Researches On Key Problems Of The Plasma Arc Butt Welding Of Pure Nickel Plates

The level of processing on nickel and nickel alloy plates in our country is overall backward,only having the production of single plate and there is no ability to make products with wide and large volume.Therefore,the comprehensive improvement of the processing level and the acceleration of structural adjustment and technological progress are very urgent.Although the production line,with the yield of 5000 tons/year,built in the efforts of all parties fills the gap in the aspect of equipment,it is still unable to produce the products with wide and large volume that meet requirements due to the limit of the manufacture technology and welding difficulties of the pure nickel.Now,the main problems are as follows: there is no special welding equipment following the production line of the nickel and nickel alloys plates as well as mature butt welding process;the poor weldability of the pure nickel with the welds being prone to defects during the welding process results in the poor strength of the joints,high fracture belt rate,and influence on the production efficiency and quality.In other words,the butt welding of the nickel and nickel alloys plates following its production line,with the guarantee of production efficiency relating to cold rolling,acid pickling and other subsequent processes,is the most critical process for the yield of the nickel and nickel alloys plates and the welding quality directly determines the quality of products with wide and large volume.As already described above,this paper is aimed at welding methods and processes of pure nickel N6,microstructure and properties of welded joints,thermo-mechanical treatment with roller compact of the weld and the mechanism of fracture belt to carry out a series of researches to propose solutions which could completely solve above problems following production lineFirst,this paper discusses the formation mechanism of defects on welding pure nickel by PAW.The formation of solidification cracking is based on the Borland stretching liquid membrane theory.Due to the pure nickel having high thermal conductivity,the solid-liquid phase temperature range being small and the liquid metal viscosity being high with poor liquidity,the escape velocity of bubbles in the small molten pool was less than the crystallization rate,resulting in forming porosity in the joints.The weldability of the pure nickel and the characteristics of porosity,which consisted of hydrogen porosity,carbon monoxide porosity,and nitrogen porosity were studied.The grain coarsening appearing in the weld and near weld zones was related to the chemical compositions of parent metal,welding methods and heat input.As the heat input was increased the grains grew up simultaneously.There were no the elements forming carbonization and nitrides in the pure nickel N6 which leaded to generate less second phase particles during the welding process.So there were almost no second phase particles to hinder the growth of austenite grain and grain boundary can move freely to equilibrium grain size.A majority of particles of carbonization and nitridesduring the PAW process were dissolved.So the pinning effect generated by those above particles on grain boundary migration was reduced.In a word,the poor appearance of weld caused by defects weakened the mechanical properties of the joints and further had influence on final production yield.Secondly,based on the weld defects during PAW process,the PAW with filler wire was proposed and utilized for joining pure nickel N6 plates.The mechanism of the substitution of the PAW with filler wire for PAW resulting in eliminating defects in the weld was studied and elaborated by the analysis of the microstructure and properties of the joints.The weld of 6mm thick pure nickel N6 plates with one-side welding with back formation was realized by PAW with filler wire.The weld formation was good and there were no defects such as porosity,cracks,undercut and an excessive penetration into the parent metal and so on.PAW with filler wire offered greater control on the process with widening the range of the specification parameters.No matter which welding methods you choose,the matrix grains at the joints consisted of austenite,which were the same as the parent metal.The tensile strength,elongation percentage and impact energy of the joints were increased by 11.75%,84.8% and31.7%,respectively.Whereas,the grains using PAW with filler wire were finer with superior anticorrosion property in various corrosion solutions.Thirdly,aiming at the large residual stress and grain coarsening in the joints welded by PAW with filler wire the thermo-mechanical treatment with roller compact of the weld were performed to get a better understanding of the influence on the microstructure and properties involved in this process.Results demonstrated that the joint tensile strength gradually increased and the elongation percentage showed the trend of increasing firstly and then decreasing with heat treatment temperature rising.So the reasonable process of thermo-mechanical treatment with roller compact could reduce the residual stress with crushing long coarse dendrites to refine grain size.In this experiment the optimum process parameters were as follows: the total grinding pressure was 21368.97 N under the rolling temperature 400?;the rolling speed was 3mm/s;the rolling times was only one with the rang of the reduction rate being 20% to 25%.The process of thermo-mechanical treatment with roller compact offered a very positive effect on optimizing the microstructure and properties of the joints with obtaining good coordination between high strength and high plasticity,especially for improving the plasticity.Finally,considering the problems of the poor strength of the welded seam and high fracture rate often appeared with welding the pure nickel N6 produced by vacuum melting process and we conducted some researches to elaborate the reason of high fracture belt rate related to the melting process.That was different production processes leading to differences in the same chemical composition.The quantity and size of non-metallic inclusions within pure nickel produced by vacuum melting process were higher than pure nickel by vacuumsmelting process.The basic reason for the reduction of the mechanical properties and the process performance induced by the non-metallic inclusions was that mechanical bonding between the brittle invariant inclusions and pure nickel was simple with weak bonding strength.There were some property differences between the brittle invariant inclusions and pure nickel such as elasticity,plasticity and linear expansion coefficient resulting in the different deformation degree during their deformation process.The matrix internal stress was redistributed to generate stress concentration,which was induced by the non-metallic inclusions with resulting in voids or cracks in the interface of between non metallic inclusion and the matrix.When the loading was continuously added with the crack continuous expansion the failure loads of joints was lower than the normal tensile strength owing to the existence of the non-metallic inclusions.Simultaneously,the pure nickel plates N6 produced by vacuum melting process should be fully used in the production line.