A material’s hardness refers to the ability of a material to resist penetration by a harder material. It is an empirical measurement of material properties signifying the ability of a material to endure indentation under a static load or scratching. The resultant hardness value relies on the testing technique.
The main techniques are:
- Brinell, which uses a tungsten carbide ball indenter with hardness value calculated from the diametrical relation between the ball and the impression made and the test force used.
- Vickers and Knoop, which uses a diamond indenter with the Vickers having a pyramid-shaped diamond and the Knoop having a rhombohedral shaped diamond indenter, the hardness value is calculated from the force applied and the length of the diagonals of the resultant indent.
- Rockwell, which uses a tungsten carbide ball or a conical diamond indenter with hardness values calculated from the relation between the indentation depth under a large load compared to that created by a pre-load.
There are a wide variety of commercial machines that perform hardness testing based on the above testing techniques. Buehler is one of the top suppliers of indentation hardness testing equipment, manufacturing machines to provide solutions for all of these techniques individually or combined in one machine.
The following article highlights Vickers testing of welded components and how its also been applied to mapping of materials. Indentation mapping relates to the ability to map out variations
in hardness across a sample because of the non-homogenous nature of materials. Non-homogeneity, displayed in variations in hardness across a test piece, is linked to processing parameters. This will be demonstrated for a welded and an additive manufactured metallic component. Mapping can also be used for materials with dual or multiphase components displaying localized variations in hardness.
Weld joints of metallic components show varying mechanical properties starting from the weld itself, which might be of the same material as the welded workpieces or might be of a different material (alloy) compared to the components being welded together. Because of the high temperature nature of the process, a fusion process occurs causing localized change in microstructures from the weld, into the base materials.
The change in microstructure around the weld region (in the base material) near the fusion line poses challenges to the reliability of the weld presenting an area prone to high residual stresses (1-2). Development of high stresses coupled with unwanted microstructure renders an area with a high probability for the welded joint to fail. This area is categorized as a heat affected zone (HAZ) in a weld and is mostly observed in the base/parent material. The different components of a weld include the base metal displaying the depth of penetration of the weld, the HAZ, the fusion line and the weld bead as illustrated in Figure 1 (a). For a multipass welded sample, the weld bead will have an equivalent HAZ, fusion lines and actual bead as the weld structure is built-up, as illustrated in Figure 1 (b), with three weld beads.
Figure 1. (a) A schematic illustration of a weld, heat affected zone and base metal and (b) shows a fillet weld with three weld passes on a low alloy steel substrate etched using 2% Nital to reveal the weld, heat affected zones and the three weld passes making up the weld.
To observe the weld microstructures, components are microscopically analyzed by doing metallographic testing involving a series of grinding/polishing stages and etching to expose the microstructure, Figure 1 (b). The degree and/or extent of HAZ can then be metallographically tested for potential areas of risky microstructural changes prone to micro-crack formation.
For quantitative comprehension of weld microstructures, indentation hardness testing is typically performed based on ISO 9015/15614, which defines how testing is performed; stipulating loads, number of indents for parent/base material, HAZs and welds, as well as the distances between indents and the depth beneath the surface of the welded joint. To meet the necessities stipulated in the standards can be time consuming, fairly challenging and subject to operator skill/experience. Buehler has streamlined the process using its own in-house software – DiaMet as shown in Figures 2 and 3 for a fillet and a butt weld respectively.
Figure 2. Illustrates hardness testing of a fillet weld with differences observed from the base metal, HAZ and weld regions.
Figure 3. Illustrates hardness testing of a butt weld, the chart shows local variation around the heat affected zone (HAZ).
Depending on the weld and base metal type, the different regions of the weld will display different microstructures. These microstructures have changing hardness levels as shown in Figures 2 and 3 above. When traversing towards the weld, start with continuous base metal, followed by a region with tempered and inter-critical HAZ, a fine grained HAZ, a coarse grained HAZ towards the weld fusion line and then the weld. The HAZ region shows a diverse microstructure and will always be prone to localized differences in hardness as illustrated on the chart, blue circles, Figure 3. It is clear that standard methods based on ISO9015 might trap local variations in hardness around the HAZ since it involves carrying out a row of indents. For a full picture of the weld, weld mapping can offer a comprehensive overview of this localized variations and potential site of high stresses.
With automated hardness testing, enhanced testing speeds and development in hardness testing software, a substitute technique of qualitative and quantitative testing of welded components is nowadays possible. This involves performing a number of indentations on a scanned area of a weld and then assigning the differences in hardness values from the base material into the weld a color code, from which the software offers a visual output of the variations in hardness. The main benefit of using the indentation mapping is the ease of identification of regions where high residual stresses occur. The maps can also be employed to qualitatively examine the HAZ, the base material and the welded regions without the necessity to etch the sample. Figure 4, shows the weld sample in Figure 1 and 2, mapped with approximately 3000 indents.
Comparing Figure 4 (b) and Figure 1 above, there is a good connection between the etched and mapped out micrographs with both techniques revealing the weld beads, the HAZ, and base/parent materials.
Figure 4. (a) Illustrates weld mapping with chart showing actual Vickers values and (b) showing map with weld passes (harder in red), heat affected area (light blue) and base metal colored blue.
Figure 5 illustrates steel butt welded samples constructed through a sequence of multi-passes (a). These types of welds are basically taken through a post weld heat treatment (PWHT) to lessen the residual stresses produced during build-up. However, residual stresses are not fully removed and examination of the weld after PWHT will also show these regions as illustrated in Figures 5(b) and (c). It is obvious from the maps where high residual stresses occur (high hardness values), chiefly around the HAZ, Cap and root areas.
Figure 5. (a) Shows a sequence of weld passes from the root to the weld cap, (b) & (c) show two butt welded components after mapping. The weld maps show high stress areas characterized by high hardness around the toes on the weld face, down the heat affected zone near the fusion line and in the root areas of the weld.
Additive Manufactured Components
Additive manufactured components are constructed by incorporating material layer by layer. These components can be made from steels, Inconel alloys, titanium and many others. Manufacturing processes include powder bed fusion (PBF) and directed energy deposition (DED) methods. The DED technique commonly has a higher build up rate compared to the PBF method with both methods resulting in residual stress development because of repeated thermal cycles during layer addition.
These residual stresses have an important role in the integrity, performance and lifetime of the components and thus the ability to assess them is vital (3-4). To map these components out, they have to first be metallographically prepared with the level of surface finish based on the load being used during Vickers indentation testing.
Figure 6. Illustrates an additive manufactured component mapped out illustrating regions that would be considered as having high residual stress (red).
Case/Induction Hardened Surfaces
Certain mechanical parts are usually surface hardened to enhance their mechanical performance by optimizing surface wear characteristics with the benefit of low core strength features to absorb stresses without cracking from recurrent usage. Examples include induction hardened cranks, gears (Figure 7) and cam shafts found for example in automotive industry. How evenly these surface treatments are performed, and their conformance to operational specified hardness limits, offers a degree of confidence on the final mechanical performance.
Traditional techniques involve performing effective case depth or case hardness depth studies on a single row of hardness indents beginning from the surface into the core of the material and noting where the hardness values go below 550 Vickers point. Figure 7 is a snapshot view of how hardening has been done. The substitute and a more complete way of assessing the case/induction hardening process is to map out an area of interest as illustrated in Figure 7 above, which emphasizes uniformity in hardness as a function of the core hardness using mapping tools in DiaMet software.
Figure 7. Illustrates a hardened gear tooth mapped out to illustrate the case hardened outer region and its uniformity across the gear tooth. High hardness regions in red have Vickers values approaching 800 Vickers whereas low hardness areas typical of core at approximately 400 Vickers.
- Weld measurements based on ISO standards can be realtively fast and easy to perform with the weld tool on DiaMet software. Traditional methods would take a longer time and were also based on operator skill/experience.
- Buehler also showed there is good correlation between etched weld samples and hardness mapped area. It is simple to identify the base/parent metal, HAZs and the weld from mapped micrograph similar to an etched weld sample. It is also clear from the micrographs that it is comparatively easy to identify weld passes in a multipass weld and how each equivalent weld bead affects the adjacent one as the weld is built up.
- Additive manufactured parts can be mapped emphasizing local variations in hardness as a way of locating areas of high residual stresses.
- Case hardened materials can be mapped thus offering a comprehensive pictorial view of uniformity of a hardening process.
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- Kumar S, Kundu A, Venkata KA, Evans A, Truman CE, Francis JA, et al. Residual stresses in laser welded ASTM A387 Grade 91 steel plate. Materials Science and Engineering A. 2013; 575:160-168.
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- Bernd B, Omer B, Rosemary G, Additive manufacturing of Ti–6Al–4V components by shaped metal deposition: Microstructure and mechanical properties, Materials & Design, Volume 31, Supplement 1, June 2010, S106-S111
- metal deposition: Microstructure and mechanical properties, Materials & Design, Volume 31, Supplement 1, June 2010, S106-S111
This information has been sourced, reviewed and adapted from materials provided by Buehler.
For more information on this source, please visit Buehler.