Dear Sir,
From the literature, I find that ER70S-6 wire confirms to AWS A5.18/A5.18M: is suitable for welding on a plate that has mill scale or surface contaminants. as this wire contains manganese and silicon deoxidizers. this deoxidizer absorbs oxygen so that it vaporizes into the arc or forms as scale oxides. it offers good fluidity & wettability. While ER 71T-1 Confirms to AWS A5.20/A5.20M: 2005 standard, Here, T-1 designates, it’ s Gas-Shielded, Flux-Cored Electrodes. Designed for welding with 100% CO2 shielding gas. Both (GMAW solid wire & FCAW Tubular wire) are suitable for ASTM A516 GR 70 to meet Tensile strength criteria. Solid wires, such as AWS ER70S-6, are an option for the root pass, while metal-cored wires and gas-shielded flux-cored wires (FCAW-G) can be used for fill and cap passes. Pl. refer the link:
https://www.hobartbrothers.com/news/304/523/Filler-Metal-Considerations-for-Pressure-Vessels.html.
Secondly, for higher temperature application. many kinds of the literature suggest that ASTM A-515 Standard Specification for Pressure Vessel Plates, Carbon Steel, for Intermediate and Higher Temperature Services however As per ASME Sec. II Part D for Sec. VIII Div. 1, both the materials have the same high-temperature limit that is 538 deg C and the working temperature is 427 deg C. A515 is produced to coarse grain practices and for use in high-temperature applications such steam piping or pressure vessels, since coarse grains good resistant for creep deformation and helps to minimize the graphitization. A516 is produced to fine grain practices in order to maximize the fracture toughness.
Hope, this explanation might be helpful to you.
Thanks
References:-
Welding Knowledge 