_TEMPERING STEEL._
Tempering may be called a mystery of the smith-shop; this operation
has that attraction which characterises every process that is
mysterious, especially such as are connected with, or belong to
mechanical manipulation. A strange and perhaps fortunate habit
of the mind is to be greatly interested in what is not well
understood, and to disregard what is capable of plain demonstration.
An old smith who has stood at the forge for a score of years will
take the same interest in tempering processes that a novice will.
When a piece is to be tempered which is liable to spring or break,
and the risk is great, he will enter upon it with the same zeal and
interest that he would have done when learning his trade.
No one has been able to explain clearly why a sudden change of
temperature hardens steel, nor why it assumes various shades of
colour at different degrees of hardness; we only know the fact, and
that steel fortunately has such properties.
Every one who uses tools should understand how to temper them,
whether they be for iron or wood. Experiments with tempered tools
is the only means of determining the proper degree of hardness,
and as smiths, except with their own tools, have to rely upon the
explanations of others as to proper hardening, it follows that
tempering is generally a source of complaint.
Tempering, as a term, is used to comprehend both hardening and
drawing; as a process it depends mainly upon judgment instead of
skill, and has no such connection with forging as to be performed
by smiths only. Tempering requires a different fire from those
employed in forging, and also more care and precision than
blacksmiths can exercise, unless there are furnaces and baths
especially arranged for tempering tools.
A difficulty which arises in hardening tools is because of the
contraction of the steel which takes place in proportion to the
change of temperature; and as the time of cooling is in proportion
to the thickness or size of a piece, it follows, of course, that
there is a great strain and a tendency to break the thinner parts
before the thicker parts have time to cool; this strain may take
place either from cooling one side first, or more rapidly than
another.
The following propositions in regard to tempering, comprehend the
main points to be observed:
The permanent contraction of steel in tempering is as the degree of
hardness imparted to it by the bath.
The time in which the contraction takes place is as the temperature
of the bath and the cross section of the piece; in other words the
heat passes off gradually from the surface to the centre.
Thin sections of steel tools being projections from the mass which
supports the edges, are cooled first, and if provision is not made
to allow for contraction they are torn asunder.
The main point in hardening and the most that can be done to avoid
irregular contraction, is to apply the bath so that it will act
first and strongest on the thickest parts. If a piece is tapering
or in the form of a wedge, the thick end should enter the bath
first; a cold chisel for instance that is wide enough to endanger
cracking should be put into the bath with the head downward.
The upflow of currents of warmed water are a common cause of
irregular cooling and springing of steel tools in hardening; the
water that is heated, rises vertically, and the least inclination
of a piece from a perpendicular position, allows a warm current to
flow up one side.
The most effectual means of securing a uniform effect from a
tempering bath is by violent agitation, either of the bath or the
piece; this also adds to the rapidity of cooling.
The effect of tempering baths is as their conducting power;
chemicals except as they may contribute to the conducting
properties of a bath, may safely be disregarded. For baths, cold or
ice water loaded with salt for extreme hardness, and warm oil for
tools that are thin and do not require to be very hard, are the two
extremes outside of which nothing is required in ordinary practice.
In the case of tools composed partly of iron and partly of steel,
steel laid as it is called, the tendency to crack in hardening
may be avoided in most cases by hammering the steel edge at a low
temperature until it is so expanded that when cooled in hardening
it will only contract to a state of rest and correspond to the iron
part; the same result may be produced by curving a piece, giving
convexity to the steel side before hardening.
Tools should never be tempered by immersing their edges or cutting
parts in the bath, and then allowing the heat to "run down" to
attain a proper temper at the edge. I am well aware that this is
attacking a general custom, but it is none the less wrong for that
reason. Tools so hardened have a gradually diminishing temper from
their point or edge, so that no part is properly tempered, and they
require continual re-hardening, which spoils the steel; besides,
the extreme edge, the only part which is tempered to a proper
shade, is usually spoiled by heating and must be ground away to
begin with. No latheman who has once had a set of tools tempered
throughout by slow drawing, either in an oven, or on a hot plate,
will ever consent to point hardening afterwards. A plate of iron,
two to two and one-half inches thick, placed over the top of a
tool dressing fire, makes a convenient arrangement for tempering
tools, besides adding greatly to the convenience of slow heating,
which is almost as important as slow drawing. The writer has by
actual experiment determined that the amount of tool dressing and
tempering, to say nothing of time wasted in grinding tools, may in
ordinary machine fitting be reduced one-third by "oven tempering."
As to the shades that appear in drawing temper, or tempering it
is sometimes called, it is quite useless to repeat any of the old
rules about "straw colour, violet, orange, blue," and so on; the
learner knows as much after such instruction as before. The shades
of temper must be seen to be learned, and as no one is likely to
have use for such knowledge before having opportunities to see
tempering performed, the following plan is suggested for learning
the different shades. Procure eight pieces of cast steel about two
inches long by one inch wide and three-eighths of an inch thick,
heat them to a high red heat and drop them into a salt bath;
preserve one without tempering to show the white shade of extreme
hardness, and polish one side of each of the remaining seven
pieces; then give them to an experienced workman to be drawn to
seven varying shades of temper ranging from the white piece to the
dark blue colour of soft steel. On the backs of these pieces labels
can be pasted describing the technical names of the shades and the
general uses to which tools of corresponding hardness are adapted.
This will form an interesting collection of specimens and accustom
the eye to the various tints, which after some experience will be
instantly recognised when seen separately.
It may be remarked as a general rule that the hardness of cutting
tools is "inverse as the hardness of the material to be cut,"
which seems anomalous, and no doubt is so, if nothing but the
cutting properties of edges is considered; but all cutting edges
are subjected to transverse strain, and the amount of this strain
is generally as the hardness of the material acted upon; hence the
degree of temper has of necessity to be such as to guard against
breaking the edges. Tools for cutting wood, for example, can be
much harder than for cutting iron, or to state it better, tools for
cutting wood are harder than those usually employed for cutting
iron; for if iron tools were always as carefully formed and as
carefully used as those employed in cutting wood, they could be
equally hard.
Forges, pneumatic machinery for blast, machinery for handling
large pieces, and other details connected with forging, are easily
understood from examples.
(1.) What causes tools to bend or break in hardening?--(2.)
What means can be employed to prevent injury to tools in
hardening?--(3.) Can the shades of temper be produced on a piece
of steel without hardening?--(4.) What forms a limit of hardness
for cutting tools?--(5.) What are the objects of steel-laying
tools instead of making them of solid steel?