What is a Slitting Chisel and What Steel Should I use For it?
A slitting chisel is a very thin flat chisel used for opening or splitting steel when it is hot. The steel has to be very resistant to heat so that it doesn't deform as it is hammered through the steel.
Most slitting chisels are about 1 inch wide and the edge less than 1/8th of an inch thick. If you are planning on making curved cuts consider making a slitting chisel about 1/2 inch wide. (The smaller the width the tighter the curve you can make!)
I usually try to make the chisel about 8 to 10 inches long. My preference for making this tool is H13 steel or S7 steel. Both of these steels are heat resistant and stand up to the heavy abuse. They are a bit difficult to forge down to size but it is worth the effort as they last much longer than other steels.
I have used coil truck spring. This is an alternative and will do in a pinch but this steel is not very heat resistant. This means that you can't work it down so thin and if you let it show a bit of color as you are hammering through the edge will deform. Then if your chisel doesn't get stuck you have to sharpen it. As I said it will work but not as well.
I have had good luck using old jack hammer bits which I believe are S5. They stand up to both the heat and the hammering.
Good luck making your chisel and it doesn't hurt to have 3 different sizes. 1 inch , 1/2 and 1/4 inch for detail work.