Damage Evolution Process And Failure Mechanism Of Carbide Insert During Milling Water Chamber Head

Water chamber heads,which are key parts of nuclear evaporators,are made from high-strength 508 III steel.Workpiece blank is formed by solid forging with weight and large-scale working allowance.Supergiant size and working allowance of parts as well as unworkability of materials,constitutes extreme manufacturing: the main cutting mode is heavy-duty milling.Material unworkability is defined as follows: material removal rate is more than 70%.During heavy-duty milling of water-chamber heads,heavy-duty milling parameters cause serious failure and short service life of carbide inserts.Therefore,it is necessary to investigate damage evolution and failure mechanism of inserts during milling of water-chamber head.Moreover,qualitative/quantitative analyses of cutting performance and failure behavior of carbide insert must be conducted.This would provide technical and theoretical foundations for increasing cutting performance and manufacturing efficiency of inserts.In-door milling simulation experiments were designed according to on-site heavy-duty milling parameters of water-chamber head.By using resistance dynamo-meter and semi-artificial thermocouple as well as cutting simulation technology,thermo-mechanical load was measured on carbide insert.Irregular cutting area and special geometrical parameters of circular inserts were considered.A mathematical model of circular inserts was constructed with asymmetric cutting force.Based on cutting heat source and heat conduction theory,the contact surface temperature model of insert-chip was developed.By performing on-site milling tests of water-chamber head,we determined dominant failure mode of insert and related damage evolution.When water chamber heads were subjected to heavy-duty milling,carbide inserts underwent a significant thermo-mechanical loading.There was serious compression and friction between rake face,flank face,and workpiece material.This resulted in obvious crater wear in rake face and flank face wear.Wear mechanisms include abrasive wear,diffusion wear,and adhesive wear.Element diffusion is one of the major causes of insert diffusion wear and adhesive wear.Element diffusion concentration model was constructed by performing quantitative analysis on element diffusion of insert-workpiece.Element diffusion tests of cemented carbide and workpiece materials were conducted to validate the model.Based on wear rate model of dominant wear mechanisms,wear rate model of crater wear was developed.Experimental results indicate that insert wear can be predicted from morphologies and model parameters of insert crater wear.Insert fatigue and impact fracture morphologies were characterized to determine the damaged state of inserts.We investigated stress conditions and regularity of crack propagation when inserts were damaged.Our main purpose was to determine critical conditions and formation mechanism of damaged inserts.Analysis was conducted on two kinds of crack propagation theories: brittle fracture and Paris fatigue.Damage constitutive model of cemented carbide was constructed according to solid constitutive behavior and damage mechanics.Damage variables were selected and coupled in the model Material performance tests of cemented carbide were conducted under thermo-mechanical loading conditions.High-temperature mechanical properties of insert materials were investigated.Data was provided for performing qualitative and quantitative analysis of insert damage and failure behaviors.Fatigue is the evolution procedure of insert damage.Impact damage randomly takes place during this procedure,and it is the final failure mode of insert fatigue damage.Based on features of cyclic loading and damage mechanics,low-cycle and high-cycle fatigue damage models were developed and fatigue evolution process of insert material was described numerically.Based on the combination of high-temperature strength test and simulation,damage parameters of cemented carbide were determined in the model.Fatigue damage limit of insert was decided under cyclic loading by performing insert fatigue simulation.Based on classical strength theory,insert impact fracture criterion of cutting parameters was established.Theoretical and technical references were provided for optimizing cutting parameters and calculating insert strength.Insert impact resistance was increased by optimizing cutting-edge parameters.Different carbide inserts were used to conduct milling tests of water chamber heads.The influence of cutting parameters on insert failure was investigated.Suitable heavy-duty milling inserts were selected thereafter.