Hardness Testing in Welded Materials17 August 2018
Welded materials are put under extraordinary stress. So far, the cooled fusing zone and depth of the weld are satisfactory, as is the shape and texture of the material. It’s even optimized been so that a perfectly profiled joint is anchoring two parts together. Everything looks good, but, as any engineer knows full well, looks can be deceiving.
The weld geometry is optimal, inside and out, and the welder has given the job a big thumbs up. But wait, as solid and well-formed as the weld might seem, there’s no way the welder, or a sharp eye, for that matter, can judge material hardness with any real authority. Welding inspectors need another handful of tools in their kit, tools that are designed to assess this crucial jointing feature. To that end, the inspector pulls out a diamond-tipped indentation instrument. Tapping the weld material with the “indenter,” a trained welding inspector soon determines the hardness of a weld joint.
Controlling Analysis Variables
The way this job has been described so far, well, it all makes the job seem a little crude. In reality, the indentation force has to be controlled, has to be applied with a measured quantity of kinetic energy. Only then, when the indentation depth produced by the tool and the force applied by the impact tool are both known, can a formula be used to produce a hardness valuation. Finally, with the gauged value known, the figure is input into a standard hardness gauging scale, such as the Vickers Hardness Scale.
The Diamond Pyramid
Again, although the test looks crude, an enormous amount of engineering know-how determines the many analysis vectors. For starters, the indenter tip is sharp, but it’s also specially profiled. Take a closer look at that fine tip. The diamond is shaped as a pyramid, a profile that has a square base and faces that extend upwards at a 136-degree angle. From here, hardness conversion charts and assessment reports dominate. A smaller indent is, of course, desirable, for it denotes a harder weld, one that has adequate tensile strength and a dense microstructure.
In the lab, intricate instruments apply pre-configured quantities of impact force, then a computerized interface instantly charts weld hardness. Out in the field, inspectors use portable electronics and indentation tools to do the same job. They take the shape of pressure-driven ball bearings or of diamond-tipped indenters. Lastly, with the non-destructive marking phase complete, the size of the mark and the known impact force measurements are used to confirm the hardness of the weld, as gauged on the Vicker’s or Brinnell scale.
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