Think of sintering as baking the magnet: it comes out of the furnace dense, solid and shaped. But a freshly sintered NdFeB magnet is not yet the strong magnet on the spec sheet — it is still magnetically weak. The trick that unlocks its power happens afterwards, in a second, gentler heating step called tempering (or aging).
What tempering does
Inside the magnet, billions of tiny magnetic grains are packed together. Right after sintering, the thin layer between the grains is messy and incomplete, so the grains can flip their magnetization too easily — which means the magnet is easy to demagnetize. Tempering reheats the part to a carefully chosen lower temperature and holds it there, letting that grain-boundary layer form properly. Each grain becomes magnetically isolated from its neighbours.
Why it matters
Isolated grains are the secret to coercivity — the magnet's resistance to demagnetization. A good tempering step can multiply coercivity two or three times. Without it, a magnet demagnetizes in heat or in an opposing field; with it, the same material earns its grade letter — the H, SH or UH that tells engineers how much heat and abuse it can take. You will find tempered magnets everywhere coercivity counts: motor rotors, sensors and generators that run hot.
That is also why tempering is done in two stages: one to tidy up the boundary layer, one to perfect it. Get the temperature slightly wrong and the layer turns patchy again and coercivity drops — so makers fine-tune the setting for every batch.
Read on: sintering: how the magnet is formed → · what grade letters like H and SH mean →