what is the relationship between hld and hrc with regardig surface hardness?

Answer 1

These are just two different scales using two different indentors and two different load weights.

They may be converted from one to another in the region that they overlap. Try www.onlineconversions.com.

Before people really appreciated what was going on with this test, various indentors and weights were developed as standards targeting certain ranges of hardness of materials. That's why we have all these different ranges today.