| Spheroidal graphite cast iron is classified as the high strength material with comprehensive performances that are close to steel,but lower in production cost than steel,and has been used to manufacture some parts that require higher strength,toughness and wear resistance.Spheroidal graphite cast iron shows great prospects in oil and gas pipelines,transportation,agricultural machine,shipbuilding,ore mining and other engineering machinery fields.Along with the increasingly growth of awareness of energy conservation and environmental protection,more attention has been paid on developing and utilizing materials with lighter weight,higher efficiency and lower cost So the excellent properties of spheroidal graphite cast iron enable it to possess the huge developing potential,and it is expected to replace the use of gray cast iron,malleable cast iron.However,the poor weldability of spheroidal graphite cast iron becomes the very factor that limits its further promotion and application greatly.In this paper,joint of a ferritic spheroidal graphite cast iron was produced by gas shielding arc welding(GMAW)technique,using ER50-6 low alloy steel wire as filler materials,under the condition of preheating to 150 ?.Joint with good welding surface appearance was obtained,and research techniques including optical microscope(OM),scanning electron microscope(SEM),micro-hardness tester and universal tensile machine were deployed to investigate the joints' microstructure and mechanical properties,and the relationship among welding process parameters,microstructure,crack morphology and mechanical properties were further studied and analyzed.Single-sided welding and double-sided welding were applied to the welding of a ferritic spheroidal graphite cast iron.The result showed that,porosity was formed mainly in bead,and that happened to all joints.Most porosity was less than 0.5 mm in diameter,the study indicated that the speed of welding could exert great influence on the forming of porosity,and the number of porosity would be effectively reduced when the welding speed was slowed down appropriately.Macro-cracks could be seen in fusion zone of joints formed by double-sided welding,these cracks originated from the toe of weld and propagated along the fusion zone,the major reasons of the formation of cracks were the effects of residual stress resulting from great differences in microstructure,in addition,fusion zone was mainly composed by brittle ledeburite with poor ability of plastic formation.Effects of welding heat input on joints' microstructure and micro-hardness were studied.The results showed that brittle microstructure,mainly high-carbon martensite and residual austenite,was produced in the later bead of double-sided welding,and its micro-hardness ranged from 600HV500g to 650HV500g?When heat input was lowered,cooling speed in double-sided welding became higher,brittle phases could also emerge in heat-affected zone.For single sided welding,its bead was mostly composed by troostite,as heat input was increased from 8.6kJ/cm to 18.2kJ/cm,it could be seen that the amount of troostite increased and the hardness in bead was lowered from 508HV500g to 444HV500g.Ledeburite in fusion zone was insensitive to the change of heat input,the increase in heat input could widen the white cast iron zone,therefore heat input should be controlled reasonably when joints' toughness is concerned.Study on the mechanical properties of joints of spheroidal graphite cast iron was carried,the results indicated joints' tensile strength reached 262MPa,and fracture occurred in fusion zone.Then the tensile fracture morphology was observed with the help of scanning electron microscope(SEM),it could be seen that there was no dimple in fracture surface,and cracks expanded along fusion zone,further analysis suggested the fracture was cleavage fracture characterized by river pattern. |
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