Sheet Metal [by: Halloweenking]
Hotrodders Bulletin Board: Knowledge Base: Body-Exterior: Articles
Yes, sheet metal. What do you know about sheet metal? Usually the average Joe (or Juliet, as the case may be), knows little to nothing about it. So before you get out in the garage and start beating a fender to death, listen up and make it easier on yourself. Learn a bit about what you're pounding or what your ailment may be. Busted, bent, and crinkled up? Patching? Maybe up to some fabricating? Let's take a closer look at that piece of metal you thought you knew. Not too close now, you'll put your eye out that way!
Is all sheet metal the same?
No, sheet metal is not all the same, however the sheet metal used on most vehicles made in the USA is steel. This steel sheet metal ends its similarity there. There are two basic types. They are outlined for you below.
Cold-rolled steel is hot-rolled steel that has been rinsed with acid and cold-rolled thin. After the rolling process, it is annealed. This type of sheet metal has better workability than hot-rolled steel due to the annealing process. It also has dependable thickness accuracy as well as a superb surface. This is due to the cold rolling process. Most uni-body vehicles are made from this type of sheet metal.
Hot-rolled steel, is just that -- rolled hot. The steel is rolled at a temperature of 1400 degrees Fahrenheit and above. This type of steel ranges from 1/16" - 5/16". It's usually used for frames and cross members and other heavy material production pieces.
Between those two major types there are also two sublevels of cold-rolled steel. Those are outlined below.
Mild Steel or Low-Carbon Steel
Mild steel has low carbon content. This makes it easily workable. It can be welded easily and formed easily without affecting its structural integrity. This type of steel is used on most body panels and non-load bearing areas.
High Strength Steel
This type of sheet metal has a higher strength than mild steel because of a heat treatment. This steel is used for structural components and load bearing areas on a vehicle such as a utility vehicle's bed.
When sheet metal is used on a vehicle, it not only has the characteristics and strengths of the type of steel used as mentioned above, but the shape also adds to its strength and structural ability. When a sheet of steel is formed or bent it takes on certain properties that harden it. (Such as seen in the early Fords. The body shape has a slight slope on the door skins to improve the steel's resistance to damage). Even though a flat panel and a curved panel are made from the same steel, the curved panel has undergone a microscopic change that increases its compressive strength. Just like any other material, sheet metal is made up of atoms. These atoms form particles that in turn form a grain and the grains form a structural grain of the material. Think of these grains at a microscopic level as thousands of playing cards erratically arranged on a table. When one of these grains or "cards" moves, they disturb the surrounding grains or cards. If one card is pushed under another, the surrounding cards get pushed up (bulge). When you change the shape of a sheet of mild steel, the grain structure rearranges. Mild steel's grain structure can withstand a considerable amount of changes before splintering or breaking. You can test this at a non-microscopic level by simply bending a piece of coat hanger back and forth. At the point of the crease the coat hanger will become very warm. This happens because each individual grain of the grain structure is rubbing against each other (friction) creating heat.
Now that you understand the microscopic structure, think of a flat sheet of steel as a single playing card. If the flat sheet (card) is held by the sides with nothing supporting the center, the card is pretty flimsy and you can push it in with little force. Now if the sheet (card) is slightly arced, it can withstand much more compressive pressure due to its new shape.
Now that you have a little insight on the structure of a sheet, its time to discuss the other side of the spectrum -- damage. Just as your sheet metal can be changed in the forming process, it can also be changed through damage much the same way. When a dent develops, the elasticity of the steel has been surpassed. The elasticity is the ability to be stretched and bounce back to its original shape. When the elasticity of the steel has been surpassed, a plastic deformation has occurred. A plastic deformation happens when the grain structure has been changed. A "dent" or a simple indentation of a panel is a plastic deformation in which the grain structure has been changed by stretching or surpassing the compressive strength of the panel. This can be simply shrunk to repair. In most vehicle collisions, severe plastic deformation occurs. Usually in front or rear impact crashes a "kink" occurs. A kink develops when the steel has been folded at more than a 90-degree angle. A kink is a severe change in the grain structure, often resulting in broken sheet metal and flaking paint. The sheet metal breaks because the fold or crease becomes work hardened, this happens when the grain structures have changed so much that each grain becomes stiff and brittle. Correcting this is much more difficult and requires the use of a heat source. Flaking paint occurs when the grain structure has changed so severely that the bonding agents in the paint cannot hold onto the grain any longer.
Once you understand the nature of the substance you're working with, it's a simple thing to repair or form.