Knife making questions
 

I am no knife maker, but I do like to watch Forged in Fire. Some questions for the blade smiths:

1) Why is quenching the blade so hit and miss? It's always a scary event for the contestants.

2) When they quench the blade, they use a file to check for hardness. What are they doing to check hardness? Looks like they are just sliding a file across the blade.

3) What makes some of the blades brittle and break?

1. Smiths use color of the steel to determine temperature the lighting in the studio and sometimes questionable materials (different alloys require different heat treat) make it hard to judge when to quench. Also they don't always show normalizing and tempering. There are more steps than just the quench.

2. The file is xx hardness if it bites into the blade it means the blades is softer than xx hardness.

3. Brittle is likely from too hard or too large metal grain size from the other heat treating steps. A way to think about this is ice-cream if you've ever noticed the texture change when it melts and refreezes you get a more icy texture due to larger crystals. Same thing can happen with metal, if the normalizing is skipped or done improperly. The larger crystals/grains make easier propagation of cracks.

It's often hit or miss due to the education of the contestant.

The brittleness is determined by the type of steel, its temperature at quench, the temperature of the quench, and the type of quench solution. A dissimilar cross section of the blade as well as fractures placed in the blade during forging and direction of insertion often cause warping and breakage at quench.

Essentially, there are multiple points of potential failure along the way. Education paired with experience is key and sometimes, Hephaestus frowns.