I am new to the pyrotechnics (apart from various failed attempts to get a weedkiller and sugar to explode when I was a kid). My question is: Does anyone know if there is a mixture which burns hot enough to cut/weld steel?

Yes, a mixture called thermite. It is still used to weld rails together for train tracks. It's a mixture of iron oxide and aluminium powder, is hard to light and burns extremely hot, producing moltern iron. If you use this, make sure the iron doesn't land on anything damp/wet or you will get a nasty steam explosion.

You were lucky not to get weedkiller/sugar to explode. That is a classic "death mix" that can (and does) spontaneously explode and possibly detonate. Sodium chlorate is to be avoided at all costs and is probably responsible for more deaths/injuries to young and naive prospective pyros than any other compound.

Paul.

Like he said, thermite.

Mix Iron Oxide and aluminum together in a 50/50 ratio by weight. Its hard as hell to light. You'll need a magnesium ribbon or some sulfuric acid. Though ive heard that sparklers can get hot enough to ignite it.

Yep, termites... They always eat throught the wood... Wait what? I know pretty much nothing about pyrotechnics.... cept theres fire.... and I like fire