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
