DMF_TomB and digger doug seem to have experience in this.
Here's some theory. You've got a chunk of 17-4 at room temperature and you want to melt it. First you need to heat it to raise it's temperature. The energy required for this is called the sensible heat change. As in you you can sense the temperature change going from room temperature to the melting point. Then, if you keep adding heat, the stuff starts to melt. As it melts, it absorbs energy. This energy is called the latent heat of fusion. After its melted, you want to raise the temperature of the melt slightly above the melting/freezing point to ensure that it doesn't start solidifying too easily. More sensible heat. You can calculate the theoretical heat values required pretty exactly.
That theoretical number is interesting but useless. As DMF_TomB points out, much of the heat involved in heating a crucible is wasted. You have to heat the crucible and the furnace lining, and heat leaks out through the insulation and escapes with any exhaust gas. If you are just using a rosebud heater, and no furnace (that is, you've no insulation) I see no way to melt what you want in any practical way. Further, when the metal becomes liquid and you keep it hot with a rosebud, there is ample opportunity for the melt to react with air. So you want a furnace with a controlled atmosphere to melt this, not a rosebud hooked to an LP tank.
You;ll also want to post cast homogenize. Here's some aspects of 17-4ph casting from eng-tips.com
EdStainless (Materials) 14 Feb 07 09:02 I have never seen cooling too fast being an issue metallurgically. I have seen it cause excessive distortion. I don't like quenching other than forced air unless there is no other way to get the part cool in 30 min.
Have you talked with the casting house? They may be able to put the hot castings into a holding funace and carry out the homogenize step right then.
Some points.
- The higher temp homogenize is important. Your casting will have significant segregation.
- Atmospher control will be critical. You can't do the high temp steps in air. Oxidation will kill you. Surface damage, alloy depleation and such.
- If low temp toughness is your goal then you are going to need a higher aging temp (lower strength). The 925F condition is not known for beign forgiving.
- The aging should be 4 hours. You adjust the temp, not the time. Playing with time can give you unstable or unrepeatable results. = = = = = = = = = = = = = = = = = = = =
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israelkk (Aerospace) 14 Feb 07 09:37 Here is a quote from MIL-HDBK-5J regarding 17-4PH castings "Alloy 17-4PH castings are produced in sand molds, investment molds, and by centrifugal casting.
While 17-4PH has good castability, it is subject to hot-tearing, so heavy X or T sections, sharp corners, and
abrupt changes in section size should be avoided. Alloy 17-4PH castings are susceptible to microshrinkage
which will decrease the ductility but have no effect on the yield or ultimate strength. During heat treatment,
care must be exercised to avoid carbon or nitrogen contamination from furnace atmospheres. Combusted
hydrocarbon and dissociated ammonia atmospheres have been sources of contamination. Air is commonly used
and both vacuum and dry argon are effective for minimizing scaling. Oxides formed during solution treating
in air may be removed by grit blasting or abrasive tumbling. Alloy 17-4PH can be heat treated to develop a wide range of properties. Heat treatment procedures
are specified in applicable material specifications and MIL-H-6875."
As to hardness correlation to yield/tensile strength, there is no direct and exact correlation. For the same yield/tensile strength there may be up to 10Rc hardness variations. Therefore, if hardness is required, for each batch you will need to test heat treatment parameters which is not practical and you may end up with non standard heat treatments.
If this part is for aerospace/military use you probably do not have the freedom to use non standard heat treatments but only those allowed by MIL-H-6875. Therefore, hardness can not be defined but only yield and tensile strength.