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 7 A principal cause for sharp yield points in hypoeutectoid plain carbon steels can be explained by the existence of hard grain boundary walls. Atomic force microscopy revealed that boundaries between adjoining grains in these steels consist of walls, probably cementite (Fe2C), that are brittle and contain longitudinal cracks. Because these walls completely enclose the grains, they inhibit the movement of dislocations between adjoining grains until the walls fracture transversely. As a result, the upper yield point is at the elastic line, followed by a large sharp drop in stress to the lower yield point. This concept supplants currently accepted theories of yielding where the Cottrell atmosphere of interstitial carbon atoms pin dislocations that must be broken to cause the upper yield point in steel. Pure iron, on the other hand, does not have continuous walls that completely surround the grains. Instead, iron has grain boundary wall segments where there is a smooth transition between elastic and plastic deformation. BACKGROUND AND EXPERIMENT There are two current theories on the cause of an upper yield point with a stress drop to a lower yield point. Cottrell proposed that interstitial carbon atoms pin edge dislocations[1]. At the upper yield point, the stress is sufficiently high that the dislocations break free from the carbon atoms, causing a drop in stress to the lower yield point. Another theory from Sun states that the upper yield point in uniaxial tension tests is strongly affected by stress concentrations[2]. To investigate further, AISI 1018 steel and polycrystalline pure iron were heated to γFe phase for 75 minutes at 1650°F, furnace cooled[3], and examined by atomic force microscopy (AFM) by Altshuler[3-5]. Figure 1 shows an AFMscan of AISI 1018 steel. This shows that the grains are surrounded by grain boundary walls, the thin yellow lines. One can see the walls at the same height as the pearlite platelets, shown at the center of the figure, see the height bar. Figure 2 shows three grain boundary walls viewed at the right side of Fig. 1. A cross section of one of these walls gives a grain boundary width of about 70 nm. Po l y c r y s t a l l i n e iron, 14 ppm C, was examined by an AFM as shown in Fig. 3. A grain boundary wall is visible between the center grain and the left one. No grain boundary wall can be seen between the center grain and the right grain. However, a grain boundary wall can be seen between the center grain and the left grain. Figure 4 is a cross-sectional view of the top portion of Fig. 3. The best estimate of the wall width at the base is about 260 nm. YIELDING RESULTS Figure 5 shows the stress-strain results for a tensile test on the AISI 1018 steel. The upper yield point was recorded at 379 MPa on the elastic line. Then there was a rapid stress drop to 360 MPa. With continued strain to 0.002208, the stress remained constant. It then dropped along the “partial break- through” line to a strain of 0.025456 at a stress of 320 MPa. At that point, there was a large drop in stress elastically to a minimum stress of GRAIN BOUNDARY WALLS CAUSE THE UPPER YIELD POINT IN STEEL Atomic force microscopy studies show that sharp yield points in steels are explained by the existence of hard grain boundary walls, contrary to current accepted theories. Thomas L. Altshuler, FASM,* Advanced Materials Laboratory, retired, Sun City Center, Florida *Member of ASM International PERSPECTIVE Fig. 1 — Atomic force microscopy of AISI 1018 steel.
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