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2 Continued on next page... When pure iron cools down from its freezing point to room temperature, something rather unusual happens – not once, but twice. At a temperature of 14000C the crystal lattice pattern changes from its original body-centered form to the face-centered form. At 9100C the lattice reverts to the body-centered form. These temperatures are termed critical temperatures. As each of the crystal rearrangements takes place, heat is released without any change in temperature taking place. The Freezing of Carbon Steel Add carbon to iron – as little at 0.1% by weight – and many things change. The freezing temperature drops. Freezing starts at one temperature and is not complete until a lower temperature has been reached. In other words, we have a freezing temperature range, not a freezing temperature. Between the limits of this range, the metal is neither solid nor liquid; it is mushy or pasty. This is very important to the welder, since he can control the metal in the mushy state much more readily then he can control completely liquid metal. Until the carbon content of the metal has reached 2%, the freezing temperature range gets broader as the carbon content increases. Beyond 2%, it starts to narrow, and finally disappears completely at 4.3% . An alloy of 95.7% iron and 4.3% carbon freezes completely at a temperature of 11300C and is called a eutectic alloy. Think of ”eutectic” as meaning ”freezes completely at a fixed temperature” and you won’t be mystified by that word. At this point, we’d like to introduce you to what is known as the ”iron- iron carbide equilibrium diagram”. A simplified form of this diagram is shown as Fig. 10-1. The two heavy lines running across the upper part of the diagram define the freezing point range for all iron-carbon alloys from 0.2% carbon to 5% carbon. Shown on the diagram are four words – austenite, ferrite, pearlite, and cementite which we shall define shortly. First, however, we ask you to look at the vertical line which picks out the 0.8% carbon alloy. We shall start at the top of that line and work our way down, describing the changes which take place as the steel cools from the liquid state to room temperature.