Feb_March_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 | F E B R U A R Y / M A R C H 2 0 1 8 5 2 2 (HTT) microstructures consisting of very fine retained mar- tensite crystals, low dislocation densities, and sometimes secondary carbide precipitation. In hardened steels, the widely used grain size for qual- ity control is austenitic grain size but amazingly, martensit- ic crystal size—the true grain size of quench and tempered martensite—has received little attention in the technical literature. This is perhaps because lath martensitic grain size is remarkably fine—most of it finer than is resolvable in the light microscope. Grain size refinement is the one microstructural parameter that increases both strength and toughness, and therefore further exploration of fine re- tained martensite crystal size would have merit, especially in HTT martensite. Martensite crystal size is just one parameter; others in- clude retainedaustenite, dislocations and twins, carbide and alloy precipitate distributions, and crystal boundaries and interfaces. Landing gear and countless other applications of hardened steel are highly successful with today’s state of the art, although failures do occur and must be explained. Se- lection of heat treatment schedules and steel composition are challenging, and perhaps even better performance can be achieved when all interactions between stress states and structure are well known and characterized. Light and elec- tron metallography as well as electron back scatter diffrac- tion and atom probe tomography are now available, and I enthusiastically encourage continued study of structure and performance in order to exploit the current capabilities of the heat treatment industry. George Krauss, FASM University Emeritus Professor Colorado School of Mines TODAY’S HEAT TREAT INDUSTRY IS STRONG, BUT MORE RESEARCH NEEDED T he application of heat treatment is in excel- lent shape. Furnace designs, systems for induc- tion hardening and carburiz- ing, and the digital control of heating and cooling rates and time at temperature have never been better. In fact, manufacturing has out- stripped a deep understand- ing of the microstructural changes induced by heat treatment. For example, the hard- ening of steel by quenching to martensite and tempering is used for vast numbers of applications, including aircraft landing gear, an application of great importance to the aero- space industry featured in this issue. However, the nano- structure and microstructural parameters of heat treated components that correlate with hardness and performance are not yet fully characterized. Martensite in low and medium carbon steels is formed by shear and consists of extremely fine crystals. These crys- tals are saturated with all the elements dissolved in parent austenite and exhibit very high densities of dislocations. Upon tempering, the crystals coarsen, dislocation densities decrease, and carbides precipitate and coarsen. Very high strengths are produced by low-temperature-tempered (LTT) microstructures with high dislocation densities and fine transition carbides. Lower (but still high) strengthswithhigh- er toughness are produced in high-temperature-tempered GUEST DITORIAL