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The Modern Hardening Room
A hardening room of today means a very different place from ...

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

Placing Of Pyrometers
When installing a pyrometer, care should be taken that it re...

Knowing What Takes Place
How are we to know if we have given a piece of steel the ver...

Surface Carburizing
Carburizing, commonly called case-hardening, is the art of pr...

Steel Can Be Worked Cold
As noted above, steel can be worked cold, as in the case of ...

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

Liberty Motor Connecting Rods
The requirements for materials for the Liberty motor connecti...

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

Cyanide Bath For Tool Steels
All high-carbon tool steels are heated in a cyanide bath. Wi...

Shrinking And Enlarging Work
Steel can be shrunk or enlarged by proper heating and cooling...

Restoring Overheated Steel
The effect of heat treatment on overheated steel is shown gra...

Forging High-speed Steel
Heat very slowly and carefully to from 1,800 to 2,000 deg.F....

Steel Before The 1850's
In spite of a rapid increase in the use of machines and the ...

Crucible Steel
Crucible steel is still made by melting material in a clay or...

Pyrometry And Pyrometers
A knowledge of the fundamental principles of pyrometry, or th...

Annealing In Bone
Steel and cast iron may both be annealed in granulated bone. ...

Carbon-steel Forgings
Low-stressed, carbon-steel forgings include such parts as car...

Ebbw Vale And The Bessemer Process
After his British Association address in August 1856, Besseme...

Hardness Testing
The word hardness is used to express various properties of me...



The Influence Of Size






Category: HEAT TREATMENT OF STEEL

The size of the piece influences the physical properties obtained in
steel by heat treatment. This has been worked out by E. J. Janitzky,
metallurgical engineer of the Illinois Steel Company, as follows:



With an increase in the mass of steel there is a corresponding
decrease in both the minimum surface hardness and depth hardness,
when quenched from the same temperature, under identical conditions
of the quenching medium. In other words, the physical properties
obtained are a function of the surface of the metal quenched for
a given mass of steel. Keeping this primary assumption in mind, it
is possible to predict what physical properties may be developed in
heat treating by calculating the surface per unit mass for different
shapes and sizes. It may be pointed out that the figures and chart
that follow are not results of actual tests, but are derived by
calculation. They indicate the mathematical relation, which, based
on the fact that the physical properties of steel are determined
not alone by the rate which heat is lost per unit of surface, but
by the rate which heat is lost per unit of weight in relation to
the surface exposed for that unit. The unit of weight has for the
different shaped bodies and their sizes a certain surface which
determines their physical properties.

For example, the surface corresponding to 1 lb. of steel has been
computed for spheres, rounds and flats. For the sphere with a unit
weight of 1 lb. the portion is a cone with the apex at the center
of the sphere and the base the curved surface of the sphere (surface
exposed to quenching). For rounds, a unit weight of 1 lb. may be
taken as a disk or cylinder, the base and top surfaces naturally do
not enter into calculation. For a flat, a prismatic or cylindrical
volume may be taken to represent the unit weight. The surfaces
that are considered in this instance are the top and base of the
section, as these surfaces are the ones exposed to cooling.

The results of the calculations are as follows:

TABLE 20.--SPHERE

Diameter Surface per
of sphere pound of steel
X Y
8 in. 2.648 sq. in.
6 in. 3.531 sq. in.
4 in. 5.294 sq. in.
3 in. 7.062 sq. in.
2 in. 10.61 sq. in.
XY = 21.185.

TABLE 21.--ROUND

Diameter Surface per
of round pound of steel
X Y
8.0 in. 1.765 sq. in.
6.0 in. 2.354 sq. in.
5.0 in. 2.829 sq. in.
4.0 in. 3.531 sq. in.
3.0 in. 4.708 sq. in.
2.0 in. 7.062 sq. in.
1.0 in. 14.125 sq. in.
0.5 in. 28.25 sq. in.
0.25 in. 56.5 sq. in.
XY = 14.124.

TABLE 22.--FLAT

Thickness Surface per
of flat pound of steel
X Y
8.0 in. 0.8828 sq. in.
6.0 in. 1.177 sq. in.
5.0 in. 1.412 sq. in.
4.0 in. 1.765 sq. in.
3.0 in. 2.345 sq. in.
2.0 in. 3.531 sq. in.
1.0 in. 7.062 sq. in.
0.5 in. 14.124 sq. in.
0.25 in. 28.248 sq. in.
XY = 7.062.

Having once determined the physical qualities of a certain specimen,
and found its position on the curve we have the means to predict the
decrease of physical qualities on larger specimens which receive
the same heat treatment.

When the surfaces of the unit weight as outlined in the foregoing
tables are plotted as ordinates and the corresponding diameters as
abscissae, the resulting curve is a hyperbola and follows the law
XY = C. In making these calculations the radii or one-half of
the thickness need only to be taken into consideration as the heat
is conducted from the center of the body to the surface, following
the shortest path.

The equations for the different shapes are as follows:

For flats XY = 7.062
For rounds XY = 14.124
For spheres XY = 21.185

It will be noted that the constants increase in a ratio of 1, 2,
and 3, and the three bodies in question will increase in hardness
on being quenched in the same ratio, it being understood that the
diameter of the sphere and round and thickness of the flat are
equal.

Relative to shape, it is interesting to note that rounds, squares,
octagons and other three axial bodies, with two of their axes equal,
have the same surface for the unit weight.

For example:

Size Length Surface Weight Surface for 1 lb.
2 in. Sq. 12 in. 96.0 sq. in. 13.60 lb. 7.06 sq. in.
2 in. Round 12 in. 75.4 sq. in. 10.68 lb. 7.06 sq. in.

Although this discussion is at present based upon mathematical
analysis, it is hoped that it will open up a new field of investigation
in which but little work has been done, and may assist in settling
the as yet unsolved question of the effect of size and shape in
the heat treatment of steel.





Next: Heat-treating Equipment And Methods For Mass Production

Previous: Care In Annealing



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