Composite Toughening And Microstructure Of Cold Stamping Die

In the course of the use,cold stamping die is subject to the mechanical of compression,shock and friction.Its life is low and the failure performance of it is usually the form of wear,deformation,fracture,etc.So the die material that always choosed high-alloy steel has a complex heat treatment process.To address this issue,the paper analyses the working conditions of cold stamping die.It points out that the main performance requirement should be good enough abrasion resistance and the strength and toughness,etc.So the boronizing process of T12 steel and GCr15 steel is researched,and the effect of carbide ball of conventional ball annealing,recycling the ball annealing,troostite treatment and the four-step heat treatment process is researched,and composite strengthen process,organization and performance of the boronizing + four-step heat treatment was further study, and the mechanism of the boriding and the toughening of steel was analysised. According to analysis and discussion of test results,received the following conclusions:(1)When the boriding temperature is 850℃,boronizing time gradually increase from 1h to 5h,the layer thickness of T12 steel gradually increase from 40μm to 93μm and the layer thickness of GCr15 steel gradually increase from 20μm to 53μm.With the extension of boriding time,the layer thickness also increase, and its increase rate gradually slows down.At the same time,the layer thickness of T12 steel is always higher than GCr15's.When boronizing time is 5h,boriding temperature gradually increase from 750℃to 950℃,the layer thickness of T12 steel gradually increase from 50μm to 163μm and the layer thickness of GCr15 steel gradually increase from 32μm to 83μm.With the extension of boriding temperature,the layer thickness also increase,and its increase rate gradually speed up.(2)When boronizing time is 5h,boriding temperature gradually increase from 750℃to 900℃,the layer hardness of T12 steel gradually increase from 1155 HV_0.1 to 1216 HV_0.1,and when boriding temperature is 950℃,the layer hardness of T12 steel is 1177 HV_0.1;the layer hardness of GCr15 steel gradually increase from 1147 HV_0.1to 1219 HV_0.1,and when boriding temperature is 950℃,the layer hardness of GCr15 steel is 1163 HV_0.1When the boriding temperature is 850℃,boronizing time gradually increase from 1h to 5h,the hardness of T12 is around 1178HV_0.1～1205HV_0.1,the hardness of GCr15 is around 1178HV_0.1～1205HV_0.1,it indict that boronizing time is less affectedt to the surface hardness of T12 steel and GCr15 steel.(3)After T12 was borided,the surface hardness of T12 is 835HV_0.1,and the sub-surface hardness is 1220HV_0.1,with the layer depth increase,the hardness gradually reduce to matrix-hardness;with the layer depth increase,the hardness of GCr15 also gradually reduce to matrix-hardness,the surface hardness of GCr15 is 835HV_0.1,and the sub-surface hardness is 1220HV_0.1.(4)The junction of the boriding layer and the transition zone is very flat,the contact of the borides and transition zone is few,that is to say the thickness of boride needle inserted into the transitional zone is very shallow(Specially GCr15 steel),so the binding force of boriding layer and metal matrix is weaken.This will lead to the boriding layer peeling off easily.But there is a concentration gradient of carbon and boron in the transition zone,so lead to the transition zone is gradually excessive to matrix organization.So there is not a clear dividing line between the boriding layer and the transition zone,this make up for these deficiencies,the borides not falling off so easy.(5)The best boronizing process of T12 steel is boriding at 900℃for 5h,layer thickness can be reached 116μm,hardness can be achieved 1216HV_0.1.The best boronizing process of GCr15 steel is boriding at 900℃for 5h,layer thickness can be reached 67μm,hardness can be achieved 1219HV_0.1.This thickness and hardness can meet the requirements.(6)A good effect of micro-carbide is got after T12 steel is carried out carbide micro-technologyl(950℃solution + 700℃high temperature tempering),the carbide is small,round,evenly distributed;a worse effect of micro-carbide is got after T12 steel is carried out carbide micro-technology 2(950℃solution +760℃ insulation two hours + 690℃insulation 3 hours),among them,some cusp carbide existence:a worst effect of micro-carbide is got after T12 steel is carried out conventional ball annealing,among them,many large and cusp carbide existence. The quenching and tempering process of four-step heat treatment,troostite heat treatment and conventional heat treatment is 760℃quenching +150℃tempering 60 min.Tempered martensite + carbide can be get.Among them,the carbide of four-step heat treatment is most small,round,evenly distributed.But some cusp carbide is got after T12 steel is carried out troostite heat treatment;many cusp, large carbide is got after T12 steel is carried out conventional heat treatment. Therefore,the strength and toughness after T12 steel carried out four-step heat treatment is better than troostite heat treatment,even better than conventional heat treatment.(7)A good effect of micro-carbide is got after GCr15 steel is carried out carbide micro-technology(950℃solution + 700℃high temperature tempering), The carbide is small,round,evenly distributed;a worse effect of micro-carbide is got after GCr15 steel is carried out annealing cycle ball,among them,some cusp carbide existence;a worst effect of micro-carbide is got after GCr15 steel is carried out conventional ball annealing,among them,many large and cusp carbide existence.The quenching and tempering process of four-step heat treatment, quenching and tempering after annealing cycle ball and conventional heat treatment is 840℃quenching +150℃tempering 60 min.Tempered martensite + carbide can be get.Among them,the carbide of four-step heat treatmentis most small,round, evenly distributed.But some cusp the carbide is got after GCr15 steel is carried out annealing cycle ball + conventional quenching and tempering,many cusp,large carbide is got after GCr15 steel is carried out conventional heat treatment. Therefore,the strength and toughness after GCr15 steel carried out four-step heat treatment is better than annealing cycle ball + conventional quenching and tempering,even better than conventional heat treatment.(8)T12 steel is carried out compound strengthen heat treatment has the following characteristics:Boriding thickness can reached 120μm,microhardness around in 1235 HV_0.1;the heart organization is tempering martensite and small, round,uniform distribution carbide,and the heart hardness is 61.5 HRC;Thus this is able to meet the small and medium-sized cold stamping die on the strength, toughness and wear-resistance requirements.(9)T12 steel's Acm is 820℃,when it is borided at 900℃for 5h,carbide decomposition and entered into the austenitic,the carbon atoms proliferate uniform and achieve the result of uniform solution after 5h proliferate,Therefore,the solution temperature is proposed dropped from 950℃to 900℃around. (10)GCr15 is steel carried out compound strengthen heat treatment has the following characteristics:Boriding thickness can reach 72μm,microhardness around in 1248HV_0.1;The heart organization is tempering martensite and small, round,uniform distribution carbide,and the heart hardness is 62.5 HRC;Thus this is able to meet the small and medium-sized cold stamping die on the strength, toughness and wear-resistance requirements.(11)There is a little hollow in the boride layer after GCr15 steel is carried out compound strengthen heat treatment,and there is no spalling phenomenon,boride layer transits relatively smooth.Not only solution can be achieved,but also overcome the phenomenon of layer "hollow",when solution temperature dropps from 1050℃to around 980℃.(12)T12 steel and GCr15 steel are be carried out compound strengthen treatment have good wear resistance and strength and toughness,it can meet the basic performance requirements when cold stamping die is used.In the actual selection the material of small and medium-sized die,it need to pay attention to the following questions:The hardenability and anti-tempering of GCr15 steel is superior than T12 steel;Cold stamping die which size is smaller than 30 mm and subject to smaller force,due to subject to relatively small pressure,don't need high hardenability,so option T12 steel to make the die,which have a 900℃boronize + four-step heat treatment compound-toughen treatment can be get good effect.Cold stamping die which size is larger than 45 mm not smaller than 30mm and slightly bigger force than the small cold stamping die,relatively slightly bigger pressure, need slightly higher hardenability,so optionl GCr15 steel to make the die,which have a 900℃boronize + four-step heat treatment compound-toughen treatment can be get good effect.