September AMP_Digital

FEATURE 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 | S E P T E M B E R 2 0 1 8 4 2 used for aerospace applications and expensive automobiles. The aerospace industry produces about 35% CFRP scrap, which is used in many under-hood automotive applications, because the chopped recycled fiber material does not have the structural strength offered by virgin long-fiber polymers for chassis and body panel applications [3] . Although heat is used to manufacture raw carbon fiber, it does not play a sig- nificant role in making the final product. One of the major obstacles facing CFRP is the end-of- life (EOP) products stated in ELV Directive 2000/53/EC. This directive issued a 2015 target applicable to Europe and Ja- pan that 95%of an automobile ready for the scrap yardmust be recoverable and recyclable. Today, it is accepted that ap- proximately 75% of an automobile is made of ferrous and nonferrous metals and the remaining 25% is made of toxic materials. Gray cast iron has been the material of choice for au- tomobile engine blocks for decades, primarily due to its low cost and adequate strength for gasoline, but not diesel, en- gines. Due to its lack of as-cast strength and the fact that it is not typically hardened by heat treatment, it has limited application for stressed components. As for heat treatment, gray iron is rarely heat treated to increase hardness, but it does receive treatments such as annealing and normalizing to enhance machinability. Stress relieving after welding is also common. Gray cast iron automotive components in- clude engine blocks, brake rotors, constant-velocity joint housings, exhaust manifolds, transmission cases, and cylin- der heads. Aluminum ismore expensive tomanufacture than steel, but it has an attractive strength-to-weight ratio and there- fore has continued to see more application in automobiles and trucks. Much of it is recycled and it also requires differ- ent forming and coating techniques. The 2 xxx , 6 xxx , and 7 xxx series grades are hardenable by solution treating, rapid wa- ter quench, and natural and artificial (heat) aging. As such, the 6 xxx series is finding application in vehicle structural and body panels. A less tangible effect of material selection is sound conduction. For example, a material can have ideal properties but transmit undesirable sound and vibration to the passenger cabin. Plain-carbon and alloy steels have the highest percent- age of application in cars and trucks simply due to their wide range of strength-to-weight relationships through diverse heat treating options. High-strength low-alloy (HSLA) grades have been used for years in the substructures of U.S. cars and trucks. The more recent use of higher hardenability al- loy steel has been directed to drivetrain components, such as transmission anddifferential gears, where the goal is to re- duce gear mass. This has resulted in the growth of using high pressure gas quenching (HPGQ) and improved distortion control. The growth of HPGQ is primarily due to increased distortion control required for more precise, less massive gearing in multispeed transmissions. In addition, OEMs real- ize that improving the hardenability, even at increased cost, can reduce or eliminate post-heat treatingmachining, grind- ing, and straightening. One of the original growth drivers for HPGQ was the use and recovery of helium. However, as the capital cost andmaintenance of compressors has increased, nitrogen is making a comeback. Nitrogen forces users to im- prove the hardenability of steels, because the horsepower for the fan motors required to equal the quench capacity of heliumwould have to be increased to impractical levels. Ductile iron (DI), specifically austempered ductile iron (ADI), has beenaround for decades, but it never gainedmain- streamacceptance compared with steel and aluminum. One reason, in this author’s opinion, is the negative perception of heat treating in salt. However, today’s salts are much more EPA-friendly than those used in the past for carburizing and cyaniding. Austempering salts consist of a 50-50 mixture of sodium nitrite and sodium nitrate compounds. A significant quantity is recycled by recovering salts frompost-wash solu- tions. Similar to steel, alloying elements play a significant role in ADI to produce properties that increase hot strength and improve tensile strength and hardness. A fundamental misunderstanding still exists regarding the microstructure formed when DI is quenched in hot salt compared with austempered steel and ferrous alloys. Years ago, investigators were credited with identifying the ferrite/ Fe 3 C/austenite microstructure as ausferrite or upper bainite, as some have called it, when DI is isothermally held above the martensite start (M s ) temperature, but not long enough to form bainite. Further, they reportedly discovered that the carbon-enriched stable austenite formed is different from what some called retained, or metastable, austenite. Lawry- nowicz [4] indicated that ausferrite is a mixture of ferrite and high-carbon austenite, and forms at austempering salt tem- peratures (Fig. 1). He further noted that bainite will not form unless theDI is heldat temperature for very long times,much longer than is perhaps practical for normal production. Fig. 1 — Ausferrite is a mixture of ferrite and high carbon austenite formed at austempering salt temperatures. Bainite will not form unless the ductile iron is held at temperature for a very long time [4] . 10