I'm a newbie of electricity and I would like to know if a piece of tool steel connected to both positive and ground pole of a welder tend to heat up red hot. If yes, at wich amperage? There are some risks involved such as overheating the welder or a huge waste of electricity due to the hight resistence of the metal. Thanks in advance.
This is a layman's view, but based on some engineering principles of which I am aware.
The tool steel as you describe would become a direct short. As such, the resistance isn't necessarily going to be high enough to make it red hot. There may be other components in the circuit (clamps, cable connections, cables, etc.) that would have higher resistance than the piece of steel.
A stick welder performs as it does because the hot lead (welding rod) and ground (work piece) are brought together only long enough to create an arc. This arc creates high temperatures to melt the rod into the work piece, as well as to heat the work piece to the correct temperature.
If you desire to heat a piece of tool steel for tempering purposes, perhaps, consider to use the electricity available in the form of inductive heating.
The linked article describes the method of creating a DIY inductive heating device with a variety of electronic components. I would not suggest that this is the precise answer for you, but to provide you with leads in a direction more effective than shorting out a welding rig.
Another similar DIY tutorial can be found on the same site. This one reads as a 12kw device, likely sufficient to perform the tasks you require.
I would not be surprised to learn that these suggestions are more complex than you would desire, considering the complexity of the latter project:
No. A welder isn't made for that. It gets a thin wire of steel hot enough to locally melt and stick to a piece of metal, it can't just heat a chunk of iron red hot. That takes enormous sustained amount of heat. If the piece has any serious mass, it will take much more heat than the welder can provide. You can heat it up. but the transformer would be molten before the steel starts to glow. I'd rather use a torch if you want to get it glowing, that's the proper tool here. Even if you have a powerful enough welder, your electrical connection will probably not be sufficient to deliver so much power. You'd just pop the fuses in your home. A torch however, can easily provide enough power/heat.
Why do you want to get it to glow? Do you need to rejoin it with another piece of steel? If you explain your plans, we might be able to help you better.