Age hardening, or precipitation hardening as it is sometimes called, is a three step heat treatment that is used to increase the strength and hardness of an alloy. Not all alloys can be effectively age hardened. We'll explain the reason for this as well as examine each of the three steps a little later, but first, we have to look at a typical phase diagram of an age hardenable alloy (and you thought you were finished with phase diagrams after the first lesson!).
Figure 1 shows a phase diagram for elements A and B (at least one of which is a metal). Pure A has a stable phase of up to a temperature of T , while pure B is a 1
phase up to a temperature of T .2
Remember the salt and glass of water in the first lesson? There we stirred a teaspoonful of salt into a glass of water. The water dissolved all the salt (i.e. the salt went into solution) and we still had a one phase system. When we continued to add salt we eventually reached a point where no more salt would dissolve: the solution had become saturated. We then had a two phase system consisting of a liquid (the salt solution) and granules of salt. By cooling the system, the solution would not be able to hold as much salt and some salt would have to come out of solution. In other words, there is a limit on the solubility of salt in water at a particular temperature and this solubility decreases with decreasing temperature.
The phase in Figure 1 can only accommodate a certain amount of element B at a given temperature (just as water can only dissolve so much salt) before a second phase, , is formed. The amount of B that the phase can contain (the solubility limit!) increases up to a temperature of T . The solvus line in Figure 1 represents the solubility3
limit of B in the phase. At temperature T , the 4 phase can hold 1% B atoms. At a higher temperature T , the 5 phase can hold 2% B atoms in solution. Summing up, there is a limit on the solubility of B in at any given temperature and this solubility limit decreases with decreasing temperature. All age hardenable alloys exhibit this behavior, however, not all alloys exhibiting this behavior are age hardenable.
Enough of the preliminaries, let's get down to business. The three steps in age hardening are: 1) solution anneal, 2) quenching, and 3) controlled reheating (aging). We'll examine the heat treatment of an age hardenable alloy consisting of 98% A and 2% B. At room temperature we have a two phase system: and .
Figure 1: Age Hardening
1. Solution Anneal - The first step in age hardening is to heat the alloy up into a one phase region so that all the will dissolve and the B atoms go into solution. This requires that we heat our alloy to a minimum temperature of T . We'll heat it up to T just to be on the safe side and5 6 be sure all the B is in solution.
2. Quench - Once we've gotten all the B into solution at T , we're going to6
quench the alloy to some temperature well below T , say T . The B5 4 atoms will still remain in solution in the phase even though the amount of B atoms (2%) exceeds the solubility limit at T (1%). By4
rapidly cooling the alloy to a low temperature, we have effectively "frozen" the B atoms in place before they had a chance to move. We now have a supersaturated solution.
3. Controlled Reheating - The final step in age hardening is the aging process itself. This consists of reheating the supersaturated solution to T , a temperature below T , holding for a period of time, and then cool-7 5
ing. This causes some of the B atoms to come out of solution
(precipitate out). These precipitates are finely dispersed throughout the
The extra strain in the crystal structure makes it difficult for dislocations to move in response to an applied load, consequently the alloy is hardened and strengthened. Note that although the B atoms do form a precipitate, this precipitate may not necessarily have the same
structure as the phase.
The aging time and temperature determine how much aging, or hardening, actually takes place. We can overage the alloy by holding it for too long a time at the aging temperature or by using too high an aging temperature. This causes the
precipitate particles to agglomerate and grow, resulting in a decrease in hardness and strength.
T-T-T DIAGRAMS
In Chapter I we discussed what happened to a carbon steel that was slowly and continuously cooled through the austenitic region down to room temperature. You'll recall that we ended up with a mixture of ferrite and pearlite. Will austenite always transform to ferrite and pearlite? Obviously a loaded question. The answer is no.
We've already talked about one case - the formation of martensite - but there are others as well.
One of the chief means of studying the decomposition of austenite is through the metallographic examination of isothermal transformation products. As an example, if we quickly quench a small test specimen that has been austenitized down to a temperature where austenite is unstable and then hold the sample at that temperature, eventually the austenite will transform into something else (isothermal here refers to the fact that we hold the austenite at a constant temperature until it transforms). We can then
quench our sample down to room temperature, examine it under a microscope, and see what the austenite decomposed into.
Suppose we take a large number of small test specimens of a carbon steel containing 0.64% carbon and 1.14% Manganese (AISI 1566) and heat them up into the austenitic region, quench them to various temperatures, hold them at those
temperatures for various lengths of time, and then quench them to room temperature. We can then look at each specimen under a microscope and determine what
microconstituents are present and in what quantities. This data will allow us to make a plot of the type and quantity of the austenite transformation products as a function of transformation temperature and holding time. This type of plot is called a T-T-T diagram (the T's stand for time-temperature-transformation). It is sometimes referred to as an isothermal transformation curve. The T-T-T diagram for our AISI 1566 carbon steel is shown in Figure 2.
The first line (from the left) in Figure 2 shows where the austenite first starts to decompose after it has been quenched to a specific temperature. The dotted line shows where 50% of the austenite has decomposed. The solid line on the right shows where the austenite transformation is complete. The horizontal line marked Ac is the1 temperature above which austenite first starts to form upon heating. The line marked
Figure 2: AISI 1566 T-T-T Diagram
shows that for our carbon steel, austenite may transform into pearlite, martensite, or something we haven't talked about yet: bainite. We'll examine each of these
transformations in some detail.
1. Pearlite - Assume we can instantaneously quench a small piece of austenitized 1566 carbon steel down to 1200(F (point 1 in Figure 2). After holding it at that temperature for about ten seconds, ferrite will start to form along the austenite grain boundaries (point 2). This is the proeutectoid ferrite that we talked about in Chapter I. Proeutectoid ferrite precedes the formation of pearlite in hypoeuctectoid steels (steel having less that the eutectoid carbon content of 0.77%). In hypereutectoid steels (steels having more than the eutectoid carbon content of 0.77%), however, the iron-carbon phase diagram in Chapter I tells us that some of the austenite will first transform to cementite before the remaining austenite transforms to pearlite.
As we continue to hold our specimen at 1200(F, more and more of the austenite transforms into ferrite until we reach point 3 after
approximately forty seconds. At this point, all the austenite that is going to transform into proeutectoid ferrite has done so. The remaining austenite has become increasingly rich in carbon because of the
Figure 3: Pearlite Formation
limited solubility of carbon in ferrite. Point 3 also marks the start of pearlite formation.
Pearlite forms when iron carbide begins to nucleate out in the
remaining austenite along grain boundaries. The subsequent growth of the carbides depletes the carbon content of the adjacent areas so that in these areas the austenite transforms into ferrite. Further carbide nucleation will result in a pearlite "colony" having alternate layers of cementite and ferrite. This process is illustrated in Figure 3. All the austenite that did not transform into proeutectoid ferrite will have
transformed to pearlite by the time we have reached point 4. No further changes in the quantities or the types of microconstituents in our
specimen will occur after point 4. The final structure in our specimen consists of proeutectoid ferrite and pearlite.
Note in Figure 2 that the lower the temperature at which pearlite forms, the "finer", or more closely spaced, the layers are. Fine pearlite is stronger and tougher than coarse pearlite.
2. Bainite - Assume we can instantaneously quench a small piece of austenitized 1566 carbon steel down to 800(F (point 5 in Figure 2). After holding our specimen for about 3 seconds at this temperature (point 6), ferrite will start to nucleate along austenite boundaries. The lattices of the ferrite grains are coherent with the austenite matrix (this means that the lattices match, see Figure 4). As the ferrite forms, the austenite adjacent to it becomes richer in carbon until it becomes saturated. At this point cementite will begin to precipitate out. The carbides form parallel to the longitudinal axis of the bainite "needle." The austenite will have completely transformed to bainite at point 7. No further transformations will occur past this point. The resulting structure has a feathery appearance under the microscope and is known as upper bainite. Figure 5 shows the formation of upper bainite. Austenite that isothermally transforms between the knee of the curve and approximately 660(F will form upper bainite.
Coherent Lattices Incoherent Lattices
Figure 4: Coherency
Figure 5: Upper Bainite Formation
Austenite that isothermally transforms between approximately 660(F and the line in Figure 2 identified "M " forms lower bainite. LowerS bainite forms when ferrite, supersaturated with carbon, forms along austenite grain boundaries. Again the ferrite is coherent with the austenite matrix. Carbides will precipitate out within the bainite
needles(see Figure 6). Lower bainite has an acicular (needle shaped) structure that is similar to tempered martensite.
The fact that the ferrite in both upper and lower bainite forms
coherently within the austenite matrix distinguishes it from the ferrite in pearlite which forms incoherently. Bainite is considerably stronger than pearlite because with coherency, perfect matching of the two lattices is seldom achieved. This results in considerable amounts of induced strain that will hinder the movement of dislocations under a load thus strengthening the material. Bainite is hard and brittle in the as-
quenched condition and consequently must be tempered.
3. Martensite - Martensite will form if we quench our 1566 carbon steel fast enough so that we miss the knee of curve and go down below the "M " (martensite start) temperature line in Figure 2. As we discussed inS Chapter 1, martensite has a distorted body centered tetragonal
structure. The austenite is so unstable at these low temperatures that it will instantaneously transform. Atomic diffusion is extremely slow at low temperatures so martensite does not form through a nucleation and growth process like bainite and pearlite do. Instead the austenite
Figure 6: Lower Bainite Formation
will transform in a shear reaction involving minimal atomic movement. The ferrite matrix that forms is distorted because it must accommodate the extra carbon atoms that were in solution in the austenite (remem- ber that carbon has a much lower solubility in a BCC structure than a FCC). This distortion is accompanied by a considerable amount of induced strain which hardens and strengthens the material. Martensite has an acicular structure.
Note in Figure 2 that the amount of austenite that transforms into martensite is dependent solely on the temperature to which we quench our austenite to. The percent of austenite that transforms is independent (unlike bainite or pearlite formation) of the time that the quenched austenite is held at temperature. Because of this, martensite formation is referred to as an athermal reaction, as opposed to the isothermal reactions that we have been talking about. The amount of austenite that transforms to martensite increases as we quench to lower temperatures until we reach the "M " (martensiteF
finish) temperature line in Figure 2. At this point all the austenite has transformed to martensite.
We can use T-T-T diagrams to show the effect of alloying elements on the heat treat response of a steel. Our T-T-T diagram for AISI 1566 steel can be thought of as three superimposed curves (see Figure 7A). By alloying with different elements, we can separate these curves in order to make it easier to obtain the desired transformation product (see Figure 7B and Figure 7C).
Increasing the carbon, manganese, nickel, and silicon content of a steel will move the pearlite and bainite curves further to right, but do not separate them to any extent. Molybdenum, chromium, and vanadium move the pearlite curve up and further to the right while moving the bainite curve to lower temperatures. Figure 8 shows these effects for an AISI 4340 steel (nominal composition, 0.40% carbon, 0.80% manganese, 1.80% nickel, 0.25% molybdenum, and 0.80% chromium).
Figure 7: T-T-T Diagrams
The significance of all this is that by moving the ferrite and pearlite curves to the right we have more time in which to quench our steel from the austenite region to the bainitic or martensitic region without having any of the austenite transform into the softer proeutectoid ferrite or pearlite. This is desirable because the rate at which we can quench our steel is limited by the quenching medium and the size of the piece we are quenching. The farther the ferrite and pearlite curves are moved to the right, the more time we have to quench our steel in order to obtain the desired martensite or bainite, the slower our cooling rate can be, and consequently the larger the piece of steel we are quenching can be and still obtain the desired microstructure. We're going to examine this a little further later on.
T-T-T diagrams are also useful for illustrating the types of heat treatments per- formed on steel. Figure 9 shows an annealing or normalizing heat treat of our 4340 steel. The steel is heated up into the austenitic region and then slowly cooled. This slow cooling will result in a microstructure of ferrite and pearlite.
Figure 10 shows an austenitize, quench, and temper heat treatment. Note that two cooling curves are shown: one for the surface and one for the center of the part being quenched. The differences in cooling rates can be appreciable depending on the size of the part being quenched and the quenching medium. Ideally the part is
quenched so that the bainite and pearlite curves are missed: only martensite is formed. After the part is cooled below M , it is heated up again for the tempering operation.F
Figure 8: AISI 4340 T-T-T Diagram
Figure 11 illustrates a martempering heat treatment. Here the part is rapidly quenched down to a temperature just above M and held until the temperatures of theS
center and surface of the part become equalized. The part is then quenched into the martensitic range. Typically this "interrupted quench" is done by quenching and holding the part in a salt bath or martempering oil until the temperature equalizes throughout the part and then is air cooled into the martensitic range. The purpose of this heat treatment is to harden the material by forming martensite, but at the same time, minimize the distortion and residual stresses associated with the martensite trans- formation. We use this heat treatment for heat treating Colmonoy® coated parts. It is always followed by tempering.
Figure 12 shows an austempering heat treatment. Here we again quench our steel down to a temperature just above M and hold it there until the austenite hasS
completely transformed into bainite. It is then air cooled. Like martempering, it is usually performed in a salt bath. The purpose of austempering is to harden the steel by forming bainite. Although this will not give the same strength as martensite, bainite formation produces much less distortion and residual stresses than martensite formation. As a consequence, warping and quench cracking are minimized. Austempering is usually followed by a tempering operation in order to restore ductility.
Figure 9: Anneal/Normalize
Figure 10:
Austenitize, Quench, & Temper
EGADS! We're finished with T-T-T diagrams. Unfortunately T-T-T diagrams are of limited use in the real world of heat treating because they are based on the
assumption that we can instantaneously quench a piece of steel down to a desired temperature and also because most heat treatments do not involve isothermal
Figure 11: Martemper
Figure 12: Austemper
reactions. But let's not be too disparaging about T-T-T diagrams. They have served their purpose well in helping us examine the decomposition of austenite.
Figure 13:
AISI 4140 CCT Diagram
cooling transformation diagrams. This, like our T-T-T diagrams, is another
transcontinental topic that metallurgists like to wax rhapsodic about. So take a deep breath before we submerge.