Your preheat may not extend in all directions and through the thickness of the part as required by the applicable welding code. Preheat has to be allowed to "soak" through the member. Based on Table 4.4 (D1.5-96), the preheat temperature is around 230°F (110°C) or higher, with 450°F (230°C) being the maximum preheat/interpass temperature. I would use a preheat that is higher than the minimum listed, while keeping it below the maximum temperature until several layers have been deposited in the groove.

A second problem may be that the joint is highly restrained. If the joint is allowed to cool, the forces that develop as a result of contraction could easily crack a single pass root pass or even a partially completed groove weld. If that is indeed the case, try to weld at least 1/3 of the joint thickness before allowing it to cool to ambient temperature which could be the case if you were to stop at the end of a shift or for lunch. My recommendation is to weld the entire joint without allowing it to cool down below the specified preheat temperature, even if it means using electric resistance heaters to maintain the required interpass temperature during off shifts. Thermal cycles are not good friends when welding high performance steels in the restrained condition.

The preheat temperature may have to be increased if the joint is highly restrained. Considering the thickness you mentioned, i.e., 150 mm, the joint thickness and the mass of the members would definitely be cause to consider the joint highly restrained. Remember that the joint has to be maintained (minimum interpass temperature) at the same temperature as the preheat temperature during the time weld is deposited. Allowing the temperature to drop below the minimum preheat temperature can result in weld cracking, especially with the high performance quenched and tempered steels.

You also have to be aware of the need to control heat input. You also need to be aware of the need to use appropriate hydrogen controls for the welding process you decide to use to reduce the susceptibility to hydrogen cracking. Don’t forget the need to use filler metals that will provide weathering characteristics if it required to match the weathering characteristics of the base metal. the need to use matching filler metals if weathering characteristics are required.

Best regards - Al