Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 213 Tougher Rails Increasing the tensile strength by 29 ksi (200 MPa) effectively doubles the wear resistance of railroad rails, which doubles their service life. Railroad rails to carry modern larger freight cars are specified as “Grade 900A.” This is a UNS G10800 alloy fabricated with fine pearlite on the wear surface. Rails made in the late 19th century had less pearlite because the carbon content was below 0.80%, and the lamellar spacing was larger because the hot-rolled rails were cooled in air. One can imagine the care necessary to direct a water spray onto the head of the rail just long enough to bring the head down to 1000°F (540°C) but not cool it further, nor cool the web or foot of the rail so quickly. Slower cooling increases the elongation and toughness, which is desirable in the web and foot of the rail. The technicians and operators making the rail need to monitor the positioning of the spray heads and the water flow to assure that both hardness in the rail head and toughness in the web and foot are maintained. PRACTICAL METALLURGY PRACTICAL METALLURGY The Isothermal Transformation (IT) Diagram Soaking many samples at different temperatures and times reveals that the amount of time before the a ustenite begins to transform depends on the hold temperature of the salt bath, as shown in Figure 10-20. This figure, an isothermal transformation (IT) diagram, is a graphic representation of the transformations forming the microstructures in a steel alloy based on the hold temperature and time. When a sample drops below the A1 temperature to the hold temperature, it does not begin to transform instantaneously. Rather, the new phases and microstructures begin to grow after a second or minutes, and this delay time depends on the hold temperature. Likewise, most transformations require some time to complete. Once the transformations are complete, the sample will not change on further cooling. On the IT diagram, the shortest time to begin transformation, or the leftmost point on the transformation band in Figure 10-20, at about 1000°F (540°C), is called the pearlite nose, or simply the nose, of the transformation. For UNS G10800 steel, the nose is less than one second. Only samples cooled to the nose temperature in less than one second, before transformation begins, will have the finest possible pearlite. 10.4.2 More Rapid Cooling What happens to a UNS G10800 steel when it is cooled rapidly to temperatures below 1000°F (540°C)? In Figure 10-21, four c ooling paths are shown. When a part is “cooled rapidly,” it means the part is cooled to the soak temperature before any transformation from austenite to pearlite begins. 1 200 600 1000 1400 °F 10 100 Hold time (seconds) 1 minute Austenite to martensite Austenite to bainite Austenite to pearlite End of transformation Transformation band 50% transformation A1 transformation temp. Begin transformation Pearlite “nose” 1 hour 1 day 1 week 1,000 10,000 100,000 Goodheart-Willcox Publisher Figure 10-20. The start, midpoint, and finish times for pearlite and other transformations in G10800 steel are shown in this isothermal transformation (IT) diagram. Small samples were heated to 1500°F (820°C), then placed in salt baths at different temperatures. The temperature is shown on the y-axis. The x-axis reports the time that the part was held at the new lower temperature, in seconds. Above the A1 temperature, 1341°F (727°C), no transformation occurs. Copyright Goodheart-Willcox Co., Inc.