Study On Inclusion Control Technology Of GH5605 Centrifugal Cast Tube

GH5605 alloy,a Co Cr-based solid solution strengthened deformation alloy,is renowned for its exceptional corrosion resistance and biocompatibility.These qualities have established GH5605 as a predominant material for cardiovascular stents in a variety of clinical implantable biomaterials.The manufacturing of cardiovascular stents from this alloy involves a complex process,typically segmented into three main stages:tube preparation,tube blank rolling,and successive cold drawing.The quality of the stents is critically affected by the presence of inclusions in the tube billets.Conventionally,the GH5605 master alloy is produced using a vacuum induction melting method.To enhance purity,secondary processes such as electroslag remelting or vacuum consumable remelting are employed,althoug h these techniques have not yet achieved the inclusion control level of foreign competitors.Additional challenges in the production process include a low yield from hot piercing,susceptibility to cracking during rolling,and suboptimal cold drawing performance.Addressing these issues,this study proposes the adoption of vacuum continuous casting for the preparation of high-purity parent alloy bars,coupled with vacuum centrifugal casting for the direct formation of tubes.Moreover,the application of centrifugal force facilitates the movement and removal of inclusions,thus significantly elevating the purity of the seamless tube blanks.This paper concludes with process validation,as well as purity and performance assessments of the tubes produced using optimized centrifugal casting parameters.The principal findings and conclusions of this research are outlined as follows:This study addresses the issue of large-sized inclusions in continuous casting mother alloy bars.It employs Abaqus finite element software to analyze the stress field distribution near the inclusions,assesses the impact of typical non-metallic inclusions in GH5605 seamless pipes,and suggests the use of vacuum centrifugal casting technology to enhance control and removal of these inclusions.The findings indicate that for inclusions measuring 2μm,the stress at the pipe’s outer diameter is approximately double that of the middle section.Inclusions smaller than 2μm do not significantly influence stress distribution based on their type.However,inclusions measuring 30μm markedly increase the stress field within the matrix,while when the size of the inclusion is around 10μm,the maximum stress value increases rapidly with the increase of size.Consequently,to ensure successful rolling of seamless tube billets,it is advisable to maintain inclusion sizes below 10μm.Theoretical calculations and numerical simulations were utilized to investigate the movement patterns of inclusions during the centrifugal casting process of pipes and to predict their final positions.The dynamics of melt flow and heat transfer were modeled using the Navier-Stokes flow equation and the continuity equation within Procast software.The trajectory of inclusion particles was delineated via a particle trajectory model,facilitating the prediction of their ultimate resting places.As a result,a correlation was formulated between the precipitation separation time and variables such as inclusion size,billet wall thickness,casting temperature,and centrifugal speed.In the centrifugal casting process,the concentration area of inclusions is predicted based on the trajectories of their movement and their precipitation separation times.A relationship is established correlating various pouring temperatures（viscosity）,wall thicknesses,and rotational speeds to the extent of the inclusion concentration area.This relationship is subsequently utilized to refine the parameters of the centrifugal casting process.Moreover,leveraging the control of inclusions,further research is undertaken to optimize the microstructure of centrifugal cast pipes.By manipulating the temperature gradient of the mold,the proportion of the columnar crystal structure is enhanced,aiming to achieve superior mechanical properties.The findings suggest that with an increase in wall thickness,a rise in viscosity,and a reduction in centrifugal speed,the inclusion concentration area expands gradually.The ideal parameters for managing inclusions in GH5605 centrifugal cast pipes have been identified as a pouring temperature of 1580°C and a centrifugal speed of 2800 rpm.At a mold temperature of500°C,a uniform and refined columnar crystal structure is obtained.Under these conditions,the centrifugal tube billet maintains a wall thickness variation of 0.2 mm,ensuring a dense internal structure free from macroscopic cracks,shrinkage cavities,and other metallurgical defects.The inclusion content was assessed through microscopic examination to confirm the effectiveness of inclusion removal.The mechanical performance of the centrifugal tube billet was evaluated via tensile testing.The optimized GH5605 centrifugal tube billet underwent a series of rolling trials to examine the form,evolutionary behavior,and deformation mechanisms of non-metallic inclusions during the rolling process.The analyses reveal that over 95%of inclusions are smaller than 2.5μm,with Class D inclusions rated at 0.5 grade,and the prevalence of other inclusions attaining levels below 0.5 grade,surpassing the purity standards of imported pipes.The tube billet formed through centrifugal casting exhibits exemplary performance,with no issues of coarse inclusions impacting the surface quality.During the rolling process,irregular Si O₂-Al₂O₃-Cr₂O₃（Mn O+Ca O）inclusions in the cast pipe transform into spherical inclusions enriched with Al（Al₂O₃）,Si（Si O₂）,and Cr and Ca（Cr₂O₃,Mn O+Ca O）.This transformation is primarily attributed to the diffusion of elements within the oxide and the interfacial reactions between the oxide inclusions and the base alloy elements during intermediate annealing.The number density and area fraction of inclusions initially increase and then decrease as rolling progresses.The average size of inclusions in seamless pipes gradually diminishes,with the critical size threshold for inclusions that resist further breakage identified at approximately 0.5μm.