A NullPointerException happens when something directly to the left of a "." or "[" operator is null at the time the line of code executes. Here, that could be the variables "nt" or "newSteelSize". Carefully check the code elsewhere in your application that's supposed to set those to some value.
One beginner mistake that's very likely to be the cause of this: make sure that in your class's constructor, you're not accidentally declaring local variables with the same names as these members. You should be writing
nt = ...
Whatever nt = ...
The stack trace should tell you exactly which line the error occurred on; it's probably that "setSteel" line, since both variables are used there.