Vacuum Quench vs. Air Quench: What the Furnace Does to Your Blade

What actually happens in the furnace

Heating steel to 1,020–1,050 °C in open air does two kinds of damage. Oxygen reacts with the surface and strips carbon out of it — a decarburised skin of 0.05–0.1 mm forms, and that skin will never reach full hardness no matter how carefully it is quenched. The fix used to be grinding the skin off afterwards, but thin blades have no grinding allowance to spare.

A vacuum furnace removes the air before heating. No oxygen, no decarburisation, no scale. The surface that goes into the quench is the surface you specified.

Distortion: the cost you do not see on the certificate

Quenching in oil or polymer from open air creates uneven cooling — the thin edge cools faster than the thick body, and the blade warps. On a 2,000 mm paper knife, that warp shows up as a step in every cut. Vacuum furnaces quench with high-pressure nitrogen gas, which cools evenly and keeps long blades straight. Our 4-metre knives come out of gas quench needing minimal straightening.

Why we temper three times

Quenched steel is hard but full of internal stress, like a tightened spring. One temper relaxes most of it, but high-alloy steels like SKD-11 keep residual austenite that converts later — sometimes on your machine, where it shows up as a dimension shift or a crack. Triple tempering converts that structure fully. It adds furnace hours, and it is the reason some quotes are cheaper than ours.

How to check a supplier without a lab

Ask one question: "Is the heat treatment done in your own plant, and is it vacuum?" If the answer is a pause, the blades are being sent out to a job-shop furnace, and nobody at the seller can tell you the actual tempering cycle. Consistent hardness across a batch — ±1 HRC from first piece to last — only comes from controlled furnaces with calibrated thermocouples.