Hardness Testing

Hardness testing is a destructive testing method that measures the resistance of a material to permanent plastic deformation caused by an indenter, far harder than the material itself, pressed into its surface under a controlled force. Physically, hardness is defined by the ratio between the applied force and the size of the indentation this force leaves on the material surface: the smaller the indentation produced under the same load, the harder the material. Although hardness is not an independent fundamental material property, it correlates strongly with yield strength, tensile strength, wear resistance and the microstructure obtained by heat treatment, and it is therefore used in manufacturing and maintenance processes as a rapid and reliable indicator of material quality. The most common static methods are Vickers (HV), Brinell (HBW) and Rockwell (HRC/HRB), which differ in indenter geometry and load magnitude.

In the Vickers method the indenter is a square-based diamond pyramid with an included angle of 136°. The specified load (generally between 1 kg and 100 kg, from HV1 to HV100) is applied free of vibration and impact, removed after a dwell time of 10–15 seconds, and the two diagonals of the rhombic indentation remaining on the surface are measured optically. The hardness value is calculated from the ratio of the applied force to the surface area of the indentation. The main advantage of Vickers is that, because the same diamond indenter is used across the entire load range, it gives results on a single continuous scale for both soft and very hard materials, and its small indentations allow it to be applied to thin sections, coatings and narrow regions. The Brinell method uses a hard metal ball and, because it leaves a large indentation, is preferred for average hardness measurement on heterogeneous, coarse-grained materials such as castings and forgings. The Rockwell method measures the indentation depth directly, providing a rapid reading, and is widely used in series production control.

The reliability of the measurement depends largely on surface preparation and instrument calibration. Because the indentation diagonals are read optically in Vickers and microhardness measurements, the surface must be ground and polished and, where necessary, etched; rough surfaces distort the diagonal reading and produce high uncertainty. It is essential that the test load is selected to suit the specimen thickness (the indentation depth must not exceed a defined proportion of the section), that the spacing between indentations and the distance to the edge are kept at the minima prescribed by the standards, and that the test temperature is controlled. Instruments are verified regularly with traceable reference hardness blocks; the indenter, the load cell and the optical or depth measuring system are subject to periodic calibration. Measurements can also be made in the field with portable dynamic (Leeb) or UCI instruments; since these methods carry higher uncertainty than laboratory measurements, the conditions of application are evaluated carefully.

One of the most critical fields of application of hardness testing is welded joints. A hardness profile taken across the weld metal, the heat-affected zone (HAZ) and the parent metal reveals the hard and brittle martensitic structures created by the thermal cycle. Particularly in sour service conditions containing H2S in the oil and gas sector, a hardness limit (for example 248 HV10 / 22 HRC) is applied to prevent sulphide stress cracking, and exceeding this limit calls for post-weld heat treatment (PWHT) or a re-evaluation of the welding parameters. Heat treatment verification, determination of surface hardening (carburising, nitriding) depth and material acceptance inspections are further principal uses of hardness testing.

Reporting covers the method and load used (for example 320 HV10), the measurement locations, applicable conversion values and the conformity assessment against the acceptance criteria of the relevant standard. The report states clearly that converting a hardness value to another scale or to tensile strength is only approximate and valid within defined standard tables, and that it does not replace the measured value. TÜV AUSTRIA SILA KALİTE performs hardness testing within the scope of its TS EN ISO/IEC 17025 accreditation, with traceable calibration and evaluation of measurement uncertainty; the results thus provide impartial and repeatable assurance recognised both nationally and internationally.