Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 217 As a practical matter, most tempering cycles call for one hour at temperature because two or three minutes more or less does not change the microstructure or properties significantly. The amount of microstructure change is controlled by the tempering temperature, not time. More change is produced at higher temperatures. Higher tempering temperature reduces hardness, as shown in Figure 10-25. Yield strength remains high until the tempering temperature is 1000°F (540°C). A processing advantage of tempered martensite is that heat treatment is relatively easy to control. The part is heated, quenched to room temperature as fast as possible, then reheated to a moderate temperature of 300°F to 600°F (150°C to 320°C) for one hour. By comparison, to produce bainite, the part must be quenched and held at an intermediate temperature. Thus, only when properties of an alloy heat-treated to bainite are superior for a selected application can the use of specialized equipment be justified. Tempered martensitic G10800 steel makes excellent cutting tools for woodworking. Wood chisels and cutting blades for hand planes are often made with high-carbon alloys that are heat-treated to form tempered martensite. Each tempered martensite tool can hold an edge well, yet withstand repeated hammering impacts. Automotive structural parts such as frame cross members, door and side intrusion beams, and bumper reinforcements all help protect people inside the car from the effects of a crash. These parts—plus rocker panel reinforcements, side sill reinforcements, springs, and clips—all may use tempered martensite. 0 100 30 40 50 60 70 20 200 400 600 800 1000 1200 200 300 400 Tempering temperature, °C Hardness, HRC Tempering temperature, °F 500 600 700 Goodheart-Willcox Publisher Figure 10-25. The hardness of quenched and tempered UNS G10800 steel declines as the tempering temperature increases. In addition, the yield strength obtained in a given production process will also depend on heating and cooling time, as well as the size of the workpiece. During process development for a new product, adjustments may be necessary to obtain the desired properties. Copyright Goodheart-Willcox Co., Inc.