Comparison of Steel Grades

In order to determine which statements are true, let's analyze each option separately:

a) 7C4 grade steel is more ductile than 10C4
Ductility is the ability of a material to undergo plastic deformation without breaking. It is typically measured by the percentage elongation or reduction in area during a tensile test. In general, higher carbon content makes the steel harder and less ductile. Therefore, it can be concluded that 7C4 grade steel (with lower carbon content) is more ductile than 10C4 grade steel.

b) Yield strength of 40C8 is greater than of 30C8
Yield strength is the stress at which a material begins to deform plastically. It is an important mechanical property as it indicates the maximum stress a material can withstand without permanent deformation. However, the numerical values provided (40C8 and 30C8) do not provide sufficient information to determine the yield strength. The numbers before the "C" represent the carbon content, which is not directly related to the yield strength. Therefore, it cannot be concluded which steel grade has a higher yield strength based solely on the information provided.

c) Hardness of 30C8 is greater than of 40C8
Hardness is a measure of a material's resistance to localized plastic deformation, such as scratching or indentation. It is often correlated with other mechanical properties, such as strength. Higher carbon content generally leads to higher hardness in steels. Therefore, it is likely that 30C8 grade steel (with higher carbon content) has greater hardness than 40C8 grade steel.

Conclusion
Based on the analysis, the only true statement is that 7C4 grade steel is more ductile than 10C4. The other statements cannot be determined based on the information provided.