Types Of Iron and Steel
There are many different kinds and grades of iron and steel used in implements and other farm equipment.
To be better enabled to repair such equipment, a mechanic should know something about the different kinds
of iron and steel and their properties and uses.
Pig Iron. The first step in the manufacture of iron and steel is to extract the iron from the iron ore,
which is mined in various parts of the world. This is done by means of the modern blast furnace. The molten
iron accumulates at the bottom of the furnace and is drawn off into sand molds and allowed to cool and form
short, thick bars known as pig iron. Pig iron is then used as the source from which other kinds of iron and
steel are made.
Wrought Iron. Wrought iron is practically pure iron with only very small amounts of carbon or
impurities. It is made by removing the carbon and impurities from pig iron. The best grade of wrought iron
comes from Norway and Sweden where the purest iron ores are mined. Wrought iron was formerly used
extensively by blacksmiths, but, because of its high price, its use at present is quite limited. Wrought iron has
about 0.04 per cent carbon.
Cast Iron. To make castings, the pig iron is remelted, together with small amounts of scrap iron, and
poured into molds of the desired shape and then allowed to solidify. Cast iron is used extensively because it
is cheap and can be readily molded into complicated shapes. It is hard and brittle and cannot be bent. It
cannot be forged or welded in the forge fire, but it can be welded with the oxyacetylene torch. It crumbles
when it is heated to a bright red or white heat. It can be drilled and sawed easily and also filed easily after the hard outer shell is removed. The quality of cast iron can be controlled by varying the amounts of scrap iron
and steel mixed with pig iron when it is melted.
Chilled Iron. Chilled iron is cast iron that has been made in special molds, sometimes water-cooled
molds, that cool the outer portions of the casting rapidly, thus making the surface of the casting very hard
and wear resistant. Chilled iron is used for bearings on certain farm machines and for shares and moldboards
of plows that are to be used in gravelly or stony soils.
Malleable Iron. Malleable iron is cast iron of special composition that has been treated, after casting,
by heating for a long period. This prolonged heating removes some of the carbon from the surface of the
casting and reduces its brittleness. Malleable castings are softer and tougher than plain castings and can be
bent a certain amount without breaking. They are also more shock resistant.
Mild Steel. Mild steel, also known variously as machine steel, low-carbon steel, soft steel, and
blacksmith iron, is the common material used by blacksmiths. It is made by removing practically, but not
quite, all the carbon from pig iron. To remove it all would be much more expensive. It contains from about
0.1 to 0.3 per cent carbon, not enough to enable it to be hardened to any appreciable extent by heating and
quenching in water. It can be bent and hammered cold to some extent and can be forged and welded in the
forge. It is a little more difficult to weld than wrought iron.
Tool Steel. Tool steel is made from pig iron by first removing all the carbon and practically all the
impurities and then adding a definite, known amount of carbon. Tool steel contains from about 0.5 to about
1.5 per cent carbon. It is granular in structure instead of fibrous or stringy. It must not be heated higher than a bright-red or low-orange heat, or it will become honeycombed and therefore weak and brittle. The higher the
percentage of carbon the harder the steel may be tempered, an the more difficult it is to weld. Blacksmiths’
tools, such as hammers and cold chisels, are commonly made of steel having from 0.5 to 0.9 per cent carbon.
Taps and dies and such tools are made of steel having 1 to 1.25 per cent carbon. The carbon content of iron
and steel is designated by points, one point being one-hundredth of 1 percent of carbon. Thus a 50-point
carbon steel contains 50/100 or one-half of 1 per cent of carbon.
Distinguishing between Grades of Steel. A good way to distinguish between the various grades of
steel is to grind them on a grinding wheel and note the sparks that are given off. Sparks from wrought iron
are light yellow or red and follow straight lines. Sparks from mild steel are similar but more explosive or
sprangled. Tool steel gives off sparks that are lighter in color and still more explosive. The higher the
percentage of carbon in steel the brighter and more explosive are the sparks.