difference between SA106 GR B and SA 516 GR 70N

One is a Seamless Tube and another plate (unless I am mistaken).

Both are used in high temperature services (heat exchangers of thermal power plants at much higher temperatures)

However for hot oil services you must check up the relevant code. You may check the ASME Sec II

Please confirm that you are looking for the material specifically for the tube, and not another component such as the shell, channel, etc ......

Also please confirm the metal temperature of the tube.

yes it is for the tube

the tube material is between 450 F to 500 F

You should use an actual "tube" product form, for example, in this type of relatively low temperature:

SA 192 - Standard Specification for Seamless Carbon Steel Boiler Tubes for High-Pressure Service

Or if you were going to a higher temperature/superheated:

SA 209 - Standard Specification for Seamless Carbon-Molybdenum Alloy-Steel Boiler and Superheater Tubes

The biggest reason for going with a 'tube' form is the tighter tolerances on outside diameter and ovality for attachment into machined/reamed tubesheets.

You can find the relevant allowable stresses in ASME Section II - Part D

Note that SA-516-70 is a 'plate' form and would unnecessarily require rolling and welding to make into a tube form. And SA-106-B is a 'pipe' form with looser tolerances.

what about the temperature

which is standing temperature more

You need not worry about any of those relevant product forms for your stated temperature (500F), for example

SA-192:

• Maximum allowable temperature = 1000 F (Sec I & VIII)
• Creep effects at temperatures greater than 750 F
• Not recommended for extended service above 800 F due to graphitization

and I suggest looking @ ASME Sec II-D for further examples/numbers/allowable stresses.

My apologies - I had in my mind water boilers, but you mentioned simply "heater". And perhaps you are looking at API 12K, where they specifically mention coils of A106-B without tubesheets, so in this case you would definitely pick a tubular (pipe) product because you wouldn't want to roll plate (516-70)

Here the queation is not what can withstand more temperature.

case 1 - seamless tube

case-2- plate rolled and then welded to form the tubes.

The codes will not allow you the second case even if the parent metal has a higher temperature withstanding capacity.