At the very least spindles are made of cast steel, (some are forged steel) I know this because I have welded them to change the steering geometry on dirt cars. Spindles on trailer axles are also made from steel as they are welded to hollow mild steel tubing to make trailer axles. When you have a trailer spindle overheat and get gouged up from no lube, (seized wheel bearings) you can cut them off an re-weld a new unit to the axle tube with mild steel rod.
Same is true for front I beam truck axles, they are made of steel. If you have ever changed king pins you have to beat the eye of the I beam the same way to get a frozen king pin to release.
FWIW, you can weld cast iron with special high nickel rod, the process is completely different requiring preheating. Cast steel can be welded with any mild steel filler rod requiring no preheat for stress relief, or slow cooling such as being buried in sand required for cast iron welding. I would never trust a piece of cast iron to be used as a steering knuckle, nor would I trust one that was cut apart and rewelded as I have done on my race cars. I surely wouldn't be welding any Pontiac connecting rods together either...…
Shocking the tapered hole with 2 hammers to get the taper to release isn't going to compromise a modern spindle. Driving the pot hole filled roads of Ohio and PA for many years hasn't broken any spindles, I did bend one during the winter one year. Sliding into a 12 inch curb back in 1975, with my 73 Grand Am.
After bending the spindle a few months later the lower ball joint stem snapped off, and I put another ball joint in, only to have the stem snap off again on the second ball joint within a month. That convinced me that I needed to change the spindle, and for 5 years after that I never snapped another ball joint stem off. The ball joints are forged steel and the spindle was still strong enough to snap the stems off them because it was deformed from the accident putting the ball joint stem in a bind during steering.
Comparing the used spindle I got from the wrecking yard to the one on the car, you could see that the bottom 90 degree portion was bent from hammering that curb. A lot more violent than freeing a ball joint taper with 2 hammers is.
Searching online there are basically 3 ways that are consistent to split the taper from the knuckle.
1) Shock with a hammer.
2) Split with a pickle fork.
3) Press with the pivot press tool.
The most widely recommended by far is shocking the joint with a hammer. Second is the pickle fork, and last is the pivot press. The pivot press is also recommended to put pressure on the stud, then whack the joint with a hammer so the tool isn't subjected to undue pressure deforming the tool.
Probably way more information than most people require about separating a ball joint from a steering knuckle or spindle.
