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Conclusions
Martien was probably never a serious contender for the honor ...

Carburizing By Gas
The process of carburizing by gas, briefly mentioned on page ...

Mushet And Bessemer
That Mushet was "used" by Ebbw Vale against Bessemer is, perh...

Bessemer Process
The bessemer process consists of charging molten pig iron int...

The Quenching Tank
The quenching tank is an important feature of apparatus in c...

Non-shrinking Oil-hardening Steels
Certain steels have a very low rate of expansion and contract...

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

Protectors For Thermo-couples
Thermo-couples must be protected from the danger of mechanica...

Fatigue Tests
It has been known for fifty years that a beam or rod would fa...

The Penetration Of Carbon
Carburized mild steel is used to a great extent in the manufa...

Tempering Round Dies
A number of circular dies of carbon tool steel for use in too...

Annealing Of High-speed Steel
For annealing high-speed steel, some makers recommend using g...

The Forging Of Steel
So much depends upon the forging of steel that this operation...

Instructions For Working High-speed Steel
Owing to the wide variations in the composition of high-speed...

Leeds And Northrup Optical Pyrometer
The principles of this very popular method of measuring tempe...

Rate Of Cooling
At the option of the manufacturer, the above treatment of gea...

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

Flange Shields For Furnaces
Such portable flame shields as the one illustrated in Fig. 1...

Correction By Zero Adjustment
Many pyrometers are supplied with a zero adjuster, by means ...

Introduction Of Carbon
The matter to which these notes are primarily directed is the...



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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