A hardness test can be used to give a rough estimate of a material's strength. A tensile test, in which a specimen is strained to complete failure, is obviously a much more accurate test of strength. Why would hardness tests ever be used instead of tensile tests?

The hardness test is preferred to tensile test because it is simple, easy, and relatively nondestructive.  

Tensile testing is a destructive test process that checks tensile strength, yield strength, and ductility of the metallic material.

Explanation:

Hardness test is a test carried out to check the resistance of a material to withstand permanent deformation such as indentation, wear, abrasion, scratch. It evaluates a material’s properties, such as strength, ductility, permanent deformation and wears resistance, and so helps you determine whether a material or material treatment is suitable for the purpose you require. This test is carried out by penetration of the material through another harder material.

Tensile testing is one of the most common mechanical testing techniques used to test the strength of material and also how much it can be stretched before it breaks. It is a destructive test process that provides information about the tensile strength, yield strength, and ductility of the metallic material.