Influence of Reheating Temperature on Residual Stress in Nitrided Hot Work Die Steel (Hl3)

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1 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol Influence of Reheating Temperature on Residual Stress in Nitrided Hot Work Die Steel (Hl3) Koji Yatsushiro, Masahiko Hihara Yamanashi Industrial Technology Center, Kofu, Yamanashi, , Japan and Makoto Kuramoto Polytechnic University, Sagan&am, Kanagawa, 229-l 196, Japan Abstract This paper reports on the changes which occurred in the nitride layer of die casting die steel over a reheating range from 673K(400 C) to 973K(700 C). These changes were measured by X-ray stress measurement method of the stress constant K, residual stress distribution, half-value breadth (ha&height peak intensity) measurement, microphotogmphic observations, hardness testing, and the profiles of X- ray difhaction. The depth profiles of the distribution of residual stress in the nitride layer varied greatly for di&rent reheating tempemture. Thus, the cxfe 211 di.feaction and s-fe;?,n 103 difhaction were used during the study. A decrease in compressive residual stress, and a correlation of h&value breadth and hardness was observed when a reheating temperature higher than 823K(550 C) was used. It was shown that longer lasting nitrided die casting die can be produced using specific heating temperature. 1. Introduction In the case of die casting dies, thermal forging dies and some other dies, such defects as heat checking, erosion, soldering and abrasion occur. As a result, the life of thermal fatigue of the dies is shortened considerably I)+). In recent years, as a means to increase fatigue life, nitriding is often applied to the surface of die steel. Nitriding has been used because it was believed that it would not only increase the hardness of the surface layer, but also add compressive residual stress, and thereby improve thermal propertiesq. The compound layer of nitriding formed on the surface is decomposed, when reheated fi-om 773K(500 C) to 873K(600 C) during the thermal fatigue test process; and some nitrogen in the compound layer is transformed into nitrogen gas and discharged into the atmosphere. Nitrogen also influences the difhrsion layer by penetrating into the matrix. It is speculated that during the reheating of nitrided specimens, both the surface hardness and the compressive residual sttess values decrease, and fatigue strength of declines at high temperatures. This has not, as yet, been proven. In a previous paper ), we reported results that specimens that were made of hot work die steel (H13) which had undergone nitriding in different conditions. Measurement of the X-ray stress constant K, thermal fatigue tests and other tests were also conducted. The changes by decomposition of the compound layer and compressive residual stress of the surface during thermal fatigue test and others were closely studied. In the current study, we applied the same nitriding process to hot work die steel, and after that, reheated it -horn 673K(400 C) to 973K(700 C). Furthermore, microphotographic observations and measurements of hardness distributions of the compound layer and the difhtsion layer were made. The decomposition of compound layer was observed during these tests. After that,

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3 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol measurements were made of the K, the l/s,, 2/S,, compressive residual stress distribution and X-ray d.eraction profiles. We investigated how these conditions improve the thermal fatigue life of die casting dies, and reported the results. 2. Experimental Procedure 2.1. Specimen preparation Table 1 Chemical composition of H13 (wt %) The material used in this experiment is a hot C Si Mn Cr MO V work die steel. Table 1 shows the chemical composition of the material. The material was ground to the size of : 1Omm W, 120mm L, 3mm T. The specimens were given a quenching [1293K(1020 C), air cooled], and after that, the material was tempered [S63K(590 C) for 4 hours and 853K@O C) for 3 hours] to a hardness of 482 1HRC. This specimen is expressed as non-nitrided specimen. Other specimens were nitrided at 823K(550 C) for 3 hours. The following test was done to understanding a phenomenon such as decomposition of compound layer and re-difiion of nitrogen in diffusion layer. The nitrided specimens that were made under the above-mentioned conditions were reheated at 673K(400 C), 773K(500 C), 823K(550 C), 873K(600 C), and 973K(700 C) inside the electric fixnace at atmosphere pressure. The specimens were heated to these temperatures for one hour and cooled to ambient temperature in the fiunace. These specimens are referred to reheated specimens. The specimens, after machining, underwent stress relief annealing Study of the nitriding layer of the reheated specimens Decomposition in the compound layer and the change in the dib?gon layer of the reheated specimens were investigated by the following measurements and observations. X-ray d&-action profiles, Vickers hardness, and microphotographic observation of the sectional area were studied. 23. X-ray stress measurement Table 2 Conditions of X-ray stress Table 2 shows the exposure conditions during X-ray measurement stress measurement. A compound layer exists on the surface of both nitrided specimens, and the reheated specimens at 673K(400 C), and 773K(500 C). The stress was measuxd using the E-F~-~N 103 difeaction and Cr-Ka radiation, a Y-goniometer, the parallel beam method, and a 0-20 scan. The stress of reheated specimens at 873K(600 C) and 973K(700 C), of the non-nitrided specimen and a stress relief annealed specimen, were measured with the a-goniometer using the cxfe 211 difeaction ordinarily used for steel. The residual stress profile from the surf&e of specimens down was measured by the same method, by removing the surf&e of specimens by electro polishing. The measurement of X-ray elastic constants l/s,, 2/s, and others were made in order to measure the changes in the nitrided material. X-ray stress measurements

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol were made on the compressive stress side of the specimens that were set up on a four-point-bending device. The applied stress was measured by an electric strain gauge (gauge length = 2 mm) cemented on the tensile stress side. The mechanical longitudinal elastic constant E mech,, necessary to calculate from applied strain to applied stress, was measured by this device. The E meek of each specimen was measured by this method. 3. Results and discussion 3.1. Observation of the nitriding layer of the specimens Fig. 1 (a) and (b) show m&photographs of the cross section of the nitrided specimen. Fig. l(b) is an enlarged photograph of Fig. l(a). On the surface layer, a compound layer of about 5pm thick existed as shown in Fig. l(b). Beneath the surface layer, the diffusion layer in the nitrogen showed a black color to a depth of about 50~. Fig. 2 shows the Vickers hardness distribution curves from inside the specimen Fig. 1 Microphotogtaphs of nitrided specimen to the surface of the nitrided specimen. The.g hardness value near the su.rfw was about ; OOHv. Moreover, the hardness radically pooo - decreased from the surface down into the = matrix. The hardness was about 5OOW, 2i about the same as the non-nitrided specimen ; of the position at a depth of 150~. As g seen in these results, the depth of the % ,,,..,, hardened layer of the n&tided specimen was.g > ~ ;O about 15Opn-r. Depth from the surface, x (Km) Fig. 3 shows the X-ray diffraction profile that Fig.2 Vickers hardness distribution curves of nittided was observed in the surface of the nitrided and non-nitrided specimens specimen using Cr-Ka radiation. In this specimen, the s--f%-~n and +Fe,N 1200 difhaction existed because a compound layer ;; 1000 formed The peak intensity of the afe diffraction showed a small value, and the 600 E--F%~~N 200 and the afe 211 difsaction 400 were overlapped Diffraction angle, 2 B (deg) Fig.3 X-ray dieaction profile of nitrided specimen

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol Microphotographs, hardness, and X-ray profiles of reheated specimens Fig. 4 (a), (b), and (c) contain microphotogrzlphs of the cross section of the reheated specimens at 773K(500 C), 823K(550 C) and 873K (600 C). Near the surface of the reheated specimen at 773K(500 C), a white compound layer exists as shown in Fig.1. Also, this change was not observed in the dife&ion layer. On one side, in the case of the reheated specimen at 823K(550 C), a part of the (a) 773K(5oo c3 co> 823K(55OV (c) 873Ko Fig.4 Microphotographs of reheated specimens compound layer was transformed into nitrogen gas, and discharged into the atmosphere. The difl%sion layer deepened more than in the z,200 previous specimen for the reason that the other % forms of nitrogen penetrated into the diflgsion g Ooo layer. Furthermore, in the case of the ^ specimen at 873K(600 C), compared with i? K (5OO C), the depth of diffusion layer z ; 400 increased f?om the surf&.x into the matrix. 6 Fig. 5 shows Vickers hardness distribution z ; 200 o curve of nitrided and reheated specimens. In the case below 773K(500 C), the change of the Depth from the surface, x (wm) hardness of the &ion layer near the surface Fig.5 Vickers hardness distribution curves of nilrided was almost not observed. Above 823K and reheated specimens (55O C), the badness showed a marked tendency to decrease. Above of 973K(700 C), the hardness value was less compared to the mentioned reheated specimens. The matrix of the specimens softened similar to that of the stress relief annealed specimen. In short, the decomposition of the compound layer occumzd when reheating was applied nearby A,, critical point of steel. It became clear that the base metal softened as a result of reheating. Fig. 6 shows a change of the X-ray dife-a&on profiles of nibided specimens. The E-F%-~N and the y - Fe,N difeact.ion existed in the nitrided specimen before reheating. In the case of the reheated specimen at 673K(400 C), the dieaction of a new oxide produced by oxidation of surface was observed. The &-F%-,,N and the y -Fe,N dieaction peak intensities, decreased markedly at 823K (550 C). The E -Fe.mSN and the y -Fe,N difftaction disappeared during high temperature reheated at 873K(600 C) and 973K(700 C), the cxfe 211 digaction was clearly observed. Fig.7 shows the change of hardness (measured at depths 10-3Opm l?om the surf e) with changes in the reheating temperature. For simplicity, only the Kelvin scale was used. A difexence in hardness was not found when the reheating temperature was below 773K(500 C) from room temperature. However, hardness radically decreased at high temperatures above 823K(550 C). In short, the

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol hardness decreased rnarkedly near the surface with an increase of temperature. A di.feerence in hardness was observed about 15OIIV compared with 973K(700 C). It was concluded that the difference of the hardness in nitride layer showed a thermal decomposition z of the compound layer and a change in the 5 difilsion layer. 4 (13 z 3.3. Measurement of X-ray elastic constants s -cj and others ii Fig.8 is a diagram of the 20-sin$ scan by a Q- g goniometer obtained from a stress relief annealed specimen. A good linear relationship was obtained over the range of applied stress o,. The changes that relate to each linear gradient M, the 2Ze,, on 28 axis and applied stress ox are shown in Fig.9. 1 * II l Fezm3N x Fe,N O&Fe nfe203 I I I I I I Fig.10 is a diagram of a 28-si.n$ scan with a Diffraction angle, 2 6 (deg) Y-goniometer that was obtained from a nitrided Fig.6 Change in X-ray difliaction profiles specimen before reheating. A good linear ofnitrided andreheated specimens relation in applied stress was obtained in the 1200 stress relief aunealed specimen. Fig. 11 shows -,., o. To -.._... the changes related to each linear gradient M, $., ooo - the 2S,, on 28 axis and applied stress o,. $ 9oo : Table 3 shows the value of X-ray elastic E 8oo - constants l/s,, 2/S,, longitudinal elastic constant E E,,, Poisson s ratio v,,, X-ray stress constant e 6oo : K and Z& of non-strain calculated for the above f 5oo : mention reheated specimens, a non-&tided z 4oo -.,.,,,,,. (,,,,, specimen, and a stress relief annealed specimen Furthermore, the value of the 24 of the non- Temperature, T (K) strain was the average value calculated from the cross point of the linear line in each applied Fig.7 Change in Vickers hardness at different depth by reheating stress CT, in the diagram of the 20-sin*v. No remarkable change was observed in the X-ray elastic constants l/s,, 2/S, of the E-F~~=N 103 dieaction of the reheated specimen at 773K(500 C). These showed nearly the same value as that from the nonnitrided specimen Changes in l/s,, 2/S, were observed in the reheated specimen at 873K(600 C) and 973K(700 C). The ExaY obtained from the X-ray stress measurement was 270GPa in the case of the reheated specimen at 873K(600 C). The E,, of a reheated specimen at 973K(700 C) showed almost the same value compared with the one of the stress relief annealed and non-nitrided specimen The

7 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol , I I I Applied strain (X 1 Om6) gi 0.70 $ b m 0.40 e s P 3 15s-7s 2 ii cn Fig * I sin*$ Fig.8 2Gn$ diagmm of stress relief annealed specimen c Applied strain(x X lo 6, sin * 20-sin2~ diagram of nitrided specimen Applied stress, uz (MPa) E Fig.9 Gradient M and 20 vs. applied stress ox of stress relief annealed specimen $S 0.85 % 0.80 g s $ 0.60 : 0.55 ; 0.50 g 0.45 L Fi:::::: 5 / *. *a_..-_. *. =O so it 3 5 Applied stress, 0; (MPa) Fig. 11 Gradient M and 20 vs. applied stress IS, of nitrided specimen.g Table 3 X-ray elastic constants, stress constant and others

8 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol Poisson s ratio v x-ray also showed a decrease compared with the value of the other specimens, and it was near the value of the stress relief annealed specimen. The penetration depth of the X-ray beam of the afe 211 difhaction used in the measurement of l/s,, 2/S, was about 5pm. The di@erence of the l/s,, 2/S, obtained by above mentioned condition., is caused by the decomposition of the compound layer at high temperatures and the structure change by the formation of a vacancy near the surface. On the reheated specimen at 973K(700 C), nitrogen was removed completely, and it showed nearby the same value as the l/s,, 2/S, obtained f?om a stress relief annealed and a non-nitrided specimens. It is considered that these measurements are proper results for these reasons. In the previous paper ), we studied nitrided materials under difzerent nitriding conditions. As the result, the K, the l/s,, 2/S,, the E,,, and Poisson s ratio v,, were different in all cases. When materials were nitrided the same way, the value of l/s,, 2/S,, the E,,, and Poisson s ratio vxw did not show much change. However, a different value was shown in K when difl%mnt difhaction methods were used. Fig.12 shows the half-value breadth distribution by the Q: Fe 211 dieaction of reheated specimens. The halfvalue breadth ofthe peak was used at ~=6.25 in the measurement of l Nitrided specimen 8 residual stress. The half-value breadth of the 5 7 n 873K(600 2) peak from the surface before reheating is about i+ 6 g This value is fairly large. It became $4 clear that the half-value breadth decreased z 3 markedly to 7.8 with an increase in temperature, at 823K(550 C); 5.6, at K(600 C); and 3.1, at 973K(700 C). It Depth from the surface, x (w m) also became clear that a large value was shown Fig.12 H&value breadth distribution curves of compared to the value at the surface because of nitriding and reheated specimens the effect of nitrogen, on the half-value breadth at a depth near 2Onm-5Opm from the surface at 823K(550 C); and 873K(600 C). However, the value of the half-value breadth of reheating at 973K(700 C) markedly showed a decrease compared with the above-mentioned two specimens. It is considered that the effect of remaing nitrogen is small. The change in ha&value breadth shows a correspondence to changes of the hardness distribution curves. Fig.13 shows the result of the measurement of depth profiles of residual stress distribution curves for nittided and reheated specimens. In the fitigure, the E-F%-~N 103 diffraction was used for the measurement of the residual stress, for the mason that the compound layer existed on the surface of the nitrided specimen, reheated at 673K(400 C) and 773K(500 C). Residual stress was measured in all areas of the diffusion layer except in the above mentioned specimen surface. For the surface of the reheated specimen at 823K(550 C); 873K(600 C); and 973K(700 C), the cxfe 211 dieaction was used. The values shown in Table 3 were used to measure the X-ray stress constant K. The value of K = -639 MEWdeg was used for the E- F%msN 103 difeaction. The K= -318MPa/deg was used in the case of (1: Fe 211 difi?action. Only values of residual stress in the surface were shown in the figure, because a large Wererice was not observed compared to a residual stress distribution before reheating at

9 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol K(400 C) and 773K(500 C). The residual 400 stress in the surface of a nitrided specimen g = (compound layer) before reheating showed a value be -200 of about -200 MPa The value of the compressive f -400 residual stress increased about -9OOMPa at a depth of e -600 about 4Ol..~m fi-om the surface. It showed the ; -800 tendency to become nearly 0 at a depth of about 3 -I opm. The decrease in residual stress was observed inside the reheated specimen at Depth from the surface, x (urn) 873K(600 C). The area of the residual stress was Fig. 13 Residual stress distribution curves of deeper inside. This tendency shows a similar result nitrided and reheated specimens the microphotographic observation and in the distribution curve of hardness. We believe this caused nitrogen di&sing toward the inside of the specimen as a result of reheating. The value of residual stress in the surface was nearly 0 in the case of a reheated specimen at 973K(700 C). The maximum value of tensile residual stress of about 18OMPa was shown at a depth position 4Opm fi-om the surface, but it showed nearly 0 in other areas. 4. Conclusion The summary of our results is as follows. (1) Microphotographs showed decomposition of the compound layer beginning at above 823K(550 C), and the inside diffusion layer was extended by the re-di&&on of nitrogen caused by reheating above 873K(600 C). (2) The Vickers hardness in a diffusion layer at 873K(600 C) decreased, and the base metal softened by reheating at 973K(700 C). (3) In the case of X-ray dieaction profiles, E -F%mxN 103 difh-action decreased markedly when the reheating temperature increased above 823K(550 C), and when the afe 211 difhaction was observed. (4) X-ray stress constant K values were measured by using dieaction techniques with changing reheating temperatures. However, the change of X-ray elastic constants l/s,, US,, longitudinal elastic constant E,,,, and Poisson s ratio vx++, were not observed. These values showed a small differences when the reheating temperature increased at 873K(600 C) and 973K(700 C). (3 The measurement of residual stress distribution in the depth direction of the specimen showed the same tendency as the reheated specimen below 823K(550 C). However, compressive residual stress decreased above 873K(600 C) in the difl%sion layer. A good correlation was obtained between half-value breadth distribution and hardness distribution. In this study, it became clear that comlxxxrd and diffusion layers changed as a result of reheating of a nitrided specimen. A good result was obtained for the evaluation of material and longer life for nitrided die casting dies and thermal forging dies. We believe that nitrided dies and thermal forging dies can be evaluated by the methods described

10 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol in this study. It may also be possible to improve processing of these dies, and increase their usefkl lifetime by using the above tests to assist in processing these types of materials. Reference [ 11 K. Yatsushiro, M. Hihara, K. T agawa and M. Kuramoto, Aa v X-ray Anal., 40 (1998) [2] M. Hihara, K. Fujiwara, Y Mukoyama and I. Ogata, J Japan Sot. Prec. Eng., 55,lO (1989) 1869-l 873 (In Japanese) [3] S. Iwanagq Y Sakakibara, T. Konagq M. Nakamura and T. Kamiya, J Sbc. Material Science Japan, 36,405 (1987) (In Japanese) [4] S. Iwanaga, Y Sakakibara, T. Konaga, M. Nakamura and T. Kamiyq J Sot. Material Science.Japan, 36,411(1987) (In Japanese) [5] A. Schinder, A. Kuhnbeg and J.H. Stuhl, 9th, Society of Die Casting Engineers Congress, G- T77-065, (1977)