Metallurgical Principles in the Heat Treatment of Steels
Heat treatment of steels is carried out for achieving the desired changes in the metallurgical structure properties of the steels. By heat treatment, steels undergo intense changes in the properties. Normally very stable steel structures are obtained when steel is heated to the high temperature austenitic state and then slowly cooled under near equilibrium conditions. This type of heat treatment, normally known as annealing or normalizing, produces a structure which has a low level of the residual stresses locked within the steel, and the structures can be predicted from the Fe (iron)- C (carbon) equilibrium diagram. However, the properties which are mostly required in the steels are high strength and hardness and these are generally accompanied by high levels of residual stresses. These are due to the metastable structures produced by non-equilibrium cooling or quenching from the austenitic state.
Crystal structure and phases
The crystal structure of pure Fe in the solid state is known to exist in two allotropic states. From the ambient temperature and up to 910 deg C, Fe possesses a body centered cubic (bcc) lattice and is called alpha-Fe. At 910 deg C, alpha-Fe crystals turn into gamma-Fe crystals possessing a face-centered cubic (fcc) lattice. The gamma crystals retain stability up to temperature of 1400 deg C. Above this temperature they again acquire a bcc lattice which is known as delta crystals. The delta crystals differ from alpha crystals only in the temperature region of their existence. Fe has two lattice constants namely (i) 0.286 nm for bcc lattices (alpha-Fe, delta-Fe), and (ii) 0.364 nm for fcc lattices (gamma- Fe). At low temperatures, alpha-Fe shows strong ferromagnetic characteristic. This disappears when it is heated to around 770 deg C, since the lattice loses its ferromagnetic spin ordering. The state of Fe above 770 deg C is called beta-Fe. The lattice of paramagnetic beta crystals is identical to the lattice of alpha crystals.
While undergoing from one form to another, Fe is capable of undercooling. This causes a difference in the position of transformation points on heating and cooling. The difference depends on the cooling rate and is termed hysteresis. The letters ‘c’ and ‘r’ indicate whether the transformation is due to heating or cooling. Further, the change in the density of alpha-Fe as it transforms to gamma-Fe results in an abrupt change in the volume of the material. Sometimes it gives rise to stresses which exceed the elastic limit and lead to failure. The density of gamma-Fe is around 4 % higher than that of alpha-Fe.
Iron-carbon equilibrium diagram
The structure of steels, which are Fe-C alloys, can contain either pure C (graphite) or a chemical compound known as cementite (Fe3C) as the C enriched constituent. Cementite is present even in relatively slowly cooled steels (a long holding at higher temperatures is usually needed to decompose Fe3C to Fe and C). For this reason the Fe-C equilibrium diagram is frequently treated as the Fe-Fe3C equilibrium diagram. The Fe-C diagram is stable, while the Fe- Fe3C diagram is metastable. The Fe- C equilibrium diagram incorporating both the Fe-C stable diagram and Fe- Fe3C metastable diagram is given in Fig 1. Dashed lines stand for the stable Fe-C diagram, and solid lines denote the metastable Fe-Fe3C diagram.
In the metastable Fe–Fe3C diagram, the lattices of allotropic forms of Fe (delta, gamma, and alpha) serve as sites of formation of delta, gamma, and solid solutions of C in Fe. When C depleted steels crystallize, crystals of the delta solid solution precipitate at the liquidus AB and solidus AH. The delta solid solution has a bcc lattice. At the maximum temperature of 1490 deg C, the delta solution contains 0.1 % C (point H). At 1490 deg C, a peritectic reaction takes place between the saturated delta solution and the liquid containing 0.5 % C (point B). As a result, the gamma solid solution of C in gamma Fe is formed. It contains 0.18% C (point I).
If the C content is higher than 0.5 %, the gamma solid solution crystallizes directly from the liquid (at the liquidus BC and solidus IE). At 1130 deg C the limiting solubility of C in gamma Fe is close to 2.0 % (point E). Decreasing the temperature from 1130 deg C leads to lowering the C solubility in gamma- Fe at the line ES. At 723 deg C the solubility of C is 0.8 % (point S). The line ES corresponds to precipitation of Fe3C from the gamma solution.
As the C content is increased, the temperature at which the gamma lattice transforms to the alpha lattice lowers, and the transformation takes place over the temperature interval corresponding to the curves GS and GP. The alpha phase precipitation curve GS intersects the Fe3C precipitation curve ES. The point S is a eutectoid point with the coordinates 723 deg C and 0.80 % C. At this point a saturated alpha solution and Fe3C precipitate simultaneously form the eutectoid concentration gamma solution. The lattice of the alpha solid solution is identical to the lattice of the delta solid solution. At the eutectoid temperature of 723 deg C the alpha solid solution contains 0.02 % C (point P).
Further cooling leads to lowering of the C solubility in alpha-Fe, and at room temperature it equals a small fraction of a percent (point D). When the C content is 2 % – 4.3 %, crystallization starts with precipitation of the gamma solution at the line BC. An increase in the C content to above 4.3 % causes precipitation of Fe3C at the line CD. Precipitation of the surplus primary phase in all iron alloys containing over 2.0 % C is followed by a eutectic crystallization of the gamma solution and Fe3C at point C, whose coordinates are 1130 deg C and 4.3 % C. The line Ao is associated with a magnetic transformation which is a transition from the ferromagnetic to the paramagnetic state.
In case of the stable Fe–C equilibrium diagram, because of very low rates of cooling, C (graphite) can crystallize directly from the liquid. In this case, a eutectic mixture of austenite and graphite is formed instead of the eutectic of austenite and cementite. The dashed lines In Fig 1 symbolize the Fe-graphite system. These lines are at higher temperatures than the lines of the Fe-Fe3C system. This affirms to the greater stability and closeness to a full equilibrium of the Fe-graphite system. This is also supported by the fact that heating of high C steels with a large amount of Fe3C leads to its decomposition shown by the equation Fe3C = 3Fe + C.
At intermediate rates of cooling, part of the steel can crystallize according to the graphite system and the other part according to the cementite system. Phase equilibrium lines in the diagrams of both the systems can be displaced depending on particular cooling rates. A pronounced displacement can be seen for the lines of precipitation of the C solid solution in gamma-Fe (austenite). For this reason the diagram holds completely true only with respect to the steels which are exposed to a relatively slow cooling rate.
Influence of carbon
A maximum solubility of C in alpha-Fe is witnessed at 721 deg C and is equal to 0.018 % C. Subject to quenching, C can remain in the alpha solid solution, but soon precipitation of phases commences, by an aging mechanism. In a solid solution, C can form either (i) a homogeneous solution, a statically uniform interstitial distribution which is a rare case, or (ii) an inhomogeneous solution; with the formation of clusters at places where the crystal lattice structure is disturbed (grain boundaries, dislocations). The latter is the most likely state of the solid solution. The clusters thus formed represent an obstacle to the movement of dislocations during plastic deformation and are responsible for an inhomogeneous development of the deformation at the beginning of plastic flow.
To analyze the influence of the C content on Fe – C alloys, every structural component is required to be considered. Slowly cooled steels comprise ferrite and cementite or ferrite and graphite.
Ferrite is plastic. In the annealed state, ferrite has large elongation (around 40 %), is soft (Brinell hardness is 65 -130 depending on the crystal dimension), and is strongly ferromagnetic up to 770 deg C. At 723 deg C, 0.22 % C dissolves in ferrite, but at room temperature only thousandths of a percent of C is left in the solution.
Cementite is brittle and shows higher hardness (Brinell hardness is around 800). It is weakly magnetic up to 210 deg C and is a poor conductor of electricity and heat. It has a complicated rhombic lattice. Normally a distinction is made between (i) primary Fe3C, which crystallizes from the liquid at the line CD, (ii) the secondary Fe3C, which precipitates from the gamma solution at the line ES, and (iii) tertiary Fe3C, which precipitates from the a solution at the line PQ.
Graphite is soft. It is a poor conductor of electricity but transfers heat well. Graphite does not melt even at temperatures of 3000 deg C to 3500 deg C. It possesses a hexagonal lattice with the axis relation c/a higher than 2.
Austenite is soft (but is harder than ferrite) and ductile. Elongation of austenite ranges from 40 % to 50 %. It has lower conductivity of heat and electricity than ferrite, and is paramagnetic. Austenite possesses an fcc lattice.
The structure of the steel containing 0 % – 0.02 % C consists of ferrite and tertiary Fe3C. A further increase in the C content leads to the appearance of a new structural component which is a eutectoid of ferrite and Fe3C (pearlite). Pearlite appears first as separate inclusions between ferrite grains and then, at 0.8 % C, occupies the entire volume. Pearlite characterizes a two phase mixture, which generally has a lamellar structure. As the C content of steel is increased to a value higher than 0.8 %, secondary Fe3C is formed along with pearlite. The secondary Fe3C is shaped as needles. The amount of Fe3C increases as the C content is increased. At 2 % C, it occupies 18 % of the field of vision of the microscope. A eutectic mixture appears when the C content exceeds 2 %. In rapidly cooled steels, not all the surplus phase (ferrite or Fe3C) has time to precipitate before a eutectoid is formed.
Alloys with 3.6 % C contain ledeburite (a eutectic mixture of C solid solution in gamma-Fe and Fe3C). The alloys are more properly classified with hypoeutectic white cast irons.
Critical (transformation) temperatures
Carbon has a noticeable effect on transformations of Fe in the solid state. The position s of the lines GS and NL in the Fe- C equilibrium diagram show that an increase in the C content leads to lowering of the point A3 and raising of the point A4 with respect to their counterparts shown in Fig 1. Hence C extends the temperature range of the delta-phase.
When a eutectoid (pearlite) is formed, heating and cooling curves show a stop. This is labeled as the point A1 (Ac1 on heating and Ar1 on cooling). This phenomenon takes place at 0 .9 % C (point S in the Fe– C diagram). Precipitation of ferrite in hypo-eutectoid steels (on crossing the line GOS) shows up in heating and cooling curves as an inflection which is denoted by the point A3. The point corresponds to the gamma to alpha transformation in pure iron. Precipitation of Fe3C (crossing of the line ES), which precedes the eutectoid precipitation, is seen in the cooling curve as a weak inflection designated as the point Acm (Ac,cm on heating and Ar,cm on cooling ). Addition of C has little influence on the magnetic transformation temperature (point A2). Hence, the line MO corresponds to the magnetic transformation in steels with a low C content. In alloys containing higher amounts of C, this transformation occurs at the line GOS, which corresponds to the onset of ferrite precipitation. If the C content is higher than the one corresponding to point S, then the magnetic transformation coincides with the temperature A1.
Cementite undergoes a magnetic transformation. Whatever the C content, the transformation takes place at a temperature of 210 deg C–220 deg C. It occurs without a marked hysteresis, as does the magnetic transformation of pure Fe at point A2.
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