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July, 1925
ly obtained by the quick open fire method, used oil
tools that can be ground.
The temper at 1100 deg. F. as compared to no
temper increases the scleroscope hardness from 2 to
5 points and at the same time reduces the Rockwell
hardness from 2 to 3 points on the C-scale, with practically
no change in Brinell hardness. It also increases
the toughness to a marked degree, which is
easily demonstrated by breaking a thin section say
y X y2 inch. French and Strauss show this very
clearly in their paper on "Effect of Heat Treatment
on Lathe Tool Performance and Some Other Properties
of High Speed Steels." (September, W23, issue
of Transactions.) They show an increase in fibre
stress and deflection after a 1100 deg. F. temper. The
author believes that the high temper is necessary for
die work when hardened by this method, due to the
added toughness which it gives, which is so important
in dies for punch-press work. Hardness values are
fixed and no further heating up to 900 deg. F (485
deg. C.) (which is as high as the author has tried)
for a short or long period wall change them. The
high temper also helps to prevent grinding cracks
which often means failure of the die.
Precautions.
The precautions which should be taken into account
in this method of heat treatment are as follows :
1. Have high-heat furnace equipped with a good
pyrometer, preferably with platinum, platinum-rhodium
thermocouple, and checked often.
2. Before using new graphite crucibles, anneal
them carefully and heat to 1600 cleg. F. (870 deg. C.)
with lids removed to burn out surface graphite to prevent
it falling on the work to be hardened.
3. See that the lids fit tightly before starting
work.
4. Always have crucibles side by side when making
transfer and do not have lids removed longer than
necessary.
5. Transfer work quickly from the preheat crucible,
and likewise from the high-heat crucible to the
quenching oil.
6. Do not allow the hardened pieces to lie around
long before tempering. The sooner they are tempered,
the less the danger of breakage.
7. After tempering in the salt bath, be sure to
let the pieces get cool before placing them in the hot
water to remove the salt.
Dimensional Changes.
This method, since the finished surfaces are not
scaled and pitted, as in open fire work, has given the
author an excellent opportunity to study dimensional
changes. Not on any certain size test bars, protected
by steel clamps and caps as other investigators have
clone, but on production dies and tools of varying dimensions.
Pieces made from round rolled high speed steel in
the annealed condition have always expanded slightly
on the linear dimension, when treated by this method,
and shrunk slightly on the cross-sectional dimension,
regardless of the varying proportions of these dimensions.
A piece \y2 inches in diameter by 3 inches longwill
usually shrink more on the ends than in the center.
The center on such a piece usually shows no
movement at all. If, however, the dimensions are
reversed, that is, 3 inches in diameter by 1^ inches
Forging- Stamping - Heat Treating
239
thick, the piece gets slightly smaller on the 3-inch
dimension and larger on the lj^-inch dimension, with
a slight bulging in the center. The average expansion
in the linear dimension on some 50 pieces measured,
was 0.0009 inch per inch of length, and the
average shrinkage in the cross sectional dimension
on the same pieces was 0.0007 inch per inch.
This linear expansion of 0.0009 inch per inch
checks very ciosely with curves given by (Jrossmann
in bis paper, "The Change in Dimensions of High
Speed Steel in Heat Treatment" (May, 1922, issue of
Transactions). Mr. (jrossmann, however, did not
take into consideration the cross sectional changes.
My experience has shown that dies made from rectangular
sections show much smaller dimensional
changes than those made from round bars. They
usually expand slightly in the center on the linear
dimension, the edges showing little movement. On
the width they usually expand slightly in the center
and shrink slightly on the ends. On thickness they
practically always expand, the expansion being more
pronounced in the center; here again showing the
bulging effect previously mentioned.
Some data along this line may be of interest to
show the small change that rectangular stock undergoes
in hardening by this method. A few 6-slot motor
laminae dies made from bar stock \y± X 4 inches were
ground on all sides, measured carefully, hardened and
tempered and again measured, showed the following
results:
TABLE I — DIMENSIONAL CHANGES OF HIGH SPEED
RECTANGULAR BAR STOCK
Size Size
Before Treat ment After Tteatinent
Selerosi'ipe Rockwell
Inches In Center At Ends Reading Reading
Length 5.072 5.072 1
Width 3.943 3.9435 3.942 1- 92 C-63
Thickness ...1.1235 1.1247 J
Length 5.933 5.933 j
Width 3.724 3.725 3.724 j 93 C-61
Thickness ...1.063 1.0635 J
Length 5.883 5.883 1
Width 3.422 3.422 3.420 | 92 C-61
Thickness ...1.0595 1.060 J
Length 5.941 5.938 j
Width 3.591 3.591 ,1.588 j 90 C-61
Thickness ...1.070 1.071 J
Treatment—Preheated 1600 deg. F. (870 deg. C.) for 30 mm.
Transferred; in high-heat furnace for 13 min. at 2300 deg. F.
(1260 deg. C). Quenched in oil. In oil at 400 deg. F. for 15
min. In salt at 1100 deg. F. (595 deg. C.) for 25 min. Cooled
in oil.
Discussion.
You no doubt have observed that the main difference
between the method outlined and standard practice
is the use of praphite crucibles and lids, with
about y inch of a mixture of 1/3 charcoal and 2/3
silica sand by volume in the crucibles in which the
work is heated. The reason for the use of graphite
crucible is that the gases therein are reducing at all
times, thus preventing the formation of scale. These
gases do not carburize at the pre-heat temperature,
and show only a slight carburizing effect at the high
heat temperature, this carburization being well under
the eutectoid range.