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tools — load line & knee analysis

Magnet Grade Selector

N42 is rated to 80 °C. A thin N42 disc can start losing flux permanently at 60 °C; a compact N42 cylinder survives past 130 °C. Identical material — the difference is geometry. This tool computes the permeance coefficient of your part, finds where its working point crosses the knee of the demagnetization curve, and reports the temperature each grade actually holds.

selector — geometry, circuit, temperature

why the grade table lies

Every magnet catalogue, including the reference tables in our own knowledge base, prints a maximum operating temperature against each grade suffix: N to 80 °C, H to 120 °C, SH to 150 °C. Those numbers are a property of the material, measured at a favourable reference geometry. They are not a property of your part.

What actually causes irreversible loss is the magnet's own demagnetizing field. A magnet generates a field that opposes its own magnetization, and the strength of that self-demagnetizing field is set entirely by shape. A short, wide magnet drives itself hard into the second quadrant; a long, narrow one barely does. Heat then pushes coercivity down until the working point falls past the knee of the curve — and whatever crosses that line does not come back on cooling.

The practical consequence. Below roughly a 0.35 thickness-to-diameter ratio, a standard NdFeB disc in open circuit will demagnetize below its rated temperature. Above it, the same material comfortably exceeds the rating. This is why two parts cut from the same block can behave completely differently in the same oven — and why "it's rated to 80 °C" is not an answer to "will it survive?"

The other half of the picture is the circuit. Putting the magnet against a steel plate roughly doubles its permeance coefficient, which can move the usable temperature by 30 °C or more without changing the part at all. A magnet that fails on the bench can be entirely safe once it is assembled.

method

steptreatment
permeance coefficient For a cylinder, Pc = 2L/D. Blocks and rings use an equivalent radius from pole area, R = √(A/π). A single steel plate doubles the effective magnet length by the image method; a full yoke roughly quadruples it.
properties at temperature Br(T) = Br₂₀·(1 + α·ΔT) and Hcj(T) = Hcj₂₀·(1 + β·ΔT). For NdFeB α ≈ −0.12 %/°C and β ≈ −0.60 %/°C, with β becoming less severe for high-coercivity suffixes.
working point The load line B = Pc·|H| meets the normal curve B = Br + μrec·H at |H| = Br / (Pc + μrec), with recoil permeability μrec ≈ 1.05. Any external opposing field adds directly to this.
knee Taken at 90% of Hcj — a squareness ratio typical of good sintered NdFeB. The magnet is safe while the working field stays below the knee, and the temperature at which they meet is the real limit.
Calibrated against published behaviour. For a ½″ diameter × ⅛″ thick N42 disc in open circuit the model returns 64 °C against a published figure of about 60 °C; for a ¼″ N42 cylinder with diameter equal to height it returns 130 °C against about 140 °C. It also reproduces, without being told, the industry rule that a diameter-to-thickness ratio below about 4 lets a magnet exceed its nominal rating.

what it can't see

Qualify before production. This is a screening tool for narrowing grade candidates. Anything safety-critical or automotive should be validated with a soak test at maximum temperature followed by flux measurement, per the control plan discussed in magnets 201.