Deep drawing does two things to a solenoid housing at once: it gives the part its shape, and it damages the magnetic properties the part was specified for. Magnetic annealing is the process step that buys those properties back.
What does magnetic annealing do?
Magnetic annealing heats a formed component to a controlled temperature in a controlled atmosphere and cools it at a defined rate, relieving the cold work introduced by drawing, bending or machining. In soft magnetic materials this raises permeability and lowers coercivity, so the component behaves closer to the base material's magnetic specification. The cycle must be matched to the material grade and to the amount of deformation: too little relief leaves the magnetics degraded, while an incorrect cycle can cause scaling, distortion, grain growth or a change in mechanical properties. Because bulk heat treatment is often outsourced, the furnace process control, atmosphere and traceability of each batch are part of the supplier's quality system.
Why the forming operation degrades magnetics
Cold work introduces dislocations that pin magnetic domain walls. Permeability falls, coercivity rises, and residual magnetism increases — which in a solenoid means slower release when the current is switched off. The effect scales with the amount of deformation, so parts drawn further from the same material end up worse than parts drawn less. That is why the material specification and the process route have to be decided together rather than in sequence.
How to specify magnetic requirements so they can be produced
- Name the parameter: permeability at a defined field strength, maximum coercivity, or saturation flux density — not "soft magnetic material"
- Say where it applies: the finished, annealed part, or the incoming material
- Define the measurement: if you intend to verify it, agree the method and the sample before tooling
- State the sequence: whether machining, plating or welding happens after annealing, because each reintroduces local cold work or heat
- Accept the limits: a deep drawn thin wall cannot match the magnetic properties of a fully annealed solid ring
The process chain that actually delivers the specification
- The drawing states the magnetic parameter and the measurement method
- The material grade is confirmed by certificate and, where required, by test
- The forming route is designed so that the final anneal can relieve the deformation
- Annealing is performed to a documented cycle with atmosphere control and batch traceability
- Secondary operations are sequenced so they do not undo the anneal
- Verification happens on a defined sample, and the record travels with the lot
Key point
"Soft magnetic material" is not a specification. A number, a measurement method and a place in the process chain are — and those three make the requirement producible and auditable.
Frequently asked questions
Can a solenoid housing meet a magnetic specification without annealing?
Rarely. Forming introduces cold work that raises coercivity, so where magnetics are specified, annealing is normally part of the process route.
Who performs the annealing?
Bulk heat treatment is commonly outsourced in the stamping industry. What matters is furnace control, atmosphere, batch traceability and how the subcontractor is managed by the supplier.
Does plating after annealing affect the magnetics?
The plating process itself generally does not, but any machining or handling that cold works the material afterwards will reintroduce local degradation.