At that level of C I would preheat and post-weld heat treat as a general rule of thumb. You can use the carbon equivalent if you have chemistry to help understand weldibility of you component.
Preheating and heat treatment is mandatory. But at that level, i'm very affraid of cracks or lack of fusions. I have to check every weld ultrasonically.
Use adequate preheat and soak time before welding along with low hydrogen weld electrodes. Selection of PWHT temperature should be based on original heat treatment, if applicable, and remain below the tempering temperature.
