Eutectoid

For detecting and percentage evaluation of the participation of the amount of austenite in the quenched structure of hyper-eutectoidal steel, devices manufactured by CMP type WIROTEST 202 and WIROTEST 12 finish (Table 1.) are applied. These devices allow to detea and evaluate the content of residual austenite as well as form the signal for part segregation with austenite content above the allowed amount, as well as parts with grinding burning... 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

Fig. 19. Effect of cooling rate on structure of a eutectoid steel. = austenitizing temperature > = austenite phase.

Table 2. Properties of Steel Structures for a Eutectoid Steel...

AJ—Zn. Aluminum-rich binary ahoys (Fig. 18) are not age hardenable to any commercial significance, and 2inc [7440-66-6] Zn, additions do not significantly increase the abhity of aluminum to strain harden. Al—Zn ahoys find commercial use as sacrificial claddings on high strength Al—Cu—Mg—Zn aircraft ahoy sheet. The eutectoid composition near 78% Zn has found use as a superplastic sheet ahoy. 

A nonaHoyed carbon steel having 0.76% carbon, the eutectoid composition, consists of austenite above its lowest stable temperature, 727°C (the eutectoid temperature). On reasonably slow cooling from above 727°C, transformation of the austenite occurs above about 550°C to a series of parallel plates of a plus cementite known as peadite. The spacing of these plates depends on the temperature of transformation, from 1000 to 2000 nm at about 700°C and below 100 nm at 550°C. The corresponding BrineU hardnesses (BHN), which correspond approximately to tensile strengths, are about BHN... 

When a eutectoid steel is slowly cooled from the austenite range, the ferrite and cementite form in alternate layers of microscopic thickness. Under the microscope at low magnification, the diffraction effects from this mixture of ferrite and cementite give an appearance similar to that of a pearl, hence the material is called peadite. 

Changes on Heating and Cooling Hypoeutectoid Steel. Hypoeutectoid steels are those that contain less carbon than the eutectoid steels. If the steel contains more than 0.02% carbon, the constituents present at and below 727°C are usually ferrite and peadite. The relative amounts depend on the carbon content. As the carbon content increases, the amount of ferrite decreases and the amount of peadite increases. 

On slow cooling the reverse changes occur. Ferrite precipitates, generally at the grain boundaries of the austenite, which becomes progressively richer in carbon. Just above A, the austenite is substantially of eutectoid composition, 0.76% carbon. 

Manganese and nickel lower the eutectoid temperature, whereas chromium, tungsten, siUcon, molybdenum, and titanium generally raise it. AH these elements seem to lower the eutectoid carbon content. 

Fig. 17. Isothermal transformation (IT) diagram for a plain carbon eutectoid steel (1). Ae is A temperature at equiUbnum BHN, BrineU hardness number ...

Ma.rtensite, Martensite is the hardest and most bntde microstmcture obtainable in a given steel. The hardness of martensite increases with increasing carbon content up to the eutectoid composition. The hardness of martensite at a given carbon content vanes only very slightly with the cooling rate. 

Materials and Process. The steel chosen for tire cord is a eutectoid carbon steel containing 0.7% carbon, 0.5% manganese, 0.2% siUcon, and a very low amount of sulfur and phosphoms (9,48). The steel rod is cleaned with acid, rinsed, drawn through tungsten carbide dies to reduce its diameter from 5.5 to - 3.0 mm, heat treated (patented) to increase ductihty for further drawing to - 1 mm, then patented again. 

Fig. 6. Effect of alloying elements on the phase diagram of titanium (a) a-stabilized system, (b) P-isomorphous system, and (c) P-eutectoid system.

The important (3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tantalum, and niobium of the P-isomorphous type and manganese, iron, chromium, cobalt, nickel, copper, and siUcon of the P-eutectoid type. The P eutectoid elements, arranged in order of increasing tendency to form compounds, are shown in Table 7. The elements copper, siUcon, nickel, and cobalt are termed active eutectoid formers because of a rapid decomposition of P to a and a compound. The other elements in Table 7 are sluggish in their eutectoid reactions and thus it is possible to avoid compound formation by careful control of heat treatment and composition. The relative P-stabilizing effects of these elements can be expressed in the form of a molybdenum equivalency. Mo (29) ... 

Table 7. /5-Eutectoid Elements in Order of Increasing Tendency to Form Compounds ...

Element Eutectoid composition, wt % Eutectoid temperature, °C Composition for P-retention on quenching, wt %... 

Alloys of the P type respond to heat treatment, are characterized by higher density than pure titanium, and are more easily fabricated. The purpose of alloying to promote the P phase is either to form an aE-P-phase aEoy having commercially useful quaUties, to form aEoys that have duplex a- and P-stmcture to enhance he at-treatment response, ie, changing the a and P volume ratio, or to use P-eutectoid elements for intermetallic hardening. The most important commercial P-aEoying element is vanadium. 

The austenite phase which can contain up to 1.7 wt% of carbon decomposes on cooling to yield a much more dilute solution of carbon in a-iroii (b.c.c), Fenite , together with cementite, again rather diaii the stable carbon phase, at temperatures below a solid state eutectoid at 1013 K (Figure 6.3). 

The higher solubility of carbon in y-iron than in a-iroii is because the face-ceiiued lattice can accommodate carbon atoms in slightly expanded octahedral holes, but the body-centred lattice can only accommodate a much smaller carbon concentration in specially located, distorted tetrahedral holes. It follows that the formation of fenite together with cementite by eutectoid composition of austenite, leads to an increase in volume of the metal with accompanying compressive stresses at die interface between these two phases. 

In the most frequently used steels, having less dran the eutectoid content of carbon (about 0.8 wt%), die vaiious forms in which the cementite phase can be produced in dispersion in fenite depend upon die rate of cooling to... 

The copper-antimony phase diagram contains two eutectic reactions and one eutectoid reaction. For each reaction ... 

We can find a good example of this diffusion-controlled growth in plain carbon steels. As we saw in the "Teaching Yourself Phase Diagrams" course, when steel is cooled below 723°C there is a driving force for the eutectoid reaction of... 

Fig. 6.7. How pearlite grows from undercooled y during the eutectoid reaction. The transformation is limited by diffusion of carbon in the y, and driving force must be shared between all the diffusionol energy barriers. Note that AH is in units of J kgn2 is the number of carbon atoms that diffuse from or to Fe3C when 1 kg of y is transformed. (AH/njKfT - 7]/TJ is therefore the free work done when a single carbon atom goes from or to Fe,C.

Fig. 8.11. The TTT diagram for a 0.8% carbon (eutectoid) steel. We will miss the nose of the 1% curve if w quench the steel at = 200°C s. Note that if the steel is quenched into cold water not all the i/will transform to martensite. The steel will contain some "retained" / which can only be turned into martensite if the steel is cooled below the Mf temperature of -50°C.

To make martensite in pure iron it has to be cooled very fast at about 10 °C s h Metals can only be cooled at such large rates if they are in the form of thin foils. How, then, can martensite be made in sizeable pieces of 0.8% carbon steel As we saw in the "Teaching Yourself Phase Diagrams" course, a 0.8% carbon steel is a "eutectoid" steel when it is cooled relatively slowly it transforms by diffusion into pearlite (the eutectoid mixture of a + FejC). The eutectoid reaction can only start when the steel has been cooled below 723°C. The nose of the C-curve occurs at = 525°C (Fig. 8.11), about 175°C lower than the nose temperature of perhaps 700°C for pure iron (Fig. 8.5). Diffusion is much slower at 525°C than it is at 700°C. As a result, a cooling rate of 200°C s misses the nose of the 1% curve and produces martensite. 

Carbon steels as received "off the shelf" have been worked at high temperature (usually by rolling) and have then been cooled slowly to room temperature ("normalised"). The room-temperature microstructure should then be close to equilibrium and can be inferred from the Fe-C phase diagram (Fig. 11.1) which we have already come across in the Phase Diagrams course (p. 342). Table 11.1 lists the phases in the Fe-FejC system and Table 11.2 gives details of the composite eutectoid and eutectic structures that occur during slow cooling. 

Peorlite The composite eutectoid structure of alternating plates of or and FejC produced when... 

Figures 11.2-11.6 show how the room temperature microstructure of carbon steels depends on the carbon content. The limiting case of pure iron (Fig. 11.2) is straightforward when yiron cools below 914°C a grains nucleate at y grain boundaries and the microstructure transforms to a. If we cool a steel of eutectoid composition (0.80 wt% C) below 723°C pearlite nodules nucleate at grain boundaries (Fig. 11.3) and the microstructure transforms to pearlite. If the steel contains less than 0.80% C (a hypoeutectoid steel) then the ystarts to transform as soon as the alloy enters the a+ yfield (Fig. 11.4). "Primary" a nucleates at y grain boundaries and grows as the steel is cooled from A3...

Fig. 11.3. Microstructures during the slow cooling of a eutectoid steel from the hot working temperature. As a point of detail, when peorlite is cooled to room temperature, the concentration of carbon in the a decreases slightly, following the a/a + FejC boundary. The excess carbon reacts with iron at the or-FejC interfaces to form more FejC. This "plates out" on the surfaces of the existing FejC plates which become very slightly thicker. The composition of Fe3C is independent of temperature, of course.

Fig. 11.4. Microstructures during the slow cooling of a hypoeutectoid steel from the hot working temperature. A3 is the standard labelling for the temperature at which or first appears, and A, is standard for the eutectoid temperature. Hypoeutectoid means that the carbon content is below that of a eutectoid steel (in the same sense that hypodermic means "under the skin" ).

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