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A comparison of tool–repair methods using CO2 laser surfacing and arc surfacing
Applied Surface Science 208±209 (2003) 424±431
A comparison of tool±repair methods using CO2 laser surfacing and arc surfacing J. Grum*, J.M. Slabe Faculty of Mechanical Engineering, University of Ljubljana, AsÏkercÏeva 6, 1000 Ljubljana, Slovenia
Abstract The life of loaded machine elements and the vital parts of tools can be successfully extended by systematic maintenance and the timely repair of damaged surfaces. It has been proved that with the regular maintenance of tool parts the cost of the tool in the price of a ®nished product can be considerably reduced. It is a very economical practise to manufacture certain parts from lowcost, tough structural steel on which a layer of wear-resistant alloy has been surfaced. In such a case the volume fraction of the surfaced layer is usually much lower than 10% of the total volume of the tool or the machine element. In this paper, we report some of our latest results involving comparative studies of repair surfacing on maraging steel and the cladding of common structural steel with a Ni±Co±Mo alloy similar to the maraging steel using a laser process and submergedarc surfacing. The results are based on micro-structural and micro-chemical analyses of the surfaced layer and are supported by analyses of the micro-hardness and the residual stresses, carried out on suitably adapted ¯at specimens. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser cladding; Submerged-arc surfacing; Maraging steel
1. Introduction Precipitation-hardened maraging steels are suitable for the manufacture of machine elements and tool parts that are subjected to high thermo-mechanical loads as a result of their exceptionally favourable mechanical properties, particularly their excellent fracture toughness combined with their high tensile strength and hardness [1,2]. Grum and ZupancÏicÏ [3,4] have investigated the in¯uence of various temperature/ time conditions of precipitation hardening on the mechanical properties of these steels. Tool manufacturers have been successfully introducing maraging steels in the manufacture of die-casting tools for *
Corresponding author. Tel.: 386-1-4771-203; fax: 386-1-2518-567. E-mail address: [email protected] (J. Grum).
aluminium and magnesium alloys, for which a high resistance to cyclic thermo-mechanical loads is required. An exceptionally good weldability of maraging steels in all states, i.e. prior to and after precipitation annealing [1], permits the successful repair of damage to tools in order to extend the operating life of the tools and preserve the quality of the die-cast parts. Gehricke [5] proved that the weld or the surfacing weld did not show inclusions and pores, which guarantees the high quality of a product. Precipitation annealing of the weld and the heat-affected zone ensures that the nominal hardness of the material recommended for the operation of the tools can be achieved. Laser cladding, which involves the preliminary deposition of a powder of a certain chemical composition and grain size on the workpiece surface, is the simplest way of improving the surface properties of a, usually cheap, parent metal. The selected method of
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(02)01427-7
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431
425
steel cladding was performed using a Ni±Co±Mo alloy similar to maraging steel with the following chemical composition: 7.5% Ni, 6.25% Co, 6.25% Mo, 0.6% Ti, 0.1% Al, 3.8% Mn, 1.9% Si, 0.04% C. Based on the chosen cladding composition, an ALV±NiCoMo-2 powder was developed and manufactured. The chemical compositions of the parent metals and the ALV± NiCoMo-2 powder are given in Table 1. The laser-cladding tests were performed using a CO2 laser (wavelength of 10.6 mm) with a maximum working power of 500 W and a Gaussian distribution of the energy in the beam. The experiments were performed with a laser power (P) of 360 W and a defocusing degree of 22 mm. The focal length of the lens used was 127 mm. The laser-beam diameter on the specimen surface was 1.4 mm. The laser-beam travelling speed (vb) was 6, 8, and 10 cm/min. The different travelling speeds of the laser beam across the specimen surface produced different energy inputs in the workpiece surface, i.e. Ed 257:14 J/mm2 (vb 6 cm/min), Ed 192:86 J/mm2 (vb 8 cm/min), and Ed 154:29 J/mm2 (vb 10 cm/min). The cladding was carried out using argon as an inert shielding gas with a ¯ow rate of 3 l/min. For the tests, specimens with a uniform shape and a size of 27 mm 27 mm 12 mm were selected. With each combination of parent metal and travelling speed of the laser beam three repetitions of the laser cladding were performed. As a result, 18 specimens were produced. For the laser cladding the central part of the specimen surface, 20 mm 20 mm in size, was chosen. The laser beam was moving in a square spiral, starting in the specimen centre. The ALV±NiCoMo-2 powder with a grain size of 0.06±0.1 mm was deposited on the specimen surfaces prior to treatment. The
cladding the parent metal results in the minimum dilution of the parent metal with the ®ller material [6,7]. Using theoretical modelling the authors studied the movement of the fusion front and established a relatively fast propagation of the molten pool through the thermal hindrance represented by the powder layer. When the molten pool reached the boundary between the parent metal and the ®ller material, the thermal load, due to high temperature gradients, increased, which resulted in the rapid solidi®cation of the molten pool. During the cladding of structural steels with a high-quality alloy, it is of extreme importance that the dilution of the cladding due to the melting of the parent metal is minimised as much as possible. The parent metal should be remelted within certain limits to ensure the faultless bonding of the ®ller material with the substrate. In high-productivity submerged-arc surfacing, the dilution of the ®ller material with the parent metal can be effectively reduced by using alloyed agglomerated ¯uxes and/or cored wires. Practice has shown that the penetration can be effectively reduced by submergedarc welding using a multiple-wire electrode. With correctly chosen welding parameters, i.e. low current per wire and a relatively high welding voltage, this process permits high-productivity one-layer surfacing of structural steels [8]. 2. Experimental procedure 2.1. Laser cladding For the laser cladding we chose two kinds of parent metal: a 1.2799 maraging steel and a St 44-3 structural
Table 1 Chemical compositions of the parent metals and the ®ller materials used in laser cladding and submerged-arc surfacing Material
Chemical composition (%) Ni
Co
Mo
Ti
Al
1.2799 (DIN) St 44-3 (DIN) ALV±NiCoMo-2 S±NiCoMo-2
12
8
8
0.5
6.25 6.49
6.25 6.49
0.63 2.6
0.05 0.027 0.12 0.52
Material
SiO2
FeO
Al2O3
MnO
FB 12.2
17
1
20
5
7.5 7.73
Mn
Si
C
BaF2
0.42 3.81 0.21
0.21 1.86 0.11
0.02 0.15 0.08 0.05
CaO
MgO
CaF2
CaSi
Na2O
6
25
23
1
2
1.24
SiO2
0.48
Na2O
0.2
P
S
0.011
0.010
426
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431
powder adhesion to the specimen surface was achieved by depositing a thin layer of high-temperature colour (Henotherm Silicon Schwarz SM, made by Henelit), which also increased the absorption of the laser beam by the material. The silicon colour was applied to the surface with a ®ne brush. The thickness of the powder deposit ranged between 0.25 and 0.3 mm. 2.2. Submerged-arc surfacing
the type and grain size of the micro-structure formed. The etching time determined the quality of the micro-graphs for the individual micro-structural zones. Fig. 1 shows micro-graphs of the characteristic zones after laser cladding of the specimens made of 1.2799 maraging steel. The micro-graphs are from the specimen with an energy input (Ed) equal to 257.14 J/mm2. Based on an analysis of the micro-
Only the St 44-3 structural steel was submerged-arc clad. A Ni±Co±Mo alloy with the same composition as used for the laser cladding was employed. With reference to the chosen composition of the surfacing weld, a S±NiCoMo-2 cored wire with a diameter of 2 mm was developed and manufactured. Its composition is given in Table 1. The compositions of the ALV± NiCoMo-2 powder used in the laser cladding and that of the S±NiCoMo-2 cored wire differed slightly because the burn-off of the alloying elements during the laser cladding with a preliminary deposit of the powder in the Ar shielding was weaker than in the submerged-arc surfacing. The submerged-arc surfacing was accomplished with an automatic welding device that included gravitation supply of the shielding ¯ux. The submergedarc surfacing was achieved with the single, homemade S±NiCoMo-2 cored wire and a commercial FB12.2 ¯ux, the composition of which is given in Table 1. The surfacing was carried with a current (I) of 167 A and a voltage (U) of 37 Von a workpiece with a size of 250 mm 27 mm 12 mm. 3. Experimental results 3.1. Analysis of the laser-clad specimens A micro-structural analysis of the laser-clad specimens of maraging and structural steels was very dif®cult due to great through-thickness differences of the specimen micro-structures. The specimens were etched with nital for the micro-structural analysis. An optical microscope was used to study their micro-structures. Because of the differing throughdepth micro-structures of the clad specimens, different concentrations of nital were used. The concentration of the etching agent was chosen with reference to
Fig. 1. Micro-structure in the transverse cross-section of laser-clad specimen made of 1.2799 maraging steel, P 360 W, vb 6 cm/ min, Ed 257:14 J/mm2 and measuring points for micro-hardness measurement.
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431
graphs in Fig. 1, ®ve characteristic zones can be distinguished: zone ``a'': a remelted and rapidly solidified clad alloyed surface with a typical dendritic micro-structure due to fast solidification; zone ``b'': the strongly heat-affected zone with an overaged micro-structure (Ni martensite matrix with coarse precipitates and reverse austenite) formed as a consequence of a high heat input; zone ``c'': the heat-affected precipitation zone I with a Ni martensite matrix and a high density of precipitates; zone ``d'': the heat-affected precipitation zone II with a Ni martensite matrix and a medium density of precipitates; zone ``e'': the heat-affected precipitation zone III with a Ni martensite matrix and a small density of precipitates. The size of the precipitates also decreases from zone ``c'' to zone ``e''. Based on reports in the literature [1,4] we assume that the precipitates in the heat-affected zones are primarily Ni3Mo, Ni3Ti, Fe2Mo, Fe2Ti and TiC. In the heat-affected precipitation zones, the content and the size of the precipitated phases was related to the laser-cladding conditions. Thus, three precipitation zones in the heat-affected zone were found only with the highest energy input
427
(Ed), i.e. 257.14 J/mm2, which was achieved with a power (P) of 360 W, and a laser-beam travelling speed (vb) of 6 cm/min. With the lowest laser-beam travelling speed the depth of the heat-affected zone was considerably greater. Consequently, a more uniform micro-structure at the transitions between the individual zones and a more uniform through-depth microhardness variation at the three measuring points in the clad specimen was obtained. The through-depth Vickers micro-hardness was measured at three points: in the centre of the transverse cross-section, x 0; at a distance x of 5 mm to the right of the centre; and at a distance x of 5 mm to the left of the centre (Fig. 1). Fig. 2 shows the characteristic micro-hardness variation as a function of depth in the laser-clad specimens made of the 1.2799 maraging steel. The data in the diagram refer to the specimen processed with a travelling speed vb of 6 cm/min. The micro-hardness of the laser-clad layer was approximately 400 HV0.1. The micro-hardness of the heataffected zone with the overaged micro-structure reaching to a depth of around 2.5 mm was approximately 350 HV0.1, which corresponds to the expected hardness of the 1.2799 maraging steel in the as-delivered state. In the depth range from 3 to 5 mm there was a strongly precipitation-hardened zone with an average micro-hardness of 500 HV0.1. At depths greater than 5 mm the micro-hardness gradually decreased to the hardness of the parent metal.
Fig. 2. Through-depth variation of micro-hardness in laser-clad specimen made of 1.2799 maraging steel, P 360 W, vb 6 cm/min, Ed 257:14 J/mm2.
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J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431
Fig. 4. Through-depth variation of micro-hardness in laser-clad specimen made of St 44-3 structural steel, P 360 W, vb 6 cm/ min, Ed 257:14 J/mm2. Fig. 3. Micro-structure in the transverse cross-section of laser-clad specimen made of St 44-3 structural steel, P 360 W, vb 6 cm/ min, Ed 257:14 J/mm2 and measuring points for micro-hardness measurement.
Fig. 3 shows a micro-graph of the characteristic zones obtained from laser cladding of the St 44-3 structural steel and a micro-structure of the cladding magni®ed 1000 times. The micro-graphs are from the specimen with the energy input (Ed) equal to 257.14 J/ mm2. Based on an analysis of the micro-graphs in Fig. 3, four characteristic zones in the micro-structure of the clad specimen made of structural steel can be distinguished: zone ``a'': a remelted and rapidly solidified clad alloyed surface with a typical fine dendritic microstructure due to the fast solidification; zone ``b'': a strongly heat-affected zone with a coarse-grained ferritic±pearlitic micro-structure;
zone ``g'': a transition zone between the heataffected zone and the parent metal with a finegrained ferritic±pearlitic micro-structure; zone ``d'': the parent metal with a typical ferritic± pearlitic micro-structure. Fig. 4 shows the through-depth variation in the micro-hardness of the laser-clad structural steel specimen. The Vickers micro-hardness was measured at three points: at x 0, x 5 mm and x 5 mm (Fig. 3). The data in the diagram refer to the specimen processed with a travelling speed (vb) of 6 cm/min. The micro-hardness of the clad layer was in a range around 450 HV0.1. The micro-hardness values of zones ``b'', ``g'' and ``d'' were the same and in a range around 200 HV0.1, which was approximately the hardness of St 44-3 steel in the as-delivered state. The transition from the surfaced layer to the parent metal
Table 2 The actual and target chemical compositions of submerged-arc surfaced layer of the Ni±Co±Mo alloy on the specimen made of the St 44-3 structural steel Ni
Co
Mo
Ti
Al
Mn
Si
Target chemical composition (%) 7.5 6.25 6.25
0.6
0.1
3.8
1.9
Actual chemical composition (%) 6.17 6.76 6.18
0.42
0.08
1.49
1.32
Cr
Nb
C 0.04
0.33
0.09
0.08
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431
was very sharp; from this we were able to infer that during the laser cladding only a minimal amount of mixing of the ®ller material and the parent metal occurred. 3.2. Analysis of the submerged-arc clad specimens For the micro-structural and micro-chemical analyses and the Vickers hardness measurement, the submerged-arc surfaced specimen was cut in a direction transverse to the surfacing weld. The specimens for the micro-structural analysis with the optical microscope were prepared in the same way as the laser-clad samples. With the submerged-arc surfaced specimens of structural steel, four zones from the surfacing weld surface through-depth were similar to the laser-clad structuralsteel specimens. The micro-structure of the surfaced layer, however, was not dendritic, as a result of a slower cooling rate. The micro-chemical analysis of the surfaced layer showed that the actual composition agreed well with the target composition (Table 2). The lower Mn content was due to a lower ``pick-up'' of Mn from the MnO during welding than we expected. We should increase the content of Mn in the cored wire. The through-depth micro-hardness was measured in the middle of the surfacing weld. The diagram in Fig. 5 shows the results of the measurement of the Vickers
Fig. 5. Through-depth variation in submerged-arc surfaced specimen (I 167 A, U 37 V) made of St 44-3 structural steel after surfacing and after different temperature/time conditions of precipitation annealing.
429
micro-hardness immediately after surfacing as well as after different heat-treatment conditions applied after the surfacing. A comparison of the diagrams in Figs. 4 and 5 shows that the micro-hardness of the submerged-arc surfaced layer was comparable to that of the laser-clad layer and equal to around 450 HV0.1. The transition of the surfaced layer to the parent metal is very sharp. This implies that during the submerged-arc surfacing only minimal mixing of
Fig. 6. Location of measurement of residual stresses and variation of the principal residual stresses in specimen made of 1.2799 maraging steel (a) and specimen made of St 44-3 structural steel (b) after laser surfacing; P 360 W, vb 8 cm/min, Ed 192:86 J/mm2.
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the ®ller material and the parent metal occurred. The hardness below the surfaced layer, as with the laserclad specimens, did not change considerably and was equal to approximately 200 HV0.1, which was almost the same as the hardness of the St 44-3 steel in the asdelivered condition. The hardness of the submergedarc surfaced layer was increased to approximately 550 HV0.1 by a process of precipitation annealing at 550 8C for 8 h. 3.3. Residual stresses The measurements of the residual stresses were carried out after laser cladding by means of a relaxation hole-drilling strain-gauge method. The experimental set-up for the measurement of the residual stresses included a compressed-air drilling device with a 1.6 mm TiC-coated milling cutter. The points were the residual stresses were measured and the results obtained are shown in Fig. 6. Fig. 6a shows the variation of the principal residual stresses as a function of depth for the maraging-steel specimen, and Fig. 6b shows the results for the structural steel specimen, both processed with a travelling speed vb of 8 cm/min. With both specimens in the entire zone of measurement, compression residual stresses were obtained. Compression residual stresses in the surface layer reduce the risk of forming and propagating surface cracks. Such a stress condition is very favourable for parts subjected to dynamic loads. With the maragingsteel specimen the compression residual stresses at a depth between 0.025 and 0.05 mm increased from an initial value of around 370 MPa to a maximum value of around 530 MPa. At a depth between 0.05 and 0.3 mm they varied and then became stable at around 500 MPa. With the structural-steel specimen, the compression residual stresses were the highest just below the surface and were equal to approximately 530 MPa. With increasing depth the stresses were reduced and converged towards zero. 4. Conclusions Maraging steels show an excellent weldability and can be laser clad in a relatively simple way. Good weldability is of major importance when it comes to repairing tool damage without having to dismantle the
tool from the machine, which reduces stoppages on the production line. The second important ®nding is that with the laser cladding of maraging steel, a very favourable through-depth variation of micro-hardness in the laser cladding and the heat-affected zone is achieved. The through-depth variation of micro-hardness can be additionally improved by subsequent precipitation annealing so that the nominal hardness recommended for the operation of the tools can be achieved. The third very important ®nding is that with the laser cladding of maraging steel, a very favourable variation of the compression residual stresses in the surface layer is achieved, which contributes considerably to the extended operating life of the tools. The results obtained are particularly important for industrial applications since major savings can be expected as a result of an extended operating life of the tools made of maraging steel. Very interesting results were also obtained with the laser cladding and submerged-arc surfacing of common structural steel. The mixing of the ®ller material and the parent metal is kept to a minimum with both processes, which ensures favourable mechanical properties of the cladding and the surfacing weld. The respective mechanical properties of the cladding and the surfacing weld can be additionally improved by a subsequent precipitation annealing. The micro-chemical analysis of the submerged-arc surfaced specimen showed good agreement between the desired and actual compositions of the surfaced layer. Surfacing of the Ni±Co±Mo alloy on the structural steel is interesting in the cases where surface resistance to thermo-mechanical loads is required. The laser cladding of structural steel provided, just as with the cladding of maraging steel, a very favourable variation of compression residual stresses in the surface layer. References [1] R.F. Decker, S. Floreen, Maraging steelsÐthe ®rst 30 years, In: R.K. Wilson (Ed.), Maraging Steels: Recent Developments and Applications, The Minerals, Metals and Materials Society, 1988, pp. 1±37. [2] H.J. Becker, K.D. Fuchs, E. Haberling, Maraging Tool Steels, Thyssen Edelstahl Technischer Bericht, 1990, pp. 53±60. [3] J. Grum, M. ZupancÏicÏ, Suitability assessment of replacement of conventional hot-working steels with maraging steel, part 1, mechanical properties of maraging steel after precipitation hardening treatment, Z. Met.kd. 93 (2002) 164±170.
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431 [4] J. Grum, M. ZupancÏicÏ, Suitability assessment of replacement of conventional hot-working steels with maraging steel, part 2, micro-structure of maraging steel after precipitation hardening treatment, Z. Met.kd. 93 (2002) 171±176. [5] B. Gehricke, Development, properties and characteristics of a new maraging steel for die casting dies, in: Proceedings of the Conference on the Die Casting Technology in Harmony with the Environment, Cleveland, 18±21 October 1993, pp. 209±217.
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[6] J. Powell, P.S. Henry, W.M. Steen, Laser cladding with preplaced powder: analysis of thermal cycling and dilution effects, Surf. Eng. 4 (1988) 141±149. [7] W.M. Steen, J. Powell, Laser surface treatments, Mater. Eng. 2 (1993) 208±312. [8] R. KejzÏar, Submerged-arc surfacing with multiple-wire electrode and alloyed agglomerated ¯uxes, in: Proceedings of the International Conference on the Joining of Materials, JOM-7, Helsingùr, 1995, pp. 273±279.
A comparison of tool±repair methods using CO2 laser surfacing and arc surfacing J. Grum*, J.M. Slabe Faculty of Mechanical Engineering, University of Ljubljana, AsÏkercÏeva 6, 1000 Ljubljana, Slovenia
Abstract The life of loaded machine elements and the vital parts of tools can be successfully extended by systematic maintenance and the timely repair of damaged surfaces. It has been proved that with the regular maintenance of tool parts the cost of the tool in the price of a ®nished product can be considerably reduced. It is a very economical practise to manufacture certain parts from lowcost, tough structural steel on which a layer of wear-resistant alloy has been surfaced. In such a case the volume fraction of the surfaced layer is usually much lower than 10% of the total volume of the tool or the machine element. In this paper, we report some of our latest results involving comparative studies of repair surfacing on maraging steel and the cladding of common structural steel with a Ni±Co±Mo alloy similar to the maraging steel using a laser process and submergedarc surfacing. The results are based on micro-structural and micro-chemical analyses of the surfaced layer and are supported by analyses of the micro-hardness and the residual stresses, carried out on suitably adapted ¯at specimens. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser cladding; Submerged-arc surfacing; Maraging steel
1. Introduction Precipitation-hardened maraging steels are suitable for the manufacture of machine elements and tool parts that are subjected to high thermo-mechanical loads as a result of their exceptionally favourable mechanical properties, particularly their excellent fracture toughness combined with their high tensile strength and hardness [1,2]. Grum and ZupancÏicÏ [3,4] have investigated the in¯uence of various temperature/ time conditions of precipitation hardening on the mechanical properties of these steels. Tool manufacturers have been successfully introducing maraging steels in the manufacture of die-casting tools for *
Corresponding author. Tel.: 386-1-4771-203; fax: 386-1-2518-567. E-mail address: [email protected] (J. Grum).
aluminium and magnesium alloys, for which a high resistance to cyclic thermo-mechanical loads is required. An exceptionally good weldability of maraging steels in all states, i.e. prior to and after precipitation annealing [1], permits the successful repair of damage to tools in order to extend the operating life of the tools and preserve the quality of the die-cast parts. Gehricke [5] proved that the weld or the surfacing weld did not show inclusions and pores, which guarantees the high quality of a product. Precipitation annealing of the weld and the heat-affected zone ensures that the nominal hardness of the material recommended for the operation of the tools can be achieved. Laser cladding, which involves the preliminary deposition of a powder of a certain chemical composition and grain size on the workpiece surface, is the simplest way of improving the surface properties of a, usually cheap, parent metal. The selected method of
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(02)01427-7
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431
425
steel cladding was performed using a Ni±Co±Mo alloy similar to maraging steel with the following chemical composition: 7.5% Ni, 6.25% Co, 6.25% Mo, 0.6% Ti, 0.1% Al, 3.8% Mn, 1.9% Si, 0.04% C. Based on the chosen cladding composition, an ALV±NiCoMo-2 powder was developed and manufactured. The chemical compositions of the parent metals and the ALV± NiCoMo-2 powder are given in Table 1. The laser-cladding tests were performed using a CO2 laser (wavelength of 10.6 mm) with a maximum working power of 500 W and a Gaussian distribution of the energy in the beam. The experiments were performed with a laser power (P) of 360 W and a defocusing degree of 22 mm. The focal length of the lens used was 127 mm. The laser-beam diameter on the specimen surface was 1.4 mm. The laser-beam travelling speed (vb) was 6, 8, and 10 cm/min. The different travelling speeds of the laser beam across the specimen surface produced different energy inputs in the workpiece surface, i.e. Ed 257:14 J/mm2 (vb 6 cm/min), Ed 192:86 J/mm2 (vb 8 cm/min), and Ed 154:29 J/mm2 (vb 10 cm/min). The cladding was carried out using argon as an inert shielding gas with a ¯ow rate of 3 l/min. For the tests, specimens with a uniform shape and a size of 27 mm 27 mm 12 mm were selected. With each combination of parent metal and travelling speed of the laser beam three repetitions of the laser cladding were performed. As a result, 18 specimens were produced. For the laser cladding the central part of the specimen surface, 20 mm 20 mm in size, was chosen. The laser beam was moving in a square spiral, starting in the specimen centre. The ALV±NiCoMo-2 powder with a grain size of 0.06±0.1 mm was deposited on the specimen surfaces prior to treatment. The
cladding the parent metal results in the minimum dilution of the parent metal with the ®ller material [6,7]. Using theoretical modelling the authors studied the movement of the fusion front and established a relatively fast propagation of the molten pool through the thermal hindrance represented by the powder layer. When the molten pool reached the boundary between the parent metal and the ®ller material, the thermal load, due to high temperature gradients, increased, which resulted in the rapid solidi®cation of the molten pool. During the cladding of structural steels with a high-quality alloy, it is of extreme importance that the dilution of the cladding due to the melting of the parent metal is minimised as much as possible. The parent metal should be remelted within certain limits to ensure the faultless bonding of the ®ller material with the substrate. In high-productivity submerged-arc surfacing, the dilution of the ®ller material with the parent metal can be effectively reduced by using alloyed agglomerated ¯uxes and/or cored wires. Practice has shown that the penetration can be effectively reduced by submergedarc welding using a multiple-wire electrode. With correctly chosen welding parameters, i.e. low current per wire and a relatively high welding voltage, this process permits high-productivity one-layer surfacing of structural steels [8]. 2. Experimental procedure 2.1. Laser cladding For the laser cladding we chose two kinds of parent metal: a 1.2799 maraging steel and a St 44-3 structural
Table 1 Chemical compositions of the parent metals and the ®ller materials used in laser cladding and submerged-arc surfacing Material
Chemical composition (%) Ni
Co
Mo
Ti
Al
1.2799 (DIN) St 44-3 (DIN) ALV±NiCoMo-2 S±NiCoMo-2
12
8
8
0.5
6.25 6.49
6.25 6.49
0.63 2.6
0.05 0.027 0.12 0.52
Material
SiO2
FeO
Al2O3
MnO
FB 12.2
17
1
20
5
7.5 7.73
Mn
Si
C
BaF2
0.42 3.81 0.21
0.21 1.86 0.11
0.02 0.15 0.08 0.05
CaO
MgO
CaF2
CaSi
Na2O
6
25
23
1
2
1.24
SiO2
0.48
Na2O
0.2
P
S
0.011
0.010
426
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431
powder adhesion to the specimen surface was achieved by depositing a thin layer of high-temperature colour (Henotherm Silicon Schwarz SM, made by Henelit), which also increased the absorption of the laser beam by the material. The silicon colour was applied to the surface with a ®ne brush. The thickness of the powder deposit ranged between 0.25 and 0.3 mm. 2.2. Submerged-arc surfacing
the type and grain size of the micro-structure formed. The etching time determined the quality of the micro-graphs for the individual micro-structural zones. Fig. 1 shows micro-graphs of the characteristic zones after laser cladding of the specimens made of 1.2799 maraging steel. The micro-graphs are from the specimen with an energy input (Ed) equal to 257.14 J/mm2. Based on an analysis of the micro-
Only the St 44-3 structural steel was submerged-arc clad. A Ni±Co±Mo alloy with the same composition as used for the laser cladding was employed. With reference to the chosen composition of the surfacing weld, a S±NiCoMo-2 cored wire with a diameter of 2 mm was developed and manufactured. Its composition is given in Table 1. The compositions of the ALV± NiCoMo-2 powder used in the laser cladding and that of the S±NiCoMo-2 cored wire differed slightly because the burn-off of the alloying elements during the laser cladding with a preliminary deposit of the powder in the Ar shielding was weaker than in the submerged-arc surfacing. The submerged-arc surfacing was accomplished with an automatic welding device that included gravitation supply of the shielding ¯ux. The submergedarc surfacing was achieved with the single, homemade S±NiCoMo-2 cored wire and a commercial FB12.2 ¯ux, the composition of which is given in Table 1. The surfacing was carried with a current (I) of 167 A and a voltage (U) of 37 Von a workpiece with a size of 250 mm 27 mm 12 mm. 3. Experimental results 3.1. Analysis of the laser-clad specimens A micro-structural analysis of the laser-clad specimens of maraging and structural steels was very dif®cult due to great through-thickness differences of the specimen micro-structures. The specimens were etched with nital for the micro-structural analysis. An optical microscope was used to study their micro-structures. Because of the differing throughdepth micro-structures of the clad specimens, different concentrations of nital were used. The concentration of the etching agent was chosen with reference to
Fig. 1. Micro-structure in the transverse cross-section of laser-clad specimen made of 1.2799 maraging steel, P 360 W, vb 6 cm/ min, Ed 257:14 J/mm2 and measuring points for micro-hardness measurement.
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graphs in Fig. 1, ®ve characteristic zones can be distinguished: zone ``a'': a remelted and rapidly solidified clad alloyed surface with a typical dendritic micro-structure due to fast solidification; zone ``b'': the strongly heat-affected zone with an overaged micro-structure (Ni martensite matrix with coarse precipitates and reverse austenite) formed as a consequence of a high heat input; zone ``c'': the heat-affected precipitation zone I with a Ni martensite matrix and a high density of precipitates; zone ``d'': the heat-affected precipitation zone II with a Ni martensite matrix and a medium density of precipitates; zone ``e'': the heat-affected precipitation zone III with a Ni martensite matrix and a small density of precipitates. The size of the precipitates also decreases from zone ``c'' to zone ``e''. Based on reports in the literature [1,4] we assume that the precipitates in the heat-affected zones are primarily Ni3Mo, Ni3Ti, Fe2Mo, Fe2Ti and TiC. In the heat-affected precipitation zones, the content and the size of the precipitated phases was related to the laser-cladding conditions. Thus, three precipitation zones in the heat-affected zone were found only with the highest energy input
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(Ed), i.e. 257.14 J/mm2, which was achieved with a power (P) of 360 W, and a laser-beam travelling speed (vb) of 6 cm/min. With the lowest laser-beam travelling speed the depth of the heat-affected zone was considerably greater. Consequently, a more uniform micro-structure at the transitions between the individual zones and a more uniform through-depth microhardness variation at the three measuring points in the clad specimen was obtained. The through-depth Vickers micro-hardness was measured at three points: in the centre of the transverse cross-section, x 0; at a distance x of 5 mm to the right of the centre; and at a distance x of 5 mm to the left of the centre (Fig. 1). Fig. 2 shows the characteristic micro-hardness variation as a function of depth in the laser-clad specimens made of the 1.2799 maraging steel. The data in the diagram refer to the specimen processed with a travelling speed vb of 6 cm/min. The micro-hardness of the laser-clad layer was approximately 400 HV0.1. The micro-hardness of the heataffected zone with the overaged micro-structure reaching to a depth of around 2.5 mm was approximately 350 HV0.1, which corresponds to the expected hardness of the 1.2799 maraging steel in the as-delivered state. In the depth range from 3 to 5 mm there was a strongly precipitation-hardened zone with an average micro-hardness of 500 HV0.1. At depths greater than 5 mm the micro-hardness gradually decreased to the hardness of the parent metal.
Fig. 2. Through-depth variation of micro-hardness in laser-clad specimen made of 1.2799 maraging steel, P 360 W, vb 6 cm/min, Ed 257:14 J/mm2.
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Fig. 4. Through-depth variation of micro-hardness in laser-clad specimen made of St 44-3 structural steel, P 360 W, vb 6 cm/ min, Ed 257:14 J/mm2. Fig. 3. Micro-structure in the transverse cross-section of laser-clad specimen made of St 44-3 structural steel, P 360 W, vb 6 cm/ min, Ed 257:14 J/mm2 and measuring points for micro-hardness measurement.
Fig. 3 shows a micro-graph of the characteristic zones obtained from laser cladding of the St 44-3 structural steel and a micro-structure of the cladding magni®ed 1000 times. The micro-graphs are from the specimen with the energy input (Ed) equal to 257.14 J/ mm2. Based on an analysis of the micro-graphs in Fig. 3, four characteristic zones in the micro-structure of the clad specimen made of structural steel can be distinguished: zone ``a'': a remelted and rapidly solidified clad alloyed surface with a typical fine dendritic microstructure due to the fast solidification; zone ``b'': a strongly heat-affected zone with a coarse-grained ferritic±pearlitic micro-structure;
zone ``g'': a transition zone between the heataffected zone and the parent metal with a finegrained ferritic±pearlitic micro-structure; zone ``d'': the parent metal with a typical ferritic± pearlitic micro-structure. Fig. 4 shows the through-depth variation in the micro-hardness of the laser-clad structural steel specimen. The Vickers micro-hardness was measured at three points: at x 0, x 5 mm and x 5 mm (Fig. 3). The data in the diagram refer to the specimen processed with a travelling speed (vb) of 6 cm/min. The micro-hardness of the clad layer was in a range around 450 HV0.1. The micro-hardness values of zones ``b'', ``g'' and ``d'' were the same and in a range around 200 HV0.1, which was approximately the hardness of St 44-3 steel in the as-delivered state. The transition from the surfaced layer to the parent metal
Table 2 The actual and target chemical compositions of submerged-arc surfaced layer of the Ni±Co±Mo alloy on the specimen made of the St 44-3 structural steel Ni
Co
Mo
Ti
Al
Mn
Si
Target chemical composition (%) 7.5 6.25 6.25
0.6
0.1
3.8
1.9
Actual chemical composition (%) 6.17 6.76 6.18
0.42
0.08
1.49
1.32
Cr
Nb
C 0.04
0.33
0.09
0.08
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was very sharp; from this we were able to infer that during the laser cladding only a minimal amount of mixing of the ®ller material and the parent metal occurred. 3.2. Analysis of the submerged-arc clad specimens For the micro-structural and micro-chemical analyses and the Vickers hardness measurement, the submerged-arc surfaced specimen was cut in a direction transverse to the surfacing weld. The specimens for the micro-structural analysis with the optical microscope were prepared in the same way as the laser-clad samples. With the submerged-arc surfaced specimens of structural steel, four zones from the surfacing weld surface through-depth were similar to the laser-clad structuralsteel specimens. The micro-structure of the surfaced layer, however, was not dendritic, as a result of a slower cooling rate. The micro-chemical analysis of the surfaced layer showed that the actual composition agreed well with the target composition (Table 2). The lower Mn content was due to a lower ``pick-up'' of Mn from the MnO during welding than we expected. We should increase the content of Mn in the cored wire. The through-depth micro-hardness was measured in the middle of the surfacing weld. The diagram in Fig. 5 shows the results of the measurement of the Vickers
Fig. 5. Through-depth variation in submerged-arc surfaced specimen (I 167 A, U 37 V) made of St 44-3 structural steel after surfacing and after different temperature/time conditions of precipitation annealing.
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micro-hardness immediately after surfacing as well as after different heat-treatment conditions applied after the surfacing. A comparison of the diagrams in Figs. 4 and 5 shows that the micro-hardness of the submerged-arc surfaced layer was comparable to that of the laser-clad layer and equal to around 450 HV0.1. The transition of the surfaced layer to the parent metal is very sharp. This implies that during the submerged-arc surfacing only minimal mixing of
Fig. 6. Location of measurement of residual stresses and variation of the principal residual stresses in specimen made of 1.2799 maraging steel (a) and specimen made of St 44-3 structural steel (b) after laser surfacing; P 360 W, vb 8 cm/min, Ed 192:86 J/mm2.
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the ®ller material and the parent metal occurred. The hardness below the surfaced layer, as with the laserclad specimens, did not change considerably and was equal to approximately 200 HV0.1, which was almost the same as the hardness of the St 44-3 steel in the asdelivered condition. The hardness of the submergedarc surfaced layer was increased to approximately 550 HV0.1 by a process of precipitation annealing at 550 8C for 8 h. 3.3. Residual stresses The measurements of the residual stresses were carried out after laser cladding by means of a relaxation hole-drilling strain-gauge method. The experimental set-up for the measurement of the residual stresses included a compressed-air drilling device with a 1.6 mm TiC-coated milling cutter. The points were the residual stresses were measured and the results obtained are shown in Fig. 6. Fig. 6a shows the variation of the principal residual stresses as a function of depth for the maraging-steel specimen, and Fig. 6b shows the results for the structural steel specimen, both processed with a travelling speed vb of 8 cm/min. With both specimens in the entire zone of measurement, compression residual stresses were obtained. Compression residual stresses in the surface layer reduce the risk of forming and propagating surface cracks. Such a stress condition is very favourable for parts subjected to dynamic loads. With the maragingsteel specimen the compression residual stresses at a depth between 0.025 and 0.05 mm increased from an initial value of around 370 MPa to a maximum value of around 530 MPa. At a depth between 0.05 and 0.3 mm they varied and then became stable at around 500 MPa. With the structural-steel specimen, the compression residual stresses were the highest just below the surface and were equal to approximately 530 MPa. With increasing depth the stresses were reduced and converged towards zero. 4. Conclusions Maraging steels show an excellent weldability and can be laser clad in a relatively simple way. Good weldability is of major importance when it comes to repairing tool damage without having to dismantle the
tool from the machine, which reduces stoppages on the production line. The second important ®nding is that with the laser cladding of maraging steel, a very favourable through-depth variation of micro-hardness in the laser cladding and the heat-affected zone is achieved. The through-depth variation of micro-hardness can be additionally improved by subsequent precipitation annealing so that the nominal hardness recommended for the operation of the tools can be achieved. The third very important ®nding is that with the laser cladding of maraging steel, a very favourable variation of the compression residual stresses in the surface layer is achieved, which contributes considerably to the extended operating life of the tools. The results obtained are particularly important for industrial applications since major savings can be expected as a result of an extended operating life of the tools made of maraging steel. Very interesting results were also obtained with the laser cladding and submerged-arc surfacing of common structural steel. The mixing of the ®ller material and the parent metal is kept to a minimum with both processes, which ensures favourable mechanical properties of the cladding and the surfacing weld. The respective mechanical properties of the cladding and the surfacing weld can be additionally improved by a subsequent precipitation annealing. The micro-chemical analysis of the submerged-arc surfaced specimen showed good agreement between the desired and actual compositions of the surfaced layer. Surfacing of the Ni±Co±Mo alloy on the structural steel is interesting in the cases where surface resistance to thermo-mechanical loads is required. The laser cladding of structural steel provided, just as with the cladding of maraging steel, a very favourable variation of compression residual stresses in the surface layer. References [1] R.F. Decker, S. Floreen, Maraging steelsÐthe ®rst 30 years, In: R.K. Wilson (Ed.), Maraging Steels: Recent Developments and Applications, The Minerals, Metals and Materials Society, 1988, pp. 1±37. [2] H.J. Becker, K.D. Fuchs, E. Haberling, Maraging Tool Steels, Thyssen Edelstahl Technischer Bericht, 1990, pp. 53±60. [3] J. Grum, M. ZupancÏicÏ, Suitability assessment of replacement of conventional hot-working steels with maraging steel, part 1, mechanical properties of maraging steel after precipitation hardening treatment, Z. Met.kd. 93 (2002) 164±170.
J. Grum, J.M. Slabe / Applied Surface Science 208±209 (2003) 424±431 [4] J. Grum, M. ZupancÏicÏ, Suitability assessment of replacement of conventional hot-working steels with maraging steel, part 2, micro-structure of maraging steel after precipitation hardening treatment, Z. Met.kd. 93 (2002) 171±176. [5] B. Gehricke, Development, properties and characteristics of a new maraging steel for die casting dies, in: Proceedings of the Conference on the Die Casting Technology in Harmony with the Environment, Cleveland, 18±21 October 1993, pp. 209±217.
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[6] J. Powell, P.S. Henry, W.M. Steen, Laser cladding with preplaced powder: analysis of thermal cycling and dilution effects, Surf. Eng. 4 (1988) 141±149. [7] W.M. Steen, J. Powell, Laser surface treatments, Mater. Eng. 2 (1993) 208±312. [8] R. KejzÏar, Submerged-arc surfacing with multiple-wire electrode and alloyed agglomerated ¯uxes, in: Proceedings of the International Conference on the Joining of Materials, JOM-7, Helsingùr, 1995, pp. 273±279.