- Email: [email protected]
Influence of an intermediate layer on the residual stress field in a laser clad
Surface and Coatings Technology, 45 (1991) 435-441
435
I n f l u e n c e of a n i n t e r m e d i a t e l a y e r on t h e r e s i d u a l stress field in a l a s e r c l a d A. F r e n k , C. F. M a r s d e n a n d J . - D . W a g n i 6 r e Laboratoire de Mdtallurgie Physique, CTML, EPFL, CH-1015 Lausanne (Switzerland)
A. B. V a n n e s MMP, CALFETMAT, Ecole Centrale Lyon, F-69131 Ecully Cedex (France)
M. L a r a c i n e a n d M. Y. L o r m a n d CALFETMAT, Bat. 403, INSA, F-69621 Villeurbanne Cedex (France)
Abstract Large residual stresses are generally produced during laser surface treatments owing to their associated high thermal gradients and cooling rates. In laser clads the high tensile residual stresses usually obtained can produce cracking or limit the practical use of these deposits. To investigate the potential of intermediate layers in controlling the residual stresses, multilayer clads were produced on a high chromium martensitic steel, with an austenitic stainless steel as an intermediate layer and stellite 6, a cobalt-based alloy, as the surfacing material. The stress profiles were calculated using the measurements of the deformation resulting from the successive electrochemical removal of thin layers. It was observed that the sign of the residual stresses in the surface alloy was not changed by one or more intermediate layers.
1. I n t r o d u c t i o n Residual stresses are k n o w n to affect the fatigue, creep a n d brittle f r a c t u r e p r o p e r t i e s of e n g i n e e r i n g c o m p o n e n t s as well as t h e i r s t r u c t u r a l stability, w e a r a n d c o r r o s i o n b e h a v i o u r . The p r o b l e m of residual stress g e n e r a t i o n d u r i n g laser surface t r e a t m e n t s is a s s o c i a t e d with h i g h t h e r m a l g r a d i e n t s a n d cooling r a t e s [1, 2]. In most cases w h e n m e l t i n g o c c u r s (laser remelting, alloying and cladding) the resolidified surface is u n d e r very h i g h tensile residual stresses w h i c h can p r o d u c e c r a c k i n g as well as limit the p r a c t i c a l use of laser-treated samples. Different t e c h n i q u e s can be used to modify the residual stresses, i n c l u d i n g p r e h e a t t r e a t m e n t , post-heat treatm e n t a n d s h o t p e e n i n g . For laser c l a d d i n g it has also been p r o p o s e d t h a t the deposition of an i n t e r m e d i a t e l a y e r or layers (also by laser cladding) m i g h t r e d u c e the stresses [3]. The i n t e r m e d i a t e l a y e r s h o u l d p l a s t i c a l l y deform to a c c o m m o d a t e the strains g e n e r a t e d as the s u r f a c i n g alloy cools to a m b i e n t t e m p e r a t u r e . By a c t i n g as a t h e r m a l barrier, an i n t e r m e d i a t e l a y e r m a y also suppress a n y of the solid state p h a s e t r a n s f o r m a t i o n s t h a t could o c c u r in the s u b s t r a t e (e.g. the m a r t e n s i t i c t r a n s f o r m a t i o n in steels), w h i c h o w i n g to t h e i r a s s o c i a t e d volume changes, w o u l d result in stress g e n e r a t i o n w i t h i n the s u r f a c i n g alloy. Elsevier Sequoia/Printed in The Netherlands
436
To investigate the potential of intermediate layers in reducing the residual stress state in laser-cladded samples, multilayer clads were produced on a high chromium martensitic steel, with an austenitic stainless steel as an intermediate layer and stellite 6, a cobalt-based alloy, as the surfacing alloy. This paper reports the results obtained.
2. Experimental details An X 2 CrNiMo 18 14 austenitic stainless steel and an X 20 Cr 13 high chromium martensitic steel in the fully tempered state were chosen as base materials. A stellite-type cobalt-based alloy (Co 28Cr-4W 1C) was used as the surfacing alloy because of the potentially large industrial interest in processing these by laser owing to their proven resistance to wear and corrosion. Austenitic stainless steel 304 ( F e - 1 9 C r 10Ni) was used for the intermediate layers since it has good ductility, a yield point which drops rapidly with temperature, a high coefficient of thermal expansion and a fusion temperature domain close to t hat of the surfacing alloy, which reduces any problems of dilution. It also has a low thermal conductivity, which can be useful when the intermediate layer is to act as a thermal barrier. Furthermore, stainless steels are easy to clad and readily available in powder form. The clads were produced using a 1.5 kW continuous wave CO2 laser. The samples were displaced under the stationary beam using a numerically controlled X - Y table and the clads were realized using the direction injection technique proposed by Weerasinghe and Steen [4]. Argon was used to convey and protect the alloy powder, which was injected towards the molten pool via a nozzle. Monolayer clads were produced on rect angul ar samples in order to determine the stresses produced by each individual clad and to act as control specimens with which to relate the final stress distributions in the multilayer specimens. The parameters used for the treatments were as follows: 1500 W nominal power, 2 mm beam diameter, 800 mm min 1 scanning speed, 0.6 mm step and 6 g min -1 powder feed rate. The clads produced were 0.9 mm thick, crack and pore free, with a metallurgical bond and a dilution of less th an 10%. The residual stress field was obtained by measuring the deformation resulting from the successive removal of thin layers [5]. By measuring the strains induced on the free surface by the redistribution of the stress field in the remaining part of the specimen, it was possible to obtain the original stress field by iteration using elastic theory. The results obtained were the so-called "first-order" residual stresses, which are nearly homogeneous across several grains of a material and are in equilibrium over the bulk of the material [6]. Although this technique was destructive, it offers the advantages of generating no additional residual stresses, since the removal of material was done electrochemically, and of keeping the original symmetry of the specimen. In order to avoid any modification of the residual stress field
437
after the laser treatment, the cladded samples were analysed without machining of the treated surfaces.
3.
Results
Figures 1-3 show residual stress profiles obtained for the samples cladded without intermediate layers. These curves confirm results obtained earlier with similar alloy couples using the X-ray diffraction technique [1, 2]. In each case the surface layer was under tension. The evolution of the stresses as a function of depth was similar for the transverse (T) and longitudinal (L) components, the magnitude of the latter being generally slightly higher.
Stellite6/X2C
r Ni M o 1814
1200 ~, a.
1000 800 600
•
T
400
L
200
•o
-200
o
n"
-400
~
(1)
~ E
-600 0.0
0.5
. . . . , . . . . , . . . . , . . . . , .... , . . . . , . . . . , . . . . 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Depth
[mrn]
Fig. 1. Transverse (T) and longitudinal (L) residual stress profiles
vs.
depth for specimen 1,
vs.
depth for specimen 2.
Stellite6/X20Cr13 1200 .
1 ooo -
~'
8oo600 -
}3
~
400 -" '~
200 -
"~
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[mm]
Fig. 2. Transverse (T) and longitudinal (L) residual stress profiles
438 304/X20Cr13 1200
W
1000
IE
800
a.
600 400
T
•
L
2000
-200 -400 -600
$ m E ,,i
0.0 0.5
. . . .
i
. . . .
i
. . . .
i . . . .
i . . . .
i . . . .
i
. . . .
i
. . . .
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Depth [ram]
Fig. 3. Transverse (T) and longitudinal (L) residual stress profiles
vs.
depth for specimen 3.
Stellite6/304/X20Cr13 1200' 1000 800 600
P
400
•
T
~ _
200
~
L
~ ' r.~_
0 "0
I
-200 -400 ~ .-
m
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1.0 1.5 2.0 2.5 3.0 3 . 5 4.0 4.5 5.0
Depth [mm]
Fig. 4. Transverse (T) and longitudinal (L) residual stress profiles Stellite6/304/304/X2OCrl
vs.
depth for specimen 4.
vs.
depth for specimen 5.
3
1200 -
i
800 6O0 400
•
200
L
-~
"~
o
"o
-200
®
-4oo- ~
•~
T
i
~
~,
~
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-600 '" .., .... , .... , .... , .... 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4 . 0 4.5 5.0
Depth
[ram]
Fig. 5. Transverse (T) and longitudinal (L) residual stress profiles
439
Figures 4 and 5 show the curves obtained from the martensitic steel samples cladded with stellite 6 plus an intermediate layer. The sign of the residual stresses in the surfacing alloy was not changed by the presence of one or more intermediate layers. The maximum tensile stress was close to the first interface under the surface, with the level at the surface being much lower, generally less than half of this value.
4. D i s c u s s i o n A clear quantitative analysis of the results presented here is difficult for several reasons. Firstly, the phenomena involved in the cladding process are complex. Secondly, the thermophysical data and the mechanical properties of most alloys are poorly known, especially at high temperature. Thirdly, the high thermal gradients and cooling rates render difficult an estimation of the phase transformations. Therefore only qualitative trends will be discussed below. The cladding of stellite 6 onto an austenitic stainless steel (Fig. 1) results in moderate tensile stresses in the surface and subsurface regions, with a smooth transition from tension to compression with depth. Earlier studies [2] have shown that it was possible to control these stresses using a post-heat treatment which allows a relaxation of the stresses through creep. On cooling, the larger contraction of the base material will produce compressive stresses in the cladded layer, the magnitude of which will depend on the post-heat treatment temperature. The cladding of stellite 6 onto a martensitic steel (Fig. 2) results in a more severe stress state. This emphasizes the important role played by the martensitic transformation within the heat-affected zone (HAZ) of the base material. Two tensile peaks can be observed in Fig. 2; one in the cladded layer and the other in a region below the HAZ. In the HAZ itself a compressive stress was measured. During the deposition of the layer, tensile stresses are generated in the clad owing to the restrained thermal contraction. The base material just below the clad is re-austenitized and undergoes a martensitic transformation on cooling. This transformation is accompanied by a volume expansion (about 3%) which introduces compressive residual stresses in the new martensitic layer and tensile stresses in both the deposited layer and in the base material below the HAZ. Therefore the tensile stresses in the clad originate from both the restrained contraction of the clad and from the expansion associated with the martensitic transformation. The aim of introducing an intermediate layer of austenitic stainless steel between the martensitic substrate and the hardfacing alloy was to obtain a stress level in the surfacing alloy comparable with t h a t obtained in Fig. 1. The stress fields resulting from each deposition are discussed below. The stress field in the martensitic Steel sample clad with an austenitic layer is shown in Fig. 3. The differences between this and the previous case (Fig. 2) are mainly due to the fact that most mechanical properties of the
440
austenitic steel drop rapidly with temperature whereas in the case of the ste]lite alloy they are preserved up to high temperatures (about 700 °C) [7]. Therefore the austenitic layer will plastically deform during cooling and the tensile stresses in the layer will be much reduced. It should be noted that the first measured point within the cladded layer can be ignored, the measurement having been made too close to the surface. Owing to the surface roughness that exists after laser processing, the thickness of the first removed layer is not known precisely and therefore the corresponding stress value is inaccurate. The deposition of a stellite 6 layer on a sample already clad with an austenitic steel results in the stress field shown in Fig. 4. High tensile stresses are measured in both layers, though the tensile peak in the region below the HAZ is smaller than that shown in Fig. 3. This is due to a tempering effect of the HAZ during the deposition of the second layer. However, the deposition of a third layer was necessary to eliminate the tensile peak, as shown in Fig. 5. Unfortunately, the relaxation of the tensile stresses generated in the hardfacing alloy during cooling, which was expected due to plastic deformation of the intermediate layer, was not observed in these specimens. The reason for this is not clearly understood but it is thought to be related to the pre-existing stress field in the intermediate layer (see Fig. 3) as well as its limited thickness (approximately I mm). Doubling the number of layers leads qualitatively to the same result. Another way of visualizing these results is shown in Fig. 6. This diagram shows the elastic energy stored in the specimens in the form of tensile stresses only and gives an indication of the mechanical properties of the different clad specimen. The values given correspond to the integral of the positive part of the stress profiles shown in Figs. 1-5 (average between the transverse and longitudinal components). The contribution from each layer is indicated differently. Note t h a t the tensile stresses must be equilibrated in the sample by compressive stresses of equal total energy. The negative influence of an intermediate layer on the tensile energy stored in the
1000 .9
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600
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200
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4
5
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[]
Stellite 6
[]
304
• •
X2CrNiMo1814 X20Cr13
441 s p e c i m e n s c a n be c l e a r l y s e e n a n d t h i s effect is e v e n m o r e p r o n o u n c e d w h e n using two i n t e r m e d i a t e layers.
5. Conclusions I n t e r m e d i a t e l a y e r s of a u s t e n i t i c s t a i n l e s s s t e e l do n o t r e d u c e t h e t e n s i l e s t r e s s e s i n l a y e r s of s t e l l i t e 6 d e p o s i t e d o n a m a r t e n s i t i c steel, as l e a s t n o t w i t h o u t a s u b s e q u e n t h e a t t r e a t m e n t a l l o w i n g t h e i n t e r m e d i a t e l a y e r s to c r e e p a n d r e l a x t h e s t r e s s e s b e f o r e c o o l i n g . N o b e n e f i c i a l effect w a s o b s e r v e d , t h e f i n a l s t r e s s field a l w a y s b e i n g h i g h l y t e n s i l e for t h e s u r f a c i n g a l l o y s . T h e r e f o r e t h i s t e c h n i q u e does n o t s e e m to b e v e r y p r o m i s i n g to s o l v e t h e p r o b l e m of r e s i d u a l s t r e s s e s i n l a s e r s u r f a c e t r e a t m e n t s .
Acknowledgments T h e a u t h o r s w o u l d l i k e to t h a n k M. R a p p a z a n d M. G r e m a u d for t h e i r c o m m e n t s as w e l l as t h e " C o m m i s s i o n p o u r l ' E n c o u r a g e m e n t de l a R e c h e r c h e S c i e n t i f i q u e " , B e r n a n d S u l z e r Bs Ltd., W i n t e r t h u r for f i n a n c i a l s u p p o r t .
References 1 M. Roth, R. Hauert, A. Frenk, M. Pierantoni and E. Blank, Residual stress formation in laser treated surfaces, in B. Waidelich (ed.), Laser '89 Cong., Munich, Springer, Berlin, 1989, pp. 532 537. 2 R. Dekumbis, Controlling residual stress formation in laser treated surfaces, in W. M. Steen (ed.), Proc. 6th Int. Conf. Lasers in Manufacturing (Lim-6), Birmingham, IFS/Springer, Kempston, U.K. 1989, pp. 185-192. 3 A. B. Vannes and J. Hernandez, Les contraintes r6siduelles et les d6p6ts laser: approche ph6nomenologique, J. Phys. (Paris), Colloq. C7, 48 (1987) 139 145. 4 V. M. Weerasinghe and W. M. Steen, Laser cladding with pneumatic powder delivery, in E. A. Metzbower (ed.), Lasers in Materials Processing, ASM, Metals Park, OH, 1983, pp. 166-174. 5 M. Pilloz, C. Sahour and A. B. Vannes, Study of the parameters of laser coatings and residual stress field created by these coatings, in CEMUL (ed.), Proc. 2nd Int. Seminar on Surface Engineering with High Energy Beams, Lisbon, Center of Mechanics and Materials of the Technical University of Lisbon, Lisbon, 1989, pp. 387-413. 6 E. Macherauch, H. Wohlfahrt and U. Wolfstieg, Zur Zweckm~issigen Definition von Eigenspannungen, H~irterei Technische Mitteilungen, 28 (1973) 201 211. 7 O. Knotek, E. Lugscheider and H. Eschnauer, Hartlegierungen zum Verschleiss-Schutz, in Stahleisen Biicher, Band 20, Stahleisen MBH, D6sseldorf, 1975, pp. 46 76.
435
I n f l u e n c e of a n i n t e r m e d i a t e l a y e r on t h e r e s i d u a l stress field in a l a s e r c l a d A. F r e n k , C. F. M a r s d e n a n d J . - D . W a g n i 6 r e Laboratoire de Mdtallurgie Physique, CTML, EPFL, CH-1015 Lausanne (Switzerland)
A. B. V a n n e s MMP, CALFETMAT, Ecole Centrale Lyon, F-69131 Ecully Cedex (France)
M. L a r a c i n e a n d M. Y. L o r m a n d CALFETMAT, Bat. 403, INSA, F-69621 Villeurbanne Cedex (France)
Abstract Large residual stresses are generally produced during laser surface treatments owing to their associated high thermal gradients and cooling rates. In laser clads the high tensile residual stresses usually obtained can produce cracking or limit the practical use of these deposits. To investigate the potential of intermediate layers in controlling the residual stresses, multilayer clads were produced on a high chromium martensitic steel, with an austenitic stainless steel as an intermediate layer and stellite 6, a cobalt-based alloy, as the surfacing material. The stress profiles were calculated using the measurements of the deformation resulting from the successive electrochemical removal of thin layers. It was observed that the sign of the residual stresses in the surface alloy was not changed by one or more intermediate layers.
1. I n t r o d u c t i o n Residual stresses are k n o w n to affect the fatigue, creep a n d brittle f r a c t u r e p r o p e r t i e s of e n g i n e e r i n g c o m p o n e n t s as well as t h e i r s t r u c t u r a l stability, w e a r a n d c o r r o s i o n b e h a v i o u r . The p r o b l e m of residual stress g e n e r a t i o n d u r i n g laser surface t r e a t m e n t s is a s s o c i a t e d with h i g h t h e r m a l g r a d i e n t s a n d cooling r a t e s [1, 2]. In most cases w h e n m e l t i n g o c c u r s (laser remelting, alloying and cladding) the resolidified surface is u n d e r very h i g h tensile residual stresses w h i c h can p r o d u c e c r a c k i n g as well as limit the p r a c t i c a l use of laser-treated samples. Different t e c h n i q u e s can be used to modify the residual stresses, i n c l u d i n g p r e h e a t t r e a t m e n t , post-heat treatm e n t a n d s h o t p e e n i n g . For laser c l a d d i n g it has also been p r o p o s e d t h a t the deposition of an i n t e r m e d i a t e l a y e r or layers (also by laser cladding) m i g h t r e d u c e the stresses [3]. The i n t e r m e d i a t e l a y e r s h o u l d p l a s t i c a l l y deform to a c c o m m o d a t e the strains g e n e r a t e d as the s u r f a c i n g alloy cools to a m b i e n t t e m p e r a t u r e . By a c t i n g as a t h e r m a l barrier, an i n t e r m e d i a t e l a y e r m a y also suppress a n y of the solid state p h a s e t r a n s f o r m a t i o n s t h a t could o c c u r in the s u b s t r a t e (e.g. the m a r t e n s i t i c t r a n s f o r m a t i o n in steels), w h i c h o w i n g to t h e i r a s s o c i a t e d volume changes, w o u l d result in stress g e n e r a t i o n w i t h i n the s u r f a c i n g alloy. Elsevier Sequoia/Printed in The Netherlands
436
To investigate the potential of intermediate layers in reducing the residual stress state in laser-cladded samples, multilayer clads were produced on a high chromium martensitic steel, with an austenitic stainless steel as an intermediate layer and stellite 6, a cobalt-based alloy, as the surfacing alloy. This paper reports the results obtained.
2. Experimental details An X 2 CrNiMo 18 14 austenitic stainless steel and an X 20 Cr 13 high chromium martensitic steel in the fully tempered state were chosen as base materials. A stellite-type cobalt-based alloy (Co 28Cr-4W 1C) was used as the surfacing alloy because of the potentially large industrial interest in processing these by laser owing to their proven resistance to wear and corrosion. Austenitic stainless steel 304 ( F e - 1 9 C r 10Ni) was used for the intermediate layers since it has good ductility, a yield point which drops rapidly with temperature, a high coefficient of thermal expansion and a fusion temperature domain close to t hat of the surfacing alloy, which reduces any problems of dilution. It also has a low thermal conductivity, which can be useful when the intermediate layer is to act as a thermal barrier. Furthermore, stainless steels are easy to clad and readily available in powder form. The clads were produced using a 1.5 kW continuous wave CO2 laser. The samples were displaced under the stationary beam using a numerically controlled X - Y table and the clads were realized using the direction injection technique proposed by Weerasinghe and Steen [4]. Argon was used to convey and protect the alloy powder, which was injected towards the molten pool via a nozzle. Monolayer clads were produced on rect angul ar samples in order to determine the stresses produced by each individual clad and to act as control specimens with which to relate the final stress distributions in the multilayer specimens. The parameters used for the treatments were as follows: 1500 W nominal power, 2 mm beam diameter, 800 mm min 1 scanning speed, 0.6 mm step and 6 g min -1 powder feed rate. The clads produced were 0.9 mm thick, crack and pore free, with a metallurgical bond and a dilution of less th an 10%. The residual stress field was obtained by measuring the deformation resulting from the successive removal of thin layers [5]. By measuring the strains induced on the free surface by the redistribution of the stress field in the remaining part of the specimen, it was possible to obtain the original stress field by iteration using elastic theory. The results obtained were the so-called "first-order" residual stresses, which are nearly homogeneous across several grains of a material and are in equilibrium over the bulk of the material [6]. Although this technique was destructive, it offers the advantages of generating no additional residual stresses, since the removal of material was done electrochemically, and of keeping the original symmetry of the specimen. In order to avoid any modification of the residual stress field
437
after the laser treatment, the cladded samples were analysed without machining of the treated surfaces.
3.
Results
Figures 1-3 show residual stress profiles obtained for the samples cladded without intermediate layers. These curves confirm results obtained earlier with similar alloy couples using the X-ray diffraction technique [1, 2]. In each case the surface layer was under tension. The evolution of the stresses as a function of depth was similar for the transverse (T) and longitudinal (L) components, the magnitude of the latter being generally slightly higher.
Stellite6/X2C
r Ni M o 1814
1200 ~, a.
1000 800 600
•
T
400
L
200
•o
-200
o
n"
-400
~
(1)
~ E
-600 0.0
0.5
. . . . , . . . . , . . . . , . . . . , .... , . . . . , . . . . , . . . . 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Depth
[mrn]
Fig. 1. Transverse (T) and longitudinal (L) residual stress profiles
vs.
depth for specimen 1,
vs.
depth for specimen 2.
Stellite6/X20Cr13 1200 .
1 ooo -
~'
8oo600 -
}3
~
400 -" '~
200 -
"~
0""
/1~
A
"e
-200-
~°
•
-400-
~
""
-600'" 0.0
•
~ 0.5
T L
~/1
~
~
mE
. , J . . . , . . . . , . . . . , . . . . , . . . . , . . . . , . . . . ~. . . . 1.0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 . 0 Depth
[mm]
Fig. 2. Transverse (T) and longitudinal (L) residual stress profiles
438 304/X20Cr13 1200
W
1000
IE
800
a.
600 400
T
•
L
2000
-200 -400 -600
$ m E ,,i
0.0 0.5
. . . .
i
. . . .
i
. . . .
i . . . .
i . . . .
i . . . .
i
. . . .
i
. . . .
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Depth [ram]
Fig. 3. Transverse (T) and longitudinal (L) residual stress profiles
vs.
depth for specimen 3.
Stellite6/304/X20Cr13 1200' 1000 800 600
P
400
•
T
~ _
200
~
L
~ ' r.~_
0 "0
I
-200 -400 ~ .-
m
m
m E • i
0.0 0.5
--
. . . .
i . . . .
! . . . .
! . . . .
i
. . . .
I
. . . .
1.0 1.5 2.0 2.5 3.0 3 . 5 4.0 4.5 5.0
Depth [mm]
Fig. 4. Transverse (T) and longitudinal (L) residual stress profiles Stellite6/304/304/X2OCrl
vs.
depth for specimen 4.
vs.
depth for specimen 5.
3
1200 -
i
800 6O0 400
•
200
L
-~
"~
o
"o
-200
®
-4oo- ~
•~
T
i
~
~,
~
~
~.~
-600 '" .., .... , .... , .... , .... 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4 . 0 4.5 5.0
Depth
[ram]
Fig. 5. Transverse (T) and longitudinal (L) residual stress profiles
439
Figures 4 and 5 show the curves obtained from the martensitic steel samples cladded with stellite 6 plus an intermediate layer. The sign of the residual stresses in the surfacing alloy was not changed by the presence of one or more intermediate layers. The maximum tensile stress was close to the first interface under the surface, with the level at the surface being much lower, generally less than half of this value.
4. D i s c u s s i o n A clear quantitative analysis of the results presented here is difficult for several reasons. Firstly, the phenomena involved in the cladding process are complex. Secondly, the thermophysical data and the mechanical properties of most alloys are poorly known, especially at high temperature. Thirdly, the high thermal gradients and cooling rates render difficult an estimation of the phase transformations. Therefore only qualitative trends will be discussed below. The cladding of stellite 6 onto an austenitic stainless steel (Fig. 1) results in moderate tensile stresses in the surface and subsurface regions, with a smooth transition from tension to compression with depth. Earlier studies [2] have shown that it was possible to control these stresses using a post-heat treatment which allows a relaxation of the stresses through creep. On cooling, the larger contraction of the base material will produce compressive stresses in the cladded layer, the magnitude of which will depend on the post-heat treatment temperature. The cladding of stellite 6 onto a martensitic steel (Fig. 2) results in a more severe stress state. This emphasizes the important role played by the martensitic transformation within the heat-affected zone (HAZ) of the base material. Two tensile peaks can be observed in Fig. 2; one in the cladded layer and the other in a region below the HAZ. In the HAZ itself a compressive stress was measured. During the deposition of the layer, tensile stresses are generated in the clad owing to the restrained thermal contraction. The base material just below the clad is re-austenitized and undergoes a martensitic transformation on cooling. This transformation is accompanied by a volume expansion (about 3%) which introduces compressive residual stresses in the new martensitic layer and tensile stresses in both the deposited layer and in the base material below the HAZ. Therefore the tensile stresses in the clad originate from both the restrained contraction of the clad and from the expansion associated with the martensitic transformation. The aim of introducing an intermediate layer of austenitic stainless steel between the martensitic substrate and the hardfacing alloy was to obtain a stress level in the surfacing alloy comparable with t h a t obtained in Fig. 1. The stress fields resulting from each deposition are discussed below. The stress field in the martensitic Steel sample clad with an austenitic layer is shown in Fig. 3. The differences between this and the previous case (Fig. 2) are mainly due to the fact that most mechanical properties of the
440
austenitic steel drop rapidly with temperature whereas in the case of the ste]lite alloy they are preserved up to high temperatures (about 700 °C) [7]. Therefore the austenitic layer will plastically deform during cooling and the tensile stresses in the layer will be much reduced. It should be noted that the first measured point within the cladded layer can be ignored, the measurement having been made too close to the surface. Owing to the surface roughness that exists after laser processing, the thickness of the first removed layer is not known precisely and therefore the corresponding stress value is inaccurate. The deposition of a stellite 6 layer on a sample already clad with an austenitic steel results in the stress field shown in Fig. 4. High tensile stresses are measured in both layers, though the tensile peak in the region below the HAZ is smaller than that shown in Fig. 3. This is due to a tempering effect of the HAZ during the deposition of the second layer. However, the deposition of a third layer was necessary to eliminate the tensile peak, as shown in Fig. 5. Unfortunately, the relaxation of the tensile stresses generated in the hardfacing alloy during cooling, which was expected due to plastic deformation of the intermediate layer, was not observed in these specimens. The reason for this is not clearly understood but it is thought to be related to the pre-existing stress field in the intermediate layer (see Fig. 3) as well as its limited thickness (approximately I mm). Doubling the number of layers leads qualitatively to the same result. Another way of visualizing these results is shown in Fig. 6. This diagram shows the elastic energy stored in the specimens in the form of tensile stresses only and gives an indication of the mechanical properties of the different clad specimen. The values given correspond to the integral of the positive part of the stress profiles shown in Figs. 1-5 (average between the transverse and longitudinal components). The contribution from each layer is indicated differently. Note t h a t the tensile stresses must be equilibrated in the sample by compressive stresses of equal total energy. The negative influence of an intermediate layer on the tensile energy stored in the
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5. Conclusions I n t e r m e d i a t e l a y e r s of a u s t e n i t i c s t a i n l e s s s t e e l do n o t r e d u c e t h e t e n s i l e s t r e s s e s i n l a y e r s of s t e l l i t e 6 d e p o s i t e d o n a m a r t e n s i t i c steel, as l e a s t n o t w i t h o u t a s u b s e q u e n t h e a t t r e a t m e n t a l l o w i n g t h e i n t e r m e d i a t e l a y e r s to c r e e p a n d r e l a x t h e s t r e s s e s b e f o r e c o o l i n g . N o b e n e f i c i a l effect w a s o b s e r v e d , t h e f i n a l s t r e s s field a l w a y s b e i n g h i g h l y t e n s i l e for t h e s u r f a c i n g a l l o y s . T h e r e f o r e t h i s t e c h n i q u e does n o t s e e m to b e v e r y p r o m i s i n g to s o l v e t h e p r o b l e m of r e s i d u a l s t r e s s e s i n l a s e r s u r f a c e t r e a t m e n t s .
Acknowledgments T h e a u t h o r s w o u l d l i k e to t h a n k M. R a p p a z a n d M. G r e m a u d for t h e i r c o m m e n t s as w e l l as t h e " C o m m i s s i o n p o u r l ' E n c o u r a g e m e n t de l a R e c h e r c h e S c i e n t i f i q u e " , B e r n a n d S u l z e r Bs Ltd., W i n t e r t h u r for f i n a n c i a l s u p p o r t .
References 1 M. Roth, R. Hauert, A. Frenk, M. Pierantoni and E. Blank, Residual stress formation in laser treated surfaces, in B. Waidelich (ed.), Laser '89 Cong., Munich, Springer, Berlin, 1989, pp. 532 537. 2 R. Dekumbis, Controlling residual stress formation in laser treated surfaces, in W. M. Steen (ed.), Proc. 6th Int. Conf. Lasers in Manufacturing (Lim-6), Birmingham, IFS/Springer, Kempston, U.K. 1989, pp. 185-192. 3 A. B. Vannes and J. Hernandez, Les contraintes r6siduelles et les d6p6ts laser: approche ph6nomenologique, J. Phys. (Paris), Colloq. C7, 48 (1987) 139 145. 4 V. M. Weerasinghe and W. M. Steen, Laser cladding with pneumatic powder delivery, in E. A. Metzbower (ed.), Lasers in Materials Processing, ASM, Metals Park, OH, 1983, pp. 166-174. 5 M. Pilloz, C. Sahour and A. B. Vannes, Study of the parameters of laser coatings and residual stress field created by these coatings, in CEMUL (ed.), Proc. 2nd Int. Seminar on Surface Engineering with High Energy Beams, Lisbon, Center of Mechanics and Materials of the Technical University of Lisbon, Lisbon, 1989, pp. 387-413. 6 E. Macherauch, H. Wohlfahrt and U. Wolfstieg, Zur Zweckm~issigen Definition von Eigenspannungen, H~irterei Technische Mitteilungen, 28 (1973) 201 211. 7 O. Knotek, E. Lugscheider and H. Eschnauer, Hartlegierungen zum Verschleiss-Schutz, in Stahleisen Biicher, Band 20, Stahleisen MBH, D6sseldorf, 1975, pp. 46 76.