I have not posted on here very much, and only look from time to time.
I have a small shop foundry, and have melted around 26 tons of scrap into castings over 44 years now.
I use clay graphite crucibles.
I will tell my proceedures for making machinable castings.
Most of what I made was not critical for strength.
I am not a real authority, but most of my castings were machinable.
Having machinable castings is the most important issue when selling castings.
I like sharing my knowledge, have learned that saying "hardspots" when talking about castings is as a high spirited topic as any politics can be. If one lets people say your castings are junk hard, you will loose business.
Quality control was my most important agenda when casting.
I may have to make more than one reply for time constraints.
To start with, things I learned to do and not do.
1. An important issue when casting from crucibles is to use clay graphite crucibles opposed to silicon carbide ones.
2. I use additives from a local foundry. I add graphite and fero silicon. The silicon some how disappears when remelting iron and must be readded..
Silicon is very strange in chemical composition. Under 2.5%, it makes castings machinable, while 7% makes white-hard iron, which is then heated in coal furnaces for about a week, to make maleable iron. One foundry melts old bath tubs to get higher silicon numbers.
If one lets his tool get dull when maching it, it rehardens the casting.
Much like if machining stress proof, which is air hardening, with manganese, one simply heats a piece, and it gets hard.
Once I cut 47 teeth of a 48 tooth gear stick, to part off model gears, when the cutter got dull, which hardened the stick, and wrecked my work.
3. Point of view, as has been stated in previous replies, the temperature heated to makes a difference in ones intended use.
When talking about tempering a spring or tool like a knife blade, alot of people go nuts if I refer to tempering as annealing. If this erks a reader here, just read the deffinition of anneal.
What I shoot for when annealing is to Normalize my castings.
I do this by placing all my castings from the last pour, into the furnace after the next poor. They turn orange, and that eliminates most hard spots.
Here I am mostly dealing with chilled castings. Improper chemical properties require or benifit from different temperatures and time heated.
Once I had to remove a casting and reheat it 3 times until the very center would machine.
I must stop here and do another reply.
