temperatu temper ature re of 600 C (11 (1112 12 F) an and d a so soak akin ing g ti time me of 10 h arechos arechosen en fo forr st stre ressss-rel reliefanne iefanneal alin ing, g, 2 the residual stresses will, after this annealing, be reduced to a maximum of 70 N/mm . Higher temperatures and longer times of annealing may reduce residual stresses to lower levels, as can be seen from Figure 6.53. 6.53. As in al alll he heat at tr trea eatm tmen entt pr proc oces esses ses wh wher eree Ho Holl llom omon on’s ’s pa para rame mete terr is in invo volv lved ed,, se sele lect ctio ion n of a higher hig her tem temper peratu ature re may dra dramat matica ically lly sho shorte rten n the soa soakin king g tim timee and con contri tribut butee sub substa stanti ntiall ally y to thee ec th econ onom omy y of th thee an anne neal alin ing g pr proc ocess ess.. Dealin Dea ling g wit with h str struct uctura urall ste steels els for har harden dening ing and tem temper pering ing,, the str stressess-rel relief ief pro process cess and the temp te mperi ering ng pr proc ocess ess ca can n be pe perf rform ormed ed sim simul ulta tane neou ousl sly y as on onee op oper erat atio ion, n, be beca caus usee Ho Holl llom omon on’s ’s para pa rame mete terr is al also so ap appl plica icabl blee to te temp mperi ering ng.. In su such ch a ca case se th thee st stre ressss-re reli lief ef di diag agra ram m ma may y be us used ed in co com mbi bin nat atio ion n wi witth the te tem mpe peri ring ng dia iagr gram am to op opti timi mize ze bot oth h the ha hard rdne ness ss an and d the le leve vell of reduce red uced d resi residua duall str stresse esses. s. Thee re Th resi sidu dual al st stre ress ss le leve vell af afte terr st stre ress ss-r -rel elie ieff an anne neal alin ing g wi will ll be ma main inta tain ined ed on only ly if th thee co cool ol-dow own n fr from om th thee an ann nea eali lin ng te tem mpe pera ratu ture re is co cont ntro roll lled ed an and d sl slo ow en eno oug ugh h th that at no new in intter erna nall stre st resse ssess ar aris ise. e. Ne New w st stres resse sess th that at ma may y be in indu duce ced d du duri ring ng co cool olin ing g de depe pend nd on th thee co cool olin ing g ra rate te,, on the cr cro oss ss--se sect ctio iona nall si size ze of the wo work rkpi piec ece, e, an and d on the co comp mpos osit itio ion n of the st stee eell. Fi Figu gure re 6. 6.54 54 show sh owss th thee ef effe fect ct of co cool olin ing g ra rate te an and d cr cros osss-sec secti tion onal al di diam amet eter er of fo forg rgin ings gs ma made de of a Cr CrMo MoNi NiV V stee st eell on th thee le leve vell of ta tang ngen enti tial al re resi sidu dual al st stre resse ssess af afte terr st stre ressss-re reli lief ef an anne neal alin ing. g. A ge gene nera rall co conc nclu lusi sion on ab abou outt st stre ress ss-r -rel elie ieff an anne neal alin ing g is th thee fo foll llow owin ing: g: In th thee te temp mper erat atur uree rang ra ngee 45 450– 0–65 650 0 C (8 (842 42–1 –120 200 0 F), the yield strength of unalloyed and loww-a alloyed st steeels is lowe lo were red d so mu much ch th that at a gr grea eatt de deal al of re resi sidu dual al st stre ress ss ma may y be re redu duce ced d by pl plas asti ticc de defo form rmat atio ion. n. Thee in Th infl flue uenc ncee of th thee st stee eell co comp mpos osit itio ion n on th thee le leve vell of re resi sidu dual al st stre ress sses es af afte terr an anne neal alin ing g ca can n be cons co nsid ider erab ablle. Whi hile le un unal allo loye yed d an and d lo loww-a all llo oy st stee eells wit ith h Ni Ni,, Mn, an and d Cr aft fter er st stre ress ss--re reli lief ef anne an neal alin ing g ab abov ovee 50 500 0 C (932 F) may get the residual stre ress ssees reduced to a low level, stee eells allo al loye yed d wi with th Mo or Mo þ V wi will ll re reta tain in a mu much ch hi high gher er le leve vell of th thee re resid sidua uall st stres resse sess af afte terr st stres resssrelief annealing at the same temperature beca cau use of their much higher yield strength at elevated eleva ted tempe temperature rature.. 8
8
8
8
8
6.2 .2..2
8
NORMALIZING
Normal Norm aliz izin ing g or no norm rmal aliz izin ing g an anne neal alin ing g is a he heat at tr trea eatm tmen entt pr proc ocess ess co cons nsist istin ing g of au aust sten enit itiz izin ing g at temper era atures of 30–80 C (86 86–1 –176 76 F) above the Ac3 tr tran ansf sfor orma mati tion on te temp mpera eratu ture re (f (for or 8
2
m m / N , s e s s e r t s l a u d i s e r l a i t n e g n a T
8
120
m m 0 m 0 m 0 0 1 0 = 8 . m a i D
100 80 60
m m 0 0 6
m m 0 0 4 m 0 m 2 0
40 20 0
0
10
20 20
30 30
40 40
50 50
60 60
80
Average cooling rate to 400, C/h Њ
Tang Ta ngen enti tial al re resi sidu dual al st stre ress sses es in a Cr CrMo MoNi NiV V al allo loy y st stee eell de depe pend ndin ing g on th thee co cool olin ing g ra rate te an and d crosscros s-se sect ctio ion n di diam amet eter er.. (F (Fro rom m G. Sp Spur ur an and d T. St Sto o¨ ferle (Eds. (Eds.), ), Handbuch der Fertigungstechnik, Vol. 4/2, ¨ rmebehandeln Wa¨ rmebehandeln, Ca Carl rl Ha Hans nser er,, Mu Muni nich ch,, 198 1987. 7.)) FIGURE 6.54
ß
2006 200 6 by Tay Taylor lor & Fra Franci nciss Gro Group, up, LL LLC. C.
1000
b
Ac3 c
800
C , e r 600 u t a r e p 400 m e T
Ac1
Њ
a
d
200 0 Time
Time–temperature regime of normalizing. a, Heating; b, holding at austenitizing temperature; c, air cooling; d , air or furnace cooling. (From G. Spur and T. Sto¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.) FIGURE 6.55
hypoeutectoid steels) followed by slow cooling (usually in air), the aim of which is to obtain a fine-grained, uniformly distributed, ferrite–pearlite structure. Normalizing is applied mainly to unalloyed and low-alloy hypoeutectoid steels. For hypereutectoid steels normalizing is performed only in special cases, and for these steels the austenitizing temperature is 30–80 C (86–176 F) above the Ac1 transformation temperature. Figure 6.55 shows the thermal cycle of a normalizing process, and Figure 6.56 shows the range of austenitizing temperatures for normalizing unalloyed steels depending on their carbon content. The parameters of a normalizing process are the heating rate, the austenitizing temperature, the holding time at austenitizing temperature, and the cooling rate. Normalizing treatment refines the grain of a steel that has become coarse-grained as a result of heating to a high temperature, e.g., for forging or welding. Figure 6.57 shows the effect of grain refining by normalizing a carbon steel of 0.5% C. Such grain refinement and 8
8
1200
1147 C Њ
E
1100 γ
1000 C , e r u t a r e p m e T
G
Њ
900 γ + Fe3C
800 α
+ γ
700 P
+
Pearlite
500
0
Њ
K
S
a
600
723 C
0.4
e t i l r a e P
Pearlite + Fe3C
0.8 1.2 1.6 Carbon content, %
2.0
2.4
Range of austenitizing temperatures for normalizing unalloyed steels depending on their carbon content. (Temperature range above the line S –E is used for dissolution of secondary carbides.) ¨ferle (Eds.), Handbuch der Fertigunga, ferrite; g , austenite; Fe3C, cementite. (From G. Spur and T. Sto stechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.) FIGURE 6.56
ß
2006 by Taylor & Francis Group, LLC.
Effect of grain refining by normalizing a carbon steel of 0.5% C. (a) As-rolled or forged, grain size ASTM 3 and (b) normalized, grain size ASTM 6. Magnification 500 Â. (From K.E. Thelning, Steel and Its Heat Treatment, 2nd ed., Butterworths, London, 1984.) FIGURE 6.57
homogenization of the structure by normalizing is usually performed either to improve the mechanical properties of the workpiece or (previous to hardening) to obtain better and more uniform results after hardening. In some cases, normalizing is applied for better machinability of low-carbon steels. A special need for normalizing exists with steel castings because, due to slow cooling after casting, a coarse-grained structure develops that usually contains needlelike ferrite (Widmannsta¨tten’s structure), as shown in Figure 6.58. A normalizing treatment at 780–950 C (1436–1742 F) (depending on chemical composition) removes this undesirable structure of unalloyed and alloyed steel castings having 0.3–0.6% C. After hot rolling, the structure of steel is usually oriented in the rolling direction, as shown Figure 6.59. In such a case, of course, mechanical properties differ between the rolling in direction and the direction perpendicular to it. To remove the oriented structure and obtain the same mechanical properties in all directions, a normalizing annealing has to be performed. After forging at high temperatures, especially with workpieces that vary widely in crosssectional size, because of the different rates of cooling from the forging temperature, a heterogeneous structure is obtained that can be made uniform by normalizing. 8
8
Structure of a steel casting (a) before normalizing and (b) after normalizing. (From H.J. Eckstein (Ed.), Technologie der Wa¨ rmebehandlung von Stahl , 2nd ed., VEB Deutscher Verlag fu¨ r Grundstoffindustrie, Leipzig, 1987.) FIGURE 6.58
ß
2006 by Taylor & Francis Group, LLC.
Structure of DIN 20MnCr5 steel (a) after hot rolling and (b) after normalizing at 880 C. Magnification 100Â. (From G. Spur and T. Sto¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.) FIGURE 6.59
8
From the metallurgical aspect the grain refinement and the uniform distribution of the newly formed ferrite–pearlite structure during normalizing treatment can be explained with the following mechanism. At normalizing, the steel is subjected first to a a ! g (ferrite– pearlite to austenite) transformation, and after the holding time at austenitizing temperature, to a recurring g ! a (austenite to ferrite–pearlite) transformation. The effect of normalizing depends on both austenitization and cooling from the austenitizing temperature. During austenitizing a far-reaching dissolution of carbides is aimed at, but this process competes with the growth of austenite grains after complete carbide dissolution, which is not desirable. Besides the carbide dissolution, the degree of homogenization within the austenite matrix is important for obtaining a new arrangement of ferrite and pearlite constituents in the structure after normalizing. Both dissolution and homogenizing are time- and temperaturedependent diffusion processes that are slower when the diffusion paths are longer (higher local differences in carbon concentration) and the diffusion rates are smaller (e.g., increasing amounts of alloying elements). Therefore, especially with alloyed steels, lower austenitizing temperatures and longer holding times for normalizing give advantages taking into account the austenite grain growth. As shown in Figure 6.60, high austenitizing temperatures result in a coarse-grained austenite structure, which yields a coarse structure after normalizing. Holding time at austenitizing temperature may be calculated using the empirical formula t ¼ 60 þ D
(6:35)
where t is the holding time (min) and D is the maximum diameter of the workpiece (mm). When normalizing hypoeutectoid steels (i.e., steels with less than 0.8% C), during cooling from the austenitizing temperature, first a preeutectoid precipitation of ferrite takes place. With a lower cooling rate, the precipitation of ferrite increases along the austenite grain boundaries. For the desired uniform distribution of ferrite and pearlite after normalizing, however, a possibly simultaneous formation of ferrite and pearlite is necessary. Steels having carbon contents between 0.35 and 0.55% C especially tend to develop nonuniform ferrite distributions as shown in Figure 6.61. The structure in this figure indicates overly slow cooling in the temperature range of preeutectoid ferrite precipitation between Ar3 and Ar1. On the other hand, if the cooling through this temperature region takes place too fast, with steels having carbon contents between 0.2 and 0.5%, formation of an undesirable needlelike ferrite (oriented at austenite grain boundaries), the so-called Widmannsta¨tten’s structure, may result as shown in Figure 6.62. Formation of pearlite follows only after complete
ß
2006 by Taylor & Francis Group, LLC.
T 2 > T 1 > A1
T 2
Austenite T 1 A1
Pearlite
Heating
Cooling
Schematic presentation of the influence of austenitizing temperature on the grain size of the structure of a eutectoid steel after normalizing. (From H.J. Eckstein (Ed.), Technologie der Wa¨ rmebehandlung von Stahl , 2nd ed., VEB Deutscher Verlag fu¨r Grundstoffindustrie, Leipzig, 1987.) FIGURE 6.60
precipitation of ferrite by transformation of the remaining austenite structure at temperature Ar1. It starts first at the boundaries of ferrite and austenite and spreads to the interior of the austenite grains. The greater the number of the pearlitic regions formed, the more mutually hindered the pearlite grains are in their growth, and consequently the finer the grains of the normalized structure. The influence of alloying elements on the austenite to ferrite and pearlite transformation may be read off from the relevant CCT diagram. Care should be taken to ensure that the cooling rate within the workpiece is in a range corresponding to the transformation behavior of the steel in question that results in a pure ferrite–pearlite structure. If, for round bars of different diameters cooled in air, the cooling curves in the core have been experimentally measured and recorded, then by using the appropriate CCT diagram for the steel grade in question, it is possible to predict the structure and hardness after normalizing. To superimpose the recorded cooling curves onto the CCT diagram, the time–temperature scales must be equal to those of the CCT diagram. Figure 6.63 shows, for example, that the unalloyed steel DIN Ck45 will attain the desired ferrite–pearlite structure in the core of all investigated bars of different diameters cooled in
Nonuniform distribution of ferrite and pearlite as a consequence of unfavorable temperature control during normalizing of unalloyed DIN C35 steel. Magnification 100 Â. (From G. Spur and T. Sto¨ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.) FIGURE 6.61
ß
2006 by Taylor & Francis Group,LLC.
Formation of needlelike ferrite at grain boundaries after normalizing of the unalloyed steel DIN C35, because of too fast a cooling rate. Magnification 500 Â. (From G. Spur and T. Sto¨ ferle (Eds.), Handbuch der Fertigungstechnik, Vol. 4/2, Wa¨ rmebehandeln, Carl Hanser, Munich, 1987.) FIGURE 6.62
air. On the other hand, as s hown in Figure 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 M s
2 15
55
d i a m . = 6 1 0 0 0 0 0 m m
85
20
300 200
20
15
Ac1
45
40 30 Ferri te 60 70 Pearl ite
3 0
7 5
1 5 0
3 0 0
3
Martensite
100 722
0 Q1
702 654 576438 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 diagram of the unalloyed steel DIN Ck45 (austenitizing temperature 850 C), 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.) FIGURE 6.63
ß
2006 by Taylor & Francis Group, LLC.
8
