CHAPTER 5
THE BLACKSMITH'S MATERIALS
WROUGHT IRON
Wrought iron has long been regarded as the traditional material worked by
the blacksmith, and its replacement by mild steel for most forging is still
a matter of regret amongst older craftsmen whose early experience did not
include the forging and welding of mild steel, which requires modified techniques.
Wrought iron, which is produced by puddling pig iron in a special hearth,
is more expensive than mild steel. Commercial wrought iron contains approximately
0.04 per cent carbon and 0.2 per cent slag which, during the process of manufacture,
is hammered or squeezed throughout the mass of the metal, producing the well-known
fibrous structure which makes wrought iron so easily recognizable when broken
across the grain. Two of the vital qualities possessed by wrought iron are
its ability to be drawn out - 'ductility', and its ability to be hammered
into shapes
malleability'.
Iron which is malleable is not necessarily ductile and iron which possesses
either of these qualities when cold does not necessarily possess them when
hot, and vice versa.
Iron which is apt to break when cold is called 'cold short' and iron which
is apt to break when hot is called 'red short'. These most undesirable qualities
are aused by impurities in the metal, excess of phosphorus and sulphur making
it cold short and excess of silicon making it red short.
Due to its low carbon content, wrought iron is highly malleable and ductile
and is easily forged and welded by the blacksmith. It was once used extensively
for all types of constructional work, but it is now confined to a few special
uses in industry where ductility and resistance to corrosion are required
rather than high tensile strength.
It is these special characteristics, however, which make wrought iron eminently
suitable for decorative iron work, for which it is still preferred by many
blacksmiths. The texture of wrought iron improves with careful forging and
it can be worked and welded into the most intricate, delicate and graceful
shapes with greater ease than mild steel.
Wrought iron can be worked at a wider range of heat than is possible with
mild steel so that despite its high cost, a small stock of wrought iron is
particularly valuable for tricky jobs. For example, a small collar for welding
onto a steel shank, is better made of wrought iron as it is able to withstand
the extra heat which the collar naturally gets before the whole job is at
the correct temperature for welding.
Steel is a general term applied to alloys of pure iron and carbon.
The quality of steel is determined by the selection of iron and alloying
ingredients used in its manufacture and not by its carbon content, or by the
appearance of the fracture. The temper of steel refers to the carbon
content and has nothing to do with quality. The term should not be confused
with 'tempering', which is used in connection with the Heat Treatment described
in Chapter 4. (Detailed
information on the tempering of carbon steels will be available on this site
soon)
MILD STEEL
Mild steel which contains from 0.2 per cent to 0.3 per cent carbon, can be
readily forged and welded within a narrower range of temperatures than wrought
iron. It is less ductile and malleable than wrought iron but possesses greater
tensile strength, which is an essential characteristic for the majority of
forgings. It cannot be hardened or tempered and when fractured it shows a
granular or non-fibrous structure.
MEDIUM CARBON STEEL
Medium carbon steel contains from 0.5 per cent to 0.6 per cent carbon and
is harder and stronger than mild steel, being readily forged but not so easily
welded. While it cannot be tempered to hold a cutting edge, it can be hardened
to a certain degree.
HIGH CARBON STEEL
High carbon steel contains from 0.75 per cent to 1.5 per cent carbon and can
be hardened to a high degree and tempered to retain a cutting edge.
When working high'carbon steels in the forge, great care must be taken because
they have a comparatively narrow range of forging and heat treatment temperatures,
being easily oxidized or burnt beyond recovery.
High carbon steel is usually graded into six tempers according to its carbon
content. Various manufacturers have their own system of classification; the
following is probably most common and is used by Arthur Balfour & Co.
Ltd.
Temper No. 5. (0.875 per cent carbon) is the one most suitable
for blacksmiths' and fitters' cutting tools, such as cold chisels, sets, etc.
Temper No. 6. (0.75 per cent carbon) is used for making hammer
heads, hot sets, punches and drifts.
ALLOY STEELS
Alloy steels include a great variety of steels containing, in addition to
carbon, other constituents which give them special characteristics.
It is not within the scope of this site to describe the chemical contents
or the physical and mechanical properties of the many types of steel and the
student is recommended to read special books on this subject.
Note: The word 'metal' is used throughout this book to
denote either wrought iron or steel. If either is intended as distinct from
the other, it is mentioned by name.
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