Superalloy pour temperature in investment casting

[skailup]

In the investment casting of superalloy such as IN718 and Stellite 21, lower pour temperature gives smaller grain size than higher pour temperature does. Given that the face coat can be zircon and backing is silica, the cooling rate is relatively modest compared to what are achievable by other solidification such as steel mold or welding. Supercool (or undercooling) is thus insignificant.

The melt from a higher pour temperature will have to cool down past a lower temperature before solidification starts at the same point on liquidus. So my question is, how does a higher pour temperature result in a larger grain size?

Thank you very much!

 

[EdStainless]

The entire mass (mold plus metal) will be hotter if you use a higher pour temp.
It will take longer to cool through the critical temp range where grain growth occurs.
Many of these alloys can be grain refined with proper heat treatment, but some cannot.
course grain = better creep resistance
fine grain = better fatigue resistance
You need to know what the goal is.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[arunmrao]

If your goal is to get fine grains in the structure, more than the pouring temperature, the addition of suitable grain refiners are important. Also please remember that the ceramic shells are preheated before the melt is poured. Thus any effect of faster cooling, producing fine grains is eliminated. Hope, I have answered your question.

 

[skailup]

Thank you!

 

[blacksmith37]

The mold temperature and deoxidation practice will also affect grain sizes.