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* aichican STATE university Ueranies . edhe
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This thesis was sent to us py lir. W. L. Nies
when he contributed his copy. In hunting for
his he discovered that he had this copy by
"F, L. Barrows and G M. O'Dell" in his possess-
ion. As Mr. Nies was a member of 1913, he
evidently borrowed this thesis and did not re-
turn it to the owner or the college.
Feb. 18, 1918.

 
THESTIS

EFFECT OF CASE HARDENING
on |
TENSILE STRENGTH OF COLD ROLLED STEEL.

FP. Le Barrows.
G. ¥. O'Dell.
Rast Lansing, Mich. 1912.
THESIS
Preface.

The subject of this thesis was selected because of the
prominent place case hardening and heat treatment of
ateels is now cocupying in the manufacturing world par-
ticularly in the manufacture of automobiles where lighte-
ness is required without the sacrifice of strength.

It was evidently impossible to cover the entire field
including the heat treatment of special steels owing, not
only to the shortness of time But also to the difficulty of
obtaining the necessary materials, so we finally decided to
limit our investigations to the relation of case hardening
to tensile strength.

As preliminary reading matter we referred principally
to "“etallurgy of Iron and Steel® by Bradley Stoughton and
a thesis entitled "Tool Steel® by BE. J. Kumnse. We also re-
ceived many practical sucgestions from Mr. Frank Sallows
who has <*arge of the heat-treating department of the Reo
Moter Car Company, from a booklet “Heat Treating of Steels"
by the Tate, Jones Company of Pittsburgh, Pa., and a series
of dbulletins published by the Hoskins Manufacturing Company
of Detroit, Michi; an, manufacturers of pyrometers.
APPARATUS

The apparatus used consisted of a “Stuart Combination
Gas Purnace® manufactured by the Chicago Flexible Shaft
Company of Chicago, Illinois. (See phote.) The fuel used
was illuminating gas supplied by a main of the Lansing City
Gas Company.

The carbonizing material was a mixture consisting
principally of charred leather and known commercially as
"Blaich Modern Carboniser"(#1 & #3 mixed) made by Alfred
O. Blaich of Chicago, Illinois.

The pyrometer used was kindly loaned us by the Chemis-
try Department and was of the portable type of the Hoskins
Therme-electric pyrometer made by the Hoskins. Manufacturing
Company of Detroit, Michigan.

The yyrometer was calibrated and compared with the
stationary pyraneter in the lecture room of the chenioal
laboratory.

PRELDIINARY WORK.

The preliminary work consisted of turning up a number
of standard specimens from cold rolled steel of the size
shown in fig. 1. Holding the diameter of the test length
within .002 of .S inch. In addition it became necessary to
make patterns and have cast, iron bexes in which to pack
the specimens. These boxes were 15° in length by 9° wide
and 4 1/2" deep and a; proximately 3/16" thick, this being
NT He

 

 

 

 

Apparatuse

Pig. 1.
o5ea

the largest sise which could be used in the furnace. We
would recormend smaller sises than this for similar work
because of the -reater ease with which they might be handled
end what is more important, that if made of the correct
proportions as many as three might be placed in the furnace
at ene time. In this case when heating for different lengths
of time they could be drawn as required without disturbing
the rest, heating would be continuous and the time and gas
required to bring the furnace up to the proper temperature
for each run would be saved.

A series of chemical tests by the eolorimetric method
were run on samples of the steel to determine approximately
the carbon content which was found to average about .26 per=-

cent.
PACKING.

The packing was done by first placing a layer of the
carbdonising material in the bottom of the box to a depth of
about one inch then the specimens were placed on this
layer and at least one inch from the sides of the box and
ene half ineh apart, another layer of the carbonising
material was added, a jayer of specimens then more of the
material and eo on until the box was filled. The iren cover
was then set in place and sealed with fire clay. The cover
hed a 5/16 * hole drilled in it so that a 1/4® test wire
could be inserted and drawn from time to tixe in order to

determine when the box had been heated through.
HEATIUUG.

After sealing, the boxes were placed in the eaabustion
chamber of the muffiey furnace and brought up to the desired
temperature. (This required from one half to three quarters
of an hour.) The tests were run for a definite length of
time and the beginning of this period was taken when the
test wire was drawn and indicated that the box was heated
through. The period through which the heating was continued
at the desired temperature was called the "time of carbcnisa-
tion®. Seme diffieulty was encountered in maintaining a
eonstant temperature due to the varying quality of the gas
and our inexperience as to the proper amount of air to |
supply. This latter difficulty was overcome as ve became
more efficient in the handling of the furnace thovgh we were
unable to get as high temperatures as we wished due to the
inferior quality of the gas. The highest temperature
attainable varying as much as 100° ¥. on a@ifferent é@ays.

Ve found a wide variation in the temperature of ear-
bonisation used and recommended by different authorities
(1200° ¥, - 1832° ¥.) and it was our aim to try to determine

which of these temperatures will give the best results.
REHRATING.

After carbonizsing the specimens were allowed to cool
in the carbonizing material, then removed and slowly heated
in the furnace, each specimen of each set being heated to
@ different temperature than the rest, for here also we

found disagreement as to the proper temperature.

QUENCHING.

After reaching the proper temperature the specimens
were quickly removed from the furnace and dipped in lin-
seed cil. O11 being used because it gives a tougher struc-
ture than when brine is used as the quenching medium. Tt
might be well to mention here some rules for hardening:*

1. Heat slowly.

2 Heat uniformly throughout the piece.

5. De not heat too hot.

4. Do not let the piece soak after it has been
heated evenly ard theroughly. Remove from the
furnace.

S. Avoid drafts of air and contact of any cold
metal before or just after dipring into tha
eocling agent.

6. Dip the piece vertically to avoid warping.

7. Avoid quick moving of the piece in the cooling

agent to avoid warping.
* Prom "Tool Steel*® by EB. J. Kunse.
 

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-6-

There was a certain maount of oxidization in the re-
heating process, an objection common to all gas furnaces
which would be serious should the plece be a finished part
but with our specimens the oxidization was so slight that
we disregarded it.

THSTING .

The testing was done in the 100,000 Riehle Testing
Machine in the engineering laboratory. Special grips were
designed to hold the teat bars (Fig. 2). The important
feature of these grips is the "ball and sooket joint® effect
procured by the spherical bushings. The object being to
eliminate any bending of the hardened piece and insure
failure by tension.

Bo extensometers were available so the elastic Limit
could be determined only approximately by the action of the
machine. However, it is certain that the elastio ratio is
high for the case-hnardened piecea. Following are the tabu-
lated results of the test and a set of curves showing the
relation of the strength to the time of carbonization. These
curves are plotted from t’e results obt«ined from the speci-
mens carbdonised at 1300°F. for different lengths of time and

reheated to different temperatures.
PRELIMINARY TZ8T.

*carbdonized at 1562°?. Yor 2 hours.
Specimen Fo. Reheat Temp. Max. Load (# sq. In.)
Ag 1100°F. 73800
Ag 1200 78400
As 1300 80100
Ap 1455 70180
Ay 1455 68450

*thie temperature wae not held for the entire two hours
but was reached duri:¢ the latter part of the pericde

TEREST Neo. l.
Carbonised at 1500°F. Yor 3 hours.
Specimen No. Reheat Temp. Maxe Load (# eq. in.)

Bs Not reheated. 69800
A. 1300°F. 74500
A. 1400 73650
3, 1450 71.760
Bs 1500 72250
TEST No. 2.

Carbonizsed at 1300°F. Yor 4 hours.

Specimen Ko. Reheat Temp. Max. Load (# aq. in.)
Be 1300°F. 73000
Bs 1400 71900
Be 1450 78050
Be 1500 74450
Bg | 1500 76300
TEST No. 3.
Carbonised at 1300°F. For & hours.
Specimen Ko. Reheat Temp. Max. load (#eq. in.)
Bo 1300°F. 81600
Cc, 1380 81000
Co 1400 71500
Cy 1450 79760
Cy 1500 87500

C 5 1500 83950
TRST No. 4.

Carbonized at 1300°F. For 6 hours.

Specimen No. Reheat Temp. Max. Load (# sq. in.)
Ce 1300°F. 69500
Co 1350 70400
C, 1400 67450
C. 1456 69800
D 1500 70600
D) 1500 78000
TRST Ho. 5.
Carbonized at 1400°F. Yor 2 hours.
Spesinen No. Rehcated Penp. Max. Load (# sq. ii.)
Dy 1300°F. 71780
Dy 1350 77600
D, 1400 84450
Ds 1450 92400
Dy 1500 85900

Da 1500 88000
TEST Noe 66

Carbonised at 1500°F. For 2 hours.

Specinen No. Reheat Temp. Max. Load (#sq. in.)
De 1300°F. 73680
Dy 1550 72500
E 1400 71900
E, 1450 70850
ER, 21500 81200
Ry 1500 78900

The curves represent the following conditions:

Curve 1. Carbdenised at 1500°F. from 3 to 6 hours and re-
heated to 1300°F.

Curve 2. Carbdonised at 1300°F. from § to 6 hours and re-
heated to 1350°?.

Curve 3. Carbdonised at 1300°F. from 3S to 6 hours and re-
heated to 1400°F.

Curve 4. Carbonised at 1300°F. from 3 to 6 hours and re-
heated to 1450°F.

Curve 5. Carbdonised at 1300°F. fram 3 to 6 hours and re-
neated to 1600°F.
IMENT MTA

§5040

80000

 

60000
85000

80000

60000

3
ee OF CARBONIZATION JN HAS.

 
 

Broken Specimens.

Explanation:

Specimens no's. 1 and 5 show the crystal texture,
and reduction of area due to the load.

No's. 2 and 3 show the coarse porous cyrstal of
specimens which were not properly rerated. No. 3 also
shows the canes
No. 4 shows a properly reheated specimen.

No. 6 shows the way in which a trexted specimen

breaks, there being no reduction of area.
@lie-

CONCLUSION.
EFFECT OF TIM® ON TENSILE STRENGTH.

A study of the curves shows that the tensile strength |

increases (except in specimens reheated to 1400°F.) with

the time of carbonization up to and including five hours

but when the oarbonizing was continued for six hours the
specinens without exception showed the same or less strength
than those carbonized for three hours. It would be interest-
ing and instructive to continue the heating for a longer
time than six hours and determine whmetha@ the continued
heating is detrimental. If so, it would seem that the
“sonking effect is the same as ocours when tempering or

hardening tool steel.
PROPER CARBONIZING TEMPERATURES.

Prom a comercial standpoint the results show that long
eontinued carbonising at a low temperature is not nearly so
effective as carbonizing for a short time at a high tempera-
ture, for the results show that those pieces (Test Ne. 5)
carbonized at 1400°F. are uniformly atronger than those
heated at 1300°F. for five hours (The strongest specimens
carbonized at 1500°F.) and furthermore there would be a saving
in time and fuel if the higher temperature were used. Some

authorities recommend as high a carbonising temperature as
-]2—

1832°F. but our investira'ions do not seem to confirm their
opinions. Probably different steels would give best results
at different temperatures but for eold rolled steel we would
recommend 1400°F. as the rroper carbonising temperature.

EYFECT OF REVEATING
ro: "eu. ATURE.

 

A further study of the results and curves shows that
reheating is almost na necessity if the strength of the steel
is to be retained. Specimen No. By was not reheated after
carbonising but was dip: ed immediately upon the removal of
the box from the furnace. It showed a tensile strength almost
10,0007 per aq. in. lewer than the weakest rele ated specing.
This shcws the value of the rereat.

Our stron;:est specimens in almost every case were those
treated from 1400°F. to 1500°F. Here we agree with other
experimenters who recomzend 1472°F. as the proper reheat
temperature,

EBFYRCE OF CASK-HARDENING ON TENSILE STRENGTH.

The results show that as a rule the case~hardened steel
will be weaker than the original though in one or two cases”
the case-hardened plece showed greater strength. Te be on the
safe side we would recormend a greater factor of safety for a
case~hardened part than that which would be allowed for the
eame part if used without case-hardening, particularly if the
piece is to be in tension.
LY.

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