Welding Chrome Moly (CrMo) Steel
This steel is mostly used in power stations. It is also called creep resistant steel which means that it doesn’t sag even at high temperatures.
It should be welded with similar consumables and you should follow a very precise method. Preheat and post weld heat treatment are almost always involved. The most common consumables according to AWS have a suffix of B2, B3 (something like E8018-B2) and are designed only for welding CrMo steels. Dont use these consumables for any other steels. They are highly crack sensitive.
The photo shows a power station steam pipe. Insulation covers heating coils were used to pre-heat the pipe to 250°C followed by PWHT of around 700-730°C.The welding consumables were TIG for the root deposit (AWS ER90S-B3, 2.4mm wire),then MMA 3.2 and 4.00mm electrodes for the fill and cap (AWS E9018-B3). Often FCAW is used for filling and capping these large bore pipes (AWS E91T1-B3), using 1.2mm wire and Ar/CO2 gas mix.
Thin Walled Tube T45 or 4130 grade
A common question is what to use to weld thin walled tube T45 and 4130 used on roll cages and automotive parts. These are also CrMo steels.
If you read the books it will tell you that you need to preheat, slow cool and post weld heat treat. However, because they are thin walled tube you don’t need the preheat especially when you TIG weld. The heat of welding is enough. Use A15 or A18 wire, the strength levels don't match but the weld will be thicker than the tube to make up for it.