Engineers often specify 316L stainless steel specifically because it is non-magnetic — critical for MRI-compatible medical devices, sensitive electronic sensors, and magnetic proximity applications. But after MIM processing, some parts exhibit weak magnetic attraction. Why?
The short answer: MIM 316L is non-magnetic in the fully annealed, as-sintered condition. Slight magnetism can appear due to cold work (shot blasting, coining) or ferrite formation during sintering. What causes magnetism in MIM 316L:| Cause | Mechanism | How to Avoid |
|---|---|---|
| Cold work (shot blasting, tumbling) | Deforms austenite, forming strain-induced martensite (α'-martensite) | Use glass bead blasting instead of steel shot; solution anneal after cold work |
| Ferrite formation during sintering | Rapid cooling through solidification range creates δ-ferrite | Control cooling rate; ensure Ni content is on the high end of specification |
| Contamination with magnetic particles | Iron or steel particles embedded during blasting | Use stainless steel media; magnetic separation after finishing |
| Low Ni content | 316L at the low end of Ni spec (10-10.5%) is less stable | Specify mid-range Ni content (11-13%) for critical non-magnetic applications |
| Condition | Magnetic Permeability (μr) | Meets Medical Spec? |
|---|---|---|
| As-sintered, no post-processing | 1.002-1.005 | Yes (μr < 1.01 for MRI compatibility) |
| After aggressive shot blasting | 1.01-1.10 | Possibly exceeds limit |
| After coining or sizing | 1.005-1.02 | Marginal |
| After solution annealing (1050°C + rapid cool) | 1.002-1.003 | Yes |
| Incorrectly processed (high ferrite) | 1.05-1.50 | No |
Properly processed MIM 316L is non-magnetic with permeability below 1.01 — suitable for MRI-compatible devices. If post-processing involves significant cold work, a solution annealing step restores full non-magnetic structure.
For applications requiring guaranteed non-magnetic performance, specify a solution annealing step after any cold-working operations.