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Residual stresses induced in steel by laser melting
Materials Science and Engineering A 174 (1994) L51 -L54
L5 1
Letter
Residual stresses induced in steel by laser melting R. Krfilovfi k~wulty of Nuclear Science and Physical Enginering, Czech Technical UniversiO', Biehovd 7, 115 19 Prague 1 (Czech Republic') (Received June 28, 1993)
Abstract The residual macrostresses in laser-treated low carbon steel as a function of the scanning velocity are investigated by X-ray diffraction. The scanning velocity is found to influence strongly the magnitude and sign of the longitudinal residual stresses in laser-treated regions.
1. Introduction One of the most important ways to control the physical properties of surface layers is to use a thermal treatment by a laser beam, which has the character of programmable heating, melting and cooling. This allows for the application of thermal treatment which is sharply confined in space and time, so that the interior of the sample remains unperturbed [1-3]. The use of laser radiation in the regime of melting, with a power density of about 107-108 k W m -2, leads to the appearance of residual stresses in the remelted layer. There are several factors which contribute to the final stress state, namely the volume difference on solidification, different coefficients of thermal expansion and various transformations in both the case and the core. Also, external laser parameters such as the distribution of the energy in the laser beam, the laser beam diameter and the scanning velocity will influence the stress state. It is possible for compressive or tensile stresses to be produced depending on the interplay of the various effects [4-6].
2. Material and experiment In this work the residual stresses are measured after the laser treatment of low carbon steel of Czech 0921-5093/94/$7.00
standard CSN 11373 (0.15% C) as a function of the scanning velocity, which is the velocity of the sample shift with respect to the laser beam. A plate specimen of size 80 x 50 x 5 mm 3, the geometry of which is shown in Fig. 1, was ground, annealed for 3 h at 650 °C in hydrogen and subsequently cooled very slowly to a temperature of 150 °C. For the laser treatment a Control Laser Limited CO 2 laser was used. The laser treatment parameters of the sample were as follows. The maximum power of the laser was 2 kW, the focal length of the lens was 200 mm and the distance of the focal point below the surface of the sample was 15 ram, resulting in a laser spot about 1 mm in diameter. In each run of the laser treatment, two tracks (A and B in Fig. 1) were remelted under identical conditions. The tracks were remelted in pairs in order to check the reproducibility of the measured data. The pairs differed from one another in the scanning velocity used. The scanning velocities of pairs 1-6 were 4.2, 4.2, 8.3, 12.5, 25 and 50 mm s ~ respectively. The distance between the centres of two tracks within a single pair was 5 ram, while neighbouring pairs were separated by a non-remelted zone approximately 8 mm wide. The width of the melted region depends on the scanning velocity and was about 2.5 mm at the lowest velocity, while at the highest velocity it was about 1.5 mm.
The residual stresses were determined by the X-ray single-exposure method [7]. Cr K a radiation was used. The incident primary beam, directed by a cylindrical collimator 1 mm in diameter, reached the surface at % = 4 5 ° ; the irradiated area amounted to approxi-
cLr 1
tHtHtNHH 2
3
5
AB
Fig. I. Shape of the specimen with the pairs of individual tracks (A, B). The black dot marks the position at which o~ and crv a r e measured. © 1994 - Elsevier Sequoia. All rights reserved
L52
Letter
mately 1.5 mm 2. The calculation of stresses from the measured changes in the lattice spacing d2~i was done using the {211} diffraction line. This approach assumes a biaxial stress state (we consider only the principal stresses o~ and (72 in the surface layer) and a material without texture. Owing to the small penetration depth of Cr K a radiation in ferrous material (about 5.5 pm), it is possible to consider o~ as zero. From the shape of the {211} diffraction line we conclude that the remelted region was not textured. The longitudinal residual stresses aL along the tracks and the transverse residual stresses o~r perpendicular to the tracks were measured (Fig. 1). Also, the halfwidth W of the analysed diffraction line {211} was measured. In order to obtain absolute values of residual stresses, we must know the interplanar spacing dE1 ]0 of a stress-free sample and also the X-ray values of the Young modulus E rtg and the Poisson ratio ~rtg of the layer. Instead of d2~,°, we can u s e dE jl n measured at normal incidence of Cr K a radiation on the surface without introducing a large error [7]. It is not easy to obtain reliable data, either experimental or theoretical, on E rtg and 1,prtg in the laser-treated surface region. Usually bulk values of E and v are used. This means, however, that the residual stresses obtained have the" character of relative values.
'Or ,MPa A
200-
1000 -100-
B 1 1
I 2
,7~ ~ ~, I 3~
number I °f'track 6
I 5
-200-300-400-500-
V
1IW, °
2
0 ~
I
Y ~
number
I
1
2
I
3
I
4
I
S
I °f
track
6
Or,MPa 200. 100-
3. Results and discussion
t±aaL
300-
0
T+-ao,
~ n u m b e r A ] of track
-100The values of residual macroscopic stresses oL measured in the individual pairs of tracks are presented in Fig. 2a. At low scanning velocities (4.2 and 8.3 mm s-~, pairs 1-3) the stresses have positive values. At velocities of 12.5, 25 and 50 mm s 1, however, the stresses change to compressive ones, with the maximum value at 25 mm s-1. The corresponding distribution of the halfwidth of the {211} diffraction line reaches a maximum at the same scanning velocity (Fig. 2b). Owing to the statistical uncertainty of the measurements, the data obtained for o T (Fig. 2c) do not allow us to make a definite conclusion about the sign and magnitude of this quantity. Nevertheless, we observe that OT is practically zero. The presence of residual stresses in laser-treated surface is usually considered to be due to two mechanisms, namely [8] (i) plastic deformation due to thermal expansion and (ii) volume changes due to phase transformation. (i) The material in the remelted track is rapidly cooled from the melting temperature. The surface and subsurface (interior) layers are then contracting at different rates. The interior of the melted region cools faster because of the thermal sink of the bulk, which
-200Fig. 2. Distribution of oL (a), W (b) and off (c)in tracks 1-6 with the respective scanning velocitiesof 4.2, 4.2, 8.3, 12.5, 25 and 50 mm s- ~.Vertical bars denote experimental precision.
remains at room temperature. After some time this difference tends to be equalized by plastic flow. Since the surface is warmer than the interior, its subsequent contraction during cooling is greater. This results in a constrained interior of the sample and in tensile stresses in the surface layer at room temperature. (ii) When the material undergoes a phase transformation such as a martensitic phase transformation, the interior transforms to martensite during cooling and expands, since the interior reaches the transformation temperature first. The surface layer, composed of low strength austenite, deforms plastically to accommodate this change. At some time the surface transforms to martensite, producing an expansion which is resisted by the high strength martensitic interior. At room temperture the surface is forced into compression by the
Letter
interior, producing surface residual compressive stresses. It is well known that the important point is whether the stresses are tensile or compressive. T h e resulting stress state in the laser-treated region depends upon which of the above mechanisms plays the major role. It appears that, depending on the conditions, tensile or compressive stresses can be found in ferrous materials in both longitudinal and perpendicular directions. This is illustrated in Table 1, in which qualitative characteristics of the stress state measured on single-track laser-treated ferrous materials by several authors are presented. T h e results given in Fig. 2 show that an important parameter, changes in which decide whether the stresses are either tensile or compressive, is the scanning velocity. This will be interpreted in what follows. T h e ratio of the laser beam diameter together with the scanning velocity v define the interaction time r. T h e interaction times of the individual tracks of our sample are given in Table 2. In low carbon steel the martensitic transformation requires a cooling rate of about 103 K s ~ [11]. T h e shorter the interaction time, the faster will be the cooling rate in the laser-treated region. It can be expected that at longer interaction times, i.e. at low scanning velocities, the tensile residual stresses observed in tracks 1-3 have their origin in the plastic deformation caused by the different time dependences of cooling of the surface and subsurface
TABLE 1. Qualitative residual stresses in ferrous materials. P is the CO2 laser output; C, compressive; T, tensile Material
P (kW)
t' ok (rams i)
ox
Reference
Ck 22 Ck 45 AISI 11118 AISI 1040 Steel 0.2%C Steel 0.2%C C22 CK 60
0.6 0.6 5 5 2 2 1.3 1.3
11 8 42 42 16 33 10 10
----C C T T
[5] [51 [8] [8] [9] [9] [10l [101
C T T C -----
TABLE 2. Interaction times of tracks Track
t, (mm s- ')
r(s)
1
4.2 4.2 8.3 12.5 25 5O
(/.24 (/.24 0.12 O.O8 0.04 O.O2
2 3 4 5 6
L 53
regions. However, when the interaction time decreases below a value of about 0.1 s, the cooling rate is sufficiently fast for the martensitic transformation to occur and compressive stresses are observed at the surface (tracks 4 - 6 in Fig. 2a). This interpretation is found to be in agreement with the increase in the {21 11 diffraction line halfwidth (Fig. 2b), which is an implication of the presence of lattice imperfections and of the fine structure which develops in the course of the martensitic transformation. We observed that the compressive residual stress OL in track 6 (Fig. 2a) is much lower than that in track 5. This can be explained in the following way. T h e depth of the remelted and heat-affected zone decreases with decreasing interaction time for a given diameter of the laser beam [12, 13]. When the interaction time decreases, the thickness of the subsurface region decreases as well and the compressive ability of the subsurface region is reduced. This effect is manifested as a decrease in the compressive residual stress in the surface layer in track 6 with respect to the value found in track 5. As already stated above, the values of o 1- are very small or zero in all tracks. Comparison of Fig. 2a with Fig. 2c clearly shows that the absolute values of o L are much larger than the absolute values of o> A m o n g various explanations for this difference we can consider the possibility of relaxation of o f due to interaction with the neighbourhood of the track in the case of narrow tracks. Intuitively, the critical track width, beyond which the relaxation of o f should not be expected to occur, should be comparable with the depth of the remelted region, which typically amounts to 1 mm or less [5, 8, 12] in similar experiments. In fact, measurements performed on low carbon steel [9] on a track with a width of about 6 mm reveal that o f is approximately between - 3 0 0 and - 4 5 0 MPa, which are the usual values of o L measured. In summary, longitudinal, o r , and transverse, Of, residual stresses were measured in low carbon steel by X-ray diffraction. T h e scanning velocity was found to have a strong influence on the development of the tensile and compressive residual stresses OL in the laser-treated surface. This influence was discussed from the point of view of thermal expansion and phase transformation. T h e observed p h e n o m e n o n of loll "* lOLl was discussed in comparison with other works.
References
1 T.R. Anthony and H. Cline, J. Appl. l'hys., 48( 197713888. 2 C. Chabrol and A. Vannes, NATOASISer. E, 115(1986) 26. 3 V. S. Vclikich, V. R Gon6arenko, A. F. Zverev and V. S. Kartavcev, Metall. Term. Obr. Memll., 4(1985) 9 (in Russian).
L54
Letter
4 D. Miiller, J. Domes and H. W. Bergmann, HiirtereiTechnische Mitteilungen, 47 (1992) 2. 5 B. Scholtes and B. L. Mordike, in E. Macherauch and V. Hauk (eds.), Residual Stresses in Science and Technology, Vol. 2, DGM Informationsgesellschaft, Oberursel, 1987, p. 655. 6 B. L. Mordike, in Advances in Surface Treatment, Vol. 5, Pergamon, New York, 1987, p. 381. 7 I. C. Noyan and J. B. Cohen, Residual Stress, Springer, New York, 1987. 8 M. R. James, D. S. Gnanamuthu and R. J. Moores, Scr. Metall., 18(1984) 357.
9 N. Ganev, I. Kraus and J. Trp6evsk~i, Phys. Status Solidi, 115 (1989)K13. 10 B. A. Van Brussel and J. Th. M. De Hosson, Mater. Sci. Eng. A, 161 (1993)83. 11 Z. Nishiyama, in M. E. Fine, M. Meshii and C. M. Wayman (eds.), Martensitic Transformation, Academic, New York, 1978, p. 152. 12 Y. Nilsson, in B. H. Kear, B. C. Giessen and M. Cohen (eds.), Rapidly Solidified Amorphous and Crystalline Alloys, Vol. 8, North-Holland, Amsterdam, 1982, p. 517. 13 Ch. Maier, P. Schaaf and U. Gauser, Mater. Sci. Eng. A, 150 (1992)271.
L5 1
Letter
Residual stresses induced in steel by laser melting R. Krfilovfi k~wulty of Nuclear Science and Physical Enginering, Czech Technical UniversiO', Biehovd 7, 115 19 Prague 1 (Czech Republic') (Received June 28, 1993)
Abstract The residual macrostresses in laser-treated low carbon steel as a function of the scanning velocity are investigated by X-ray diffraction. The scanning velocity is found to influence strongly the magnitude and sign of the longitudinal residual stresses in laser-treated regions.
1. Introduction One of the most important ways to control the physical properties of surface layers is to use a thermal treatment by a laser beam, which has the character of programmable heating, melting and cooling. This allows for the application of thermal treatment which is sharply confined in space and time, so that the interior of the sample remains unperturbed [1-3]. The use of laser radiation in the regime of melting, with a power density of about 107-108 k W m -2, leads to the appearance of residual stresses in the remelted layer. There are several factors which contribute to the final stress state, namely the volume difference on solidification, different coefficients of thermal expansion and various transformations in both the case and the core. Also, external laser parameters such as the distribution of the energy in the laser beam, the laser beam diameter and the scanning velocity will influence the stress state. It is possible for compressive or tensile stresses to be produced depending on the interplay of the various effects [4-6].
2. Material and experiment In this work the residual stresses are measured after the laser treatment of low carbon steel of Czech 0921-5093/94/$7.00
standard CSN 11373 (0.15% C) as a function of the scanning velocity, which is the velocity of the sample shift with respect to the laser beam. A plate specimen of size 80 x 50 x 5 mm 3, the geometry of which is shown in Fig. 1, was ground, annealed for 3 h at 650 °C in hydrogen and subsequently cooled very slowly to a temperature of 150 °C. For the laser treatment a Control Laser Limited CO 2 laser was used. The laser treatment parameters of the sample were as follows. The maximum power of the laser was 2 kW, the focal length of the lens was 200 mm and the distance of the focal point below the surface of the sample was 15 ram, resulting in a laser spot about 1 mm in diameter. In each run of the laser treatment, two tracks (A and B in Fig. 1) were remelted under identical conditions. The tracks were remelted in pairs in order to check the reproducibility of the measured data. The pairs differed from one another in the scanning velocity used. The scanning velocities of pairs 1-6 were 4.2, 4.2, 8.3, 12.5, 25 and 50 mm s ~ respectively. The distance between the centres of two tracks within a single pair was 5 ram, while neighbouring pairs were separated by a non-remelted zone approximately 8 mm wide. The width of the melted region depends on the scanning velocity and was about 2.5 mm at the lowest velocity, while at the highest velocity it was about 1.5 mm.
The residual stresses were determined by the X-ray single-exposure method [7]. Cr K a radiation was used. The incident primary beam, directed by a cylindrical collimator 1 mm in diameter, reached the surface at % = 4 5 ° ; the irradiated area amounted to approxi-
cLr 1
tHtHtNHH 2
3
5
AB
Fig. I. Shape of the specimen with the pairs of individual tracks (A, B). The black dot marks the position at which o~ and crv a r e measured. © 1994 - Elsevier Sequoia. All rights reserved
L52
Letter
mately 1.5 mm 2. The calculation of stresses from the measured changes in the lattice spacing d2~i was done using the {211} diffraction line. This approach assumes a biaxial stress state (we consider only the principal stresses o~ and (72 in the surface layer) and a material without texture. Owing to the small penetration depth of Cr K a radiation in ferrous material (about 5.5 pm), it is possible to consider o~ as zero. From the shape of the {211} diffraction line we conclude that the remelted region was not textured. The longitudinal residual stresses aL along the tracks and the transverse residual stresses o~r perpendicular to the tracks were measured (Fig. 1). Also, the halfwidth W of the analysed diffraction line {211} was measured. In order to obtain absolute values of residual stresses, we must know the interplanar spacing dE1 ]0 of a stress-free sample and also the X-ray values of the Young modulus E rtg and the Poisson ratio ~rtg of the layer. Instead of d2~,°, we can u s e dE jl n measured at normal incidence of Cr K a radiation on the surface without introducing a large error [7]. It is not easy to obtain reliable data, either experimental or theoretical, on E rtg and 1,prtg in the laser-treated surface region. Usually bulk values of E and v are used. This means, however, that the residual stresses obtained have the" character of relative values.
'Or ,MPa A
200-
1000 -100-
B 1 1
I 2
,7~ ~ ~, I 3~
number I °f'track 6
I 5
-200-300-400-500-
V
1IW, °
2
0 ~
I
Y ~
number
I
1
2
I
3
I
4
I
S
I °f
track
6
Or,MPa 200. 100-
3. Results and discussion
t±aaL
300-
0
T+-ao,
~ n u m b e r A ] of track
-100The values of residual macroscopic stresses oL measured in the individual pairs of tracks are presented in Fig. 2a. At low scanning velocities (4.2 and 8.3 mm s-~, pairs 1-3) the stresses have positive values. At velocities of 12.5, 25 and 50 mm s 1, however, the stresses change to compressive ones, with the maximum value at 25 mm s-1. The corresponding distribution of the halfwidth of the {211} diffraction line reaches a maximum at the same scanning velocity (Fig. 2b). Owing to the statistical uncertainty of the measurements, the data obtained for o T (Fig. 2c) do not allow us to make a definite conclusion about the sign and magnitude of this quantity. Nevertheless, we observe that OT is practically zero. The presence of residual stresses in laser-treated surface is usually considered to be due to two mechanisms, namely [8] (i) plastic deformation due to thermal expansion and (ii) volume changes due to phase transformation. (i) The material in the remelted track is rapidly cooled from the melting temperature. The surface and subsurface (interior) layers are then contracting at different rates. The interior of the melted region cools faster because of the thermal sink of the bulk, which
-200Fig. 2. Distribution of oL (a), W (b) and off (c)in tracks 1-6 with the respective scanning velocitiesof 4.2, 4.2, 8.3, 12.5, 25 and 50 mm s- ~.Vertical bars denote experimental precision.
remains at room temperature. After some time this difference tends to be equalized by plastic flow. Since the surface is warmer than the interior, its subsequent contraction during cooling is greater. This results in a constrained interior of the sample and in tensile stresses in the surface layer at room temperature. (ii) When the material undergoes a phase transformation such as a martensitic phase transformation, the interior transforms to martensite during cooling and expands, since the interior reaches the transformation temperature first. The surface layer, composed of low strength austenite, deforms plastically to accommodate this change. At some time the surface transforms to martensite, producing an expansion which is resisted by the high strength martensitic interior. At room temperture the surface is forced into compression by the
Letter
interior, producing surface residual compressive stresses. It is well known that the important point is whether the stresses are tensile or compressive. T h e resulting stress state in the laser-treated region depends upon which of the above mechanisms plays the major role. It appears that, depending on the conditions, tensile or compressive stresses can be found in ferrous materials in both longitudinal and perpendicular directions. This is illustrated in Table 1, in which qualitative characteristics of the stress state measured on single-track laser-treated ferrous materials by several authors are presented. T h e results given in Fig. 2 show that an important parameter, changes in which decide whether the stresses are either tensile or compressive, is the scanning velocity. This will be interpreted in what follows. T h e ratio of the laser beam diameter together with the scanning velocity v define the interaction time r. T h e interaction times of the individual tracks of our sample are given in Table 2. In low carbon steel the martensitic transformation requires a cooling rate of about 103 K s ~ [11]. T h e shorter the interaction time, the faster will be the cooling rate in the laser-treated region. It can be expected that at longer interaction times, i.e. at low scanning velocities, the tensile residual stresses observed in tracks 1-3 have their origin in the plastic deformation caused by the different time dependences of cooling of the surface and subsurface
TABLE 1. Qualitative residual stresses in ferrous materials. P is the CO2 laser output; C, compressive; T, tensile Material
P (kW)
t' ok (rams i)
ox
Reference
Ck 22 Ck 45 AISI 11118 AISI 1040 Steel 0.2%C Steel 0.2%C C22 CK 60
0.6 0.6 5 5 2 2 1.3 1.3
11 8 42 42 16 33 10 10
----C C T T
[5] [51 [8] [8] [9] [9] [10l [101
C T T C -----
TABLE 2. Interaction times of tracks Track
t, (mm s- ')
r(s)
1
4.2 4.2 8.3 12.5 25 5O
(/.24 (/.24 0.12 O.O8 0.04 O.O2
2 3 4 5 6
L 53
regions. However, when the interaction time decreases below a value of about 0.1 s, the cooling rate is sufficiently fast for the martensitic transformation to occur and compressive stresses are observed at the surface (tracks 4 - 6 in Fig. 2a). This interpretation is found to be in agreement with the increase in the {21 11 diffraction line halfwidth (Fig. 2b), which is an implication of the presence of lattice imperfections and of the fine structure which develops in the course of the martensitic transformation. We observed that the compressive residual stress OL in track 6 (Fig. 2a) is much lower than that in track 5. This can be explained in the following way. T h e depth of the remelted and heat-affected zone decreases with decreasing interaction time for a given diameter of the laser beam [12, 13]. When the interaction time decreases, the thickness of the subsurface region decreases as well and the compressive ability of the subsurface region is reduced. This effect is manifested as a decrease in the compressive residual stress in the surface layer in track 6 with respect to the value found in track 5. As already stated above, the values of o 1- are very small or zero in all tracks. Comparison of Fig. 2a with Fig. 2c clearly shows that the absolute values of o L are much larger than the absolute values of o> A m o n g various explanations for this difference we can consider the possibility of relaxation of o f due to interaction with the neighbourhood of the track in the case of narrow tracks. Intuitively, the critical track width, beyond which the relaxation of o f should not be expected to occur, should be comparable with the depth of the remelted region, which typically amounts to 1 mm or less [5, 8, 12] in similar experiments. In fact, measurements performed on low carbon steel [9] on a track with a width of about 6 mm reveal that o f is approximately between - 3 0 0 and - 4 5 0 MPa, which are the usual values of o L measured. In summary, longitudinal, o r , and transverse, Of, residual stresses were measured in low carbon steel by X-ray diffraction. T h e scanning velocity was found to have a strong influence on the development of the tensile and compressive residual stresses OL in the laser-treated surface. This influence was discussed from the point of view of thermal expansion and phase transformation. T h e observed p h e n o m e n o n of loll "* lOLl was discussed in comparison with other works.
References
1 T.R. Anthony and H. Cline, J. Appl. l'hys., 48( 197713888. 2 C. Chabrol and A. Vannes, NATOASISer. E, 115(1986) 26. 3 V. S. Vclikich, V. R Gon6arenko, A. F. Zverev and V. S. Kartavcev, Metall. Term. Obr. Memll., 4(1985) 9 (in Russian).
L54
Letter
4 D. Miiller, J. Domes and H. W. Bergmann, HiirtereiTechnische Mitteilungen, 47 (1992) 2. 5 B. Scholtes and B. L. Mordike, in E. Macherauch and V. Hauk (eds.), Residual Stresses in Science and Technology, Vol. 2, DGM Informationsgesellschaft, Oberursel, 1987, p. 655. 6 B. L. Mordike, in Advances in Surface Treatment, Vol. 5, Pergamon, New York, 1987, p. 381. 7 I. C. Noyan and J. B. Cohen, Residual Stress, Springer, New York, 1987. 8 M. R. James, D. S. Gnanamuthu and R. J. Moores, Scr. Metall., 18(1984) 357.
9 N. Ganev, I. Kraus and J. Trp6evsk~i, Phys. Status Solidi, 115 (1989)K13. 10 B. A. Van Brussel and J. Th. M. De Hosson, Mater. Sci. Eng. A, 161 (1993)83. 11 Z. Nishiyama, in M. E. Fine, M. Meshii and C. M. Wayman (eds.), Martensitic Transformation, Academic, New York, 1978, p. 152. 12 Y. Nilsson, in B. H. Kear, B. C. Giessen and M. Cohen (eds.), Rapidly Solidified Amorphous and Crystalline Alloys, Vol. 8, North-Holland, Amsterdam, 1982, p. 517. 13 Ch. Maier, P. Schaaf and U. Gauser, Mater. Sci. Eng. A, 150 (1992)271.