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.
 | When cast iron (and other metals) are heated, they expand. |
| Expansion is cubic (or equal in all directions) unless the heated metal is contained. |
| When cast iron (and other metals) cool, they contract. |
| Contraction is cubic (or equal in all directions) unless the cooling metal is contained. |
Pretty simple, huh? Follow the ideas and illustrations below for a thorough understanding.
Click here
for a streaming video demonstration.
Free expansion results in the cast iron cube becoming physically larger in all directions when heated.
Free contraction results in the cast iron cube returning to its exact physical dimensions when cooled following free expansion.
Restricted 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.
Free
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.
Restricted
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.
When
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.
When
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.
Once you
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
temperature.
