Figure 2: Effect of alloying elements on TTT diagram (H. Bhadeshia, Michaelmas)
Now a days, steel have become a commercial use product in our daily life especially at construction and electric component field. This is because steel can have excellent strength by changing its heating and cooling temperature. When the steel (C contain<2.03%) being heat up to around 950? to form face central cubic (Fcc) and quench rapidly, it will form a metastable structure, martensite. Martensite starts forming during the cooling at a temperature, Ms and end at another lower temperature, Mf. Martensite formation are due to the carbon was trap inside the Face central cubic (Fcc) austenite structure.
Introduction of Tempering
Tempering is a very important heat treatment in quenched steel. It is a low heat treatment process done after neutral hardening, double hardening, atmospheric carburising, carbonitriding, quenching or induction hardening. This heat treatment is often being use to improve the characteristics of a quenched metal especially steel. During the quenching process, the steel had store a lot of internal stress due to rapid cooling. Tempering can be easily done by heating up to a critical temperature (depends the steel grade, usually will be 160? to 500? or higher) and hold the sample for a period of time to unsure the whole metal had reach the critical temperature. Then, let the metal slowly cool down to room temperature by air cooling. In this process it allow the carbon diffuse out from Body central tetragonal (BCT) hence increase the ductility and reduce the internal stress.

Tempering can be divided into 3 main group (Low temperature (160? -300?), spring steel (300?-500?), and high temperature (500? and above)). For quenched steel usually will be high temperature tempering. For some types of steels the holding time at the tempering temperature is of great importance; an extended holding time will correspond to a higher temperature. Depending on the steel grade a phenomenon known as temper brittleness can occur in certain temperature intervals.

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Effect of temper embrittlement
Temper brittleness will affect the notch toughness this is because when temper embitter happen, there will have transition carbide nuclear grow along with the grain boundary. The carbide formation causing the steel become softer and weaker.

Besides, temper embrittlement can be categories in 2 group. Which is reversible and irreversible. Irreversible temper embrittlement is due to the formation of carbides on decomposition of martensite, in particular, precipitation of carbides in the form of films at grain boundaries. At higher temperatures of tempering, this film disappears and cannot be restored on repeated heating at 250-400°C. Silicon in low-alloy steels can prevent irreversible temper embrittlement by retarding the decomposition of martensite.

For reversible temper embrittlement process is a steel embrittled through tempering at a temperature above 600°C with subsequent slow cooling or through tempering at 450-600°C (with any rate of cooling) is again heated above 600°C and cooled quickly, its impact toughness will restore to the initial value. If the steel then again enters the dangerous interval of tempering temperatures, it is again embrittled. A new heating at a temperature above 600°C, followed with quick cooling, can eliminate the embrittling effect, and so on. This is why the phenomenon discussed is called reversible embrittlement.

However, Plain carbon steels with less than 0.5% Mn are not susceptible to temper embrittlement. However, additions of Ni, Cr and Mn will cause greater susceptibility to temper embrittlement. Small additions of W and Mo can inhibit temper embrittlement, but this inhibition is reduced with greater additions.

Furthermore, a carbon steels with less than 0.5% Mn are not prone to reversible temper embrittlement. The phenomenon can only appear in alloy steels. Alloying elements may have different effects on steel after tempering at the steel proneness to temper embrittlement. Unfortunately, the most widely used alloying elements, such as chromium, nickel, and manganese, promote temper embrittlement. When taken separately, they produce a weaker effect than in the case of combined alloying. The highest embrittling effect is observed in Cr-Ni and Cr-Mn steels. Small additions of molybdenum (0.2-0.3%) can diminish temper embrittlement, while greater additions enhance the effect. A fundamental fact is that alloy steels of very high purity are utterly unsusceptible to temper embrittlement which is caused by the presence of various impurities, in the first place of phosphorus, tin, antimony and arsenic, in commercial steels.

With different type of microstructure will also require a different parameter of tempering. The most commonly is the martempering and austempering. Martempering is a common heat treatment process that quenches the material to an intermediate temperature just above the martensite start temperature (????) and then cools air through the martensitic transformation range to room temperature 1–4. It is important to air-cool throughout the transformation range since rapid cooling through this range is required to produce residual stress patterns similar to those produced by a direct quench and negate any advantages of the process. Austempering is a method of hardening steel by quenching from the austenitizing temperature into a heat extracting medium (usually molten salt) which is maintained at specific temperature level between 200? C and 400? C and holding the steel in this medium until austenite is transformed to bainite. ?is method is used to increase strength, toughness, and to reduce distortion
Recommendation for temper embrittlement
Introduction of Secondary hardening
Secondary hardening is when a steel being quenched and tempered, tend to soften a bit after tempering at low temperatures, and to reharden at intermediate tempering temperatures before finally softening to low hardness. This behavior in tempering has been studied by many investigators, chiefly with respect to the secondary hardening of high-speed steels. The initial softening has usually been ascribed to decomposition of martensite and growth of iron carbide particles. The secondary hardening has been explained by formation of fresh martensite from residual austenite, formation of very fine alloy carbide particles and by precipitation hardening of metallic compounds in the ferritic matrix. However, factors derived for the calculation of tempered hardness in low-alloy steels1 indicated only a tendency toward retarded softening rather than a discontinuous rehardening. 
Efffect of Secondary hardening
Review of thermo-physical properties, wetting and heat transfer characteristics of nanofluids and their applicability in industrial quench heat treatment
AP3, Hardenability of Steel (University of Cambridge)


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