What happens to you when you break a law, even if you don't know about
that particular law? Usually something you don't
like, right? Well, cast iron will obey the laws of physics, even if you
don't know them. So, it makes a lot of sense to learn the laws before
something happens that you don't like. Right?
First, let's talk about expansion and
contraction. This is extremely important and very easy. Understanding these
principles can prevent you from having welding catastrophes.
Pretty simple, huh? Follow the ideas and
illustrations below for a thorough understanding.
for a streaming video demonstration.
expansion results in the cast iron cube becoming physically larger in all
directions when heated.
contraction results in the cast iron cube returning to its exact physical
dimensions when cooled following free expansion.
expansion occurs when cast iron is confined on two or more sides when
heated. The cube will only be able to expand up and down plus side to
side, not against the vise. Heating the cube while restricted by the vise
will change its physical dimensions permanently.
contraction following restricted expansion will result in the cast iron cube
contracting equally in all directions. Measuring the cube after cooling
you will find that it is now taller, longer, and shorter than it was before
heating. Heating the cube in the vise has permanently changed its shape.
expansion with restricted contraction is the number one cause of cracking when
people attempt to weld cast iron. As the temperature increases in the
heat-affected area, the cast iron cannot expand equally because it is contained
by the colder iron around it. This forces the iron to grow in the only
direction that it can: thickness.
expansion is forced in a confined condition the shape is permanently
changed. It is now thicker than it was and it will not return to its
original thickness when it cools. Just like the cube in the vise, the
change is permanent and it cannot be reversed. It also does not matter
what type of welding rod, wire or powder you apply, or even who manufactured
it. It's the heat applied to the base iron that causes all of the stress,
hardening and cracking.
the casting cools, the cast iron shrinks equally in all directions. The
difference here is that the heated area is attached to the surrounding cast
iron. When contraction occurs, stress builds up and most often relieves
itself by cracking. The bottom line is: if a weld cracks, the
casting was too cold; if the base iron gets hard, it cooled too fast. To
avoid these heat related problems never electric weld on cast iron and stay out
of the middle of the parts. Unless the entire casting is preheated to at
least 900°F, only weld on corners and ears.
have this information stored away in your mind for quick and easy reference, it
is time to add another piece to the puzzle. Next, read about cast iron's critical