The quality of molten iron has a great influence on the lost cast iron parts. The influence of improper misalignment temperature on the defects of lost cast iron parts and the requirements of hot metal quality of different cast iron parts are analyzed. The measures and adjustment methods to prevent casting defects are analyzed. Lost Foam Casting Because the foaming mold (white mold) gasification consumes the heat of the molten iron, it is required to increase the temperature of the molten iron. Therefore, the iron collision smelting must be properly adjusted to obtain the same or better castings as the sand burning. organization.
1. Increase the pouring temperature
After the molten iron is introduced into the cavity, the casting white mold (EPS, STMMA) with the lifting system must first be gasified, decomposed and cracked. For this reason, the temperature of the sewing is generally 30-50″C higher than that of the sand casting. The squash iron piece is even increased to 80 ° C. The ductile iron temperature range is: 1380-1 480 . C. Gray cast iron is 1 360- 1 420″ C, alloy cast iron (anchor white iron) is 1 380- 1450 .C. Increasing the heat of the scratching temperature should be just consumed by burning the white touch. After that, the temperature of the molten iron should be lowered to the temperature of the sand casting, so that the qualified castings can be guaranteed. Therefore, in the actual production process, the process must be based on the unit. Conditions such as equipment find the appropriate tip temperature.
(1) Defects caused by excessive pouring temperature
a. sticky sand
Excessive shovel temperature is likely to cause chemical sticking and mechanical sticking.
Chemical grit: The sand in the sand contains fine sand, dust, especially quartz sand, which is easy to react with the iron smelt to produce chemical grit. It is extremely difficult to clean $ mechanical sand: excessive iron temperature The white paint layer peels off, cracks, softens and ruptures, the molten iron passes through cracks, cracks in the gaps, and the speed of the threading is fast, the temperature of the molten iron is high, and the degree of sticking sand is more serious. The most prone to occur is the bottom or side of the casting and the hot joint zone. The sand is not easy to be compacted, especially at the corners. The joints of the string castings are easy to form mechanical sand with mechanical mixing of the iron and the sand.
b. Back spray
The gasification mode EPS (or STMMA) pattern casts a fierce pyrolysis reaction under the action of excessively high temperature iron.
75 – 1 64 ‘C : Thermal deformation, high elastic state, the mold begins to soften and expand and deform. The air and foaming agent in the cell begin to escape, the volume shrinks, and the cell loses its viscous flow. Polystyrene steroid
164 – 316″C : Melting, the molecular weight of the flow state is unchanged g
316 – 576 “C: depolymerization, gasification state, when the weight begins to change, the long-chain polymer breaks into a short-chain low-molecular polymer, and the gasification reaction begins, producing a polyethylene monomer and its Small molecular weight derivatives constitute a vaporous product;
567-700″C: cracking, gasification combustion, precipitation gas increased significantly, low molecular weight polymer cracked into a small amount of hydrogen (0.6%), C02, CO small molecular weight saturated, unsaturated hydrocarbon g
700-1350’C: Extremely cracked, gasified and burned, and the decomposition of low molecular weight polymer is gradually completed. At the same time as the production of a large number of small molecular hydrocarbons, hydrogen and solid carbon are decomposed; the hydrogen content is precipitated at 1 350 ‘C. Up to 32%; in the presence of aerobic conditions with the presence of free carbon and flames
150- 1SS0 ‘C: Rapid cracking, combustion gasification, rapid cracking of low molecular weight polymer, precipitation of hydrogen up to 48%; at the same time the combustion process is more intense, and a large amount of free carbon and a flame generated by volatile gases are precipitated. If the pouring temperature is too high, the decomposition and cracking will be rapid, and the amount of gas will increase sharply. If the vacuum pump is too late to suck and discharge, the gas will not be able to escape, which will cause back-spraying, which may cause injury and cause accidents.
It can be seen from the above that the white mold is decomposed and cracked by the heat of the molten iron, and a large amount of gas is generated. When the temperature is too high, the well is rushed to generate gas, and the gas dispersion expands into the cavity, and the sand type cannot be discharged in time to enter the molten iron. Stomata, which is large and numerous (cluster) and is accompanied by carbon black. Too high pouring temperature, excessive heat burning sand type makes the cavity, sand type produces more gas, and can not be discharged from the mold and iron in time, it will produce pores.
Injecting ductile iron, using white mold STMMA (EPMMA), its gas generation is larger than EPS, more, more concentrated – the time zone is very cracked, more attention should be paid to the sharp generation of pores and timely exhaust (adjustment) Vacuum pump suction speed, control of iron flow and speed). In addition, the amount of gas generated by the decomposition of the white mold is large, rapid, rapid, the exhaust speed of the mold is not enough, the vacuum pump suction, the speed is insufficient, the gas impact mold, resulting in mold anger, the collapse of the casting can not be a good product . It can also cause other defects in lost foam casting: nodule, shrinkage, shrinkage, hot slag hole, etc.
(2) Defects caused by low temperature of the scratching
The main influence is that the pouring temperature is too low and the heat is insufficient. The decomposition, cracking and pores cannot be completed. The white mold pyrolysis is not complete, the gas phase product is reduced, and the liquid phase and solid phase product increase are more conducive to the appearance of wrinkled carbon deposits. The temperature of the iron liquid is lower than 1420 – 1480 ‘C, which is more likely to produce wrinkles, carbon deposits and carbon black for thin-walled iron castings. Cold insulation (for fire), heavy leather, pouring white mold is heated and decomposed, to absorb a large amount of heat, too low pouring temperature provides insufficient heat to decompose the white mold, so it is necessary to absorb heat from the iron shovel, so that the iron shovel The temperature is too low (often appearing in the wall thickness of the casting, the distance is long); the gas produced increases to prevent the filling of the molten iron, which in turn reduces the fluidity of the molten iron, thus causing cold separation, heavy skin, and pouring. When the two streams of molten iron are filled with the top of the casting type, the temperature of the iron shovel has been lowered to a lower level and cannot be fused. When the casting is started, the cold separation is likely to occur when the temperature of the casting is lower. When the pouring temperature is low, a thin iron shell (film) is formed near the casting surface, and after the subsequent iron filling, there is not enough heat to melt the film (shell), and a heavy skin defect occurs. In addition, the temperature of the pouring is too low, and the molten iron in the cavity does not have sufficient heat, so that impurities, slags, and wastes in the fast liquid cannot be raised to the top surface in a timely manner, thereby forming defects such as inclusions and slag inclusions.
2, adjust the iron liquid
Although there are differences in the heat capacity (specific heat) of different types of dry sand, the cooling rate of the mold is slower than that of sand casting. For gray cast iron, there is less tendency to white mouth. For cast iron, dry sand casting The rigidity of the type is not as good as that of the metal type (or the type of sand-covered metal). When casting the white cast iron of the anchor, the surface of the casting is not as good as the hard shell of the casting formed by the metal type, so the iron shovel or corresponding measures should be adjusted.
For lost foam casting, in order to increase the temperature of the threading, it is generally smelted by induction furnace or cupola-induction furnace.
(1) Gray cast iron
a. Cast iron parts mainly based on toughness, iron liquid plus inoculation treatment of 75% Si-Fe, or adding a small amount of aluminum, aluminum, copper for microalloying.
b. When the mechanical properties of stiffness and strength are the main requirements, reduce the carbon content, increase the amount of spheroids, and micro-synthesis of Cr and Mo to promote the increase of the amount of spheroids.
(2) Ductile iron
Smelting in an induction furnace increases the temperature of the molten iron. It is necessary to use ductile iron and spheroidizing agent suitable for induction furnace melting.
(3) Anchor anti-wear cast iron
Due to the slow cooling rate of the lost foam casting, the structure and properties of the white iron are changed and refined by the heavy rare earths; copper, front dart, silver iron microalloying
Good matrix structure performance; if the wear resistance is insufficient, adjust the size, shape and distribution of the matrix carbide to change (by adding Ming, button, sensitive, sputum, etc.).
Castings of various cast irons are affected by the slow cooling rate of the lost foam casting, which can be adjusted by the corresponding measures mentioned above.