- Email: [email protected]
Repair welding of polymer injection molds manufactured in AISI P20 and VP50IM steels
Journal of Materials Processing Technology 179 (2006) 244–250
Repair welding of polymer injection molds manufactured in AISI P20 and VP50IM steels Wilson Tafur Preciado a,∗ , Carlos Enrique Ni˜no Bohorquez b a
Mechanical Engineering Department, Federal University of Santa Catarina - UFSC, Campus Universit´ario, Cx. Postal 476, CEP 88049-900, Florian´opolis, SC, Brazil b Mechanical Engineering Department, Federal University of Santa Catarina - UFSC, Brazil
Abstract The objective of this work was to determine the best practices to repair by welding AISI P20 and VP50IM steels during the manufacturing of polymer injection molds, in such a way to obtain similar characteristics in the weld and base metal, to uniformly respond to the processes of polishing and texturing. The welds by the wire feed TIG process were done using a similar filler metal for P20 steel, AWS A5.28-96 ER 80S-B2, and two filler metals for VP50IM steel, similar and dissimilar AWS A5.28-96 ER 80S-B6. The welds were deposited with different heat inputs (3.6–9.8 kJ/cm), using constant amplitude DC, and they were evaluated in relation to the quality obtained in the mirrored and textured surfaces. Those results were correlated with the chemical compositions, microstructures and hardness. © 2006 Elsevier B.V. All rights reserved. Keywords: Repair welding; Injection molds; TIG; Polishing; Texturing
1. Introduction Polymer injection molds frequently require some form of repair by welding: (a) during the manufacture, by errors in machining or by some change in the design of the part to be injected and (b) during the service, by the incidence of failures in the mold. For those repairs, TIG process is used frequently, because it allows the deposition of small amounts of material, without spatter, in such a way to obtain the complex geometries of the mold. Special processes and welding procedures are used due to the tight tolerances required and the high cost of the mold. The required quality of the weld is not only related to the suitable mechanical properties, but also to the weld and the heat affected zone (HAZ) behavior when finishing processes are applied to give a uniform and adequate texture to the mold cavity. Nowadays, the welding procedures applied for the repair of molds are elaborated having as a basis the recommendations given by the manufacturers of steel, but it remains not clear what are the involved metalurgical phenomena, whose knowledge is necessary to define rational procedures, suitable
for different materials, sizes and geometries of the molds to repair. The vast majority of the information available in the literature about the repair of these steels focuses the aspects of weld techniques, without considering metallurgical aspects [1,2]. The efforts of mold steels manufacturing companies has led to the development of base materials and their filler metals as a way of increasing the weldability and dimensional stability of the mold after heat treatments. This is the case of VP50IM steel studied in this work. The P20 steel, also focused in this study, has a long successful history in the industry of polymer injection molds, although with severe problems when recovered by welding. 2. Objectives The primary objective of this work is to establish optimal conditions repair by welding P20 and VP50IM using the wire feed TIG process, in such a way to obtain that the weld metal (WM) and heat affected zone have behaviour similar to that of the base metal (BM) when the surface finishing processes are applied. 3. Theoretical revision
∗
Corresponding author. E-mail address: [email protected] (W.T. Preciado).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.03.101
After mechanizing the mold to obtain the geometry that makes it functional, its cavity is subjected to finishing processes
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
of finished so the texture characteristics obtained are the same desired part that will be injected in the mold. The common finishing processes are three: polishing, mirroring and texturing. 3.1. Polishing and mirroring of the mold Some characteristics of the cavity of the mold are particularly important: the surface must have a correct geometry, without ondulations; the mirrored surface must be free of pores, inclusions and other defects. The finished surface generally is evaluated by visual exam (to the naked eye). Nevertheless, as that involves certain difficulties, in more demanding cases the finished surface is evaluated by means of techniques like optical interferometry [3]. According to [3], for an injection mold that must be polished after welding, it is essential that the weld metal does not have significant differences in the composition and hardness with respect to the base metal. Otherwise, in the contour of the weld could remain, after polishing, marks that would be reproduced in the injected part. 3.2. Texturing of a welded surface The texturing treatment can be applied to ferrous and nonferrous metals. It consists of a selective chemical attack, obtained frequently in the following form: the surface is covered with a photosensitive resin; over that it is placed a sheet with a drawing of the texture; the piece is exposed to an intense light, that fixes the unprotected parts of the surface. With a solvent the resin nonexposed to the light (that is to say, that part of it not fixed) is removed and then the surface is subjected to attack by acid [1,4]. According to [5], a weld in a surface to be texturized must be done using a filler metal of similar composition, with a carbon content slightly smaller than the one of the BM, and the weld hardness must be similar to that of the BM. The preheating around the area helps to diminish the differences of hardness between the base and weld metals. According to [6], filler metals are specifically recommended by the base metal manufacturers in order to hace a satisfactory surface, and must be only used with suitable welding procedures for each steel in particular, with the objective to reduce the variations in the chemical composition and/or hardness, that could impair the texturing process significantly. Some materials are more resistant to acid attack than others, due to the differences in the chemical composition, so it is better to choose a filler metal of chemical composition similar to that of the base metal. Unfortunately, the chemical composition is not the only factor that determines the depth of dissolution by the acid. The microstructure also has an influence. As it becomes harder, greater will be the resistance to the action of acid [1]. 4. Methodology In order to obtain the proposed objectives, welds were made in two base materials, followed by polishing and texturing treatments. The quality of the obtained surfaces was correlated with
245
the chemical composition, microstructures and hardness of the different regions of the weld. The base materials used were the P20 and VP50IM steels, that are Cr–Ni–Mo steels refined by vacuum degassing. The AISI P20 is a hardenable steel used frequently in the manufacturing of polymer injection molds. It is provided in the quenched and tempered state, with hardness between 30 and 34 HRC. The VP50IM is a steel hardened by precipitation, developed by the Villares Metals [7]. It is furnished in the solution treated state, with hardness between 30 and 35 HRC. In order to increase its resistance it must be aged by heat treatment at 500 ◦ C. For the P20 steel weld was used a dissimilar filler metal, AWS A5.28-96 ER 80S-B2, that is a Cr–Mo steel used in the welding of boilers, pressure vessels and pipelines that work at high temperatures. For the VP50IM steel two filler metals were used • A similar one, developed by the same manufacturer of that base metal. • A dissimilar one, AWS A5.28-96 ER 80S-B6, that is a 5Cr–0,5Mo steel used for the welding of components that works at high temperatures and in atmospheres containing hydrogen. The chemical compositions of the base and filler materials are shown in Table 1. At first, welds in SAE 1020 steel plates were done in order to determine the appropriate conditions to deposit sound welds by wire feed TIG. After that, in plates of 150 mm × 150 mm × 12 mm of P20 and VP50IM steels there were deposited six beads with three levels of average current (88, 106 and 124 A) and two speed levels (10 and 14 cm/min). That resulted in six heat input values, ranging from 3.6 to 9.8 kJ/cm, as shown in Table 2. The following factors were maintained constant in the tests: • • • • • • • •
Preheating temperature: 225 ± 25 ◦ C. Shielding gas: Argonium, 12 l/min. Electrode: W + 2% ThO2 , diameter 2.4 mm, 60◦ tip angle. Arc length: 4 mm. Wire diameter: 1.2 mm. Nozzle diameter: 10 mm. Wire feeding angle: 25◦ in relation to the plate surface. Torch incidence angle: 15◦ in relation to the surface normal.
From the six beads deposited with each filler metal, two samples were removed, as shown in Fig. 1. A specimen was kept to apply on it the grinding and polishing processes, and parts of the polished surface were subjected to texturing and mirroring. The other specimen was for microstructural analysis and hardness measurement of the weld cross-section. Based on the uniformity of textured and polished surfaces, it was selected only one specimen for each filler metal to make the mirroring, because it is a time-consuming process. The specimens for metalography and hardness were polished with 13 m sandpaper and 3 m diamond paste. Later they were
246
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
Table 1 Chemical compositions of the base and filler metals, and their respective calculated CEiiw Element
P20 BM
Carbon Silicon Manganese Chromium Nickel Molybdenum Phosphor Sulphur Copper Aluminum Niobium Cobalt Titanium Vanadium Tungsten CEiiwc a b c
VP50IM P20a
Dissim FM AWS A5.28 E
0.37 0.39 1.4 1.89 0.77 0.18 0.03 0.0055 0.12 0.002 0.005 0.025 0.002 0.012 0.011 1.08
80S-B2b
0.09 0.58 0.54 1.33 0.04 0.51 0.01 0.006 0.03 – – – – – – 0.55
BM VP50IMa
Similar FMa
Dissimilar FM-AWS A5.28 E 80S-B6b
0.17 0.21 1.4 0.27 2.95 0.28 0.03 0.09 0.9 0.85 0.0048 0.05 0.0046 0.09 0.01 0.79
0.14 0.10 1 0.24 2.92 0.27 <0 0.13 1 0.35 0.0039 0.024 0.0034 0.064 0.036 0.68
0.08 0.39 0.53 5.9 0.06 0.54 0.004 0.013 0.07 – – – – – – 1.46
Compositions measured by opthical spectrometry. Compositions furnished by the manufacturer in quality certificates of the product. IIW carbon equivalent, CEiiw = %C + %Mn/6 + %(Cr + Mo + V)/5+%(Ni + Cu)/15.
Table 2 Welding conditions for wire feed TIG Run
Currenta (A)
Voltage (V)
Welding speed (cm/min)
Heat input (kJ/cm)
Arc power (W)
Wire feed (m/min)
E1 E2 E3 E4 E5 E6
88 106 124 106 124 106
10 10 10 10 10 10
14 14 14 10 10 7
3.78 4.5 5.25 6.4 7.54 9.08
880 1060 1240 1060 1240 1060
0.4 0.4 0.5 0.4 0.4 0.4
a
All the welds were done with constant value DC current.
attacked with Nital 2% solution. There were measured Vickers hardness profiles, with 1 kg load and 0.2 mm interval between indentations, in a transverse direction to the weld, including the WM, HAZ and BM. In order to evaluate the polished and textured surfaces in a more objective form that can be done through visual exam, there were made measurements in an optical rugosimeter.
The mirroring was done on VP50IM steel welded specimens and the hardness profile was measured on the mirrored surfaces, in a transverse direction to the weld. An analysis was made of the mirrored surfaces by visual examination and optical microscopy. These results were correlated with the microstructure and chemical composition of the three regions (WM, HAZ and BM). The conditions used in the grinding,
Fig. 1. Specimen removal and tests done.
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250 Table 3 Process conditions for surface finishing Processo Grinding Grinding wheel Workpiece speed Grinding wheel speed Depth
Condic¸o˜ es Norton 3SGK46 KVSP (ABNT NB33) 10 m/min 2500 rpm 0.25 mm/pass
Polishing Rough finish grinding stone Smooth finish sandpaper (VP50IM) Smooth finish stone (P20)
#150 #320, 400, 600 #320, 400, 600
Texturizing Time of attack Method Type of bath
7 min Strip Jet
Mirroring Polishing time Smooth finish sandpapers Machine remotion Manual remotion with cotton
1 h on 8 cm2 #600, 800, 1000, 1200, 2000 Diamond paste 2–4 m, 7000 rpm Diamond paste 1/4 m
polishing, texturing and mirroring processes are described in Table 3. 5. Results and analysis 5.1. Evaluation of response to polishing and mirroring After making the grinding of the surface (to remove the reinforcement of the weld and to reduce the roughnesss), the polishing had to be done in a different way in both of the materials: whereas in steel VP50IM was possible to use sandpaper, in the P20 steel the use of it produced a not uniform removal in the three regions of the weld, due to the higher hardness of the HAZ (650 HV) compared with that of the BM (300 HV). Therefore, it was necessary to make the removal with abrasive stones that allowed to obtain the same finishing quality produced by the sandpapers. In order to evaluate the polished surfaces there were made scans parallel to the longitudinal direction of the weld, through
247
a 5.6 mm length, going from the centerline of the bead to the base metal. In all the specimens, the peak to valley distances were similar in the evaluated zones (WM, HAZ and BM), as it is illustrated in Fig. 2. Based on that, the Ra roughness parameter was calculated in all the measurement length, and that value was related to the base materials and level of heat input, as it is shown in Fig. 2b. It is observed that the Ra value did not vary significantly with heat input, being in the interval between 0.45 and 0.80 m, satisfactory for a polishing made as a preparation for texturing or mirroring. At the left side of Fig. 3 it can be observed that in the aswelded condition, the WM and HAZ of the VP50IM steel welds using both types of filler metals (similar and dissimilar) had similar hardness (430 HV), lower than those of WM and HAZ of welds in P20 steel (450 and 650 HV), and slightly greater than the one of VP50IM and P20 base metals (305 HV). In the case of the P20 steel, the high differences of hardness between the WM, HAZ and BM turn difficult the polishing treatment. Taking into account that, it was not made mirroring (that is a lengthy process). The great differences of hardness turned necessary to use more rigid cloths, in order to maintain its form and allow a uniform removal in the regions of the weld and BM. This was a decisive factor to not apply the mirroring on P20 steel, because there would form reliefs in the various regions of the weld. Therefore, it is important to reduce the hardness variations on the weld of a steel of high hardenability as P20, through a postweld heat treatment or through controlled multipass welding techniques as double layer and temper bead, in which it is taken advantage of welding thermal cycles to heat treat the previously hardened zones [7]. Welds made on VP50IM steel were subjected to an aging treatment before mirroring. Two specimens welded with similar and dissimilar filler metals were selected for this purpose. Those specimen were welded with the same heat input of weld (E5 = 7.54 kJ/cm of the Table 2) chosen to give a greater dilution (as will be shown in Fig. 7b). In Fig. 4 are shown the hardness profiles measured in the mirrored surfaces. By comparing the results shown in Figs. 3 and 4, it can be verified that the differences of hardness between the
Fig. 2. (a) Typical roughness profile obtained after polishing and (b) Ra roughness values measured in a transverse direction to the welds, for different heat inputs.
248
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
Fig. 3. Hardness profiles, macrographies, textured surfaces and micrographies (BM, WM and CG-HAZ) of specimens welded with 7.54 kJ/cm heat input.
WM and BM in mirrored specimens were smaller (55 and 40 HV) that the observed ones before the aging treatment (130 HV). This explains the recommendation given by the manufacturer of VP50IM steel of doing the mirroring after the aging treatment of the weld. The visual evaluation of the surface by means of light reflectance allowed to verify the excellent behavior of steel VP50IM welded with a similar filler metal, not being so good equal in the case of dissimilar filler metal weld. Although the differences of hardness between the weld and the BM were nearly equal (55 and 40 HV for the similar and dissimilar filler metals, respectively), in the weld with dissimilar filler metal was formed a step between the WM and HAZ that is shown in Fig. 5. That can be due to the lower sulfur content of the dissimilar filler metal and, therefore, of the WM obtained with it, that causes a reduction of the remotion rate.
5.2. Evaluation of the texturized surface After the polishing, a part of the surface was subjected to texturing. As reported in the previous item, in all the specimens the polished surface showed a uniform roughness in the three weld zones (WM, HAZ and BM), which means that there was no influence of the polishing on the results of the texturing. Fig. 6 shows a typical roughness profile of a textured surface, measured throughout a length of 12.5 mm, including the WM, HAZ and BM. Through the visual exam it is easy to verify that in P20 welds the BM was more intensively attacked than WM and HAZ (the valleys correspond to the zones dissolved by acid). When attempting to characterize that in a more objective form by means of roughness measurements, it was verified that Ra value is not a good indicator of the textured surface quality,
Fig. 4. Hardness profiles in mirrored surfaces after welding and aging treatment (Hi = 7.54 kJ/cm, Im = 124 A, Um = 10 V, WS = 10 cm/min, WF = 0.4 m/min).
Fig. 5. Relief in the region of the WM in the VP50IM steel weld, observed by means of light reflectance: (a) weld with dissimilar FM and (b) weld with similar FM.
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
249
Fig. 6. Roughness profiles of textured specimens in a transverse direction to weld bead, in textured specimens: (a) uniform acid attack in VP50IM and (b) not uniform acid attack in P20.
because it depends to a large extent on the texture pattern that was used (Ra is an average of the squares of the deviations from a reference line). A better indicator could be the Rz value, that here was obtained as the average of five peak to valley distances). Then, to characterize the textured surface there were measured roughness profiles of WM, HAZ and BM separately, along lines parallels to the weld bead, through a measurement lenght of 5.6 mm. In Fig. 7 are shown the Rz roughness and hardness values measured in the WM, HAZ and BM. It is possible to observe that in steel VP50IM the Rz roughness values of WM, HAZ and BM were always similar, although WM and HAZ had
hardness greater than the one of BM (around 400 HV and 300 HV, respectively). From those results, obtained with both of the base metals, it is possible to conclude that the texture depth does not have a direct relation with the hardness of the material. For example, in the case of VP50IM steel the obtained textured surface was uniform, despite the significant hardness differences between HAZ and BM. In the case of the P20 steel, altough the HAZ was harder than the WM, the texture was uniform. In order to explain that, it is necessary to remember that P20 BM is furnished in the quenched and tempered condition. When subjected to a weld thermal cycle, the material is quenched again
Fig. 7. (a) (Rz) roughness and Vickers hardness in HAZ, WM and BM for each heat input of weld and (b) values of dilution for each material and heat input.
250
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
(the hardness of 600 HV corresponds to that of martensite with 0.37 %C) and becomes more resistant to the acid attack [8]. In order to evaluate the reasons for the behavior of the WM, the dilution was determined by means of added and fused areas measured in the weld cross section. The dilution was always high (between 50 and 70 %), which was not expected when applying TIG process (a process of low thermal efficiency) with relatively low currents. Because of that, a considerable portion of the WM is formed by fused BM. In Table 1 it is possible to observe that the used filler metal to weld the P20 steel has lower C and CEiiw than that of the BM. That reduces the hardenability (but not enough to avoid the formation of 100% martensite with the welding conditions applied) and reduces the hardness of martensite. This effect can be verified by the higher hardness obtained with higher dilutions (530 HV for 71.5% dilution, 400 HV for 62% dilution). On the other hand, the hardness differences did not have effect on the Rz roughness of the texturized surfaces. That suggests that the answer to the chemical attack depends on the form in which the alloy elements are present (in solid solution or as part of precipitates). In WM and HAZ the material in the tempered state is hard, with the martensite supersaturated in C and other alloy elements. In the BM, due to the tempering treatment, it occurs the formation of carbides. Then, to obtain uniformity in the texturized surface it would be necessary to promote the tempering of WM and HAZ, which could be done by means of a conventional heat treatment (with deleterious effects on surface finish and dimensional stability) or by means of double layer and temper bead welding techniques [8]. 5.3. Macrostructure, hardness distribution and microstructure In order to complete the analysis, in the right side of Fig. 3 macrographies, textured surfaces and micrographies (including the WM, HAZ and BM) are shown. The following comments can be done: (a) In relation to the P20 steel: - The microstructure of the BM, WM and HAZ is martensitic. At BM the martensite is tempered. At HAZ it has a high hardness (650 HV), with diferent prior austenite sizes, coarser at the region next to the fusion line (coarse grain HAZ). At WM, besides martensite, it is possible to observe some retained austenite. - The BM shows a deeper texture relief than that of the WM and HAZ. (b) In relation to the VP50IM steel: - The BM had a martensitic microstructure, with the alloy elements in solid solution. - The microstructure of the HAZ and WM is martensitic, but their hardness is lower than the obtained in the P20 steel, due to the lower carbon content. - The hardness of WM and HAZ are slightly higher than that of the BM.
6. Conclusions From the tests and analyses the following comments can be done: (a) In relation to the P20 steel: - A satisfactory polished surface can only be obtained by using a polishing stone instead of sandpaper, to guarantee the flatness. - The application of mirroring is not advisable in the aswelded state, unless the tempering of the HAZ and WM can be promoted through double layer and temper bead welding techniques. - The textured surface was not uniform, being deeper the chemical attack in the BM, because it is on the quenched and tempered condition. (b) In relation to the VP50IM steel: - It is possible to obtain a uniform polished surface, even with the use of sandpaper. - In the mirrored surface of the weld made with the similar filler metal a totally flat surface was obtained. However, in the weld done with dissimilar metal was possible to perceive relief differences. - The textured surfaces were uniform for all the welding conditions. (c) In general, for both types of base materials: - The uniformity obtained in the polished and mirrored surfaces depends mainly on the uniformity of the hardness throughout the weld. - The uniformity in the texture of surfaces does not depend on the hardness, but on the local chemical composition and the condition of the alloy elements (in solid solution or as part of precipitates). References [1] S. Thompson, Handbook of mould, tool and die repair welding, first ed., Abington Publishing Limited, London, 1999, p. 224. [2] M. Vedani, Microstructural evolution of tool steels after Nd-YAG laser repair welding, J. Mater. Sci., Milan 39 (2004) 241–249. [3] Uddeholm, Polishing Mold Steel. Treatment of Tool Steel. Available in:. Accessed at September 21st, 2004. [4] D. Schauf, Reproducing textures from the cavity surface to the surface of the thermoplastic moulding. Application technology information. Available in: . Accessed at September 21, 2004. [5] The akron metal etching. Mold Texturing. Available in: . Accessed at September 21, 2004. [6] Mold-tech. Pre-Texturing Mold Finishes Required. About texturizing. Available in: . Accessed at september 21, 2004. [7] R.A. Mesquita, C.A. Barbosa, C. A. Ac¸os para moldes de pl´astico com melhores propriedades de manufatura., in: Proceedings of “Usinagem 2004”, October 27–29, S˜ao Paulo- SP, Brazil, 2004 (in CD ROM). [8] C. E. Ni˜no, Especificac¸a˜ o de procedimentos de reparo por soldagem sem tratamento t´ermico posterior: efeito de revenimento produzido pelos ciclos t´ermicos. PhD Thesis- Postgraduation Course on Mechanical Engineering, Federal University of Santa Catarina, Florian´opolis-SC, Brazil, 2001.
Repair welding of polymer injection molds manufactured in AISI P20 and VP50IM steels Wilson Tafur Preciado a,∗ , Carlos Enrique Ni˜no Bohorquez b a
Mechanical Engineering Department, Federal University of Santa Catarina - UFSC, Campus Universit´ario, Cx. Postal 476, CEP 88049-900, Florian´opolis, SC, Brazil b Mechanical Engineering Department, Federal University of Santa Catarina - UFSC, Brazil
Abstract The objective of this work was to determine the best practices to repair by welding AISI P20 and VP50IM steels during the manufacturing of polymer injection molds, in such a way to obtain similar characteristics in the weld and base metal, to uniformly respond to the processes of polishing and texturing. The welds by the wire feed TIG process were done using a similar filler metal for P20 steel, AWS A5.28-96 ER 80S-B2, and two filler metals for VP50IM steel, similar and dissimilar AWS A5.28-96 ER 80S-B6. The welds were deposited with different heat inputs (3.6–9.8 kJ/cm), using constant amplitude DC, and they were evaluated in relation to the quality obtained in the mirrored and textured surfaces. Those results were correlated with the chemical compositions, microstructures and hardness. © 2006 Elsevier B.V. All rights reserved. Keywords: Repair welding; Injection molds; TIG; Polishing; Texturing
1. Introduction Polymer injection molds frequently require some form of repair by welding: (a) during the manufacture, by errors in machining or by some change in the design of the part to be injected and (b) during the service, by the incidence of failures in the mold. For those repairs, TIG process is used frequently, because it allows the deposition of small amounts of material, without spatter, in such a way to obtain the complex geometries of the mold. Special processes and welding procedures are used due to the tight tolerances required and the high cost of the mold. The required quality of the weld is not only related to the suitable mechanical properties, but also to the weld and the heat affected zone (HAZ) behavior when finishing processes are applied to give a uniform and adequate texture to the mold cavity. Nowadays, the welding procedures applied for the repair of molds are elaborated having as a basis the recommendations given by the manufacturers of steel, but it remains not clear what are the involved metalurgical phenomena, whose knowledge is necessary to define rational procedures, suitable
for different materials, sizes and geometries of the molds to repair. The vast majority of the information available in the literature about the repair of these steels focuses the aspects of weld techniques, without considering metallurgical aspects [1,2]. The efforts of mold steels manufacturing companies has led to the development of base materials and their filler metals as a way of increasing the weldability and dimensional stability of the mold after heat treatments. This is the case of VP50IM steel studied in this work. The P20 steel, also focused in this study, has a long successful history in the industry of polymer injection molds, although with severe problems when recovered by welding. 2. Objectives The primary objective of this work is to establish optimal conditions repair by welding P20 and VP50IM using the wire feed TIG process, in such a way to obtain that the weld metal (WM) and heat affected zone have behaviour similar to that of the base metal (BM) when the surface finishing processes are applied. 3. Theoretical revision
∗
Corresponding author. E-mail address: [email protected] (W.T. Preciado).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.03.101
After mechanizing the mold to obtain the geometry that makes it functional, its cavity is subjected to finishing processes
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
of finished so the texture characteristics obtained are the same desired part that will be injected in the mold. The common finishing processes are three: polishing, mirroring and texturing. 3.1. Polishing and mirroring of the mold Some characteristics of the cavity of the mold are particularly important: the surface must have a correct geometry, without ondulations; the mirrored surface must be free of pores, inclusions and other defects. The finished surface generally is evaluated by visual exam (to the naked eye). Nevertheless, as that involves certain difficulties, in more demanding cases the finished surface is evaluated by means of techniques like optical interferometry [3]. According to [3], for an injection mold that must be polished after welding, it is essential that the weld metal does not have significant differences in the composition and hardness with respect to the base metal. Otherwise, in the contour of the weld could remain, after polishing, marks that would be reproduced in the injected part. 3.2. Texturing of a welded surface The texturing treatment can be applied to ferrous and nonferrous metals. It consists of a selective chemical attack, obtained frequently in the following form: the surface is covered with a photosensitive resin; over that it is placed a sheet with a drawing of the texture; the piece is exposed to an intense light, that fixes the unprotected parts of the surface. With a solvent the resin nonexposed to the light (that is to say, that part of it not fixed) is removed and then the surface is subjected to attack by acid [1,4]. According to [5], a weld in a surface to be texturized must be done using a filler metal of similar composition, with a carbon content slightly smaller than the one of the BM, and the weld hardness must be similar to that of the BM. The preheating around the area helps to diminish the differences of hardness between the base and weld metals. According to [6], filler metals are specifically recommended by the base metal manufacturers in order to hace a satisfactory surface, and must be only used with suitable welding procedures for each steel in particular, with the objective to reduce the variations in the chemical composition and/or hardness, that could impair the texturing process significantly. Some materials are more resistant to acid attack than others, due to the differences in the chemical composition, so it is better to choose a filler metal of chemical composition similar to that of the base metal. Unfortunately, the chemical composition is not the only factor that determines the depth of dissolution by the acid. The microstructure also has an influence. As it becomes harder, greater will be the resistance to the action of acid [1]. 4. Methodology In order to obtain the proposed objectives, welds were made in two base materials, followed by polishing and texturing treatments. The quality of the obtained surfaces was correlated with
245
the chemical composition, microstructures and hardness of the different regions of the weld. The base materials used were the P20 and VP50IM steels, that are Cr–Ni–Mo steels refined by vacuum degassing. The AISI P20 is a hardenable steel used frequently in the manufacturing of polymer injection molds. It is provided in the quenched and tempered state, with hardness between 30 and 34 HRC. The VP50IM is a steel hardened by precipitation, developed by the Villares Metals [7]. It is furnished in the solution treated state, with hardness between 30 and 35 HRC. In order to increase its resistance it must be aged by heat treatment at 500 ◦ C. For the P20 steel weld was used a dissimilar filler metal, AWS A5.28-96 ER 80S-B2, that is a Cr–Mo steel used in the welding of boilers, pressure vessels and pipelines that work at high temperatures. For the VP50IM steel two filler metals were used • A similar one, developed by the same manufacturer of that base metal. • A dissimilar one, AWS A5.28-96 ER 80S-B6, that is a 5Cr–0,5Mo steel used for the welding of components that works at high temperatures and in atmospheres containing hydrogen. The chemical compositions of the base and filler materials are shown in Table 1. At first, welds in SAE 1020 steel plates were done in order to determine the appropriate conditions to deposit sound welds by wire feed TIG. After that, in plates of 150 mm × 150 mm × 12 mm of P20 and VP50IM steels there were deposited six beads with three levels of average current (88, 106 and 124 A) and two speed levels (10 and 14 cm/min). That resulted in six heat input values, ranging from 3.6 to 9.8 kJ/cm, as shown in Table 2. The following factors were maintained constant in the tests: • • • • • • • •
Preheating temperature: 225 ± 25 ◦ C. Shielding gas: Argonium, 12 l/min. Electrode: W + 2% ThO2 , diameter 2.4 mm, 60◦ tip angle. Arc length: 4 mm. Wire diameter: 1.2 mm. Nozzle diameter: 10 mm. Wire feeding angle: 25◦ in relation to the plate surface. Torch incidence angle: 15◦ in relation to the surface normal.
From the six beads deposited with each filler metal, two samples were removed, as shown in Fig. 1. A specimen was kept to apply on it the grinding and polishing processes, and parts of the polished surface were subjected to texturing and mirroring. The other specimen was for microstructural analysis and hardness measurement of the weld cross-section. Based on the uniformity of textured and polished surfaces, it was selected only one specimen for each filler metal to make the mirroring, because it is a time-consuming process. The specimens for metalography and hardness were polished with 13 m sandpaper and 3 m diamond paste. Later they were
246
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
Table 1 Chemical compositions of the base and filler metals, and their respective calculated CEiiw Element
P20 BM
Carbon Silicon Manganese Chromium Nickel Molybdenum Phosphor Sulphur Copper Aluminum Niobium Cobalt Titanium Vanadium Tungsten CEiiwc a b c
VP50IM P20a
Dissim FM AWS A5.28 E
0.37 0.39 1.4 1.89 0.77 0.18 0.03 0.0055 0.12 0.002 0.005 0.025 0.002 0.012 0.011 1.08
80S-B2b
0.09 0.58 0.54 1.33 0.04 0.51 0.01 0.006 0.03 – – – – – – 0.55
BM VP50IMa
Similar FMa
Dissimilar FM-AWS A5.28 E 80S-B6b
0.17 0.21 1.4 0.27 2.95 0.28 0.03 0.09 0.9 0.85 0.0048 0.05 0.0046 0.09 0.01 0.79
0.14 0.10 1 0.24 2.92 0.27 <0 0.13 1 0.35 0.0039 0.024 0.0034 0.064 0.036 0.68
0.08 0.39 0.53 5.9 0.06 0.54 0.004 0.013 0.07 – – – – – – 1.46
Compositions measured by opthical spectrometry. Compositions furnished by the manufacturer in quality certificates of the product. IIW carbon equivalent, CEiiw = %C + %Mn/6 + %(Cr + Mo + V)/5+%(Ni + Cu)/15.
Table 2 Welding conditions for wire feed TIG Run
Currenta (A)
Voltage (V)
Welding speed (cm/min)
Heat input (kJ/cm)
Arc power (W)
Wire feed (m/min)
E1 E2 E3 E4 E5 E6
88 106 124 106 124 106
10 10 10 10 10 10
14 14 14 10 10 7
3.78 4.5 5.25 6.4 7.54 9.08
880 1060 1240 1060 1240 1060
0.4 0.4 0.5 0.4 0.4 0.4
a
All the welds were done with constant value DC current.
attacked with Nital 2% solution. There were measured Vickers hardness profiles, with 1 kg load and 0.2 mm interval between indentations, in a transverse direction to the weld, including the WM, HAZ and BM. In order to evaluate the polished and textured surfaces in a more objective form that can be done through visual exam, there were made measurements in an optical rugosimeter.
The mirroring was done on VP50IM steel welded specimens and the hardness profile was measured on the mirrored surfaces, in a transverse direction to the weld. An analysis was made of the mirrored surfaces by visual examination and optical microscopy. These results were correlated with the microstructure and chemical composition of the three regions (WM, HAZ and BM). The conditions used in the grinding,
Fig. 1. Specimen removal and tests done.
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250 Table 3 Process conditions for surface finishing Processo Grinding Grinding wheel Workpiece speed Grinding wheel speed Depth
Condic¸o˜ es Norton 3SGK46 KVSP (ABNT NB33) 10 m/min 2500 rpm 0.25 mm/pass
Polishing Rough finish grinding stone Smooth finish sandpaper (VP50IM) Smooth finish stone (P20)
#150 #320, 400, 600 #320, 400, 600
Texturizing Time of attack Method Type of bath
7 min Strip Jet
Mirroring Polishing time Smooth finish sandpapers Machine remotion Manual remotion with cotton
1 h on 8 cm2 #600, 800, 1000, 1200, 2000 Diamond paste 2–4 m, 7000 rpm Diamond paste 1/4 m
polishing, texturing and mirroring processes are described in Table 3. 5. Results and analysis 5.1. Evaluation of response to polishing and mirroring After making the grinding of the surface (to remove the reinforcement of the weld and to reduce the roughnesss), the polishing had to be done in a different way in both of the materials: whereas in steel VP50IM was possible to use sandpaper, in the P20 steel the use of it produced a not uniform removal in the three regions of the weld, due to the higher hardness of the HAZ (650 HV) compared with that of the BM (300 HV). Therefore, it was necessary to make the removal with abrasive stones that allowed to obtain the same finishing quality produced by the sandpapers. In order to evaluate the polished surfaces there were made scans parallel to the longitudinal direction of the weld, through
247
a 5.6 mm length, going from the centerline of the bead to the base metal. In all the specimens, the peak to valley distances were similar in the evaluated zones (WM, HAZ and BM), as it is illustrated in Fig. 2. Based on that, the Ra roughness parameter was calculated in all the measurement length, and that value was related to the base materials and level of heat input, as it is shown in Fig. 2b. It is observed that the Ra value did not vary significantly with heat input, being in the interval between 0.45 and 0.80 m, satisfactory for a polishing made as a preparation for texturing or mirroring. At the left side of Fig. 3 it can be observed that in the aswelded condition, the WM and HAZ of the VP50IM steel welds using both types of filler metals (similar and dissimilar) had similar hardness (430 HV), lower than those of WM and HAZ of welds in P20 steel (450 and 650 HV), and slightly greater than the one of VP50IM and P20 base metals (305 HV). In the case of the P20 steel, the high differences of hardness between the WM, HAZ and BM turn difficult the polishing treatment. Taking into account that, it was not made mirroring (that is a lengthy process). The great differences of hardness turned necessary to use more rigid cloths, in order to maintain its form and allow a uniform removal in the regions of the weld and BM. This was a decisive factor to not apply the mirroring on P20 steel, because there would form reliefs in the various regions of the weld. Therefore, it is important to reduce the hardness variations on the weld of a steel of high hardenability as P20, through a postweld heat treatment or through controlled multipass welding techniques as double layer and temper bead, in which it is taken advantage of welding thermal cycles to heat treat the previously hardened zones [7]. Welds made on VP50IM steel were subjected to an aging treatment before mirroring. Two specimens welded with similar and dissimilar filler metals were selected for this purpose. Those specimen were welded with the same heat input of weld (E5 = 7.54 kJ/cm of the Table 2) chosen to give a greater dilution (as will be shown in Fig. 7b). In Fig. 4 are shown the hardness profiles measured in the mirrored surfaces. By comparing the results shown in Figs. 3 and 4, it can be verified that the differences of hardness between the
Fig. 2. (a) Typical roughness profile obtained after polishing and (b) Ra roughness values measured in a transverse direction to the welds, for different heat inputs.
248
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
Fig. 3. Hardness profiles, macrographies, textured surfaces and micrographies (BM, WM and CG-HAZ) of specimens welded with 7.54 kJ/cm heat input.
WM and BM in mirrored specimens were smaller (55 and 40 HV) that the observed ones before the aging treatment (130 HV). This explains the recommendation given by the manufacturer of VP50IM steel of doing the mirroring after the aging treatment of the weld. The visual evaluation of the surface by means of light reflectance allowed to verify the excellent behavior of steel VP50IM welded with a similar filler metal, not being so good equal in the case of dissimilar filler metal weld. Although the differences of hardness between the weld and the BM were nearly equal (55 and 40 HV for the similar and dissimilar filler metals, respectively), in the weld with dissimilar filler metal was formed a step between the WM and HAZ that is shown in Fig. 5. That can be due to the lower sulfur content of the dissimilar filler metal and, therefore, of the WM obtained with it, that causes a reduction of the remotion rate.
5.2. Evaluation of the texturized surface After the polishing, a part of the surface was subjected to texturing. As reported in the previous item, in all the specimens the polished surface showed a uniform roughness in the three weld zones (WM, HAZ and BM), which means that there was no influence of the polishing on the results of the texturing. Fig. 6 shows a typical roughness profile of a textured surface, measured throughout a length of 12.5 mm, including the WM, HAZ and BM. Through the visual exam it is easy to verify that in P20 welds the BM was more intensively attacked than WM and HAZ (the valleys correspond to the zones dissolved by acid). When attempting to characterize that in a more objective form by means of roughness measurements, it was verified that Ra value is not a good indicator of the textured surface quality,
Fig. 4. Hardness profiles in mirrored surfaces after welding and aging treatment (Hi = 7.54 kJ/cm, Im = 124 A, Um = 10 V, WS = 10 cm/min, WF = 0.4 m/min).
Fig. 5. Relief in the region of the WM in the VP50IM steel weld, observed by means of light reflectance: (a) weld with dissimilar FM and (b) weld with similar FM.
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
249
Fig. 6. Roughness profiles of textured specimens in a transverse direction to weld bead, in textured specimens: (a) uniform acid attack in VP50IM and (b) not uniform acid attack in P20.
because it depends to a large extent on the texture pattern that was used (Ra is an average of the squares of the deviations from a reference line). A better indicator could be the Rz value, that here was obtained as the average of five peak to valley distances). Then, to characterize the textured surface there were measured roughness profiles of WM, HAZ and BM separately, along lines parallels to the weld bead, through a measurement lenght of 5.6 mm. In Fig. 7 are shown the Rz roughness and hardness values measured in the WM, HAZ and BM. It is possible to observe that in steel VP50IM the Rz roughness values of WM, HAZ and BM were always similar, although WM and HAZ had
hardness greater than the one of BM (around 400 HV and 300 HV, respectively). From those results, obtained with both of the base metals, it is possible to conclude that the texture depth does not have a direct relation with the hardness of the material. For example, in the case of VP50IM steel the obtained textured surface was uniform, despite the significant hardness differences between HAZ and BM. In the case of the P20 steel, altough the HAZ was harder than the WM, the texture was uniform. In order to explain that, it is necessary to remember that P20 BM is furnished in the quenched and tempered condition. When subjected to a weld thermal cycle, the material is quenched again
Fig. 7. (a) (Rz) roughness and Vickers hardness in HAZ, WM and BM for each heat input of weld and (b) values of dilution for each material and heat input.
250
W.T. Preciado, C.E.N. Bohorquez / Journal of Materials Processing Technology 179 (2006) 244–250
(the hardness of 600 HV corresponds to that of martensite with 0.37 %C) and becomes more resistant to the acid attack [8]. In order to evaluate the reasons for the behavior of the WM, the dilution was determined by means of added and fused areas measured in the weld cross section. The dilution was always high (between 50 and 70 %), which was not expected when applying TIG process (a process of low thermal efficiency) with relatively low currents. Because of that, a considerable portion of the WM is formed by fused BM. In Table 1 it is possible to observe that the used filler metal to weld the P20 steel has lower C and CEiiw than that of the BM. That reduces the hardenability (but not enough to avoid the formation of 100% martensite with the welding conditions applied) and reduces the hardness of martensite. This effect can be verified by the higher hardness obtained with higher dilutions (530 HV for 71.5% dilution, 400 HV for 62% dilution). On the other hand, the hardness differences did not have effect on the Rz roughness of the texturized surfaces. That suggests that the answer to the chemical attack depends on the form in which the alloy elements are present (in solid solution or as part of precipitates). In WM and HAZ the material in the tempered state is hard, with the martensite supersaturated in C and other alloy elements. In the BM, due to the tempering treatment, it occurs the formation of carbides. Then, to obtain uniformity in the texturized surface it would be necessary to promote the tempering of WM and HAZ, which could be done by means of a conventional heat treatment (with deleterious effects on surface finish and dimensional stability) or by means of double layer and temper bead welding techniques [8]. 5.3. Macrostructure, hardness distribution and microstructure In order to complete the analysis, in the right side of Fig. 3 macrographies, textured surfaces and micrographies (including the WM, HAZ and BM) are shown. The following comments can be done: (a) In relation to the P20 steel: - The microstructure of the BM, WM and HAZ is martensitic. At BM the martensite is tempered. At HAZ it has a high hardness (650 HV), with diferent prior austenite sizes, coarser at the region next to the fusion line (coarse grain HAZ). At WM, besides martensite, it is possible to observe some retained austenite. - The BM shows a deeper texture relief than that of the WM and HAZ. (b) In relation to the VP50IM steel: - The BM had a martensitic microstructure, with the alloy elements in solid solution. - The microstructure of the HAZ and WM is martensitic, but their hardness is lower than the obtained in the P20 steel, due to the lower carbon content. - The hardness of WM and HAZ are slightly higher than that of the BM.
6. Conclusions From the tests and analyses the following comments can be done: (a) In relation to the P20 steel: - A satisfactory polished surface can only be obtained by using a polishing stone instead of sandpaper, to guarantee the flatness. - The application of mirroring is not advisable in the aswelded state, unless the tempering of the HAZ and WM can be promoted through double layer and temper bead welding techniques. - The textured surface was not uniform, being deeper the chemical attack in the BM, because it is on the quenched and tempered condition. (b) In relation to the VP50IM steel: - It is possible to obtain a uniform polished surface, even with the use of sandpaper. - In the mirrored surface of the weld made with the similar filler metal a totally flat surface was obtained. However, in the weld done with dissimilar metal was possible to perceive relief differences. - The textured surfaces were uniform for all the welding conditions. (c) In general, for both types of base materials: - The uniformity obtained in the polished and mirrored surfaces depends mainly on the uniformity of the hardness throughout the weld. - The uniformity in the texture of surfaces does not depend on the hardness, but on the local chemical composition and the condition of the alloy elements (in solid solution or as part of precipitates). References [1] S. Thompson, Handbook of mould, tool and die repair welding, first ed., Abington Publishing Limited, London, 1999, p. 224. [2] M. Vedani, Microstructural evolution of tool steels after Nd-YAG laser repair welding, J. Mater. Sci., Milan 39 (2004) 241–249. [3] Uddeholm, Polishing Mold Steel. Treatment of Tool Steel. Available in: