Experimental study on improving the service life o

Experimental study on improving the service life of die-casting punches

Abstract: starting with the analysis of the wear mechanism of punches, 45 medium carbon steel is used. After boronizing, the surface hardness and toughness are improved, and its service life can be increased by times on the original basis

the working conditions of die-casting punches are very poor and their service life is very low. A few factories use heat-resistant alloy steel punches, and their service life is only more than ten shifts. The service life of using nodular cast iron as punch material is lower. The economic losses caused by shutdown, taking a medium-sized die-casting unit as an example, amount to tens of thousands of yuan per year, which has not been included in losses such as material, working hours and power consumption. The main purpose of this research is to change the original punch material, abandon those ductile iron whose performance is not stable and it is difficult to supply in time, and use No. 45 medium carbon steel material with rich sources, guaranteed quality, reasonable price and unified specifications. Through surface strengthening treatment, the surface hardness and toughness are improved, and its service life is doubled on the original basis, so as to achieve good economic and social benefits

1 mechanism analysis of punch wear

when the surfaces of two materials slide relatively, adhesion will appear on the contact surface due to the suction between the atoms on the surface of the material. When the two surfaces are close together and separate along the normal direction or move along the tangential direction, the atomic suction tends to pull one surface material towards the other surface to form wear debris. The size of its wear amount can be expressed by J. the pressure is also large. F. Archard formula: the wear amount (wear volume V) is directly proportional to the load L and distance x, and inversely proportional to the wear surface hardness P, that is:

v=klx/3P (1)

where: K is the friction coefficient. K is related to lubrication conditions and matching friction metal

in addition, there is an inverse square relationship between the wear volume and the different hardness of the friction materials. For example, let vs and PS represent the wear volume and surface hardness of the softer material in the friction pair respectively; VH and pH represent the wear volume and surface hardness of harder materials, and the relationship between them can be expressed in the following formula

(vs/VH) 2 = (pH/PS) 2 (2)

from the friction coefficient K and formula (2): in order to reduce adhesive wear, attention should be paid to when selecting materials: 1) it is required that hard materials should be selected for the surface to reduce wear; 2) Pairing materials with small or incompatible interaction between surfaces shall be selected as friction pairs

therefore, the focus of our research work is: in order to reduce adhesive wear, we can solve it by improving the hardness, changing the type of matching materials and using lubricants

2 test conditions

2.1 basis for selecting workpiece materials for test

the workpiece (punch) adopts 45 medium carbon steel, which belongs to quenched and tempered steel and is most suitable for boronizing treatment. The sawtooth boride can be obtained on the microstructure surface after boronizing, which is firmly combined with the matrix and has good wear resistance. In addition, the strength of the matrix can be improved by quenching and tempering after boronizing. The quenching temperature of 45 medium carbon steel is in the range of 840^860 ℃, which is close to the existing boronizing temperature, so that the workpiece after boronizing can be directly quenched, reducing heating times, saving energy and reducing costs

2.2 penetrant material used in the test

solid powder penetrant is used in this test, which has the advantages of simple and convenient operation, no need to add special equipment, easy separation from the workpiece after infiltration, and easy promotion and application in production

the main components of powder penetrating agent include boron carbide (B4C). In order to provide the main source of boron atoms, potassium fluoborate

(kbf4) is used as activator to accelerate the penetrating process, and silicon carbide (SIC) is used as carrier (filler)

the principle of solid powder boronizing belongs to gas phase boronizing of gaseous catalytic reaction. Potassium fluoborate is a highly active component, which can be decomposed at 5300c, and it will be completely decomposed at 800 OC. The gaseous and solid borides decomposed by potassium fluoborate are important substances to promote boronizing, and are the most important gases to improve the activity of boronizing agent and participate in boronizing chemical reaction. The reaction formula of gaseous BF3 formed by the thermal decomposition of kbf4 at the boronizing temperature is as follows:

kbf4> 530 0C KF + BF3 ↑

BF3 has a strong reduction reaction with B203 formed by oxidation in B4C at the boronizing temperature to generate low-cost secondary boron oxide (B202), which promotes the boronizing process, that is:

2 B203 +2bf3 =202 + 3f2 ↑

B202 here is a very unstable oxide, which will be decomposed according to the following formula, Form stable b2c3 and release active boron atoms at the same time:

3b203 =2b203+2 [b]

BF3 generated by the thermal decomposition of potassium fluoroborate can directly produce the following reactions with B4C, separating boron atoms and low-cost bf2:

2bf3+b4c=3bf2 ↑ +3 [b] +c

potassium fluoroborate also reacts with B and C with the participation of oxygen, Directly conduct the following chemical reactions to precipitate boron atoms:

2kbf4+b4c+02 =k20+4bf2 ↑ +2 [b] +co ↑

BF2 is generated in the reaction products of the above two formulas, and BF2 decomposes due to instability and precipitates boron atoms. From this, it can be determined that the boronizing agent with B4C as the boronizing agent has good activity and fast boronizing speed, which is also the theoretical basis for our selection of boronizing agent.

2.3 the process parameters of the test

generally depend on the specific requirements of the parts. Those requiring small deformation and thin boronizing layer can be selected at the critical point (ACL) Proceed as follows, otherwise the boronizing temperature can be increased. The treatment temperature selected in this test is 9000c and the holding time is 4 hours

2.4 workpiece sample standard

the workpiece sample is selected from the most commonly used J1113 die casting machine Φ.. 08 punch as standard, see Figure 1

2.5 wear sample standard

this wear sample adopts the national unified standard

3 equipment specification for the test

the heating equipment for the test is SX box electric furnace, a series of products produced by Shanghai experimental electric furnace factory. The model of wear testing machine is MM-200

under the action of large enough alternating stress

4 control of relevant process in boronizing process

4 1. Control of boronizing layer thickness

the thickness of boronizing layer mainly depends on the boronizing temperature and holding time, and there is a linear relationship between them. Taking this workpiece as an example, according to its service conditions and from the damaged parts, it is analyzed that the main form of failure is adhesive wear, so it is concluded that in order to improve the residual compressive stress on the surface, increase the toughness and improve the heat-resistant fatigue performance, it is appropriate to adopt single-phase Fe2B with an average boronizing thickness of 70~80 μ M. Fe2B is square crystal with boron content of 8 3%, the melting point is 1389 0C, and the hardness is about HV

4. 2 rule of workpiece size change before and after boronizing

the rule of size change before and after Boronizing for No. 45 medium carbon steel punch is shown in Table 1. It can be seen from the table that: 1) the size of the front of the punch changes greatly after boronizing; 2) The size of the rear part of the punch changes little after boronizing; 3) The size change range of punch before and after boronizing is generally about 0.022 mm

judge the outer diameter of 45 medium carbon steel after grinding and before boronizing according to the requirements of the matching clearance between the original ductile iron punch and the pressure chamber. Under chamber m, the inner diameter of the pressure chamber is taken as Φ 40+0. 027, The outer diameter of the punch is (d 40 dragon, the gap between them is 0.077^0.107 mm. After both are affected by high-temperature metal, the diameter of the former is reduced, and the outer diameter of the latter is expanded. The gap size under thermal state cannot be detected, and can only be obtained through theoretical calculation.

according to the actual use, the pressure chamber material is made of the original 3Cr2W8V steel, and its heat treatment process and size change law are also constant. However, the punch material and heat treatment process are determined by After changing from ductile iron to 45 medium carbon steel, its thermal expansion has changed. To make the outer diameter of 45 steel punch after thermal expansion exactly equal to that of ductile iron punch after thermal expansion, the following conditions must be met. That is, when the working temperature of the punch is at 180 ℃, the thermal expansion coefficients of the two materials are respectively:

a 45 steel = 12.32 ×／ 0C (at 20 ~ 2000c)

a ductile iron =11.85 × ／ 0C

according to the expansion formula:

d the coherent material =d is reviewed. [1] 1＋a（t-t。）］

in the formula: D - the last diameter, m

do - the diameter of the real object, mm

a - the coefficient of thermal expansion

t - the working temperature, ℃

t - the room temperature, ℃

in order to achieve the matching clearance required by the original production, it must be:

through the above calculation, it shows that the outer diameter of the punch after boronizing should be Φ.. 083。 According to the actual data obtained after boronizing, the outer diameter of the boronizing punch has expanded by about 0.022 mm, so the outer diameter of the punch should be controlled within after the last grinding during machining. Φ.. 105 i.e Φ Within the range of 39.895~39.925 mm, it can meet the needs of actual production

4. 3. Surface corrosion resistance of boronizing layer

boronizing can improve the corrosion resistance of 45 medium carbon steel by 50, 60, 15 and 4 times in dilute aqueous solutions of sulfuric acid, hydrochloric acid, citric acid and acetic acid respectively; It is increased by 4 and 3.6 times in KOH and NaOH aqueous solutions, and 0.9 times in NaCl aqueous solutions

for the results of the interaction between the boronized surface and molten pure aluminum, aluminum silicon alloy and zinc alloy, see table 245 interaction between the boronized surface of steel and molten alloy

from the above test data, it can be seen that the boronized surface has good corrosion resistance and low affinity with the alloy, which is helpful to improve the service life of the punch. Therefore, as long as the boronized surface works under the condition of 700 0C, it can create necessary conditions for the die-casting production of zinc alloy and aluminum alloy with large quantity and wide range. If the punch is water-cooled, its working conditions will be improved, which will be more beneficial to the improvement of its service life

4.4 relative wear test results of boronized samples and nitrided samples

in the wear test, the disc samples after nitriding and boronizing represent the actual situation of the pressure chamber and punch respectively. After being installed in pairs, they are grinded under a load of 50 kg, and each time their weight loss is weighed with a precision balance with a measurement accuracy of 1/10000 to test their wear degree. After 223889 rotations and weighing, the total weight loss of nitriding sample is 0.6782g, while that of boronizing sample is 1.3414g, that is, the wear of boronizing sample is twice that of nitriding sample, which solves the concern of whether the pressure chamber will be worn in advance after use, and lays a good foundation for the popularization and application of boronizing punch

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