welding - SA516 Gr 70 & SA240 TYPE 304/304L

By the perspective of the weld itself, a 304L rod could do. Some nickel consumable either, even without post treatment (304 is austenitic, really forgiving in terms of stress - if you keep the weld absolutely clean...). But I'm afraid the mechanical properties and corrosion resistance of the joint would not be that much. SA516 as far as I am aware includes grain size specs to allow toughness and low temp performance that you are just throwing away with such weld. So, take a closer look over your design before going on...

304 rod does not exist, when you've to weld a SS 304 you'll use a 308 filler metal. usually when you have to make a dissimilar weld, carbon steel to stainless steel, you first make a buttering layer on the carbon steel with a 309 filler metal and then you'll finish with a 308. regarding the corrosion resistance pay attention to the possibility to have galvanic corrosion

galvanic corrosion (http://www.corrosionist.com/Galvanic_Corrosion.htm) (Sorry! Link no longer available.)

welding stainless steel (http://www.corrosionist.com/Materials_Stainless_Steel_Welding.htm) (Sorry! Link no longer available.)

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corrosion & rust prevention

For welding dissimilar base materials in which one is carbon steel such as SA 516 Gr.70 and the other SS-304/304L such as SA240 Type 304/304L one can safely use the electrode E-309-15/16 (coated electrode) or the filler material ER-309 (filler wire with Argon shielding). There is no need for any particular precautions. Type 309 belongs to 25 Cr-12 Ni material composition and adequately compensates for the loss of alloying elements Chromium & Nickel percentage in the weldpool to ensure that it stays austenitic in structure. No Post weld heat treatment is required.