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Problems in laser repair-welding a surface-treated tool steel
Surface & Coatings Technology 201 (2007) 4518 – 4525 www.elsevier.com/locate/surfcoat
Problems in laser repair-welding a surface-treated tool steel M. Vedani a , B. Previtali a,⁎, G.M. Vimercati a , A. Sanvito b , G. Somaschini c a
Politecnico di Milano, Dipartimento di Meccanica, Via La Masa 34, 20156 Milano, Italy b Sanvito e Somaschini SpA, Via Francesco Petrarca 4, 20055 Renate (MI), Italy c Utensiltempra SrL, Via Rivabella 98, 20045 Besana In Brianza (MI), Italy Received 14 June 2006; accepted in revised form 14 September 2006 Available online 31 October 2006
Abstract A research work is described on the development of defects in laser repair welding of a surface-treated tool steel. Repair welding tests were carried out on plasma-nitrided and on chrome-plated type 1.2738 steel plates. Welding defects in the chrome-plated samples were mainly due to chrome and oxygen overalloying of the weld metal, leading to extensive hot cracking. Cracks of the surface chrome deposit in the HAZ due to welding stresses were also detected. In the nitrided samples, welding resulted in the copious formation of gas pores due to nitrogen release during weld metal solidification. A procedure for combined laser pre-treating and laser welding of the nitrided samples was thus presented. The procedure is aimed at minimising the negative effect of the laser beam on the structural integrity of the base and welded material. The results of the microstructural analyses revealed that a significant reduction in defects could be achieved by the proposed method in the nitrided samples. © 2006 Elsevier B.V. All rights reserved. Keywords: [X] Nitrided tool steel; [X] Chrome plated tool steel; [X] Laser repairing welding
1. Introduction Steel dies for injection moulding of metal alloys (diecasting) and plastics are products of strategic importance in the industrial world. They strongly affect the productivity of goods of very large mass production such as car body and engine parts, glass and polymer containers, and household appliances. Depending on mould application, the typical damage and failure mechanisms may differ. In diecasting tools, the main reason for mould failure is surface cracking promoted by thermal fatigue phenomena. Other common reasons for damage are tension cracks caused by constructional notches and steel erosion promoted by the cast molten metal flow [1–3]. Moulds for polymer injection are subjected to lower working temperatures but their pressure cycles are significantly more demanding. Therefore, mechanical-fatigue damage and overload failures might occur. Normally, plastics are not regarded as chemically aggressive materials but during their production, aggressive agents can be released (hydrochloric acid in PVC processing, acetic acid in cellulose ⁎ Corresponding author. E-mail address: [email protected] (B. Previtali). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.051
acetate processing, hydrofluoric acid in plastics containing fluorine) and localised or uniform corrosion attacks might also occur [4]. Service life of dies can be further improved by using proper surface treatments such as nitriding and chrome plating. Despite the widespread application in several tooling sectors, there is still a debate in published literature on the effectiveness of plasma nitriding or of duplex treatments (combined plasma nitriding and PVD coating) on wear and thermal fatigue resistance in diecasting tools and rolling mill equipments [5–9]. On the contrary, there is full agreement on the marked improvement achievable by functional chrome plating in polymer moulding dies and other tools subjected to wear and corrosive environments [10–12]. When damage appears during the life of the tool, processes based on fusion welding are frequently adopted to rebuild worn or cracked surfaces. According to industrial practice, repairing of the tool is done by first removing the damaged parts by milling or grinding and by rebuilding the missing volume by welding with a suitable filler metal [13]. Pre-heating and postweld heat treating are generally carried out to avoid solidification cracking and excessive residual stress induced by welding
M. Vedani et al. / Surface & Coatings Technology 201 (2007) 4518–4525 Table 1 Chemical composition (mass %) of the 1.2738 tool steel and of the filler metals C
Si
Mn
Cr
Mo
Ni
Ti
S
P
1.2738 steel 0.48 0.25 1.67 2.20 0.23 1.26 – 0.0026 0.013 Filler metal wire 0.35 0.3 1.2 7.0 2.0 – 0.3 – – type GS2 Filler metal wire 0.10 0.6 1.2 – 0.5 – – – – type GS20
[14]. The need to modify the shape of existing tools after prototype evaluation or due to product requirement is a further reason for tool refurbishing by welding [15]. Repair welding of tool steels represents a technical practice with considerable metallurgical concerns due to low weldability of the hardened, often highly alloyed tool steels. Gas Tungsten Arc Welding (GTAW) is the most common process currently in use for repair welding due to great process control suited for small areas, thin sections and sharp edges [16]. For distortion sensitive tools, high-density heat input processes such as plasma arc and laser beam welding are preferred [17]. The lower net heat input required to produce the weld reduces tool distortions and allows thermally sensitive steel grades to be repaired without additional pre- and post-weld thermal treatments. When the tools to be repaired are surface treated, additional difficulties in welding arise. Nitrogen release and gas pores formation occur in nitrided tools whereas Cr overalloying and cracking of the weld metal are often found in chrome plated tools [18]. A research work is described in this paper on the development of defects in laser repair-welding of a surface treated tool steel. A procedure for combined laser pre-treating and welding of the tool is also presented. 2. Experimental procedures 2.1. Materials and surface treatments The investigation was carried out on 25 mm thick samples of a quenched and tempered type 1.2738 tool steel having the chemical composition given in Table 1. Repair welding was carried out by a pulsed Nd-YAG laser with commercial type 0.5 mm diameter filler wires, whose nominal chemical compositions are also reported in Table 1.
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Before the welding tests, some of the samples were surface treated in industrial plants. A first set of the plates was plasma nitrided at 510 °C for 12 h in a 80N2/20H2 atmosphere at 350 Pa, whereas a second set of samples was electrolytically plated in a hexavalent chromic acid solution bath to produce a functional coating with a nominal thickness of 50 μm. 2.2. Laser welding parameters The weld beads were initially deposited on the plates using several sets of process conditions. A few reference conditions were then selected on the basis of resulting weld bead size (height and depth) and absence of defects. Table 2 gives the main process parameters adopted. In the table, due to the pulsed nature of the laser beam, the heat input rate was calculated as: HI ¼ ðPdsdf =DdmÞ ½J=mm2 where HI is the heat input rate, P the pulse power, τ the pulse duration, f the pulse frequency, D the spot diameter and ν the beam feed rate. Repair welding tests were carried out by producing multipass deposits obtained by parallel beads with a moderate degree of lateral overlapping. As a rule, the deposits were produced using the GS20-type filler wires (see Table 1). However, for the chrome-plated samples and for specific trials on the asmachined samples, a GS2-type filler wire was also tested in the attempt to achieve higher hardness properties of the as welded deposits. To improve the weldability of the plasma nitrided steel, surface remelting laser passes were carried out on a set of nitrided samples before laser deposition. The surface remelting passes were obtained by using the same laser source but without the use of the filler metal. Table 2 shows that surface remelting was done with two different heat input rates, obtained by changing the spot size. As a result, the height-to-depth ratio and the volume of the molten pool were modified. The following deposition phase, performed with the filler metal, was kept at constant welding conditions, corresponding to set C parameters. 2.3. Experimental analyses Analyses were carried out on the described samples to characterize microstructural properties of the weld deposits and
Table 2 Process conditions adopted for the production of laser weld beads and laser remelting passes
Type of process Pulse duration (ms) Pulse power (kW) Pulse frequency (Hz) Spot diameter (mm) Travel speed (mm/min) Heat input rate (J/mm2) Shielding gas Filler wire
Set A (HP)
Set B (LP)
Set C
Set D (T1)
Welding 7.2 0.88 9 0.55 0.65 9570 Ar GS2/GS20
Welding 7.2 0.52 9 0.55 0.65 5655 Ar GS2/GS20
Welding 8.0 0.76 7.5 0.55 0.65 7653 Ar GS20
Treatment 8.0 0.76 7.5 0.55 0.65 7653 Ar –
Set E (T2) Welding 8.0 0.76 7.5 0.55 0.65 7653 Ar GS20
Treatment 8.0 0.76 7.5 1.10 0.65 3827 Ar –
Welding 8.0 0.76 7.5 0.55 0.65 7653 Ar GS20
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to define chemical composition profiles across the surface treated samples and in the weld deposits. Microhardness profiles were measured along the vertical axis of the deposits, starting from the upper surface, by selecting a load of 1.96 N (HV 200) on the Vickers indenter and performing at least four indentations for each depth level. For the evaluation of the case depth in the nitrided samples and at chrome layer-substrate interface, additional measurements were carried out with a load of 0.098 N (HV 10), moving from external surface toward the centre of the samples. 3. Results 3.1. Microstructure of the surface-treated plates Representative optical micrographs of the heat treated 1.2738 tool steel plate in the as-machined condition and after the two surface treatments here investigated are shown in Fig. 1. The bulk structure of the steel was formed by tempered martensite with a prior austenite grain size of about of 250 μm (Fig. 1a). Inspection of the nitrided samples (Fig. 1b) allowed to observe the presence of a thin compound layer and of a diffusion layer, extending for about 50 μm from the surface. Fig. 2a shows the microhardness profile measured on a cross-
Fig. 1. Representative optical micrographs of the 1.2738 steel. (A) As machined; (B) plasma nitrided; (C) chrome plated.
to highlight possible improvements achievable in repair welding of surface-treated tools. Specimens for microstructural examinations and microhardness profile determinations were cut using a precision diamond saw and polished by conventional metallographic techniques. When required, the microstructure was revealed using the Nital 2% or the Vilella reagents. Optical and scanning electron microscope (SEM) observations were carried out on the prepared specimens. An EDS microprobe fitted to the SEM was also used
Fig. 2. Microhardness profiles. (A) Plasma nitrided, (B) chrome-plated samples.
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the welds was extremely narrow, about 50 μm, when evaluated on the basis of microstructural alteration visible in optical micrographs. Scanning electron microscope (SEM) analyses revealed that the weld metal structure was made up of tiny cellular dendrites with a spacing ranging from 2 to 4 μm (Fig. 4b). 3.3. Microstructure of welded chrome-plated samples The weld deposits produced on chrome-plated samples revealed to be particularly critical since the weld metal zone featured a large number of cracks, as depicted in Figs. 5a and 6a. In addition, the Cr layer located in the HAZ of the welded samples always showed transversal cracks as a result of tensile stresses promoted by weld metal solidification. Microprobe analyses carried out in the weld metal zone showed that the chemical composition was reasonably homogeneous and that a significant enrichment in Cr (up to 11.8% in single pass deposits) was achieved. Cr is believed to be mainly supplied by the dilution effect of the molten Cr layer. Elemental line profiles shown in Fig. 6b also suggested that the chrome-plated layer features a higher oxygen content that can be released in the weld metal zone during welding.
Fig. 3. Weld beads produced by Nd-YAG laser deposition on the as machined 1.2738 steel by type C conditions. (A) Macro views of the top surface; (B) section of a multipass deposit obtained under type B conditions.
section of a nitrided sample. The hardness close to the surface was about 750 HV. A case depth of about 400 μm was evaluated as the distance from the surface where the microhardness exceeds by 10% the core hardness value. The chrome plating layer featured a thickness of about 50 μm, as depicted in Fig. 1c. Microhardness profiles measured across the deposited chrome layer showed a hardness ranging from 800 to 1000 HV in the chrome layer and a sharp deviation to the unaffected tool steel hardness beneath it (Fig. 2b). 3.2. Microstructure of the welded untreated samples The average size of the weld bead samples here investigated was about 10 mm in length and 0.6 mm in width for the single pass deposits, as depicted in Fig. 3a. Multipass weld beads featured a deliberate transversal overlapping of about 40% in width (Fig. 3b) to increase weld bead homogeneity. Inspection of the weld bead microstructure showed evidence of a complex convective flow of the liquid metal within the weld metal zone. Solidification microcracks (Fig. 4a) were often observed when the filler metal rich in C and Cr (GS2 type) was used. As expected, the heat affected zone (HAZ) of
Fig. 4. Representative optical micrographs of weld beads produced under type B conditions with GS2 filler metal. (A) Double pass deposit; (B) weld metal microstructure showing intergranular hot cracks.
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tiveness was qualitatively assessed by the amount of gas pores exposed on the upper surface of the treated samples observed by SEM (compare Fig. 9a and b). The production of a weld deposit using a filler metal on such remelted steel regions, generally allowed the amount of gas pores in the final weld bead to be reduced, even though residual voids remained in the remelted zones that were not subjected to the successive deposition by the filler metal. The microstructure of the remelted and welded samples also featured a relatively wide extension of the HAZ, as confirmed by microhardness measurements. Fig. 10a depicts a representative microhardness profile recorder along the thickness of a sectioned surface treated and welded nitrided sample (set D conditions). The soft
Fig. 5. Cross-sectional optical micrographs of the weld deposits. (A) Chromeplated sample under type A conditions with GS20 filler metal; (B) nitrided sample under type C conditions.
3.4. Microstructure of the welded nitrided samples Welding of the nitrided samples led to the release of a large amount of nitrogen within the molten pool and resulted in a deposit structure with a significant number of gas pores, as depicted in Figs. 5b and 7a. Also in this case, small hot cracks were detected in the weld deposit structure. These defects often took their origin from gas pores. SEM images of the upper surface of the weld beads (Fig. 7b) suggested that the pores intersecting the surface are preferentially located at the edges of each pass. 3.5. Effects of surface-remelting and weld deposition on nitrided samples For the deposits performed on the nitrided steels, the formation of gas pores was considered to be the most limiting factor, strongly impairing the reliability of weld-repaired moulds. For this reason, a new approach was investigated [19], consisting of an initial surface remelting pass, carried out using the Nd-YAG laser source without the filler metal, and of a second pass, aimed at producing a new weld deposit on the previously remelted and degassed metal portion. Fig. 8 shows a collection of cross sectional views of multipass samples obtained according to the above mentioned approach, with the two different parameter settings defined in Table 2. The corresponding upper surface of the samples are reported in Fig. 9. The two parameter settings here selected for the surface remelting pass differed in their focus distance and therefore in the laser beam spot size (0.55 mm diameter for type D and 1.10 mm for type E). The shape of the remelted zone of the samples varied accordingly, giving a shallow but more intensive degassing effect for the type E condition. The degassing effec-
Fig. 6. Deposit obtained on chrome-plated samples under type A welding condition with GS20 filler metal. (A) Optical micrograph; (B) elemental line profile of oxygen, iron and chromium across the unmelted region of the chrome layer (along the arrow shown in the micrograph.
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From industrial experience it is also well known that repair welding of surface treated tools is more critical [13]. In the present investigation, for the chrome-plated samples a Cr-rich filler metal was initially selected in order to better match the surface composition and preserve the high hardness properties of the chrome-plated tools. Large defects formed due to hot cracking and the weld metal revealed to be excessively enriched in chromium and oxygen due to elemental pickup from the chrome-plated layer. It is supposed that improved weldability
Fig. 7. Multipass weld deposit produced on a nitrided sample under type C condition. (A) Enlarged view of a porous region of the weld bead; (B) macro view of the multi-pass weld bead.
weld deposit obtained by the use of a low-alloyed filler is recognized within the first 300 μm from surface. Beneath this region, a high-hardness HAZ can be recognized (corresponding to the previously remelted region of the 1.2738 steel), and only after about 800–1000 μm in depth, the unaffected parent metal can be found. Cracks mainly formed within the HAZ and at gas pores, (Fig. 10b) due to the high hardness and possible brittleness of this steel region combined with the tensile stress induced by the following deposition pass. 4. Discussion Pulsed laser welding is receiving great attention as a welding process for tool refurbishing operations requiring great precision and flexibility [17,20–23]. Previous investigations on repair welding of tool steels [24] already showed that satisfactory results could be obtained using a laser source on several tool steel grades. From present results it was confirmed that welding a 1.2738 steel generates a limited extension of the HAZ and produces a refined weld metal structure, thus potentially reducing the cracking tendency and increasing hardness.
Fig. 8. Cross-sectional micrographs of the nitrided samples. (A) Surface treatment under set D conditions; (B) surface treatment under set E conditions; (C) weld bead on surface treated sample under set D conditions; (D) weld bead on surface treated sample under set E conditions.
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Fig. 9. Upper view of the laser-processed nitrided samples. (A) Surface treatment under set D conditions; (B) surface treatment under set E conditions; (C) weld bead on surface treated sample under set D conditions; (D) weld bead on surface treated sample under set E conditions.
could have been achieved by the use of low-alloyed filler metal wires. This produces a dilution effect of the harmful elements but could also generate softer deposits with inappropriate properties. A second limiting aspect consisted in the systematic cracking of the unmelted Cr layer in the HAZ close to the weld deposit due to welding stresses. Although the above harmful effects could be reasonably reduced with a further selection of materials and welding conditions, it is believed that no reliable repair welding can be performed in chrome-plated samples without a preliminary removal of the surface layer by mechanical methods. Repair welding of the nitrided samples led to copious development of gas porosity due to nitrogen release from the molten weld metal, thus impairing the structural integrity of repair welds. A laser remelting treatment before the weld deposition was therefore investigated in order to promote gas release within the molten nitrided layer. The potential improvement of the subsequent welding operation within this partially degassed region was then experimentally evaluated. It is worth noting that since this treatment is aimed at reducing the amount of nitrogen from surfacial layers, it also fully destroys the nitrided layer of the tool. However, the preservation of the nitrided layer is of
secondary importance when compared to the need of restoring the geometrical surface of the tool (rebuilding of cracked and worn surfaces). In addition, the rapid solidification conditions experienced by the weld metal can potentially contribute to increase the surface hardness of the tool and partially counterbalance the lack of the nitriding treatment in the repaired regions. The production of a weld deposit using a filler metal on such remelted steel regions, generally allowed the amount of gas pores in the final weld bead to be significantly reduced, even though residual voids remained in the remelted zones that were not subjected to the successive deposition by the filler metal. Microstructural observations also confirmed that the relatively wide high-hardness HAZ formed by remelting a larger volume of steel generates a larger volume of brittle material that is prone to cracking on further welding. The results suggest that a reduction of the gas pores can be obtained in the nitrided and welded samples. Technological parameters have to be carefully adjusted since an optimally balanced volume of remelted zone as a function of the desired size and shape of the weld deposit is required. Experimental investigations are currently in progress to find laser treatment
M. Vedani et al. / Surface & Coatings Technology 201 (2007) 4518–4525
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metal solidification. Small cracks starting from these voids were also detected. ○ A procedure for laser pre-treating was thus developed, acting as a degassing surface treatment for the nitrided samples. Two different parameter settings were investigated that produced a different extension of the degassed volume. ○ The combination of the above laser remelting and the subsequent welding using a filler wire revealed that a significant reduction of defects could be achieved in the nitrided samples. A further improvement in deposit quality can be achieved by a greater control of technological parameters to produce an optimal volume and shape of the remelted region and by a more precise location of the weld deposit within this volume. Acknowledgements The authors would like to express their acknowledgement to the skillful experimental work carried out by Mr P. Pellin, Dr. A. Ciolfi, Dr. D. Colombo and Mr A. Masi. References
Fig. 10. Deposit performed under set D conditions on a surface treated nitrided sample. (A) Microhardness profile taken along the vertical axis (B) microstructure of a weld deposit.
conditions that produce a deep but more uniform volume of the degassed steel. Aesthetic passes will also be evaluated in order to attain a smooth and tempered surface of the degassed volume. 5. Conclusions The investigation on the development of defects in repairwelding tests by pulsed Nd-YAG laser deposition on surface treated 1.2738 steel samples allowed the following conclusions to be drawn. ○ Extensive hot cracking in the fine cellular-dendritic structure of the weld metal was detected in the chrome-plated samples. These welding defects were accounted for by chrome and oxygen overalloying of the weld metal due to chrome-plating layer dissolution. Cracks of the surface chrome deposit in the HAZ due to welding stress were also detected. These unavoidable effects suggested that repair welding in chromeplated samples of the 1.2738 steel grade is hardly feasible. ○ Welding of the nitrided samples resulted in the copious formation of gas pores due to nitrogen release during weld
[1] A. Persson, S. Hogmark, J. Bergstrom, Surf. Coat. Technol. 191 (2–3) (2005) 216. [2] M. Nagasawa, K. Kubota, Y. Tamura, H. Yokoo, Proc. Fifth Int Tooling Conf., Leoben, 1999, p. 225. [3] P. Ried, J. Moore, J. Lin, S. Carrera, Die Cast. Eng. 49 (5) (2005) 40. [4] B. Gehricke, I. Schruff, Proc. Fifth Int Tooling Conf., Leoben, 1999, p. 83. [5] M. Uma Devi, T.K. Chakraborty, O.N. Mohanty, Surf. Coat. Technol. 116–119 (1999) 212. [6] M. Pellizzari, A. Molinari, G. Straffelini, Surf. Coat. Technol. 142–144 (2001) 1109. [7] W. Blech, M. Pant, Wear 259 (2005) 377. [8] M. Uma Devi, O.N. Mohanty, Surf. Coat. Technol. 107 (1998) 55. [9] N. Dingremont, E. Bergmann, P. Collignon, Surf. Coat. Technol. 72 (1995) 157. [10] K.M. Yin, C.M. Wang, Surf. Coat. Technol. 114 (1999) 213. [11] J. Simao, D.K. Aspinwall, J. Mater. Process. Technol. 92–93 (1999) 281. [12] K.R. Newby, Metal Finish. 97 (1999) 227. [13] S. Thompson, Mold, Tool and Die Repair Welding, 1th.ed, Wiliam Andrew Publishing, 1999. [14] T. Lant, D.L. Robinson, B. Spafford, S. Storesund, Int. J. Press. Vessels Piping 78 (11–12) (2001) 813. [15] J.Y. Jeng, M.C. Lin, J. Mater. Process. Technol. 110 (1) (2001) 98. [16] W.T. Preciado, C.E.N. Bohorquez, J. Mater. Process. Technol. 179 (2006) 244. [17] E. Capello, B. Previtali, J. Mater. Process. Technol. 174 (2006) 223. [18] K.L. Baumert, Mater. Perform. 33 (11) (1994) 45. [19] E. Capello, B. Previtali, M. Vedani, Italian Patent N. MI2005A000575 (2005). [20] J. Kim, Y. Peng, Opt. Lasers Eng. 33 (2000) 299. [21] J. Kim, Y. Peng, J. Mater. Process. Technol. 104 (2000) 284. [22] Y. Sun, H. Sunada, N. Tsujii, ISIJ Int. 41 (9) (2001) 1006. [23] Y. Sun, S. Hanaki, M. Yamashita, H. Uchida, H. Tsujii, Vacuum 73 (2004) 655. [24] M. Vedani, J. Mater. Sci. 39 (2004) 241.
Problems in laser repair-welding a surface-treated tool steel M. Vedani a , B. Previtali a,⁎, G.M. Vimercati a , A. Sanvito b , G. Somaschini c a
Politecnico di Milano, Dipartimento di Meccanica, Via La Masa 34, 20156 Milano, Italy b Sanvito e Somaschini SpA, Via Francesco Petrarca 4, 20055 Renate (MI), Italy c Utensiltempra SrL, Via Rivabella 98, 20045 Besana In Brianza (MI), Italy Received 14 June 2006; accepted in revised form 14 September 2006 Available online 31 October 2006
Abstract A research work is described on the development of defects in laser repair welding of a surface-treated tool steel. Repair welding tests were carried out on plasma-nitrided and on chrome-plated type 1.2738 steel plates. Welding defects in the chrome-plated samples were mainly due to chrome and oxygen overalloying of the weld metal, leading to extensive hot cracking. Cracks of the surface chrome deposit in the HAZ due to welding stresses were also detected. In the nitrided samples, welding resulted in the copious formation of gas pores due to nitrogen release during weld metal solidification. A procedure for combined laser pre-treating and laser welding of the nitrided samples was thus presented. The procedure is aimed at minimising the negative effect of the laser beam on the structural integrity of the base and welded material. The results of the microstructural analyses revealed that a significant reduction in defects could be achieved by the proposed method in the nitrided samples. © 2006 Elsevier B.V. All rights reserved. Keywords: [X] Nitrided tool steel; [X] Chrome plated tool steel; [X] Laser repairing welding
1. Introduction Steel dies for injection moulding of metal alloys (diecasting) and plastics are products of strategic importance in the industrial world. They strongly affect the productivity of goods of very large mass production such as car body and engine parts, glass and polymer containers, and household appliances. Depending on mould application, the typical damage and failure mechanisms may differ. In diecasting tools, the main reason for mould failure is surface cracking promoted by thermal fatigue phenomena. Other common reasons for damage are tension cracks caused by constructional notches and steel erosion promoted by the cast molten metal flow [1–3]. Moulds for polymer injection are subjected to lower working temperatures but their pressure cycles are significantly more demanding. Therefore, mechanical-fatigue damage and overload failures might occur. Normally, plastics are not regarded as chemically aggressive materials but during their production, aggressive agents can be released (hydrochloric acid in PVC processing, acetic acid in cellulose ⁎ Corresponding author. E-mail address: [email protected] (B. Previtali). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.051
acetate processing, hydrofluoric acid in plastics containing fluorine) and localised or uniform corrosion attacks might also occur [4]. Service life of dies can be further improved by using proper surface treatments such as nitriding and chrome plating. Despite the widespread application in several tooling sectors, there is still a debate in published literature on the effectiveness of plasma nitriding or of duplex treatments (combined plasma nitriding and PVD coating) on wear and thermal fatigue resistance in diecasting tools and rolling mill equipments [5–9]. On the contrary, there is full agreement on the marked improvement achievable by functional chrome plating in polymer moulding dies and other tools subjected to wear and corrosive environments [10–12]. When damage appears during the life of the tool, processes based on fusion welding are frequently adopted to rebuild worn or cracked surfaces. According to industrial practice, repairing of the tool is done by first removing the damaged parts by milling or grinding and by rebuilding the missing volume by welding with a suitable filler metal [13]. Pre-heating and postweld heat treating are generally carried out to avoid solidification cracking and excessive residual stress induced by welding
M. Vedani et al. / Surface & Coatings Technology 201 (2007) 4518–4525 Table 1 Chemical composition (mass %) of the 1.2738 tool steel and of the filler metals C
Si
Mn
Cr
Mo
Ni
Ti
S
P
1.2738 steel 0.48 0.25 1.67 2.20 0.23 1.26 – 0.0026 0.013 Filler metal wire 0.35 0.3 1.2 7.0 2.0 – 0.3 – – type GS2 Filler metal wire 0.10 0.6 1.2 – 0.5 – – – – type GS20
[14]. The need to modify the shape of existing tools after prototype evaluation or due to product requirement is a further reason for tool refurbishing by welding [15]. Repair welding of tool steels represents a technical practice with considerable metallurgical concerns due to low weldability of the hardened, often highly alloyed tool steels. Gas Tungsten Arc Welding (GTAW) is the most common process currently in use for repair welding due to great process control suited for small areas, thin sections and sharp edges [16]. For distortion sensitive tools, high-density heat input processes such as plasma arc and laser beam welding are preferred [17]. The lower net heat input required to produce the weld reduces tool distortions and allows thermally sensitive steel grades to be repaired without additional pre- and post-weld thermal treatments. When the tools to be repaired are surface treated, additional difficulties in welding arise. Nitrogen release and gas pores formation occur in nitrided tools whereas Cr overalloying and cracking of the weld metal are often found in chrome plated tools [18]. A research work is described in this paper on the development of defects in laser repair-welding of a surface treated tool steel. A procedure for combined laser pre-treating and welding of the tool is also presented. 2. Experimental procedures 2.1. Materials and surface treatments The investigation was carried out on 25 mm thick samples of a quenched and tempered type 1.2738 tool steel having the chemical composition given in Table 1. Repair welding was carried out by a pulsed Nd-YAG laser with commercial type 0.5 mm diameter filler wires, whose nominal chemical compositions are also reported in Table 1.
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Before the welding tests, some of the samples were surface treated in industrial plants. A first set of the plates was plasma nitrided at 510 °C for 12 h in a 80N2/20H2 atmosphere at 350 Pa, whereas a second set of samples was electrolytically plated in a hexavalent chromic acid solution bath to produce a functional coating with a nominal thickness of 50 μm. 2.2. Laser welding parameters The weld beads were initially deposited on the plates using several sets of process conditions. A few reference conditions were then selected on the basis of resulting weld bead size (height and depth) and absence of defects. Table 2 gives the main process parameters adopted. In the table, due to the pulsed nature of the laser beam, the heat input rate was calculated as: HI ¼ ðPdsdf =DdmÞ ½J=mm2 where HI is the heat input rate, P the pulse power, τ the pulse duration, f the pulse frequency, D the spot diameter and ν the beam feed rate. Repair welding tests were carried out by producing multipass deposits obtained by parallel beads with a moderate degree of lateral overlapping. As a rule, the deposits were produced using the GS20-type filler wires (see Table 1). However, for the chrome-plated samples and for specific trials on the asmachined samples, a GS2-type filler wire was also tested in the attempt to achieve higher hardness properties of the as welded deposits. To improve the weldability of the plasma nitrided steel, surface remelting laser passes were carried out on a set of nitrided samples before laser deposition. The surface remelting passes were obtained by using the same laser source but without the use of the filler metal. Table 2 shows that surface remelting was done with two different heat input rates, obtained by changing the spot size. As a result, the height-to-depth ratio and the volume of the molten pool were modified. The following deposition phase, performed with the filler metal, was kept at constant welding conditions, corresponding to set C parameters. 2.3. Experimental analyses Analyses were carried out on the described samples to characterize microstructural properties of the weld deposits and
Table 2 Process conditions adopted for the production of laser weld beads and laser remelting passes
Type of process Pulse duration (ms) Pulse power (kW) Pulse frequency (Hz) Spot diameter (mm) Travel speed (mm/min) Heat input rate (J/mm2) Shielding gas Filler wire
Set A (HP)
Set B (LP)
Set C
Set D (T1)
Welding 7.2 0.88 9 0.55 0.65 9570 Ar GS2/GS20
Welding 7.2 0.52 9 0.55 0.65 5655 Ar GS2/GS20
Welding 8.0 0.76 7.5 0.55 0.65 7653 Ar GS20
Treatment 8.0 0.76 7.5 0.55 0.65 7653 Ar –
Set E (T2) Welding 8.0 0.76 7.5 0.55 0.65 7653 Ar GS20
Treatment 8.0 0.76 7.5 1.10 0.65 3827 Ar –
Welding 8.0 0.76 7.5 0.55 0.65 7653 Ar GS20
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to define chemical composition profiles across the surface treated samples and in the weld deposits. Microhardness profiles were measured along the vertical axis of the deposits, starting from the upper surface, by selecting a load of 1.96 N (HV 200) on the Vickers indenter and performing at least four indentations for each depth level. For the evaluation of the case depth in the nitrided samples and at chrome layer-substrate interface, additional measurements were carried out with a load of 0.098 N (HV 10), moving from external surface toward the centre of the samples. 3. Results 3.1. Microstructure of the surface-treated plates Representative optical micrographs of the heat treated 1.2738 tool steel plate in the as-machined condition and after the two surface treatments here investigated are shown in Fig. 1. The bulk structure of the steel was formed by tempered martensite with a prior austenite grain size of about of 250 μm (Fig. 1a). Inspection of the nitrided samples (Fig. 1b) allowed to observe the presence of a thin compound layer and of a diffusion layer, extending for about 50 μm from the surface. Fig. 2a shows the microhardness profile measured on a cross-
Fig. 1. Representative optical micrographs of the 1.2738 steel. (A) As machined; (B) plasma nitrided; (C) chrome plated.
to highlight possible improvements achievable in repair welding of surface-treated tools. Specimens for microstructural examinations and microhardness profile determinations were cut using a precision diamond saw and polished by conventional metallographic techniques. When required, the microstructure was revealed using the Nital 2% or the Vilella reagents. Optical and scanning electron microscope (SEM) observations were carried out on the prepared specimens. An EDS microprobe fitted to the SEM was also used
Fig. 2. Microhardness profiles. (A) Plasma nitrided, (B) chrome-plated samples.
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the welds was extremely narrow, about 50 μm, when evaluated on the basis of microstructural alteration visible in optical micrographs. Scanning electron microscope (SEM) analyses revealed that the weld metal structure was made up of tiny cellular dendrites with a spacing ranging from 2 to 4 μm (Fig. 4b). 3.3. Microstructure of welded chrome-plated samples The weld deposits produced on chrome-plated samples revealed to be particularly critical since the weld metal zone featured a large number of cracks, as depicted in Figs. 5a and 6a. In addition, the Cr layer located in the HAZ of the welded samples always showed transversal cracks as a result of tensile stresses promoted by weld metal solidification. Microprobe analyses carried out in the weld metal zone showed that the chemical composition was reasonably homogeneous and that a significant enrichment in Cr (up to 11.8% in single pass deposits) was achieved. Cr is believed to be mainly supplied by the dilution effect of the molten Cr layer. Elemental line profiles shown in Fig. 6b also suggested that the chrome-plated layer features a higher oxygen content that can be released in the weld metal zone during welding.
Fig. 3. Weld beads produced by Nd-YAG laser deposition on the as machined 1.2738 steel by type C conditions. (A) Macro views of the top surface; (B) section of a multipass deposit obtained under type B conditions.
section of a nitrided sample. The hardness close to the surface was about 750 HV. A case depth of about 400 μm was evaluated as the distance from the surface where the microhardness exceeds by 10% the core hardness value. The chrome plating layer featured a thickness of about 50 μm, as depicted in Fig. 1c. Microhardness profiles measured across the deposited chrome layer showed a hardness ranging from 800 to 1000 HV in the chrome layer and a sharp deviation to the unaffected tool steel hardness beneath it (Fig. 2b). 3.2. Microstructure of the welded untreated samples The average size of the weld bead samples here investigated was about 10 mm in length and 0.6 mm in width for the single pass deposits, as depicted in Fig. 3a. Multipass weld beads featured a deliberate transversal overlapping of about 40% in width (Fig. 3b) to increase weld bead homogeneity. Inspection of the weld bead microstructure showed evidence of a complex convective flow of the liquid metal within the weld metal zone. Solidification microcracks (Fig. 4a) were often observed when the filler metal rich in C and Cr (GS2 type) was used. As expected, the heat affected zone (HAZ) of
Fig. 4. Representative optical micrographs of weld beads produced under type B conditions with GS2 filler metal. (A) Double pass deposit; (B) weld metal microstructure showing intergranular hot cracks.
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tiveness was qualitatively assessed by the amount of gas pores exposed on the upper surface of the treated samples observed by SEM (compare Fig. 9a and b). The production of a weld deposit using a filler metal on such remelted steel regions, generally allowed the amount of gas pores in the final weld bead to be reduced, even though residual voids remained in the remelted zones that were not subjected to the successive deposition by the filler metal. The microstructure of the remelted and welded samples also featured a relatively wide extension of the HAZ, as confirmed by microhardness measurements. Fig. 10a depicts a representative microhardness profile recorder along the thickness of a sectioned surface treated and welded nitrided sample (set D conditions). The soft
Fig. 5. Cross-sectional optical micrographs of the weld deposits. (A) Chromeplated sample under type A conditions with GS20 filler metal; (B) nitrided sample under type C conditions.
3.4. Microstructure of the welded nitrided samples Welding of the nitrided samples led to the release of a large amount of nitrogen within the molten pool and resulted in a deposit structure with a significant number of gas pores, as depicted in Figs. 5b and 7a. Also in this case, small hot cracks were detected in the weld deposit structure. These defects often took their origin from gas pores. SEM images of the upper surface of the weld beads (Fig. 7b) suggested that the pores intersecting the surface are preferentially located at the edges of each pass. 3.5. Effects of surface-remelting and weld deposition on nitrided samples For the deposits performed on the nitrided steels, the formation of gas pores was considered to be the most limiting factor, strongly impairing the reliability of weld-repaired moulds. For this reason, a new approach was investigated [19], consisting of an initial surface remelting pass, carried out using the Nd-YAG laser source without the filler metal, and of a second pass, aimed at producing a new weld deposit on the previously remelted and degassed metal portion. Fig. 8 shows a collection of cross sectional views of multipass samples obtained according to the above mentioned approach, with the two different parameter settings defined in Table 2. The corresponding upper surface of the samples are reported in Fig. 9. The two parameter settings here selected for the surface remelting pass differed in their focus distance and therefore in the laser beam spot size (0.55 mm diameter for type D and 1.10 mm for type E). The shape of the remelted zone of the samples varied accordingly, giving a shallow but more intensive degassing effect for the type E condition. The degassing effec-
Fig. 6. Deposit obtained on chrome-plated samples under type A welding condition with GS20 filler metal. (A) Optical micrograph; (B) elemental line profile of oxygen, iron and chromium across the unmelted region of the chrome layer (along the arrow shown in the micrograph.
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From industrial experience it is also well known that repair welding of surface treated tools is more critical [13]. In the present investigation, for the chrome-plated samples a Cr-rich filler metal was initially selected in order to better match the surface composition and preserve the high hardness properties of the chrome-plated tools. Large defects formed due to hot cracking and the weld metal revealed to be excessively enriched in chromium and oxygen due to elemental pickup from the chrome-plated layer. It is supposed that improved weldability
Fig. 7. Multipass weld deposit produced on a nitrided sample under type C condition. (A) Enlarged view of a porous region of the weld bead; (B) macro view of the multi-pass weld bead.
weld deposit obtained by the use of a low-alloyed filler is recognized within the first 300 μm from surface. Beneath this region, a high-hardness HAZ can be recognized (corresponding to the previously remelted region of the 1.2738 steel), and only after about 800–1000 μm in depth, the unaffected parent metal can be found. Cracks mainly formed within the HAZ and at gas pores, (Fig. 10b) due to the high hardness and possible brittleness of this steel region combined with the tensile stress induced by the following deposition pass. 4. Discussion Pulsed laser welding is receiving great attention as a welding process for tool refurbishing operations requiring great precision and flexibility [17,20–23]. Previous investigations on repair welding of tool steels [24] already showed that satisfactory results could be obtained using a laser source on several tool steel grades. From present results it was confirmed that welding a 1.2738 steel generates a limited extension of the HAZ and produces a refined weld metal structure, thus potentially reducing the cracking tendency and increasing hardness.
Fig. 8. Cross-sectional micrographs of the nitrided samples. (A) Surface treatment under set D conditions; (B) surface treatment under set E conditions; (C) weld bead on surface treated sample under set D conditions; (D) weld bead on surface treated sample under set E conditions.
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Fig. 9. Upper view of the laser-processed nitrided samples. (A) Surface treatment under set D conditions; (B) surface treatment under set E conditions; (C) weld bead on surface treated sample under set D conditions; (D) weld bead on surface treated sample under set E conditions.
could have been achieved by the use of low-alloyed filler metal wires. This produces a dilution effect of the harmful elements but could also generate softer deposits with inappropriate properties. A second limiting aspect consisted in the systematic cracking of the unmelted Cr layer in the HAZ close to the weld deposit due to welding stresses. Although the above harmful effects could be reasonably reduced with a further selection of materials and welding conditions, it is believed that no reliable repair welding can be performed in chrome-plated samples without a preliminary removal of the surface layer by mechanical methods. Repair welding of the nitrided samples led to copious development of gas porosity due to nitrogen release from the molten weld metal, thus impairing the structural integrity of repair welds. A laser remelting treatment before the weld deposition was therefore investigated in order to promote gas release within the molten nitrided layer. The potential improvement of the subsequent welding operation within this partially degassed region was then experimentally evaluated. It is worth noting that since this treatment is aimed at reducing the amount of nitrogen from surfacial layers, it also fully destroys the nitrided layer of the tool. However, the preservation of the nitrided layer is of
secondary importance when compared to the need of restoring the geometrical surface of the tool (rebuilding of cracked and worn surfaces). In addition, the rapid solidification conditions experienced by the weld metal can potentially contribute to increase the surface hardness of the tool and partially counterbalance the lack of the nitriding treatment in the repaired regions. The production of a weld deposit using a filler metal on such remelted steel regions, generally allowed the amount of gas pores in the final weld bead to be significantly reduced, even though residual voids remained in the remelted zones that were not subjected to the successive deposition by the filler metal. Microstructural observations also confirmed that the relatively wide high-hardness HAZ formed by remelting a larger volume of steel generates a larger volume of brittle material that is prone to cracking on further welding. The results suggest that a reduction of the gas pores can be obtained in the nitrided and welded samples. Technological parameters have to be carefully adjusted since an optimally balanced volume of remelted zone as a function of the desired size and shape of the weld deposit is required. Experimental investigations are currently in progress to find laser treatment
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metal solidification. Small cracks starting from these voids were also detected. ○ A procedure for laser pre-treating was thus developed, acting as a degassing surface treatment for the nitrided samples. Two different parameter settings were investigated that produced a different extension of the degassed volume. ○ The combination of the above laser remelting and the subsequent welding using a filler wire revealed that a significant reduction of defects could be achieved in the nitrided samples. A further improvement in deposit quality can be achieved by a greater control of technological parameters to produce an optimal volume and shape of the remelted region and by a more precise location of the weld deposit within this volume. Acknowledgements The authors would like to express their acknowledgement to the skillful experimental work carried out by Mr P. Pellin, Dr. A. Ciolfi, Dr. D. Colombo and Mr A. Masi. References
Fig. 10. Deposit performed under set D conditions on a surface treated nitrided sample. (A) Microhardness profile taken along the vertical axis (B) microstructure of a weld deposit.
conditions that produce a deep but more uniform volume of the degassed steel. Aesthetic passes will also be evaluated in order to attain a smooth and tempered surface of the degassed volume. 5. Conclusions The investigation on the development of defects in repairwelding tests by pulsed Nd-YAG laser deposition on surface treated 1.2738 steel samples allowed the following conclusions to be drawn. ○ Extensive hot cracking in the fine cellular-dendritic structure of the weld metal was detected in the chrome-plated samples. These welding defects were accounted for by chrome and oxygen overalloying of the weld metal due to chrome-plating layer dissolution. Cracks of the surface chrome deposit in the HAZ due to welding stress were also detected. These unavoidable effects suggested that repair welding in chromeplated samples of the 1.2738 steel grade is hardly feasible. ○ Welding of the nitrided samples resulted in the copious formation of gas pores due to nitrogen release during weld
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