Hello.

Can you provide the thickness(es) of the base materials.

The first material is a creep resistant steel (group 5.2 acc. to EN ISO 15608), and the second one is a carbon steel (group 1.2 acc. to EN ISO 15608).

From what I learn and what I saw in my little experience, when you weld this two different materials, it's recommended to use the filler metal for the lowest alloy steel, and the technology after the higher alloyed steel.

In this case you can use ER70S-3, and use the technology (preheat - minimum 180degress acc. to ASME B31.3, parameters - i think the intensity it's very high - maybe maximum 130A, and PWHT - min 704 - max 760, 2.4mm/min, min time 2h, acc. to ASME B31.3).

Also, for slow cooling rate, it should be used the insulation at about 300 degrees (we kept the insulation for 1h).

I hope this will be helpfull.

Best regards!

In general, you have 3 ways to reduce the hardness. Slowing the cooling rate is one. The ways to do that are to increase the preheat or heat input of the weld for a given thickness. The second way is to adjust the composition of the base metal. Can you get one with lower carbon? The carbon content is directly related to the maximum attainable hardness. The third is to temper the material after welding. This can be done with a PWHT, or temper bead technique. Look into the options, and see what works best for your situation.