LIANG Gaofei,YAN Yuanyuan,LUO Xiaoliang,LIN Changqing,FU Yan’an and SHAO Zongqiang

1)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China;2)Manufacturing Management Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China;3)Customer & Product Service Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

Causes of progressive stamping die damage and retooling measures to reduce carbide punch fracture

LIANG Gaofei1),YAN Yuanyuan1),LUO Xiaoliang1),LIN Changqing2),FU Yan’an3)and SHAO Zongqiang3)

1)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China;2)Manufacturing Management Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China;3)Customer & Product Service Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

We investigated the reasons for frequently appearing early failure of carbide punches in a progressive stamping die when they were used to produce magnetic shielding steel parts.The failure modes of the carbide punches were statistically classified.Edge chipping and shoulder fracturing were the two types of failure modes that were found.The composition,hardness,and microstructure of the punches were analyzed.The cobalt content was found to be too low to ensure sufficient toughness.The brittle phase of tungsten carbide(WC)was prone to breaking off during the shear cutting process,and resulted in edge chipping.In addition,the material chips were easily bonded to the punch edge when there was a lack of press oil lubricity.It resulted in an uneven blanking clearance,which caused a lateral force and finally resulted in shoulder fracturing of the punches.Improvement measures were introduced to address the abovementioned issues.By using the carbide punches with greater toughness and shear cutting oil with high lubricity,the early damage problems of the punches are effectively solved,which ensures normal production of the magnetic shielding steel parts.

progressive die; shear cutting; failure; fracture

1 Introduction

A progressive stamping die is a type of complex,precise shear cutting die having high efficiency,high precision,and long service life.It is widely used in the production of precise or small electronic components.The high-speed,continuous,and extended working conditions of the progressive die require extremely high strength and wear resistance of the mold[1].WC-Co cemented carbide is a type of composite material that is produced by a powder metallurgy method with two basic components of the refractory metal carbides and the binding metal cobalt(Co).As Co has good lubricity and cohesion with hard phase WC,the solubility of WC in Co is also high.The combined effect makes the strength of WC-Co composite higher than that of any single component.With the qualities of high strength,high hardness,and high wear resistance,WC-Co carbide is an ideal material and is widely used in a progressive die[2].

An electronic products company uses a progressive stamping die to produce magnetic shielding steel parts,and the carbide punches frequently fracture at different locations.According to statistics from their production record,the average life of the punches is found to be approximately 10 000 parts.The pro-duction must then stop because the die needs to be repaired and adjusted.This results in lower efficiency and a higher cost of production.As a result,the magnetic shielding parts cannot be batch produced.Therefore,the causes of early failure of the punches are analyzed and the corresponding solutions are presented.By using the carbide punches with greater toughness and shear cutting oil with high lubricity,the service life of the die is increased to more than 800 000 parts and it effectively relieves the pressure of the magnetic shielding parts production.

2 Materials and methods

First,an investigation is conducted at the production site,including the properties of the steel sheet,punch material,shear cutting oil,and any other information related to the shear cutting process.Second,a statistical classification is made according to the failure mechanisms of the punches.Next,the microstructure,hardness,surface morphology,and crack and fracture morphology of the punches are observed by scanning electron microscopy(SEM).Finally,the causes of the carbide punch failure in the progressive stamping die are comprehensively analyzed based on the analysis results obtained.The corresponding improvement measures are presented and the effects are verified.

3 Results and discussion

3.1 Production site investigation

To understand the service conditions of the carbide punches in the progressive stamping die,an investigation is conducted at the production site.The 1.8 mm steel sheet(Table 1)is shear cut.The punches made from WC-Co cemented carbide material are produced using an electrical discharge machine.The shear cutting oil is a type of colorless transparent volatile oil that is chosen by the electronics company.According to the production record of the recent 100 000 parts,statistical analysis is used to get some information about the frequency and locations of the punch failures,as shown in Fig.1.The punch fractures occur 10 times(1#positions break simultaneously:1 time;2#position:4 times;3#position:2 times;4#position:3 times),and there is no obvious reason for the fracture position.Edge chipping and shoulder fracturing are the two types of failure modes.

Table 1 Mechanical properties of the steel sheets

3.2 Edge chipping analysis

Fig.2 shows the surface morphology observed on the chipped edge of the punches in the progressive die.SEM results show that many small pits exist at the cutting edge of a punch.The most important evaluation criteria for the performance of WC-Co carbide are the hardness and the fracture toughness.The determination of hardness is based on the resistance to micro-plastic deformation,and the hardness reflects the wear resistance of cemented carbide.In general,the hardness increases with the decrease of Co content and the decrease of WC crystal size,whereas the change in fracture toughness and hardness tend to be opposite[3].The chemical composition(Table 2)and physical properties(Table 3)of the failed punches are analyzed.The Co content of the material is 11.2%,the HRA hardness is 89.2,and the bending strength is 3 680 MPa.From Fig.3,the binding phase is seen to distribute uniformly and no locally absent Co or abnormally coarse WC particles exist.After eliminating the reasons for microstructural defects,the crescent-like chippings appear to be caused by an improper selection of the WC-Co material grades.It is advised to enhance the Co content and properly lower the hardness of the material,thereby enhancing the fracture toughness of the punches and avoiding edge chipping.

%

Table 3 Physical properties of the failed punches

3.3 Shoulder fracture analysis

Fig.4 is a typical photograph of a shoulder fracture for a failed punch.No crescent-like chipping is found at the cutting edge of the punch;however,there are intermittent adhesive substances within the area of 2 mm from the edge,as shown in Fig.5.The adhesives are not attached by oil because they cannot be removed with acetone or by ultrasonic cleaning.They are tightly adhered to the surface of the punch and can only be removed by a fine oil stone.The 1#and 2#regions are magnified and EDX is used to get the elemental composition of the adhesives.Fe,C,O,and W are detected onthe surface.W and C are the intrinsic elements of the cemented carbide.Fe and O are derived from the steel sheets that are shear cut,indicating that adhesion and transfer may occur between the WC-Co carbide and the workpiece material.

Fig.4 Location of the failure site of the shoulder fracture on the punch

During the progressive die shear cutting process,the thickness of the steel sheet is 1.8 mm,and the blanking clearance is only 10% of the sheet thickness.The continually gathered chips gradually reduce the blanking clearance between the punch and the die.This results in uneven blanking clearance,introduces a lateral force,and finally results in shoulder fractures of the punches.To provide an effective solution to this problem,it is necessary to find the cause of this phenomenon.

Shear cutting is the process by which the mold exerts pressure on the steel sheet to force it to separate or deform plastically.In this process,the newly ruptured surface is so clean that surface energy is rather high.Direct contact with the mold results in high pressure and high temperature,causing electron migration of the interfacial atoms.The binding enrygy can be as high as that of the metal atoms,resulting in a strong local adhesion.When the binding strength of the bonding part is higher than the inner strength of any side,chipping is likely to occur on the side with weaker inner strength while the adhesion occurs on the surface of the side with higher inner strength.In this case,the hardness of the steel sheet is much lower than the cemented carbide[4-5].The steel chips easily bond with the punch surface in the shear cutting process,gradually accumulate,generate a lateral force,and finally cause the punch shoulder to fracture.

To reduce the friction between the two relative surfaces,shear cutting oil is introduced to separate the two side materials.In doing this,the high surface energy in direct contact of the two side materials is replaced by the low surface energy of oil,and reduces or even avoids the effect of adhesive wear.Selecting the right shear cutting oil is extremely critical.During the shear cutting process,the oil arrives at the friction surface through the initial surface contact of the two materials.The thickness of the oil filmHcan be approximately calculated by the following formula:

(1)

wherev0is the relative velocity of the mold,v1is the moving speed of the material to be machined,αis the half angle of the die,P0is the inlet pressure of the oil,P1is the initial oil pressure,andηis the viscosity of the lubricating oil.Thus,increased oil viscosity increases the oil film thickness in the punching process.

3.4 Improvement proposals

Through the above analysis,the following improvement proposals are given:using the carbide punches with higher toughness and using the shear cutting oil with high lubricity.

3.4.1 Using carbide punches with greater toughness

The chemical composition of the new WC-Co punch is shown in Table 4.The Co content of the new punch is slightly improved compared with the original punch material in Table 2.The physical properties of the new punch are shown in Table 5.Compared with the failed punch materials in Table 3,the hardness of the new punch is slightly reduced,whereas the bending strength is increased by approximately 120 MPa.These properties easily alleviate or even avoid the occurrence of edge chipping.

Table 4 Chemical composition of the new punches %

Table 5 Physical properties of the new punches

3.4.2 Using shear cutting oil with high lubricity

The investigation finds that the shear cutting oil used by the company is a colorless transparent volatile cutting oil with a kinematic viscosity of 3 cSt at 40 ℃.Based on the understanding of the role of cutting oil,a new type of oil with a kinematic viscosity of 10 cSt is introduced to obtain good lubrication and appropriate strength of the oil film.These measures are used to avoid adhesives on the punches,and used as a solution to the shoulder fracturing problem.

3.5 Evaluation of the improvement effect

After introducing the above improvements,the problems of edge chipping and shoulder fracturing are effectively solved.The service life of the die is increased to more than 800 000 parts,which effectively relieves the pressure of the magnetic shielding parts production,and significant economic benefits are achieved.

4 Conclusions

(1) The cobalt content of the failed punches was too low to ensure sufficient toughness,and the brittle phase of WC was prone to breaking off during the shear cutting process,which resulted in edge chipping.The lack of press oil lubricity resulted in the uneven blanking clearance,introduced a lateral force,and finally resulted in shoulder fractures of the punches.

(2) The early damage problems of the punches are effectively solved by using carbide punches with greater toughness and shear cutting oil with high lubricity.The service life of the die is increased to more than 800 000 parts,which ensures normal production of the magnetic shielding steel parts.

[1] HUANG Z Y,LIU W J,ZHOU C,etal.Wear of carbide alloy progressive die in high speed stamping[J].Die & Mould Industry,2007,33(1):71-73.

[2] ZHANG W B,LIU X Z,CHEN Z H,etal.Latest development of WC-Co cemented carbide[J].Chinese Journal of Rare Metals,2015(2):178-184.

[3] HUANG Z Y,GUO L,ZHOU C,etal.WC-Co fracture failure investigation of WC-Co cemented carbides precision progressive die[J].Electromachining & Mould,2007(3):34-37.

[4] ZHANG X.The present station and development trend of the stamping oil used in the car[J].Automobile Technology & Material,2012(7):52-56.

[5] WANG H C,DONG C S and LIU Y.Lubrication mechanism in metal plastic working[J].Die & Mould Industry,1998(1):36-40.

LIANG Gaofei; E-mail: lianggf@baosteel.com

10.3969/j.issn.1674-3458.2016.04.005