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High-carbon Machinery Steel
The carbon content of this steel is above 30 points and is ha...

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

Annealing Work
With the exception of several of the higher types of alloy s...

Complete Calibration Of Pyrometers
For the complete calibration of a thermo-couple of unknown e...

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

Connecting Rods
The material used for all connecting rods on the Liberty engi...

Take Time For Hardening
Uneven heating and poor quenching has caused loss of many ve...

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

Manganese
MANGANESE is a metal much like iron. Its chemical symbol is M...

Using Illuminating Gas
The choice of a carburizing furnace depends greatly on the fa...

Hardening High-speed Steels
We will now take up the matter of hardening high-speed steels...

Machineability
Reheating for machine ability was done at 100 deg. less than ...

The Pyrometer And Its Use
In the heat treatment of steel, it has become absolutely nece...

Steel For Chisels And Punches
The highest grades of carbon or tempering steels are to be re...

Compensating Leads
By the use of compensating leads, formed of the same materia...

Hardening
Steel is hardened by quenching from above the upper critical....

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

Pyrometers
Armor plate makers sometimes use the copper ball or Siemens' ...

Typical Oil-fired Furnaces
Several types of standard oil-fired furnaces are shown herew...

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



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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