Bryan A. Webler

UHCSDB: UltraHigh Carbon Steel Micrograph DataBase

We present a new microstructure dataset consisting of ultrahigh carbon steel (UHCS) micrographs taken over a range of length scales under systematically varied heat treatments. Using the UHCS dataset as a case study, we develop a set of visualization tools for interacting with and exploring large microstructure and metadata datasets. Based on generic microstructure representations adapted from the field of computer vision, these tools enable image-based microstructure retrieval, as well as spatial maps of both microstructure and related metadata, such as processing conditions or properties measurements. We provide the microstructure image data, processing metadata, and source code for these microstructure exploration tools. The UHCS dataset is intended as a community resource for development and evaluation of microstructure data science techniques and for creation of microstructure data science teaching modules.

Coarsening of Inter- and Intra-granular Proeutectoid Cementite in an Initially Pearlitic 2C-4Cr Ultrahigh Carbon Steel

We have examined spheroidization and coarsening of cementite in an initially pearlitic 2C-4Cr ultrahigh carbon steel containing a cementite network. Coarsening kinetics of spheroidized cementite and growth of denuded zones adjacent to the cementite network were investigated by analyzing particle sizes from digital micrographs of water-quenched steel etched with Nital. Denuded zones grew at a rate proportional to t1/4–t1/5. Spheroidization of pearlite was completed within 90 minutes at 1073 K and 1173 K (800 °C and 900 °C), and within 5 minutes at 1243 K (970 °C). Bimodal particle size distributions were identified in most of the samples and were more pronounced at higher temperatures and hold times. Peaks in the distributions were attributed to the coarsening of intragranular and grain boundary particles at different rates. A third, non-coarsening peak of particles was present at 1073 K (800 °C) only and was attributed to particles existing prior to the heat treatment. Particle sizes were plotted vs time to investigate possible coarsening mechanisms. The coarsening exponent for the growth of grain boundary carbides was closest to 4, indicating grain boundary diffusion control. The coarsening exponent was closest to 5 for intragranular carbides, indicating suppression of volumetric diffusion (possibly due to reduced effective diffusivity because of Cr alloying) and control by dislocation diffusion.

Precipitation behavior of titanium nitride on a primary inclusion particle during solidification of bearing steel

Titanium nitride precipitation on a primary inclusion particle during solidification of bearing steel has been tracked by varying temperature in a confocal scanning violet laser microscope. Upon precipitation, an obvious growth of titanium nitride on a primary inclusion particle was observed due to the rapid solute diffusion in liquid steel. The onset of titanium nitride precipitation did not change with primary inclusion particle size, but the time of growth was greater for a smaller primary inclusion particle. Meanwhile, the particle size displayed little influence on the total precipitated amount of titanium nitride on it under the same conditions. At the later period of solidification, almost no change occurred in inclusion size, but the inclusion shape varied from circle to almost square in two-dimension, or cubic in three-dimension, to attain the equilibrium with steel.

Effects of Nb Modification and Cooling Rate on the Microstructure in an Ultrahigh Carbon Steel

In this study, two different melting methods were used to investigate effects of Nb modification on microstructure in ultrahigh carbon steel (UHCS). Nb-free and Nb-modified UHCS samples were produced by melting and resolidifying an industrially produced base UHCS with and without addition of Nb powder. Microstructure was characterized using scanning electron microscopy, X-ray diffraction, and electron dispersive spectroscopy. Equilibrium computations of phase fractions and compositions were utilized to help describe microstructural changes caused by the Nb additions. Nb combined with C to form NbC structures before and during austenite solidification, reducing the effective amount of carbon available for the other phases. Cementite network spacing in the Nb-free samples was controlled by the cooling rate during solidification (faster cooling led to a more refined network). Network spacing in the Nb-modified UHCS could be enlarged by NbC structures that formed cooperatively with austenite.

Reduction of Slag and Refractories by Aluminium in Steel and Inclusion Modification

As Al contents in Advanced High Strength Steels (AHSS) increase, the possibility exists that Al will reduce CaO and MgO from slag or refractory. Excessive Ca or Mg transfer would form solid inclusions that can cause nozzle clogging. This study documents experimental observations of reduction of CaO and MgO from slag and refractory in steels containing 2, 0.5 and 0.1 wt% Al at 1600°C or 1700°C. Mg transfer was observed in all experiments, while Ca transfer was only noticed under certain conditions and less intense when comparing with Mg transfer. These observations were consistent with considering the rate of reaction to be controlled by Mg and Ca transfer from slag/crucible to liquid steel.

Investigating the Effects of a Heat Treatment on Microstructure of an Ultrahigh Carbon Steel through SEM and In Situ CLSM studies

Without special thermomechanical processing and/or chemical additives, ultrahigh carbon steels (UHCS, 1 2.1 wt% C) develop an extensive network of brittle carbides. The network carbides can serve as initiation sites and propagation pathways for cracks. Thus, the high volume fraction of interconnected carbides will greatly reduce toughness/ductility, but does result in very good abrasion resistance. As a consequence, UHCS have been successfully used in rolling mills at least as far back as 1913 despite their brittleness. In the years following the discovery in the early 1970s that UHCS could be made superplastic, there has been significant progress in making them usable for many more applications through break-up of the carbide networks [1]. An important question is whether network breakup can only be accomplished through heavy mechanical deformation or additives. For some applications these methods are not feasible, and an alternative would be desirable.