Quality And Structure
Category: HIGH-SPEED STEEL
The quality of high-speed steel is dependent to a very great extent
upon its structure. The making of the structure begins under the
hammer, and the beneficial effects produced in this stage persist
through the subsequent operations, provided they are properly carried
out. The massive carbides and tungstides present in the ingot are
broken down and uniformly distributed throughout the billet.
To accomplish this the reduction in area must be sufficient and the
hammer blows should be heavy, so as to carry the compression into
the center of the billet; otherwise, undesirable characteristics
such as coarse structure and carbide envelopes will exist and cause
the steel treater much trouble. Surface defects invisible in the
ingot may be opened up under the hammering operation, in which
event they are chipped from the hot billet.
Ingots are first hammered into billets. These billets are carefully
inspected and all surface defects ground or chipped. The hammered
billets are again slowly heated and receive a second hammering,
known as cogging. The billet resulting therefrom is known as
a cogged billet and is of the proper size for the rolling mill
or for the finishing hammer.
Although it is not considered good mill practice, some manufacturers
who have a large rolling mill perform the very important cogging
operation in the rolling mill instead of under the hammer. Cogging
in a rolling mill does not break up and distribute the carbides and
tungstides as efficiently as cogging under the hammer; another objection
to cogging in the rolling mill is that there is no opportunity to
chip surface defects developed as they can be under the trained eye
of a hammer-man, thereby eliminating such defects in the finished
The rolling of high-speed steel is an art known to very few. The
various factors governing the proper rolling are so numerous that
it is necessary for each individual rolling mill to work out a
practice that gives the best results upon the particular analysis
of steel it makes. Important elements entering into the rolling
are the heating and finishing temperatures, draft, and speed of
the mill. In all of these the element of time must be considered.
High-speed steel should be delivered from the rolling mill to the
annealing department free from scale, for scale promotes the formation
of a decarbonized surface. In preparation of bars for annealing,
they are packed in tubes with a mixture of charcoal, lime, and
other material. The tubes are sealed and placed in the annealing
furnace and the temperature is gradually raised to about 1,650 deg.F.,
and held there for a sufficient length of time, depending upon the
size of the bars. After very slow cooling the bars are removed
from the tubes. They should then show a Brinnell number of between
235 and 275.
The inspection department ranks with the chemical and metallurgical
departments in safeguarding the quality of the product. It inspects
all finished material from the standpoint of surface defects, hardness,
size and fracture. It rejects such steel as is judged not to meet
the manufacturer's standard. The inspection and metallurgical
departments work hand in hand, and if any department is not functioning
properly it will soon become evident to the inspectors, enabling
the management to remedy the trouble.
The successful manufacture of high-speed steel can only be obtained
by those companies who have become specialists. The art and skill
necessary in the successful working of such steel can be attained
only by a man of natural ability in his chosen trade, and trained
under the supervision of experts. To become an expert operator
in any department of its manufacture, it is necessary that the
operator work almost exclusively in the production of such steel.
As to the heat treatment, it is customary for the manufacturer
to recommend to the user a procedure that will give to his steel
a high degree of cutting efficiency. The recommendations of the
manufacturer should be conservative, embracing fairly wide limits,
as the tendency of the user is to adhere very closely to the
manufacturer's recommendations. Unless one of the manufacturer's
expert service men has made a detailed study of the customer's
problem, the manufacturer is not justified in laying down set rules,
for if the customer does a little experimenting he can probably
modify the practice so as to produce results that are particularly
well adapted to his line of work.
The purpose of heat-treating is to produce a tool that will cut so
as to give maximum productive efficiency. This cutting efficiency
depends upon the thermal stability of the complex hardenites existing
in the hardened and tempered steel. The writer finds it extremely
difficult to convey the meaning of the word hardenite to those that
do not have a clear conception of the term. The complex hardenites
in high-speed steel may be described as that form of solid solution
which gives to it its cutting efficiency. The complex hardenites are
produced by heating the steel to a very high temperature, near the
melting point, which throws into solution carbides and tungstides,
provided they have been properly broken up in the hammering process
and uniformly distributed throughout the steel. By quenching the
steel at correct temperature this solid solution is retained at
It is not the intention to make any definite recommendations as to
heat-treating of high-speed steel by the users. It is recognized
that such steel can be heat-treated to give satisfactory results
by different methods. It is, however, believed that the American
practice of hardening and tempering is becoming more uniform. This
is due largely to the exchange of opinions in meetings and elsewhere.
The trend of American practice for hardening is toward the following:
First, slowly and carefully preheat the tool to a temperature
of approximately 1,500 deg.F., taking care to prevent the formation
of excessive scale.
Second, transfer to a furnace, the temperature of which is
approximately 2,250 to 2,400 deg.F., and allow to remain in the furnace
until the tool is heated uniformly to the above temperature.
Third, cool rapidly in oil, dry air blast, or lead bath.
Fourth, draw back to a temperature to meet the physical requirements
of the tool, and allow to cool in air.
It was not very long ago that the desirability of drawing hardened
high-speed steel to a temperature of 1,100 deg. was pointed out, and it
is indeed encouraging to learn that comparatively few treaters have
failed to make use of this fact. Many treaters at first contended
that the steel would be soft after drawing to this temperature and
it is only recently, since numerous actual tests have demonstrated
its value, that the old prejudice has been eliminated.
High-speed steel should be delivered only in the annealed condition
because annealing relieves the internal strains inevitable in the
manufacture and puts it in vastly improved physical condition. The
manufacturer's inspection after annealing also discloses defects
not visible in the unannealed state.
The only true test for a brand of high-speed steel is the service that
it gives by continued performance month in and month out under actual
shop conditions. The average buyer is not justified in conducting a
test, but can well continue to purchase his requirements from a
reputable manufacturer of a brand that is nationally known. The
manufacturer is always willing to cooperate with the trade in the
conducting of a test and is much interested in the information
received from a well conducted test. A test, to be valuable, should be
conducted in a manner as nearly approaching actual working conditions
in the plant in which the test is made as is practical. In conducting
a test a few reputable brands should be allowed to enter. All tools
entered should be of exactly the same size and shape. There is much
difference of opinion as to the best practical method of conducting
a test, and the decision as to how the test should be conducted
should be left to the customer, who should cooperate with the
manufacturers in devising a test which would give the best basis
for conclusions as to how the particular brands would perform under
actual shop conditions.
The value of the file test depends upon the quality of the file and
the intelligence and experience of the person using it. The file
test is not reliable, but in the hands of an experienced operator,
gives some valuable information. Almost every steel treater knows
of numerous instances where a lathe tool which could be touched
with a file has shown wonderful results as to cutting efficiency.
Modern tool-steel practice has changed from that of the past, not
by the use of labor-saving machinery, but by the use of scientific
devices which aid and guide the skilled craftsman in producing a
steel of higher quality and greater uniformity. It is upon the
intelligence, experience, and skill of the individual that quality
of tool steel depends.
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