Steel Heat Treatment - Metallurgy and Technologies 2nd ed - G. Totten (CRC_ 2010) WW Part 3 pdf

stage, the recrystallization of martensite lath is more complete, and there is a tendency for the formation of equiaxed grains and extensive grain growth. 2.4.2 A USTENITE F ORMATION The early work on austenite formation by Robert and Mehl [35] focused on the nucleation of austenite from a ferrite–pearlite mixture. Various researchers have since shown the complex- ity of austenite formation from a two-phase mixture of ferrite and cementite and have made attempts at modeling austenite formation as a function of composition and microstructure [36–41]. On heating steel, with a spheroidized ferrite þcementite mixture, the austenite phase nucleates at the ferrite–cementite boundary. With further heating, the austenite phase con- sumes the cementite and then grows into ferrite through diffusion-controlled growth. In a pearlitic microstructure, the austenite may nucleate in the cementite and grow into the colony by dissolving both ferrite and cementite. A typical micrograph of austenite growth into a pearlite colony is shown in Figure 2.12a. A schematic illustration of the growth is shown in Figure 2.12b. Recent work has shown that it is possible to model the above phenomenon with computational tools [42]. It is possible to construct TTT diagrams for the austenite growth for any steel to evaluate the effect of the initial microstructure. These diagrams do not show a C-curve behavior because both the driving force for austenite formation and the diffusivity increase with temperature. This results in monotonic acceleration of austenite formation as the temperature increases. The rate of austenite formation also increases with the presence of residual austenite in the microstructure, as demonstrated by Yang and Bhadeshia, who studied the austenite growth kinetics in a bainitic microstructure [43,44]. The microstructure contained residual austenite films, and there was no requirement for nucleation of austenite and the austenite films grew with an increase in the temperature. After the completion of austenite formation, continued heating leads to grain growth of austenite. The grain growth is also affected by the presence of fine carbonitride precipitates. With the presence of these precipitates the grain boundaries are pinned and therefore, grain growth characteristics are sluggish. However, on heating above the dissolution temperature of these precipitates, the austenite grain may coarsen at an accelerated rate [45,46]. Most of the austenite formation from ferrite occurs by a diffusion-controlled reconstructive transformation mechanism. However, at rapid rates, the transformation of ferrite to austenite may occur by interface controlled or by displacive transformation [47]. 2.5 SUMMARY OF STEEL MICROSTRUCTURE EVOLUTION An overview of all microstructure evolution through reconstructive or displacive transform- ation mechanisms during heating and cooling of steel can be classified as shown in Figure 2.13 [4]. Reconstructive transformation involves substitutional diffusion. The reconstruction of a parent lattice into a product lattice occurs through a noncoordinated motion of atoms across the interface. The growth mostly occurs by nucleation and growth of product phases. Reconstructive transformations are slow below 6008C. The formation of allotriomorphic ferrite, idiomorphic ferrite, massive ferrite, pearlite, carbide, and austenite all belong to the category of reconstructive transformation. Displacive transformation involves coordinated atom, causing a change from a parent crystal to a product crystal. This change is achieved by IPS deformation with a large shear component, leading to a plate or lath shape. During this transformation, the substitutional atoms do not diffuse. However, displacive transformations occurring at high temperatures (above M s ) may involve varying amounts of interstitial carbon diffusion during nucleation and growth. In the case of Widmansta ¨ tten ferrite formation, both nucleation and growth involve carbon diffusion. In contrast, in bainitic ferrite formation, ß 2006 by Taylor & Francis Group, LLC. stage, the recrystallization of martensite lath is more complete, and there is a tendency for the formation of equiaxed grains and extensive grain growth. 2.4.2 A USTENITE F ORMATION The early work on austenite formation by Robert and Mehl [35] focused on the nucleation of austenite from a ferrite–pearlite mixture. Various researchers have since shown the complex- ity of austenite formation from a two-phase mixture of ferrite and cementite and have made attempts at modeling austenite formation as a function of composition and microstructure [36–41]. On heating steel, with a spheroidized ferrite þcementite mixture, the austenite phase nucleates at the ferrite–cementite boundary. With further heating, the austenite phase con- sumes the cementite and then grows into ferrite through diffusion-controlled growth. In a pearlitic microstructure, the austenite may nucleate in the cementite and grow into the colony by dissolving both ferrite and cementite. A typical micrograph of austenite growth into a pearlite colony is shown in Figure 2.12a. A schematic illustration of the growth is shown in Figure 2.12b. Recent work has shown that it is possible to model the above phenomenon with computational tools [42]. It is possible to construct TTT diagrams for the austenite growth for any steel to evaluate the effect of the initial microstructure. These diagrams do not show a C-curve behavior because both the driving force for austenite formation and the diffusivity increase with temperature. This results in monotonic acceleration of austenite formation as the temperature increases. The rate of austenite formation also increases with the presence of residual austenite in the microstructure, as demonstrated by Yang and Bhadeshia, who studied the austenite growth kinetics in a bainitic microstructure [43,44]. The microstructure contained residual austenite films, and there was no requirement for nucleation of austenite and the austenite films grew with an increase in the temperature. After the completion of austenite formation, continued heating leads to grain growth of austenite. The grain growth is also affected by the presence of fine carbonitride precipitates. With the presence of these precipitates the grain boundaries are pinned and therefore, grain growth characteristics are sluggish. However, on heating above the dissolution temperature of these precipitates, the austenite grain may coarsen at an accelerated rate [45,46]. Most of the austenite formation from ferrite occurs by a diffusion-controlled reconstructive transformation mechanism. However, at rapid rates, the transformation of ferrite to austenite may occur by interface controlled or by displacive transformation [47]. 2.5 SUMMARY OF STEEL MICROSTRUCTURE EVOLUTION An overview of all microstructure evolution through reconstructive or displacive transform- ation mechanisms during heating and cooling of steel can be classified as shown in Figure 2.13 [4]. Reconstructive transformation involves substitutional diffusion. The reconstruction of a parent lattice into a product lattice occurs through a noncoordinated motion of atoms across the interface. The growth mostly occurs by nucleation and growth of product phases. Reconstructive transformations are slow below 6008C. The formation of allotriomorphic ferrite, idiomorphic ferrite, massive ferrite, pearlite, carbide, and austenite all belong to the category of reconstructive transformation. Displacive transformation involves coordinated atom, causing a change from a parent crystal to a product crystal. This change is achieved by IPS deformation with a large shear component, leading to a plate or lath shape. During this transformation, the substitutional atoms do not diffuse. However, displacive transformations occurring at high temperatures (above M s ) may involve varying amounts of interstitial carbon diffusion during nucleation and growth. In the case of Widmansta ¨ tten ferrite formation, both nucleation and growth involve carbon diffusion. In contrast, in bainitic ferrite formation, ß 2006 by Taylor & Francis Group, LLC. 0.1 300 400 500 600 Temperature (8C) 700 800 900 10 Time (s) Grain size: 6.0 ASTM Austenitization: 819.61 C 1000 Ferrite (0.1%) Pearlite (0.1%) Pearlite (99.9%) Bainite (0.1%) Bainite (99.9%) 100.0 C/s 10.0 C/s 1.0 C/s 0.1 C/s Composition (wt%): Fe: 97.93 Mn: 1.45 Si: 0.25 C: 0.37 Transitions: Pearlite: 710.69 C Bainite: 565.04 C Ferrite: 769.62 C Martensite: 337.78 C CCT HSLAS(b) Grain size: 6.0 ASTM Austenitization: 819.61 C Composition (wt%): Fe: 97.93 Mn: 1.45 Si: 0.25 C: 0.37 Transitions: Pearlite: 710.68 C Bainite: 565.04 C Ferrite: 769.61 C Martensite: 337.77 C Ferrite (0.1%) Pearlite (0.1%) Pearlite (99.9%) Bainite (0.1%) Bainite (99.9%) 0.1 300 400 500 600 Temperature (8C) 700 800 10 Time (s) TTT HSLAS(a) 1000 TTT CCT B a M a ORNL 10 −1 10 0 10 1 10 2 Time (s) Temperature (K) 600 700 800 900 1000 1100 10 3 10 4 (c) FIGURE 2.20 Calculated (a) time temperature transformation (TTT) diagram and (b) continuous cooling transformation (CCT) diagram for a steel using JMatPro 1 software for an Fe–0.37C–0.25Si– 1.45Mn steel. (c) TTT and CCT transformation diagram is calculated using the tool found on the Internet at http:== engm01.ms.ornl.gov. ß 2006 by Taylor & Francis Group, LLC. 3 Fundamental Concepts in Steel Heat Treatment Alexey V. Sverdlin and Arnold R. Ness CONTENTS 3.1 Introduction 122 3.2 Crystal Structure and Phases 122 3.2.1 Crystal Structure of Pure Iron 122 3.2.2 Iron–Carbon Equilibrium Diagram 123 3.2.2.1 Metastable Fe–Fe 3 C Equilibrium Diagram 123 3.2.2.2 Stable Fe–C Equilibrium Diagram 124 3.2.3 Effect of Carbon 125 3.2.4 Critical (Transformation) Temperatures 127 3.3 Structural Transformations in Steel 128 3.3.1 Austenite–Pearlite Transformation 128 3.3.2 Structure of Pearlite 129 3.3.3 Transformation of Austenite in Hypo- and Hypereutectoid Steels 130 3.3.4 Martensite Transformation 131 3.3.5 Morphology of Ferrous Martensites 133 3.3.6 Bainite Transformation 134 3.3.7 Morphology of the Bainite Transformation 135 3.3.8 Tempering 135 3.4 Kinetics of Austenite Transformation 137 3.4.1 Isothermal Transformation Diagrams 137 3.4.2 Continuous-Cooling Transformation Diagrams 138 3.4.2.1 Transformations That Take Place under Continuous Cooling of Eutectoid Steels 139 3.4.2.2 Transformations of Austenite on Cooling in the Martensite Range 141 3.4.3 Derivation of the Continuous-Cooling Transformation Diagram from the Isothermal Transformation Diagram 142 3.4.4 Continuous-Cooling Transformation Diagram as a Function of the Bar Diameter 143 3.4.5 Definition of Hardenability 145 3.5 Grain Size 146 3.5.1 Structure of Grain Boundaries 146 3.5.1.1 Structural Models 147 3.5.2 Determination of Grain Size 149 3.5.3 Austenite Grain Size Effect and Grain Size Control 150 3.5.4 Grain Size Refinement 152 3.6 Strengthening Mechanism in Steel 153 ß 2006 by Taylor & Francis Group, LLC. 4 Effects of Alloying Elements on the Heat Treatment of Steel Alexey V. Sverdlin and Arnold R. Ness CONTENTS 4.1 Effects of Alloying Elements on Heat Treatment Processing of Iron–Carbon Alloys 166 4.1.1 g- and a-Phase Regions 166 4.1.2 Eutectoid Composition and Temperature 169 4.1.3 Distribution of Alloying Elements 171 4.1.4 Alloy Carbides 172 4.2 Effect of Alloying Elements on Austenite Transformations 174 4.2.1 Influence of Alloying on Ferrite and Pearlite Interaction 175 4.2.2 Effect on Martensite Transformation 177 4.2.3 Retained Austenite 179 4.2.4 Effect on Bainite Transformation 181 4.2.5 Transformation Diagrams for Alloy Steels 183 4.3 Hardening Capacity and Hardenability of Alloy Steel 185 4.3.1 Hardness and Carbon Content 185 4.3.2 Microstructure Criterion for Hardening Capacity 187 4.3.3 Effect of Grain Size and Chemical Composition 189 4.3.4 Boron Hardening Mechanism 193 4.3.5 Austenitizing Conditions Affecting Hardenability 195 4.4 Tempering of Alloy Steels 196 4.4.1 Structural Changes on Tempering 196 4.4.2 Effect of Alloying Elements 197 4.4.3 Transformations of Retained Austenite (Secondary Tempering) 198 4.4.4 Time–Temperature Relationships in Tempering 199 4.4.5 Estimation of Hardness after Tempering 199 4.4.6 Effect of Tempering on Mechanical Properties 200 4.4.7 Embrittlement during Tempering 201 4.5 Heat Treatment of Special Category Steels 201 4.5.1 High-Strength Steels 201 4.5.2 Boron Steels 202 4.5.3 Ultrahigh-Strength Steels 202 4.5.4 Martensitic Stainless Steels 204 4.5.5 Precipitation-Hardening Steels 205 4.5.5.1 Structural Steels 205 4.5.5.2 Spring Steels 206 4.5.5.3 Tool Steels 207 4.5.5.4 Heat-Resistant Alloys 208 4.5.6 Transformation-Induced Plasticity Steels 208 ß 2006 by Taylor & Francis Group, LLC. . Martensite Transformation 131 3. 3.5 Morphology of Ferrous Martensites 133 3. 3.6 Bainite Transformation 134 3. 3.7 Morphology of the Bainite Transformation 135 3. 3.8 Tempering 135 3. 4 Kinetics of Austenite. Transformations in Steel 128 3. 3.1 Austenite–Pearlite Transformation 128 3. 3.2 Structure of Pearlite 129 3. 3 .3 Transformation of Austenite in Hypo- and Hypereutectoid Steels 130 3. 3.4 Martensite. 137 3. 4.1 Isothermal Transformation Diagrams 137 3. 4.2 Continuous-Cooling Transformation Diagrams 138 3. 4.2.1 Transformations That Take Place under Continuous Cooling of Eutectoid Steels 139 3. 4.2.2