The main differences between the two are as follows:
Grade C: Has a higher maximum carbon content (up to 0.35%, compared to 0.30% for Grade B) and higher tensile and yield strengths. Its advantage lies in its higher allowable stress, which allows for thinner wall thicknesses; its disadvantage is poorer weldability, requiring preheating and post-weld heat treatment.
Grade B: The standard grade for most high-temperature piping; it features a relatively simple welding process and is more suitable for routine on-site construction.