Correlation between Hardness and Tensile Strength

[ashfaqanwer]

Hardness and Tensile Strength are two very basic mechanical properties of any material.
Many a times, I have read that the correlation between hardness and tensile strength is very rough and there is not any perfect relation between them.
Like what I normally see in many of the reference books an estimated conversion of hardness value to TS is:
Tensile Strength (psi) = 500 x HB

How true this relation is on practical grounds? Any experimentation etc.? Please share your knowledge & experience.

[qaisarabbas]

Dear Ashfaq,

Yes! You are absolutely right that there is not a true & exact relationship between hardness and tensile strength in general for all metals. The conversion you have mentioned has a theoretical basis and may be used only an approximation. This is also evident from a graph (attached) in which handbook data of H & UTS (for different metals) is plotted to testify the relationship. This proved to be true to some extent upto 200 BHN but not for all metals.

Therefore, some literature also narrates following relationships for best approximation basis:
TS (psi) = 515 HB (for HB<175)
TS (psi) = 490 HB (for HB>175)

But above all, it must be worth mentioning that even ASM Metals Handbook does not specify any direct relationship between Hardness and Tensile Strength. Rather a table is referred by ASM, giving different relations for different metals based on actual hardness data. These are called Taylor's Equations & give a range of multiplying factor with hardness values (HB / HV).

Hope this answers your query. Regards - Qaisar

[ashfaqanwer]

Thanks Qaisar for the valuable inputs.

[mechcolor]

My personal experience with correlation between hardness and tensile strength is very confusing.
Years back, I got two weld joints tested on the basis of low hardness value but the tensile tests were appreciably acceptable. Actually, hardness (surface hardness) is a very crude method of estimating the actual mechanical properties of the material. Correlating hardness with tensile test may be good in some instances but not recommended. You must not be assuming any thing based just on the surface hardness results. If you have any confusions, take the part, get it the micro hardness test done or go for mechanical tests tensile, impact, fatigue (whichever required).