulated and treated to be used as the inner shaft of a linear solenoid.

Solenoids are, at heart, simple machines. You can make one yourself using copper wire, a drinking straw, a battery, and any magnetic metal. You can even use an ordinary nail for the shaft (or armature, as it is known). Coil up the wire around the straw, put the nail inside and attach the two loose ends of the wire to a battery. That's it. Watch the nail move in and out of the straw as the electrified wire creates a magnetic field that attracts or repels the shaft.


Just as all digital data is stored and transmitted in ones and zeroes, ons and offs, linear solenoids begin with such simplicity. The armature is either moving in or out based on whether or not current runs through the wire. It moves in one direction or another based on the polarity of the magnetic force. Imagine the possibilities of that simple motion, though. Digital data can be used simply to tell you what time it is, or it can be used to beat humans on 'Jeopardy'. Solenoids can be used to lock the door of your car, or they can be used to control a person's blood flow through a kidney dialysis machine. Or to play beautiful music on the new Steinway Spirio grand piano.

Any magnetic metal will work in a solenoid for demonstration purposes, but for a real-world application the choice of metal alloy is very important. 430F and 430FR stainless steels are used most often in the solenoids we use in our daily lives.

The ASTM A 838 standard covers "free-machining ferritic stainless soft magnetic alloy produced or supplied expressly in cold-finished bar form for use in magnetic cores and other parts requiring a high permeability, low-coercivity stainless steel".1 The spec defines two chemical compostions as "Alloy Type 1" and "Alloy Type 2". Type 1 corresponds to 430F and Type 2, 430FR. Generally our customers ask for "430F" or "430FR Solenoid Quality" and then refer to the ASTM A838 type. It's interesting, though, that A838 does not specifically name one type or another as 430FR. When ordering material, it's best to insure you specify the 'type' and also, the 'grade' within the type. 'Grade' refers to the annealing and drawing processes. Both have an important effect on the consistency of magnetic properties.

430FR has became the 'go to' alloy of the two. It's increased silicon content makes it slightly harder and more able to withstand the repeated impact solenoiods are subject to. When properly annealed, at the mill or after your parts are machined, it has very stable bar to bar magnetic properties. Combined with it's ease of machining, these characteristics have made 430FR the alloy we've chosen to stock for job shops that are machining solenoids.

Our 430FR round bar is certified to ASTM A 838, Alloy Type 2 Grade 1. It is produced by the finest DFARs compliant mills, including Carpenter and Schmolz + Bickenbach.

If it's 430F that you need, we also stock a good range of round bar sizes.

Short Facts:

  • The higher percentage of silicon in 430FR (compared to 430F) ups the electrical resistivity, hence the 'R' in 430FR.
  • ASTM A 838 only covers material between .250" and 1.625" diameter. This is because anything larger or smaller cannot hold the specified low coercivity when mill annealed.
  • The specification also states that if material is supplied from different heat lots, it must not only be identified separately, but must be packaged separately.
  • To avoid confusion, it is stated in A838 that all purchase orders must refer to the Alloy Type and Grade when ordering.
  • There is 430FR and 430F in the metals distribution network that isn't certified to A 838, but instead covered by ASTM A 582. This material can be used for any purpose.
  • "Soft Magnetic" is the term used for metals that can be quickly magnetized and demagnetized. Metals that hold magnetism for a long period of time are called "Hard Magnetic".
  • The "F" in 430F and 430FR refers to 'free-machining'. It forms small chips when it's machined. These fall away from your tools so the waste metal doesn't clog up your machine.

1 ASTM International

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