Why E9015-B91 preferred over E9018-B91 for P91 welding?

Welding P91 material, a high-strength, heat-resistant alloy often used in high-temperature applications such as power plants, requires careful consideration of the welding process and the choice of welding electrodes. Both E9015-B91 and E9018-B91 electrodes can be used for this purpose, but E9015-B91 is preferred.