Both 304 and 316 are usually low-magnetic in the solution-annealed condition; neither grade name guarantees low permeability after fabrication. 316 often shows less deformation-induced magnetic response than 304, but the actual heat composition and processing route matter. If a sensor, instrument housing or other assembly has a strict limit, qualify the finished condition rather than selecting by a hand magnet alone.
The central purchasing question is therefore not “Which grade is always non-magnetic?” It is “Which grade and process can meet the required corrosion, strength and magnetic limits in this part?” Separating those requirements prevents an expensive material change that does not solve the problem.
| Comparison point | 304 | 316 |
|---|---|---|
| Solution-annealed wrought material | Generally very low magnetic response | Generally very low magnetic response |
| Cold forming or drawing | Can develop strain-induced martensite and increased attraction | Can also change; often less sensitive than 304, but not guaranteed |
| Welded component | Weld metal can contain magnetic ferrite | Weld metal can also contain ferrite; grade alone does not eliminate it |
| Chloride exposure | Assess carefully against environment and surface condition | Molybdenum improves resistance in many chloride conditions; not immune to corrosion |
| Acceptance for a sensitive assembly | Require an agreed finished-condition test | Require the same clarity; do not rely on the higher alloy cost as proof |
The table describes selection tendencies, not measured batch values or a universal permeability ranking. Comparing annealed 316 sheet against heavily drawn 304 wire does not isolate the effect of grade. Match the form, dimensions, deformation, heat treatment and measurement conditions before interpreting the difference.
Magnetic response comes from the phases present and their behaviour, not from a simple one-element rule. Nickel and other austenite stabilisers influence the resistance to deformation-induced transformation. The molybdenum addition in 316 is important to corrosion selection, but does not justify a blanket claim that 316 becomes more magnetic than 304 after working.
Published comparative experiments describe particular compositions and processing conditions. Do not transplant one measured permeability value to every bar, sheet, tube or wire of that grade. Two compliant heats can differ, and local strain in a formed part can differ from the average reduction quoted for the starting material.
A low-permeability requirement does not replace the corrosion review. Specify the chemical exposure, chlorides, temperature, cleaning regime, drainage and surface condition. While 316 is often considered where 304 provides insufficient chloride resistance, neither is universally suitable for stagnant seawater or crevices that retain salts.
Likewise, a magnetic response is not a corrosion test. Surface contamination, weld heat tint and deposits need separate controls. Where failure would be consequential, the engineer should assess the complete environment and approve the alloy rather than treating “marine grade” or “non-magnetic” as a complete specification.
Relative permeability describes response to an applied field. Residual field describes retained magnetisation. State which one affects the equipment and identify the applicable limit, instrument or method, field conditions and test locations. A purchase specification that mixes these two quantities is difficult to verify and can lead to conflicting inspection results.
Will testing occur on incoming stock, after drawing, after forming, after welding or after final assembly? For a bent bracket, measuring only the flat centre can miss a different response at the corner. For a welded enclosure, identify whether welds are included in the limit. Include representative high-strain and joined areas in the agreed inspection plan.
A hand magnet can flag an unexpected response, but it cannot identify 304 versus 316 or certify the absence of magnetic phases. Review the certificate and use appropriate chemistry testing for grade verification. Documented magnetic measurement is a separate acceptance step when the drawing requires it. Maintain heat and component traceability through both checks.
If cold work is the cause, reducing strain or considering a qualified solution treatment may help, but neither change is automatic. A solution treatment can affect strength, finish and dimensional accuracy. It must not remove strength that the drawing deliberately obtains through cold work. Welding requirements also remain governed by a qualified procedure, not by a generic instruction to eliminate all ferrite.
Degaussing is relevant when retained magnetisation is the issue. It does not turn strain-induced martensite back into austenite. When a tight permeability limit is unavoidable, compare candidate material routes using representative trials and an acceptance plan before committing to the production quantity.
Food equipment, pharmaceutical equipment and instrument housings may share a stainless grade while having very different surface, fabrication and inspection requirements. A low-magnetic result is not an approval for medical use or MRI safety. Such applications require the responsible designer’s separate qualification.
For a machined part, the 304 round bar specification can help establish the starting product requirements. For a formed or welded housing, begin with the stainless steel sheet and plate range, then add the final magnetic acceptance criteria and fabrication route to the inquiry.
Technical reference: ASSDA magnetic-effects guidance and condition-specific comparison. Reference information is not a test certificate for a supplied batch.
Send the candidate grade, product form, dimensions, quantity, corrosion environment and fabrication sequence. Include the magnetic limit, measurement method and final inspection stage so alternatives can be compared on the same basis.