Heat Affected Zone
Due to the heat generated in flamecutting, the steel along the cut
edge is changed in a couple of different ways. First and foremost,
the very surface of the edge will be a thin layer of brittle, resolidified
steel, altered in chemistry. This is referred to as the decarb (for
decarburized) layer. Depending on the grade of plate and the end-use
application, it may be necessary to remove the decarb layer before
the flamecut part is used; this is often done by grinding in preparing
plate for welding, or as part of a machining operation.
The decarb layer is part of an area near the cut that is known as
the heat affected zone (HAZ). The HAZ is that portion of the steel
plate that got hot enough in the flamecutting process to cause a change
in the metallurgical structure of the steel. Heated to a temperature
of 1600 degrees or more and then rapidly cooled afterwards as that
heat is conducted into the rest of the plate, this narrow zone along
the edge is hardened by the process.
The depth of the HAZ will vary with the carbon level of the plate
grade and the specific alloy. Plate thickness is also a factor, as
the cutting torch moves slower in thicker plate, putting more heat
into the steel along the cut path. Lower carbon grades such as A36,
A572, or A516 will have a HAZ of 1/32” on thin plate to 1/8” on
heavy plate. This can double in depth on medium carbon steels like
1045. Alloy plate grades like 4140 and 4340 can have a HAZ of 5/16” or
more on heavy plate (thicknesses 6” and above).
Hardness of the HAZ is primarily determined by the grade of steel.
Low carbon steels such as A36, A572, A516, or C1020 will have a flamecut
edge that, below the decarb layer, is not much harder than the surface
of the plate. Plate grades like 1045, 4140, or 4340 typically have
edges with as-burned hardness of 500 Bhn or more. If these steels are
flamecut without proper preheating, the as-burned edge will be an extremely
hard and brittle, possibly as hard as 650 or 700 Bhn. Without further
thermal treatment such as annealing or quench-and- tempering, parts
flamecut from alloy plate or 1045 plate often develop cracks that arise
from this hardened edge.