-Talking about the production practice of vermicular graphite cast iron brake drum.

I. Preface (Current Situation of Brake Drum)

Since 20 10, China's automobile production has exceeded180,000, surpassing the United States and ranking first in the world. Car ownership has also reached 65.438+0.2 billion. The extraordinary growth of automobile production has brought prosperity and chaos to the automobile parts market. Automobile brake drum is an important part of automobile driving system and belongs to axle assembly. At present, Axle Company is only a parts company, a subsidiary company, or just an OEM supplier of automobile factory, and its profit margin is far less than that of automobile factory, so the competition is extremely fierce. In order to reduce the cost, Axle Company also purchased some parts with simple appearance or meager profit, such as truck brake drums, forcing some powerful big-generation factories to give up producing brake drums or simplify the production process, while some small factories took advantage of it, resulting in the current situation that "big factories are unwilling to do it, but small factories can't do it well", which leads to the decline of brake drum quality and makes it difficult to invest in further research on brake drums. Some axle companies could have imposed "50,000 km compensation" on the axle, but they dared not mention it because of the brake drum.

It is understood that the service life of the brake drum varies greatly due to different road conditions, overload or not, driver's operating habits and manufacturers. Generally speaking, if a car is equipped with the original factory, its service life can exceed 654.38+10,000 kilometers, but if it is equipped with accessories in the market, it is often only 50,000 kilometers. For trucks, the original factory configuration can reach 30,000-50,000 kilometers or even higher (driving in plain areas with few brakes and good road conditions can reach more than 80,000 kilometers), while the drums in the parts market are mostly below 30,000 kilometers or even less than 6,543.8+0,000 kilometers.

Frequent replacement of the brake drum not only wastes a lot of financial and material resources, but also is a terrible accident that the failure of the brake drum may cause car crash and death during driving. Therefore, it should be the responsibility of automobile industry and foundry industry to improve the material of brake drum and improve its service life and reliability.

What is the best material for making brake drums? To answer this question, we must first understand the function of the brake drum and the failure form of the brake drum.

Second, the function of the brake drum and the basic requirements for materials

The use function of automobile brake drum and the basic requirements for materials are as follows:

1, which bears heavy objects and bears strong mechanical impact when braking. The more the load, the faster the speed and the greater the impact, so the brake drum is required to have sufficient mechanical properties;

2. When braking, kinetic energy is quickly converted into heat energy through dry sliding friction and dissipated as soon as possible, so the brake drum is required to have good thermal conductivity;

3. Frequent braking or long-term downhill braking in mountainous areas makes the temperature of the brake drum rise continuously, and sometimes it needs to be chilled with water, or when driving in rainy days, rain will splash on the hot brake drum, so it is required that the brake drum has good thermal fatigue performance;

4. Braking depends on the friction between the inner circular surface (friction surface) of the brake drum and the friction plate of the brake shoe, so the brake drum is required to have good wear resistance and bite resistance.

5. Modern cars pursue ride comfort, which requires reducing vibration and noise during braking. Therefore, it is required that the braking size is stable, the internal structure is uniform and the shock absorption is good.

Third, the failure form of the brake drum

At present, most of the automobile brake drums on the market are made of gray cast iron, and there are also a few composite inlaid drums. There are also a number of vermicular cast iron brake drums put on the market for trial use.

Zhao Yongqi and others of Haoxin Company [1] made a statistical analysis on the failure forms of the brake drum of 35,500 gray cast iron freight cars compensated by "Three Guarantees". The failure reasons are shown in the following table:

Cause of failure

crack

Full of cracks

Put down the bottom

There is no obvious failure.

Abnormal wear

excessive wear

total

Quantity (piece)

26492

5764

183 1

947

330

136

35500

Proportion (%)

74.63%

16.23%

5. 16%

2.67%

0.93%

0.38%

100

Zhao Yongqi and others pointed out that, in fact, most cracked parts are cracked first. Because the "three guarantees" compensation service personnel are unclear, cracks are often counted as cracks.

From the statistics, it can be seen that cracking and cracking account for more than 90% of the total compensation of "Three Guarantees", while wear (the sum of normal wear and abnormal wear) only accounts for 1.3% of the total.

I don't know the company's "Three Guarantees" mileage or "Three Guarantees" payout ratio. The author has made an investigation in the area from northern Guizhou to Qijiang, Chongqing, and found that the local parts dealer asked to process a circle of grooves with a depth of about 1mm on the friction surface of the brake drum, and agreed with the user that if the grooves are still visible and the brake drum is damaged (such as cracking, falling off the bottom, etc.). ), it will be replaced free of charge, and it will not be compensated if the groove is worn out of sight. According to the parts dealer, this groove is smooth and generally needs to travel about 30 thousand kilometers. In the local area, the payout ratio of the brake drum during the "Three Guarantees" period is: the accessories provided by the main engine factory company are about 10%, and the accessories provided by small enterprises are 15-20%.

To sum up, the main reason for the early failure of the automatic drum stopper within 30,000 km is cracking and cracking (90.9%), followed by falling bottom (5. 16%), which is far from the degree of wear failure.

The reason for the failure is clear, that is, the material is unqualified, the tensile strength is too low, or the local wall thickness is too thin. This is mostly because the production process is out of control, or the "parallel imports" produced by small enterprises. And cracking (cracking) is caused by thermal shock. When an automobile is braked, the huge braking thermal load and thermal shock cause a great temperature gradient on the brake drum, resulting in high thermal stress. Due to the huge thermal energy generated by braking, the friction zone will generate high temperature enough to cause phase transformation, thus generating phase transformation stress. The existence of thermal stress and phase transformation stress makes the mechanical properties of materials uneven. Under the action of frequent braking load, cracks occur. Most of these cracks are intermittent or continuous, and continuously extend into a network (i.e. cracks). In severe cases, cracks are wide and long, which is cracks.

Fourth, the material of the brake drum

At present, there are three main materials for manufacturing automobile brake drums, namely gray cast iron, vermicular cast iron and composite materials. Cast steel or forged steel, which is commonly used as the material of train brake disc, has not been reported in automobile brake drum.

1, gray cast iron

Gray cast iron has excellent thermal conductivity, low elastic modulus, good shock absorption, high strength, good wear resistance and low price, and is still the first choice for automobile brake drums. However, with the acceleration of the car and the increase of the load, the kinetic energy of the car increases geometrically, and the thermal shock generated during braking also increases greatly, resulting in frequent early failure of the brake drum and a sharp increase in cracking. Therefore, foundry workers took two countermeasures:

(1) Improve the strength of materials while maintaining good thermal conductivity.

Because the shape and size of graphite in gray cast iron are very important to the thermal conductivity, in order to ensure that there are enough grade 3-5 A graphite in the structure, it is necessary to maintain a high carbon equivalent (generally above 3.8%), so the tensile strength of cast iron can only be improved by low alloying. Usually, the added alloys and their ranges are: Cr 0. 15-0.45%, Mo 0. 15-0.65%, Cu 0.4-0.8%, Ni 0.2-0.5%, and some of them are added with Sn and V, but the combined addition effect is better. The tensile strength of low alloy gray cast iron can reach 220-260Mpa. At present, most commercial vehicle brake drums adopt this measure to improve the service life of brake drums.

Data [2] introduced that adding 0. 1%Nb to the sub-crystalline cast iron brake disc with 3.93% carbon equivalent, 0.27% Cr and 0.68% Cu, the tensile strength reached 222 MPa, and the length of A graphite was 4-5, which met the requirements of TL0 1 1 standard. This material has been used in the brake discs of ordinary cars.

Although low alloying can make the tensile strength of gray cast iron exceed 260Mpa (only slightly reduce the carbon equivalent), graphite is often fine, the thermal conductivity decreases, and the machinability becomes worse, which is not conducive to improving the service life of brake drums and is not suitable as a material for brake drums. In other words, the strength that can be improved is limited under the premise of maintaining good thermal conductivity.

② On the premise of ensuring a certain strength, improve the thermal conductivity of the material.

Some luxury cars require not only long service life of the brake disc, but also low noise and no vibration when braking, so high carbon equivalent gray cast iron with excellent shock absorption and thermal conductivity is selected as the material of the brake disc. Information [2] introduces that Volkswagen adopts * * * crystal gray cast iron as the brake disc of limousine (standard number TL048), which requires carbon equivalent to be controlled at about 4.4-4.5%, A-type graphite with a length of 3-5, and a small amount of A-type graphite with a length of 2 and individual C-type graphite are allowed, with friction surface hardness of 150-200HB and tensile strength. According to the data [3], in order to solve the problem of automobile braking noise and jitter, Toyota Motor Corporation selected high-carbon equivalent gray cast iron, and through controlled inoculation, the length of type A graphite reached 100μm or more (equivalent to the graphite length in the national standard 1), and the tensile strength reached 175-225Mpa.

This kind of gray cast iron which requires high graphite shape and size is very difficult to produce, and not all foundries can produce it. In addition, due to its low strength, this material is not suitable for the brake drum of commercial vehicles that usually use drum brakes.

2. Composite materials

At present, there are some inlaid brake drums in the spare parts market, the friction surface (inner ring) is made of high-strength steel, and the outer ring is made of gray cast iron with good thermal conductivity, and the two materials are inlaid (it is said that they are also fused). This material is beneficial to reduce the cracking of early brake drum. However, due to the different thermal expansion coefficients of the two materials, the inlay becomes loose after repeated cold and hot shocks, so the service life is not very long. Coupled with the high price, it is not popular.

3 vermicular graphite cast iron

As early as 1970s and 1980s, in view of the excellent thermal fatigue resistance of vermicular cast iron, domestic foundry workers developed the vermicular cast iron brake drum. However, perhaps because of the small number of cars, low load and slow speed at that time, the production stability of vermicular graphite cast iron was difficult to control, and the brake drum made of vermicular graphite cast iron did not get the attention and recognition of product designers. After entering the new century, although the quantity, load and speed of cars have made a qualitative leap, the improvement of brake drum material has been kidnapped by short-term interests, and "parallel imports" have washed away the market, and high-quality products have not been compensated. Coupled with the widespread serious overload, people can't tell whether it is the brake drum itself or the user's problem. At the same time, in recent 20 years, the technicians of the railway department have carried out fruitful research and development work on the series of brakes, applied vermicular graphite cast iron brake discs to passenger cars with a speed less than or equal to160 km/h, and formulated industry standards, such as TB/T2444- 1993 "General technical conditions of vermicular graphite cast iron for railway rolling stock" and TB.

As long as we compare vermicular graphite cast iron with gray cast iron according to the working conditions of automobile brake drum and the basic requirements for materials (namely, tensile strength, thermal conductivity and wear resistance), we can draw a better conclusion-of course, this has to be verified by actual use.

① tensile strength

The tensile strength of gray cast iron used as brake drum material is only 150-260Mpa, while vermicular iron can reach 350-450Mpa. The high temperature strength of vermicular iron is much higher than that of gray cast iron.

② Thermal conductivity

The thermal conductivity of vermicular graphite cast iron is between gray cast iron and nodular cast iron, and the higher the vermicular rate, the closer it is to gray cast iron. The carbon equivalent of vermicular cast iron is higher than that of gray cast iron, that is, it contains more graphite with excellent thermal conductivity; The graphite in vermicular graphite cast iron is interconnected, not isolated like spherical graphite. Although the graphite between the * * * crystal clusters of vermicular graphite cast iron is not intertwined like gray cast iron, the number of * * * crystal clusters of vermicular graphite cast iron is very small (generally, the number of * * * crystal clusters of gray cast iron is 10-20, that of vermicular graphite cast iron is 20-50, and that of nodular cast iron is 65430).

③ Wear resistance

Zhang Yongzhen and others [5] have studied the friction characteristics of cast iron for a long time, and think that vermicular graphite cast iron has the lowest wear rate, the highest friction coefficient and the smallest attenuation of friction coefficient compared with ductile iron and gray cast iron under dry sliding friction conditions. The three-dimensional morphology of the friction surface of vermicular graphite cast iron is different from that of nodular cast iron and gray cast iron. The main feature of the friction surface between nodular cast iron and gray cast iron is the shape of groove ridges formed by cutting, but the groove of gray cast iron is wide and deep, and the groove of nodular cast iron is narrow and shallow, while the main feature of vermicular cast iron is the peeling pit-island convex peak formed by cutting, without obvious cutting groove. Vermicular graphite cast iron has good wear resistance.

Even though vermicular cast iron is a good material for making automobile brake drums, people still can't reach an agreement on whether the vermicular rate is high or low and the pearlite content is more or less. Du's foundry company in Jianyang City, Fujian Province has been developing vermicular iron brake drums since 2009, and thousands of pieces of vermicular iron brake drums have been used in mountainous areas in northern Guizhou. According to these production and use experiences, the author has the following views:

On creep rate

The definition of vermicular graphite cast iron in ISO standard and national standard is ≥80%, but it is also pointed out that the cooling rate has a great influence on the vermicular graphite cast iron, so it is not always required that the vermicular graphite cast iron be above 80%, and the best vermicular graphite cast iron can be determined according to the casting structure and actual use. The brake drum has a uniform wall thickness and requires good thermal conductivity, so the creep rate is higher, but flake graphite must not appear. According to the production experience of Du Foundry Company, castings with creep rate ≥80% can be obtained stably even by flushing method. There is no obvious difference between the creep rate of ≥70% and the creep rate of ≥80% in processing and practical use, so according to the enterprise standard of Du Fu Company, the creep rate of brake drum is ≥70%.

On pearlite content

Some people think that the brake drum is a wear-resistant part, and the pearlite content of the original gray iron brake drum is mostly above 97%, so the production of vermicular cast iron brake drum should also have a high pearlite content. Actually, not necessarily. There are three reasons: a) Data [5] points out that vermicular graphite cast iron has better wear resistance than ductile iron and gray cast iron. It can also be considered that the wear resistance of vermicular graphite cast iron with mixed matrix is equivalent to that of ductile iron and gray cast iron with pearlite matrix. B) As-cast vermicular graphite cast iron is easier to produce ferrite. Different from ductile iron and gray iron, as-cast iron with high pearlite content can only be obtained by adjusting the composition of conventional elements. In order to obtain as-cast vermicular graphite cast iron with high pearlite, it is necessary to add more pearlite promoting elements, such as Cu, Cr, Mn, Mo and Sn, which will inevitably lead to the increase of manufacturing cost. C) Failure analysis of brake drum shows that wear is not the main reason for failure, and it is a waste of resources to spend a lot of money to improve the wear resistance of existing remaining functions.

Du's company's enterprise standard "Technical Conditions for Truck Vermicular Cast Iron Brake Drum Castings" stipulates three grades of RnT350, RnT400 and RnT450, all of which require creep rate ≥70%, but the pearlite content is ≥35%, ≥50% and ≥60% respectively. None of the vermicular graphite cast iron brake drums put on the market were scrapped due to wear.

Verb (abbreviation of verb) production practice of vermicular cast iron brake drum

Du's foundry company in Jianyang City, Fujian Province was originally a professional foundry for casting alloy production, producing nodulizer and inoculant for some domestic manufacturers, with an annual production capacity of10.5 million tons. In 2006, we expanded our business and set up a casting company to produce ductile iron castings such as differential housing and wheel housing of construction machinery for the use of bridge box company of Xiamen construction machinery company. In 2009, the vermicular cast iron brake drum was developed, with an annual output of 1 10,000 tons and 50,000 vermicular cast iron brake drums.

1 means of production

1. 1 melting

Use 1t intermediate frequency furnace to melt molten iron. The charge is scrap steel+carburizing agent+recycled charge, and the amount of pig iron is less than 20% (in most cases, pig iron is not added). All expenses must be weighed. The amount of carbon and silicon in front of the furnace is controlled by thermal analysis, and the temperature is measured quickly by thermocouple. The tapping temperature is1500 200℃

1.2 Wormlike treatment

Adopt 1t spheroidizing treatment package (common to both spheroidizing treatment packages), dam-type treatment and creeping treatment.

Adopt self-made patented vermicular agent. Because it is treated with ductile iron, there is no Ti in the vermicular agent (and there is no Ti in the original molten iron). The vermicular agent contains rare earth element 14- 16%, magnesium element 3-5%, and some calcium, aluminum, silicon and iron elements. 0.6-0.8% (based on the weight of molten iron).

Secondary inoculation is adopted, and 0.2% inoculant (75 ferrosilicon containing Ba4%) and 0.3-0.4% stream inoculation (both 75 ferrosilicon containing Ba4%) are added to the bottom of the bag. No more stream inoculation when pouring.

Because the burden has not changed much, it has been accurately weighed, and the creeping treatment is to empty the whole furnace and strictly control the tapping temperature of molten iron, so the creeping rate is very stable. After vermiculation, the pouring time should be controlled within 65438 05 minutes.

1.3 modeling

Adopt Z 148 molding machine, green sand mold, 1 piece/type.

The pouring system adopts top injection and multiple inner gates to ensure the uniform structure of the brake drum and no shrinkage porosity and shrinkage cavity inside.

2. Production stability

The monthly production data of 2011.5.20-6.18 were counted, during which 77 boilers were started.

2. 1 creep rate (VG%): See the table below.

Date of production

Sampling times m

Creep rate statistics

VG concentration ≥70%

VG concentration ≥80%

When the confidence is 95%, the range of creep rate is `x 2 б.

Mean value/mean value

difference

Standard deviation

` x

rare

б

20 1 1 year 5.20-6. 18

77

84.90%

20%

7.47%

100%

80.70%

70.0-99.9

Note: ① Metallographic samples are taken from Y-shaped test blocks poured during normal production in the factory.

② Creep rate: 0.6-0.8% of molten iron.

As can be seen from the table, the stability of creep rate is very high.

2.2 mechanical properties:

Tensile strength: 350-506 MPa,

Elongation: 1.5-6.0%

Hardness: HB 143-2 1 1.

Pearlite content: 25%-65%

Due to different grades and alloy compositions, the data of mechanical properties are scattered, which is not suitable for statistical analysis.

3. Standard:

In 20 1 1 year, Du's company formulated the first enterprise standard "Technical Conditions for Automobile Vermicular Cast Iron Brake Drum Castings" in China, which includes nine aspects, such as scope, normative reference documents, chemical composition, mechanical properties, metallographic structure, dimensional tolerance, casting defects, casting identification, surface protection packaging and storage and transportation requirements. There are three grades specified in the standard, namely, RuT350, RuT400 and RuT450, and the pearlite content in the matrix is ≥35%, ≥ 50% and ≥60% respectively.

4. Weight loss and market reaction

Du's company has carried out weight reduction tests on some brake drums. The weight reduction part is mainly the wall thickness of the drum. The experimental drums are 153 brake drum and 1094 brake drum. Rotate the excircle by 2mm to reduce the wall thickness from 16mm to 12mm. The original weight of 153 drum decreased from 47.6kg to 40kg, and the weight loss rate was 16%. The weight loss rate of 1094 drum decreased from 4 1 kg to 38kg, and the weight loss rate was 7%. Now it has been put into market testing. After half a year, the market responded well.

According to the follow-up investigation of Du's company, the "three guarantees" refund rate of vermicular graphite cast iron brake drum varies with different models. The best one is Jiuping Chai, and the refund rate is only 1.8%. The worst one is Steyr brake drum, and the refund rate is 8.2%. But overall, it is much better than the gray iron bucket, and the initial investment market return rate is only 1/5- 1/3 of the gray iron bucket. I believe that by improving market feedback, the chances of retreat will be further reduced.

Ending of intransitive verbs

1. Among the materials for making automobile brake drums, vermicular cast iron is the best, with the longest service life, the production cost is equal to or slightly higher than that of low-alloy gray cast iron, and the cost performance is the best.

2. The creep rate of automobile vermicular cast iron brake drum should be ≥70%. Too low will reduce the thermal conductivity, and too strict will increase the production cost. Under the current conditions, as long as the working procedure is well controlled, the requirement of VG≥70% can be achieved stably even if the work is rushed.

3. There is no need to pursue high pearlite content. RuT400 (≥50%P+F), RuT450 (≥60%P+F) or RuT350(≥35%P+F) can be selected according to different vehicles and different service conditions.

4. The vermicular cast iron brake drum has the potential of reducing weight 15-20%. When the wall thickness decreases, attention should be paid to the smooth transition with the flange to ensure dynamic balance. The thinning of the wall is beneficial to reduce the temperature gradient and thermal stress of the casting

5. Carefully select the casting process, eliminate the defects such as shrinkage cavity and porosity in the casting, and make the internal structure uniform, so as to reduce internal stress and braking jitter.