Carbon steel3/22/2023 ![]() ![]() Note: L is the longitudinal direction and T is the transverse direction (1): CFRE is carbon fiber reinforced epoxy (2): GFRE is glass fiber reinforced epoxy. With greater emphasis on vehicle weight reduction, it is expected that other lightweight materials, such as magnesium alloys, titanium alloys and carbon fiber composites will find several niche applications in future automobiles ( Powers, 2000). Lighter components can be produced with carbon fiber composites, but because of their high cost, carbon fiber composites are not used in today’s automobiles except in a few low-production volume, high-cost vehicles. Among the polymer matrix composites, glass fiber composites are selected for most interior applications today, but they are also found in some exterior body panel or structural applications. The growth in the use of polymer matrix composites has also occurred due to their lower density. Much of this growth in the use of aluminum alloys has occurred at the expense of cast iron in engine and transmission components and copper-based alloys in radiators but aluminum alloys, because of their lower density than steel’s, are also making inroads in body panels and structures. For example, the use of aluminum alloys in North American automobiles has increased from 2% of the curb weight in 1970 to nearly 8.8% in 2010 and is projected to reach 10% or higher in 2020. There is also an increasing use of aluminum alloys and polymer matrix composites. As a result, the amount of high strength and advanced high strength steels has increased in recent years, while the amount of plain carbon steels has decreased ( Table 2.2). As shown in Table 2.1, even today steel is used in much larger quantities than any other material however, high strength steels and advanced high strength steels, on account of their significantly higher strength, are now replacing plain carbon steels in several body structure and chassis applications. Plain carbon steel and cast iron were the workhorse materials in the automotive industry prior to 1970s. Figures 12.7 and 12.8 show a real microstructure-from a polished and etched section of a low-carbon steel. ![]() These structural differences are summarized in Figure 12.6. ![]() ![]() If the steel contains more than 0.80% C (a hypereutectoid steel), then we get a room temperature microstructure of primary Fe 3C plus pearlite instead ( Figure 12.5). The room temperature microstructure is then made up of primary α+pearlite. At A 1 the remaining γ (which is now of eutectoid composition) transforms to pearlite as usual. “Primary” α nucleates at γ grain boundaries and grows as the steel is cooled from A 3 to A 1. If the steel contains less than 0.80% C (a hypoeutectoid steel), then the γ starts to transform as soon as the alloy enters the α+γ field ( Figure 12.4). If we cool a steel of eutectoid composition (0.80 wt% C) below 723 ☌ pearlite nodules nucleate at grain boundaries ( Figure 12.3) and the microstructure transforms to pearlite. The limiting case of pure iron ( Figure 12.2) is straightforward: when γ iron cools below 914 ☌ α grains nucleate at γ grain boundaries and the microstructure transforms to α. Figures 12.2–12.6 show how the room temperature microstructure of carbon steels depends on the carbon content. ![]()
0 Comments
Leave a Reply.AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |