The previous article dealt with the conventional Vickers and Brinell hardness tests. This second article reviews micro-hardness and portable hardness testing. The investigation of metallurgical problems in welds often requires the determination of hardness within a very small area or on components in service or too large to be able to test in a laboratory environment.
Micro-hardness testing may be carried out using any one of three common methods and, as with the macro-hardness tests, measure the size of the impression produced by forcing an indentor into the specimen surface under a dead load, although many of the new test machines use a load cell system.
The three most common tests are the Knoop test, the Vickers test and the ultrasonic micro-hardness test.
The Knoop test uses a pyramidal indentor that gives an elongated diamond shaped impression with an aspect ratio of around 7:1, the Vickers test uses the pyramidal indentor described in the previous article (January/February 2005).
The Knoop test is rarely used in Europe where the Vickers test is the preferred method. The loads used for the tests vary from 1gmf to 1kgf and produce impressions that need to be measured by using a microscope with magnifications of up to 100X, although modern machines may be equipped with an image analysis system that enables the process to be automated.
The ultrasonic hardness test does not rely upon measuring the size of an impression. Instead, the test uses a Vickers diamond attached to the end of a metal rod. The rod is vibrated at its natural frequency by a piezoelectric converter and then brought into contact with the specimen surface under a small load. The resonant frequency is changed by the size of the impression produced and this change can be measured and converted to a hardness value.
The size of the impression is extremely small and the test may be regarded as non-destructive since it is non-damaging in most applications.
The micro-hardness test has a number of applications varying from being a metallurgical research tool to a method of quality control. The test may be used to determine the hardness of different micro-constituents in a metal, as shown in Fig.1. Where an impression would be damaging, for instance on a finished product, micro-hardness tests, particularly the ultrasonic test, may be used for quality control purposes. Micro-hardness testing also finds application in the testing of thin foils, case hardened items and decarburised components.
Portable hardness tests may be used where the component is too large to be taken to the testing machine or in on-site applications. It is useful on-site, for example, for checking that the correct heat treatment has been carried out on welded items or that welded joints comply with the hardness limits specified by NACE for sour service. There are three principal methods - dynamic rebound, Brinell or Vickers indentation or ultrasonic testing.
The Leeb hardness test uses dynamic rebound where a hammer is propelled into the test piece surface and the height of the rebound is measured. This gives a measure of the elasticity of the material and hence its hardness.
This type of test is typified by the 'Equotip' test, Fig.2, a trademark of Proceq SA. The Equotip tester comprises a hand-held tube that contains a spring loaded hammer. The device is cocked by compressing the hammer against the spring, the device is then positioned vertically on the test surface and the release button is pressed. The hammer strikes the surface, rebounds and the result displayed digitally. Generally the average of five readings is taken.
To obtain a valid result, the position of the device, the flatness of the surface and the flexibility of the component all affect the accuracy of the results. Needless to say the skill and experience of the operator is one of the key factors in producing accurate hardness figures. The results are generally converted to give a hardness in Vickers or Brinell units.
The other type of portable hardness test in common use is the ultrasonic method described above. Commercially available machines are typified by the Microdur unit supplied by GE Inspection Technologies as shown in Fig.3. This type of equipment is electronically based and can be programmed to give hardness readings of any type - Vickers, Brinell, or Rockwell. Needless to say, any of these methods of hardness testing require regular calibration of the equipment, fully trained operators and well prepared surfaces.
Although there are several different methods of hardness testing the results can be compared and converted. The ASTM specification E140 contains conversion tables for metals - ferritic and austenitic steels, nickel alloys, copper and brass- for converting Vickers to Brinell or Rockwell or vice versa.
To end this article on hardness testing let us look at the significance of the results.
Hardness is related to tensile strength - multiplying the Vickers hardness number of a carbon steel by 3.3 will give the approximate ultimate tensile strength in N/mm2 . A hardness traverse across a weld and its HAZs will therefore reveal how the tensile strength varies. In carbon or low alloy steels a hardness of above approximately 380HV suggests that the hard brittle microstructure, martensite, has been formed leading to the possibility of cold cracking during fabrication or brittle fracture in service. This fact has been recognised in the specification EN ISO 15614 Part 1 so that a maximum hardness of 380HV is permitted on a hardness traverse of a macro-section from a carbon steel procedure qualification test.
ASTM E 10 Brinell Hardness of Metallic Materials
ASTM E 140 Hardness Conversion Tables for Metals.
ASTM E 110 Portable Hardness Testing.
ASTM E 384 Microhardness Testing of Metallic Materials.
ASTM E 103 Rapid Indentation Hardness Testing.
ASTM E 18 Rockwell Hardness Testing.
ASTM E 92 Vickers Hardness of Metallic Materials.
This article was written by Gene Mathers.