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1 A. Heat Treatments 1. Preheating and Postheating High Carbon or Alloy Steel Heat treatments are seldom required for low carbon or structural steels, although they are occasionally used to prevent warpage or to assure low hardness for machining. In welding high carbon or alloy steels, however, there is a danger that the weld deposit and heat-affected zone will contain a high percentage of martensite, a hard, brittle form of steel. Such welds have extreme hardness and low ductility, and may even crack while cooling. The purpose of preheating and postheating is to keep the martensite content of the weld at a minimum. Improved ductility, low hardness, and less chance of cracking during cooling result from both treatments. The martensite is actually formed during the cooling of the weld and the heat-affected zone. The amount of martensite formed can be limited by slowing down the rate of cooling of the weld. The heat treatments raise the temperature of the metal surrounding the weld so that there is less temperature difference between the weld and the surrounding metal. The result is that the heated weld zone cools more slowly since the rate of cooling is directly proportional to the temperature difference between the hot and cold masses. Whether these heat treatments should be used depends upon the amount of carbon and other alloying elements in the metal being welded. If test welds without heat treatment are found to have too low ductility or too high hardness, the need for preheating or postheating is indicated. a. Preheating A simple method is available to determine if preheat is required in welding a steel. The hardenability of a steel is approximately related to its carbon content plus the content of certain other alloying elements. The approximate amount of the other alloying elements which produce the same hardness as 1% carbon is known. Thus, an indication of the hardenability, called an “carbon equivalent”, can be calculated as follows: Making the Weld Preparations for Welding