Iron.--Iron, in its pure state, is a soft, white, easily worked
metal. It is the most important of all the metallic elements, and is, next
to aluminum, the commonest metal found in the earth.
Mechanically speaking, we have three kinds of iron: wrought iron, cast iron
and steel. Wrought iron is very nearly pure iron; cast iron contains carbon
and silicon, also chemical impurities; and steel contains a definite
proportion of carbon, but in smaller quantities than cast iron.
Pure iron is never obtained commercially, the metal always being mixed with
various proportions of carbon, silicon, sulphur, phosphorus, and other
elements, making it more or less suitable for different purposes. Iron is
magnetic to the extent that it is attracted by magnets, but it does not
retain magnetism itself, as does steel. Iron forms, with other elements,
many important combinations, such as its alloys, oxides, and sulphates.
Image Figure 1.--Section Through a Blast Furnace
Cast Iron.--Metallic iron is separated from iron ore in the blast
furnace (Figure 1), and when allowed to run into moulds is called cast
iron. This form is used for engine cylinders and pistons, for brackets,
covers, housings and at any point where its brittleness is not
objectionable. Good cast iron breaks with a gray fracture, is free from
blowholes or roughness, and is easily machined, drilled, etc. Cast iron is
slightly lighter than steel, melts at about 2,400 degrees in practice, is
about one-eighth as good an electrical conductor as copper and has a
tensile strength of 13,000 to 30,000 pounds per square inch. Its
compressive strength, or resistance to crushing, is very great. It has
excellent wearing qualities and is not easily warped and deformed by heat.
Chilled iron is cast into a metal mould so that the outside is cooled
quickly, making the surface very hard and difficult to cut and giving great
resistance to wear. It is used for making cheap gear wheels and parts that
must withstand surface friction.
Malleable Cast Iron.--This is often called simply malleable iron. It
is a form of cast iron obtained by removing much of the carbon from cast
iron, making it softer and less brittle. It has a tensile strength of
25,000 to 45,000 pounds per square inch, is easily machined, will stand a
small amount of bending at a low red heat and is used chiefly in making
brackets, fittings and supports where low cost is of considerable
importance. It is often used in cheap constructions in place of steel
forgings. The greatest strength of a malleable casting, like a steel
forging, is in the surface, therefore but little machining should be done.
Wrought Iron.--This grade is made by treating the cast iron to
remove almost all of the carbon, silicon, phosphorus, sulphur, manganese
and other impurities. This process leaves a small amount of the slag from
the ore mixed with the wrought iron.
Wrought iron is used for making bars to be machined into various parts. If
drawn through the rolls at the mill once, while being made, it is called
"muck bar;" if rolled twice, it is called "merchant bar" (the commonest
kind), and a still better grade is made by rolling a third time. Wrought
iron is being gradually replaced in use by mild rolled steels.
Wrought iron is slightly heavier than cast iron, is a much better
electrical conductor than either cast iron or steel, has a tensile strength
of 40,000 to 60,000 pounds per square inch and costs slightly more than
steel. Unlike either steel or cast iron, wrought iron does not harden when
cooled suddenly from a red heat.
Grades of Irons.--The mechanical properties of cast iron differ
greatly according to the amount of other materials it contains. The most
important of these contained elements is carbon, which is present to a
degree varying from 2 to 5-1/2 per cent. When iron containing much carbon
is quickly cooled and then broken, the fracture is nearly white in color
and the metal is found to be hard and brittle. When the iron is slowly
cooled and then broken the fracture is gray and the iron is more malleable
and less brittle. If cast iron contains sulphur or phosphorus, it will show
a white fracture regardless of the rapidity of cooling, being brittle and
less desirable for general work.
Steel.--Steel is composed of extremely minute particles of iron and
carbon, forming a network of layers and bands. This carbon is a smaller
proportion of the metal than found in cast iron, the percentage being from
3/10 to 2-1/2 per cent.
Carbon steel is specified according to the number of "points" of carbon, a
point being one one-hundredth of one per cent of the weight of the steel.
Steel may contain anywhere from 30 to 250 points, which is equivalent to
saying, anywhere from 3/10 to 2-1/2 per cent, as above. A 70-point steel
would contain 70/100 of one per cent or 7/10 of one per cent of carbon by
weight. The percentage of carbon determines the hardness of the steel, also
many other qualities, and its suitability for various kinds of work. The
more carbon contained in the steel, the harder the metal will be, and, of
course, its brittleness increases with the hardness. The smaller the grains
or particles of iron which are separated by the carbon, the stronger the
steel will be, and the control of the size of these particles is the object
of the science of heat treatment.
In addition to the carbon, steel may contain the following:
Silicon, which increases the hardness, brittleness, strength and difficulty
of working if from 2 to 3 per cent is present.
Phosphorus, which hardens and weakens the metal but makes it easier to
cast. Three-tenths per cent of phosphorus serves as a hardening agent and
may be present in good steel if the percentage of carbon is low. More
than this weakens the metal.
Sulphur, which tends to make the metal hard and filled with small holes.
Manganese, which makes the steel so hard and tough that it can with
difficulty be cut with steel tools. Its hardness is not lessened by
annealing, and it has great tensile strength.
Alloy steel has a varying but small percentage of other elements mixed with
it to give certain desired qualities. Silicon steel and manganese steel are
sometimes classed as alloy steels. This subject is taken up in the latter
part of this chapter under Alloys, where the various combinations
and their characteristics are given consideration.
Steel has a tensile strength varying from 50,000 to 300,000 pounds per
square inch, depending on the carbon percentage and the other alloys
present, as well as upon the texture of the grain. Steel is heavier than
cast iron and weighs about the same as wrought iron. It is about one-ninth
as good a conductor of electricity as copper.
Steel is made from cast iron by three principal processes: the crucible,
Bessemer and open hearth.
Crucible steel is made by placing pieces of iron in a clay or
graphite crucible, mixed with charcoal and a small amount of any desired
alloy. The crucible is then heated with coal, oil or gas fires until the
iron melts, and, by absorbing the desired elements and giving up or
changing its percentage of carbon, becomes steel. The molten steel is then
poured from the crucible into moulds or bars for use. Crucible steel may
also be made by placing crude steel in the crucibles in place of the iron.
This last method gives the finest grade of metal and the crucible process
in general gives the best grades of steel for mechanical use.
Image Figure 2.--A Bessemer Converter
essemer steel is made by heating iron until all the undesirable
elements are burned out by air blasts which furnish the necessary oxygen.
The iron is placed in a large retort called a converter, being poured,
while at a melting heat, directly from the blast furnace into the
converter. While the iron in the converter is molten, blasts of air are
forced through the liquid, making it still hotter and burning out the
impurities together with the carbon and manganese. These two elements are
then restored to the iron by adding spiegeleisen (an alloy of iron, carbon
and manganese). A converter holds from 5 to 25 tons of metal and requires
about 20 minutes to finish a charge. This makes the cheapest steel.
Image Figure 3.--An Open Hearth Furnace
Open hearth steel is made by placing the molten iron in a receptacle
while currents of air pass over it, this air having itself been highly
heated by just passing over white hot brick (Figure. 3). Open hearth steel
is considered more uniform and reliable than Bessemer, and is used for
springs, bar steel, tool steel, steel plates, etc.
Aluminum is one of the commonest industrial metals. It is used for
gear cases, engine crank cases, covers, fittings, and wherever lightness
and moderate strength are desirable.
Aluminum is about one-third the weight of iron and about the same weight as
glass and porcelain; it is a good electrical conductor (about one-half as
good as copper); is fairly strong itself and gives great strength to other
metals when alloyed with them. One of the greatest advantages of aluminum
is that it will not rust or corrode under ordinary conditions. The granular
formation of aluminum makes its strength very unreliable and it is too soft
to resist wear.
Copper is one of the most important metals used in the trades, and
the best commercial conductor of electricity, being exceeded in this
respect only by silver, which is but slightly better. Copper is very
malleable and ductile when cold, and in this state may be easily worked
under the hammer. Working in this way makes the copper stronger and harder,
but less ductile. Copper is not affected by air, but acids cause the
formation of a green deposit called verdigris.
Copper is one of the best conductors of heat, as well as electricity, being
used for kettles, boilers, stills and wherever this quality is desirable.
Copper is also used in alloys with other metals, forming an important part
of brass, bronze, german silver, bell metal and gun metal. It is about
one-eighth heavier than steel and has a tensile strength of about 25,000 to
50,000 pounds per square inch.
Lead.--The peculiar properties of lead, and especially its quality
of showing but little action or chemical change in the presence of other
elements, makes it valuable under certain conditions of use. Its principal
use is in pipes for water and gas, coverings for roofs and linings for vats
and tanks. It is also used to coat sheet iron for similar uses and as an
important part of ordinary solder.
Lead is the softest and weakest of all the commercial metals, being very
pliable and inelastic. It should be remembered that lead and all its
compounds are poisonous when received into the system. Lead is more than
one-third heavier than steel, has a tensile strength of only about 2,000
pounds per square inch, and is only about one-tenth as good a conductor of
electricity as copper.
Zinc.--This is a bluish-white metal of crystalline form. It is
brittle at ordinary temperatures and becomes malleable at about 250 to 300
degrees Fahrenheit, but beyond this point becomes even more brittle than at
ordinary temperatures. Zinc is practically unaffected by air or moisture
through becoming covered with one of its own compounds which immediately
resists further action. Zinc melts at low temperatures, and when heated
beyond the melting point gives off very poisonous fumes.
The principal use of zinc is as an alloy with other metals to form brass,
bronze, german silver and bearing metals. It is also used to cover the
surface of steel and iron plates, the plates being then called galvanized.
Zinc weighs slightly less than steel, has a tensile strength of 5,000
pounds per square inch, and is not quite half as good as copper in
conducting electricity.
Tin resembles silver in color and luster. Tin is ductile and
malleable and slightly crystalline in form, almost as heavy as steel, and
has a tensile strength of 4,500 pounds per square inch.
The principal use of tin is for protective platings on household utensils
and in wrappings of tin-foil. Tin forms an important part of many alloys
such as babbitt, Britannia metal, bronze, gun metal and bearing metals.
Nickel is important in mechanics because of its combinations with
other metals as alloys. Pure nickel is grayish-white, malleable, ductile
and tenacious. It weighs almost as much as steel and, next to manganese, is
the hardest of metals. Nickel is one of the three magnetic metals, the
others being iron and cobalt. The commonest alloy containing nickel is
german silver, although one of its most important alloys is found in nickel
steel. Nickel is about ten per cent heavier than steel, and has a tensile
strength of 90,000 pounds per square inch.
Platinum.--This metal is valuable for two reasons: it is not
affected by the air or moisture or any ordinary acid or salt, and in
addition to this property it melts only at the highest temperatures. It is
a fairly good electrical conductor, being better than iron or steel. It is
nearly three times as heavy as steel and its tensile strength is 25,000
pounds per square inch.
ALLOYS
An alloy is formed by the union of a metal with some other material, either
metal or non-metallic, this union being composed of two or more elements
and usually brought about by heating the substances together until they
melt and unite. Metals are alloyed with materials which have been found to
give to the metal certain characteristics which are desired according to
the use the metal will be put to.
The alloys of metals are, almost without exception, more important from an
industrial standpoint than the metals themselves. There are innumerable
possible combinations, the most useful of which are here classed under the
head of the principal metal entering into their composition.
Steel.--Steel may be alloyed with almost any of the metals or
elements, the combinations that have proven valuable numbering more than a
score. The principal ones are given in alphabetical order, as follows:
Aluminum is added to steel in very small amounts for the purpose of
preventing blow holes in castings.
Boron increases the density and toughness of the metal.
Bronze, added by alloying copper, tin and iron, is used for gun metal.
Carbon has already been considered under the head of steel in the section
devoted to the metals. Carbon, while increasing the strength and hardness,
decreases the ease of forging and bending and decreases the magnetism and
electrical conductivity. High carbon steel can be welded only with
difficulty. When the percentage of carbon is low, the steel is called "low
carbon" or "mild" steel. This is used for rods and shafts, and called
"machine" steel. When the carbon percentage is high, the steel is called
"high carbon" steel, and it is used in the shop as tool steel. One-tenth
per cent of carbon gives steel a tensile strength of 50,000 to 65,000
pounds per square inch; two-tenths per cent gives from 60,000 to 80,000;
four-tenths per cent gives 70,000 to 100,000, and six-tenths per cent
gives 90,000 to 120,000.
Chromium forms chrome steel, and with the further addition of nickel is
called chrome nickel steel. This increases the hardness to a high degree
and adds strength without much decrease in ductility. Chrome steels are
used for high-speed cutting tools, armor plate, files, springs, safes,
dies, etc.
Manganese has been mentioned under Steel. Its alloy is much used for
high-speed cutting tools, the steel hardening when cooled in the air and
being called self-hardening.
Molybdenum is used to increase the hardness to a high degree and makes the
steel suitable for high-speed cutting and gives it self-hardening
properties.
Nickel, with which is often combined chromium, increases the strength,
springiness and toughness and helps to prevent corrosion.
Silicon has already been described. It suits the metal for use in
high-speed tools.
Silver added to steel has many of the properties of nickel.
Tungsten increases the hardness without making the steel brittle. This
makes the steel well suited for gas engine valves as it resists corrosion
and pitting. Chromium and manganese are often used in combination with
tungsten when high-speed cutting tools are made.
Vanadium as an alloy increases the elastic limit, making the steel
stronger, tougher and harder. It also makes the steel able to stand much
bending and vibration.
Copper.--The principal copper alloys include brass, bronze, german
silver and gun metal.
Brass is composed of approximately one-third zinc and two-thirds copper. It
is used for bearings and bushings where the speeds are slow and the loads
rather heavy for the bearing size. It also finds use in washers, collars
and forms of brackets where the metal should be non-magnetic, also for many
highly finished parts.
Brass is about one-third as good an electrical conductor as copper, is
slightly heavier than steel and has a tensile strength of 15,000 pounds
when cast and about 75,000 to 100,000 pounds when drawn into wire.
Bronze is composed of copper and tin in various proportions, according to
the use to which it is to be put. There will always be from six-tenths to
nine-tenths of copper in the mixture. Bronze is used for bearings,
bushings, thrust washers, brackets and gear wheels. It is heavier than
steel, about 1/15 as good an electrical conductor as pure copper and has a
tensile strength of 30,000 to 60,000 pounds.
Aluminum bronze, composed of copper, zinc and aluminum has high tensile
strength combined with ductility and is used for parts requiring this
combination.
Bearing bronze is a variable material, its composition and proportion
depending on the maker and the use for which it is designed. It usually
contains from 75 to 85 per cent of copper combined with one or more
elements, such as tin, zinc, antimony and lead.
White metal is one form of bearing bronze containing over 80 per cent of
zinc together with copper, tin, antimony and lead. Another form is made
with nearly 90 per cent of tin combined with copper and antimony.
Gun metal bronze is made from 90 per cent copper with 10 per cent of tin
and is used for heavy bearings, brackets and highly finished parts.
Phosphor bronze is used for very strong castings and bearings. It is
similar to gun metal bronze, except that about 1-1/2 per cent of phosphorus
has been added.
Manganese bronze contains about 1 per cent of manganese and is used for
parts requiring great strength while being free from corrosion.
German silver is made from 60 per cent of copper with 20 per cent each of
zinc and nickel. Its high electrical resistance makes it valuable for
regulating devices and rheostats.
Tin is the principal part of babbitt and solder. A
commonly used babbitt is composed of 89 per cent tin, 8 per cent antimony
and 3 per cent of copper. A grade suitable for repairing is made from
80 per cent of lead and 20 per cent antimony. This last formula should not
be used for particular work or heavy loads, being more suitable for
spacers. Innumerable proportions of metals are marketed under the name of
babbitt.
Solder is made from 50 per cent tin and 50 per cent lead, this grade being
called "half-and-half." Hard solder is made from two-thirds tin and
one-third lead.
Aluminum forms many different alloys, giving increased strength to whatever
metal it unites with.
Aluminum brass is composed of approximately 65 per cent copper, 30 per cent
zinc and 5 per cent aluminum. It forms a metal with high tensile strength
while being ductile and malleable.
Aluminum zinc is suitable for castings which must be stiff and hard.
Nickel aluminum has a tensile strength of 40,000 pounds per square inch.
Magnalium is a silver-white alloy of aluminum with from 5 to 20 per cent of
magnesium, forming a metal even lighter than aluminum and strong enough to
be used in making high-speed gasoline engines.
HEAT TREATMENT OF STEEL
The processes of heat treatment are designed to suit the steel for various
purposes by changing the size of the grain in the metal, therefore the
strength; and by altering the chemical composition of the alloys in the
metal to give it different physical properties. Heat treatment, as applied
in ordinary shop work, includes the three processes of annealing, hardening
and tempering, each designed to accomplish a certain definite result.
All of these processes require that the metal treated be gradually brought
to a certain predetermined degree of heat which shall be uniform throughout
the piece being handled and, from this point, cooled according to certain
rules, the selection of which forms the difference in the three methods.
Annealing.--This is the process which relieves all internal strains
and distortion in the metal and softens it so that it may more easily be
cut, machined or bent to the required form. In some cases annealing is used
only to relieve the strains, this being the case after forging or welding
operations have been performed. In other cases it is only desired to soften
the metal sufficiently that it may be handled easily. In some cases both of
these things must be accomplished, as after a piece has been forged and
must be machined. No matter what the object, the procedure is the same.
The steel to be annealed must first be heated to a dull red. This heating
should be done slowly so that all parts of the piece have time to reach the
same temperature at very nearly the same time. The piece may be heated in
the forge, but a much better way is to heat in an oven or furnace of some
type where the work is protected against air currents, either hot or cold,
and is also protected against the direct action of the fire.
Image Figure 4.--A Gaspipe Annealing Oven
Probably the simplest of all ovens for small tools is made by placing a
piece of ordinary gas pipe in the fire (Figure 4), and heating until the
inside of the pipe is bright red. Parts placed in this pipe, after one end
has been closed, may be brought to the desired heat without danger of
cooling draughts or chemical change from the action of the fire. More
elaborate ovens may be bought which use gas, fuel oils or coal to produce
the heat and in which the work may be placed on trays so that the fire will
not strike directly on the steel being treated.
If the work is not very important, it may be withdrawn from the fire or
oven, after heating to the desired point, and allowed to cool in the air
until all traces of red have disappeared when held in a dark place. The
work should be held where it is reasonably free from cold air currents. If,
upon touching a pine stick to the piece being annealed, the wood does not
smoke, the work may then be cooled in water.
Better annealing is secured and harder metal may be annealed if the cooling
is extended over a number of hours by placing the work in a bed of
non-heat-conducting material, such as ashes, charred bone, asbestos fiber,
lime, sand or fire clay. It should be well covered with the heat retaining
material and allowed to remain until cool. Cooling may be accomplished by
allowing the fire in an oven or furnace to die down and go out, leaving the
work inside the oven with all openings closed. The greater the time taken
for gradual cooling from the red heat, the more perfect will be the results
of the annealing.
While steel is annealed by slow cooling, copper or brass is annealed by
bringing to a low red heat and quickly plunging into cold water.
Hardening.--Steel is hardened by bringing to a proper temperature,
slowly and evenly as for annealing, and then cooling more or less quickly,
according to the grade of steel being handled. The degree of hardening is
determined by the kind of steel, the temperature from which the metal is
cooled and the temperature and nature of the bath into which it is plunged
for cooling.
Steel to be hardened is often heated in the fire until at some heat around
600 to 700 degrees is reached, then placed in a heating bath of molten
lead, heated mercury, fused cyanate of potassium, etc., the heating bath
itself being kept at the proper temperature by fires acting on it. While
these baths have the advantage of heating the metal evenly and to exactly
the temperature desired throughout without any part becoming over or under
heated, their disadvantages consist of the fact that their materials and
the fumes are poisonous in most all cases, and if not poisonous, are
extremely disagreeable.
The degree of heat that a piece of steel must be brought to in order that
it may be hardened depends on the percentage of carbon in the steel. The
greater the percentage of carbon, the lower the heat necessary to harden.
Image Figure 5.--Cooling the Test Bar for Hardening
To find the proper heat from which any steel must be cooled, a simple test
may be carried out provided a sample of the steel, about six inches long
can be secured. One end of this test bar should be heated almost to its
melting point, and held at this heat until the other end just turns red.
Now cool the piece in water by plunging it so that both ends enter at the
same time (Figure 5), that is, hold it parallel with the surface of the
water when plunged in. This serves the purpose of cooling each point along
the bar from a different heat. When it has cooled in the water remove the
piece and break it at short intervals, about 1/2 inch, along its length.
The point along the test bar which was cooled from the best possible
temperature will show a very fine smooth grain and the piece cannot be cut
by a file at this point. It will be necessary to remember the exact color
of that point when taken from the fire, making another test if necessary,
and heat all pieces of this same steel to this heat. It will be necessary
to have the cooling bath always at the same temperature, or the results
cannot be alike.
While steel to be hardened is usually cooled in water, many other liquids
may be used. If cooled in strong brine, the heat will be extracted much
quicker, and the degree of hardness will be greater. A still greater degree
of hardness is secured by cooling in a bath of mercury. Care should be used
with the mercury bath, as the fumes that arise are poisonous.
Should toughness be desired, without extreme hardness, the steel may be
cooled in a bath of lard oil, neatsfoot oil or fish oil. To secure a result
between water and oil, it is customary to place a thick layer of oil on top
of water. In cooling, the piece will pass through the oil first, thus
avoiding the sudden shock of the cold water, yet producing a degree of
hardness almost as great as if the oil were not used.
It will, of course, be necessary to make a separate test for each cooling
medium used. If the fracture of the test piece shows a coarse grain, the
steel was too hot at that point; if the fracture can be cut with a file,
the metal was not hot enough at that point.
When hardening carbon tool steel its heat should be brought to a cherry
red, the exact degree of heat depending on the amount of carbon and the
test made, then plunged into water and held there until all hissing sound
and vibration ceases. Brine may be used for this purpose; it is even better
than plain water. As soon as the hissing stops, remove the work from the
water or brine and plunge in oil for complete cooling.
Image Figure 6.--Cooling the Tool for Tempering
In hardening high-speed tool steel, or air hardening steels, the tool
should be handled as for carbon steel, except that after the body reaches
a cherry red, the cutting point must be quickly brought to a white heat,
almost melting, so that it seems ready for welding. Then cool in an oil
bath or in a current of cool air.
Hardening of copper, brass and bronze is accomplished by hammering or
working them while cold.
Tempering is the process of making steel tough after it has been
hardened, so that it will hold a cutting edge and resist cracking.
Tempering makes the grain finer and the metal stronger. It does not affect
the hardness, but increases the elastic limit and reduces the brittleness
of the steel. In that tempering is usually performed immediately after
hardening, it might be considered as a continuation of the former process.
The work or tool to be tempered is slowly heated to a cherry red and the
cutting end is then dipped into water to a depth of 1/2 to 3/4 inch above
the point (Figure 6). As soon as the point cools, still leaving the tool
red above the part in water, remove the work from the bath and quickly rub
the end with a fine emery cloth.
As the heat from the uncooled part gradually heats the point again, the
color of the polished portion changes rapidly. When a certain color is
reached, the tool should be completely immersed in the water until cold.
For lathe, planer, shaper and slotter tools, this color should be a light
straw.
Reamers and taps should be cooled from an ordinary straw color.
Drills, punches and wood working tools should have a brown color.
Blue or light purple is right for cold chisels and screwdrivers.
Dark blue should be reached for springs and wood saws.
Darker colors than this, ranging through green and gray, denote that the
piece has reached its ordinary temper, that is, it is partially annealed.
After properly hardening a spring by dipping in lard or fish oil, it should
be held over a fire while still wet with the oil. The oil takes fire and
burns off, properly tempering the spring.
Remember that self-hardening steels must never be dipped in water, and
always remember for all work requiring degrees of heat, that the more
carbon, the less heat.
Case Hardening.--This is a process for adding more carbon to the
surface of a piece of steel, so that it will have good wear-resisting
qualities, while being tough and strong on the inside. It has the effect of
forming a very hard and durable skin on the surface of soft steel, leaving
the inside unaffected.
The simplest way, although not the most efficient, is to heat the piece to
be case hardened to a red heat and then sprinkle or rub the part of the
surface to be hardened with potassium ferrocyanide. This material is a
deadly poison and should be handled with care. Allow the cyanide to fuse on
the surface of the metal and then plunge into water, brine or mercury.
Repeating the process makes the surface harder and the hard skin deeper
each time.
Another method consists of placing the piece to be hardened in a bed of
powdered bone (bone which has been burned and then powdered) and cover with
more powdered bone, holding the whole in an iron tray. Now heat the tray
and bone with the work in an oven to a bright red heat for 30 minutes to an
hour and then plunge the work into water or brine.
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