Principles of Heat Treating of Steels Abstract:
Steels can exhibit a wide variety of properties depending on composition as well as the phases and micro-constituents present, which in turn depend on the heat treatment. The basis for the understanding of the heat h eat treatment of steels is the Fe-C phase diagram. It actually includes two diagrams; the stable iron-graphite diagram and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. Steel is usually defined as an alloy of iron and carbon with the carbon content between a few hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5 wt% in low-alloy steels and h igher in more highly alloyed steels such as tool steels, stainless steels (>10.5%) and heat resisting CrNi steels (>18%). Steels can exhibit a wide variety of properties depending on composition as well as the phases and micro-constituents present, which in turn depend on the heat treatment.
The Fe-C Phase Diagram The basis for the understanding of the heat h eat treatment of steels is the Fe-C phase diagram (Fig 1). Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or metastable equilibrium) for different different combinations of carbon concentration and temperature. We distinguish at the low-carbon end ferrite (α-iron),which can at most dissolve 0.028% C, at 727°C (1341°F) and austenite -iron, which can dissolve 2.11 wt% C at 1148°C (2098°F). At the carbon-rich side we find cementite (Fe3C). Of less interest, except for highly alloyed steels, is the δ-ferrite existing at the highest temperatures. Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + δ-ferrite. In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names: •
A1, the so-called eutectoid temperature, which is the minimum temperature for austenite
•
•
A3, the lower-temperature boundary of the austenite region at low carbon contents, that is, the γ/γ + α boundary Acm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe3C boundary
The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content (0.77 wt% C). The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic cha racteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that degenerates into cementite particles dispersed with a ferrite matrix after extended holding close to A1.
Fig. 1. The Fe-Fe3C diagram.
The Fe-C diagram in Fig 1 is of experimental origin. The knowledge of the thermodynamic principles and modern thermodynamic data now permits very accurate calculations of this diagram. This is particularly useful when phase bound aries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to develop. If alloying elements are added to the iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries and the eutectoid composition are changed. It suffices here to mention that 1. all importa important nt alloying alloying elements elements decrease decrease the eutecto eutectoid id carbon carbon content, content, 2. the austenit austenite-stab e-stabiliz ilizing ing elements elements manganes manganesee and nickel nickel decrease decrease A, A, and 3. the ferriteferrite-stabi stabilizi lizing ng elements elements chromium, chromium, silicon, silicon, molybdenum molybdenum,, and tungsten tungsten increase A1.
Transformation Diagrams The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. The metastable phase martensite and the morphologically metastable microconstituent bainite, which are of extreme importance to the properties of steels, can generally form with comparatively rapid cooling to ambient temperature. That is when the diffusion of carbon and alloying elements is suppressed or limited to a very short range.
Bainite is a eutectoid decomposition that is a mixture of ferrite and cementite. Martensite, the hardest constituent, forms during severe quenches from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to about 0.7 wt%. If these unstable metastable products are subsequently heated to a moderately elevated temperature, they decompose to more stable distributions of ferrite and carbide. The reheating process is sometimes known as tempering or annealing. The transformation of an ambient temperature structure like ferrite-pearlite or tempered martensite to the elevated-temperature structure of austenite or austenite-carbide is also of importance in the heat treatment of steel. One can conveniently describe what is happening during transformation with transformation diagrams. Four different types of such diagrams can be distinguished. These include: •
•
• •
Isothermal transformation diagrams describing the formation of austenite, which will be referred to as ITh diagrams Isothermal transformation (IT) diagrams, also referred to as time-temperaturetransformation (TTT) diagrams, describing the decomposition of austenite Continuous heating transformation (CRT) diagrams Continuous cooling transformation (CCT) diagrams
Isothermal Transformation Diagrams This type of diagram shows what happens when a steel is held at a constant temperature for a prolonged period. The development of the microstructure with time can be followed by holding small specimens in a lead or salt bath and quenching them one at a time after increasing holding times and measuring the amount of phases formed in the microstructure with the aid of a microscope. ITh Diagrams (Formation of Austenite). During the formation of austenite from an original microstructure of ferrite and pearlite or tempered martensite, the volume decreases with the formation of the dense austenite phase. From the elongation curves, the start and finish times for austenite formation, usually defined as 1% and 99% transformation, respectively, can be derived. IT Diagrams (Decomposition of Austenite). The procedure starts at a high temperature, normally in the austenitic range after holding there long enough to obtain homogeneous austenite without undissolved carbides, followed by rapid cooling to the desired hold temperature. The cooling was started from 850°C (1560°F). The A1 and A3 temperatures are indicated as well as the hardness. Above A3 no transformation can occur. Between A1 and A3 only ferrite can form from austenite.
CRT Diagrams In practical heat treatment situations, a constant temperature is not required, but rather a continuous changing temperature during either cooling or heating. Therefore, more directly applicable information is obtained if the diagram is constructed from dilatometric data using a continuously increasing or decreasing temperature. Like the ITh diagrams, the CRT diagrams are useful in predicting the effect of short-time austenitization that occurs in induction and laser hardening. One typical question is how high the maximum surface temperature should be in order to achieve complete austenitization for a given heating rate. To high a temperature may cause unwanted austenite grain growth, which produces a more-brittle martensitic microstructure.
CCT Diagrams As for heating diagrams, it is important to clearly state what type o f cooling curve the transformation diagram was derived from. Use of a constant cooling rate is very common in experimental practice. However, this regime rarely occurs in a practical situation. One can also find curves for so-called natural cooling rates according to Newton’s law of coo ling. These curves simulate the behavior in the interior of a large part such as the cooling rate of a Jominy bar at some distance from the quenched end. Close to the surface the characteristics of the cooling rare can be very complex. Each CCT diagram contains a family of curves representing the cooling rates at different depths of a cylinder with a 300 mm (12 in.) diameter. The slowest cooling rate represents the center of the cylinder. The more severe the cooling medium, the longer the times to which the C-shaped curves are shifted. The M, temperature is unaffected.
Fig.2. CCT (a) and TTT (b) diagrams.
It should be noted, however, that transformation diagrams can not be used to predict the response to thermal histories that are very much different from the ones used to construct the diagrams. For instance, first cooling rapidly to slightly above Ms and then reheating to a higher temperature will give more rapid transformation than shown in the IT diagram because nucleation is greatly accelerated during the introductory quench. It should also be remembered that the transformation diagrams are sensitive to the exact alloying content within me allowable composition range.
Quench hardening of steel Abstract:
Hardening of steel is obtained by a suitable quench from within or above the critical range. The temperatures are the same as those given for full annealing. The soaking time in air furnaces should be 1,2 min for each mm of cross-section or 0,6 min in salt or lead baths. Uneven heating, overheating and excessive scaling should be avoided.The quenching is necessary to suppress the normal breakdow n of austenite into ferrite and cementite, and to cause a partial decomposition at such a low temperature to produce martensite. To obtain this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements, which therefore cause hardening with mild quenching (e.g. oil and hardening steels). Hardening of steel is obtained by a suitable quench from within or above the critical range. The temperatures are the same as those given for full annealing. The soaking time in air furnaces should be 1,2 min for each mm of cross-section or 0,6 min in salt or lead baths. Uneven heating, overheating and excessive scaling should be avoided. The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite, and to cause a partial decomposition at such a low temperature to produce martensite. To obtain this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements, which therefore cause hardening with mild quenching (e.g. oil and hardening steels). Steels with less than 0,3 % carbon cannot be hardened effectively, while the maximum effect is obtained at about 0,7 % due to an increased tendency to retain austenite in high carbon steels Fig. 1.
Figure 1. Variation of hardness of martensite and bainite with carbon content Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the article. Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidise and form sludge with consequent lowering of efficiency. The quenching velocity of oil is much less than water. Ferrite and troostite are formed even in small sections. Intermediate rates between water and oil can be obtained with water containing 10-30 % Ucon, a substance with an inverse solubility which therefore deposits on the object to slow rate of cooling. To minimise distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated. Fully hardened and tempered steels develop the best combination of strength and notchductility.
Tempering and toughening The martensite of quenched tool steel is exceed ingly brittle and highly stressed. Consequently cracking and distortion of the object are liable to occur after quenching. Retained austenite is unstable and as it changes dimensions may alter, e.g. dies may alter 0,012 mm. It is necessary, therefore, to warm the steel below the c ritical range in order to relieve stresses and to allow the arrested reaction of cementite precipitation to take place. This is
known as tempering . •
•
•
150-250°C. The object is heated in an oil bath, immediately after quenching, to prevent related cracking, to relieve internal stress and to decompose austenite without much softening. 200-450°C. Used to toughen the steel at the expense of hardness. Brinell hardness is 350-450. 450-700°C. The precipitated cementite coalesces into larger masses and the steel becomes softer. The structure is known as sorbite, which at the higher temperatures becomes coarsely spheroidised. It etches more slowly than troostite and has a Brinell hardness of 220-350. Sorbite is commonly found in heat-treated constructional steels, such as axles, shafts and crankshafts subjected to dynamic stresses. A treatment of quenching and tempering in this temperature range is frequently referred to as toughening, and it produces an increase in the ratio of the elastic limit to the ultimate tensile strength.
The reactions in tempering occur slowly. Reaction time as well as temperature of heating is important. Tempering is carried out to an increasing extent under pyrometric control in oil, salt (e.g. equal parts sodium and potassium nitrates for 200-600°C) or lead baths and also in furnaces in which the air is circulated by fans. After the tempering, the objects may be cooled either rapidly or slowly, except for steels susceptible to temper brittleness. Temper colours formed on a cleaned surface are still used occasionally as a guide to temperature. They exist due to the interference effects of thin films of oxide formed during tempering, and they act similarly to oil films on water. Alloys such as stainless steel form thinner films than do carbon steels for a given temperature and hence produce a colour lower in the series. For example, pale straw corresponds to 300°C, instead of 230°C (Table 1). Table 1.
Temper Colour
Temperature °C
Pale straw
230
Planing and slotting tools
Dark straw
240
Milling cutters, drills
Brown
250
Taps, shear blades for metals
Brownish-purple
260
Punches, cups, snaps, twist drills, reamers
Purple
270
Press tools, axes
Dark purple
280
Cold chisels, setts for steel
Blue
300
Saws for wood, springs
Blue
450-650
Toughening for constructional steels
Objects
For turning, planing, shaping tools and chisels, only the cutting parts need hardening. This is frequently carried out in engineering works by heating the tool to 730°C, followed by quenching the cutting end vertically. When cutting end gets cold, it is cleaned with the stone and the heat from the shank of the tool is allowed to temper the cutting edge to the correct colour. Then the whole tool is quenched. Oxidation can be reduced by coating the tool with charcoal and oil.
Changes during tempering The principles underlying the tempering of quenche d steels have a close similarity to those of precipitation hardening. The overlapping changes, which occur when high carbon martensite is tempered, are shown in Fig. 2 and as follows: •
•
•
•
Stage 1. 50-200°C. Martensite breaks down to a transition precipitate known as c-carbide (Fe2,4C) across twins and a low carbon martensite which results in slight dispersion hardening, decrease in volume and electrical resistance. Stage 2. 205-305°C. Decomposition of retained austenite to bainite and decrease in hardness. Stage 3. 250-500°C. Conversion of the aggregate of low carbon martensite and ccarbide into ferrite and cementite precipitated along twins, which gradually coarsens to give visible particles and rapid softening, Fig. 3. Stage 4. Carbide changes in alloy steel at 400-700°C. In steels containing one alloying addition, cementite forms first and the alloy diffuses to it. When sufficiently enriched the Fe3C transforms to an alloy carbide. After further enrichment this carbide may be superseded by another and this formation of transition carbides may be repeated several times before the equilibrium carbide forms. In chromium steel, changes are: Fe3C→Cr7C3→Cr23C6. In steels containing several carbide-forming elements the reactions are often more complex, and the carbides which decompose are not necessarily followed by carbides based on the same alloy elements. The transformation can also occur in situ by gradual exchange of atoms without any appreciable hardening; or by resolution of existing iron carbides and fresh nucleation of coh erent carbide with considerable hardening that counteracts the normal softening that occurs during tempering. In some alloy steels, therefore, the hardness is maintained constant up to about 500°C or in some cases it rises to a peak followed by a gradual drop due to breakdown of coherence and coalescence of the carbide particles. This agehardening process is known as secondary hardening and it enhances high temperature creep properties of steel (e.g. steel E in Fig. 2). Chromium, for an example, seems to stabilise the size of the cementite particles ov er a range 200500°C. Vanadium and molybdenum form a fine dispersion of coherent precipitates (V4C3Mo2C) in a ferrite matrix with considerable hardening. When over-ageing starts the V4C3 grows in the grain boundaries and also forms a Widmanstätten pattern of plates within the grain.
Figure 2.Tempering curves for 0,35 % C steel and die steel
a) As quenched. Laths with high density of dislocation
b) Tempered 300°C. Widmanstätten precipitation of carbides within laths
c) Tempered 500°C. Recovery of dislocation structure into cells with laths
d) Tempered 600°C. Recrystallisation cementite re-nucleated equioxed ferrite boundaries
e) High C twinned martensite
f) Tempered 100°C. Fine e-carbides across twins
g) Tempered 200°C. Coherent cementite along twins. c-carbides dissolve
h) Tempered 400°C. Breakdown of twinned structure. Carbides grow and spheroidise
Figure 3. Low carbon lath martensites have a high Ms temperature and some tempering often occurs on cooling, i.e. autotempering. Tempering at 300°C causes precipitation of carbides within the laths in Widmanstätten form (Fig. 3). Tempering at 500°C promotes the recovery of the dislocation tangle into cells within the laths with carbides precipitated along boundaries. Tempering at 600°C gives rise to recrystallisation into equioxed ferrite with carbides re-nucleated at the boundaries.
Quench cracks The volume changes, which occur when austenite is cooled, are: a) expansion when gamma iron transforms to ferrite; b) contraction when cementite is precipitated; c) normal thermal contraction. When steel is quenched these volume changes occur very rapidly and unevenly throughout the specimen. The outside cools most quickly, and is mainly martensitic, in which contraction (b) has not occurred. The centre may be troostitic and contraction (b) started. Stresses are set up which may cause the metal either to distort or to crack if the ductility is insufficient for plastic flow to occur. Such cracks may occ ur some time after the quenching or in the early stages of tempering. Quench cracks are liable to occur: a) due to presence of non-metallic inclusions, cementite masses, etc.; b) when austenite is coarse grained due to high quenching temperature; c) owing to uneven quenching; d) in pieces of irregular section and when sharp re-entrant angles are present in the design. The relation of design to heat-treatment is very important. Articles of irregular section need special care. When steel has been chosen which needs a water-quench, then the designer must use generous fillets in the corners and a uniform section should be aimed at. This can sometimes be obtained by boring out metal from bulky parts without materially affecting the design; examples are given in Fig. 4. A hole drilled from the side to meet a central hole may cause cracking and it should be drilled right through and temporarily stopped up with asbestos wool during heattreatment. A crack would also form at the junction of the solid gear with the shaft. There is a serious danger of cracks at the roots of the teeth, owing to the great change in size of section. This design could be improved by machining the metal away under the rim to
make a cross-section of uniform mass.
Figure 4. The relation of design to heat-treatment
Fundamentals of heat-treatment Heat-treatment of steel involves the change of austenite, a face-centred cubic iron lattice containing carbon atoms in the interstices, into a body-centred cubic ferrite with a low solubility for carbon. The carbon atoms segregate into areas to form cementite. This involves mobility or diffusion of the carbon atoms and both time and temperature are important. Atomic movements are rapid at high temperatures but increasingly sluggish as the temperature decreases. As the rate of cooling of an austenitised steel increases the time allowed for the changes is shortened and the reactions are incomplete at 600-700°C. Residual austenite, therefore, transforms at lower temperatures, with shorter movements of atoms and finer structures. At temperatures below about 250°C diffusion is so slow that another transition structure is formed. The effect of rapid cooling on the critical points is complex (Fig. 5). Increase in the rate of cooling has the following effects: 1. Arrest temperatures are depressed. 2. Ar3 merges with Ar1 producing a single depressed point known as Ar". Fine laminated troostite is formed. 3. Accelerated cooling causes another arrest to appear at 350-150°C, known as Ar". Troostite and martensite are formed. 4. Rapid quenching causes Ar" to merge into Ar". Martensite is formed.
5. The arrest due to the formation of bainite at 500-250°C does not usually appear with carbon steel, but is present with many alloy steels.
Figure 5. Effect of cooling rate on the transformation of austenite
Annealing Abstract:
The purpose of annealing may involve one or more of the following aims: 1. To soften the steel and to improve machinability. 2. To relieve internal stresses induced by some previous treatment (rolling, forging,uneven cooling). 3. To remove coarseness of grain. The treatment is applied to forgings, cold-worked sheets and wire, and castings. The operation consists of: a. heating the steel to a certain temperature, b. "soaking" at this temperature for a time sufficient to allow the necessary changes to occur, c. cooling at a predetermined rate. The purpose of annealing may involve one or more of the following aims:
1. To soften the steel and to improve machinability. 2. To relieve internal stresses induced by some previous treatment (rolling, forging, uneven cooling). 3. To remove coarseness of grain. The treatment is applied to forgings, cold-worked sheets and wire, and castings. The operation consists of: a. heating the steel to a certain temperature, b. "soaking" at this temperature for a time sufficient to allow the necessary changes to occur, c. cooling at a predetermined rate.
Sub-critical Anneal It is not always necessary to heat the steel into the critical range. Mild steel products which have to be repeatedly cold worked in the processes of manufacture are softened by annealing at 500° to 650°C for several hours. This is known as "process" or "close" annealing, and is commonly employed for wire and sheets. The recrystallisation temperature of pure iron is in the region of 500°C consequently the higher temperature of 650°C brings about rapid recrystallisation of the distorted ferrite Since mild steel contains only a small volume of strained pearlite a high degree of softening is induced. As shown, Fig. 1b illustrates the structure formed consisting of the polyhedral ferrite with elongated pearlite (see also Fig. 2). Prolonged annealing induces greater ductility at the expense of strength, owing to the tendency of the cementite in the strained pearlite to "ball-up" or spheroidise, as illustrated in Fig. 1c. This is known as "divorced pearlite". The ferrite grains also become larger, particularly if the metal has been cold worked a critical amount. A serious embrittlement sometimes arises after prolonged treatment owing to the formation of cementitic films at the ferrite boundaries. With severe forming operations, cracks are liable to start at these cementite membranes.
Figure 1. Effect of annealing cold-worked mild steel
Figure 2. Effect of annealing at 650°C on worked steel. Ferrite recrystallised. Pearlite remains elongated (x600) The modern tendency is to use batch or continuous annealing furnaces with an inert purging gas. Batch annealing usually consists of 24-30 hrs 670°C, soak 12 hrs, slow cool 4-5 days. Open coil annealing consists in recoiling loosely with controlled space between wraps and it reduces stickers and discoloration. Continuous annealing is used for thin strip (85% Red) running at about 400 m/min. The cycle is approximately up to 660°C 20 sec, soak and cool 30-40 sec. There is little chance for grain growth and it produces harder and stiffer strip; useful for cans and panelling. "Double reduced" steel is formed by heavy reduction (~50%) after annealing but it suffers from directionality. This can be eliminated by heating between 700-920°C and rapidly quenching.
Full Anneal and Normalising Treatments For steels with less than 0,9% carbon both treatments consist in h eating to about 2550°C above the upper critical point indicated by the Fe-Fe3C equilibrium diagram (Fig. 3). For higher carbon steels the temperature is 50°C above the lower critical point.
Figure 3. Heat-treatment ranges of steels Average annealing and hardening temperatures are:
Carbon, %
0.1
0.2
0.3
0.5
0.7
0.9 to 1.3
Avg.temp. °C
910
860
830
810
770
760
These temperatures allow for the effects of slight variations in the impurities present and also the thermal lag associated with the critical changes. After soaking at the temperature for a time dependent on the thickness of the article, the steel is very slowly cooled. This treatment is known as full annealing , and is used for removing strains from forgings and castings, improving machinability and also when softening and refinement of structure are both required. Normalising differs from the full annealing in that the metal is allowed to cool in still air . The structure and properties produced, however, varying with the thickness of metal treated. The tensile strength, yield point, reduction of area and impact value are higher than the figures obtained by annealing.
Changes on Annealing Consider the heating of a 0,3% carbon steel. At the lower critical point (Ac1) each "grain" of pearlite changes to several minute austenite crystals and as the temperature is
raised the excess ferrite is dissolved, finally disappearing at the upper critical point (Ac3), still with the production of fine austenite crystals. Time is necessary for the carbon to become uniformly distributed in this austenite. The properties obtained subsequently depend on the coarseness of the pearlite and ferrite and their relative distribution. These depend on: a) the size of the austenite grains; the smaller their size the better the distribution of the ferrite and pearlite. b) the rate of cooling through the critical range, which affects both the ferrite and the pearlite. As the temperature is raised above Ac3 the crystals increase in size. On a certain temperature the growth, which is rapid at first, diminishes. Treatment just above the upper critical point should be aimed at, since the austenite crystals are then small. By cooling slowly through the critical range, ferrite commences to deposit on a few nuclei at the austenite boundaries. Large rounded ferrite crystals are formed, evenly distributed among the relatively coarse pearlite. With a higher rate of cooling, many ferrite crystals are formed at the austenite boundaries and a network structure of small ferrite crystals is produced with fine pearlite in the centre.
Overheated, Burnt and Underannealed Structures When the steel is heated well above the upper critical temperature large austenite crystals form. Slow cooling gives rise to the Widmanstätten type of structure, with its characteristic lack of both ductility and resistance to shock. This is known as an overheated structure, and it can be refined by reheating the steel to just above the upper critical point. Surface decarburisation usually occurs during the overheating. During the Second World War, aircraft engine makers were troubled with overheating (above 1250°C) in drop-stampings made from alloy steels. In the hardened and tempered condition the fractured surface shows dull facets. The minimum overheating temperature depends on the "purity" of the steel and is substantially lower in general for electric steel than for open-hearth steel. The overheated structure in these alloy steels occurs when they are cooled at an intermediate rate from the high temperature. At faster or slower rates the overheated structure may be eliminated. This, together with the fact that the overheating temperature is significantly raised in the presence of high contents of MnS and inclusions, suggests that this overheating is conected in some way with a diffusion and precipitation process, involving MnS. This type of overheating can occur in an atmosphere free from oxygen, thus emphasising the difference between overheating and burning. As the steel approaches the solidus temperature, incipient fusion and oxidation take place at the grain boundaries. Such a steel is said to be burnt and it is characterised by the presence of brittle iron oxide films, which render the steel unfit for service, except as
scrap for remelting.
Annealing of casting Abstract:
In the case of steel castings full annealing is the only way for completely effacing the coarse grains and Widmanstätten structure, with its associated brittleness.The Widmanstätten structure can be modified into a "feathery" arrangement of the ferrite by the influence of silicon. This is shown in Fig. 1 which consists of a portion of the boundary-ferrite, Widmanstätten and feathery structures. Fig. 2 shows the macroform of the primary crystals, revealed by the segregation of the impurities. The effects of this segregation have to be effaced as much as possible by annealing and this necessitates temperatures higher than those used for worked steels. In the case of steel castings full annealing is the only way for completely effacing the coarse grains and Widmanstätten structure, with its associated brittleness. The following properties of a 0,3 % carbon steel illustrate the improvement obtained by annealing: Table 1
BH
YP MPa
TS MPa
El Cold % Bend
Izod J 12
Cast
160 309
448
6
45°
Annealed 880°C, 6hr, furnace cooled
153 247
479
24 165°
32
The Widmanstätten structure can be modified into a "feathery" arrangement of the ferrite by the influence of silicon. This is shown in Fig. 1 which consists of a portion of the boundary-ferrite, Widmanstätten and feathery structures. Fig. 2 shows the macroform of the primary crystals, revealed by the segregation of the impurities. The effects of this segregation have to be effaced as much as possible by annealing and this necessitates temperatures higher than those used for worked steels.
Figure 1. Micro-structure of same steel showing part of ferrite network, Widmanstätten Figure 2. Macro-structure of cast steel and feathery structure. Ferrite-white. revea-ling large prirmary austenite crystals Pearlite-dark ( x 80) due to presence of impurities (x 4) An imperfect anneal is illustrated by Fig. 3, in which the original cast structure is still outlined by the deposition of the ferrite in the old positions, especially around the inclusions. They can be prevented by double annealing. Double annealing consists of heating the steel to a temperature considerably over the A3 point, cooling rapidly to a temperature below the lower critical range and then immediately reheating to a point just over the upper critical point (Ac3), followed by slow cooling. This method is particularly useful for castings. The high temperature treatment effaces the strains, coalesces the sulphide films in the ferrite which embrittle the steel and produces homogenity by rapid diffusion. The quick cooling prevents the coarse deposition of the ferrite in the large grains, but tends to harden the metal. The second heating refines the coarse grains and leaves the steel in a softened condition. A typical structure, which is shown in Fig. 4, should be compared with Fig. 3.
Figure 3. Same steel imperfectly annealed ferrite formed in masses outlining original cast structure (x80)
Figure 4. Same steel properly annealed: ferrite and perlite uniform and fine (x80)
Softening Tool and Air-hardening Steels To soften high carbon and air-hardening steels, in order to allow machining operation to be carried out, they are heated just below the lower change point (650-700°C), causing. the cementite to balls-up into rounded masses (i.e. spheroidising anneal). When the cementite is in this condition high carbon steels can be cold drawn; but too high a temperature causes pearlite to be reformed, with consequent high resistance to deformation. It should be remembered, however, that coarse laminated cementite spheroidises extremely slowly, and the above treatment is therefore carried out on a "hardened" material, obtained by suitable cooling from above A3 or after cold working. A short cycle anneal consists of heating just above Ac1, cooling below Ar1 and then raising temperature just below Ac1 for 8 hours. Although the softest condition is obtained when the large globules of cementite are embedded in the ferrite, a smooth machined surface is difficult to obtain due to tearing. Groups of large globules cause failure of sharp-edged cutting tools. During the subsequent hardening operations, the time required to dissolve fine spheroidised cementite is less than for the lamellar type. This property is being used in hardening thin sections, such as safety razor blades and needles, in order to reduce decarburisation.
Annealing and Hardening Temperatures for Tool Steels The annealing or hardening temperatures of steels containing more than 0,9% carbon is just above the lower critical point (730-790°C) instead of above the upper range. Fig. 5 shows the appearance of a 1,3% carbon steel cast, in which the cementite exists as
brittle networks and plates. This type of structure must be replaced by rounded particles of cementite in a fine pearlite before hardening, otherwise cracks will propagate through the continuous masses of brittle cementite.
Figure 5. As cast: cementite network and plates in pearlite (x 100)
Figure 6. Heated to 1050°C and quenched in water. Large grains (x 100)
Figure 7. Cementite globules in properly hot-worked steel (x 200)
Figure 8. Cementite globules in martensite, in hardened steel (x 200)
The upper critical line rises steeply with increasing carbon content above 0,87% and an excessively high temperature is required to dissolve all the free cementite. This tends to develop coarse austenite crystals which cause the steel to become brittle and cracks to form on quenching. This grain growth is shown on Fig. 6, which is the structure of steel quenched from 1050°C. On the other hand, particles of cementite restrain grain growth. Forging is, therefore, carried out through the critical range in order to disperse the free cementite. This is followed by annealing at about 760°C (just above Ac1), to ball-up the free cementite and to remove strains. Fig. 7 shows the structure formed. Even a fine cementite network structure would cause trouble when drastic quen ching is used, such as for files. Fig. 8 shows the structure of a steel hardened from 760°C, consisting of
particles of cementite dispersed in a matrix of martensite.
Products of Quenching: Cnstituents of Hardened Steel The equilibrium diagram indicate the changes which occur under the slow cooling conditions of annealing. The rapid rates of cooling necessary to harden a steel cause the austenite to persist to a lower temperature and to transform into a variety of microconstituents discussed below. Austenite is sometimes present with martensite, in drastically quenched steels (Fig. 9). It cannot be completely retained in carbon steels by even drastic quenching, but suitable additions of alloying elements allow the retention of this constituent, for example 18/8 austenitic stainless steel. This austenite consists of p olyhedral grains, showing twins (Figs. 9 and 10). It is non-magnetic and soft. Martensite is the hardest constituent obtained in a given steel, but the hardness increases with the carbon content of the steel up to 0,7%. The micro-structure exhibits a fine needle-like structure, which becomes more pronounced when the steel is quenched from high temperatures. See Figs. 9 and 11.
Figure 9. Forms of carbide in micro-constituents in steel
Figure 10. Martensite needles (dark) in austenite (x1200)
Figure 11. Steel (C, 0,4) quenched from between A1 and A3. Undissolved ferrite
arou-nd inclusion in martensite (x 100)
Figure 12. Martensite and quench crack. Steel (C, 0,5) quenched in water from 900°C(x400)
Figure 13. Nodular troostite in martensite (x400)
Figure 14. Sorbite in quenched and tempered (600°C) steel (C, 0,5) (x500)
Figure 15. Case-hardened screw. Cracked martensitic case (white), martensite and ferrite core (x30)
The nature of martensite has not been definitely agreed upon, but for the present purpose it might be considered to be highly stressed a-iron which is supersaturated with carbon. The fc.c. lattice of g-iron is equivalent to a body centred tetragonal lattice with ratio 1,414. The tetragonal lattice of martensite is formed from this by compressing its height and increasing its cross-section. A slight further compression to give a ratio of 1 gives rise to a-iron. Bainite, which occurs in alloy steels, has a rapidly etched needle-like structure, somewhat resembling tempered martensite. As the temperature of its formation becomes higher the acicular nature becomes less accentuated, the needles increasing in size Troostite (nodular) rapidly etches black and is practically unresolvable under the ordinary microscope. Special microscopic technique has shown that nodular troostite is a mixture of radial lamellae of ferrite and cementite. Therefore it differs from pearlite only
in degree of fineness and carbon content which is the same as that in the austenite from which it is formed. Figs. 9 and 13 show a typical martensite-troostite structure; nodules outline the boundaries of the original austenite grains. Troostite is softer than martensite and small amounts in the steel lessen the risks of cracking and distortion. Some confusion arises as to the nomenclature of micro-constituents found in hardened and tempered steels. The terms troostite and sorbite are frequently used to indicate constituents formed during quenching and also du ring tempering. In the former case (quenching) the cementite always occurs in a laminated form, while in the latter (tempering) it has a granular form. Hence, the term troostite has been adopted in this book for constituents possessing a laminated structure. Sorbite is used to denote the granular structures.
Constant temperature transformation TTT curves Abstract:
The structures formed during the continuous cooling of steel from above Ac3 can be understood best by studying the constant-temperature (isothermal) transformation of austenite, thus separating the two variables: time and temperature.One method consists of heating small specimens above Ac3 to form austenite, then quenching into a suitable bath (e.g. liquid tin) at some constant sub-critical temperature. After holding for selected periods of time, the specimens are withdrawn from the bath and rapidly quenched in cold water. This converts any untransformed austenite into martensite the volume of which can be estimated microscopically. Another method consists in measuring length changes caused by the decomposition of austenite at the constant temperature by means of a dilatometer.
The structures formed during the continuous cooling of steel from above Ac3 can be understood best by studying the constant-temperature (isothermal) transformation of austenite, thus separating the two variables: time and temperature. One method consists of heating small specimens above Ac3 to form austenite, then quenching into a suitable bath (e.g. liquid tin) at some constant sub-critical temperature. After holding for selected periods of time, the specimens are withdrawn from the bath and rapidly quenched in cold water. This converts any untransformed austenite into martensite the volume of which can be estimated microscopically. Another method consists in measuring length changes caused by the decomposition of austenite at the constant temperature by means of a dilatometer. When carbon steel is quenched in the baths at constant temperatures, the velocity of austenite transformation is found to depend on temperature. The time for the beginning and completion of the transformation of the austenite is plotted against the temperature to give the Bain "S-curve", shown in Fig. 1, now called TTT-curve (time-temperature-transformation).
Figure 1. Ideal TTT-curve for 0,65% carbon steel depicting time interval required for beginning, 50% and 100% transformation of austenite at a constant temperature A= Austenite F= Ferrite P = Pearlite B = Bainite The logarithmic scale of time is used to condense results into a small space. Ae1 and Ae3 lines represent the equilibrium transformation temperatures. Austenite is completely stable above Ae3 and partially unstable between Ae3 and Ae1. Below Ae1 austenite is completely unstable and transforms in time. Two regions of rapid transformation occur about 550° and 250°C. The form of each of the curves and their positions with respect to the time axis depend on the composition and grain size of the austenite which is transforming. The TTT-curve is most useful in presenting an overall picture of the transformation behaviour of austenite. This enables the metallurgist to interpret the response of steel to any specified heat-treatment, to plan practical heattreatment operations and to control limited hardening or softening and the time of soaking. The decomposition of austenite occurs according to three separate but sometimes overlapping mechanisms and results in three different reaction products: pearlitic, bainitic, martensitic.
Pearlitic The upper dotted curve in Fig. 1 represents the beginning of the formation of ferrite. The curve just below it indicates the beginnings of the breakdown of the austenite remnant into a ferrite-carbide aggregate. In the hightemperature pearlitic range in Fig. 1 the process resembles the solidification of crystals from a liquid by the formation and growth of nuclei of carbide followed by ferrite by side nucleation with side and edge growth, Fig. 2a and b. At 700°C the formation of nuclei is slow (i.e. incubation period), then growth proceeds rapidly to form large pearlite colonies covering several austenite grains in some cases. As the transformation temperature is lowered to 500°C the incubation period decreases and the pearlite becomes increasingly fine. Large numbers of nuclei form in the austenite boundaries, but growth is slower and this produces nodular
troostite, Fig. 2a. In the case of medium carbon steels the excess ferrite decreases in volume and begins to show an acicular or Widmanstätten type of distribution. The relative amounts of free ferrite to be expected after a given heat-treatment is indicated by the size of the "austenite and ferrite" field and by the temperature interval between Ae1 and Ae3.
Bainitic Between about 500° and 350°C initial nuclei are ferrite which is coherent with the austenite matrix. Cementite then precipitates from the carbon-enriched layer of austenite, allowing further growth of the ferrite as shown in Fig. 2c. The carbides tend to lie parallel to the long axis of the bainite needle to form the typical open feathery structure of upper bainite. Below 350°C coherent ferrite, supersaturated with carbon, forms first and is then followed by the precipitation of carbide within the ferrite needle, transversely at an angle of 55°. A proportion of the carbide is Fe2,4C and the ferrite contains a little dissolved carbon. This lower bainite structure is somewhat similar to lightly tempered martensite (Fig. 2d).
Figure 2. (a) Effect of different speeds of nucleation and growth on formation of pearlite colonies; (b), (c), (d) diagrammatic representation of formation of pearlite, upper bainite and lower bainite
Martensitic In quenching down to about 250°C, the temperature drops rapidly through the interval in which "nucleation" could take place, to a temperature so low that the molecular mobility, i.e. diffusion, becomes too small for the formation of nuclei. In the third stage, therefore, the austenite changes incompletely into a distorted body-centred structure, with little or no diffusion of the carbon into particles of cementite, to form martensite the plates of which are formed at a high speed (less than 0,002 sec). This suggests that the mechanism of formation of this structure is not nucleation and growth but a shearing process. This resembles the process of mechanical twinning and involves very little atomic movement, but considerable internal stress due to the shear and to the position of the carbon atoms. As the temperature decreases the elastic energy increases and eventually causes a shear in a part of the matrix, which stabilises the rest. Further shear can only occur when the temperature is lowered and more energy gained. The amount of martensit e formed, therefore, is practically independent of time and depends principally on the temperatures at which the steel is held. Hence a proportion of austenite is usually retained in quenched steel which can be reduced in amount by a decrease in temperature. This fact is used in sub-zero quenching. The temperature at which martensite begins to form (Ms) is progressively lowered as the carbon content of the steel increases, e.g. C%
0,02
0,2
0,4
0,8
1,2
Ms °C
520
490
420
250
150
The temperature is also affected by the alloy content, but the following empirical formula (Steven and Haynes) can be used for calculating Ms from the chemical analyses, provided all carbides have been dissolved in the austenite: Ms in °C = 561 - 474 (% C) - 33 (% Mn) - 17 (% Ni) - 17(% Cr) - 21 (% Mo). Mf is about 215°C below the Ms. Plastic and elastic stresses promote the formation of martensite, but it is retarded when cooling is interrupted. When cooling is resumed after such a stabilisation arrest martensite only begins to form again after cooling to a lower temperature. The rate and extent of stabilisation (depression) depend on the temperature and time of holding, amount of prior transformation and alloy content. Two forms of martensite have been identified depending on carbon content. In low carbon steels laths containing many dislocations are found, while in high carbon steels the plates are heavily twinned, Fig. 3(a) and (b).
