Every weld tells a story, and the microstructure is the chapter where that story is written. For a welding engineer, reading a polished and etched cross-section is like decoding a narrative of thermal cycles, cooling rates, and phase transformations.
Start with the heat-affected zone (HAZ). The grain size gradient from fusion line to base metal reveals peak temperature exposure. Coarse grains near the fusion line indicate overheating; fine grains farther away suggest a lower thermal impact. Next, examine the weld metal itself. Dendritic structures point to solidification direction and cooling rate. Equiaxed grains often signal proper interpass temperature control or the use of grain refiners.
A common starting point is the base metal microstructure. Ferrite-pearlite bands in carbon steel can indicate rolling direction and potential anisotropy. In stainless steels, the presence of delta ferrite in an austenitic matrix tells you about the Schaeffler or WRC-1992 diagram prediction. Etching with different reagents (e.g., Nital for carbon steels, Kalling's for stainless) brings out different phases.
To build this skill, take a known WPS and examine a weld cross-section under a microscope. Compare what you see with the expected microstructure from the cooling rate and chemistry. Over time, you will learn to spot anomalies like untempered martensite, sigma phase in duplex, or hydrogen-induced cracks before they become failures.
How do you approach a new microstructure sample for the first time?