In recent years, along with the production and development of science and technology, domestic and foreign steel in terms of the development is very fast, a lot of new material and new smelting technology, casting technology is also in unceasing enhancement. This requires us to constantly on the exploration of new, good methods to realize the fast development of steel castings, for the country to make greater contributions in large steel castings, some defects such as crack on must make adjustments. During the casting process, we must constantly improve the method, to refine on, so in the world manufacturing industry in the remain invincible.
Casting is a casting mold manufacturing, metal smelting, and pouring a molten metal, pressure or suction mould cavity, after the solidification of a certain shape and properties of the blank or parts of the molding method. Casting the obtained blank or parts for castings. Large steel casting is a casting a, wherein, large steel castings defect formation and casting design, sand ( including paint ), steel, plastic, finishing, welding and heat treatment process. In these processes, if improper control will form the defects, some defects are not a cause of formation. In order to eliminate or reduce defects must be comprehensive treatment can receive good results, at present, some defects still difficult to completely avoid. When the casting stress of metal over the tensile strength of the material, casting will crack. Crack is a serious defect, must try to prevent. According to the temperature difference, crack can be divided into hot and cold cracks of two.
1 hot crack
Hot cracking is the solidification period, high temperature metal strength is low, the metal to produce larger shrinkage, and by the mold or core block, the mechanical stress occurring over the temperature of metal tensile strength, and heat cracking. The shape feature is: the shorter dimension, gap is wider, zigzagging shape, slot presenting with severe oxidation color.
2 internal stress
Casting stress is mainly due to casting in solid contraction caused by blocked. These obstacles include mechanical and heat block. Mechanical resistance induced by internal stress and easy to understand, such as the core, casting or casting riser for casting shrinkage of block. The resulting stress is temporary, once the mechanical stress elimination, disappears.
Thermal stress is difficult to understand, it is related to the structure and casting. Non uniform wall thickness of casting, cooling processes in various parts of the cooling rate is not a thin wall portion due to cold, fast, took the lead from the stage of plastic deformation of the elastic deformation stage, at this time, because the thick part is still in the stage of plastic deformation, thick, thin between two parts can not generate stress; when a thick wall portion from plastic deformation stage into the elastic deformation stage of elastic contraction, due to the two parts as a whole, thick wall part of the elastic contraction is bound by the thin wall portion of the elastic block, in order to maintain their common length, thick wall part of thin wall part of it by tensile stress, and the thin wall part is opposite to the force — pressure stress. Therefore, must make the uniform wall thickness of the casting, avoiding the metal local accumulation, in order to reduce the thermal stress.
Casting stress will lead to casting deformation, even deformation. Therefore, the primary task is to eliminate internal stress.
3 hot cracks
The alloy properties. Metal casting crystallization characteristics and chemical composition on thermal crack has significant influence. Crystallization temperature range is wide, solidification shrinkage heals big, hot cracking tendency of alloys are larger. Gray cast iron and ductile cast iron due to solidification shrinkage is small, so the tendency of hot tear is smaller. Cast steel, cast aluminum alloy white cast iron, some hot cracking tendency of large. Such as: sulfur, phosphorus and other harmful elements content of high heat of formation of crack in steel; inclusion and segregation is easy to form stress concentration.
The 4 mold resistance
Mold, core concessions on hot tear formation has important influence. Concessions as possible, mechanical stress and smaller, formation of hot cracking possibility also smaller. Such as: steel liquid shrinkage is larger, hot crack tendency is greater; casting design bad structure is easy to generate local stress concentration, can also cause thermal crack; molten steel pouring temperature is high, easy to produce hot crack; casting riser arrangement, position is undeserved, cause contraction of suffocate suffocate and hot crack; sand Chung tight, yielding undesirable, hinder contraction, increasing hot cracking tendency; casting cooling speed too fast or loose box time is not reasonable, it will result in greater cooling stress, increasing the hot cracking tendency; casting pouring riser cutting, undeserved, can also cause thermal cracking.
5 prevention crack measure
Chemical composition of alloy. For steel castings, in the technical conditions allow, in the lower carbon steel, withω C 0.21%-0.25% for medium carbon steel instead of low carbon steel, especially steel castings with complex structure when it is necessary, in order to avoid serious heat crack defect. The sulfur content of steel, phosphorus should be controlled inω s<0.025%,ω p<0.03%.
The grain refinement element is to prevent hot cracking and effective measures. For example, for the carbon steel and alloy steel castings for micro alloying and modification treatment can make the first crystal grain refinement, reducing casting hot cracking tendency. Commonly used micro alloying elements (Nb ) with niobium, vanadium ( V ), ( Ti ) titanium, zirconium and rare earth ( Zr ) ( RE ) and other elements.
In addition, appropriate adjustment of hot cracking tendency of alloy composition, lower alloy linear shrinkage, reduce as much as possible, shrinkage and porosity defects such as inclusions, but also help to reduce casting hot cracking tendency.
6 casting process
From the prevention of casting heat cracking ways, in the actual production should be according to the structure characteristics, the right to take various casting technology, comprehensive preventing casting crack. To choose reasonable materials, so that the sand has good high-temperature concessions, so that the casting solidification shell free shrinkage, reduce the shrinkage stress of casing. The correct design of gating system, single ingate sectional area should not be too large, as far as possible the use of decentralized multichannel ingate, avoid the sprue and runner formed hot section castings. Casting, casting riser and joint should have suitable fillet, riser shape and position should not hinder the normal casting contraction, and to ensure that all parts of the casting solidification rate to converge. Improvement of casting structure design, and try to make the uniform wall thickness of the casting, avoid wall thickness mutation and multiple connection. The two wall junction should be sufficiently large fillet radius, to avoid the figure ten intersecting hot spot, it changed to T type hot day. Is easy to pull stress part setting reasonable anti-crack rib and cold iron.
Casting wall connection should avoid metal accumulation and internal stresses. Casting wall corner should be generally rounded structure. Casting round the size of casting wall thickness to adapt. Usually should make the corner casting fillet R adjacent wall thickness less than 1.5 times, while too large to increase shrinkage hole. In order to reduce the casting on the hot spot and the phenomenon of stress concentration, to prevent hot cracking, casting wall and wall should be avoided between acute angle connection, thick, thin wall are connected should be a gradual transition, avoiding mutation.
In order to prevent hot cracking, in casting crack easily adding crack control reinforcement, the crack control reinforcement can play the anti-cracking effect, rib direction and mechanical stress direction, and the rib thickness should be connected to the wall thickness of 1/4-1/3. The crack control reinforcement is very thin, therefore in the cooling process and solidification has higher strength, thus increasing the wall connection stress. Crack control reinforcement used in theand so easy to hot cracking of alloy.
In the corner and easy to crack, the reasonable placement of external iron chill, can make this part first accelerated cooling, solidification, so as to ensure the casting shrinkage, the site has sufficient tensile strength to withstand stress to avoid heat crack.
Appropriate pine box time can make also are in the condition of high temperature casting reduces due to pressure tank or box of fastening the superposed part of the stress, thereby reducing the casting heat cracking opportunity; at the same time appropriately extended casting pouring in situ after retention time to avoid casting hot cracks have a certain effect.
For large wheel spokes and web plate of steel castings, in different forms, the anti-cracking effect is also different. Should try to avoid even symmetrically aligned spokes and a horizontal plate. If even the spokes are connected, when the alloy shrinkage and hub, wheel rim and spokes thickness difference is bigger, its different cooling rate, systolic time inconsistency, thus forming the large internal stress. Even spoke to the casting stress by deformation to relaxation, the spokes and the rim is connected fixedly often produces crack. The odd number of spokes are connected, for each wheel location relative to the site for the rim, the stress through the rims of the trace deformation to relaxation, thereby avoiding crack.
Summary. Steel application in industry is very broad. The new century steel industry is facing the problem of technology, management, economy and ecological environment in four aspects of the challenge, so its development characteristic is no longer yield and quantity increase, but crop is relatively stable or slightly downward trend, but the steel casting quality, variety, performance and special alloy steel and steel ratio increasing. Thehas high strength, ductility and toughness, resulting in heavy mechanics for the manufacture of heavy load heavy parts, for example, bolster, side frame are used in steel castings. In order to be able to cast a good steel castings, must adopt reasonable and perfect casting process, in the casting process, not only to ensure that the steel castings in shape and size to meet the technical requirements, and special attention should be paid on casting solidification process control, to ensure the casting has good internal quality. On casting solidification process are correct and effective control, in order to get tight organization, no shrinkage and cracking of internal defects in sound castings.
Through the analysis of a large steel casting heat cracking reasons, the prevent measures are analyzed, and its feasibility was confirmed, we hope that the research can help to obtain high quality castings.