In recent years, the domestic automobile industry has developed rapidly. Major companies have introduced automobile products processing equipment in an all-round way, in order to launch competitive new products in a short time, quickly occupy the market. Casting products generally adopt advanced self-made methods. Among them, Gray cast iron cylinder blocks are typical castings with large output. In the process of localization, when imported domestic cylinder blocks are processed on the same production line, the production efficiency of domestic castings cutting processing is only 40% to 80% of imported parts. The low life span seriously affects the localization process of cylinder block castings. Dongfeng Motor Corporation, FAW some other large domestic automobile manufacturers all have similar problems experiences. Therefore, improving the processing performance of gray cast iron cylinder blocks has become an urgent problem. Due to the many factors that affect the machinability of gray cast iron castings, it has increased the difficulty of improving the machinability. The author summarizes the current research results of gray cast iron machinability at home abroad, analyzes the influencing factors that affect the machinability of gray cast iron cylinder blocks. In order to improve gray cast iron The processing performance of the cylinder block provides a reference.

1 Gray cast iron cylinder material its processing performance

Gray cast iron cylinders are generally made of materials equivalent to the national standard HT250. For example, the Shenlong TU3F cylinder body adopts the French PSA Group standard, which is a cylinder body without liner; the cylinder body mainly checks the top surface the cylinder tube hardness at 50mm the top surface, the hardness is required to reach 207~241HBS, no clear strength requirements are proposed; the cylinder wall The upper graphite is flake type A graphite, grade 4, type C is allowed; the matrix structure is pearlite ≥ 95%, free carbide ≤ 2%; during the localization process, the composition of the casting blank the imported cylinder body casting are the same. The hardness of the parts is slightly lower than that of imported castings; in conventional metallographic inspections, imported castings domestic castings with different grinding times have no obvious difference in graphite matrix structure at the same processing position, but when processing on automatic lines, domestic parts The honing boring time of the cylinder top surface is longer than that of imported parts, the tool life is lower.

The main products of Dongfeng Motor Co., Ltd. Casting Branch are Cummins 6B cylinder block cylinder head. The material is HT250, which are processed on the E processing line the W processing line. The E processing line is a domestic automatic processing line, the roughing line speed is generally less than 100m/min, the domestic cylinder block casting material has good processing performance, the tool life can meet the production requirements; the W processing line is an imported high-speed automatic processing line, the roughing line speed can be Up to 870m/min, the domestic cylinder block casting material has poor machinability, the tool life is only about 30% of imported castings. This reduces production efficiency significantly increases tool wear. This has caused complaints processing plants, it is strongly demanded to improve cylinder block cylinder head castings. Material processing performance.

Compared with foreign castings under the same processing conditions, the poor processing performance of domestic castings is the common cause of large deviations in the size of castings, large machining allowances, poor surface quality, white corners, hard spots, etc. in the traditional sense, but When the strength, hardness, metallographic structure of the casting material are abnormal, problems such as large tool consumption, long processing time, poor surface roughness of the castings generally occur in the finishing process.

2 Analysis of factors affecting the processing performance of gray cast iron

2.1 The influence of the chemical composition of gray cast iron

2.1.1C Si

The general machinability theory believes that when high-speed steel tools are used the cutting speed is less than 150m/min, the graphite is coarser the tool life increases.As the relative volume of graphite in gray cast iron increases, the size of the graphite flakes relatively increases, the machinability index increases. Trend, but the smaller the amount of graphite, the smaller the surface roughness after processing. In cemented carbide tools, 350m/min high-speed cutting conditions, the change in the amount of graphite makes the tool life change less. Therefore, it can be inferred that the use of ceramic tools under 870m/min ultra-high-speed cutting conditions, high w(C) has little effect on improving the life of ceramic tools. The test of Dongfeng Motor Casting Branch a found that the amount of w(C) in the cylinder block was increased 3.31% to 3.42%, the tool life of the rough boring cylinder bore of the cylinder block was increased 20 to 22 pieces, indicating that the amount of w(C) is affected. The main factor of gray cast iron.

Si is an element that strongly promotes graphitization, which can reduce the whitish tendency of molten iron reduce carbides in the structure. There was a factory that used high Si/C ratio to produce high-strength, low-stress machine bed other gray cast iron parts. However, Si also has a negative effect. Increasing the amount of w (Si) will increase the amount of ferrite in the structure, the high w (Si) will increase the temperature of A1. Pearlite is formed at a higher temperature, the spacing of the sheets is large, the pearlescent The strength of the body is low. Therefore, when strengthening the inoculation effect, the amount of w(Si) should be strictly controlled.

2.1.2 Mn S

The understanding of the role of S in gray cast iron is a gradual improvement process. thinking that S is a harmful element, to adding a certain amount of S to gray cast iron to improve the inoculation effect graphite morphology, thereby improving the cutting performance, gradually realize In gray cast iron, w(S) is beneficial in a certain range, which is 0.08%~0.12%. When w(S) is less than 0.05%, S must be increased to improve the inoculation effect.

AdrianaAnaPereira et al. studied the effect of S on the workability of FC25 (equivalent to HT250) gray cast iron. The test conditions are: medium frequency induction electric furnace smelting. The charge is mainly scrap steel reheat charge. Graphite is used to increase C. The w(C) w(Si) of the molten iron are 3.40% 2.15%, respectively. Pyrite (FeS2) is used. The increase in S, w(S) was controlled at 0.065%, 0.12%, 0.15% 0.18% respectively. Use FeSiCaAl inoculation treatment. Microstructure performance analysis experiments show that the change of w(S) has no significant effect on the hardness the number of eutectic clusters of FC25 gray cast iron, the difference in strength of the specimen is 17MPa. The samples with different w(S) are all type A graphite, the length distribution of graphite the pearlite structure do show obvious differences due to the difference of w(S). When the amount of w(S) is 0.065%, it is type I sulfide; when the amount of w(S) is 0.12%, the sample is mainly type I sulfide with a small amount of type III sulfide; when the amount of w(S) is 0.15% , The amount of type Ⅰ type Ⅲ sulfide is equal; when w(S) is 0.18%, the sulfide of the sample is mainly type Ⅲ a small amount of type Ⅰ.

The cutting test results show that there is no significant difference in the machinability of FC25 when the cutting speed is 100m/min, w(S) is 0.12% 0.18%; when the cutting speed is 150m/min 200m/min, w(S) is 0.12 There is no significant difference in processability at %, 0.15% 0.18%. Comparing the samples with w(S)0.065% w(S)0.18%, we can see that the main factor affecting the cutting performance is the area ratio of sulfide, the form of sulfide. Compared with the w(S)0.12% sample, the tool life of the w(S)0.065% sample is reduced by 24%, 32% 38% at cutting speeds of 100, 150 200m/min, respectively. Among the four kinds of w(S) samples, only 0.065% of w(S) samples had severe cutting sticking. The above data shows that adding S to gray cast iron to form a certain amount of MnS can improve the cutting performance of gray cast iron. The author has analyzed the microstructure of imported Shenlong cylinder found that more fine MnS is polygonally distributed in the pearlite matrix, but whether the processing performance of imported parts is related to MnS remains to be tested verified.

2.2 The influence of alloying elements

In order to improve the mechanical properties (especially the hardness) thermal fatigue properties of the cylinder block, a small amount of alloying elements are often added in the cylinder block production. These alloying elements are mainly: Cr, Cu, Sn, Sb, etc. The following analyzes the influence of these alloying elements on the workability of gray cast iron.

2.2.1Cr gray cast iron cylinder Cr alloying can improve stabilize the hardness of the cylinder block, but Cr is a strong carbide forming element, which will increase the tendency of molten iron to supercool form a white mouth structure at the thin wall of the cylinder block , Making processing difficult. FAW Foundry Co., Ltd. Foundry No. 2 in the Jetta cylinder block test found that the sample obtained by adding Cr, Mn, Si a small amount of Sb in the inoculant is better than adding Cr alloy to the charge alone.

2.2.2Cu

Cu is a medium graphitizing element, which can reduce the eutectoid transformation temperature, promote refine pearlite, hinder the formation of ferrite. Cu is rarely used alone, is usually used in combination with carbide forming elements for better results. Elements paired with Cu include Cr, Mo, Mn, V so on. FAW's test results show that when the amount of Cu is increased, B-type graphite a certain amount of sorbite are easy to appear in the structure, both of which are unfavorable to the processing performance. Compared with A-type graphite, B-type graphite is conducive to the lubrication chip breaking of the tool aggravates the wear of the tool; sorbite is a high-strength phase in the cast iron structure, which increases the wear of the tool. When w(Cu)>0.3%, the wear of the tool increases sharply, the reasonable amount of w(Cu) added is 0.15%~0.25%.

2.2.3Sn

Sn can increase the amount of pearlite increase the strength hardness of cast iron. Sn has little effect on the eutectic reaction does change the shape of graphite, but it has a great influence on the eutectoid reaction. Sn strongly promotes the pearlization of the matrix, its effect is ten times that of copper. It is a very effective element to improve the strength, hardness, uniformity wear resistance of cast iron. However, w(Sn) should be less than 0.1% in the gray cast iron cylinder block, the actual addition amount is about 0.035%.

2.2.4Sb

Sb is a low-melting surface active substance in cast iron. It hinders the diffusion migration of carbon atoms during the first crystallization. It is difficult for the ink eutectic group to grow up, the tendency of molten iron to be supercooled increases. Sb is a strong pearlitizing element, twice stronger than tin 100 times stronger than copper, but Sb is prone to intergranular segregation. The test results of Dongfeng Motor Company show that adding w(Sb) 0.004% can significantly increase the hardness improve the section sensitivity; adding a small amount of w(Sb) will affect the scale, white mouth graphite morphology; when the amount of w(Sb) When it exceeds 0.03%, the cast iron becomes brittle the deflection begins to decrease. Therefore, the recommended amount of w(Sb) is 0.004%~0.006%.

2.3 The influence of smelting inoculation treatment

The smelting process is directly related to the quality of molten iron, has an important influence on the structure performance of cylinder castings. Due to the difference in the smelting process, the shape size of the graphite, the amount of free ferrite, the refinement of pearlite the amount of carbides are different. These microstructure characteristics determine the strength, hardness cutting of the cylinder body casting. performance.

It is better to use cupola-induction furnace double smelting for the molten iron of the cylinder, so that only can there be a sufficient amount of molten iron, but also high temperature stable composition of molten iron can be obtained. For example, the French SEPTFONS foundry uses water-cooled hot air (400~800℃) cupola coreless power frequency induction electric furnace for double smelting, uses air pressure insulation ladle for pouring. The pouring temperature is controlled at 1420±10℃, the molten iron is flowed. Bred. The German KHD Harbo foundries both use double smelting, the cupola furnace is rich in oxygen air is supplied. The quality of coke is good, the molten iron temperature can reach 1550 ℃ more. When the power frequency furnace is used as the smelting equipment, the molten iron will have a greater tendency to white mouth shrinkage. At this time, S[w(S)>0.08%] must be increased sufficient incubation should be carried out.

In terms of charge, foreign charge mostly uses scrap + carbon silicon additives, no less use of pig iron, that is, "synthetic cast iron". In the case of the same carbon equivalent, the tensile strength of synthetic cast iron is 15-30MPa higher than that of ordinary gray cast iron, the hardness is about 10HB. The graphite is all type A, the uniformity of the structure is better. Table 1 shows the test results of Jiangsu University on the amount of scrap steel. The results in the table show that increasing the amount of scrap steel can improve the hardness uniformity of the gray cast iron.

The use of flow inoculation also has a certain effect on improving the hardness. It can increase the hardness value by about 5-10HB, while reducing the sensitivity of the casting section, making the hardness distribution of the top surface of the cylinder the cylinder tube more uniform, thereby improving the gray cast iron Cutting performance. However, the amount of inoculation should be too large, otherwise it will increase the amount of ferrite improve the toughness of the material, which is detrimental to the chip breaking of high-speed cutting.

The amount of w (Ca) in the inoculant has a certain effect on the workability of gray cast iron. When the amount of w (Ca) in the inoculant is between 1.5% 3.0%, a CaO-Mn2O3-SiO2 ternary is formed on the surface of the tool tip. Compound oxide, with a melting point of 1553K, softens at the tip of the tool to provide lubrication. When the amount of w(Ca) exceeds this range, the shape of the composite oxide of the tool tip changes, the melting point rises, it cannot be softened for lubrication, which is unfavorable for processing performance.

2.4 The influence of gray cast iron structure hardness

According to the technical standards of Shenlong cylinder block Jetta cylinder block, the graphite form of gray cast iron cylinder block should be uniformly distributed type A graphite, type C graphite is allowed. B-type graphite is conducive to the lubrication chip breaking of the tool aggravates the wear of the tool. the perspective of matrix structure, sorbite is a high-strength phase in cast iron, which will aggravate tool wear. Figure 3 shows the matrix structure of imported domestic Shenlong cylinders. It can be seen the figure that the pearlite sheets of imported castings are evenly spaced, while the pearlite sheets of domestic castings have poor uniformity, there is a sorbite structure locally. Therefore, the uneven structure of pearlite the presence of sorbite should be one of the reasons for the increase in tool wear.

Refining the eutectic cluster of gray cast iron can refine the graphite in the eutectic cluster. The fine A-type graphite is evenly distributed on the pearlite matrix. During the cutting process, the chance of contact between the tool the graphite increases. The notch function improves the workability of gray cast iron. The refinement of the eutectic cluster increases the strength hardness of gray cast iron. Therefore, the increase in hardness does necessarily deteriorate the workability of gray cast iron. The anatomical analysis of the imported domestic Cummins cylinder block by the Casting Branch of Dongfeng Motor Corporation found that the grain boundary of the imported cylinder block eutectic cluster is relatively easy to corrode, the grain boundary is relatively clear. The grain boundaries of domestic cylinders are relatively wide fuzzy. The hardness of imported parts is higher than that of domestic ones, the eutectic clusters are small in size large in quantity.

2.5 The impact of hard points

Hard spots such as carbides nitrides are found in the matrix structure of domestic gray cast iron cylinder blocks. It is generally believed that the hard spots will hit the knife, thus affecting the processing performance of gray cast iron. Therefore, to improve the cutting performance, it is necessary to take measures to reduce the hard spots as much as possible, the less the number, the better. Therefore, the alloying is usually mainly based on adding Cu Sn, increasing the inoculation amount. It is believed that this can reduce the number of hard spots is beneficial to the cutting performance.

However, the hardness of tiny carbides, phosphorus eutectic TiC TiN particles are very high. The resistance of these hard spots to peel off the matrix is very large. During high-speed cutting, they are easy to peel off the matrix. Tool wear has little effect. If alloying with Cu Sn as the main part is used, when the intergranular carbides, phosphorus eutectic other hard point particles are greatly eliminated by powerful inoculation measures, the toughness is improved, the chip breaking during cutting is good, resulting in cutting temperature The increase will accelerate the wear of the tool reduce the service life of the tool.

3 concluding remarks

At present, the material performance of the domestic gray cast iron cylinder block has reached the level of imported parts, it can be mass-produced. The processing performance of the cylinder block has also been improved to a certain extent after a large number of domestic studies. However, poor processing performance is still one of the main factors restricting the development of domestic gray cast iron cylinder blocks. Although the reasons for the processing performance of domestic gray cast iron cylinders are more complicated, the research results understanding are completely unified, the author found that the composition performance of domestic imported parts are basically