Temper Embrittlement
Temper embrittlement is inherent in many steels and can be characterized by reduced impact toughness. The state of temper embrittlement has practically no effect on other mechanical properties at room temperature. Many alloy steels have two temperature intervals of temper embrittlement. For instance, irreversible temper brittleness may appear within the interval of 250-400°C and reversible temper brittleness, within 450-650°C.
Figure 1 shows schematically the effect of temperature on impact toughness of alloy steel which is strongly liable to temper embrittlement. Many alloy steels have two temperature intervals of temper embrittlement. For instance, irreversible temper brittleness may appear within the interval of 250-400°C and reversible temper brittleness, within 450-650°C.
The impact toughness of quenched steel after tempering at 250-400°C is lower than that obtained on tempering at temperatures below 250°C. If brittle steel tempered at 250-400°C is heated above 400°C and transferred into a tough state, a second tempering at 250-400°C cannot return it to the brittle state. The rate of cooling from the tempering temperature within 250-400°C has no effect on impact toughness.
Steel in the state of irreversible temper embrittlement has a bright intercrystalline fracture at boundaries of former austenitic grains. This type of brittleness is inherent to some extent to all steels, including carbon grades. For that reason medium-temperature tempering is, as a rule not employed in practice, though it can ensure a high yield limit.
Irreversible temper embrittlement is thought to be 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.
The embrittlement on high-temperature tempering may manifest itself in two different ways:
- as a result of heating at 450-600°C (irrespective of the rate of subsequent cooling) and effect of temperature, and
- as a result of tempering at temperatures above 600°C with subsequent slow cooling within the range of 600-450°C.
The most important feature of embrittlement on high-temperature tempering is that the process is reversible. If 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.
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.
The rate and degree of development of temper embrittlement depend on the temperature and time of holding steel within the dangerous temperature interval (450-600°C). With a certain temperature of tempering within this interval, the initial stages of embrittlement appear appreciably sooner than at a higher or a lower temperature.
Many scientists adhered for a long time to the "solution precipitation" hypothesis, according to which the loss in impact toughness was caused by precipitation of some phases, such as phosphides, at grain boundaries. These phases were thought to pass into the á-solution on heating up to approximately 650°C and to precipitate from the solution and embrittle the steel on slow cooling; quick cooling should prevent the precipitation of embrittling phases. As has been found by electron-microscopic analysis, however, there are no special precipitates at grain boundaries in embrittled steel, so that the "solution precipitation" hypothesis turned to be inconsistent.
Another hypothesis explained temper embrittlement by an increased concentration of impurities in boundary layers of the solid solution. This was proved by an increased etchability of grain boundaries in embrittled steel by picric acid. The hypothesis on the leading role of impurity segregates has been fully confirmed in the recent years by a brilliant series of research work using Auger spectroscopy, a method enabling determination of concentrations of elements in monatomic surface layers. Using this method makes it possible to detect segregations of phosphorus and other impurity elements at the fracture surface in embrittled steel and measure their concentrations (as also the concentrations of alloying elements) at the fracture surface. It has also been shown that the development of temper embrittlement is directly linked with the rise of impurity concentration near the prior austenite boundaries.
Owing to equilibrium segregation, the concentration of harmful impurities at the surface of a fracture may exceed tens or hundreds times their average concentration in the steel. The concentration of impurities in commercial purity steels is usually a few thousandths or hundredths of a percent, but amounts to a few percent at the surface of fracture.
As the temperature increases, the diffusion process of grain boundary segregation is accelerated, with the absolute value of equilibrium segregation being simultaneously decreased owing to thermal motion. At temperatures above 600-650°C, the segregation of impurities either disappears fully (Sb) or drops to a very low level (P). On subsequent cooling of the steel in water, the segregates have no time to restore.
The role of alloying elements in the development of temper embrittlement is not less than that of impurities. The segregation of harmful impurities in iron-carbon alloys is so small that causes no temper embrittlement. In the presence of alloying elements (Ni, Cr or Mn), the segregation of impurities increases appreciably. 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 á-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. Namely in this way behave P and Ni, P and Cr, Sb and Ni, Sb and Mn and other "impurity - alloying element" pairs. A second alloying element can additionally enhance segregation of an impurity. For instance, nickel and chromium, when present together in steel, can cause a greater segregation of antimony than might be expected from simple summation of their separate effects.
An increased concentration of harmful impurities in boundary layers of the solid solution, which may be caused by the effect of alloying additions, weakens the intergranular bondage and is one of the main causes why alloy steels containing Ni, Cr or Mn are highly susceptible to temper embrittlement. The main measures to prevent temper embrittlement are as follows:
- of the content of harmful impurities in steel;
- accelerated cooling from the temperature of high-temperature tempering (above 600°C);
- alloying of steel with small additions of molybdenum (0.2-0.3%); and
- subjecting the metal to high-temperature thermo-mechanical treatment.