THE EFFECT OF TEMPERING TEMPERATURE ON ... - Konference

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Confer. Poznań-. Kołobrzeg, 30 Sept.-4 Oct. 1981, Carbides, nitrides, borides, pp. 228-238. 9. NYKIEL T., PhD Thesis, Politechnika Poznańska, Poznań 1982.
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THE EFFECT OF TEMPERING TEMPERATURE ON CARBIDES AND MATRIX OF STEEL OF TYPE 217H12WF/D3. PART 1. SEPARATION OF CARBIDES Tadeusz Nykiel, Tadeusz Hryniewicz Politechnika Koszalinska, Faculty of Mechanical Engineering Racławicka 15-17, 75-620 Koszalin [email protected] Abstract The subject of investigation was the steel of type 217H12WF/AISI D3. The main purpose of the studies was to establish effect of tempering temperature throughout 120 min on the changes of contents of carbides. The tempering was carried out for hardened steel samples after austenitizing in temperatures of 950, 1050, 1150 °C during 30 min. It was found that during tempering of hardened steel in the temperature range of 100-400 °C after austenitizing at 950 °C and 1050 °C the intensity of release/separation of carbides is small and it increases strongly with the temperature rise. In the hardened steel after austenitizing at 1150 °C the carbides begin to separate/release not before the tempering at about 400 °C and very intensively above 500 °C tempering. After tempering at 700 °C the contents of carbides in the steel is less than the contents of carbides in the annealed steel in the degree the more so as the higher has been the austenitizing temperature in the hardening operation. That means after 120 min of tempering of steel at 700 °C one does not obtain equivalent to the annealed steel the state of equilibrium of ferrite-carbides. Key words: D3 tool steel, tempering, carbides 1. INTRODUCTION In the soft annealed steels containing about 2% C and 12% Cr, on the substrate of ferrite, the chromium-iron carbides of type M7C3 [1] occur with the percentage share dependent on the contents of carbon in steel and varies in quite a narrow range [2]. During hardening the percentage share of carbides, due to their dissolution in austenite, decreases with the rise of temperature and time of austenitizing up to 30 min [3]. Dissolution of carbides leads to the growth of contents of carbon and chromium in austenite and with the same after hardening of higher contents of these elements in martensite and residual austenite [4]. Apart from this it is known that with the increase of contents of carbon and chromium in austenite during austenitizing the content of residual austenite rises in steel after hardening. It is also known that the structure of hardened steel is unstable and that during tempering the process of carbides precipitating from the matrix occurs. During tempering of steel of type 195H12/NC11 hardened after austenitizing at 950, 1050, and 1150 °C the carbides begin to precipitate at temperature of about 300 °C and then with the temperature increase (up to 700 °C) the intensity of carbides precipitation rises distinctly [5]. In the steel of this type with the addition of 0.06% nitrogen (experimental heat/melt) hardened after austenitizing at 950, and 1050 °C the carbides begin to precipitate from the matrix (martensite) during tempering above 100 °C, whereas the steel hardened after austenitizing at 1150 °C the carbides begin to precipitate not before the temperature above about 400 °C. In steel of type 165H12/NC10 (contents of 1.64% C, 12.35% Cr) hardened after austenitizing at 1000 °C throughout 15 min the contents of carbides rise successively with the rise of

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temperature of tempering from 100 to 550 °C whereas it slightly decreases in the temperature range of 550 to 750 °C [6]. The process of precipitation of carbides from matrix during tempering of steel of type 217H12WF/AISI D3 (2.04% C, 12.2% Cr, 1.33% W, 0.29% V [7, 8]). In that steel hardened after austenitizng at 950 °C, during tempering the percentage share of carbides rises successively with the growth of temperature of tempering of 100-700 °C, whereas in steel hardened after austenitizing at 1050 °C the process of carbides precipitation during tempering in the temperature range of 400-700 °C is distinctly higher than in temperatures up to 400 °C. During tempering of the steel hardened after austenitizing at 1150 °C the carbides begin to precipitate practically not before the temperature above about 500 °C. It should be noticed that in home and/or foreign literature there is no complex elaborations concerning kinetics of precipitation of carbides during tempering of steel of type of about 2% C and 12% Cr with the additives of tungsten and vanadium. The aim of the investigation, with the results presented in this work, was to: • establish changes of carbides contents in function of tempering temperature in the steel of type 217H12WF (of the composition given in Table 1) • compare our own results with the results of other steels of type of about 2% C and 12% Cr, in these also with the additives of tungsten and vanadium • investigate of displacement of Cr, W, and V in the samples of hardened steel after austenitizing at 1050°C and tempering at 100, 400, and 700 °C by means of X-ray microanalyzer (see Part 2). 2.

MATERIAL AND STUDY METHOD

2.1. Chemical steel composition and heat treatment The studies were carried out for steel of type 217H12WF/AISI D3 of chemical composition given in Table 1. The samples for the studies were made of rods, forged and soft annealed coming from the same heat. Table 1. Chemical composition of the studied steel C 1.95 Cr 11.56 W 1.32 V 0.31

Mo Ni Cu Si

0.05 0.122 0.073 0.27

Mn 0.44 P 0.024 S 0.022 N2 0.016

2.2. Heat treatment (a) Hardening. The steel was austenitized throughout 30 min at 950, 1050, and 1150 °C in the salt oven with the salt (BaCl2+3-5%SiO2+Al2O3) and then cooled down in hardening oil (b) Tempering. Annealing of samples during 120 min at 100-250 °C has been carried out in ultrathermostat filled with a glycerin and in temperatures of 300-700 °C in the bath with liquid lead placed in electric chamber furnace. After annealing the samples were cooled down in quiet air. The method of tempering has been given in [3]. 2.3. Determination of carbides contents using the electrolytic isolation method The electrolytic isolation of carbides has been carried out for samples: (a) hardened after austenitizing at 1050 °C and tempering at temperatures of 100, 150, 200, …, 600, and 700 °C. The percentage share of carbides after tempering at 100,

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200, 300, …, 600, and 700 °C has been determined based on six isolations out of 4 samples and after tempering at 150, 250, …550 °C based on four isolations (b) hardened after austenitizing at 950 and 1150 °C and tempering at temperatures of 100, 200, …700 °C the weight share of carbides has been determined on the basis of isolation of four samples. After heat treatment of samples (of diameter 11 mm and 60 mm long) assigned for carbides isolation the layer of about 0.5 mm thick has been removed by means of grinding (on a centreless grinder). The electrolytic isolation of carbides was carried out in aqueous 5% solution of HCl (sp.grav. 1.19 g/cm3) at the current density of 10 mA/cm2. The isolation time was about 20 hours. After anodic dissolution of steel matrix the isolate from the samples surface was recovered mechanically with the use of 5% solution of CH3OH in H2O and next undergoing treatment by: − double rinsing with methyl alcohol − scavenging with 5% NH3 in H2O − scavenging with methyl alcohol − drying at 100 °C throughout 60 min. Each time after rinsing the isolate was centrifuged with the rotation of 6000 rev/min. To determine percentage contents of carbides the samples before the isolation and after it, and the isolates after drying and cooling down to the ambient temperature, were weighed with the accuracy of ± 0.1 mg. (The weight share of carbides in steel was calculated acc. to the relationship: w[wt%] = {mw/(m1−m2)}x100%, where mw is the weight of dried isolate, m1 is weight of sample prior to isolation, m2 is weight of sample after isolation. In calculations of confidence intervals, the significance level was assumed to 1−α = 0.95, (α=0.05). 2.4. Investigation of surface and linear distributions Investigation of surface and linear distributions of Fe, Cr, W, and V in the hardened steel after austenitizing at 1050 °C throughout 30 min and tempering was performed by means of X-ray microanalyzer JXA by JEOL. The results of this investigation has been presented in Part 2. 3. INVESTIGATION RESULTS 3.1. Effect of tempering temperature on the carbides contents in steel Variations of carbides contents in the studied steel in function of austenitzing and tempering temperatures is presented in Fig. 1. It results from Fig. 1 that for the studied hardened steel after its austenitizing at 950 and 1050 °C one may differentiate two ranges of temperatures differing by the intensity of carbides precipitation, that is the range up to about 400 °C, and from about 400 to 700 °C. During tempering in temperatures up to 400 °C the carbides precipitate from martensite and their amount is not high. During tempering at that temperature of hardened steel after austenitizing at 950 °C only 1.81% of carbides precipitate, and after austenitizing at 1050 °C even less, i.e. 1.3%. Distinctly higher growth of carbides contents in function of tempering temperature in the temperature range of 400-700 °C results first of all from the residual austenite transformation and higher diffusion rate of carbon and alloying elements.

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26 24.8% carbides in annealed steel Carbide contents, %

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950°C 1050°C 1150°C

22 20 18 16 14 0

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Tempering temperature, °C Fig. 1. Effect of tempering temperature on the carbides contents in steel of type 217H12WF/AISI D3 hardened after austenitizing in temperatures of 950, 1050, and 1150 °C The matrix of hardened steel after austenitizing at 1150 °C during 30 min consists of alloying austenite on which the substrate occurred the big primary M7C3 carbides and small amounts of fine secondary carbides which did not dissolve during austenitizing [3-9]. During tempering of steel of such structure the carbides begin to precipitate not before the temperature of about 400 °C, and very intensively above 500 °C. The characteristic phenomenon is that after tempering at 700 °C throughout 120 min of the studied steel hardened after austenitizing at 950, 1050, and 1150 °C, the contents of carbides are lower than in annealed steel. For the austenitizing temperatures as given above, the carbides contents are less than 1.26, 1.41, and 2.11%, respectively. The investigation results concerning effect of tempering temperature on the contents of carbides in steels of type of 2% C and 12% Cr, hardened after austenitizing in temperatures of 950, 1050, and 1150 °C during 30 min, are presented in Figs. 2, 3, and 4. In the studied steels no increase of carbides contents after tempering at 100 °C was found. It results from Fig. 2 that weight share of carbides in steels with additives tungsten and vanadium in 217H12WF/AISI D3 is higher than in steel 195H12/NC11, and lower than in steel NC11N (of increased contents of nitrogen). In steels of type NC11N hardened after austenitizing at 950 and 1050 °C the weight share of carbides rises successively up to 500 °C and then after a deflection rises again from 600 to 700 °C. In hardened steels after austenitizing at 1050 °C the course of changes of carbides contents in function of tempering temperature is different, i.e. in NC11N steel the intensity of carbides precipitation during tempering in temperatures up to 300 °C is very small. A similar phenomenon occurs in steels of type 217H12WF/AISI D3 but in temperature ranges up to 400 °C. After tempering at that

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Contents of carbides in annealed steels: 26.7% NC11N 25.5% 217H12WF(a) 24.8% 217H12WF(b) 23.0% NC11(195H12)

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Carbides, wt %

26 24 22 20 18 16 14 0

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Tem pering tem perature, ºC NC11

NC11N

AISI D3(a)

AISI D3(b)

Fig. 2. Effect of tempering temperature on the carbides contents in steels: 195H12/NC11 (2.14% C, 11.5% Cr, 0.017% N2)[4, 5], and of increased nitrogen contents NC11N (2.20% C, 12.48% Cr, 0.06% N2 [4, 5], and of type 217H12WF/AISI D3(a) [7, 8], and AISI D3(b) [3, 10]. Hardening temperature 950 °C, austenitizing time 30 min, tempering time 120 min

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Carbides, wt %

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Tempering tem perature, ºC NC11

NC11N

AISI D3(a)

AISI D3(b)

Fig. 3. Effect of tempering temperature on the carbides contents in steels as in Fig. 2. Hardening temperature 1050 °C, austenitizing time 30 min, tempering time 120 min temperature the following amount of carbides 1.6% D3(a) and 1.3% D3(b) precipitates from martensite, and 0.9% from NC11N (after tempering at 300 °C). Quite different is the course of changes of carbides contents in the investigated steels hardened after austenitizing at 1150 °C. In steel 195H12/NC11 the carbides begin to precipitate from austenite not before during tempering in temperature above 300 °C, and in NC11N and 217H12WF/AISI D3 above 400 °C.

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28 26

Carbides, wt %

24 22 20 18 16 14 0

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200

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Tem pering tem perature, ºC NC11

NC11N

AISI D3(a)

AISI D3(b)

Fig. 4. Effect of tempering temperature on the carbides contents in steels as in Fig. 2. Hardening temperature 1150 °C, austenitizing time 30 min, tempering time 120 min One may assume that present in steels of type 2% C and 12% Cr tungsten and vanadium alike the nitrogen in steel NC11N is shifting the temperature of intensive precipitation of carbides during tempering into higher temperatures. Tungsten is believed to play a dominating part because its concentration in the matrix of hardened steel is considerably higher than the vanadium concentration. For example, the matrix of steels of type 217H12WF/AISI D3 hardened after austenitizing at 1050 °C during 30 min contains 0.82% of tungsten and only 0.023% of vanadium, and after austenitizing at 1150 °C, 1.1% and 0.065%, respectively [7, 8]. Similar phenomenon concerning tungsten and vanadium contents in the matrix was also was found in another steel of this type 217H12WF/AISI D3(b) hardened after austenitizing at 900 to 1150 °C [9, 10]. Some characteristic phenomenon is that after tempering at 700 °C throughout 120 min in each of the analyzed steels, apart from NC11N steel, the contents of carbides are lower than in the steels under soft-annealed state. In the steel NC11N after tempering at 700 °C the carbides contents are insignificantly higher that the contents in the steel under annealed state. 4. CONCLUSIONS Based on the investigations carried out, the following conclusions may be formulated: (1) The intensity of carbides precipitation during tempering of the studied steel 217H12WF/AISI D3 hardened after austenitizing at 950 and 1050 °C distinctly rises at temperatures above 400 °C. On the other hand, in the hardened steel after austenitizing at 1150 °C the carbides begin to precipitate during tempering at 400 °C, and very intensively above 500 °C. The contents of carbides in the studied steel hardened after austenitizing at 950, 1050, and 1150 °C throughout 30 min and tempering at 700 °C and time 120 min are lower than the contents of carbides in the soft-annealed state. (2) The contents of carbides in the studied steel in annealed, hardened, and state after tempering at 100-700 °C are lower than the contents occurring in steels NC11N (of higher nitrogen contents) and 217H12WF/AISI D3(a) and higher than in steel

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195H12/NC11. The course of changes of carbides in function of tempering temperature is qualitatively similar to course changes in steels 217H12WF/AISI D3 and NC11N and in steel 195H12/NC11 but for tempering temperatures above 300 °C. (3) The tungsten and vanadium present in the hardened steel matrix result in shifting the temperature of the beginning of intensive precipitation of carbides into higher temperatures of about 100 °C in comparison with the beginning of intensive precipitation of carbides in the steels195H12/NC11 and NC11N. BIBLIOGRAPHY 1. HECZKO T.H., DANNŐL W., Stahl und Eisen, 69, 1949, 85 2. SATO T., HONDA Y., NISHIZAWA T., Tetsu To Hagane, 44(12), 1956, 1118-1122 3. NYKIEL T., HRYNIEWICZ T., Proc. of 17th Int.Conf. on Metallurgy and Materials, Metal 2008, 13-15.05.2008, Hradec nad Moravici, 1-8 4. GŁOWACKI Z., Badania nad węglikami stali chromowych, V Zebr.Sprawozd. Komitetu Hutnictwa PAN, Zakopane, 25-27 kwietnia, 1968, 1-17 5. GŁOWACKI Z., Praca doktorska, AGH, Kraków, 1965 6. KOWALSKI W., Prace Instytutu Mechaniki Precyzyjnej, 1966, Rok XIV, z. 4, 1-17 7. KAŁUŻA K., Praca doktorska, Politechnika Poznańska, Poznań 1979 8. GŁOWACKI Z., KAŁUŻA K., BARANOWSKI J., 2nd Intern.Confer. PoznańKołobrzeg, 30 Sept.-4 Oct. 1981, Carbides, nitrides, borides, pp. 228-238 9. NYKIEL T., PhD Thesis, Politechnika Poznańska, Poznań 1982 10. NYKIEL T., HRYNIEWICZ T., Proc. 11th Congress of the Intern.Feder. for Heat Treatment and Surface Engineering, and 4th ASM Heat Treatment and Surface Engineering Confer. in Europe, Florence, Italy, 19-21 October 1998, pp. 87-96

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