A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 2 2 9 proposed by Altshuler[4,5] as a result of AFM examination of steel. If the grain boundaries consisted only of disordered Fe atoms, due to their higher entropy compared to that of the ordered crystal lattice of Fe atoms within the grain, the boundaries should etch at a more rapid rate than the grains themselves. Thus, the grain boundaries should be depressed compared to the adjacent grains, which is generally believed to be true. However, the grain boundaries formwalls, which are at an equal height as that of the pearlite platelets, above the ferrite portion of the grains as measured by the AFM scans, Fig. 1. This means that the walls must consist of stable molecules, and etch at the same low rate as that of the cementite platelets. These walls must also consist of carbide since the amount of carbon in the steel is by far the most abundant element present that could form an ionic crystal with iron, Table 1. The surface of the steel was observed with an optical microscope using differential interference contrast by Altshuler[5], suggested by Wells[11]. Both the pearlite platelets and the grain boundary walls rotated polarized light by the same amount, indicating that they probably consist of the same intermetallic compound, steels. Then pearlite platelets begin to grow from the grain boundaries[14-15]. • ~400°C the pearlite platelets have essentially completed their growth • 20°C ferrite grains have less than 0.5 ppm carbon in solid solution[7] For the carbon by weight in the grain walls, Cow, the grain boundary wall thickness, t, and the grain diameter, d, then from equation (1), see derivation in (equation 1[3]), Cow = 0.2148 t/d (Eq 1) The grain diameter is 15.9 µm for the AISI 1018 steel[3], and the grain boundary wall thickness is 0.070 µm. For AISI 1018 steel, which has 1900 ppm carbon, Cow = 946 ppm is required for the grain boundary walls to surround completely the grains. This shows that grain boundary walls surround completely the grains of ferritic hypoeutectoid plain carbon steels containing sufficient carbon. The sharp upper yield point occurs when stress concentrations cause dislocations to break through the grain boundary walls. In order for this to happen, the grain boundary walls must completely surround all the grains. Otherwise, dislocations would pass around namely cementite, which is brittle. The walls are seen as brittle due to the cracks within the walls. McMahon and Cohen[12] also observed cracks in carbide particles in the grain boundaries of iron. CARBIDE WALLS SURROUND THE GRAINS The following discussion shows why the grain boundary walls surround completely each grain in hypoeutectoid steels. This is due to the formation of these walls as steel cools from 900°C to room temperature, which occurs in the following sequence: • 900°C steel grains are austenite • 822°C ferrite nuclei begin to form with subsequent growth of grains[13] • 727°C is the eutectoid temperature. The solute carbon is 0.0218 wt%. • 716°C the solute carbon is 0.019 wt% in AISI 1018 steel. Ferrite grains are fully formed. • ~715°C solute carbon is below 0.019 wt% and the excess carbon atoms diffuse to the grain boundaries and combine with iron atoms forming Fe3C (cementite) • 639°C grain boundary walls are fully formed for ferritic hypoeutectoid Fig. 4 — Polycrystalline iron, 14 ppmC, cross section. TABLE 1 — COMPOSITION OF STEEL AND IRON, PPM Metal Al C Cr Co Cu Mn Mo Ni P S Si Polycrystal AISI 1018 steel 300 1900 1100 500 7300 100 60 13 1400 Polycrystal iron, 14 ppm C 1.5 2.4 6.7 0.9 0.61 0.22 1.5 69
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