Form rmat atio ion n of ne need edle leli like ke fe ferr rrit itee at gr grai ain n bo boun unda dari ries es af afte terr no norm rmal aliz izin ing g of th thee un unal allo loye yed d FIGURE FIG URE 6.6 6.62 2 Fo steell DI stee DIN N C3 C35, 5, be beca caus usee of to too o fa fast st a co cool olin ing g ra rate te.. Ma Magn gnif ific icat atio ion n 50 500 0 Â. (From G. Spur and T. Sto¨ ferle ¨ rmebehandeln Handbuch h der Fertigungstechnik Fertigungstechnik,, Vol. 4/2, Wa¨ rmebehandeln, Ca (Eds.), Handbuc Carl rl Ha Hans nser er,, Mu Muni nich ch,, 198 1987. 7.))
air. On the other hand, as s hown in Figure 6.64, 6.64, the alloyed steel DIN 55NiCrMoV6 cooled in the same way in air will transform to martensite and bainite. In this case, to obtain a desired structure and hardness after normalizing, a much slower cooling of about 10 C/h (50 F/h), i.e., furnace cooling, has to be applied from the austenitizing temperature to the temperature at which the formation of pearlite is finished ( %600 C (%1100 F)). 8
8
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6.2.3
8
ISOTHERMAL ANNEALING
Hypoeutectoid low-carbon steels for carburizing as well as medium-carbon structural steels for hardening and tempering are often isothermally annealed, for best machinability, because
1000 Hardness HV 900 Ac3
800 700
C , e 600 r u t a r 500 e p m400 e T Њ
Austenite
10 3 1 35 10
85
80
1 0
Bainite 2 15
M s
55
d i a m . = 6 1 0 0 0 0 0 m m
85
20
300 200
20
15
Ac1
45
40 ite 30 Ferr te ite 60 0 Pearl te 70 7
3 0
7 5
1 5 0
3 0 0
3
Martensite
100 722
0 Q1
702 654 576 438 348
101
1 Time, s
278
244
102 1
2
228
213
103 4
8 15 min
174
104
105
60 1 2
4
8
16 24
h
CCT T di diag agra ram m of th thee un unal allo loye yed d st stee eell DI DIN N Ck Ck45 45 (a (aus uste teni niti tizi zing ng te temp mper erat atur uree 85 850 0 C) C),, wi with th FIGURE FIG URE 6.6 6.63 3 CC 8
superi supe rimp mpos osed ed co cool olin ing g cu curv rves es me meas asur ured ed in th thee co core re of ro roun und d ba bars rs of di diff ffer eren entt di diam amet eter erss co cool oled ed in ai air. r. ¨ rmebehandeln rmebehandeln, Ca (Frrom G. Spu (F purr an and d T. Sto¨ ferle (Eds. (Eds.), ), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ Carl rl Hanser Han ser,, Mun Munich ich,, 1987 1987.) .)
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2006 200 6 by Tay Taylor lor & Fra Franci nciss Gro Group, up, LL LLC. C.
900 Ac3
800
Ac1
700 100
A P
600 C , e r 500 u t a r 400 e p m e 300 T Њ
1 0
3 0
7 5
1 5 0
3 0 0 B
M s
D 35 93 i a m . = 6 1 0 0 0 0 0 m m 53 5 75
20
200 M
100
796
Hardness HV
0
1
870
101 Time, s
102 1
796
786 782
103 101
753 772
743
104 102 min
454
363 370
105 103
285
106
s 104
FIGURE 6.64 CCT diagram of the alloyed steel DIN 55NiCrMoV6 (austenitizing temperature 950 C), 8
with superimposed cooling curves measured in the core of round bars of different diameters cooled in air. (From G. Spur and T. Sto¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.)
a well-differentiated, nontextured ferrite–pearlite structure is the optimum structure for machinability of these steels. If low-carbon steels are soft annealed, they give long shavings when turned and a bad surface appearance (sometimes called ‘‘smearing’’ or ‘‘tearing’’) because of the accumulation of the material on the tool’s cutting edge. On the other hand, nonannealed workpieces, having harder structural constituents like bainite, result in heavy wear of the cutting edge when machined. An isothermally annealed structure should have the following characteristics: 1. 2. 3. 4. 5.
High proportion of ferrite Uniformly distributed pearlite grains Fine lamellar pearlite grains Short pearlite lamellae Coarse ferrite grains
Figure 6.65 shows the structure of a thin-wall die forging made of low-alloy steel for carburizing (DIN 16MnCr5) after a normalizing anneal (Figure 6.65a) and after an isothermal annealing process (Figure 6.65b). The desired ferrite–pearlite structure originates during an isothermal annealing, the principle of which is explained by Figure 6.66. This figure shows an IT diagram of a low-alloy steel for carburizing (DIN 15CrNi6) with superimposed cooling curves for different cooling rates at continuous cooling. The slowest cooling rate of 3 K/min relates to a furnace cooling, and the fastest cooling rate of 3000 K/min relates to a quenching process. From the diagram in Figure 6.66 it can be clearly seen that bainite formation can be avoided only by very slow continuous cooling, but with such a slow cooling a textured (elongated ferrite) structure results (hatched area in Figure 6.66). There is only one way to avoid both the formation of bainite and the formation of a textured structure (see the open arrow in Figure 6.66), and this is the isothermal annealing process, which consists of
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2006 by Taylor & Francis Group, LLC.
FIGURE 6.65 Structure of a forging made of low-carbon steel for carburizing (DIN 16MnCr5) (a) after
normalizing and (b) after isothermal annealing. Magnification 200 Â. (From G. Spur and T. Sto¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.)
austenitizing followed by a fast cooling to the temperature range of pearlite formation (usually about 650 C (1200 F)), holding at this temperature until the complete transformation of pearlite, and cooling to room temperature at an arbitrary cooling rate. The temperature–time diagram of an isothermal annealing is given in Figure 6.67. The metallurgical mechanism of a good isothermally annealed structure depends on the austenitizing conditions as well as on the temperature and time of the isothermal transformation and on cooling from the austenitizing temperature to the isothermal transformation temperature. The austenitizing temperature and time should be high enough to completely dissolve all carbides, to homogenize the austenite matrix, to stabilize the austenite structure, and achieve a coarse-grained ferrite–pearlite structure after cooling. The undesired textured structure originates by preeutectoid ferrite precipitation along stretched phases acting as germs, for instance manganese sulfides, carbon segregations, or aluminum nitride precipitations. These phases have been stretched as a consequence of a preliminary hot-forming process. To avoid the textured structure the steel has to contain as little sulfur, nitrogen, and aluminum as possible, and during austenitizing a complete dissolution of nitride precipitations and carbides should be achieved. Therefore the austenitizing temperature is adequately high, i.e., about 100 C (212 F) above Ac3, and the holding times are usually about 2 h. 8
8
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Field of textured structure 1000 3 K/min 30 300 C , e r u t 500 a r e p m e T
P
3000
Њ
F
A B
M 400
0 10−2
Isothermal annealing
10−1
320
1
10 Time, min
250
170 HV
102
103
FIGURE 6.66 The principle of isothermal annealing. TTT diagram of the low-alloy steel for carburizing DIN 15CrNi6. (From J. Wu¨nning, Ha¨ rterei-Tech. Mitt. 32:43–49, 1977, pp. 43–49 [in German].)
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1000 Ac3
C800 , e r u 600 t a r e 400 p m e T200 Њ
Ac1
0 Time
FIGURE 6.67 Temperature–time cycle of isothermal annealing. (From G. Spur and T. Sto¨ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.)
Another very important condition to avoid a textured structure is to realize a minimum cooling rate between the austenitizing temperature ( %950 C (%1750 F)) and the isothermal transformation temperature ( %650 C (1200 F)). Thus, about 300 C (572 F) decrease should pass through at a minimum cooling rate of 20–40 K/min. This means that the whole batch of treated workpieces should be cooled from about 950 C (1750 F) to about 650 C (1200 F) in less than 10 min. During this cooling process an undercooling below the chosen isothermal transformation temperature must be avoided to prevent the formation of bainite. The physical mechanism that accounts for the manner and magnitude of ferrite precipitation is the carbon diffusion during cooling from the austenitizing temperature. To achieve a good structure after isothermal annealing, all measures that reduce the carbon diffusion rate or restrict the diffusion time for carbon atoms during cooling are useful. Figure 6.68 shows three structures after isothermal annealing of the low-alloy steel DIN 16MnCr5. It can be seen that cooling too slowly from the austenitizing temperature to the transformation temperature results in an undesirable textured s tructure of ferrite and pearlite, and if during this cooling process an undercooling takes place (i.e., the transformation temperature has been chosen too low) before the pearlite formation, then bainite will be present in the structure, which is not allowed. Big companies usually have internal standards to estimate the allowable degree of texturing of the isothermally annealed structures, with respect to machinability, as shown in Figure 6.69. The transformation temperature and the necessary transformation time for the steel in question may be determined by means of the appropriate I T diagram. Figure 6.70 shows such a diagram for the steel DIN 17CrNiMo6. As can be seen, the lower the transformation temperature chosen, 8
8
8
8
8
8
8
8
8
8
FIGURE 6.68 Different structures after isothermal annealing of the low-alloy steel DIN 16MnCr5 (left).
Well-distributed ferrite–pearlite; correct annealing (center). Textured ferrite–pearlite structure; too slow cooling from the austenitizing to the transformation temperature (right). Ferrite – pearlite þ bainite; undercooling before pearlite transformation. (From J. Wu¨ nning, Ha¨ r terei-Tech. Mitt. 32:43–49, 1977, pp. 43–49 [in German].)
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2006 by Taylor & Francis Group, LLC.
FIGURE 6.69 Internal standard of the German company Edelstahlwerke Buderus A.G.-Wetzlar for
estimation of the allowable degree of texturing of the structure after isothermal annealing. Magnification 100Â. (From G. Spur and T. Sto¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.)
the sooner the transformation starts, up to a temperature (the so-called pearlite nose) at which the shortest time to start the transformation is achieved. Below this temperature, longer times are again necessary to start the transformation. In the range of the pearlite nose temperature, fine lamellar pearlite will be formed, and the time to complete pearlite transformation is the shortest. For unalloyed steels, the pearlite nose temperatures are between 550 and 580 C (1022 and 1076 F), while for alloyed steels they are between 640 and 680 C (1184 and 1256 F). The optimum isothermal annealing temperature is 10–20 C (50–68 F) above the pearlite nose temperature. The necessary transformation time depends on the alloying elements in the steel. In the practice of isothermal annealing the holding time at the transformation temperature includes an adequate reserve because of compositional tolerances in different steel heats. Usually for low-alloy steels for carburizing and structural steels for hardening and tempering the transformation times are below 2 h. From the technical standpoint, when a batch of workpieces has to be isothermally annealed, the biggest problem is to realize sufficiently fast cooling from the austenitizing 8
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2006 by Taylor & Francis Group, LLC.
8
8
900
Ac3
880
Start of ferrite transformation
Ac1
700 C600 , e r 500 u t a r e 400 p m e 300 T
Start of pearlite transformation
Austenite
Њ
Ms
33
84
91
81
93
Pearlite
95
End of transformation
Start of transformation 35
Bainite
31
Martensite
200
Hardness HRC
100 0 1
Hardness HRB
46
102
10 Time, s
1
2
103 4
8 min
15
104
105
106
60 1
2
4
8
24
h 1
2
3 5 days
10
FIGURE 6.70 Isothermal transformation (IT) diagram of the steel DIN 17CrNiMo6. Austenitizing temperature 870 C. (From G. Spur and T. Sto¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.) 8
temperature to the chosen transformation temperature without any undercooling. This cooling process depends on several factors, and the main factors include the workpiece crosssectional size, the loading arrangement, the temperature difference between the austenitizing temperature and the temperature of the cooling medium, and the heat transfer coefficient between the workpieces’ surface and the ambient.
6.2.4
SOFT ANNEALING (SPHEROIDIZING ANNEALING)
Soft or spheroidizing annealing is an annealing process at temperatures close below or close above the Ac1 temperature, with subsequent slow cooling. The microstructure of steel before soft annealing is either ferrite–pearlite (hypoeutectoid steels), pearlite (eutectoid steels), or cementite–pearlite (hypereutectoid steels). Sometimes a previously hardened structure exists before soft annealing. The aim of soft annealing is to produce a soft structure by changing all hard constituents like pearlite, bainite, and martensite (especially in steels with carbon contents above 0.5% and in tool steels) into a structure of spheroidized carbides in a ferritic matrix. Figure 6.71 shows the structure with spheroidized carbides (a) after soft annealing of a medium-carbon low-alloy steel and (b) after soft annealing of a high-speed steel. Such a soft structure is required for good machinability of steels having more than 0.6% C and for all coldworking processes that include plastic deformation. Whereas for cold-working processes the strength and hardness of the material should be as low as possible, for good machinability medium strength or hardness values are required. Therefore, for instance, when ball bearing steels are soft annealed, a hardness tolerance is usually specified. In the production sequence, soft annealing is usually performed with a semiproduct (after rolling or forging), and the sequence of operations is hot working, soft annealing, cold forming, hardening, and tempering. The required degree of spheroidization (i.e., 80–90% of globular cementite or carbides) is sometimes specified. To evaluate the structure after soft annealing, there are sometimes internal standards, for a particular steel grade, showing the percentage of achieved globular
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2006 by Taylor & Francis Group, LLC.
