Influence of cryogenic treatment on the dimensional stability of Gear Steel-En 353

(1) Bensely A., (2) Pete Paulin, (3) Nagarajan G., Mohan Lal D
(1) 3012 Shadowood Pkwy SE, Atlanta, Georgia 30339, USA.
(2) 300 Below, Inc., 2999 E. Parkway Dr., Decatur, Illinois, 62526,USA.
(3) Department of Mechanical Engineering, Anna University, Chennai, India.

Case carburized steel (En 353) when cryogenically treated showed tremendous
improvement in wear resistance [1]. This is due to the microstructural changes such as
conversion of retained austenite to martensite and precipitation of fine alloying
carbides. The equivalent grades of En 353 are BS-815M17 and IS-15NiCr1Mo12. In
an earlier research work on En 353 it was found that cryogenic treatment changes the
residual stress [2]. This research article describes the dimensional stability of En 353
steel after Conventional Heat Treatment (CHT), Shallow Cryogenic Treatment (SCT)
and Deep Cryogenic Treatment (DCT). Coefficient of linear thermal expansion is
very much used for design purpose and to determine failure by thermal stresses
when a material is subjected to temperature variations. Thermomechanical analysis
was carried out as per ASTM E 831 for studying the linear thermal expansion of En
353 [3].

Heat Treatment of En 353

Heat treatment is a combination of heating and cooling applied to a metal or alloy in
the solid state in a way that will produce desired effects on the properties. All basic
heat-treating processes for steel involve the transformation or decomposition of
austenite. The first step in the heat treatment of steel is to heat the raw material to a
temperature nearer or above the critical temperature in order to form austenite. The
nature and appearance of this transformation determines the physical and mechanical
properties of any given steel. Gears are generally subjected to carburization process to
achieve the desired properties. The basic principle of carburizing has remained
unchanged, since carburizing was first employed. The method used to introduce the
carbon into the steel has been a matter of continuous evolution. In its earliest
application, components were simply placed in a suitable container and covered with
a thick layer of carbon powder (pack carburizing). Although effective in introducing
carbon, this method was exceedingly slow, and as the demand for production grew,
a new process using liquid and gaseous atmosphere was developed. They are gas
carburizing, vacuum carburizing, plasma carburizing and salt bath carburizing.

Conventional Heat Treatment (CHT) Process

Numerous industrial components such as gears, crown wheel and pinion require a
hard wear resistant surface and a soft tough core to withstand heavy loading. This
can be achieved through case hardening process. When En 353, a low carbon steel is
subjected to carburizing, a hard wear resistant surface called the case having high
carbon content and a relatively soft-tough inside called the core having low carbon
content is produced. The two regions (case and core) have different in-service
functions to perform.

The conventional heat treatment cycle of En 353 begins with gas carburizing and is
followed by air cooling, quench hardening in oil and tempering. In the present study
liquid carburization process was employed instead of gas carburization to achieve a

case depth of 1 mm. Liquid carburized cases are higher in carbon and lower in
nitrogen. It has the advantages of freedom from oxidation, sooting problems,
production of uniform case depth and carbon content, a rapid rate of penetration and
reduction of time required for the steel to reach carburizing temperature. The
carburizing environment was created by fusing a mixture of sodium cyanide,
potassium chloride, sodium chloride and sodium cyanate. The machined raw
specimens (5 mm diameter and 5 mm height) were placed in a bath of molten
cyanide, so that carbon will diffuse from the bath into the metal and produce a hard
case. Cyanide oxidizes at the surface of the steel producing carbon monoxide, which
in turn dissociates to carbon dioxide and nascent carbon with rapid diffusion into
steel at high temperatures. In conventional heat treatment process the carburizing
temperature was 1183 K and the cycle time for carburizing was 5 hours. At this
temperature the following reaction takes place.

Fe+2CO --> Fe ( C ) + CO2

where Fe(C) represents carbon dissolved in austenite. Due to this carburization, a
surface layer of high carbon equivalent to the carbon potential (0.75 %) is quickly
built up in the case. Since the raw material has low carbon content (0.17%), the
carbon atoms try to reach equilibrium by diffusing inward. After diffusion has taken
place for the required amount of time, depending upon the case depth desired (1
mm), the component was removed from the furnace. The material after carburizing
was cooled in air. It was followed by quench-hardening process. In quenchhardening
process, the carburized specimens were heated to 1093 K (15080F) and
soaked for 30 minutes and suddenly quenched in oil at 313 K(1040F). Gears are
generally oil quenched to avoid distortion to the lowest possible level. This rapid
cooling from the hardening temperature causes the transformation of austenite,
which is soft and ductile, into martensite, which is very hard and brittle. This also
suppresses the conversion of austenite into ferrite and cementite. Hence the structure
of hardened steel consists of mainly tetragonal martensite and some amount of
retained austenite that depends on the chemical composition of steel. This
transformation is diffusionless and time independent and there is no change in
chemical composition.

The hardening of steel depends entirely on the formation of martensite. It increases
the compressive strength and wear resistance of steels, but by itself leaves the steel
very brittle. Tetragonal martensite and retained austenite are unstable. Therefore
hardened steel naturally has a tendency to pass into equilibrium or stable condition.
But this equilibrium is not reached at room temperature because of low mobility of
atoms at this temperature. So, after quench-hardening, the samples were
immediately subjected to tempering. As the temperature rises during tempering the
mobility of atoms increases causing phase and structural changes until reaching
equilibrium. For the present investigation the quench hardened samples are heated
to 423 K (3020F) and soaked for 1.5 hours. During this process the carbon atoms
separates from the space lattice of tetragonal martensite. Consequently the
martensite breaks down into a transition precipitate known as iron carbide (Fe3C)
and a low carbon martensite (soft structure). During this the internal stresses get
removed and the ductility increased. Tempering was carried out to gain toughness
and relive internal stresses at the cost of some hardness and tensile strength.

Cryogenic Treatment Process

The retained austenite present in the conventional heat treatment process after
hardening can be alleviated by means of cryogenic treatment. Since in most of the
alloy steel martensite finish temperature does not lie above room temperature the
problem of retained austenite in service still prevails. Hence the steel has to be
cooled still further down from room temperature to achieve 100% martensite.
Cryogenic treatment is an extension of conventional heat treatment to achieve 100%
martensite. This treatment alters the material microstructures that enhances strength
and wear resistance. For maximum benefits cryogenic treatment should be
introduced between hardening and tempering process. Hardened alloy steels such as
carburized gears, pinion and shafts are particularly responsive to this treatment.
Depending on the alloy composition and the pre hardening cycles the benefits
reaped are increased strength, greater dimensional stability or microstructural
stability, improved wear resistance and relief of residual stress [1, 2, 4, 5].
Presently two major types of cryogenic treatment are available in practice. They are
SCT and DCT. There is a lot of confusion among researchers in classifying the
temperature applied in both the treatments. Hence it becomes imperative to define
both in order to enable readers to identify the temperature limits referred in this
paper. In order to have clarity the details of conventional heat treatment and
cryogenic treatment parameters adopted for En 353 in the present study is shown in
Figure 1.

Shallow Cryogenic Treatment (SCT)

Due to high carbon content attained in the case during carburization there is
retention of austenite in En 353, which can transform during service. This is
detrimental to the material. Hence in the present work specimens which have
undergone conventional hardening process (i.e., oil quenching) were directly kept in
freezer at 193 K(-1120F) for 5 hours to complete the martensitic transformation. It
was followed by tempering at 423 K(3020F) for 1.5 hours . It was done to ensure
that there is no brittle untempered martensite when the component is put into
service.

Deep Cryogenic Treatment (DCT)

Similar to shallow cryogenic treatment, deep cryogenic treatment is also a
supplementary process to conventional heat treatment. Even though the martensite
finish temperature (Mf) lies nearer to 193 K(-1120F), the need for deep cryogenic
treatment is due to the increased benefits reported by Barron (1974) on other
materials when treated at 77 K(-3210F) [5]. DCT is expected to enhance the desired
metallurgical and structural properties by completing the transformation of austenite
(a softer metal phase) to martensite (a tougher, more durable metal phase). In the
present investigation the material which has undergone conventional hardening was
cooled from room temperature to 77 K (-3210F) in 3 hours, soaked for 24 hours and
heated back to room temperature in 6 hours. These very low temperatures were
achieved using computer controls in a well-insulated treatment chamber with liquid
nitrogen as working fluid. Liquid nitrogen, being cheap, inert and easily available is
highly suitable to attain 77 K (-3210F). After the specimens reach room temperature
it was immediately subjected to tempering at 423 K (3020F) for 1.5 hours in order to
attain carbide precipitation and tempered martensite.

Figure 1 - Heat treatment and cryogenic treatment steps adopted for the study

Results and Discussion

Materials expand to some extent when heated. The heat increases the average
amplitude of vibration of the atoms, which in turn increase the average separation.
Suppose an object of length L undergoes a temperature change of magnitude ΔT and
if ΔT is reasonably small, the change in length, ΔL, is proportional to L and ΔT.

Hence where α is called the coefficient of linear expansion for the material. The specimen
to be tested was machined to 5 mm in length and 5 mm in diameter and subjected to
CHT, SCT and DCT processes. The dimensional stability of En 353 after CHT, SCT
and DCT were studied using a Mettler thermo mechanical analyzer. The treated
specimen was placed in the specimen holder under the sensing probe, with the

temperature sensor in contact with the specimen. The furnace encloses the specimen
holder. An appropriate load force of 20mN wan applied to the sensing probe to
ensure that it was in contact with the specimen. The specimen was heated at a
constant heating rate of 20 K/minute from room temperature to 1103 K (1526˚F).
The change in the specimen length was recorded using linear variable differential
transformer and the data obtained helps in comparing and describing the
dimensional stability of En 353 during service. The linear expansion coefficient
estimated for the CHT, SCT and DCT samples are shown in Figure 2.

Figure 2 - Linear expansion coefficient for CHT, SCT and DCT specimens

The coefficient of thermal expansion (CTE) is low for the DCT specimens, which
can indicate that the dimensional stability is high. The CTE increases in the three
specimens till 503 K (446˚F) and remains the same, which is due to pure thermal
effect. A redistribution of the carbon atoms in the martensite occurs roughly from
room temperature to 373 K (212˚F). This can be due to both segregation and
movement of carbon atoms to the lattice defects such as dislocations and twin
boundaries or by the clustering of carbon atoms, which in turn can occur in several
ways and involve spinodal decomposition and ordering. From temperature 503 K
(446˚F) to 643 K (698˚F), a decrease in CTE is observed in SCT and DCT
specimens. During this period an increase and decrease in dimension in CHT sample
takes place. This is due to the conversion of retained austenite to bainite in CHT
specimens. X-Ray diffraction studies show that a small amount of retained austenite
still prevails in SCT and DCT specimens. Between 503 K (446˚F) to 643 K (698˚F),
SCT and DCT specimens undergo contraction. This is due to the simultaneous
conversion of retained austenite to martensite (expansion) and decomposition of
martensite into cementite (contraction). The contraction exceeds the expected
expansion due to the conversion of small amount of retained austenite left in the
specimen, which is a clear indication of large carbide precipitation in SCT and DCT
specimens. From 643 K (698˚F) to 883 K (1130˚F) the CTE remains the same for

the entire specimen. Beyond 883 K (1130˚F) the CHT, SCT and DCT specimens
undergo irregular expansion and contraction and can be attributed to the
precipitation of metal carbides and grain coarsening.

Conclusions

This study clearly shows that both shallow cryogenic treatment and deep cryogenic
treatment aid further conversion of retained austenite to martensite which on
tempering will lead to enhanced carbide precipitation. The study also concludes that
the dimensional stability of SCT and DCT specimens are higher than CHT
specimens. Therefore case carburized gear which demands high wear resistance and
good dimensional stability should be subjected to deep cryogenic treatment.

REFERENCES

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the wear resistance of case carburized steel (En 353) by cryogenic
treatment’, Cryogenics, Vol.45, No.12, December 2005, pp.747-754.
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stress in case carburized En 353 steel’, Materials Science and Engineering:
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they do’, Heat Treating, 1974, pp.14-17.