RETAINED AUSTENITE STABILIZATION

The phenomenon of retained austenite stabilization in tool steels is well  documented. When a steel with below room temperature is quenched, some austenite is retained in the microstructure. If cooling is immediately continued to a temperature below the M_p virtually all the austenite present at room temperature can be transformed to martensite. However, if there is a delay between quenchine to room temperature and the further cooling, the austenite can stabilize and cannot then be transformed by subsequent cooling.

It is important to know if this phenomenon also applies to carburized low alloy steels used to make gears. As well as reduceing hardness and wear resistance, retained austenite in the case of such components can later be transformed by applied stress, causing distortion during service. Retained austenite has also been reported to lead to cracking during grinding after heat treatment.

There is no agreement in the literature on  the occurrence of stabilization, with some reports suggesting that it does not occur in carburized low alloy steels. Others, however, report that it does occur; and some even suggest that its onset is very rapid. Where stabilization is reported, it is generally associated with high alloy content, particularly nickel, and with the presence of nitrogen in the case. The work reported here set out to show the effects of stabilization on a typical carburizing steel(SAE 8620) after carburizing in a typical industrial cycyle without any added nitrogen.

Experimental Conditions

The SAE 8620 material for the tests was in the form of gear teeth. They were carburized in an industrial sealed quench furnace with the following cycle:

• Heat to 930°C with a carbon potential of 0.4 percent and soak for more than 190 minutes
• Carburize at 930°C for 345 minutes at a carbon potential of 0.9 percent.
• Diffuse at 850°C for 150 min at a carbon potential of 0.75 percent.
• Oil quench until the samples reach 70°C.
• Cold treat at -120°C for 1 hour, after delays of 2 minutes, 1 hour, 12 hours, 24 hours, and 168 hours.

After treatment, each sample was examined for microstructure, retained austenite(by x-ray diffraction) and hardness profile.

Testing Results

The hardness profiles for the different delay times are shown in fig. 1. Photomicrographs of the case for one of the as-quenched samples that was cold treated within two minutes of quench and one cold treated after a delay of 168 hours are shown in fig. 2. The structure within 20 µm of the surfaces has been affected by the migration of alloying elements through internal oxidation, which is normal for carburizing treatments carried out in endothermically generated atmospheres. There is no visible difference between the sample cold treated after two minutes and that cold treated after 168 hours. The case depths obtained from the hardness traverses, the surface hardness, and the retained austenite levels obtained by x-ray diffraction are summarized in table 1.

Discussion of Test Results

Retained austenite is thought to be stabilized by a pinning mechanism. During aging, carbon is redistributed by diffusion out of the  martensite. the structure is then stabilized by interstitial carbon atoms pinning the austenitemartensite interface. As pinning increases with the length of time after quenching, more energy is needed to restart the transformation to martensite; i.e., a lower cold treatment temperature is needed.

In general terms, some alloying additions are known to promote stabilization, particularyly nickel, carbon, and nitrogen. Thus, the austenite in the cases of  higher alloy carburizing steel with high carbon or carbonitrided cases will be able to restart the transformation. Assessing reports on the stabilization time must therefore take into account not only the composition of the austenite, but the cold treatment temperature used.

In the case of SAE 8620, under the experimental conditions using cold treatment at -120°C, this temperature was sufficient to restart transformation after all the stabilization times tested. This effect probably explains the disparity in the results found in the literature. It is likely that for low alloy carburizing steel, -120°C will always be sufficent to restart transformation, but any higher temperature may not be.

To test this theory, a second as-quenched sample gear was allowed to stabilize for 1,680 hours, and was then subjected to cold treatment at temperatures in the range -40 to -120°C for one hour. The results are shown in fig. 3. Although not completely conclusive, as the same effect might have been found in samples immediately after quench, the results suggest that a cold treatment temperature of between -40 and -70°C was needed to restart transformation in this steel.

The quantity of retained austenite in this second sample gear in the as- quenched condition was found to be significantly lower than in the original sample gear, when it was tested shortly after quenching. To check if the retained austenite could have been reduced as a result of the time that elapsed between the two measurements, a second sample from this gear was checked. This sample was also found to contain less retained austenite(15.8 percent). To confirm that this was not simply a difference between the two gears, a sample from the first gear was retested and was found to contain 19 percent retained austenite, significantly less than when it was originally tested. The microstructure (fig. 4) appears to contain some small bainite laths that could have formed during extended aging at room temperature. This suggests that the effect is real and is probably caused by isothermally reaching the bainite start condition after protracted aging at room temperature.

Conclusions

For carburized SAE 8620, cold treatment at -120°C is sufficient to restart the transformation of retained austenite to martensite after any stabilization period. The temperature necessary to restart the transformation after stabilization is probably in the range -40 to -70°C.

Recommendations

The results of this study were combined with a review of the available literature to develop a number of recommendations for best industrial practice for the cold treatment of case carburized components to remove retained austenite. It is obviously impractical to test every carburizing and available cold treatment temperatures, so the following guidelines can be used:

• The higher the alloy content(particulary nickel), the shorter the stabilization time and the lower the cold treatment temperature that should be used.
• The higher the case carbon content, the shorter the stabilization time and the lower the cold treatment temperature that should be used. Preemptive treatment of all components is preferable to attempting to recover components after over carburizing is discovered.
• The shourter the delay between quenching and cold treatment the better.
• The colder the treatment temperature the better.

DIMENSIONAL STABILITY IN CRYOGENIC TREATMENTS

The following artilce describe the results of academic and  industrial research into the influence of cryogenic treatment on dimensial stability in gear steel

By A.Bensely, Pete Paulin, G. Nagrajan, and D. Mohan Lal

When cryogenically treatd, case carburized steel (En 353) showed tremendous improvement in wear resistance[¹]. This is due to the microsstructual changes such as coversion of retained austenite to martensite and precipitation of fine alloying carbides. The equivalent grades of En 353 are BS-815M17 and IS-15NicCr1Mo12. In an earlier research work on En 353 it was found that cryogenic treatment changes the residual stress[²]. This article desribes 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 E831 for studying the linear thermal expansion of En 353[³].

HEAT TREATMETNT 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 propertis. All basic heat-treating processes for steel involve the transformation or decomposition of austenite. The firs step in the heat treatment of steel is to heat the raw material to a 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 carburized process of 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 or continuous evolution. In its earliest application, components were simply placed in a suitable container and covered with a thick layer of carbon, this method was exceedingly slow, and  as the demand for production grew a new process using liquid adn gaseous atmophere was developed. They are gas carburizing, vacuum carburizing, plasma carburizing, and salt bath carburizing.

CONVENTIONAL HEAT TREATMENT

Numerous industrial components such as gears, crown wheels, and pinions require a hard wear resistant surface and a soft tough core to withstand heavy loading. This can be achieved through a 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 softthough 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 a 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 a reduction of the 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 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 five hours. At this temperature the following reaction takes place.

Fe+2C0 → Fe(C) + CO₂

Where Fe(C) represents carbon dissoved in austenite. Due to this carburization, a surface layer of high carbon equivalent to the carbon potential (0.75 percent) is quickly built up in the case. Since the raw material has low carbon cotent(0.17 percent), 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 quenchhardening process. In quench-hardening prcess, the carburized specimens were heated to 1093 K(1580°F) and soaked for 30 minutes and suddenly quenched in oil at 313 K(104°F). 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 is alos supresses the conversion of austenite into ferrite and cementite. Hence the structure of hardened steel consist 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(302°F) 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 is increased. Tempering was carried out to gain toughness and relive internal stressess at the cost of some hardness and tensile strength.

CRYOGENIC TREATMENT

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 services still prevails. Hence the steel has to be cooled still further down from room temperature to achieve 100-percent martensite. Cryogenic treatment is an

extension of conventional heat treatment to achive 100-percent 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, pinions, 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 microstrutural stability, improved wear resistance, and relief of residual stress [^1,2,4,5].

Presently two major types of cryogenic treatment are avaiable in practice. They are SCT and DCT. There is a lot of confusion among researchers in classifying the temperature applied in both the treatements. 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 coventional heat treatment and cryogenic treatment parameters adopted for En 353 in the present study in shown in fig. 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 a conventional hardening process (i.e., oil quenching) were directly kept in freezer at 193 K(-112°F) for five hours to complete the martensiteic transformation. This was followed by tempering at 423K(320°F) for 1.5 hours. It was done to ensure that there is no brittle untermpered 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(M_f), lies nearer to 193 k(-112°F), the need for deep cryogenic treatment is due to the increased benefits reported by Barron (1974) on other materials when treated at 77 K(-32l°F)[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 (-321°F) in three hours, soaked for 24 hours, and heated back to room temperature in six 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 77K(-321°F). After the specimens reach room temperature it was immediately subjected to tempering at 423 K(302°F) for 1.5 hours in order to attain carbide precipitation and tempered martensite(see fig. 1).

RESULTS AND DICUSSION

Materials expand to some extent when heated. The heat increases hte 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

α = ΔL/LΔT

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 thermomechanical analyer. The treated specimen was placed in the specimen holder under the sessing probe, with the temperature sensor is contact with the specimen. The furnace encloses the specimen holder. An appropriate load force of 20 mN wan applied to the sensing probe to ensure that it was in cotact with the specimen. The specimen was heated at a constant heating rate of 20K/mintue from room temperature to 1103K(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 fig.2.

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 decompositon and  ordering. From temperature 503K(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 martensite into cementite(contraction). the contraction exceeds the expected expansion due to the convertsion 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 643K (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 expansionand contraction and can be attributed to the precipitation of metal carbides and grain coarsening.

CONCLUSIONS

This study clearly shows that both shallow cryogenic treatment adn deep cryogenic treatment aid further conversion of retained austenite to martensite, which on tempering will lead to enhanced carbide precipitation. The study also conclues that the dimensional stabillity should be subjected to deep cryogenic treatment.