Springs again

[Canute Rex]

Rich,

I blush...

I taught blacksmithing during the summer at the Lake Champlain Maritime Museum for years, which honed my explaining skills. Now I teach occasionally at the Yestermorrow Design/Build School.

Doug,

I blush again, thanks.

You are right - in some cases, a blue temper may not be enough. It all depends on the steel.

The straw, bronze, purple, blue tempering spectrum is an array of what are called oxidation colors. These are the colors produced by clean steel combining with oxygen at a particular temperature. It's just a rough visual thermometer.

Every steel has its own range of hardness/flexibility produced by a range of temperatures. That's why I recommend buying known steel and getting the info sheet on it. You'll get instructions such as "Bring to 1437 degrees F and hold for 10 minutes per 0.25 inches of thickness." Some steels might spring temper properly at 680, some at 700, some at 650. Doug, you are right to distrust the books. Believe the specific info sheet, not a random chart in a general blacksmithing book. That's also why those tempering crayons are so useful.

I have never used the lead method, but I have used large blocks of steel heated in the forge. A friend of mine who makes spectacular Damascus knives (Jon Loose - Google his site and drool) uses a molten salt bath with high-temp thermistors controlling variable burners. Complicated, but he gets flawless hardening and tempering done.

Best,

Canute

[LRB]

  If one defines critical heat as  non-magnetic, that is not a good quench heat. Non-magnetic is 1414°, which is not enough heat to do a proper hardening on any steel I know of. The heat needs to be 50°, to 75° above non-magnetic in order to get a good austenitic solution, and be able to lock that solution at it's best condition to get full martensite which is the hard phase of steel.  Even 1075. A magnet is really only good for judging sub-critical heat cycling, and to tell you that the austenite phase is beginning, but is not yet in full solution. With 1075, a temper from 625°, low side, and about the lead melting point,  to 650° will give you a good spring. 675°, would in my opinion be the top side, but would still be a decent spring.