For ASTM A36 steel less than 3/4 inch thick, which SMAW low carbon filler metal classification can be used?

The appropriate classification for SMAW (Shielded Metal Arc Welding) low carbon filler metal that can be used with ASTM A36 steel less than 3/4 inch thick is indeed found in the classification E60XX and E70XX.

E60XX electrodes are designed for use in welding low carbon steels and are suitable for situations where a moderate level of strength and ductility is required. They can also provide good impact properties at low temperatures. On the other hand, E70XX electrodes are designed for higher strength applications, making them compatible with ASTM A36, which is a common structural steel specification. These electrodes maintain good weldability and offer the necessary mechanical properties for good performance in structural applications.

Using E80XX or E90XX would not necessarily be appropriate for ASTM A36 of this thickness, as these classifications are intended for higher strength materials and may not provide the required properties for lower strength steel. Likewise, E40XX and E50XX electrodes belong to different classifications that may not adhere to the requirements specified by the AWS D1.1 for this type of base material.