I have a metal - maybe iron or steel, I don't know - piece of fence or gating that I need to cut up. A tree that I want to save has partially grown into it. I have to cut the fence into pieces in order to extract it.

I'm looking at buying Tungsten Carbide Sawzall Blades. They come in 6 and 9 inch lengths. The 6 inch should be enough since the area is exposed - or is there some other factor involved. Or what about a Diamond Grit Torch Sawzall Blade? Would that make it easier for me?

Here's what I am dealing with:

It will cut faster.

The lower limit is three teeth in contact with the metal being cut, so choose a high enough tooth count blade to achieve this.

Slow speed and solid blade contact are also needed.
Starting the cut is a little harder in metal.
The contact area can be small resulting in the blade slipping on the material and losing teeth until a decent kerf is established.
Using a triangular file to start the kerf works well.
A few quick strokes with the file and the saw blade can settle in and get to work.

A circular saw would not work well for this job.
You need to be able to get close to the tree without tearing it up.

This job should not need a carbide grit blade.
Just buy a 5 or 10 pack of blades and go to town.
The blades need to be long enough to cut close to the tree without the saw shoe and body hitting it and damaging the bark.

If you start with a file kerf a longer blade length is not nearly as important and can make the job easier.

They are usually used only when the material is so hard a regular blade cannot cut at all, and at that point the grit works well.

On softer materials grit blades are very slow since they spend a lot of time smearing the material back and forth before it breaks off.

Soft but 'gummy' material can even rip the grit off the blade in short order (like a concrete diamond blade hitting rebar).

Soft steel just needs the correct tooth count bi-metal blade.