It is possible to combine the use of E6013 and E7018 if there is no requirement that a low hydrogen electrode be used.

What comes to mind is ASTM A36 carbon steel plate and shapes. Per AWS D1.1, A36 up to 3/4 inch thick can be welded with either non-low hydrogen electrodes or low hydrogen electrodes if higher preheat is used. However, if the base metal is more than 3/4 inch thick, only low hydrogen electrodes can be used.

Another case would be ASTM A992; the ASTM specification and AWS D1.1 requires only low hydrogen be used, thus the WPS must not include A992 or any other base metal that requires the use of low hydrogen electrodes.

Whether it makes sense to write one WPS that permits either E6013 or E7018 is a decision that requires some forethought. One must consider the ramifications and the possibility of welders making mistakes such as using E6013 on materials that require low hydrogen electrodes. However, if the WPS is developed for a specific application (a widget) that can be welded with non-low hydrogen electrodes, using E7018 would not be a problem. There is no problem if the WPS allows either E6013 or E7018. One application that comes to mind is safety railing. There would not be problem unless the railing was welded to, let's say a A992 structural member.

Best regards - Al