Temper-Brittleness versus Secondary Hardening

Temper-Brittleness versus Secondary Hardening: Their Effects on Steel Properties and Design Parameters
BTMT2074 Physical Metallurgy
YANG EU ZHI
Introduction
Temper-brittleness
Temper-brittleness also can be known as temper embrittlement, where temper embrittlement is also belongs to the method of tempering. Temper Embrittlement is a method reduces the impact toughness of steels. In many alloy steels, temper embrittlement also can classify in 2 methods which is irreversible temper brittleness and reversible temper brittleness. Where usually irreversible temper brittleness happens in the range of temperature 250-400°C and reversible temper brittleness happens in the range of 450-650°C. Temper-brittleness this method has the greatest effect on Martensite structures. As comparison of Martensite, Banite and Pearlite, Martensite will be on rank 1 follow by Banite then to Pearlite. It appears to be associated with the segregation of solute atoms to the grain boundaries by lowering the boundary strength.

Secondary Hardening
Secondary hardening is another type of hardening techniques, it tempered the alloy steel at certain temperatures and the resulting hardness is greater than the same steel which is obtained by tempering for the same time. Secondary hardening also can be known as the fourth class or tempering process. In some specific steels, the hardness value of part improved instead of decreasing by tempering, this phenomenon is call secondary hardening. Secondary hardening is usually promoted by precipitation of fine alloy carbides. It usually happens in tool steels because when the tempering resistance of tool steels is higher the strong the effect of the secondary hardening. Usually, steels tool that contain tungsten, chromium and vanadium will show the phenomenon of secondary hardening.

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Effect on Steel properties
Temper-Brittleness
When steel are in the state of irreversible temper embrittlement, the steel usually has a bright intercrystalline fracture and austenitic grain formed. This phenomenon inherits to all steels to some extent. In other word, this also can be known as the formation of carbides on decomposition of martensite. While the formation of carbides happens, carbides tend to precipitate in the form of films at grain boundaries. While the temperature of tempering increases to the range of 250-400°C, this film disappears and cannot be restored on repeated heating. In order to prevent the irreversible temper embrittlement to be happened, retard the decomposition of martensite will prevent the phenomenon to be happened. Another very important temper embrittlement is reversible temper brittleness, it is a feature of embrittlement on high-temperature tempering at a temperature above 600°C followed by slow cooling or through tempering at 450-600°C is again heated above 600°C and cooled quickly, the impact toughness of the steel will back 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. Carbon steels with less than 0.5% of Mn are not prone to reversible temper embrittlement. This phenomenon usually only happen in alloy steels, by alloying has the probability of having different effects on steel after tempering at the steel proneness to temper embrittlement. Chromium, Nickel and manganese are the most widely use alloying elements and unfortunately always promote temper-brittleness. 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. Another hypothesis also explained that temper embrittlement will happen more frequent by increasing the concentration of impurities in boundary layers of solid solution. This was proven by increasing the etchability of grain boundaries in embrittled steel by picric acid. Segregations of phosphorus and other impurities at the embrittled steel fracture surface causes the temper embrittlement and their concentrations are measured .The development of temper embrittlement is directly linked with the impurity concentration near the austenite boundaries. As the temperature increases, diffusion process usually happens in the grain boundary and the segregation process also being accelerated, with the absolute value of equilibrium segregation being simultaneously decreased owing to thermal motion. Usually alloying elements develop temper-brittleness will be more than impurities because of the segregation of impurities is so small in iron-carbon alloy and causes no temper embrittlement. The segretation of impurities increases when there is the presence of Nickel, Chromium or Manganese. In this process, the alloying elements themselves, which cause no equilibrium segregation in high-purity steels, segregate at grain boundaries in the presence of harmful impurities. Therefore, we can assume that an alloying element and impurity interact with each other in the ?-solution and thus mutually promote their segregation. It can be also assumed that if atoms of an impurity and alloying element attract one another stronger than atoms of that impurity and iron, the segregation of the impurity and alloying element will be mutually enhanced.
J-factor=Mn+SiP+Sn×104J-factor is a factor that related to the amount of elements indicated above, its value shows the sensitivity of steel to temper embrittlement. J-factor is a function indicating the composition of Mn, Si, P and Sn, these elements are the impurities that effecting on the steel properties and causing segretation of impurities that lead to temper embrittlement.
Effect on Steel properties
Secondary Hardening
The effect of secondary hardening on steel usually are the Martensite decomposition and the precipitation of alloy carbides, as I mentioned above, the more intense of carbide precipitation, the stronger the effect of secondary hardening on steel. Martensite decomposition is happens when a heat treatment of martensite in steels undergoes few stages. At the first stage, excessive of carbon in solid solution segregates or form clusters within the solid solution then follow by precipitation into either as cementite or transition iron carbides in either low-carbon steels or high-carbon alloy. While at the stage 2, almost all excess carbon in the solid solution gets precipitated and the carbides are all converted into cementite. Continuation of heat treatment will leads to carsening of carbides and finally to the recrystallization of ferrites into equiaxed grains. Most of the iron carbide grow at low temperature or we can say that it precipitate at low temperature, this is because they grow by a displacive mechanism which does not require the redistribution of substitutional atoms. Martensite is a supersaturated solid solution with carbon when the concentration exceed the equilibrium solubility of another phase. However, the equilibrium solubility depends on the phase. The solubility will be larger when the martensite is in equilibrium with a metastable phase such as ? carbide. Some 0.25 wt% of carbon is said to remain in solution after the precipitation of ?-carbide is completed.
During tempering, if the temperature range are 200-300°C for 1 hour duration, austenite inside the steel will decompose into a mixture of cementite and ferrite. Further tempering leads to coarsening of cementite particle and this dislocation structure tends to recover. This recovery is not very obvious when in the steels containing alloying elements such as molybdenum and chromium. The dislocation structure and migration of dislocation cell and martensite boundaries causes the recovery not successful and causes increase in the crystallographic misorientation between adjacent plates. Precipitation of alloy carbides is also a procedure in secondary hardening, these carbides in order to precipitate, it requires a sufficient time and temperature to proceed because these carbides require the long-range diffusion of substitutional atoms. In order to eliminate the cementite completely, the concentration of strong carbide forming elements such as Mo,Cr,Ti,V,Nb must be large because these alloy carbides relatively grow at the expense of the less stable cementite.  Carbides like cementite therefore have a kinetic advantage even though they may be metastable. Tempering at first causes a decrease in hardness as cementite precipitates at the expense of carbon in solid solution, but the hardness begins to increase again as the alloy carbides form. Hence the term secondary hardening. Coarsening eventually causes a decrease in hardness at high tempering temperatures or long times, so that the net hardness versus time curve shows a secondary hardening peak. CITATION Tem18 l 1033 (Tempered Martensite, 2018)References
BIBLIOGRAPHY Tempered Martensite. (2018, July 22). Retrieved from Phase-trans.msm.cam.ac.uk: Phase-trans.msm.cam.ac.uk