Eutectoid Carbide Content

The eutectoid transformation is the point where, during cooling, pearlite forms directly from austenite. In welding, this matters because rapid cooling in the heat-affected zone means you blow past this point and get harder, more brittle structures like martensite. If you want to normalize the structure again, you have to heat the area specifically and let it cool slowly. Make sure the carbon content is right, otherwise you'll get an uneven structure that's prone to cracking.