The heat-affected zone caused by flamecutting in certain grades like 1045, 4140,
or 4340 presents a risk for possible cracking; it is also desirable and often
necessary to reduce the hardness in the heat-affected zone before machining.
This is usually accomplished by annealing the parts.
Conventional annealing, also called solution or full annealing, involves
heating the parts to a temperature above the range for grain transformation.
For 1045, 4140, and 4340 this is generally 1550º F. The temperature
is held long enough to ensure uniform heating, and the parts are then
allowed to cool in the closed furnace.
This process as generally applied does not provide any specific microstructure,
but it does ensure a uniform and more homogeneous structure, eliminating
the danger of cracks from the stresses in the heat-affected zone, and
improving the overall machinability.
Spherodize annealing can be done when desired to achieve a specific
spheroidal or globular grain structure. This involves repeated heating
and cooling cycles at specific temperatures and times. It is time-consuming
and expensive and usually done only for exceptional applications.