As a rule of thumb, you can take the brinell hardness devided by 2, and that is your approximate tensile strength. At 150 KSI, you have some pretty hard material there. I would venture to say this material is probably quenched and tempered to achieve that strength. I don't think normalized material would be that strong. The 3" thick section doesn't help anything, as it will provide high joint restraint, and a high cooling rate if not preheated properly.

The highest tensile strength we have welded in this material (acutally it is a casting equivalent to 8625) without a water quench and temper PWHT is about 120 ksi. We used a E120T5-K4 electrode, and/or E12018M. We preheat to 400F, and PWHT at 1100F. I imagine that this PWHT temperature though, would soften your base metal significantly. Usually, you do not want to PWHT at more than 25 degrees minimum below the tempering temperature of the base metal.

If you need the weld metal strength to match the base metal, I would recommend one of the ESAB T-4130 4130LN FCAW wires, with a Water quench and temper PWHT. Other companies make similar products, like Weldmold, and Cor-Met. These companies (the last two) do a lot in the tool and die industry, and may be able to point you in the right direction.

I definately cannot recommend welding this mateial without some sort of PWHT, whether a full heat treatemnt, or just a stress relief. It may also be necessary to PWHT before letting it cool down to room temperature from your preheat. It is hard to know without trying.

Additionally, are you needing to qualify a procedure for this welding? If so, I don't think you will be able to pass a 180 degree bend test. You will need to perform a base metal bend, and subsequent weld metal bends will need to bend within 5 degrees of the base metal. That is if your code allows this. (ASME IX, for instance, does not).

Good luck