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On the weldability of grey cast iron using nickel based filler metal
Materials and Design 31 (2010) 3253–3258
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
On the weldability of grey cast iron using nickel based filler metal M. Pouranvari * Young Researchers Club, Islamic Azad University, Dezful Branch, Dezful, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2009 Accepted 6 February 2010 Available online 23 February 2010 Keywords: D. Welding F. Microstructure Grey cast iron
a b s t r a c t Shielded metal arc welding process using nickel based filler metal was used to join grey cast iron. The effect of post weld heat treatment (PWHT) on the microstructure and hardness was studied. PWHT included heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. By using nickel based filler metal, formation of hard brittle phase (e.g. carbides and martensite) in the fusion zone is prevented. Before PWHT, heat affected zone exhibited martensitic structure and partially melted zone exhibited white cast iron structure plus martensite. Applied PWHT resulted in the dissolution of martensite in heat affected zone and graphitization and in turn the reduction of partially melted zone hardness. Results showed that welding of grey cast iron with nickel based filler metal and applying PWHT can serve as a solution for cast iron welding problems. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cast iron is generally considered as a difficult material to be welded. This is basically due to two reasons: (i) inherent brittleness of the cast iron and (ii) the effect of weld thermal cycle on the metallurgical structure of the cast iron. Typically, four distinct regions are formed when cast iron is welded, as follows: (i) Fusion zone (FZ) which is melted during welding process and is resolidified upon cooling. (ii) Partially melted zone (PMZ) which is the area immediately outside the FZ where liquation can occur during welding. (iii) Heat affected zone (HAZ) which is not melted but undergoes microstructural changes. (iv) Base metal (BM) which its structure remains unaffected during weld thermal cycle. Fig. 1 shows relationship between Fe–C phase diagram and the temperature experienced by each microstructural zone [1]. High carbon content of the cast irons leads to formation of hard brittle phases, namely martensite and carbides in the FZ, the PMZ and the HAZ. Both carbide and martensite, being hard and brittle, are detrimental to the ductility, toughness and machineability of the weld and also may cause cracking in the joint [2–5]. Weldability of cast iron depends on the several factors including [4–11]: (i) type of the cast iron, (ii) chemical composition of the cast iron, (iii) chemical composition of filler metal, (iv) original * Address: Materials and Metallurgical Engineering Department, School of Engineering, Islamic Azad University, Dezful Branch, Dezful, Iran. Tel.: +98 91 24075960; fax: +98 21 88522421. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.02.034
matrix structure and (v) welding process and preheat/post heat treatment. Grey cast iron is inherently brittle and often cannot withstand stresses set up by a cooling weld. As the lack of ductility is caused by the coarse graphite flakes, the graphite clusters in malleable irons, and the nodular graphite in ductile cast irons, give significantly higher ductility which improves the weldability [12]. Ductile cast irons and malleable irons are less susceptible to form martensite in HAZ, therefore, they are more readily weldable, particularly if the ferrite content in their matrix is high. White cast iron which is very hard and contains iron carbides, is normally considered to be unweldable [12,13]. Arc welding processes and oxyacetylene welding are two most common welding processes which are used to cast iron welding [11,14,15]. However, the application of diffusion bonding, friction welding and electron beam welding are evaluated by some researchers [16–20]. There are generally three type available filler metals for welding cast irons: mild/low carbon steel filler metal, cast iron filler metal and nickel/nickel–iron based filler metal. Some researchers used mild steel electrode for welding grey cast iron. The main driving force for using mild steel electrodes is their low cost [21]. However, these electrodes suffer from some metallurgical problems including: (i) Steel shrinks more than grey cast iron during solidification; therefore, tensile stresses generated in FZ can make it susceptible to shrinkage cracking [13,14]. (ii) In spite of dilution of mild steel electrode with high carbon cast iron, the carbon content of FZ is sufficient to formation hard and brittle product in FZ. This reduces the impact
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2. Experimental procedure
Fig. 1. Temperatures experienced by various microstructural zones in a cast iron weld [1].
properties of the weldment. Moreover, inability of FZ to yield and relieve welding stresses can result in cracking in the adjacent cast iron heat affected zone. Therefore, the use of steel electrodes should be restricted to application where the joint is not loaded in tension or in bending. (iii) Due to high hardness in FZ and HAZ, preheating and post weld heat treatment (PWHT) is required. Preheating reduces cooling rate and therefore leave softer FZ and HAZ. The preheat temperature is usually in the range of 300–600 °C. Preheating cannot be used, however, when minimum heat is to be applied to avoid distortion of the parts being welded [22]. According to work of Kumar [22], a preheat temperature of 540 °C is required to significant reduction of FZ and HAZ hardness. However, for more improvement in machinability of welded cast iron a PWHT is also required. When a color match FZ is required the best choice is cast iron rods. Also when pressure tightness and uniform thermal expansion is required such as on cylinder heads or steam turbines or pump cases the oxyacetylene welding with a cast iron rod is preferred process [15]. In the case of cast iron filler metal, it is reported that the weld cracking due to formation of brittle phase in FZ is highly probable when preheat temperature is lower than 300 °C. Nickel based electrodes offer the highest crack resistance weld mainly because of their desirable mechanical properties and their ability to precipitates the carbon picked up from the base in its free form as graphite. Therefore, successful welding of cast iron requires more sophisticated understanding of interaction between the composition/microstructure of cast iron, filler metal composition and weld thermal cycle. This paper addresses the microstructure of grey cast iron welds in as-welded condition and after applying a post weld heat treatment (PWHT). In this work, preheat heat treatment was not performed. This was due to facts that a high preheat temperature as 540–600 °C is required to minimize the martensite formation and reduce the hardness in HAZ. In addition, it is reported that very high preheat temperature of 818 °C (1500 F) is required to reduce solidification cooling rate to a level such that iron carbide does not form in PMZ. However, such high preheat temperature can cause undesirable distortion of workpieces. Moreover, it is reported by Kotecki [11] that preheat temperature grater than 316 °C (600 F) increases the carbides size and their continuity in HAZ and PMZ. Therefore, in this research it was tried to (1) solve the problem of FZ by using a nickel based electrode and (2) solve the problems of HAZ and PMZ simultaneously by applying a PWHT.
A grey cast iron was used as the base metal in this study. The chemical composition of the base metal is: Fe–2.85C–1.75Si– 0.10P–0.82Mn–0.05S (wt.%). Shielded metal arc welding (SMAW) process using nickel electrode was used to join a grey cast iron. The chemical composition of the filler metal is: 98.5 Ni–0.71 C– 0.42 Si–0.37 Mn. A single V-shaped groove with an angle of 60° was considered as the preferred joint design. Welding parameters are given in Table 1. After welding, some specimens were immediately transferred to an electric furnace, kept there at 870 °C for 1 h and then furnace cooled to the room temperature. Cutting of the samples for metallographic examination was carried out while avoiding excessive heating that might have led to local alterations in the microstructure. Standard metallographic procedure was used for microstructural examinations. Nital 2% solution was used to reveal various microstructure constituents in the weldment. Vickers microhardness test using 200 g load was carried out to obtain average hardness of various microstructural zones in the weldment. 3. Results and discussion 3.1. Microstructure in the as-welded condition 3.1.1. FZ microstructure As can be seen in Fig. 2a, BM exhibits a ferritic matrix plus flake graphite phase. In FZ, BM is melted and mixed with filler metal. High cooling rates in this zone lead into very hard and brittle ledeburitic carbides in as-welded condition, if a Fe–C alloy filler metal is used [23]. To reduce the risk of the formation of brittle phase in FZ, nickel based filler metal is used to join cast irons. Fig. 2b demonstrates that FZ microstructure consists of mainly an austenitic matrix plus small amount of dispersed graphite particles. Nickel filler metal is able to precipitate carbon, picked up from the BM, in its free form as graphite. During welding the carbon of the BM is diluted with the filler metal in the FZ. The carbon content in FZ is well above the solubility limit of carbon in Ni (0.5% at eutectic temperature of Ni–C system). Therefore, the excess carbon is precipitated as graphite during FZ solidification. It should be noted that nickel carbide (Ni3C) is much less stable than iron carbide (Fe3C) and under most solidification conditions the carbon separates in the form of graphite [24]. Therefore, the use of nickel base filler metal has the following advantages: (i) Formation of hard and brittle micro-constituents in the FZ is prevented [25]. (ii) Precipitation of graphite in the FZ increases its volume. This can further reduce the weld metal shrinkage during solidification, which in turn, leads to the minimization of the residual stress in FZ and HAZ causing the reduction of the cracking susceptibility of the cast iron joint [8]. (iii) The high ductility of the nickel based FZ can play an important role in the absorption of tensile stress created during welding. This contributes to the minimization of the crack susceptibility of joint [4].
Table 1 Welding parameters used in this study. Current (A)
Polarity
Speed (mm/min)
Electrode diameter (mm)
120
DCEP
100
4
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Fig. 2. Microstructure of various regions in grey cast iron weld in as-welded condition: (a) base metal, (b) fusion zone, (c) fusion boundary, (d) partially melted zone, and (e) heat affected zone.
(iv) The nickel base FZ exhibits good machineability [8]. (v) The nickel base FZ has a grater tolerance for sulfur and phosphorus. This can increase the FZ resistance to hot cracking [4,8]. Consequently, by using nickel based electrodes, the welding problems are not in FZ but they are in PMZ and HAZ. Therefore, the impact properties of the cast iron welded joint depend on the amount and distribution of massive carbides and martensite in HAZ and PMZ. 3.1.2. PMZ microstructure Fig. 2c and d shows the PMZ microstructure indicating that this region consists of eutectic ledeburit and martensite. In PMZ, the portion of the matrix of the base metal near the pri-
mary graphite melted during the welding, while the remainder of the matrix transformed to austenite. The PMZ is enriched from carbon. Therefore, at the fast cooling rate typical of welding, coarse carbides solidify directly from the liquid as a result of the eutectic reaction (i.e. white cast iron structure is formed). These can form a continuous brittle network along the weld fusion line. The austenitic matrix surrounding these carbides can also transform to martensite at lower temperatures. Martensite is also hard and brittle particularly when its carbon content is high. The phase transformation sequences during the cooling of the PMZ can be summarized as:
L þ Graphite ) Eutectic ledeburit ðc þ Fe3 CÞ þ Austenite ðcÞ ) Ledeburit þ Martensite
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3.1.3. HAZ microstructure Fig. 2e shows that the microstructure contains of high amount of martensite plus some graphite in HAZ. During the weld thermal cycle, HAZ experiences a temperature higher than A1 (eutectoid temperature in Fe–C phase diagram). Therefore, at this high temperature, the graphite flakes begin to dissolve in the austenite producing a carbon gradient from the graphite flakes into the austenite and resulting in the gradation of transformation products within this region. The transformation rate of graphite to austenite depends on the diffusion rate of carbon. The amount of austenite formation depends on the following factors [10,11,19,26,27]: (i) Time and temperature experienced by each point at HAZ: the higher the temperature and the longer the time available for carbon to diffuse, the higher the volume fraction of martensite. (ii) The primary matrix microstructure of cast iron: typically, the rate of the austenite formation in a pearlitic structure is higher than the rate in a ferritic structure due to its lamellar structure (i.e. the diffusion distance for carbon atoms from iron carbide to ferrite is shorter). It is interesting to note that martensite start temperature (Ms) of ferritic cast iron is lower than pearlitic one due to its much lower carbon content. This is advantageous since it is known that the volume change associated with martensitic transformation is greater with lower Ms temperatures. Also the hardness of martensite is directly proportional to the carbon content, again favoring the ferritic grades [22].
(iii) The shape of the graphite phase: the flake graphite has higher tendency to dissolve than nodular graphite due to its higher surface to volume ratio. (iv) The volume fraction of graphite: higher amount of graphite favors the austenite formation. During fast cooling rate typical of welding, HAZ microstructure transforms to a brittle structure. Since, the cooling rate is high; the graphitization process cannot be accomplished. Moreover, the formed austenite is transformed into a hard brittle martensite due to the inherent high hardenability of this austenite.
3.2. Microstructure after PWHT PWHT is necessary in most cases in order to eliminate the massive carbides and martensite in HAZ and PMZ and thus reduce hardness and brittleness. Two most common PWHTs for cast iron weldments are subcritical tempering and full (ferritizing) annealing. Low temperature tempering can reduce hardness of martensite; however, higher tempering temperature is required to graphitize the eutectic carbides. Secondary graphitization reduces the brittleness of PMZ and HAZ and improves impact properties. However, excessive graphitization can reduce ductility of weldment [27,28]. Askeland and Birer [26] studied the secondary graphitization and they concluded that the full annealing PWHT rather than subcritical tempering gives better microstructure control and the excessive graphitization and formation of chain-like graphite is prevented. Therefore, in this study a full annealing
Fig. 3. Microstructure of various regions in grey cast iron weld after PWHT: (a) fusion zone, (b) fusion zone boundary, and (c) heat affected zone.
M. Pouranvari / Materials and Design 31 (2010) 3253–3258
PWHT was chosen including heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. Fig. 3 shows various microstructural zones in the weldment after PWHT. As can be seen, the microstructure of FZ is remained unchanged after PWHT thermal cycle. However, HAZ microstructure significantly is affected by PWHT. As can be seen in Fig. 3c, HAZ consists of graphite flakes in a ferritic matrix. Holding in 870 °C for 1 h provides sufficient driving force to dissolve of eutectic carbide and the martensite phases formed during the welding. During the slow furnace cooling, graphite is formed in a ferritic matrix but not Fe3C. Therefore, the applied PWHT can reduce the formation of brittle phases in HAZ.
3.3. Effect of PWHT on weld hardness Hardness variation across the weldment of the cast irons is one of the most important factors controlling the quality of cast iron welds. Strength/hardness mismatch between various microstructural regions significantly affects the fracture behavior of cast iron weldment [29,30]. Fig. 4. shows the hardness value corresponding to FZ, PMZ, HAZ and BM, before and after PWHT. The data points for hardness are average of five points in each zone. As can be seen in the as-welded condition, PMZ has the highest hardness value. This can be related to its brittle microstructure consisting of hard eutectic ledeburit and martensite. High hardness value of HAZ can be related to the presence of high amount of martensite in this region. The low value of hardness in FZ, comparable to BM hardness, is due to its austenitic structure. Both carbide and martensite formation in FZ can be prevented by using a nickel cast iron. Therefore, nickel filler metal can reduce the hardness of FZ in cast iron, as one of the big problems of cast iron welding. The soft nickel filler metal yields more easily during cooling than does the harder filler metals, lessening the strain on the HAZ, reducing the HAZ cracking. However, the use of nickel filler metal is incapable of altering the chemical composition of the HAZ and PMZ. Therefore, attempts to overcome the tendency to form white cast iron in PMZ and to form martensite in both HAZ and PMZ have directed toward the control of the weld thermal cycle. One of the ways to control the weld thermal cycle is through designing a proper PWHT. As can be seen in Fig. 4. HAZ and PMZ hardness significantly reduce after applied PWHT. FZ hardness remains unchanged after PWHT. It should be noted that the PWHT may cause reduction in strength of BM [19]. However, as can be seen the reduction of BM hardness after PWHT is not negligible. Therefore, it can be concluded that welding of grey cast iron using nickel electrode and designing a proper PWHT can serve as a solution for cast iron problems.
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4. Summary and conclusions Welding grey cast irons show serious problems in FZ, PMZ and HAZ, formation of hard and brittle product during solidification and post solidification eutectoid transformation. Formation of martensite and carbide in FZ can be controlled via controlling of cooling rate and the chemical composition of FZ which is in turn governed by BM and filler metal composition and degree of dilution. Results of the current study showed that by using nickel filler metal, the formation of brittle martensite and carbides in FZ is prevented. The FZ exhibited an austenitic structure plus some graphite nodules leaving a ductile and tough FZ which improve weld ductility and toughness. Moreover, tough austenite can absorb shrinkage and thermal stresses effectively reducing the risk of cracking. Therefore, by using nickel filler metal the problems of FZ are completely solved without need to controlling the dilution and the weld thermal cycle (i.e. without need to preheat or post heat treatment). It is of note that, the nickel filler metal has low coefficient of thermal expansion, therefore, it strains the cast iron HAZ much less than other filler metals, helping in reducing the risk of HAZ cracking. It was shown that HAZ microstructure of grey cast iron exhibited martensite. Also, PMZ microstructure exhibited hard eutectic carbide and martensite. To guard against this problem it is generally advisable to reducing cooling rate via preheating to prevent martensite and carbide formation or post heat treatment to decompose martensite and carbides to softer micro-constituents. Preheating enables to avoid the formation of martensite, however high preheat temperature which is sufficient for prevention of martensite formation induces other problems: distortion of the workpieces (in the case of local preheating) and formation of larger carbides with higher continuity which can impair the weld quality. Only, very slow cooling rates can prevent the eutectic–carbide formation during solidification in PMZ, the typical preheat temperatures often tend to increase the amount and continuity of the carbides rather than prevent their formation. Therefore, in this research preheating was not use. As an alternate solution, full annealing including heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling was used. This PWHT was successful in the dissolution of martensite in HAZ and graphitization in this zone. Also, this heat treatment was successful in the reduction of PMZ hardness. Moreover, unlike high temperature subcritical tempering, excessive secondary graphitization which can reduce weld ductility was not observed. Applied PWHT was successful in producing a weld with nearly uniform hardness profile. Therefore, according to the results presented in this paper, it can be concluded that welding of grey cast iron with a nickel filler metal coupled with applying a proper full annealing (ferritizing) PWHT can serve as solution for grey cast iron welding problems. References
Fig. 4. Average hardness value of various microstructure zones in as-welded condition and after PWHT.
[1] Kou S. Welding metallurgy. 2nd ed. New Jersey: John Wiley and Sons; 2003. [2] Huke EE, Udin H. Welding metallurgy of nodular cast iron. Weld J 1953;32:378s–85s. [3] Nippes EF. The heat affected zone of arc welded ductile iron. Weld J 1960;39:465–72. [4] Pease GR. The welding of ductile iron. Weld J 1960;39:1–9. [5] Kiser SD. Production welding of cast iron. AFS Trans 1977;85:37–42. [6] Kiser SD, Irving B. Unraveling the mysteries of welding cast iron. Weld J 1993;72:39–44. [7] Fujii N, Honda H, Fukase A, Yasuda K. Comparison of strength characteristic of nodular graphite cast iron welded joints by various welding process. Quart J Jpn Weld Soc 2007;25:261–7. [8] Voight RC, Loper Jr CR. A study of heat affected zone structure in ductile cast iron. Weld J 1983;62:82s–8s. [9] Martinez RA, Sikora JK. Perlitic nodular cast iron: can it be welded. Weld J 1995:65–70. [10] El-Banna EM. Effect of preheat on welding of ductile cast iron. Mater Lett 1999;41:20–6.
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[11] Kotecki DJ, Braton NR, Loper NR. Preheat effects on gas metal arc welding ductile cast iron. Weld J 1969;48:161s–6s. [12] TWI website.. [13] Voight RC, Loper CR. Welding metallurgy of grey and ductile cast irons. AFS Trans 1986;94:133–46. [14] Hogaboom AG. Welding of gray cast iron. Weld J 1977;56:17–21. [15] Klimek J, Morrison AV. Gray cast iron welding. Weld J 1977:29–33. [16] Hatate M, Shiota T, Abe N, Amano M, Tanaka T. Bonding characteristics of spheroidal graphite cast iron and mild steel using electron beam welding process. Vacuum 2004;73:667–71. [17] Kolukisa S. The effect of the welding temperature on the weldability in diffusion welding of martensitic (AISI 420) stainless steel with ductile (spheroidal graphite-nodular) cast iron. J Mater Process Technol 2007;186:33–6. [18] Zdemir NO, Aksoy M, Orhan N. Effect of graphite shape in vacuum-free diffusion bonding of nodular cast iron with gray cast iron. J Mater Process Technol 2003;141:228–33. [19] Ciszewski G. New experimental concept for the fabrication of cast iron to cast iron and cast iron to steel joints by means of friction welding and the mechanical and plastic properties of these joints. Weld Cutting 2007;6:288–97.
[20] Sawada Y-K, Nakamura M. Lapped friction stir welding between ductile cast irons and stainless steels. Quart J Jpn Weld Soc 2009;27:176–82. [21] Bishel RA, Bielenberg JG, Dybas RJ, Johnson KL, Malizio AB. Cast irons. In: Welding handbook, vol. 4, 7th ed.; 1982. [22] Kumar RL. Welding grey iron with mild steel electrodes. Foundry; 1968. [23] Sun DQ, Gu XY, Liu WH, Xuan ZZ. Welding consumable research for austempered ductile iron (ADI). Mater Sci Eng A 2005;402:9–15. [24] Forrest RD. Welding ductile iron casting. Casting Engineering and Foundry World; 1983. [25] Pascual M, Cembrero J, Salas F, Martínez MP. Analysis of the weldability of ductile iron. Mater Lett 2008;62:1359–62. [26] El-Banna EM, Nageda M, Saddat M. Study of restoration by welding of pearlitic ductile cast iron. Mater Lett 2000;42:311–20. [27] Askeland DR, Birer N. Secondary graphite formation in tempered nodular cast iron weldments. Weld J 1979;58:337s–41s. [28] Askelnad DR, Hirota Y. The effect of base metal microstructure of impact properties of cast ductile iron weldments. AFS Trans 1979:73–9. [29] Cetinel H. Fracture behavior of overmatched ductile iron weldment. Int J Mater Res 2007;198:128–36. [30] Cetinel H, Uyulgan B, Aksoy T. The effect of yield strength mismatch on the fracture behavior of welded nodular cast iron. Mater Sci Eng A 2004;387– 389:357–60.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
On the weldability of grey cast iron using nickel based filler metal M. Pouranvari * Young Researchers Club, Islamic Azad University, Dezful Branch, Dezful, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2009 Accepted 6 February 2010 Available online 23 February 2010 Keywords: D. Welding F. Microstructure Grey cast iron
a b s t r a c t Shielded metal arc welding process using nickel based filler metal was used to join grey cast iron. The effect of post weld heat treatment (PWHT) on the microstructure and hardness was studied. PWHT included heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. By using nickel based filler metal, formation of hard brittle phase (e.g. carbides and martensite) in the fusion zone is prevented. Before PWHT, heat affected zone exhibited martensitic structure and partially melted zone exhibited white cast iron structure plus martensite. Applied PWHT resulted in the dissolution of martensite in heat affected zone and graphitization and in turn the reduction of partially melted zone hardness. Results showed that welding of grey cast iron with nickel based filler metal and applying PWHT can serve as a solution for cast iron welding problems. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cast iron is generally considered as a difficult material to be welded. This is basically due to two reasons: (i) inherent brittleness of the cast iron and (ii) the effect of weld thermal cycle on the metallurgical structure of the cast iron. Typically, four distinct regions are formed when cast iron is welded, as follows: (i) Fusion zone (FZ) which is melted during welding process and is resolidified upon cooling. (ii) Partially melted zone (PMZ) which is the area immediately outside the FZ where liquation can occur during welding. (iii) Heat affected zone (HAZ) which is not melted but undergoes microstructural changes. (iv) Base metal (BM) which its structure remains unaffected during weld thermal cycle. Fig. 1 shows relationship between Fe–C phase diagram and the temperature experienced by each microstructural zone [1]. High carbon content of the cast irons leads to formation of hard brittle phases, namely martensite and carbides in the FZ, the PMZ and the HAZ. Both carbide and martensite, being hard and brittle, are detrimental to the ductility, toughness and machineability of the weld and also may cause cracking in the joint [2–5]. Weldability of cast iron depends on the several factors including [4–11]: (i) type of the cast iron, (ii) chemical composition of the cast iron, (iii) chemical composition of filler metal, (iv) original * Address: Materials and Metallurgical Engineering Department, School of Engineering, Islamic Azad University, Dezful Branch, Dezful, Iran. Tel.: +98 91 24075960; fax: +98 21 88522421. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.02.034
matrix structure and (v) welding process and preheat/post heat treatment. Grey cast iron is inherently brittle and often cannot withstand stresses set up by a cooling weld. As the lack of ductility is caused by the coarse graphite flakes, the graphite clusters in malleable irons, and the nodular graphite in ductile cast irons, give significantly higher ductility which improves the weldability [12]. Ductile cast irons and malleable irons are less susceptible to form martensite in HAZ, therefore, they are more readily weldable, particularly if the ferrite content in their matrix is high. White cast iron which is very hard and contains iron carbides, is normally considered to be unweldable [12,13]. Arc welding processes and oxyacetylene welding are two most common welding processes which are used to cast iron welding [11,14,15]. However, the application of diffusion bonding, friction welding and electron beam welding are evaluated by some researchers [16–20]. There are generally three type available filler metals for welding cast irons: mild/low carbon steel filler metal, cast iron filler metal and nickel/nickel–iron based filler metal. Some researchers used mild steel electrode for welding grey cast iron. The main driving force for using mild steel electrodes is their low cost [21]. However, these electrodes suffer from some metallurgical problems including: (i) Steel shrinks more than grey cast iron during solidification; therefore, tensile stresses generated in FZ can make it susceptible to shrinkage cracking [13,14]. (ii) In spite of dilution of mild steel electrode with high carbon cast iron, the carbon content of FZ is sufficient to formation hard and brittle product in FZ. This reduces the impact
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2. Experimental procedure
Fig. 1. Temperatures experienced by various microstructural zones in a cast iron weld [1].
properties of the weldment. Moreover, inability of FZ to yield and relieve welding stresses can result in cracking in the adjacent cast iron heat affected zone. Therefore, the use of steel electrodes should be restricted to application where the joint is not loaded in tension or in bending. (iii) Due to high hardness in FZ and HAZ, preheating and post weld heat treatment (PWHT) is required. Preheating reduces cooling rate and therefore leave softer FZ and HAZ. The preheat temperature is usually in the range of 300–600 °C. Preheating cannot be used, however, when minimum heat is to be applied to avoid distortion of the parts being welded [22]. According to work of Kumar [22], a preheat temperature of 540 °C is required to significant reduction of FZ and HAZ hardness. However, for more improvement in machinability of welded cast iron a PWHT is also required. When a color match FZ is required the best choice is cast iron rods. Also when pressure tightness and uniform thermal expansion is required such as on cylinder heads or steam turbines or pump cases the oxyacetylene welding with a cast iron rod is preferred process [15]. In the case of cast iron filler metal, it is reported that the weld cracking due to formation of brittle phase in FZ is highly probable when preheat temperature is lower than 300 °C. Nickel based electrodes offer the highest crack resistance weld mainly because of their desirable mechanical properties and their ability to precipitates the carbon picked up from the base in its free form as graphite. Therefore, successful welding of cast iron requires more sophisticated understanding of interaction between the composition/microstructure of cast iron, filler metal composition and weld thermal cycle. This paper addresses the microstructure of grey cast iron welds in as-welded condition and after applying a post weld heat treatment (PWHT). In this work, preheat heat treatment was not performed. This was due to facts that a high preheat temperature as 540–600 °C is required to minimize the martensite formation and reduce the hardness in HAZ. In addition, it is reported that very high preheat temperature of 818 °C (1500 F) is required to reduce solidification cooling rate to a level such that iron carbide does not form in PMZ. However, such high preheat temperature can cause undesirable distortion of workpieces. Moreover, it is reported by Kotecki [11] that preheat temperature grater than 316 °C (600 F) increases the carbides size and their continuity in HAZ and PMZ. Therefore, in this research it was tried to (1) solve the problem of FZ by using a nickel based electrode and (2) solve the problems of HAZ and PMZ simultaneously by applying a PWHT.
A grey cast iron was used as the base metal in this study. The chemical composition of the base metal is: Fe–2.85C–1.75Si– 0.10P–0.82Mn–0.05S (wt.%). Shielded metal arc welding (SMAW) process using nickel electrode was used to join a grey cast iron. The chemical composition of the filler metal is: 98.5 Ni–0.71 C– 0.42 Si–0.37 Mn. A single V-shaped groove with an angle of 60° was considered as the preferred joint design. Welding parameters are given in Table 1. After welding, some specimens were immediately transferred to an electric furnace, kept there at 870 °C for 1 h and then furnace cooled to the room temperature. Cutting of the samples for metallographic examination was carried out while avoiding excessive heating that might have led to local alterations in the microstructure. Standard metallographic procedure was used for microstructural examinations. Nital 2% solution was used to reveal various microstructure constituents in the weldment. Vickers microhardness test using 200 g load was carried out to obtain average hardness of various microstructural zones in the weldment. 3. Results and discussion 3.1. Microstructure in the as-welded condition 3.1.1. FZ microstructure As can be seen in Fig. 2a, BM exhibits a ferritic matrix plus flake graphite phase. In FZ, BM is melted and mixed with filler metal. High cooling rates in this zone lead into very hard and brittle ledeburitic carbides in as-welded condition, if a Fe–C alloy filler metal is used [23]. To reduce the risk of the formation of brittle phase in FZ, nickel based filler metal is used to join cast irons. Fig. 2b demonstrates that FZ microstructure consists of mainly an austenitic matrix plus small amount of dispersed graphite particles. Nickel filler metal is able to precipitate carbon, picked up from the BM, in its free form as graphite. During welding the carbon of the BM is diluted with the filler metal in the FZ. The carbon content in FZ is well above the solubility limit of carbon in Ni (0.5% at eutectic temperature of Ni–C system). Therefore, the excess carbon is precipitated as graphite during FZ solidification. It should be noted that nickel carbide (Ni3C) is much less stable than iron carbide (Fe3C) and under most solidification conditions the carbon separates in the form of graphite [24]. Therefore, the use of nickel base filler metal has the following advantages: (i) Formation of hard and brittle micro-constituents in the FZ is prevented [25]. (ii) Precipitation of graphite in the FZ increases its volume. This can further reduce the weld metal shrinkage during solidification, which in turn, leads to the minimization of the residual stress in FZ and HAZ causing the reduction of the cracking susceptibility of the cast iron joint [8]. (iii) The high ductility of the nickel based FZ can play an important role in the absorption of tensile stress created during welding. This contributes to the minimization of the crack susceptibility of joint [4].
Table 1 Welding parameters used in this study. Current (A)
Polarity
Speed (mm/min)
Electrode diameter (mm)
120
DCEP
100
4
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Fig. 2. Microstructure of various regions in grey cast iron weld in as-welded condition: (a) base metal, (b) fusion zone, (c) fusion boundary, (d) partially melted zone, and (e) heat affected zone.
(iv) The nickel base FZ exhibits good machineability [8]. (v) The nickel base FZ has a grater tolerance for sulfur and phosphorus. This can increase the FZ resistance to hot cracking [4,8]. Consequently, by using nickel based electrodes, the welding problems are not in FZ but they are in PMZ and HAZ. Therefore, the impact properties of the cast iron welded joint depend on the amount and distribution of massive carbides and martensite in HAZ and PMZ. 3.1.2. PMZ microstructure Fig. 2c and d shows the PMZ microstructure indicating that this region consists of eutectic ledeburit and martensite. In PMZ, the portion of the matrix of the base metal near the pri-
mary graphite melted during the welding, while the remainder of the matrix transformed to austenite. The PMZ is enriched from carbon. Therefore, at the fast cooling rate typical of welding, coarse carbides solidify directly from the liquid as a result of the eutectic reaction (i.e. white cast iron structure is formed). These can form a continuous brittle network along the weld fusion line. The austenitic matrix surrounding these carbides can also transform to martensite at lower temperatures. Martensite is also hard and brittle particularly when its carbon content is high. The phase transformation sequences during the cooling of the PMZ can be summarized as:
L þ Graphite ) Eutectic ledeburit ðc þ Fe3 CÞ þ Austenite ðcÞ ) Ledeburit þ Martensite
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3.1.3. HAZ microstructure Fig. 2e shows that the microstructure contains of high amount of martensite plus some graphite in HAZ. During the weld thermal cycle, HAZ experiences a temperature higher than A1 (eutectoid temperature in Fe–C phase diagram). Therefore, at this high temperature, the graphite flakes begin to dissolve in the austenite producing a carbon gradient from the graphite flakes into the austenite and resulting in the gradation of transformation products within this region. The transformation rate of graphite to austenite depends on the diffusion rate of carbon. The amount of austenite formation depends on the following factors [10,11,19,26,27]: (i) Time and temperature experienced by each point at HAZ: the higher the temperature and the longer the time available for carbon to diffuse, the higher the volume fraction of martensite. (ii) The primary matrix microstructure of cast iron: typically, the rate of the austenite formation in a pearlitic structure is higher than the rate in a ferritic structure due to its lamellar structure (i.e. the diffusion distance for carbon atoms from iron carbide to ferrite is shorter). It is interesting to note that martensite start temperature (Ms) of ferritic cast iron is lower than pearlitic one due to its much lower carbon content. This is advantageous since it is known that the volume change associated with martensitic transformation is greater with lower Ms temperatures. Also the hardness of martensite is directly proportional to the carbon content, again favoring the ferritic grades [22].
(iii) The shape of the graphite phase: the flake graphite has higher tendency to dissolve than nodular graphite due to its higher surface to volume ratio. (iv) The volume fraction of graphite: higher amount of graphite favors the austenite formation. During fast cooling rate typical of welding, HAZ microstructure transforms to a brittle structure. Since, the cooling rate is high; the graphitization process cannot be accomplished. Moreover, the formed austenite is transformed into a hard brittle martensite due to the inherent high hardenability of this austenite.
3.2. Microstructure after PWHT PWHT is necessary in most cases in order to eliminate the massive carbides and martensite in HAZ and PMZ and thus reduce hardness and brittleness. Two most common PWHTs for cast iron weldments are subcritical tempering and full (ferritizing) annealing. Low temperature tempering can reduce hardness of martensite; however, higher tempering temperature is required to graphitize the eutectic carbides. Secondary graphitization reduces the brittleness of PMZ and HAZ and improves impact properties. However, excessive graphitization can reduce ductility of weldment [27,28]. Askeland and Birer [26] studied the secondary graphitization and they concluded that the full annealing PWHT rather than subcritical tempering gives better microstructure control and the excessive graphitization and formation of chain-like graphite is prevented. Therefore, in this study a full annealing
Fig. 3. Microstructure of various regions in grey cast iron weld after PWHT: (a) fusion zone, (b) fusion zone boundary, and (c) heat affected zone.
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PWHT was chosen including heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. Fig. 3 shows various microstructural zones in the weldment after PWHT. As can be seen, the microstructure of FZ is remained unchanged after PWHT thermal cycle. However, HAZ microstructure significantly is affected by PWHT. As can be seen in Fig. 3c, HAZ consists of graphite flakes in a ferritic matrix. Holding in 870 °C for 1 h provides sufficient driving force to dissolve of eutectic carbide and the martensite phases formed during the welding. During the slow furnace cooling, graphite is formed in a ferritic matrix but not Fe3C. Therefore, the applied PWHT can reduce the formation of brittle phases in HAZ.
3.3. Effect of PWHT on weld hardness Hardness variation across the weldment of the cast irons is one of the most important factors controlling the quality of cast iron welds. Strength/hardness mismatch between various microstructural regions significantly affects the fracture behavior of cast iron weldment [29,30]. Fig. 4. shows the hardness value corresponding to FZ, PMZ, HAZ and BM, before and after PWHT. The data points for hardness are average of five points in each zone. As can be seen in the as-welded condition, PMZ has the highest hardness value. This can be related to its brittle microstructure consisting of hard eutectic ledeburit and martensite. High hardness value of HAZ can be related to the presence of high amount of martensite in this region. The low value of hardness in FZ, comparable to BM hardness, is due to its austenitic structure. Both carbide and martensite formation in FZ can be prevented by using a nickel cast iron. Therefore, nickel filler metal can reduce the hardness of FZ in cast iron, as one of the big problems of cast iron welding. The soft nickel filler metal yields more easily during cooling than does the harder filler metals, lessening the strain on the HAZ, reducing the HAZ cracking. However, the use of nickel filler metal is incapable of altering the chemical composition of the HAZ and PMZ. Therefore, attempts to overcome the tendency to form white cast iron in PMZ and to form martensite in both HAZ and PMZ have directed toward the control of the weld thermal cycle. One of the ways to control the weld thermal cycle is through designing a proper PWHT. As can be seen in Fig. 4. HAZ and PMZ hardness significantly reduce after applied PWHT. FZ hardness remains unchanged after PWHT. It should be noted that the PWHT may cause reduction in strength of BM [19]. However, as can be seen the reduction of BM hardness after PWHT is not negligible. Therefore, it can be concluded that welding of grey cast iron using nickel electrode and designing a proper PWHT can serve as a solution for cast iron problems.
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4. Summary and conclusions Welding grey cast irons show serious problems in FZ, PMZ and HAZ, formation of hard and brittle product during solidification and post solidification eutectoid transformation. Formation of martensite and carbide in FZ can be controlled via controlling of cooling rate and the chemical composition of FZ which is in turn governed by BM and filler metal composition and degree of dilution. Results of the current study showed that by using nickel filler metal, the formation of brittle martensite and carbides in FZ is prevented. The FZ exhibited an austenitic structure plus some graphite nodules leaving a ductile and tough FZ which improve weld ductility and toughness. Moreover, tough austenite can absorb shrinkage and thermal stresses effectively reducing the risk of cracking. Therefore, by using nickel filler metal the problems of FZ are completely solved without need to controlling the dilution and the weld thermal cycle (i.e. without need to preheat or post heat treatment). It is of note that, the nickel filler metal has low coefficient of thermal expansion, therefore, it strains the cast iron HAZ much less than other filler metals, helping in reducing the risk of HAZ cracking. It was shown that HAZ microstructure of grey cast iron exhibited martensite. Also, PMZ microstructure exhibited hard eutectic carbide and martensite. To guard against this problem it is generally advisable to reducing cooling rate via preheating to prevent martensite and carbide formation or post heat treatment to decompose martensite and carbides to softer micro-constituents. Preheating enables to avoid the formation of martensite, however high preheat temperature which is sufficient for prevention of martensite formation induces other problems: distortion of the workpieces (in the case of local preheating) and formation of larger carbides with higher continuity which can impair the weld quality. Only, very slow cooling rates can prevent the eutectic–carbide formation during solidification in PMZ, the typical preheat temperatures often tend to increase the amount and continuity of the carbides rather than prevent their formation. Therefore, in this research preheating was not use. As an alternate solution, full annealing including heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling was used. This PWHT was successful in the dissolution of martensite in HAZ and graphitization in this zone. Also, this heat treatment was successful in the reduction of PMZ hardness. Moreover, unlike high temperature subcritical tempering, excessive secondary graphitization which can reduce weld ductility was not observed. Applied PWHT was successful in producing a weld with nearly uniform hardness profile. Therefore, according to the results presented in this paper, it can be concluded that welding of grey cast iron with a nickel filler metal coupled with applying a proper full annealing (ferritizing) PWHT can serve as solution for grey cast iron welding problems. References
Fig. 4. Average hardness value of various microstructure zones in as-welded condition and after PWHT.
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