Try a bit of the remaining steel quenched in water.

Sounds like one of my attempts! First, no point in tempering if the initial hardening step failed. After quenching try running a file across it – the file should slide off, not cut.

Very likely the form tool wasn't hot enough OR, it cooled off enough on the way to the quench to spoil it. What's needed is to heat the steel hot enough to change its internal state without burning it (ie remove carbon), and then cool it so rapidly it doesn't have time for form the usual soft structure.

Bit of a wild guess based on my failures but I reckon it's more difficult to harden small steel items than big ones because small items lose heat much more quickly than big lumps. I blast the item to get it extra hot just before quenching, but I'm not sure it's effective. Perhaps a clockmaker can advise? What's the best way to harden small parts?

Changing the cooling fluid might help. Water (provided the tool is stirred violently to disperse cooling steam) makes harder steel than oil, and salt-water even harder. Often too hard!

The purpose of tempering is to increase toughness at the expense of reducing hardness. There's a risk of untempered steel shattering because its brittle. On partly hardened steel, tempering will leave the tool too soft.

Dave

I would not bother to temper, just use it hard.

I have no problems using oil to harden parts made from gauge plate but they are very small in size. Larger parts gave problems when using the same size container filled with oil. You need a good size container for quenching in oil, better larger than smaller, otherwise you cannot get rid of the heat fast enough. Niko.

It sounds like the piece wasn't hot enough did you use a hearth to keep the heat enclosed and when you say red hot it should be more orange to be hot enough to get a decent hardening. I have a pile of fire bricks which I enclose the piece to get it hot enough and a nice large blow torch. I also have a gallon tub of oil right next to quench it in. You must vigorously move the piece in the oil to remove the heat quickly. Test with a file and it should just skid off. To temper the piece polish with emery cloth and I sometimes descale in brick cleaner and degrease as well. Heat slowly until a light brown colour is atained and then wait a minute and quench in water. Job done. This is how I harden gauge plate and silver steel I hope it helps.

David

Have a read of Wikipedia on spark testing. Simple to do, though don't jump to conclusions without practice.

When you push steel against a grinding wheel it produces a shower of sparks characteristic of the metal. Mild steel is least sparkly and usually easy to tell apart from a tool steel like gauge plate.

I doubt it's a quality problem, more likely it's heat. I've failed and succeeded hardening bits off the same Silver Steel rod. The weakest link was me!

Dave

Gauge plate needs to be bright orange in heat, Yellow is too hot , bright red is too cold. A low light room will give a better result than in day light or with bright lights on. Quench in oil is fine, you need enough oil, so that the oil does not get hotter than 30c or so, otherwise the volume is too small and the quench is inadequate. Temper in the oven at 150c to 180 c, or if heating with a torch when clean from any oil and dry, is a pale straw colour. Allow for sharpening of the edge after the heat treatment. The edge will be rounded off and burnt away from the heating of the part. If you have stainless gauze to wrap the part in for the heating will significantly protect the very sharp edges or corners. Best to remove the cage just prior to quenching, as it will hamper in the quench cooling.

Neil

Not sure if it's been mentioned already but I use a magnet to test if the steel has reached the curie point and is hot enough to quench.

I only Quench in brine if I want glass hard results and stick to Quenchall 22 oil for most jobs.

I have a chart of useful tempering temperatures in my shop for various tools, I'll have a gander to see if it mentions lathe tools.

Regards Mick

I note that a few people don't temper after hardening. That might imply that tempering isn't necessary. But it could also mean that the hardening didn't work properly. In that case tempering will have little or no effect.

Andrew

And you think we wouldn’t notice?! In my case at least all the parts ended up glass hard as determined by a file just skidding over the surface. I’ve made trigger parts and scraper tips and they’ve all worked very well.