Heat Treating

I am in the process of building an amateur seismograph. This instrument will come close to the performance of a professional instrument. To achieve this it takes more effort than just cobbing together some simple gadget.

One of the things needed for the instrument is a special leaf spring. Whenever I do a project that needs a special tool I use the project as an excuse for getting or making the tool. The spring is to be made of a special stainless steel that has to be hardened after cutting it to size. The material is 17-7 PH stainless steel. The PH stands for precipitation hardened. The process for hardening this steel is considerably different than for high carbon or tool steel. Basically for tool steel there are three heat treatments annealing, hardening and tempering. Annealing consists of heating the metal to above it’s curie temperature, the point where it becomes nonmagnetic, and then allowing it to cool very slowly. Hardening consists of heating the metal to above it’s curie temperature and cooling it very quickly. Tempering consists of heating hardened steel to a specific temperature depending on the degree of hardness desired. For the 17-7 steel there several different processes one even includes cooling to -100F and holding for 8 hours.

Luckily I need the hardest you can get called condition  CH 900 and the material I have to start with is in condition C. To get from condition C to condition CH 900 all you have to do is heat to 900 +/- 10 F and hold for one hour followed by air cooling to room temperature.

To keep the temperature within plus or minus 10F will require some sort of active temperature control.  The other day I spotted a nice all stainless steel pizza oven on sale for about half price. This will be my basic oven. It has nice Calrod heating elements rather that just some Nichrome coils. I have also ordered some ceramic blanket insulation to help it get to and survive operating at 900F.

On Ebay I spotted some nice digital temperature controllers for not too much money. Putting this all together should give me a nice heat treating oven for the springs. I also see it being useful for tempering tool steel items, annealing glass, re-flow soldering circuit boards and even still able to bake a pizza if desired.

Heat Treating Part II can be found here. http://wp.me/p12vwL-1v

This entry was posted in Amateur Science, Metal Working, Uncategorized. Bookmark the permalink.

3 Responses to Heat Treating

  1. Ron Amundson says:

    One thing to keep in mind, is that the internal wiring of most consumer units is not built to withstand really high temperatures for an extended period, nor for all that many cycles. Its a lot easier to rewire when new, than it is to deal with wire insulation binder deterioration after its been overtemped. Another thing to check out is whether you need to changed the overtemperature cutout. Some are resetable, and others are merely thermal fuses… but consumer wise, its likely to trip well under an hour with the oven at 900F. You can get new ones at Digikey or Omega for not a lot of money.

    • jimhannon says:

      I just took the oven apart the other day. It looks like the wiring will be up to the job. The temperature controller is a simple fixed thermostat attached to the oven wall. I am going to mount a switch that bypasses that and the timer so that my external controller will have full control. Another post is coming with picture when I get things put together.

  2. Pingback: Heat Treating Part II | Citizen Scientists League

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