Study of the Heat-Treatments Effect on High Strength Ductile Cast Iron Welded Joints

Abstract

Nowadays, ultimate tensile strengths above 400 MPa become usual for ductile irons, thus allowing the implementation of new design paradigms. Large concentrations of carbon and other influencing elements can negatively interfere with the welding process of ductile cast irons. Efforts made by researchers have led to the discovery of solutions which enable good enough operational results to consider welding as a viable repairing or joining method. Although these alloys have been available for quite some time, researchers have mainly focused on issues relating to microstructural phenomena tied to casting and similar processes, leaving much to explore in terms of their weldability. Thus, this work intends to investigate the effect of different heat-treatments on the weldability of a high strength ductile cast iron by assessing the mechanical properties of welded joints as well as structural modifications induced by thermal cycles imposed before and after welding. Successful weld joints were achieved showing a clear heat affected zone (HAZ) close to the joint area. This area is coincident with the fracture area of the welded samples when loaded on a tensile test bench, having obtained promising results regarding mechanical strength and strain. The hardness of the welded zone was also carefully investigated, showing clear changes throughout the joint.

Comparing the Structure and Mechanical Properties of Welds on Ductile Cast Iron (700 MPa) under Different Heat Treatment Conditions

“…A similar effect is produced when performing further welding passes relative to the former ones. In a recent study, authors found that when welding a similar high strength DCI to the one discussed in this work, a pre-heating stage of 300 • C for 2 h along with multi-pass welds promoted the best outcome in terms of mechanical properties [24]. Buttering was used successfully in the preparation of the SMAW process, conferring a higher ductility to the weldment [19].…”

“…The temperatures profiles were chosen based on the work developed in [20,24], consisting of either 300 • C or 700 • C heat cycles for pre-heating treatments and either 700 • C or 800 • C for PWHT. A map of the sequence/parameters used for each group of samples can be seen in Table 2.…”

“…This process was selected following the same line of thought discussed in [24]. The equipment utilized was an OERLIKON welder, model SAXOTIG 1600 (OERLIKON, Pfäffikon, Switzerland).…”

“…All previously-mentioned tests followed the same methodology, equipment and standards discussed in [24]. In particular, Table 3 shows the different international standards used for each test method.…”

“…Moreover, the same authors concluded that Ni alloys provide the best results in terms of mechanical properties, with stainless steel being a cheaper alternative as FM, ensuring, as well, satisfactory results. Effectively, using Ni alloys as FM when welding DCIs avoids carbide formation, contributing in this way to an increase in ductility in the joint, mainly when used together with a preheating treatment [24]. However, many years ago, Kelly et al [29] studied the suitability of Ni-Fe-Mn alloys as FM for DCI welding, concluding that these alloys may be able to be used without the necessity of preheating or PWHT processes, maintaining the properties of the parent metal in the joint, stating that the presence of some martensite and carbides in the HAZ can help tensile properties on welded DCI, concluding in this way that the microstructure of the HAZ is not enough by itself to characterize the performance of a joint.…”