heat affected zone??

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how does the coloration correspond with the HAZ? i am asking this more in relation to GTAW single pass welds on thin steel and stainless. the question stems from shielding gas is sometimes able to keep the coloration limited to almost none, and my question is does the edge of present color coincide with the end of the HAZ? or can the HAZ extend on past the end of the coloration?
Reply:no doubt better answers will follow mine... but dont confuse heat discoloration with  haz. i believe the haz is limited to an area just beyond the puddle, the area of actual melt...
Reply:The size of the HAZ will depend on the heat input to the weld puddle, the material, and the environmental conditions(temp/wind/preheat) during welding.  The discoloration on the surface does not correlate to the HAZ.   The HAZ can have chemistry, microstructure, and grain size that differ greatly from the base metal and the weld bead.  Stainless vs mild steel vs high strength low alloy(HSLA) base metals will all have different HAZ characteristics after welding.  The chemistry of the filler metal will also influence the nature of the HAZ.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:The discoloration of the SS weld and base metal is related to the temperature of the steel and the presence of oxygen while the metal was above some oxidizing temperature.  If you welded in an inert glovebox with O2 less than about 30 parts per million (ppm), your weld and heat affected zone would remain shiny silver with no discoloration.So, the heat affected zone can be determined by cross sectioning the weld, polishing, and etching to reveal the grain structure, where you could see a change in structure from the unaffected base metal to the heated portion next to the weld, and into the weld In hardenable steels, the HAZ can be quantified by doing a series of micro-hardness measurements across the weld and base metal.