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Effect of annealing between cold working and quenching on the structure and properties of steel
Journal of Mechanical Working Technology, 4 (1980) 83--88
83
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Short Communication E F F E C T OF A N N E A L I N G BETWEEN COLD WORKING AND QUENCHING ON THE STRUCTURE AND PROPERTIES OF STEEL
M. KONIECZYNSKI
Metal Forming Institute, Poznafi (Poland) (Received June 27, 1979; accepted September 19, 1979)
Industrial Summary Substantial improvement in the properties of metals may be achieved by the modification of the fibre structure in a forming operation, and a further improvement is gained by the inheriting of the structural changes caused by both hot or cold mechanical working. These effects are utilized, for example, in the thermo-mechanical treatments (TMT): high-TMT, low-TMT and iso-TMT. Such treatments have recently become well understood and are now frequently applied in industry as routine processes. However, the advantages of work hardening of ferrite have not been investigated as closely. The present study examines the qualitative influence of work hardening of ferrite on the properties of steel after quenching. Results show that work hardening of ferrite, and particularly the employment of particular annealing temperatures after cold work, have a beneficial influence on the properties and structure of steel, observed after its quenching. Investigations of a plain carbon steel and of a high-speed tool steel (T7) show that annealing within the ranges 250 to 350°C and 450 to 500°C results in an improvement in hardness and structure of the steels.
Introduction The hexagonal cutting dies in T7 steel, presented for illustration in Fig.l, are processed as follows: the working part is cold formed, followed by austenitizing in a salt bath, quenching in another salt bath and tempering. This process has n o w been supplemented by an interstage annealing treatment between the cold forming and the hardening treatments. Industrial observations have shown that the life of the tools manufactured as above is up to 80 per cent greater than that of dies processed without interstage annealing. This statement prompted the author to perform detailed investigations of the effect of the interstage annealing temperature after cold forming on the properties of steel austenitized and quenched in salt baths. The association of cold forming with hardening as schematized in Fig. 2 represents a particular type of thermo-mechanical treatment [ 1 ]. This type of TMT has a particular advantage, resulting from the fact that hardening can be carried o u t after any time delay; in the other types of TMT (high-, lowor iso-TMT) hardening must immediately follow the h o t or warm deformation.
84
Fig.1. Cutting die for hexagonal nuts in ~,eel TT. ~ formed.
. . . . . ]½ ~ " ~ - -
cutting ecllIm of t h e die are cold
Acs Trecr.
Fig.2. Schematic representation of a thermo-mechanical treatment with cold working: (1) cold work; (2) annealing; (3) fast heating to the austenitizing temperature and cluenching, (4) tempering.
Experimental procedure Steel T7 has a complex chemical composition and the observation of the effects of interstage annealing temperature is difficult. Similar investigations have therefore been performed with a plain carbon steel, which shows with more clarity the related modifications to the structure. Steel No. 1 (see Table 1 ) was delivered in the soft annealed condition and steel No. 2 in the normalized condition. The test pieces of steel No. 1 were cold upset to ( h o - h ) / h o = 30 per cent and the test pieces of steel No. 2 were cold rolled with a reduction of area ( A o - A ) / A o = 32 per cent. After cold work the test pieces of b o t h steels were annealed in a muffle furnace for 1 hour at a t e m p e r ature ranging from 200 to 650°C, and then austenitized in a salt bath furnace as follows: Steel No. 1: two-stage heating to 1200°C, holding at the temperature for 1.5 rain, quenching in a salt bath to 550°C for 1.5 rain and final cooling in air to 20°C; Steel No. 2: one-stage heating to 890°C, holding at this temperature for 1.5 rain and water-quenching. The hardness of the test pieces was measured with a Zwick hardness tester
85 TABLE 1 C h e m i c a l c o m p o s i t i o n o f the steels investigated ( p e r c e n t ) Steel
C
Mn
Si
Ni
Cr
W
V
No. 1
0.95
0.40
0.40
0.40
4.5
10.0
2.2
No. 2
0.14
0.40
0.19
0.10
0.07
--
--
employing a test load of 100 N; the integral width of the ferrite lines (211), {220) and (321) were determined using a Siemens X-ray diffractometer with MOk~ radiation and the structure was observed on a $4-10 Cambridge scanning microscope. All these observations were performed on two independent sets of test pieces, each set consisting of three repetitions, mean experimental values then being calculated from the six separate measurements. Results
Experimental results are summarized in Fig. 3. It is evident that the effect of the annealing temperature on the hardness and the integral width of the ferrite lines investigated, after hardening -- is similar in both steels. At some annealing temperatures b o t h hardness and line width attain a maximum. In the case o f steel No. 2, maximal values of hardness and width of the (220)~ line are reached after annealing at temperatures of 250--300 and 450--500°C. Steel No. 1 attains both maxima of hardness and (211)~ line width at temperatures of 350 and 500°C. In the case of steel No. 1 both maxima are coincident; in steel No. 2 the maxima of hardness and line (220)~ are relatively displaced by a b o u t 50°C. Corresponding to the modifications in the hardness and ferrite line width, changes in the structure in both steels are also observed.
%t. ~ c
~ 100
~
,Hv~o0 211~/ ._
x~... f
~:~ ;,
~ 60- .kll' st.~No.r r. ' 2 20
300
500
. 400
500
a2o
600 °C
Anneoling temperotore
Fig.3. Influence of the interstage annealing temperature on the hardness and integral width of th~ X-ray lines (220) of fertile and (9.11) of marLensJte after hardening.
86
Fig.4. Structure of the plain carbon steel after hardening treatment: (a) without cold work; (b) cold rolled; (c) cold rolled and annealed at 250°C for 1 h; (d) cold rolled and annealed at 500°C for 1 h. Etched in 10% HNO 3. Scanning microscope.
Fig.5. Structure of the T7 steel: (a) in the delivery condition; (b) after hardening treatment; (c) cold worked and annealed at 500°C for 1 h; (d) cold worked, annealed at 500°C for I h and hardening treated. Etched in Na~S203 + H20. Scanning microscope, magnification of (a) is equal to that of (c),and (b) to that of (d).
87 Figure 4 shows the structural changes in steel No. 2 and Fig. 5 those in steel No. 1. Steel No. 2, after hardening treatment with no preliminary cold work, shows (Fig. 4a) a coarse-grained structure with numerous ferrite grains whereas the preliminarily cold-worked and hardened structure (Fig. 4b) is more refined. The interstage annealing, especially at the higher temperature of 500°C, causes (Fig. 4d) a further refinement of the grains; segregations of coagulated cementite, and almost no ferrite grains, are visible. In the case of steel No. 1, the complete treatment (cold work, annealing and hardening treatment) results in some grain refinement and diminution of the martensite lamellae. Discussion
Structural changes which occur in the interstage annealing of cold-worked steel have some beneficial effect on the properties and structure of hardened steel. There are two specific annealing temperatures which ensure maximal gains in hardness, integral width of X-ray lines and structure quality after hardening. Figure 6 shows the effect of the annealing temperature after cold work on hardness and the integral width of the ferrite (321) lines of both steels investigated. The changes of the properties investigated show the same tendencies in both steels. Up to 350°C, the rise of the annealing temperature causes an enlargement of the ferrite (321)~ line, followed by its slow narrowing as the annealing temperature further rises to 650°C. In both steels the influence of the annealing temperature on hardness is similar. A diminution of the hardness of steel No. 1 begins at 450°C, and of steel No. 2 at 350°C. Both the hardness and the integral width of the (321)~ line after annealing at 650°C are greater than in the steels in the as-delivered condition [2]. The changes in the width of the integral ferrite lines correspond with the changes in the structure during annealing after cold work. The widening or narrowing of the X-ray lines are under the influence, of, for example, the subgrain size, HVIO0 32O
280
-
40
3oL~l 20 200
O0
I
I
300
LO0
L. - " - - T " " ~ oo 5OO
GO0
Annealing temperature Fig.6. Influence o f the annealing t e m p e r a t u r e after cold w o r k on the hardness and integral
width of the ferrite (321) line in plain carbon steel and in T7 steel.
88
lattice distortion and stacking faults. At the relatively low annealing temperature of 350°C the changes in the cold-worked structure are minute. The structural processes are associated with the movement of dislocations and with the interaction of intersticialatoms with dislocations. Steel No. I conrains a greater amount of carbon and therefore the accomplishment of these processes is displaced toward a higher temperature; this is the reason for the difference of the first maximum temperature for the steels (see Fig. 3). Annealing at a higher temperature (about 500°C) after cold work causes more pronounced structural changes and a constitution of a polygonized substructure. This substructure is characterized by subgralns with boundaries formed of dislocation rows. Figure 3 shows that in both steels investigated these processes occur at the same annealing temperature. The displacement of the temperature of the second maximum of the (220)a line in steel No. 2 toward a lower temperature is associated with recrystallization, which begins in this steel at a lower temperature. The decrease of the properties investigated (Fig.3) after annealing at a temperature above 500°C, is associated with the development of recrystallization nucleation. Besides the changes in the subgrain structure, during annealing at 350 to 500°C the precipitation of nitrides at dislocations and of carbides at the grain boundaries occurs. Fine dispersed precipitations at the grain boundaries (see Fig. 5c) restrain grain growth during austenitizing. After the hardening treatment of steel No. 1 with no preliminary cold work, the grain size of the prior austenite was 13 (after Snyder--Graff), and after cold work and annealing at 350 and/or 500°C and hardening it was 16 [2].
Conclusions
(1) Interstage annealing between cold work and hardening treatment beneficially affects hardness and grain refinement in steel. Two annealing temperatures are advantageous: 250--300 and 500°C. After annealing at 500°C the effects are particularly pronounced. (2) Annealing after cold work at 500°C results in the development of a polygonized substructure, which contributes in the inheriting of a part of the cold-wnrked structure by the martensite. (3) Utilization of the above observation enables an increase to be secured in the tool life of dies such as that illustrated in Fig.1. Acknowledgements The author gratefully acknowledges the permission of the Metal Forming Institute in Poland to publish this paper. References 1 M. Konieczy~ki, Obrbbka Plastyczna, 16(4) (1977) 151--162 (in Polish). 2 M. Konieczyfiski, Ibid, 18(4) (1979) (in press).
83
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Short Communication E F F E C T OF A N N E A L I N G BETWEEN COLD WORKING AND QUENCHING ON THE STRUCTURE AND PROPERTIES OF STEEL
M. KONIECZYNSKI
Metal Forming Institute, Poznafi (Poland) (Received June 27, 1979; accepted September 19, 1979)
Industrial Summary Substantial improvement in the properties of metals may be achieved by the modification of the fibre structure in a forming operation, and a further improvement is gained by the inheriting of the structural changes caused by both hot or cold mechanical working. These effects are utilized, for example, in the thermo-mechanical treatments (TMT): high-TMT, low-TMT and iso-TMT. Such treatments have recently become well understood and are now frequently applied in industry as routine processes. However, the advantages of work hardening of ferrite have not been investigated as closely. The present study examines the qualitative influence of work hardening of ferrite on the properties of steel after quenching. Results show that work hardening of ferrite, and particularly the employment of particular annealing temperatures after cold work, have a beneficial influence on the properties and structure of steel, observed after its quenching. Investigations of a plain carbon steel and of a high-speed tool steel (T7) show that annealing within the ranges 250 to 350°C and 450 to 500°C results in an improvement in hardness and structure of the steels.
Introduction The hexagonal cutting dies in T7 steel, presented for illustration in Fig.l, are processed as follows: the working part is cold formed, followed by austenitizing in a salt bath, quenching in another salt bath and tempering. This process has n o w been supplemented by an interstage annealing treatment between the cold forming and the hardening treatments. Industrial observations have shown that the life of the tools manufactured as above is up to 80 per cent greater than that of dies processed without interstage annealing. This statement prompted the author to perform detailed investigations of the effect of the interstage annealing temperature after cold forming on the properties of steel austenitized and quenched in salt baths. The association of cold forming with hardening as schematized in Fig. 2 represents a particular type of thermo-mechanical treatment [ 1 ]. This type of TMT has a particular advantage, resulting from the fact that hardening can be carried o u t after any time delay; in the other types of TMT (high-, lowor iso-TMT) hardening must immediately follow the h o t or warm deformation.
84
Fig.1. Cutting die for hexagonal nuts in ~,eel TT. ~ formed.
. . . . . ]½ ~ " ~ - -
cutting ecllIm of t h e die are cold
Acs Trecr.
Fig.2. Schematic representation of a thermo-mechanical treatment with cold working: (1) cold work; (2) annealing; (3) fast heating to the austenitizing temperature and cluenching, (4) tempering.
Experimental procedure Steel T7 has a complex chemical composition and the observation of the effects of interstage annealing temperature is difficult. Similar investigations have therefore been performed with a plain carbon steel, which shows with more clarity the related modifications to the structure. Steel No. 1 (see Table 1 ) was delivered in the soft annealed condition and steel No. 2 in the normalized condition. The test pieces of steel No. 1 were cold upset to ( h o - h ) / h o = 30 per cent and the test pieces of steel No. 2 were cold rolled with a reduction of area ( A o - A ) / A o = 32 per cent. After cold work the test pieces of b o t h steels were annealed in a muffle furnace for 1 hour at a t e m p e r ature ranging from 200 to 650°C, and then austenitized in a salt bath furnace as follows: Steel No. 1: two-stage heating to 1200°C, holding at the temperature for 1.5 rain, quenching in a salt bath to 550°C for 1.5 rain and final cooling in air to 20°C; Steel No. 2: one-stage heating to 890°C, holding at this temperature for 1.5 rain and water-quenching. The hardness of the test pieces was measured with a Zwick hardness tester
85 TABLE 1 C h e m i c a l c o m p o s i t i o n o f the steels investigated ( p e r c e n t ) Steel
C
Mn
Si
Ni
Cr
W
V
No. 1
0.95
0.40
0.40
0.40
4.5
10.0
2.2
No. 2
0.14
0.40
0.19
0.10
0.07
--
--
employing a test load of 100 N; the integral width of the ferrite lines (211), {220) and (321) were determined using a Siemens X-ray diffractometer with MOk~ radiation and the structure was observed on a $4-10 Cambridge scanning microscope. All these observations were performed on two independent sets of test pieces, each set consisting of three repetitions, mean experimental values then being calculated from the six separate measurements. Results
Experimental results are summarized in Fig. 3. It is evident that the effect of the annealing temperature on the hardness and the integral width of the ferrite lines investigated, after hardening -- is similar in both steels. At some annealing temperatures b o t h hardness and line width attain a maximum. In the case o f steel No. 2, maximal values of hardness and width of the (220)~ line are reached after annealing at temperatures of 250--300 and 450--500°C. Steel No. 1 attains both maxima of hardness and (211)~ line width at temperatures of 350 and 500°C. In the case of steel No. 1 both maxima are coincident; in steel No. 2 the maxima of hardness and line (220)~ are relatively displaced by a b o u t 50°C. Corresponding to the modifications in the hardness and ferrite line width, changes in the structure in both steels are also observed.
%t. ~ c
~ 100
~
,Hv~o0 211~/ ._
x~... f
~:~ ;,
~ 60- .kll' st.~No.r r. ' 2 20
300
500
. 400
500
a2o
600 °C
Anneoling temperotore
Fig.3. Influence of the interstage annealing temperature on the hardness and integral width of th~ X-ray lines (220) of fertile and (9.11) of marLensJte after hardening.
86
Fig.4. Structure of the plain carbon steel after hardening treatment: (a) without cold work; (b) cold rolled; (c) cold rolled and annealed at 250°C for 1 h; (d) cold rolled and annealed at 500°C for 1 h. Etched in 10% HNO 3. Scanning microscope.
Fig.5. Structure of the T7 steel: (a) in the delivery condition; (b) after hardening treatment; (c) cold worked and annealed at 500°C for 1 h; (d) cold worked, annealed at 500°C for I h and hardening treated. Etched in Na~S203 + H20. Scanning microscope, magnification of (a) is equal to that of (c),and (b) to that of (d).
87 Figure 4 shows the structural changes in steel No. 2 and Fig. 5 those in steel No. 1. Steel No. 2, after hardening treatment with no preliminary cold work, shows (Fig. 4a) a coarse-grained structure with numerous ferrite grains whereas the preliminarily cold-worked and hardened structure (Fig. 4b) is more refined. The interstage annealing, especially at the higher temperature of 500°C, causes (Fig. 4d) a further refinement of the grains; segregations of coagulated cementite, and almost no ferrite grains, are visible. In the case of steel No. 1, the complete treatment (cold work, annealing and hardening treatment) results in some grain refinement and diminution of the martensite lamellae. Discussion
Structural changes which occur in the interstage annealing of cold-worked steel have some beneficial effect on the properties and structure of hardened steel. There are two specific annealing temperatures which ensure maximal gains in hardness, integral width of X-ray lines and structure quality after hardening. Figure 6 shows the effect of the annealing temperature after cold work on hardness and the integral width of the ferrite (321) lines of both steels investigated. The changes of the properties investigated show the same tendencies in both steels. Up to 350°C, the rise of the annealing temperature causes an enlargement of the ferrite (321)~ line, followed by its slow narrowing as the annealing temperature further rises to 650°C. In both steels the influence of the annealing temperature on hardness is similar. A diminution of the hardness of steel No. 1 begins at 450°C, and of steel No. 2 at 350°C. Both the hardness and the integral width of the (321)~ line after annealing at 650°C are greater than in the steels in the as-delivered condition [2]. The changes in the width of the integral ferrite lines correspond with the changes in the structure during annealing after cold work. The widening or narrowing of the X-ray lines are under the influence, of, for example, the subgrain size, HVIO0 32O
280
-
40
3oL~l 20 200
O0
I
I
300
LO0
L. - " - - T " " ~ oo 5OO
GO0
Annealing temperature Fig.6. Influence o f the annealing t e m p e r a t u r e after cold w o r k on the hardness and integral
width of the ferrite (321) line in plain carbon steel and in T7 steel.
88
lattice distortion and stacking faults. At the relatively low annealing temperature of 350°C the changes in the cold-worked structure are minute. The structural processes are associated with the movement of dislocations and with the interaction of intersticialatoms with dislocations. Steel No. I conrains a greater amount of carbon and therefore the accomplishment of these processes is displaced toward a higher temperature; this is the reason for the difference of the first maximum temperature for the steels (see Fig. 3). Annealing at a higher temperature (about 500°C) after cold work causes more pronounced structural changes and a constitution of a polygonized substructure. This substructure is characterized by subgralns with boundaries formed of dislocation rows. Figure 3 shows that in both steels investigated these processes occur at the same annealing temperature. The displacement of the temperature of the second maximum of the (220)a line in steel No. 2 toward a lower temperature is associated with recrystallization, which begins in this steel at a lower temperature. The decrease of the properties investigated (Fig.3) after annealing at a temperature above 500°C, is associated with the development of recrystallization nucleation. Besides the changes in the subgrain structure, during annealing at 350 to 500°C the precipitation of nitrides at dislocations and of carbides at the grain boundaries occurs. Fine dispersed precipitations at the grain boundaries (see Fig. 5c) restrain grain growth during austenitizing. After the hardening treatment of steel No. 1 with no preliminary cold work, the grain size of the prior austenite was 13 (after Snyder--Graff), and after cold work and annealing at 350 and/or 500°C and hardening it was 16 [2].
Conclusions
(1) Interstage annealing between cold work and hardening treatment beneficially affects hardness and grain refinement in steel. Two annealing temperatures are advantageous: 250--300 and 500°C. After annealing at 500°C the effects are particularly pronounced. (2) Annealing after cold work at 500°C results in the development of a polygonized substructure, which contributes in the inheriting of a part of the cold-wnrked structure by the martensite. (3) Utilization of the above observation enables an increase to be secured in the tool life of dies such as that illustrated in Fig.1. Acknowledgements The author gratefully acknowledges the permission of the Metal Forming Institute in Poland to publish this paper. References 1 M. Konieczy~ki, Obrbbka Plastyczna, 16(4) (1977) 151--162 (in Polish). 2 M. Konieczyfiski, Ibid, 18(4) (1979) (in press).