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Residual stresses in borided layers
Journal of the Less-Common
Metals, 117 (1986)
RESIDUAL
STRESSES
IN BORIDED
R. PRIMMER
and W. PFEIFFER
Fraunhofer-Institute
for Materials Mechanics,
411 - 414
411
LAYERS*
D-7800
Freiburg,
Wiihlerstrasse
11 (F.R.G.)
Summary Pure iron (Armco-iron) was borided. A 100 pm thick surface layer consists of FeB at the immediate surface and FezB in deeper regions, followed by a diffusion layer. X-ray stress analysis proves to be an appropriate method to determine the residual stresses non-destructively. At the surface a residual compressive stress of 600 MPa is found. Its variation with depth below the surface is determined.
1. Introduction Boriding of steel is performed on finished products exposed to severe service conditions, especially when abrasive wear can occur. One of the disadvantages can be a reduced fatigue behaviour or crack-formation either in the boride layer or at the interphase. The layer can consist of the iron borides FeB and Fe,B, depending on the boriding conditions. Residual stresses in the boride layer can be created due to different thermal expansions of the layer and the substrate after cooling from the boriding temperature and owing to different elastic properties. The method of X-ray stress analysis was applied in order to determine the residual stresses in boride layers after boriding of pure iron.
2. Boriding Pure iron samples, consisting of Armco-iron (C < 0.02%), with dimensions of 50 mm in diameter and 5 mm thick were exposed to a boriding atmosphere (Ekabor 501, Elektroschmelzwerk Kempten) at a temperature of
*Paper presented at the 8th Nitrides and Related Compounds,
International Symposium on Boron, Tbilisi, October 8 - 12, 1984. @ Elsevier
Sequoia/Printed
Borides,
Carbides,
in The Netherlands
412
Fig. 1. Micrograph of borided layer.
1000 “C for 3 h. The metallurgical investigations reveal a 100 pm thick layer. Its outer surface was determined by means of X-ray diffraction patterns to comprise FeB, with FezB needles in deeper layers. Deeper still a diffusion layer with boride contents amounting to not more than 0.01% and about 250 pm thick is found. X-ray investigations also revealed a severe (002)-fibre texture in both layers. The diffusion layer shows a severe texture as well as precipitates, presumably consisting of FeB. 3. X-ray stress analysis The method of X-ray stress analysis, first applied by Akzenov [ 11, is of increasing applicability [2,3]. It allows the non-destructive determination of stresses in surface layers of polycrystalline samples. A monochromatic X-ray beam impinging on the surface is diffracted at lattice planes of crystallites which Eulfil the Bragg equation. When loading stresses or residual stresses are active in the surface layers of a polycrystalline sample the lattice spacing is altered in different directions and as a result of this a d~fraction angle varying with the direction of me~urement is observed, In contrast with mechanical methods of strain analysis the X-ray method allows the determination of the strain ellipsoid. The knowledge of the strain ellipsoid allows the calculation of the stress state making use of the X-ray elastic constants ‘[41. 4. Results Residual strains in borided planes of Fe,B (with tetragonal
samples were determined at the {204] lattice structure) using Fe KCYradiation (wavelength
413
-100
-
‘7
-200
-
: 5
-300
-
+ Y
z f -400
:
v)
2 -500 2 $ -600 -700
20
40
60
00
100
120
140
160
100 DEPTH
Fig. 2. Residual
stress distribution
in the boride layer and substrate
200
(pm) US.
depth.
X = 1.935 97 A) with a diffraction angle of 2&J= 161”. The measurements at the pure iron and in the diffusion layer were determined using the same radiation and the (220) lattice planes of o-Fe with a diffraction angle of 28 = 145.5”. A determination of the residual stress distribution uersus the depth under the surface is possible by electrolytical etching away of surface layers and subsequent repetition of the measurements. Fig. 2 shows the results of these investigations; compressive residual stresses are detected in the boride layers. Their highest value arises at the immediate surface, amounting to 600 N mmp2 and approaching zero at depths greater than the diffusion layer.
5. Discussion Residual stresses of a compressive nature are known to improve the service behaviour of machine parts whereas such stresses of tensile nature can be of detrimental effect. The compressive residual stresses observed in the Fe2B layers of borided Armco-iron are therefore beneficial. They also increase the capacity for static tensile loads. However, their sizes are still a matter of further investigations. In our case the measured lattice strains were transformed to stresses making use of the Youngs modulus and Poisson ratio of ferritic steel (E = 200 000 N mmm2, V = 0.2). The X-ray stress analysis method, however, is of a selective nature: crystallites, contributing to the Bragg reflections, are measured in certain crystallographic orientations. If the elastic behaviour of the single crystal is
414
anisotropic (this usually is the case) elastic anisotropy has to be taken into account. To correct for this either an experimental determination of the X-ray elastic constants has to be performed or their value has to be calculated. However, the values describing the elastic behaviour of the single crystal, the single crystal elastic coefficients Sik or elastic stiffness Cik, are not yet available for Fe2B. Therefore, further investigations are recommended in order to improve the reliability of the X-ray determination of residual and loading stresses in boride layers.
References 1 G. J. Aksenov, Measurement of elastic stress in a fine grained material, J. Appl. Phys. (USSR), 6 (1929) 3 - 16. 2 R. Glocker, Materialpriifung mit R6ntgenstrahlen, Springer Verlag, Berlin, 1971. 3 V. Hauk and E. Macherauch (eds.), Eigensponnungen und Lastspannungen, Hauser, Miinchen, 1982. 4 R. Priimmer and H. W. Pfeiffer-Vollmar, A method for X-ray stress analysis of thermochemically treated materials, Adu. X-Ray Stress Anal., 26 (1983) 225 - 231.
Metals, 117 (1986)
RESIDUAL
STRESSES
IN BORIDED
R. PRIMMER
and W. PFEIFFER
Fraunhofer-Institute
for Materials Mechanics,
411 - 414
411
LAYERS*
D-7800
Freiburg,
Wiihlerstrasse
11 (F.R.G.)
Summary Pure iron (Armco-iron) was borided. A 100 pm thick surface layer consists of FeB at the immediate surface and FezB in deeper regions, followed by a diffusion layer. X-ray stress analysis proves to be an appropriate method to determine the residual stresses non-destructively. At the surface a residual compressive stress of 600 MPa is found. Its variation with depth below the surface is determined.
1. Introduction Boriding of steel is performed on finished products exposed to severe service conditions, especially when abrasive wear can occur. One of the disadvantages can be a reduced fatigue behaviour or crack-formation either in the boride layer or at the interphase. The layer can consist of the iron borides FeB and Fe,B, depending on the boriding conditions. Residual stresses in the boride layer can be created due to different thermal expansions of the layer and the substrate after cooling from the boriding temperature and owing to different elastic properties. The method of X-ray stress analysis was applied in order to determine the residual stresses in boride layers after boriding of pure iron.
2. Boriding Pure iron samples, consisting of Armco-iron (C < 0.02%), with dimensions of 50 mm in diameter and 5 mm thick were exposed to a boriding atmosphere (Ekabor 501, Elektroschmelzwerk Kempten) at a temperature of
*Paper presented at the 8th Nitrides and Related Compounds,
International Symposium on Boron, Tbilisi, October 8 - 12, 1984. @ Elsevier
Sequoia/Printed
Borides,
Carbides,
in The Netherlands
412
Fig. 1. Micrograph of borided layer.
1000 “C for 3 h. The metallurgical investigations reveal a 100 pm thick layer. Its outer surface was determined by means of X-ray diffraction patterns to comprise FeB, with FezB needles in deeper layers. Deeper still a diffusion layer with boride contents amounting to not more than 0.01% and about 250 pm thick is found. X-ray investigations also revealed a severe (002)-fibre texture in both layers. The diffusion layer shows a severe texture as well as precipitates, presumably consisting of FeB. 3. X-ray stress analysis The method of X-ray stress analysis, first applied by Akzenov [ 11, is of increasing applicability [2,3]. It allows the non-destructive determination of stresses in surface layers of polycrystalline samples. A monochromatic X-ray beam impinging on the surface is diffracted at lattice planes of crystallites which Eulfil the Bragg equation. When loading stresses or residual stresses are active in the surface layers of a polycrystalline sample the lattice spacing is altered in different directions and as a result of this a d~fraction angle varying with the direction of me~urement is observed, In contrast with mechanical methods of strain analysis the X-ray method allows the determination of the strain ellipsoid. The knowledge of the strain ellipsoid allows the calculation of the stress state making use of the X-ray elastic constants ‘[41. 4. Results Residual strains in borided planes of Fe,B (with tetragonal
samples were determined at the {204] lattice structure) using Fe KCYradiation (wavelength
413
-100
-
‘7
-200
-
: 5
-300
-
+ Y
z f -400
:
v)
2 -500 2 $ -600 -700
20
40
60
00
100
120
140
160
100 DEPTH
Fig. 2. Residual
stress distribution
in the boride layer and substrate
200
(pm) US.
depth.
X = 1.935 97 A) with a diffraction angle of 2&J= 161”. The measurements at the pure iron and in the diffusion layer were determined using the same radiation and the (220) lattice planes of o-Fe with a diffraction angle of 28 = 145.5”. A determination of the residual stress distribution uersus the depth under the surface is possible by electrolytical etching away of surface layers and subsequent repetition of the measurements. Fig. 2 shows the results of these investigations; compressive residual stresses are detected in the boride layers. Their highest value arises at the immediate surface, amounting to 600 N mmp2 and approaching zero at depths greater than the diffusion layer.
5. Discussion Residual stresses of a compressive nature are known to improve the service behaviour of machine parts whereas such stresses of tensile nature can be of detrimental effect. The compressive residual stresses observed in the Fe2B layers of borided Armco-iron are therefore beneficial. They also increase the capacity for static tensile loads. However, their sizes are still a matter of further investigations. In our case the measured lattice strains were transformed to stresses making use of the Youngs modulus and Poisson ratio of ferritic steel (E = 200 000 N mmm2, V = 0.2). The X-ray stress analysis method, however, is of a selective nature: crystallites, contributing to the Bragg reflections, are measured in certain crystallographic orientations. If the elastic behaviour of the single crystal is
414
anisotropic (this usually is the case) elastic anisotropy has to be taken into account. To correct for this either an experimental determination of the X-ray elastic constants has to be performed or their value has to be calculated. However, the values describing the elastic behaviour of the single crystal, the single crystal elastic coefficients Sik or elastic stiffness Cik, are not yet available for Fe2B. Therefore, further investigations are recommended in order to improve the reliability of the X-ray determination of residual and loading stresses in boride layers.
References 1 G. J. Aksenov, Measurement of elastic stress in a fine grained material, J. Appl. Phys. (USSR), 6 (1929) 3 - 16. 2 R. Glocker, Materialpriifung mit R6ntgenstrahlen, Springer Verlag, Berlin, 1971. 3 V. Hauk and E. Macherauch (eds.), Eigensponnungen und Lastspannungen, Hauser, Miinchen, 1982. 4 R. Priimmer and H. W. Pfeiffer-Vollmar, A method for X-ray stress analysis of thermochemically treated materials, Adu. X-Ray Stress Anal., 26 (1983) 225 - 231.