Tempering is a heat treatment technique applied to ferrous alloys like steel or cast iron to achieve greater toughness by decreasing the hardness of the alloy. The ductility of the metal increase and brittleness decrease as the hardness reduce. Tempering is performed after quenching which is the metal undergo rapid cooling process. Tempering is accomplished by controlled heating of the quenched metal to a temperature below its lower critical temperature. At this temperature, ferrite and cementite combine to form a single-phase solid solution called austenite. Heating above lower critical temperature is avoided in order to prevent destroy martensite which is a hard quenched microstructure. There are two different type of tempering treatment which are Austempering and Martempering treatment.
Characteristic of Austempering Treatment:
Austempering is an isothermal heat treatment that is applied to ferrous metals such as notably steel and ductile iron to produce bainite structure. Austempering is performed from quenching from a temperature above the transformation range to a temperature above the upper limit of martensite formation and holding at this temperature until the austenite is completely transformed to the desired intermediate structure. It produces bainite microstructure in steel and produces ausferrite in cast iron. Ausferrite contain high carbon content and stabilized austenite. Austempering treatment is primarily used to promote and improve mechanical properties or reduce distortion. The properties including higher ductility, resistance to shock and uniform hardness.
Figure 1: TTT diagram. Red line shows the cooling curve for austempering
Advantages of Austempering:
Reduce or eliminate distortion
Uniform and consistent hardness
High impact and fatigue strength
Resistance to hydrogen embrittlement
Limitation of Austempering:
The greatest limitation is that for Austempering to work, one must be able to miss the ‘nose’ on the TTT diagram and prevent transition to structures inferior in strength to Bainite and Martensite. This means that the material must respond quickly enough to the quench medium to accomplish this. In Austempering, the quench material must also be capable of being held at a high enough temperature to arrest Martensite Start transformation. Thus, Austempering will not work with every steel type, which may in fact limit the process to only a select few options of threaded fastener grade steels.
Application of Austempering:
Austempering usually is substituted for conventional quenching and tempering for these reasons:
To improve mechanical properties
To reduce cracking and distortion
To increase wear resistance at a given hardness
To add resistance to subsequent embrittlement
At times, austempering is more economical than conventional quenching and tempering. Most likely when small parts are treated in an automated setup in competition with conventional quenching and tempering which is a three step operation: austenitizing, quenching and tempering. Austempering is a two- step process: austenitizing and isothermal transformation in an austempering bath.
Generally, applications are limited to parts made from small diameter bar or from sheet or strip which is small in cross section. The process is especially suitable for the treatment of thin section, carbon steel parts calling for exceptional toughness at hardness between 40 and 50 HRC. Reduction in the area of austempered carbon steel parts usually is much greater than it is for part conventionally quenched and tempered as indicated in Figure 3 for 5mm bars of 0.85% carbon, plain carbon steel:
Figure 3: Properties of 5mm bars of 0.85% carbon, plain carbon steel
Characteristics of Martempering Treatment:
Figure 2: TTT diagram of Martempering process
Formation of martensite occurs fairly uniformly throughout the workpiece during cooling to room temperature, thereby avoiding formation of excessive amounts of residual stress. Straightening or forming is also easily accomplished upon removal from the marquenching bath while the part is still hot. The piece will hold its shape upon subsequent cooling in fixturing or in air cooling after removal from the forming die.
Figure 3: Molten salt bath treatment
Martempering of wrought steel and cast iron consists of the following steps:
Quenching from the austenitizing temperature into a hot fluid medium (oil, molten salt. molten metal, or a fluidized particle bed) at a temperature usually above the martensitic range (Ms point)
Holding in the quenching medium until the temperature throughout a part is uniform.
Cooling usually in air, at a moderate rate to prevent large differences between temperatures on the outside and center of a section.
During cooling to room temperature, the formation of martensite throughout a part is fairly uniform which avoids excessive residual stresses. When the still-hot part is removed from the bath, it is easy to straighten or to form, and will hold its shape on subsequent cooling in a fixture, or in air cooling after removal from a forming die. Following marquenching, parts are tempered in the same manner as conventionally quenched parts. The time lapse between martempering and tempering is not so critical as it is in conventional quenching and tempering operation CITATION Tem l 18441 (Tempering Processes/Technology).
Advantages of Martempering:
The advantage of martempering lies in the reduced thermal gradient between surface and center as the part is quenched to the isothermal temperature and then is air cooled to room temperature. Residual stresses developed during martempering are lower than those developed during conventional quenching because the greatest thermal variations occur while the steel is in the relatively plastic austenitic condition and because final transformation and thermal changes occur throughout the part at approximately the same time. Martempering also reduces or eliminates susceptibility to cracking.
Another advantage of martempering in molten salt is the control of surface carburizing or decarburizing. When the austenitizing bath is neutral salt and is controlled by the addition of methane gas or proprietary rectifiers to maintain its neutrality, parts are protected with a residual coating of neutral salt until immersed in the marquench bath.
Although martempering is used primarily to minimize distortion, eliminate cracking, and minimize residual stresses, it also greatly reduces the problems of pollution and fire hazard as long as nitrate-nitrite salts are used rather than martempering oils. This is especially true where nitrate-nitrite salts are recovered from wash waters with systems that provide essentially no discharge of salts into drains. Any steel part or grade of steel responding to oil quenching can be martempered to provide similar physical properties. The quenching severity of molten salt is greatly enhanced by agitation and water additions to the nitrate-salt bath. Both techniques are particularly beneficial in heat treating of carbon steels that have limited hardenability.
Oil for Martempering:
At temperatures between 95 to 230?, quenching oil requires special handling. It must be maintained under a protective atmosphere (reducing or neutral) to prolong its life. Exposure to air at elevated temperatures speeds up the deterioration of oil. For every 10? above 60? the oxidation rate approximately doubles, causing the formation of sludge and acid, which can affect the hardness and colour of workpieces. Oil life can be extended and the production of clean work can be maintained by using bypass or continuous filter units containing suitable filtering media (clay, cellulose cartridge or waste cloth). Oils should be circulated at a rate no lower than 0.9 m/s to break up excessive vapour formed during quenching.
The advantages of using oil is oil can be used at lower temperature and is easier to handle at room temperature as compared to salt bath. However, it still have disadvantages. Firstly, the maximum operating temperature of oil is 230?. Workpiece require more time to reach temperature equalization. Next is soap or emulsifier is needed to wash off oil. Washers must be drained and refilled periodically. Oil waste present disposal problems.
Limitation of Martempering:
Due to the high temperature of the quenching medium, the quench severity is seriously hampered CITATION Pan17 l 18441 (Deval, 2017). Hence, there is a danger of intercepting the nose of the TTT curve while cooling to the temperature of the marquenching medium. Therefore hardenability of parts to be martempered should be sufficiently high CITATION Pan17 l 18441 (Deval, 2017). Also parts with very high section thickness cannot be martempered satisfactorily. Even though the residual stresses are minimised because of Martempering, tempering after marquenching is still required.
Application of Martempering:
Since martempering lowers the residual thermal stress, it is used for parts with complex geometries, diverse weights, and section changes. Martempering is used primarily to minimise distortion and eliminate cracking. Alloy steels are generally more adaptable to martempering. In general, any steel part or grade of steel responding to oil quenching can be martempered to provide similar physical properties.
Alloy steels generally are more adaptable than carbon steels to martempering. In general, any steel that is normally quenched in oil can be martempered. Some carbon steels that are normally water quenched can be martempered at 205? in section thinner than 5mm using vigorous agitation of the martempering medium. In addition, thousands of gray cast iron parts are martempered on a routine basis. The grades of steel that are commonly martempered to full hardness include 1090, 4130, 4140, 4150, 8740 and SAE 1141 CITATION Tem l 18441 (Tempering Processes/Technology). Carburizing grades such as 3312, 4620, 8620 and 9310 also are commonly martempered after carburizing CITATION Tem l 18441 (Tempering Processes/Technology). Higher alloy steels such as type 410 stainless are martempered but this is not a common practice CITATION Tem l 18441 (Tempering Processes/Technology).
Table 1: Differences between Austempering and Martempering
Austempering is not a hardening treatment Martempering is a hardening treatment
Austempering gives bainite structure Martempering gives martensite structure
Less distortion and quenching cracks More distortion and quenching cracks
Tempering is not needed after austempering Tempering is needed after martempering
It requires more time It requires less time
Low ductility and low toughness obtained Greater ductility and high toughness obtained
Martempering consists of heating a steel to above its critical transformation and then quenching into a bath held at a temperature approximately equal to that of its Mg. The steel is maintained in the hot bath until its temperature is essentially uniform and then is cooled in air. Martensite microstructure will be formed. Severe internal stresses develop in steel during hardening. Steel contracts during cooling but undergoes a marked expansion when the austenite transforms to martensite. Since a quenched steel must cool from the surface inward, various portions transform at different times. The steel is subjected to internal stress. By equalizing the temperature throughout the section before transformation takes place, and then cooling slowly through the martensite range, the internal stresses are considerably reduced. Austempering is a method of hardening steel by quenching from the austenitizing temperature into a heat extracting medium which is maintained between at 200? and 400?. Then, holding the steel until austenite is transformed to bainite. This method is used to increase strength, toughness and to reduce distortion.