Quality And Structure





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

billet.



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

atmospheric temperature.



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.





Pyrometry And Pyrometers Quenching facebooktwittergoogle_plusredditpinterestlinkedinmail

Feedback