American Welding Society Forum
While trying to explain the HAZ to a non-welder, I realized my knowledge had slipped on the subject. Could anyone give me a brief explanation of the HAZ to relate it to non-welders?
Heat Affected Zone (Hard As- Zone) Generally hard area due to dilution and rapid cooling.
Oops!!! No, there is no admixture (dilution or pick-up) in the HAZ. Yes you are correct some areas of the HAZ are generally higher in hardness as compared to the base material. But by definition, the HAZ contained NO melted material, it is simply the region beyond the fusion line, where the heat from the welding process effected the microstructre. The closer you are to the fusion line the higher the temperature. As the temperature approachs the melting point of the material, the material recrystallizes,and because of the high cooling rate, changes the size of the grains. There are several "areas" that make up the HAZ, consisting of both coarse and fine grains.
The easiest explaination of the HAZ to a non-welder is simply what the term states: Heat affected zone- a zone directly next to the weld, which has been affected or changed due to the heat of welding.
The heat of the molten weld metal changes the base metal grain structure, usually for the worse. It does not melt the base metal but you get larger grain size and usually a harder and less ductile metallurigal structural. HAZ are the result of the base metal being raised to temperatures from just below the transformation temperature to just below the melting point. Cooling rates in this area are the most rapid due to contact quenching.The use of smaller diameter electrodes, lower welding currents, and faster travel speeds will all tend to decrease the heat input and therefore increase the cooling rate. This is one reason that preheat is specified to slow the cooling rate which allows the formation of ferrite and pearlite instead of martensite.
The key thing to remember is that cooling rate affects the HAZ and a slow cooling rate is better. Also the low alloy steels are more readily affected by a rapid cooling rate and it is easy to form martensite.
I'll give you a simple explanation on what the Heat Affected Zone is, suitable for a NON WELDER, i.e., one who knows nothing (or very little) about welding. When you weld, you deposit molten metal -which is at a very high temperature- on the base metal, which is at room temperature, or at the most has received some preheating at some 150 ºF or so. It's obvious that the molten metal, when dropping onto the base metal, will melt a little part of it. When doing this, the molten metal will pick up some of the chemical elements (ingredients) of the base metal, incorporating them into its chemical composition. This is called DILUTION. Inmediately after, while the molten metal solidifies, it's also obvious that it'll heat up a narrow stripe of the base
metal, i.e., the one which is adjacent, WITHOUT MELTING IT. This is called the HEAT AFFECTED ZONE (HAZ), which suffers the metallographic changes that Mr. Johnson has brilliantly explained. The region adjacent to the HAZ has also heated up, but the temperature rise is insufficient to produce metallographic changes.
Satisfied with the explanation? If you need some further help, post an advise on this site.
Giovanni S. Crisi
Sao Paulo - Brazil
Thanks for all the great info.
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