Casting process control of alloy cast iron cylinder liner

1 Overview

The use of medium frequency induction furnace to smelt cylinder liner alloy cast iron has the following disadvantages: First, the tendency of iron liquid to be too cold is too high, and it is easy to produce D and E type graphite which affects the mechanical properties of the material; second, the iron liquid is pure, and the heterogeneous crystal core is less. , resulting in poor breeding results, casting hardness is low. In addition, the mechanical properties of ordinary cast iron cylinder liners can no longer meet the upgrade requirements of the engine for its performance. A chemical alloy composition of a high-performance alloy cast iron cylinder liner with a large increase in strength and hardness developed by a main engine factory is taken as an example. The requirements are: w _C = 3.0% to 3.4%, w _Si = 1.7% to 2.4%, w _Mn = 0.6% to 0.8%, w _P ≤ 0.20%, w _S ≤ 0.08%, w _Cr = 0.25% to 0.4% , w _Cu = 1.3% ~ 1.6%, w _B = 0.045% ~ 0.07%, mechanical properties are: tensile strength ≥ 270MPa, hardness 280 ~ 310HBW, the same cylinder liner hardness difference ≤ 30HBW, the matrix metallographic structure is pearl Body + ferrite, ferrite ≤ 5%, graphite is type I A or B, allowing D and E graphite on the outer surface of the liner, type I graphite 4 to 8 grade, using existing charge formula The above-mentioned alloy cast iron consumes a large amount of raw and auxiliary materials such as pig iron and iron alloy, and the raw material cost is high.

In view of the defects in the above-mentioned cylinder liner induction furnace melting method, our engineers and technicians have explored a medium-frequency induction furnace melting method and centrifugal casting method for high-performance alloy cast iron from many practices, and rationally utilized waste materials. The original and auxiliary materials are optimized, and the cylinder liner blanks produced can meet the requirements of the high-performance alloy cast iron material of the OEM.

2. Smelting equipment and raw material selection

The smelting equipment selected by our company is 5t/3200kW medium frequency induction furnace. The main technical parameters are: rated capacity 5t, rated working temperature 1570°C, rated temperature insulation power 180kW, melting rate 3.8t/h.

(1) Selection of metal charge 1 In the batching, according to the technical standard GB4224-1984, the block size is not more than 300mm×200mm×200mm, and the recycled material is waste cylinder liner. 2 The scrap in the batching requires no serious rust. The scrap steel with zinc, chrome and other components on the surface can be used with the better quality scrap without affecting the technical requirements of the chemical composition of the molten iron. The block size should conform to GB4224. - 1984 specifies less than 500mm × 200mm, the long material is less than 1/2 of the diameter of the furnace, that is, <550mm, and the steel pipe is made of 20 steel pipes. 3 The high silicon pig iron length in the batching is ≤400mm, the chemical composition is w _C =3.8%, w _Si =3%～5%.

(2) In the selection of alloy materials, Si75 for silicon iron, block size 200mm×100mm; FeB18C0.5A for boron iron, block size 50mm×100mm.

(3) In the selection of iron filings, the iron filings are required to be free of severe rust, moisture <3%; the carbonizing agent is graphite type; the slag removing agent is selected from perlite. The iron filings are mainly machined iron chips for the cylinder liner.

3. Feeding requirements

(1) Furnace mix iron filings 59% + scrap 28% + recycled material 5.8% + high silicon pig iron 6% +75 ferrosilicon 0.6% + boron iron 0.06% + copper 0.54%.

(2) Adding sequence recarburizer → iron filings → ferroalloy → scrap steel → returning charge.

(3) Feeding principle According to the requirements of different materials, the charging should be clean and dry. It is strictly forbidden to add oily, flammable materials and metal materials with serious pollution. Check the color of the color or bundle of debris before feeding.

(4) The smelting power electric furnace supplies more than 90% of the power within 6 to 8 minutes, and gradually increases the power to the maximum value.

(5) The smelting process control avoids the addition of partially closed containers such as beverage cans and closed pipes. Such containers may contain liquid or flammable materials, which may boil during melting and cause a violent explosion, thereby causing molten metal spray. splash. Regardless of the charge, the lower charge should be slowly injected before the previous charge is not melted. If the rust and sticky sand charge is used incorrectly, the block size and shape of the charge are poor, the charge of the charge is not tight and the scaffolding is severe, or if the cold charge is too much, the “bridge” is likely to occur. Therefore, the liquid level must be checked frequently. Once the bridging phenomenon occurs, immediately remove the “bridge” and avoid the “bridge” formation. Otherwise, the lower iron liquid will overheat, causing erosion of the lower furnace lining and even leakage of molten iron or explosion.

When the operator handles the bridge with tools, it should follow certain operating procedures to prevent the operator from falling into the furnace or being burnt by hot air or metal liquid under the bridge.

　　 (6) The slag should be slag immediately after the furnace is cleaned to prevent the formation of a “slag cover”. If it is formed into a "slag cover", it should be immediately powered off, and the slag cover will be broken and thrown out of the furnace, otherwise the lower iron will overheat, causing erosion of the lower lining and even leakage or explosion of molten iron.

(7) Feeding control should be carefully added to avoid the “cross-bridge” of the material. The ultra-high temperature of the molten metal under the “bridge” will cause the lining corrosion to accelerate.

(8) The testing equipment for testing the melting temperature before the furnace is the rapid thermocouple in front of the furnace. The melting temperature should be controlled at 1500~1560 °C, and the temperature of the molten iron should be ≥1450 °C.

(9) Component detection When the temperature of the molten iron reaches 1380~1450 °C, the sample is sampled for spectral analysis, and the chemical composition is checked by a spectrum analyzer and adjusted in time to ensure the chemical composition and quality of the molten iron.

(10) The silicon indole alloy was selected before the furnace, and the silicon content was 75%, the germanium content was 2%, the particle size was 1-3 mm, and the inoculation dose was 0.35%. Inoculation method: the inoculant is fully preheated and dried, and the molten iron flows into the ladle with the molten iron, and the melting is uniform.

4. Pouring method

The equipment adopts a horizontal mechanical die casting machine, and the tooling is a casting mold. Prepare before pouring:

(1) Coating preparation The paint was poured into the tank for 2.5 h, and the soap powder was added for 1 h before use. Coating ratio: silica sand powder 68%, pellet soil 16%, soap powder 1%, diatomaceous earth 9%, talc powder 7%, density 1.6 ~ 1.8g / cm3.

(2) Preparations Ready to operate tools: pouring nozzles, enamel, paint, cylinder hooks, paint buckets, graphite buckets, asbestos pads and brushes. Fully dry the enamel, pouring nozzle and stuffing cover, check whether the pouring machine is running normally, whether there are collisions, vibrations and other noises in all parts, whether the screws are loose, whether the rods, flying hammers, and stuffing cover are good, using the first package of molten iron The metal mold is preheated to a temperature of ≥ 280 ° C and put into normal production.

The pouring cup is bred (second gestation): the inoculant is selected from silicon germanium alloy, the Si content is 75%, the bismuth content is 2%, and the particle size is 0.5 mm. The inoculation dose is 0.05%, and the inoculant is fully preheated and dried, and then placed in a crucible.

Pouring operation: put a 3mm thick small end asbestos pad in the preheated casting mold, the asbestos pad is placed close to the small head positioning plate, then place the 3mm thick front asbestos pad, put on the front cover, and open the pouring machine start button. The casting machine is rotated clockwise, and then the coating is quantitatively applied. The thickness of the coating layer is 0.8 to 1.2 mm, and the pouring nozzle is extended into the suffocating cover by about 10 mm. Pour the molten iron into the quantitative 坩埚 from the big bag, hold the hook on the left to block the slag, turn the slag in the right hand, and pour the molten iron for 5~15s, then start to release the water to cool. When the temperature in the mold drops to 900°C, stop the water release for 40~70s. Stop and turn on the reducer switch to make the blank out of the mold. After the blank is released from the mold, the impurities in the mold and the residual iron in the crucible are cleaned, waiting for the next pouring, and the mold temperature is controlled above 280 °C.