Heat Treatment of Steels :Recrystallization annealing The carbon and alloy steels were treated at a temperature of about 700 C, which is about 20 C below the eutectoid temperature. The holding time should be long enough for recrystallization to be proceed. Heat Treatment of Steels : Spheroidize annealing The purpose of this process is to improve machinability by changing the morphology of cementite from continuous network or large plate into globular. Therefore, machining become easier. Spheroidize annealing shall be applied only to medium and high carbon steels. Ref: Linde booklet: Furnace Atmosphere No.2 Recrystallization time depends on the degree of cold deformation. The higher the degree of cold deformation, the lower recrystallization time is. 29 Ref: http://www.astonmet.com/ss/images/win_ht1207_23.html 30 Heat Treatment of Steels : Spheroidize annealing Heat Treatment of Steels : Normalizing The steel is heated to temperature just below or slightly above the eutectoid temperature. Holding for 2-6 hr followed by slow cooling The purpose of this process is to refine grain size or to produce uniform grain size. It is also used for softening the heavily cold deformed materials. It is quite similar to full annealing but the parts are cooled in air. Thus the cooling rate is faster compare to annealing. 31 32
Heat Treatment of Steels : Normalizing Normalizing is considered to be out of equilibrium since the air-cooling is considerably fast. Eutectoid temperature and composition become lower as cooling rate increase. Ar3 and Ar1 merge and full pearlitic microstructure is obtained in steels with >0.4%C A 3 A cm The purpose of quenching and tempering is to improve hardness, strength, and wear resistance of the parts. The desired microstructure is martensite or bainite or mixture of both depending on the hardness, strength, and toughness required in service. A 1 33 [Ref: Heat Treater s Guide, 1995]34 The hardness of martensite is a function of carbon contents only. The maximum hardness (fully matensite) is then a function of carbon contents in steels. The steel parts are heated to a temperature above A 3 for hypoeutectoid steel and above A 1 for hypereutectoid steel. Soaking time is about 30 minutes per 1 inch cross section. The parts are rapidly cooled (so called quench) in either brine, water, hot oil, cold oil, forced air, and still air. temperature austenitizing quenching tempering This point is important for materials selection Ref: Linde booklet: Furnace Atmosphere No.2 35 36 time
Heat Treatment of Steels : TTT and CCT Metallurgists use TTT and CCT diagrams to design the quenching severity required to achieve the target hardness value. TTT (time-temperature transformation diagram) indicates the isothermal transformation versus holding time. CCT (continuous cooling transformation diagram) indicates the transformation by the different cooling rate. 37 Ref: William D., and Jr. Callister, 2007 38 Curves on TTT Upper curves are of diffusion transformation e.g. γ α+γ γ Fe 3 C +γ γ α +Fe 3 C Lower curve are of bainitic transformation Horizontal lines are of martensitic transformation 39 40
41 42 Additions of alloying elements result in Separation of diffusion transformation and bainitic transformation curves. The position of the nose of diffusion transformation curve (upper curves). The position of the nose of bainitic transformation curve The position of M s. TTT diagram of AISI 4340 steel (JIS-SNCM 439, DIN 20CrMo5) (0.4%C, 1.0%Mn, 0.8%Cr, 0.8%Mo, 1.85%Ni) 43 Ref: William D., and Jr. Callister Important information of TTT needed to be considered The position of the nose which indicates the critical cooling rate to suppress diffusion transformation M s temperature indicating that austenite start to transform into martensite at such temperature. Fraction of martensite at a temperature below M s. This indicates that after quenching to room temperature, how much retained austenite exist in the steel. 44
Heat Treatment of Steels : CCT diagram Heat Treatment of Steels : CCT diagram CCT curves are slightly on the lower right of TTT curves Better to follow CCT since most heat treatment processes are based on continuous cooling except martempering and austempering. 45 Ref: William D., and Jr. Callister, 2007 46 Heat Treatment of Steels : CCT diagram Heat Treatment of Steels : How to read CCT %proeutectoid ferrite %pearlite %bainite Hardness TTT and CCT Diagram of AISI 4340 steel (0.4%C, 1.0%Mn, 0.8%Cr, 0.8%Mo, 1.85%Ni) Ref: William D., and Jr. Callister, 2007 47 48
Alloying elements except cobalt shift TTT to the right. This increases hardenability of the steels. Temperature Critical cooling rate A A c3 A c1 A+F+C A+F+C F+C F+C Temperature Critical cooling rate A A c3 A c1 A+F+C A+F+C F+C F+C The hardenability of high alloy steels is much better than that of carbon steels. Air-cooling is fast enough to quench the high alloy steel. Alloying elements also decrease M s and M f. Carbon strongly influences M s Ms ½ 1 2 Time Ms ½ 1 2 Time In alloy steels with >0.4%C, some retained austenite exists after quenching. HRC Distance from specimen core HRC Distance from specimen core 49 50 Ref: Linde booklet: Furnace Atmosphere No.2 As-quenched martensite is brittle because of strain accompanied with phase transformation. There is a volume expansion when austenite transforms into martensite. At high temperature Some carbon atoms are rejected from martensite and form carbide. The BCT- martensite become closer to BCC-ferrite and the hardness decreases. Retained austenite can also decompose into ferrite and carbide. These phenomena are also accompanied with the volume change. Tempering is a process used for stress-relieving of fresh martensite by heating to a temperature at a range of 150-650 C for 40-60 minutes or longer. 51 Ref:[Heat Treater s Guide] 52
Tool steels such as hot-work tool steels, cold-work tool steels, and high speed tool steels, the tempering curves exhibit secondary hardening at tempering temperatures in a range of 450-600 C. Low to medium alloy steels high alloy steels with medium carbon contents Carbon steels high alloy steels with high carbon contents Such steels need to be tempered twice or more. Ref: George E. Totten 54 Heat Treatment of Steels : Martempering Sub-zero quenching is an option treatment for high alloy steel quenching. The transformation of austenite into pearlite or bainite is delayed by addition of some alloying elements. austenitizing The purpose of this method is to remove the retained austenite. After oil or air quenching to room temperature, the parts are dipped into cryogenic media such as liquid nitrogen. The parts are then tempered afterward. temperature austenitizing quenching tempering Sub-zero quenching time 55 The high alloy steels can be isothermal holding at a temperature above M s for long time without undergoing phase transformation Large or complex high alloy steel parts can then be treated by martempering to avoid quench- cracking temperature tempering time 56
Heat Treatment of Steels : Martempering Heat Treatment of Steels : Austempering Austempering produces bainite microstructure by quenching to the temperature above M s and isothermally holding until the bainitic transformation is completed. temperature austenitizing The austempered steel exhibit high strength, high tougness, and high ductility. time quenching martempering Coil spring and disc spring are examples of steel parts treated by austempered 57 Ref: George E. Totten, 2006 58 Heat Treatment of Steels : Austempering - Not all steels can be austempered practically. - Their kinetics of bainitic transformation determined whether austempering shall be applied to such steels. Ref: George E. Totten, 2006 59 Heat Treatment of Steels : Summary of Metallurgical Factors during heat treatment Understand the concept of microstructural change due to thermal treatment Case 1 When crystallization of austenite and its transformation are the principles behind then what needed to be considered Austenitizing temperature must be proper selected depending on the chemical compositions of steels, the methods of cooling, and the final microstructure required. When austenitizing treatment is in the single austenite region such as hardening of medium carbon steels, treatment temperature and time should not be too high since austenite grain coarsening may occur. Proper cooling rate (quenching method) is required. The final microstructure required There is a limitationof miximum hardness of steel, which can be reached depending on the carbon conceration 60
Heat Treatment of Steels : Summary of Metallurgical Factors during heat treatment Case 2 When recrystallization of the same phase (α in α) is the principle behind Treatment temperature must be high enough to drive nucleation of new strain-free grains. Proper time is required but too long time has no benefit or may cause to large grain size. Only heavily cold-worked low carbon steels are treated with this method since large cold deformation is required as a drivig force for necleation. Heat Treatment of Steels : External Factors during heat treatment Oxidation Discoloration Carburization/decarburization Effectiveness of quenchant Stability of quechant temperature and its thermo-physical properties Transport of heat into/out of steel parts Parts configuration Jig Design etc 61 62 References: George E. Totten. Steel Heat Treatment Handbook : Metallurgy and Technologies, CRC Press, USA 2006 George Krauss. S T E E L S :Processing, Structure, and Performance. ASM International, USA 2005 Heat Treater s Guide: Practice and Procedure for Iron and Steel. ASM International 2 nd edition, USA 1995 H. Eallentowitz et al. Materials for future automotive body structure Business Briefing:GlobalAutomotive Manufacturing & Technology 2003 Karen Connery and Len Switzer. High Quality Heat Treatment: Atmosphere choice of critical Heat Treating Progress, September 2008 Linde booklet: Furnace Atmosphere No.1 Gas carburizing and carbonitriding. Special Edition available from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage Linde booklet: Furnace Atmosphere No.2 Neutral hardening and annealing. Special Edition available from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage Linde booklet: Furnace Atmosphere No.3 Nitriding and nitocarburizing. Special Edition available from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage William D., and Jr. Callister MATERIALS SCIENCE AND ENGINEERING: AN INTRODUCTION John Wiley & Sons, 7 th 2007 Dissemination of IT for the Promotion of Materials Science (DoITPoMS), Universiy of Cambridge, http://www.doitpoms.ac.uk/about/dissemination.html 63