Correlation of Hardness Values to Tensile Strength

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Correlation of Hardness Values to Tensile Strength
Semih Genculu, P.E.

Various procedures and approaches are utilized to determine if a given material is suitable for a certain application. The material may be tested for its ability to deform satisfactorily during a forming operation, or perhaps for its ability to operate under a certain stress level at high temperatures. For technological purposes, economy and ease of testing are important factors.

Hardness tests: In many cases it is possible to substitute for the relatively time consuming and costly tensile test with a more convenient test of the plastic deformation behavior of metals, a hardness test. Hardness is defined as resistance of a material to penetration of its surface, and the majority of commercial hardness testers force a small penetrator (indenter) into the metal by means of an applied load. A definite value is obtained as the hardness of the metal, and this number can be related to the tensile strength of the metal.

In the Rockwell test, hardness is measured by the depth to which the penetrator moves under a fixed load. The elastic component of the deformation is subtracted from the total movement. In the Brinell and Vickers/Knoop scales, on the other hand, the hardness is measured by dividing the load by the area of an indentation formed by pressing the corresponding indenters into the metal. Therefore while the Rockwell number is read directly from a gage, which is part of the tester, the Brinell and Vickers/Knoop require optical measurements of the diameters or diagonals, respectively.

While all indentation hardness tests may be thought to serve the same purpose, each one has definite advantages with some being more applicable to certain types of materials and size and shape parts than the others. Brinell is used primarily for forgings and cast irons. Its large test area allows an average representative value to be obtained in a material that contains features/phases with vastly different properties (i.e. graphite, matrix, carbides, etc.). Vickers and Knoop are used on very small and thin parts as well as for case depth determinations, and Rockwell on almost all other applications. The table below provides basic information regarding the most commonly used hardness tests.



Usual load range, kg

Rockwell-C Scale Rockwell-B Scale
Knoop (microhardness)

Diamond cone 1/16" carbide ball 10 mm carbide ball Diamond pyramid Diamond pyramid

150 100
0.5-100 0.01-1

Typical range of hardness Medium to very hard Soft to medium
Soft to hard
Very soft to very hard
Very soft to very hard

Surface preparation needed for testing Fine sanding Fine sanding
Coarse sanding
Fine polishing

Although the Rockwell test procedure is relatively straight forward, a number of items can contribute to inconsistent and incorrect readings and should not be overlooked. These items include the following:

x Cleanliness of the tested surface and the support anvil x Curvature of the surface (correction factors must be used) x Test surface not being perpendicular to the indenter x Readings taken too close to the sample edge x Readings taken too close together x Test sample too thin for the hardness scale being used x Part not supported properly x Damaged indenter

Standard method for testing metallic materials using the Rockwell scales can be found in ASTM E18.

There is no general method of accurately converting the hardness numbers determined on one scale to hardness numbers on another scale, or to tensile strength values. Nevertheless, hardness conversion tables are published by ASTM, and often by hardness equipment manufacturers in the literature. Such conversions are, at best, approximations and therefore should be treated with caution. The Standard Hardness Conversion Tables for Metals, ASTM E140, give approximate conversion values for specific materials such as steel, austenitic stainless steel, nickel and high nickel alloys, cartridge brass, copper alloys, and alloyed white cast irons. The first two tables below, which are reproduced from ASTM A370, give the approximate interrelationships of hardness values and approximate tensile strength of steels. It is possible that steels of various compositions and processing histories will deviate in hardnesstensile strength relationship from the data presented below. Also, the data in these tables should not be used for austenitic stainless steels, but have been shown to be applicable for ferritic and martensitic stainless steels. Furthermore, the data in these tables should not be used to establish a relationship between hardness values and tensile strength of hard drawn wire. Where more precise conversions are required, they should be developed specifically for each steel composition, heat treatment, and part. The third table is reproduced from SAE J417, whereas the fourth one is published by Wilson-Instron Corporation. Cautions should be exercised if conversions from these tables are used for the acceptance or rejection of a product.

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Correlation of Hardness Values to Tensile Strength