Steel Heat Treatment - Metallurgy and Technologies 2nd ed - G. Totten (CRC_ 2007) 8 pdf

Increased ductility and toughness as well as increased bendability and fatigue life are the strongest reasons to apply austempering instead of hardening and tempering. Figure 6.150 shows the relation of impact toughness and Brinell hardness (HB) of a Cr–Mn–Si steel after conventional hardening and tempering and after austempering, as a function of tempering temperature and austempering temperature, respectively. The most important difference is that a good combination of hardness and toughness after conventional hardening and tempering is possible only at high tempering temperatures, which means low hardness, whereas at austempering a good combination of hardness and impact toughness may be achieved at high hardness values. Another comparison of impact toughness of a carbon steel after hardening and tempering and after austempering, as a function of hardness, is shown in Figure 6.151. It is evident that austempering yields much better impact toughness, especially at high hardness, around 50 HRC. It is necessary to emphasize that high toughness after austempering is possible only under conditions of complete transformation of austenite to bainite. Table 6.7 shows a comparison of some mechanical properties of austempered and of hardened and tempered bars made of AISI 1090 steel. In spite of having a little higher tensile strength and hardness, austempered specimens have had remarkably higher elongation, reduction of area, and fatigue life. Figure 6.152 shows the fatigue diagram of DIN 30SiMnCr4 steel after conventional hardening and tempering and after austempering. The increase in fatigue resistance values is especially remarkable for notched specimens. Regarding bendability, Figure 6.153, from an early work of Davenport [30], shows the results of bending a carbon steel wire austempered and hardened and tempered to 50 HRC. When selecting a steel for austempering, IT diagrams should be consulted. The suitability of a steel for austempering is limited first of all with minimum incubation time (the distance of the transformation start curve from the ordinate). Another limitation may be the very long transformation time. Figure 6.154 shows the transformation characteristics of four AISI grades of steel in relation to their suitability for austempering. The AISI 1080 steel has only limited suitability for austempering (i.e., may be used only for very thin cross sections) Austempering Austempering temperature, ЊC Hardness HB Impact toughness, kg/cm 2 600 550 500 450 400 350 300 250 200 14 12 10 8 6 4 2 0 250 300 350 400 Impact toughness Hardness Hardening and tempering Tempering temperature, ЊC 300 400 500 550 600 Hardness Impact toughness FIGURE 6.150 Impact toughness and hardness (HB) of five heats of a Cr–Mn–Si steel after conven- tional hardening and tempering and after austempering, as a function of tempering temperature and austempering temperature, respectively. (From F.W. Eysell, Z. TZ Prakt. Metallbearb. 66:94–99, 1972 [in German].) ß 2006 by Taylor & Francis Group, LLC. Increased ductility and toughness as well as increased bendability and fatigue life are the strongest reasons to apply austempering instead of hardening and tempering. Figure 6.150 shows the relation of impact toughness and Brinell hardness (HB) of a Cr–Mn–Si steel after conventional hardening and tempering and after austempering, as a function of tempering temperature and austempering temperature, respectively. The most important difference is that a good combination of hardness and toughness after conventional hardening and tempering is possible only at high tempering temperatures, which means low hardness, whereas at austempering a good combination of hardness and impact toughness may be achieved at high hardness values. Another comparison of impact toughness of a carbon steel after hardening and tempering and after austempering, as a function of hardness, is shown in Figure 6.151. It is evident that austempering yields much better impact toughness, especially at high hardness, around 50 HRC. It is necessary to emphasize that high toughness after austempering is possible only under conditions of complete transformation of austenite to bainite. Table 6.7 shows a comparison of some mechanical properties of austempered and of hardened and tempered bars made of AISI 1090 steel. In spite of having a little higher tensile strength and hardness, austempered specimens have had remarkably higher elongation, reduction of area, and fatigue life. Figure 6.152 shows the fatigue diagram of DIN 30SiMnCr4 steel after conventional hardening and tempering and after austempering. The increase in fatigue resistance values is especially remarkable for notched specimens. Regarding bendability, Figure 6.153, from an early work of Davenport [30], shows the results of bending a carbon steel wire austempered and hardened and tempered to 50 HRC. When selecting a steel for austempering, IT diagrams should be consulted. The suitability of a steel for austempering is limited first of all with minimum incubation time (the distance of the transformation start curve from the ordinate). Another limitation may be the very long transformation time. Figure 6.154 shows the transformation characteristics of four AISI grades of steel in relation to their suitability for austempering. The AISI 1080 steel has only limited suitability for austempering (i.e., may be used only for very thin cross sections) Austempering Austempering temperature, ЊC Hardness HB Impact toughness, kg/cm 2 600 550 500 450 400 350 300 250 200 14 12 10 8 6 4 2 0 250 300 350 400 Impact toughness Hardness Hardening and tempering Tempering temperature, ЊC 300 400 500 550 600 Hardness Impact toughness FIGURE 6.150 Impact toughness and hardness (HB) of five heats of a Cr–Mn–Si steel after conven- tional hardening and tempering and after austempering, as a function of tempering temperature and austempering temperature, respectively. (From F.W. Eysell, Z. TZ Prakt. Metallbearb. 66:94–99, 1972 [in German].) ß 2006 by Taylor & Francis Group, LLC. 7 Heat Treatment with Gaseous Atmospheres Johann Grosch CONTENTS 7.1 General Introduction 415 7.2 Fundamentals in Common 417 7.3 Carburizing 422 7.3.1 Introduction 422 7.3.2 Carburizing and Decarburizing with Gases 422 7.3.2.1 Gas Equilibria 423 7.3.2.2 Kinetics of Carburizing 426 7.3.2.3 Control of Carburizing 428 7.3.2.4 Carbonitriding 431 7.3.3 Hardenability and Microstructures 432 7.4 Reactions with Hydrogen and with Oxygen 440 7.5 Nitriding and Nitrocarburizing 446 7.5.1 Introduction 446 7.5.2 Structural Data and Microstructures 448 7.5.2.1 Structural Data 448 7.5.2.2 Microstructures of Nitrided Iron 450 7.5.2.3 Microstructures of Nitrided and Nitrocarburized Steels 452 7.5.2.4 Microstructural Specialties 456 7.5.3 Nitriding and Nitrocarburizing Processes 457 7.5.3.1 Nitriding 457 7.5.3.2 Nitrocarburizing 460 7.5.3.3 Processing Effects on the Nitriding and Nitrocarburizing Results 461 7.6 Properties of Carburized and Nitrided or Nitrocarburized Components 463 References 469 7.1 GENERAL INTRODUCTION Heat treatment of components is to date mostly accomplished in gaseous atmospheres, the more so if plasma and vacuum are regarded as special cases of gaseous atmospheres. In comparison, heat treatment in solid or liquid media is negligible in numbers. Heat treatment in gaseous atmospheres falls into two categories: processes wi th the aim of avoiding a mass transfer between the gaseous atmosphere and the material, and processes with the aim of achieving just such a transfer. Mass transfer occurs when there is a difference in the potential between the constituents of a gaseous atmosphere and tho se of the microstructure of a component. The direction of such a mass transfer is determined by the potential difference, which leaves two fund amental possibilities with regard to the component. One is the intake ß 2006 by Taylor & Francis Group, LLC. Temperature (ЊC) Coefficient of diffusion (m 2 /s) 1500 400 01 2 3 4 10 −29 10 −24 10 −19 10 −14 10 −9 10 −4 5 200 100 25 0 50 Hydrogen Interstitial atoms N, C Substitutional atoms 10 3 T 1 K FIGURE 7.1 Diffusion coefficients of hydrogen and of interstitial and substitional elements in a-iron. (From E. Hornbogen, Werkstoffe, 2nd ed., Springer-Verlag, Berlin, 1979.) 0 1.0 2.0 3.0 4.0 10 −26 10 −24 10 −22 10 −20 10 −18 10 −16 10 −14 10 −12 10 −10 10 −8 10 −6 10 −4 10 −28 10 −26 10 −24 10 −22 10 −20 10 −18 10 −16 10 −14 10 −12 10 −10 10 −8 10 −6 D O and D C (α-Fe) (m 2 /s) D N (α-Fe) (m 2 /s) 0 in α-Fe 10 3 T 1 K FIGURE 7.2 Diffusion coefficients of C, N, and O in a-iron. (From Th. Heumann, Diffusion in Metallen, Springer-Verlag, Berlin, 1992.) ß 2006 by Taylor & Francis Group, LLC. . austempered and of hardened and tempered bars made of AISI 1090 steel. In spite of having a little higher tensile strength and hardness, austempered specimens have had remarkably higher elongation, reduction. AISI 1090 steel. In spite of having a little higher tensile strength and hardness, austempered specimens have had remarkably higher elongation, reduction of area, and fatigue life. Figure 6.152. specimens. Regarding bendability, Figure 6.153, from an early work of Davenport [30], shows the results of bending a carbon steel wire austempered and hardened and tempered to 50 HRC. When selecting a steel