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Martempering and Austempering of Steel

Martempering and Austempering of steel	September 13 2007
Steel can be heat treated to high hardness and strength levels for getting the required strength for various applications. One of those processes is tempering and that mainly contain martempering and austempering.		Steel and Cast Iron
060150L – Habarakada H. M. R.

INTRODUCTION

Hardening and Tempering

Steels can be heat treated to high hardness and strength levels. The reasons for doing this are obvious. Structural components subjected to high operating stress need the high strength of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation.

As-quenched hardened steels are so brittle that even slight impacts may cause fracture. Tempering is a heat treatment that reduces the brittleness of steel without significantly lowering its hardness and strength. All hardened steels must be tempered before use.

As-quenched hardened steels are so brittle that even slight impacts may cause fracture. Tempering is a heat treatment that reduces the brittleness of steel without significantly lowering its hardness and strength. All hardened steels must be tempered before use.

Quench and tempering processes:

Ø Martempering

Ø Austempering

Ø Conventional Heat, Quench and Temper process

Here we will consider the process of Martempering and Austempering.

MARTEMPERING (MARQUENCHING)

To overcome the restrictions of conventional quenching and tempering, Martempering process can be used. Martempering or marquenching permits the transformation of Austenite to Martensite to take place at the same time throughout the structure of the metal part. This is shown in Figure. By using interrupted quench, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the center to cool to the same temperature as the surface. Then cooling is continued through the martensite region, followed by the usual tempering.

Martempering of steel (and of cast iron) consists of,

Ø Quenching from the austenitizing temperature into a hot fluid medium (hot oil, molten salt, molten metal, or a fluidized particle bed) at a temperature usually above the martensite range (Ms point)

Ø Holding in the quenching medium until the temperature throughout the steel is substantially uniform

Ø Cooling (usually in air) at a moderate rate to prevent large differences in temperature between the outside and the center of the section

Figure: martempering process

Advantages

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. Table 1 compares the properties obtained in 1095 steel by martempering and tempering with those obtained by conventional quenching and tempering.

Figure: mechanical properties of 1095 steel heat treated by 2 methods

Suitability of Steels for Martempering

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 °C (400 °F) in sections thinner than 5 mm ( 3 16 in.), 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, 4340, 300M (4340M), 4640, 5140, 6150, 8630, 8640, 8740, 8745, SAE 1141, and SAE 52100. Carburizing grades such as 3312, 4620, 5120, 8620, and 9310 also are commonly martempered after carburizing. Occasionally, higher-alloy steels such as type 410 stainless are martempered, but this is not a common practice.
Success in martempering is based on a knowledge of the transformation characteristics (TTT curves) of the steel being considered. The temperature range in which martensite forms is especially important. Figure 4 shows the martensite temperature ranges for 14 carbon and low-alloy steels. Two trends may be observed in these data: As carbon content increases, the martensite range widens and the martensite transformation temperature is lowered; and the martensite range of a triple-alloy (nickel-chromium-molybdenum) steel is usually lower than that of either a single-alloy or a double-alloy steel of similar carbon content.

AUSTEMPERING

This is the second method that can be used to overcome the restrictions of conventional quench and tempering. The quench is interrupted at a higher temperature than for Martempering to allow the metal at the center of the part to reach the same temperature as the surface. By maintaining that temperature, both the center and the surface are allowed to transform to Bainite and are then cooled to room temperature.

It has been 20 years since the mass production of austemepered ductile iron began. Is is due to the wide variety of realizable mechanical properties.

The manufacturing process of ADI consists of 2 stages; casting and heat treatment. The casting and heat treating technologies are in close connection. Both stages are important for manufacturing the end product with the prescribed specifications. The task of foundries is to produce cast iron of such properties that it contains spherical graphite and assures the expected quality during heat treatment. Low quality iron casts are not suitable for this purpose, since smaller defects (microinclusions, microporocity, and blisters) grow and spoil the mechanical properties of molding.

As a function of the composition and the parameters of the heat treatment, the austemepered product’s mechanical properties vary within a wide range, e.g. at higher austempering temperature the strength decreases and the toughness increases.

Being a relatively resent material, austemepered ductile iron (ADI) has progressively being used for many industrial applications, as several advantages can be obtained, particularly when substituting steel parts. ADI is lighter than steel, can absorb a higher level of vibrations and can be severely reduced, as the base material is less tool consuming than high resistance steel.

Some ADIs can present high mechanical resistance (tensile strength above 1600 MPa) while others can be less resistance but present high ductility levels (rupture elongation reaching 12%), thus allowing the production of well-equilibrated materials for specific applications.

ADIs are produced by a typical heat treatment (Austempering), which is done at low temperatures, (lower than 40^oC), allowing energetic cost saving and simpler equipments and at the same time, resulting is environmental advantages relative to conventional steel surface hardening heat-treatments. Metallurgical transformations can be induced to ADI when high mechanical stress is applied. This phenomenon is responsible for martensite formation, a very effective way to increase surface and sub-surface contact-fatigue resistance.

The austempering heat treatment consists of three steps.

Ø Austinitisation in the temperature range of 840 - 950 ^oC for a time sufficient to produce fully austenitic matrix

Ø Rapid cooling of the entire part to an austempering temperature in the range of 230 – 450 ^oC without any transformations

Ø Isothermal treatment at the austempering temperature, at which during the transformation only bainitic ferrite forms in a favorable case (that is called ausferrite)

Advantages of Austempering

Ø Less distortion and cracking than Martempering,

Ø No need for final tempering (less time consuming and more energy efficient)

Ø Improvement of toughness (impact resistance is higher than the conventional quench and tempering)

Ø Improved ductility

Limitations of Austempering

Austempering can be applied to parts where the transformation to pearlite can be avoided. This means that the section must be cooled fast enough to avoid the formation of pearlite. Thin sections can be cooled faster than the bulky sections. Most industrial applications of Austempering have been limited to sections less than 1/2 in. thick. The thickness can be increased by the use of alloy steels, but then the time for completion of transformation to Bainite may become excessive.

Figure: Austempering process

In Austempering process, the end product is 100% Bainite. It is accomplished by first heating the part to the proper austenitizing temperature followed by cooling rapidly in a slat bath which is maintained between 400 and 800 ^oF. The part is left in the bath until the transformation to Bainite is complete. The steel is caused to go directly from austenite to Bainite.