Improving the Mechanical Strength of Ductile Cast Iron Welded Joints Using Different Heat Treatments

The main advantage of welding cast iron is to recover parts by repairing defects induced by casting processes (porosities, etc.), before they enter their working cycle, as well as repair cracks or fractures when already in service. This method contributes to decreased foundry industrial waste and avoids the additional energy costs of their immediate recycling. Therefore, it is necessary to have a welded joint with similar or better characteristics than the parent material. The major problem of welding cast iron is that this material has a very high content of carbon in comparison to steel (≈3%). Therefore, when it is heated by the very high temperatures from arc welding and during its process of solidification, very hard and brittle phases originate, known as ledeburite and martensite, and appear in the partially melted zone and in the heat-affected zone. Eventually, this problem can be solved by implementing heat treatments such as preheat or post weld heat treatments under specific parameters. Therefore, in this study, the aim is to collect data about the effects of heat treatments performed at different temperatures on welded joints of high strength ductile cast iron (SiboDur ® 450), and to evaluate the effects of heat treatments performed at diverse temperatures on welded joints of this type of material, using Shield Metal Arc Welding and nickel electrodes. Mechanical strength, hardness, and microstructure were analyzed, showing that the best mechanical strength in the joint (380 MPa) was obtained using two passes of E C Ni-Cl (ISO EN 1071:2015) filler metal and post weld heat treatments (PWHT) of 400 °C for two hours.

1. Introduction

Cast iron is a class of Fe-C alloys with higher C content than steels and, depending on the alloying elements and the microstructure formed during solidification, it is classified as lamellar /grey cast iron, nodular/ductile cast irons or white cast iron. Some of these cast irons can still be subjected to specific heat treatments, which give rise to other classes of cast iron, such as malleable or austempered ductile iron (ADI) [1].

Ductile cast iron (DCI) is a cast iron type which presents properties as good as some kinds of steels, such as mechanical strength, fatigue and wear resistance, and machinability. DCIs are used in diverse applications, for example, internal combustion blocks, clutches, pistons, and automotive chassis parts [1,2].

In addition, DCIs have a great number of advantages as compared with other cast irons and common steels, such as obtaining complex shapes through casting and foundry processes and have lower specific weights allowing casting of thinner-walled parts while maintaining superior mechanical properties (ductility and fracture toughness) [3,4].

Most of the properties of DCIs emerge from their microstructure, which depends on several characteristics such as chemical composition, process parameters, and inoculation type used during their production [5,6,7]. Moreover, DCIs originate from the melting of a metallic material such as steel, and then, its weight defines its chemical composition, which will help in the other additive element choice. Inoculation is usually performed immediately before casting by adding Mg (with a residual content of 0.04%) or Ce [8] which gives rise to the nodularization of the graphite. Nodular graphite allows DCIs to have mechanical properties superior to other cast irons such as higher toughness and greater resistance to plastic deformation [8].

Commonly, DCIs have a carbon equivalent value (CEV) between 2.0% and 4.0%, and therefore the volume of the graphite precipitated during the solidification completely secures all the sections of the mold. Other relevant elements are Si (2% to 2.8%), Mn (0.1% and 1.0%), Cr (<0.08%) due to its carbonic nature, and Pb (<0.008%) as it deforms the spherical shape of the graphite [9]. It is also possible to detect small amounts of P and S [8], which are the main cause of problems during production and processing DCIs as they affect the material’s mechanical properties by promoting the appearance of prejudicial structures, such as carbides and/or irregular graphite [10,11,12,13].

Some elements of the chemical composition of DCIs influence their own weldability, for example, the Si content, since it is a constituent known for its high graphitization. This has the tendency to increase austenite/graphite eutectic temperature and to lower the eutectic austenite/cementite temperature. In order to avoid the formation of cementite, the Si content of DCIs must be as high as the specification allows it to be [14,15]. Other elements to consider are Mn and Ni, where the higher the content of these, the greater the difficulties in welding and the greater the need for preheating treatments [14]. It is also important to control the P content, since it contributes to the formation of eutectic Fe-Fe3P, steadite, in the grain boundaries, which contributes to cracks caused by contraction and cooling of the material. As this compound has a low melting point, this may cause hot cracking in the weld bead or in the heat-affected zone (HAZ) area [14,15]. The element S causes brittle fractures when there are high levels of Ni. A high content of O and S accelerate the precipitation of carbides which, consequently, can also cause cracking of the joint [14].

The practice of welding DCIs is not common, even in the metalworking industry, due to problems related to its chemical composition (high amount of carbon) [16], which promotes martensite and carbides formation in the HAZ and the partially melted zone (PMZ). Other problems are related to its low ductility which does not accommodate residual stresses in the welded joint, causing fractures. In addition, the high content of P, S, and O, can lead to pores in the welded joint. Other problems associated with welding DCIs are related to their own processing method, for example, defects in casting, such as sand inclusions and contraction. These defects do not let the total fusion between the parent material and the filler metal occur [14].

Despite these challenges, welding processes are mainly used as corrective maintenance and are applied to repairing casting defects in areas with no functional requirements [17,18]. The Shield Metal Arc welding (SMAW) process is one of the most exploited for welding DCIs [19] since it has easier maneuverability, easier access of filler metal (electrodes), and it has a low welding speed which can contribute to lower cooling rates.

There are generally three types of electrodes used for welding DCIs: cast iron filler metal, Ni, and Ni-Fe filler metal alloy [20]. Electrodes are considered to be pure Ni and may cause the carbon of the parent material to dissolve in fine graphite in the welded joint, making it ductile, and consequently, machinable. In addition, Ni suppresses the diffusion of carbon in the interface between the parent material and the filler metal, reducing the formation of carbides and other harmful structures with high hardness such as martensite in the HAZ and PMZ areas. This occurs because Ni has a low coefficient of heat release, which helps the weld joint to have a slow cooling rate, [14] as shown in the Pouranvari experiments [21]. The Ni-Fe electrodes were also studied and compared with pure Ni electrodes by Pascual et al. [16], and it was concluded that the high purity Ni electrode had better results than the Ni-Fe electrodes. Therefore, welds performed with Ni electrodes have been given greater importance [22,23] due to the microstructure achieved during welding processes [24].

Several studies have been accomplished in order to find the best results for mechanical properties of welded DCIs [16,19,22,25,26]. Heat treatments are a common way to surpass the usual challenges encountered when welding DCIs [16,25,27,28]. The preheating temperature depends on the chemical composition or CEV of the alloy, which conditions the hardenability of the DCI [20]. The hardness of the HAZ can be limited using preheating treatments, followed by a slow cooling rate after welding. Preheating reduces the cooling rate of the filler metal in the HAZ, which, consequently, decreases the amount of martensite and cementite, which create the hardness in these areas [29]. It also prevents cold cracking, reduces residual stress and distortion, and improves the fluidity of the material [14,16,20]. The minimum preheating temperature required to avoid the formation of martensite should be above the formation temperature of its own (Ms temperature), 230 °C. The definition of the preheating temperature depends on the thickness of the parts, as well as the thermal energy of the process [14]. The required temperatures for preheating are the following [14,15,16]: 250 °C and 400 °C, which are suitable for cast iron where the percentage of C is equal to or greater than 3%; 300 °C, where the transformation of austenite to fine perlite may occur; 425 °C, which can be used to prevent the formation of martensite, but it is not advisable, since it may give rise to cementite in the HAZ; 500 °C, the temperature range which forms a continuous net of cementite in the melting lines; 600 °C and 650 °C, which are suitable temperatures when there is a lot of heat dissipation in the workpiece, and finally, 760 °C, which is the maximum recommended temperature for preheating, since this temperatures is close to the critical temperature of cast iron (790 °C).

Post weld heat treatments (PWHT) can lower the hardness of the HAZ but do not restore ductility and toughness to its own original values, due to the formation of a fine dispersion of secondary graphite that accompanies the decomposition of martensite. This structure in the HAZ can be tempered to a structure with less hardness [14]. The PWHT is usually advisable for the following reasons [30,31]: It improves the ductility and the machinability of the HAZ and the joint, it decomposes the carbides formed during welding processes, it transforms martensite into a less brittle structure, and it relieves residual stresses formed during the welding cycles. It is also possible to produce a similar effect to the PWHT by performing further welding passes, relative to the former ones in the welded joint.

Manifold studies about the influences of heat treatments (preheating and PWHT) were carried out by Ebrahimnia et al. [19] and El-Banna [17,27]. These studies showed good results in terms of improvement of UTS (Ultimate Tensile Strength) values, by subjecting DCIs to several preheating temperatures such as 200 °C, 300 °C, and 400 °C. Askari-Paykani et al. [25], Mandal [26] and Connor [32], studied the influence of cooling rates, it was referred by [25] that when preheating was used for DCI welding, cooling conditions did not present a significant influence on the UTS value of the joined part. However, [26,32] believe that rapid cooling rates can increase the probability of martensite appearance in the HAZ.

This work intends to estimate the effects of heat treatments performed at different temperatures (in the range of 300 °C to 700 °C), through the implementation of several thermal cycles, namely, preheating, PWHT or both procedures, on welded joints of SiboDur® 450, by the SMAW process and using 98% Ni electrode. This work is an improvement from one previously elaborated [33] in this article.