Massachusetts Institute of Technology Department of Mechanical Engineering Cambridge, MA 02139 2.002 Mechanics and Materials II Spring 2004 Laboratory Module No. 5 Heat Treatment of Plain Carbon and Low-Alloy Steels: Effects on Macroscopic Mechanical Properties. 1 Background and Objectives Iron is one of the oldest known metals, and carbon is the cheapest and most effective alloying element for hardening iron. Iron-Carb on alloys are known as “carbon steels” and account for more than 70% of the tonnage of metallic materials used in the United States for engineering applications. Carbon is added to iron in quantities ranging from 0.04 to 2 wt% to make low, medium, and high carbon steels. The microstructure and resulting mechanical properties of these steels are amenable to modification via heat treatment, and a wide range of mechanical properties can be obtained by proper vari-ations of heating and cooling cycles. Modest amounts (up to a few wt% percent each) of costlier alloying elements such as nickel, chromium, manganese, and molybdynum can be added to the composition, resulting in “low alloy” [content] steels that possess addi-tional desirable properties, including achievability of high strength and good ductility in larger sections. In this laboratory module we will demonstrate the essential steps involved in the heat treatment of a medium carbon steel, AISI 1045 (Fe + 0.45 wt % C + 0.75 wt % Mn + 0.2 wt % Si), and a low alloy steel, AISI 4140 (Fe + 0.40 wt % C + 0.75 wt % Mn + 0.2 wt % Si + 1.0 wt % Cr + 0.2 wt % Mo), and measure macroscopic mechanical properties of the materials after different heat treatments. 12 Lab Tasks In this laboratory module we will perform the following tasks: • Introduce, in elementary terms, essential steps involved in the heat treatment of carbon and low alloy steels, and briefly discuss aspects of the underlying materials science. • Demonstrate austenitizing, annealing, quenching, and tempering heat-treating procedures for AISI 1045 and/or AISI 4140 steels. • Conduct mechanical tests on specimens of 1045 and/or 4140 steels which have been subjected to four different heat treatment procedures: 1. As-received (normalized); 2. Austenitized and slow-cooled (annealed); 3. Austenitized and water-quenched; and 4. Austenitized, water-quenched, and tempered. For each material condition and associated microstructure we will perform: 1. A tensile test, 2. A Rockwell hardness test. Also, in order to get a tangible perception of the relative properties of the three material conditions, we will manually bend rods and compare the relative loads required to initiate plastic deformation. 3. A Charpy U-notch impact test. • We will discuss the correlations between the measured mechanical properties (strength, hardness, ductility, impact energy) in the various heat-treated conditions. 23 Lab Assignment: Specific Questions to Answer 1. Describe the heat treatment processes of 1045 (or 4140) steel introduced during the laboratory session. 2. Typical tensile stress-strain behavior for 1045 steel, in the four conditions con-sidered, will be generated. From these curves for each of the four heated-treated material states, obtain the following tensile properties: (a) Young’s modulus, E; (b) Tensile yield strength (0.2 percent offset), σy ; (c) Ultimate tensile strength, σUTS; (d) The reduction in area at fracture, q ≡ (A0 − Af )/A0 where A0 ≡ original cross-sectional area, and Af ≡ final cross-sectional area of the tensile test specimens. Note that q is a measure of ductility, or plastic strain to failure. (e) Based on the Rockwell hardness data collected during the lab session, esti-mate the ultimate tensile strength σ(HRC) of the alloy in the three conditions UTS considered. To complete this task use the conversion charts attached to this handout. (f) Tabulate, for each microstructural state, the tensile properties, (E, σy , σUTS, q), the Rockwell hardness and corresponding estimated tensile strength σ(HRC) ,UTS and the Charpy U-notch impact energy (E (Charpy) ) data.U 3. Identify trends in the data. In particular, how do the moduli, strength, ductility, hardness and impact energy change with heat treatment? 4. How well does the hardness-estimated tensile strength, σ(HRC) , compare with the UTS actually-measured tensile strength, σUTS? In particular, what is the source of dis-crepancy for the data from the austenitized and quenched condition? (Hint: Does the compressive yield strength correlate better with the hardness-estimated “tensile strength” in those cases where there is a major discrepancy between the estimate and the maximum measured stress in the tensile test?) 5. Over the set of lab sections, data was obtained for the quenched-and-temp ered condition using a range of tempering temperatures (see the web page for a summary of the results). Identify and discuss trends in the resulting properties with respect to the tempering temperature. 34 Introduction to Heat Treatment of Plain Carbon and Low-Alloy Steels Additional reference material: Dowling, Section 3.3 It is perhaps presumptuous to attempt even a cursory synopsis of the vast literature surrounding the heat-treatment of carbon and low alloy steels. There are at least two levels at which the subject can be approached. One level constitutes a phenomenological description of what thermal histories are applied, along with details such as temperature levels, times of holding at temperature, maximum allowable time limits for executing “rapid” temperature changes, and technical nomenclature asso ciated with the processes. A second, and more ambitious level focuses on fundamental physical processes, equi-librium