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Steel Making

Manganese
Manganese adds considerably to the tensile strength of steel,...

Tool Or Crucible Steel
Crucible steel can be annealed either in muffled furnace or b...

The Thermo-couple
With the application of the thermo-couple, the measurement of...

Open Hearth Process
The open hearth furnace consists of a big brick room with a l...

Short Method Of Treatment
In the new method, the packed pots are run into the case-har...

Uses Of The Various Tempers Of Carbon Tool Steel
DIE TEMPER.--No. 3: All kinds of dies for deep stamping, pres...

Annealing Method
Forgings which are too hard to machine are put in pots with ...

Nickel
Nickel may be considered as the toughest among the non-rare a...

Application Of Liberty Engine Materials To The Automotive Industry
The success of the Liberty engine program was an engineer...

Process Of Carburizing
Carburizing imparts a shell of high-carbon content to a low-...

The Leeds And Northrup Potentiometer System
The potentiometer pyrometer system is both flexible and subst...

Plant For Forging Rifle Barrels
The forging of rifle barrels in large quantities and heat-tre...

Gears
The material used for all gears on the Liberty engine was sel...

Quenching
It is considered good practice to quench alloy steels from th...

William Kelly's Air-boiling Process
An account of Bessemer's address to the British Association w...

Highly Stressed Parts
The highly stressed parts on the Liberty engine consisted of ...

Hints For Tool Steel Users
Do not hesitate to ask for information from the maker as to t...

Hardening
The forgings can be hardened by cooling in still air or quen...

Lathe And Planer Tools
TO FORGE.--Gently warm the steel to remove any chill is parti...

The Electric Process
The fourth method of manufacturing steel is by the electric f...



Tensile Properties






Category: COMPOSITION AND PROPERTIES OF STEEL

Strength of a metal is usually expressed in the number of pounds
a 1-in. bar will support just before breaking, a term called the
ultimate strength. It has been found that the shape of the test
bar and its method of loading has some effect upon the results,
so it is now usual to turn a rod 5-1/2 in. long down to 0.505 in.
in diameter for a central length of 2-3/8 in., ending the turn
with 1/2-in. fillets. The area of the bar equals 0.2 sq. in., so
the load it bears at rupture multiplied by 5 will represent the
ultimate strength in pounds per square inch.

Such a test bar is stretched apart in a machine like that shown
in Fig. 9. The upper end of the bar is held in wedged jaws by the
top cross-head, and the lower end grasped by the movable head.
The latter is moved up and down by three long screws, driven at
the same speed, which pass through threads cut in the corners of
the cross-head. When the test piece is fixed in position the motor
which drives the machine is given a few turns, which by proper
gearing pulls the cross-head down with a certain pull. This pull
is transmitted to the upper cross-head by the test bar, and can
be weighed on the scale arm, acting through a system of links and
levers.

Thus the load may be increased as rapidly as desirable, always
kept balanced by the weighing mechanism, and the load at fracture
may be read directly from the scale beam.

This same test piece may give other information. If light punch
marks are made, 2 in. apart, before the test is begun, the broken
ends may be clamped together, and the distance between punch marks
measured. If it now measures 3 in. the stretch has been 1 in. in 2,
or 50 per cent. This figure is known as the elongation at fracture,
or briefly, the elongation, and is generally taken to be a measure
of ductility.

When steel shows any elongation, it also contracts in area at the
same time. Often this contraction is sharply localized at the fracture;
the piece is said to neck. A figure for contraction in area is
also of much interest as an indication of toughness; the diameter
at fracture is measured, a corresponding area taken out from a
table of circles, subtracted from the original area (0.200 sq.
in.) and the difference divided by 0.2 to get the percentage
contraction.



Quite often it is desired to discover the elastic limit of the
steel, in fact this is of more use to the designer than the ultimate
strength. The elastic limit is usually very close to the load where
the metal takes on a permanent set. That is to say, if a delicate
caliper (extensometer, so called) be fixed to the side of the
test specimen, it would show the piece to be somewhat longer under
load than when free. Furthermore, if the load had not yet reached the
yield point, and were released at any time, the piece would return
to its original length. However, if the load had been excessive, and
then relieved, the extensometer would no longer read exactly 2.0
in., but something more.

Soft steels give very quickly at the yield point. In fact, if
the testing machine is running slowly, it takes some time for the
lower head to catch up with the stretching steel. Consequently at
the yield point, the top head is suddenly but only temporarily
relieved of load, and the scale beam drops. In commercial practice,
the yield point is therefore determined by the drop of the beam.
For more precise work the calipers are read at intervals of 500 or
1,000 lb. load, and a curve plotted from these results, a curve
which runs straight up to the elastic limit, but there bends off.

A tensile test therefore gives four properties of great usefulness:
The yield point, the ultimate strength, the elongation and the
contraction. Compression tests are seldom made, since the action
of metal in compression and in tension is closely allied, and the
designer is usually satisfied with the latter.





Next: Impact Tests

Previous: Properties Of Steel



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