AMP_04_May_June_2021_Digital_Edition

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 | M A Y / J U N E 2 0 2 1 6 2 S urface hardening of steels is of the utmost techno- logical importance in modern society. The ability to precisely alter the surface composition and micro- structure of steel components allows for significant improve- ments in corrosion resistance, wear properties, strength, and fatigue performance. In many instances, the surface alterations are subtle and require detailed and specialized characterization to develop a mechanistic understanding of processing-microstructure-property relationships. This article describes how researchers in the Advanced Steel Pro- cessing and Products Research Center (ASPPRC) at Colorado School of Mines have employed advanced characterization techniques to develop a deeper understanding of surface hardened steels. ATOM PROBE TOMOGRAPHY TO UNDERSTAND GRAIN SIZE CONTROL In surface hardened steels, including steels heat treated through carburizing, carbonitriding, and induction hardening, it has been established that refining the prior austenite grain size (PAGS) improves fatigue performance. For example, microalloying approaches have been used in carburizing steels to promote microstructural refinement during hot rolling and carbonitride precipitation during the carburizing heat treatment [1] . These precipitates and the initial refined structure help maintain fine grain sizes through a Zener pinning effect of the prior austenite grain boundaries, which can be particularly beneficial at elevat- ed temperatures used in vacuum carburizing. However, the coarsening resistance of the precipitates must be con- sidered as well, which can be affected through variations in the thermal processing route and changes in alloy compo- sition. Advanced characterization is required to investigate these effects. Molybdenum additions in alloys used for carburiz- ing have been evaluated because of their potential role in maintaining coarsening resistance of niobium containing microalloy precipitates [2] . It has been shown that molyb- denum additions retard niobium carbonitride precipitate coarsening and help prevent the onset of abnormal grain growth. The specific mechanism by which molybdenum interacts with these carbonitride precipitates was evalu- ated through advanced characterization. Figure 1 shows results from local electrode atom probe tomography of a THE CASE FOR ADVANCED CHARACTERIZATION TO BETTER UNDERSTAND SURFACE HARDENING OF STEELS Using advanced characterization techniques on surface hardened steels builds a richer understanding of the underlying metallurgical phenomena. J. Klemm-Toole,* M. Agnani, S.W. Thompson, K.O. Findley* Colorado School of Mines, Golden C.M. Enloe Colorado School of Mines and CBMM North America, Pittsburgh *Member of ASM International Fig. 1 — (a) Three-dimensional atom probe reconstruction of a 4120 alloy modified with molybdenum and niobium additions. The alloy was heated to 900°C and water quenched. (b) Proximity histogram showing the distribution of elements near the precipitate interface. The solid line represents the 10 at.%Nb iso-concentration surface. Taken from Enloe et al. [3] . (a) (b) 9 10