What is difference between steel Grade-40 & Grade-60? Explain briefly where its use & why use?

Grade 40 means steel having yield strength of 40Ksi and grade 60 means steel having yield strength of 60 Ksi. So grade 60 is stronger than grade 40.

According to ASTM A615 ,

Grade 40 steel must be able to carry a MINIMUM tesnsile strength of 420 MPA  ,  and MINIMUM yield strength equals to 300 MPA

FOr Grade 60 , MINIMUM tesnsile strength of 620 MPA  ,  and MINIMUM yield strength equals to 420 MPA

According to ASTM A706

Tensile strenght must be a minimun of 550 MPA for all grade types and 420 MPA yield strength . 

grade of steel is the ultimate tensile strength this material can stand for example a grade 40 steel could be used in the most common steel structure works eg beams,column and bracings conversely, grade 60 might be used in tougher environments and higher loads such as suspended bridges.

grade means that steel can overcomeKsi 

grade means that steel can overcomeKsi 

Grade 40= Steel strength 40000 psi = 275 MPa

Grade 60 = Steel strength 60000 psi = 415 Mpa

Uses vary from using as rebars or as steel built up or standard steel sections.