Obtaining Consistent Microindentation Hardness Data (part 2)
Microindentation hardness testing is a very powerful tool for characterizing materials and diagnosing problems. But it is a complicated process, and the subject is rarely taught in schools. We must decide which test – Knoop or Vickers – is best for the problem at hand and then choose the most appropriate test force to evaluate the problem. This is not a trivial task.
In our first installment, we began the discussion by focusing on Vickers and Knoop tests for materials of different hardnesses and different test forces.
Correlating Knoop Data to the Equivalent HK500 Value
The Knoop test is extremely useful but does have one very significant problem – the increase in HK with decreasing test force. It is possible to develop correction factors, although these factors have some inherent imprecision and will probably vary from operator to operator. An individual’s own variation in HK between 500 gf and lower test forces can be easily checked and compared to the data shown in Table 1 by performing a number of indents (e.g., 5) at various test loads from 500 gf and below on a certified test block, such as an HRC test block.
An example of such tests made on a number of steel tensile bars covering a wide range of HK is given in Figure 11. Figure 12 shows the approximate shift in HK as the applied force decreases and the HK500 value increases. Table 1 lists these approximate correction values as a function of the applied load and the HK500 value.
Using HK and HV at Various Loads
Carburized specimens are commonly evaluated metallographically and by microindentation hardness tests. Figure 13 shows the quenched-and-tempered carburized case of 1141 carbon steel. Figure 14 shows the microindentation hardness evaluation of the specimen using the Knoop indenter at test loads of 100, 200, 300 and 500 gf and the Vickers indenter at test loads of 200, 300 and 500 gf.
For the Vickers study, the 100-gf test force gave indent diagonals <20 µm, where measurement precision is difficult. Consequently, Vickers tests at 100 gf were not employed. As expected, the Knoop data at the four test loads shows a continuously increasing hardness trend with decreasing test force, while the Vickers data at the three test loads also shows some minor variation in HV but less than the HK data.
Both sets of data show a drop in hardness near the extreme surface, but this is defined better with the Knoop data, except for the 200-gf data. These tests were performed at different locations along the surface. Therefore, the 200-gf result at the surface could be due to a composition/microstructural difference at that location. The Knoop surface-hardness data for the 300- and 500-gf indents are identical.
Influence of Etching upon Microindentation Hardness
ASTM E 384 states that specimens to be tested using the microindentation procedure should not be etched. As every metallographer knows, this cannot be done in almost all cases. The metallographer must be able to see the microstructure, which in most cases requires etching to evaluate surface conditions such as decarburization, flame or induction hardening, carburizing, nitriding, etc.; to evaluate segregation; or try to use hardness differences to identify phases or constituents. If the specimen is deeply etched, the metallographer will be unable to see the indent tips. In general, if etching results are normal (not over-etched), there is no significant difference observed for the microindentation values in the etched versus unetched condition.
Two examples are shown in Figure 15. The first is a thick carburized case on 8620 alloy steel, which was subsequently heat treated using an isothermal hold to form lower bainite in the case while the core was tempered, low-carbon martensite. The first observance of martensite was at a depth of ~0.5 mm, and the structure was fully martensitic after a depth of ~0.69 mm. Overall, the difference in hardness between the as-polished specimen and the etched specimen was not significant, except for slightly higher HK in the core from a depth of ~1.5 to 2.25 mm.
The second example shows the measurement of decarburization depth in quenched-and-tempered 41S50 alloy steel. Overall, the differences are insignificant, although the two tests at depths of ~0.05 and 0.09 mm are slightly lower in the etched condition than in the as-polished condition. In both cases, the second run was performed near the first run. If the structure must be etched to properly locate the indents, etch as lightly as possible.
The use of microindentation tests where the test forces are ≤1,000 gf (the microindentation test force range) is a very important tool for the metallurgist for solving many characterization problems. This article has presented details about some of the inherent difficulties in obtaining precise, repeatable data. It did not illustrate the influence of specimen preparation upon the obtainment of precise, repeatable data.
As the test force decreases below 1,000 gf, specimen preparation becomes more and more important. Remnant damage introduced in sectioning and in specimen preparation (grinding and polishing) must be fully removed in order to get valid test results in the low-load range, and this becomes more critical as the force decreases below 1,000 gf. The types of tests illustrated here, which are very typical for many laboratories, require test forces well under 1,000 gf, usually in the range of 100 or 200 gf or below.
The equations defining the calculation of both Knoop and Vickers hardness have a built-in problem due to the division of the test force by the area of the indent cavity (Vickers) or the projected area of the indent (Knoop). As the diagonal length decreases below about 20 µm, we observe a very rapid increase in hardness with very small decreases in diagonal length. Thus, the usual imprecision in measuring an indent of approximately ±0.5 µm for most users can lead to very large variations in the calculated hardness. Consequently, every effort should be made, as far as possible, to use loads of ≥100 gf for Vickers and ≥50 gf for Knoop, for which this problem is not as acute (where the long diagonal or the mean diagonals, are >20 µm long).
Measurement of small diagonals (≤20 µm in length) does require a high-quality optical system and multiple objective powers. This cannot be accomplished using a single 40X or 50X objective. The illumination system must be as good as a high-quality light microscope with adjustable diaphragms.
The so-called load-hardness or “indentation size effect” problem is not due to interactions between the indenter and dislocations, as previously claimed by more than 60 publications. It is strictly a visual-perception problem. If the indents are measured with higher-magnification, high-quality objectives as their length decreases, results are far more consistent. Vickers hardness values will be statistically identical down to at least a 25-gf test load. Lower test forces, especially with high-hardness steels, cannot be accurately measured consistently with the light microscope due to its resolution limitation and image contrast for indents smaller than ~20 µm.
Case hardness was shown to be slightly better revealed using Knoop than Vickers, particularly at the extreme surface where a Knoop indent can be placed closer to the outer surface than a Vickers indent. Testing an etched surface, as long as it is not highly over-etched (and very dark), does not cause the reported measurement problems.
A procedure for correcting Knoop hardness test values made at forces <500 gf back to the approximate Knoop hardness for 500 gf, is illustrated in Fig. 7 (part 1, August 2014). Conversion tables for Knoop to other test scales, and to tensile strength, are based on the Knoop hardness at 500 gf. The Knoop hardness does increase as the test force decreases below 500 g. Therefore, such corrections can be very useful for proper comparison of tests made at different test loads.
For more information: Contact George F. Vander Voort, Vander Voort Consulting L.L.C., Consultant – Struers Inc., 24766 Detroit Rd., Westlake, OH 44145; tel: 847-623-7648; e-mail: email@example.com; web: www.struers.com and www.georgevandervoort.com.
- Vander Voort, G. F., Metallography: Principles and Practice, McGraw-Hill Book Co., NY, 1984; ASM International, Materials Park, OH, 1999, pp. 356, 357, 380 and 381.
- Vander Voort, G.F., “Results of an ASTM E04 Round Robin on the Precision and Bias of Measurements of Microindentation Hardness,” Factors that Affect the Precision of Mechanical Tests, ASTM STP 1025, ASTM, Philadelphia, 1989, pp. 3-39.
- Vander Voort, G.F., “Operator Errors in the Measurement of Microindentation Hardness,” Accreditation Practices for Inspections, Tests, and Laboratories, ASTM STP 1057, ASTM, Philadelphia, 1989, pp. 47-77.
- Vander Voort, G. F. and Fowler, R., “Low-Load Vickers Microindentation Hardness Testing,” Advanced Materials & Processes, Vol. 170, April 2012, pp, 28-33.