Additionaly I can say that there are standards that strictly regulates usage of prheating for CrMo alloyed steel welding. It depends on: thickness of welding ant conditions of welding - temperature.
Quite possible that you've measured hardness of the surface, wich is decarburised. What is the indentation depth? If you have possibilty to destruct tobe - measure hardness in cross section. If you havent this possibilty - measure hardness after deeper grinding of tube surface.
The other case...
Check out if there is the right steel tipe to be carburised.
The other point is - may be you do not reach the required temperature for chemical diffusion process.
