Welding Tips and Tricks

Like the title says.....
Why would I preheat a work piece? Is it something a back yarder like me would do if I'm welding up thin 3mm aluminium or is it more of a 3/4" thick "thing"

Is it limited to alloy and exotic stuff or would you do it with mild steel and stainless?

Lastly, are we talking oven preheat or are we talking heat gun preheat?

Thanks in advance

As far as I know (new to a lot of this and still learning) it's mostly for more brittle metals. Like cast iron for example. But a lot of others also.
I think cast iron usually needs around 650°F. Also have to do a controlled cool down.
The way I saw it down was with a rosebud on a torch. He used a temp stick to check it.

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79jasper is right about cast iron (and cast steel, cast stainless steel). Some high-chrome steels also require pre-heat (and inter-pass heat, and post-heat/controlled cool). P-91 comes to mind.

Aluminum preheat is nice any any material 1/4" and above just to help establish the initial puddle quickly and get moving, but is only "neccessary" when the thickness pushes the limits of your machine and/or will require multiple passes.

Steve S

Thanks guys, that's pretty much what I thought, I just couldn't find the info. Cheers