Saturday, February 4, 2012

The Suburban Blacksmith

My younger son decided to make a chef's knife.

He had some narrow eighteen-inch plates of O-1 (oil hardening) tool steel shipped in from New York, bought a torch, and bought some fire bricks. The plan was to construct a little forge with the fire bricks, heat the interior of the forge with a propane torch, and then insert the blade into the hot forge for heat treating. Heat treating meant getting the blade red-hot and then smacking it with a hammer. Lacking a regulation anvil, he planned to use a flat spot on our workbench vise.

The plan seemed a bit sketchy to me, but my son began executing it with characteristic confidence and efficiency.

He cut out a rough blade with tang from the steel plate. Then he constructed the forge by stacking the fire bricks into a little cube and cut a hole in the side of the forge chamber to insert the tip of the torch.

He repeatedly heated the blade to a moderate reddish glow and smacked it with a hammer until it began to cool off. The object was to draw out the blade into a gradual bevel leading down to what would be the cutting edge.

After a great deal of heating and hammering, my son had made only minor progress. The forge just wasn't sufficiently hot to make the steel malleable.

The picture above shows the knife after initial heat treating. My son filed the knife's surface to show low spots of remaining black scale (the origin of the 'black' in blacksmith).

More heat was needed. My son and I reconfigured the forge to close in the interior, leaving only a slit for insertion of the blade.

This new and improved forge did the trick. The blade got plenty hot, allowing my son to complete his hammering.

Now my son prepared to heat the blade to the perfect temperature to establish a fine crystal structure and then quench the blade in oil. The point to quenching is to freeze a fine crystal structure in place and thereby harden the steel. My son was throwing around a lot of metallurgy terms at this point: austenite, martensite, critical temperature, normalization, and so forth into incomprehensibility. He wanted to heat the blade to a temperature where the steel's internal structure would no longer attract a magnet. After some trial and error with touching my refrigerator magnet to the red-hot blade, my son was confident that he could recognize the red-orange color that marked the blade's perfect temperature. He was ready to quench.

We had improvised a quench tank by filling a stock pot full of canola oil. (Kiddies, do not try this at home.) My son plunged the red-hot blade into the oil. The odor of Chinese stir fry filled the garage. After a few minutes my son pulled the blade from the oil. The steel was coated with a lovely brown residue of burned canola oil.

Now it was time for my son to begin the long filing process to finish shaping the blade and to put a nice edge on it. However, my son looked down the line of the blade, squinting and scowling. The blade wasn't quite straight enough. It offended his standards of craftsmanship.

He decided to clamp the blade tightly between two steel plates and cook this assembly in the kitchen oven at 425 degrees for half an hour in hopes of straightening the slight bend at the middle of the blade. Unfortunately, this cooking process didn't work. He tried again, placing some nickels as shims between the blade and the steel plates, clamping everything back down, and cooking it all for another half hour. Still didn't work. Finally, he took the blade back to the garage to coax it into straightness using the vise. The result was heartbreaking.

I'm sure his next knife making attempt will have a happier outcome.

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