Steel Heat Treatment - Metallurgy and Technologies 2nd ed - G. Totten (CRC_ 2010) WW Part 6 ppsx

When comparing the curves for the start of transformation in CCT and IT diagrams for the same heat and steel grade (Figure 6.7 and Figure 6.10), we found that in the CCT diagram the curves are slightly shifted to longer times and lower temperatures. For example, in the IT diagram of Figure 6.7, the shortest time to start the transformation of ferrite is 16 s at 650 8C (1200 8F) and the corresponding time for bainite is 9 s at 480 8C (9008F). In the CCT diagram of Figure 6.10, however, the shortest transformation start time for ferrite is 32 s at 620 8C (1150 8F) and the corresponding time for bainite is 20 s at 380 8C (716 8F). This indicates that in CCT processes the transformation starts later than in IT processes. The basis of this phenomenon is related to the incubation time and can be found in a hypothesis of Scheil [6]. It should also be noted that with higher austenitizing temperature the curves denoting the start of transformation of a particular phase can be shifted to longer times. Figure 6.11 shows the CCT diagram of DIN 16MnCr5 steel after austenitizing at 870 8C (1600 8F) (a), and after austenitizing at 1050 8C (1922 8F) (b). In the latter case, the regions of ferrite and pearlite are shifted to longer times. It is necessary, therefore, when using a CCT diagram, to ascertain that the austenitizing temperature used to develop the CCT diagram corresponds to the austeni- tizing temperature of the parts treated. A CCT diagram is developed in the following way. Many small specimens (e.g., 4-mm diameterÂ2 mm for high cooling rates, and 4.5-mm diameterÂ15 mm for medium and low cooling rates) are austenitized and cooled within a dilatometer with different cooling rates. Start and finish of transformation of relevant phases with each cooling curve are recorded and these points are connected to obtain the regions of transformation for the relevant phases (see Figure 6.10). Therefore, a CCT diagram can be read only in the way in whichitwasdeveloped, i.e.,alongthecoolingcurves.AscanbeseenfromFigure 6.10,asingle- phase structure occurs only in cases of very high cooling rates (martensite) and very slow cooling rate (pearlite). In all other cooling regimes a mixture of more structural phases results. How much of each phase such a mixture contains can be read in percentage from the numbers along the cooling curve (usually marked in CCT diagrams of German origin). The numbers at the end of each cooling curve denote the relevant hardness after quenching (usually in HRC (two-digit numbers) or in HV (three-digit numbers)). For example, as shown in Figure 6.10 for grade DIN 50CrV4 steel, if cooling proceeds at the rate marked withÂ, a mixture of 10% ferrite, 30% pearlite, 30% bainite, and (the rest) 30% martensite will result at room tempera- ture, and the hardness after quenching will be 47 HRC. It should be noted that the part of the area (region) of a phase that the cooling curve intersects is by no means a measure of the amount of transformed phase. Sometimes a CCT diagram can be supplemented with a diagram showing portions of each structural phase and hardness after quenching; see the lower part of Figure 6.12. The abscissa of this diagram denoting time enables the cooling time to 500 8C (9328F) to be determined for every cooling curve. To determine the portions of structural phases and hardness after quenching, one should follow the relevant cooling curve until its intersection with the 500 8C (932 8F) isotherm and from this point down along the vertical line read the phase portions and hardness after quenching. For example, for cooling curve C, which intersects the 500 8C (932 8F) isotherm at 135 s, the readings are 4% ferrite, 7% pearlite, 78% bainite, and 11% martensite and a hardness of 34 HRC. It should be noted that every CCT diagram is exactly valid only for the heat of a steel that was used for its construction. The influence of different heats (having slightly different compositions) of the same grade of steel on the position of transformation curves in the relevant CCT diagram, as an example, is shown in Figure 6.13. ß 2006 by Taylor & Francis Group, LLC. ∅ ∅ ∅ ∅ ∅ ∅ (a) CCT diagram and Jominy hardenability curve for AISI 3410. (From Met. Prog., October 1963, p. 134.) (b) Chart showing relationship between rate of cooling at different Jominy distances and rate of cooling in moderately agitated oil of round bars of 12.5–100-mm diameter. (From K.E. Thelning, Steel and Its Heat Treatment, 2nd ed., Butterworths, London, 1984.) ß 2006 by Taylor & Francis Group, LLC. CCT diagram for rolled steel austenitized at 8608C. (From M. Atkins, Atlas of Continuous Transformation Diagrams for Engineering Steels, British Steel Corporation, BSC Billet, Bar and Rod Product, Sheffield, U.K., 1977.) ß 2006 by Taylor & Francis Group, LLC. 8 8 The CCT diagram for the core (0.22% C) of DIN 20MoCr5 steel. (From H.J. Eckstein (Ed.), Technologie der Wa ¨ rmebehandlung von Stahl, 2nd ed., VEB Deutscher Verlag fu ¨ r Grundstoffin- dustrie, Leipzig, 1987.) The CCT diagram for the case (0.88% C) of DIN 20MoCr5 steel. (From H.J. Eckstein (Ed.), Technologie der Wa ¨ rmebehandlung von Stahl, 2nd ed., VEB Deutscher Verlag fu ¨ r Grundstoffin- dustrie, Leipzig, 1987.) ß 2006 by Taylor & Francis Group, LLC. Effect of soaking time (at different temperatures) of stress-relief annealing on the reduction of residual stresses for hardening and tempering steels. (From G. Spur and T. Sto ¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa ¨ rmebehandeln, Carl Hanser, Munich, 1987.) − − − Yield strength at elevated temperatures (for 0.2% strain) calculated according to the Larson–Miller method for three grades of alloyed structural steels for hardening and tempering (designations according to DIN). (a) Calculated values and (b) experimentally obtained values. (From G. Spur and T. Sto ¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa ¨ rmebehandeln, Carl Hanser, Munich, 1987.) ß 2006 by Taylor & Francis Group, LLC. Internal standard of the German company Edelstahlwerke Buderus A.G Wetzlar for evaluation of the degree of spheroidization after soft annealing of grade DIN 100Cr6 steel. Magnifica- tion 500Â. Amount of lamellar pearlite remaining 1, 0%; 2, 8%; 3, 20%; 4, 35%; 5, 60%; 6, 80%. (From G. Spur and T. Sto ¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa ¨ rmebehandeln, Carl Hanser, Munich, 1987.) Schematic presentation of the process of transforming cementite lamella to spheroids during soft annealing. (From K.E. Thelning, Steel and Its Heat Treatment, 2nd ed., Butterworths, London, 1984.) ß 2006 by Taylor & Francis Group, LLC. . comparing the curves for the start of transformation in CCT and IT diagrams for the same heat and steel grade (Figure 6. 7 and Figure 6. 10), we found that in the CCT diagram the curves are slightly. the parts treated. A CCT diagram is developed in the following way. Many small specimens (e .g. , 4-mm diameterÂ2 mm for high cooling rates, and 4.5-mm diameterÂ15 mm for medium and low cooling rates). stress-relief annealing on the reduction of residual stresses for hardening and tempering steels. (From G. Spur and T. Sto ¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa ¨ rmebehandeln,