What is Hardness Testing of Metals in Welding Inspection?

16 July 2018

Strong alloys endure when extraordinary structural loads are applied. Durable and hard, the metal won’t fatigue, won’t deform, nor will it be impacted by abrasive forces. Clearly, just from reading that opening, metal hardness is an essential part of a welding inspector’s job. There’s no use having an ultra-strong joint if the surrounding metal is weak, after all. Anyway, just how is a welding professional meant to test for hardness?

What is Hardness Testing? 

This NDT (Non-Destructive Test) exists to ensure a weld zone or structural loading area isn’t weak. If the weld joint or structural component is weak, then plastic deformations occur. Next, just like any other measurement system, hardness testing uses several well-established engineering standards. There’s the Brinnel Scale, the Rockwell Scale, and several other notable hardness gauging standards in place. As for the test, it can be as simple as an indentation or as complex as a digitally measured readout.

Portable H.T. Testing 

Used for non-destructive metallurgical assessments, Hardness Testing (H.T.) instrumentation is available in an easy to mobilize form. The gear looks like a standard multimeter, a piece of kit found in an electrician’s toolbox, but its capabilities are entirely different. Instead of digitally measuring voltages and currents in wires, portable hardness testers use special probes to metallurgically examine metal surfaces. Ultrasonic technology is one solution here, but there are also impact bolts, moving parts that strike surfaces. In this case, it’s the rebound velocity of the spring-loaded rod that supplies hardness data to the portable tester.

Setting the Hardness Test Area 

Back in the lab, an inspector is checking gear wheels and drive components under a microscope. He’s applying precise quantities of force to a diamond-tipped probe, and there’s a large, stable anvil under the instruments. Out in the field, it’s a harder proposition. The welding inspector is checking flat and round structural pieces. Weld overlays and HAZ sites (Heat Affected Zones) are a core part of the job, so the inspector needs his wits about him, plus one of those specialized digital testers. Wielding the spring-loaded rod, a known impact force is applied, the probe rebounds, and the tensile strength of the test area is calculated by the test instrument.

It’s a fascinating field, one that’s not limited to the weld inspection profession. Those indentations are studied in great detail. The Brinnell and Rockwell measurements are also studied, then they’re recorded and sent to the project leader. Conversion tables, indentation tools, handheld instrumentation and microscopes, all of these items are loaded into a weld inspector’s hardness testing kit so that he can non-destructively determine deformation risks and the likelihood of metal fatigue.

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