Materials Week '97: Wednesday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Wednesday morning, September 17.

THERMOMECHANICAL PROCESSING AND MECHANICAL PROPERTIES OF HYPEREUTECTOID STEELS AND CAST IRONS: Session I: Processing of Ultrahigh-Carbon Steels and Cast Irons

Sponsored by: SMD Structural Materials Committee

Program Organizers: Donald R. Lesuer, Chol K. Syn, Lawrence Livermore National Laboratory, P.O. Box 808, L-342, Livermore, CA 94550; Oleg D. Sherby, Stanford University, Dept. Materials Science & Eng., Stanford, CA 94305

Room: 212

Session Chairperson: Ray Decker, USP Holdings, 717 E. Huron, Ann Arbor, MI 48104; Gordon Geiger, Qualitech Steel Corporation, 301 Merchant Bank Bldg., 11 So. Meridian St., Indianapolis, IN 46204

INTRODUCTORY REMARKS: Donald Lesuer, Lawrence Livermore National Laboratory, Livermore, CA

8:35 am KEYNOTE, INVITED

THE HISTORY OF ULTRAHIGH CARBON STEELS: Jeffrey Wadsworth¹, Oleg D. Sherby², ¹Lawrence Livermore National Laboratory, P.O. Box 808, L-342, Livermore, CA 94550; ² Stanford University, Dept. Materials Science & Eng., Stanford, CA 94305

The history and development of Ultrahigh Carbon Steels (i.e., steels containing between 1 and 2.1% C and now known as UHCS) are described. The early use of steel compositions containing carbon contents above the eutectoid level is found in ancient weapons from around the world. For example, both Damascus and Japanese sword steels are examples of hypereutectoid steels. Their manufacture and processing is of interest in developing an understanding of the role of carbon content in the development of modern steels. Although sporadic examples of UHCS compositions are found in the early part of this century, it was not until the mid 1970s that the modern study began. This study had its origin in the development of superplastic behavior in steels and the recognition that increasing the carbon content was of importance in developing that property. The compositions that were optimal for superplasticity involved the development of steels that contained higher carbon contents than conventional modern steels. It was discovered, however, that the room temperature properties of these compositions were of interest in their own right. Following this discovery a period of intense work began on understanding their manufacture, processing, and properties. The development of laminated composites containing UHCS was an important part of this history.

9:05 am INVITED

EFFECT OF Al-ADDITION ON MICROSTRUCTURAL REFINEMENT OF ULTRA-HIGH CARBON STEELS: Dong-Wha Kum, Korean Institute of Science and Technology, P.O. Box 131, Cheongryang, 130-650, Seoul, Korea

Small amounts of Al-addition to ultra-high carbon steels leads to finer grain size during thermomechanical processing, and is known to improve superplastic property of the steels. In order to understand the refining effect, 0.32-1.75 wt%Al were added to 1.2%C + 1.5%Cr steel and its effect during hot and warm working has been studied. The grain size of prior austenite after hot working decreased with Al-addition. The interlamellar spacing of pearlite was studied by isothermal transformation experiments at undercoolings of 50 to 130°C below the A_l temperature. The interlamellar spacing also decreased with Al contents at undercoolings of 100°C and 130°C, while it was independent of Al at the undercooling of 50°C. The role of Al will be interpreted by its partitioning in boundaries and ferrite.

9:25am

HOT WORKABILITY OF HYPEREUTECTOID TOOL STEELS: H.J. McQueen¹ and C. Imbert², ¹Dept. of Mechanical Engineering, Concordia University, 1455 De Maisonneuve Blvd. W., Montreal, Quebec H3G 1M8, Canada; ²University of West Indies, St. Augustine, Trinidad

The four tool steels M2, D2, A2 and W1 with a wide range of alloying additions have been subjected to torsion testing over the ranges 900-1200°C and 0.1 to 4 s^-1 in order to determine the variation of strength and ductility expressed by suitable constitutive equations. Microscopic examination clarified the role of alloy carbides and confirmed the occurrence of dynamic recrystallization resulting in grain size dependence on temperature and strain rate and correlation with the flow stress developed. In addition, simulation of multistage rolling was simulated and softening in interpass intervals was determined. After summarizing, the above characteristics of these hypereutectoid steels with quite high carbide content over the working range, comparisons will be made on one hand with HSLA, low and medium carbon steels and on the other hand with austenitic stainless steels strengthened principally by solutes.

9:45 am

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF AN ULTRAHIGH-CARBON STEEL PROCESSED BY THE DIVORCED EUTECTOID TRANSFORMATION: B. Walser¹, T. Oyama², U. Ritter², O.D. Sherby³, ¹Sulzer Brothers, Inc., Winterthur, Switzerland; ²WESGO, General Telephone and Electric, 477 Harbor Blvd., Belmont, CA 94002; ³Stanford University, Dept. Materials Science & Eng., Stanford, CA 94305

Fine spheroidized structures in a 1.5%C ultrahigh carbon steel (UHCS-1.5C) were achieved by processing routes involving hot and warm working (HWW) and the divorced eutectoid transformation (DET). These procedures eliminate the need for isothermal working required in previous processing of UHC steels. The divorced eutectoid transformation (DET) is accomplished by heating for about 60 minutes at 790°C, followed by air cooling. If deformation accompanies the air cooling step, then it is a DETWAD (Divorced Eutectoid Transformation With Associated Deformation). Quantitative scanning and transmission electron microscopy studies were performed to evaluate microstructures. It is shown that HWW or HWW + DETWAD processing results in superplastic behavior of the UHCS-1.5C material at 700°C because of the fine ferrite grain size present (~1.5 µm). The HWW + DET processed material has a ferrite grain size of 6 µm and is not superplastic at 700°C. Its room temperature properties, however, are impressive, exhibiting a tensile strength of 790 MPa and 35% elongation to failure.

10:00 am BREAK

10:20 am

THERMOMECHANICAL PROCESSING OF AUSTEMPERED DUCTILE IRON: J.D. DeLa'o, C.M. Burke, D.J. Moore, K.B. Rundman, Michigan Technological University, Metallurgical Engineering, 1400 Townsend, Houghton, MI 49931-1295

Austempered ductile cast iron (ADI) derives its beneficial properties from an ausferrite matrix (stable austenite with fine acicular ferrite) formed in a transformation that is similar to bainite formation in austempered steels. The isothermal treatment of metastable austenite at the austempering temperature lends itself to an ausforming process. To explore the potential benefits of ausforming ADI, a rolling operation was introduced into the austempering schedule at a point following the quench but preceding any significant formation of ausferrite. Variable alloy chemistries, austenitizing and austempering temperatures, austempering times and degrees of deformation were investigated. Results indicate that ausforming provides significant kinetic and microstructural benefits leading to marked simultaneous increases in yield strength and ductility for all conditions studied. A preliminary study of the metal working parameters relevant to ausforming ADI was conducted and a means of fabricating ausformed ADI components is suggested. An overview of the work is presented.

10:40 am

THE EFFECT OF ALLOYING ADDITIONS ON THE FORMABILITY OF LEDEBURITE OF WHITE CAST IRONS: Yelena Pirogov, State Metallurgical Academy of Ukraine, Ukraine

The present investigation has been made to study the plastic behavior of white cast irons with respect to alloying additions of vanadium and chromium. Annealed cast iron flat bars were hot rolled at 1050°C by reduction range from 25% to 65% at strain rates of 30 s^-1. The deformed specimens were examined using microstructure analysis and X-ray diffractometry. Dynamic structure forming mechanisms of austenite and cementite have been studied. Evolution of eutectic cementite crystallographic textures as a function of alloying additions and degree of plastic working has been determined. Mechanism of enhanced formability of vanadium alloyed white cast irons has been found.

11:00 am

AUSTEMPERING OF Mo ALLOYED DUCTILE IRONS: S. Yazdani, R. Elliott, University of Manchester and UMIST, Grosvenor St., Manchester, M17HS, UK

Measurements of ultimate tensile strength, 0.2% proof stress, elongation, hardness and impact energy are reported during austempering at 400, 375, 320 and 285°C after austenitizing at 870°C for 1- 4320 minutes for ductile irons with the chemical composition 3.55% C, 2.72% Si, 0.25% Mn, 0.25% Cu and variable Mo content in the range 0.13-0.45%. X-ray diffraction and optical microscopy were used to determine the high carbon austenite content, carbon content of austenite, untransformed austenite volume and stage I and stage II austempering kinetics. A microstructural model was used to define the heat treatment processing window and the variation of the window with austempering temperature was established. The effect of Mo on the kinetics of austempering and mechanical properties are reported. The austempering processing window is shown to be open for all the austempering temperatures studied and the optimum properties correspond with the defined window. It is shown that excellent ductility combined with high strength can be attained well in excess of the ASTM A897M:1990 standard by controlling the heat treatment parameters for a low Mo content composition.

11:20 am

THERMOMECHANICAL PROCESSING AND PROPERTIES OF A DUCTILE IRON: Chol K. Syn¹, Donald R. Lesuer¹, Oleg D. Sherby²; ¹Lawrence Livermore National Laboratory, P.O. Box 808, L-342, Livermore, CA 94550; ²Stanford University, Dept. Materials Science & Eng., Stanford, CA 94305

Thermo-mechanical processing of ductile irons is a potential method for enhancing their mechanical properties. A ductile cast iron containing 3.6%C, 2.6%Si and 0.045% Mg was continuously hot-and-warm rolled or one-step press-forged from a temperature in the austenite range (900°C-1100°C) to a temperature below the A₁ temperature. Various amounts of reduction were used (from 60% to more than 90%) and then a short heat treatment at 600°C was given. The heat treatment lead to a structure of fine graphite in a matrix of ferrite and carbides. The hot-and-warm worked materials developed a pearlitic microstructure while the press-forged materials developed a spheroidite-like carbide microstructure in the matrix. Tensile properties including tensile strength and total elongation were measured along the directions parallel and transverse to the rolling direction and along the direction transverse to the press-forging direction. The tensile ductility and strength both increased with a decrease in the amount of hot-and-warm working. The press-forged materials showed higher strength (645 Mpa) than the hot-and-warm worked materials (575 MPa) when compared at the same ductility level (22% elongation).

11:40 am

A COMPARISON OF MECHANICAL BEHAVIOR IN PEARLITIC AND SPHEROIDIZED HYPEREUTECTOID STEELS: Eric M. Taleff¹, Chol K. Syn², Donald R. Lesuer² and Oleg D. Sherby³, ¹The University of Texas at Austin, Dept. of Aerospace Eng. & Eng. Mechanics, CO600, Austin, TX 78712; ²Lawrence Livermore National Laboratory, , P.O. Box 808, L-342, Livermore, CA 94550; ³Stanford University, Dept. Materials Science & Eng., Stanford, CA 94305

Hypereutectoid steels can exhibit remarkable mechanical properties at room temperature, principally because of the ability to develop fine microstructures through thermomechanical processing. Use of the divorced eutectoid transformation (DET) allows the development of fine, equiaxed microstructures with spheroidized carbide particles. The hypereutectoid austenite-cementite to pearlite transformation provides a thermal processing method which can produce pearlitic microstructures with very fine interlamellar spacings. A combination of DET and thermal processing is shown to create extremely fine microstructures containing controlled amounts of both spheroidized carbides and pearlite with controlled interlamellar spacings. Tension tests have been conducted on such microstructures, and the resulting strengths have proven to be predictable based on several microstructural parameters. Most remarkable is that both spheroidized and pearlitic microstructures are shown to obey the same predictive relation.