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Quenching treatment of chrome molybdenum-copper low alloy gray cast iron light sphere plate
1. The chemical composition and as-cast metallographic structure of the light sphere plate
For chrome molybdenum-copper low-alloy gray cast iron ball plates, most of the manufacturers use the following chemical composition: w C = 2.9% to 3.1%, w Si = 1.5% to 1.7%, w Mn = 0.8% to 1.0%, w Cr = 0.4% to 0.6%, w Mo = 0.8% to 1.2%, w Cu = 1.0% to 1.2%, and both w s and w p are <0.06%. After conventional medium frequency induction furnace melting, inoculation, sand casting, the as-cast metallographic structure is flake graphite + flaky pearlite (or pearlite + sorbite), and a small amount of carbides and binary or ternary phosphorus Eutectic is present between the eutectic groups. According to GB7216, the graphite form is A type or A + B type or B type or D + E type, and coarse C type graphite is not allowed. The graphite has a length of 4 to 6 and a content of ≥ 7%. Carbides should be ≤3, and phosphorus eutectic should be ≤2. Poor morphology or too small amount of graphite will significantly reduce the cutting force of the light sphere plate, and the ball blank will not slip under the light ball plate. Phosphorus eutectic hard and brittle exists between eutectic (750~950HV), which reduces the toughness and strength of the light sphere plate. It is prone to flaky or flaking in the work of the light sphere plate.
2. Light ball plate quenching process and precautions
The slab of the spheroidal slab is quenched after roughing to improve strength and hardness. The quenching heating equipment mostly adopts the trolley type electric resistance furnace, the quenching heating temperature is 860~870°C, the holding time is 1.5min/mm, the quenching cooling medium is L-AN32 full loss system oil, the oil temperature is 40~80°C, the board surface is out Oil temperature ≤ 120 ° C (cooling time 5 mm / min). The ball plate should be tempered after quenching, and the tempering temperature is 180~220 °C.
A pair of light sphere plates is composed of a fixed plate and a rotating plate, and the fixed plate is seriously damaged and is damaged first. In order to coordinate the service life of the two plates, it is required that the fixed plate has a hardness of 2~3HRC higher than that of the rotating plate. For the processing of large steel balls (φ15mm or more), the light ball plate is required to have higher toughness and the hardness should be lower to prevent the ball from falling or breaking during the work. In contrast, the processing of small steel balls (<φ15mm) of the light sphere plate requires a higher hardness to improve the service life of the light sphere plate. Therefore, the tempering temperature of the photosphere plate after quenching should be selected. The rotating light sphere plate for processing large steel balls is tempered at 200 ° C, and the fixed light sphere plate for processing large steel balls is tempered at 180 ° C. The rotating light sphere plate for processing small and medium steel balls is tempered at 180 ° C, and the fixed light sphere plate for processing medium and small steel balls is tempered at 160 ° C. The tempering time is ≥ 4h, and it is air-cooled after tempering.
Light ball plate quenching should pay attention to the following matters:
(1) In order to prevent the deformation of the ball plate, the bottom plate of the furnace should be flat, the pad between the ball plates should be equal, and the force of each fulcrum should be even.
(2) The cooling oil is quenched by compressed air to prevent the formation of oil and gas film on the surface of the spherical plate and affect the cooling. After the ball plate enters the oil, it should move up and down and rotate to make the ball plate cool evenly, and prevent the oil plate from partially exposing when the ball plate cools.
(3) In order to prevent cracks and breakage during the quenching of the ribbed fixed photosphere, it should be tempered in time after quenching.
(4) Limiting the amount of furnace to prevent the ball plate that is finally quenched and quenched in the furnace for too long, resulting in uneven hardness after quenching and tempering of the ball plate heated by the same furnace.
The attached table is made of RT-75-9 trolley resistance furnace, and 8 furnaces of the same shape are used in a furnace. After heating and heat preservation, two ball plates are quenched each time, and the oil cooling time is 20 minutes. The first and last quenched spheres differed in furnace heat holding time by nearly 1.5h. The hardness change after tempering is shown in the attached table. The quenching hardness of the quenched photosphere plate is basically the same, but the hardness changes after tempering. The hardness of the quenched spherical plate is reduced after tempering, and the hardness of the quenched spherical plate is increased after tempering. This is because the ball plate has two functions in tempering. One is to deposit cementite from martensite, which reduces the squareness of martensite and decreases the hardness. The second is the transformation of retained austenite to martensite. Raise. The quenched spherical plate has a short holding time in the furnace, and the amount of retained austenite is small. The hardness after tempering is decreased more than the hardness, and the hardness is decreased. Finally, the quenched spherical plate has a long holding time in the furnace, and the amount of retained austenite is relatively large. The hardness increase after tempering is greater than the decrease in hardness, and the hardness is improved.
In order to prevent the above phenomenon, the difference in the holding time of the same light bulb in the furnace should be minimized. Reducing the amount of furnace loading or increasing the number of balls per quenching can reduce the difference in holding time of the same furnace plate in the furnace, but the latter needs to consider the bearing capacity of the quenching cooling system.
3. Light ball plate hardenability and hardenability
Chromium molybdenum copper low alloy gray cast iron light sphere plate, due to its large geometric size, is inconvenient to measure with a Rockwell hardness tester, and is mostly measured by a Leeb hardness tester and converted into HRC value. In order to know exactly the hardness and hardenability of the photosphere plate after quenching, a large number of samples were cut on the finished light sphere board for hardness measurement.
According to statistics, the hardness is ≥58HRC, there are 1 C4543 board, 3 C1347 boards, and 12 G1101180 boards. They accounted for 1.6%, 5.6%, and 14% of each test point. The difference between the highest hardness and the lowest hardness of each board test point: 9HRC for C4543 board, 8HRC for C1347 board, and 12HRC for G1101180 board, indicating that the cast structure of the spherical plate is uneven. The total average hardness of each board: 54.08HRC for C4543 board, 54.42HRC for C1347 board, and 55.10HRC for G1101180 board. It indicates that the hardness of the photosphere plate after quenching should be 54~55HRC. This is consistent with the upper limit of hardness required by many domestic and foreign light sphere boards. The hardness of the light sphere boards produced by different manufacturers is generally between 52 and 54 HRC. Therefore, the hardness of the chromium-molybdenum-copper low-alloy gray cast iron light sphere plate after quenching should be ≤55HRC.
Generally, the factory hardness of each company's light sphere board (measured by the Leeb hardness meter HLD) is 5~6HRC higher than the hardness measured by the Rockwell hardness tester. Therefore, the factory hardness of the light sphere board minus 5~6HRC is the exact Rockwell hardness value of the board. The thickness of the finished light sphere plate is mostly 100mm. The hardness of the measured plate surface is basically the same as the hardness at the plate thickness of 50 mm, indicating that the quenching depth of the spherical plate of the component is above 50 mm, which can fully meet the requirements of the use of the light sphere plate.
About the author: Wu Xianzu, Henan Xinxiang Piston Co., Ltd. grinding board branch.