What Is a Fissure-in-Ano?
Cracks, for example: 1, cracks generated by the material under stress or the environment (or both); 2, cracks; 3, the crack-like pattern intentionally made when the porcelain is fired; 4, GB-T232 -1988 Metal bending test method: micro-cracks: length less than 2mm, width less than 0.2mm; cracks; length 2-5mm, width 0.2-0.5mm; cracks: length greater than 5mm, width greater than 0.5mm; cracks: cracks across the full width . There are also cracks of different materials.
- Name
- crack
- Foreign name
- crack
- Category
- Proper noun
- Subject
- physics
- Production conditions
- Produced under stress or environment
- Classification
- Micro and macro cracks
- Cracks, for example: 1, cracks generated by the material under stress or the environment (or both); 2, cracks; 3, the crack-like pattern intentionally made when the porcelain is fired; 4, GB-T232 -1988 Metal bending test method: micro-cracks: length less than 2mm, width less than 0.2mm; cracks; length 2-5mm, width 0.2-0.5mm; cracks: length greater than 5mm, width greater than 0.5mm; cracks: cracks across the full width . There are also cracks of different materials.
Crack definition:
- 1 Cracks created by materials under stress or the environment (or both). Divided into micro cracks and macro cracks. The process of crack formation is called crack nucleation. The process of growing micro-cracks and macro-cracks under the action of stress or environment (or both), is called crack growth or crack growth. The cracks spread to a certain extent, which causes the material to break. Cracks can be divided into: fatigue cracks under alternating load; creep cracks under the combined action of stress and temperature; cracks generated during the loading process in an inert medium; stress corrosion cracking under the combined action of stress and chemical medium; Hydrogen-induced cracking. The formation process and mechanism of each type of crack are different. The appearance and propagation of cracks significantly deteriorate the mechanical properties of the material. Crack resistance is the material's ability to resist the generation and propagation of cracks, and is one of the important performance indicators of the material.
- 2 Ragged
- 3 The porcelain is intentionally made like a cracked pattern during firing.
- 4 GB-T232-1988 Metal Bend Test Method Appendix A
- Microcracks: less than 2mm in length and less than 0.2mm in width;
- Crack; length 2-5mm, width 0.2-0.5mm;
- Crack: length is greater than 5mm, width is greater than 0.5mm;
- Cracking: cracks in full width
- Crack
- A steel ingot defect. Cracks can be divided into surface cracks (Figure 1) and internal cracks (Figure 2) according to the location of the ingot. Surface cracks can be observed with the naked eye during finishing. Among them, transverse cracks can cause the rolling material to crack, and longitudinal cracks can cause the rolling material to split. Internal cracks can only be found during low magnification inspection or nondestructive testing. Causes internal cracking of rolled materials, which can cause delamination of rolled materials in severe cases. According to the period of formation, cracks can be divided into hot cracks and cold cracks. The former is caused by the thermal stress, the static pressure of the molten steel, the shrinkage resistance of the ingot shell and other external forces during or shortly after the solidification of the steel ingot; the latter is due to the phase change structure when the steel ingot is cooled to the solid phase transition. Caused by the effects of stress and thermal stress. Cold cracks have metal sounds when they are formed, so they are also called "crack cracks". Hot cracked fractures are rough and dull; cold cracked fractures are smooth and metallic.
Cause of crack
- Under the action of stress, the local actual deformation of the steel ingot exceeds its plastic limit, which causes local fracture, that is, cracks. Hot and cold cracks have different stress sources and fracture mechanisms.
Causes of thermal cracks
- During the temperature drop of the steel after solidification, it appears brittle in three temperature zones. The chemical composition, especially the carbon content, in steel affects the severity of brittleness at high temperatures and the brittleness temperature range. Generally, steel enters the first brittle temperature region shortly after solidification. At this time, the solidified ingot shell of the ingot tends to leave the support of the ingot mold wall due to condensation shrinkage, and thus bears the static pressure tensile effect of the molten steel inside. At the same time, some irregularities in the mold wall hinder the shrinkage of the steel ingot shell, and tensile stress will also be generated to affect the local shell. Because the steel is in the high temperature brittle zone, the local deformation of the shell caused by this kind of stress can easily exceed the maximum allowable deformation of the steel, which will cause surface cracks. As the temperature decreases, it will soon enter the second high temperature brittle zone of the steel. For low carbon steel, 1000 to 1100 ° C is the second brittle region. At this time, because the temperature and the rate of temperature change of each point in the steel ingot are different, it is easy to cause local deformation to exceed the allowable maximum deformation amount, thereby causing surface cracks and internal cracks, or expanding existing cracks.
Causes of cold cracks
- The specific volume of austenite in steel is the smallest, followed by ferrite, and the specific volume of martensite is the largest. So as the temperature
- Cold crack is the main defect of alloy steel, which often occurs when austenite is decomposed in the temperature range of 850 600 . In some cases, cold cracks can form due to decomposition of retained austenite below 300 ° C.
- Hot crack prevention measures Hot cracking is caused by the local deformation of the steel ingot under the thermal stress exceeding the allowable maximum deformation. Therefore, increasing the high-temperature plasticity of steel, that is, improving the high-temperature crack resistance of steel, is an important aspect of preventing cracks in steel ingots. Therefore, reducing the sulfur content in the steel, appropriately increasing the manganese content in the steel, and using modifiers or other technological measures to increase the equiaxed crystal ratio in the steel ingot can increase the crack resistance of the steel and play a role in preventing cracks.
- In addition, different external measures can be taken to prevent external causes during crack formation.
- Shrinkage resistance Cracking Shrinkage resistance acts on the surface of the ingot. Cause various types of surface cracks:
- (1) Vertical cracks and angular cracks near the fins. Due to the cracking of the inner wall of the ingot mold, molten steel penetrates to form fins, which hinders the local condensation shell from contracting and shrinking, causing cracks nearby. Therefore, the cracks on the inner wall of the ingot mold should be ground to prevent them from causing cracks in the ingot.
- (2) The surface is cracked. Because the inner wall of the steel ingot mold is cracked, the molten steel infiltrates and produces mesh-like fins, which locally hinders condensation and shrinkage, resulting in mesh-like cracks. The preventive measures are to pay attention to the grinding of the inner wall of the ingot mold and the renewal of the ingot mold.
- (3) Lower transverse crack. Due to the damage of the mold bottom brick, the bottom hole of the ingot mold was melted and stuck to the steel, which prevented the longitudinal condensation and shrinkage of the ingot, resulting in transverse cracks in the lower part of the ingot. In addition, if the inner wall of the ingot mold is too rough, it will also hinder the longitudinal shrinkage of the ingot, resulting in transverse cracking. Preventive measures are to pay attention to placing the mold bottom bricks and strengthen the ingot mold grinding management.
- (4) The head is split. Due to damage to the insulation lining, improper installation, or molten steel overflowing the ingot mold, the ingot is suspended, causing the head to undergo transverse cracks under the action of the ingot heavy tensile force. In order to prevent this kind of horizontal crack, install and place the heat insulation cap correctly, pay attention to the sealed connection between the heat insulation cap and the steel ingot mold to prevent the molten steel from penetrating the gap and causing the suspension. At the same time, do not spill the steel and cause suspension.
- (5) Flash at the bottom. Because the ingot mold and the chassis are not tightly connected, the molten steel penetrates to form a flash and prevents the bottom from shrinking. Therefore, it should be noted that the bottom surface of the mold is ground and the mold operation is good.
- (6) The bump blocks the crack. Due to the indentation of the ingot surface caused by the mold wall dents, the partial shrinkage of the blank shell is hindered, resulting in cracks. The pits on the mold wall need to be ground.
- (7) The heavy skin is adjacent to the crack. Due to the heavy skin's hindrance to the local shrinkage of the blank shell, shrinkage hindering stress is generated. At the same time, due to the dissatisfaction of molten steel penetration at the edge of the heavy skin, surface pits were formed, where heat conduction deteriorated, and the green shell was thin. Therefore, cracks occur near the heavy skin. In order to prevent such cracks from pouring, the injection rate should be properly controlled, the change is slow, the pouring is stable, and the formation of heavy skin is prevented.
- (8) Viscous mold hinders cracks. Because the mold temperature or injection temperature is too high, the green shell sticks to the mold, hinders shrinkage, and causes cracks. The preventive measures are to properly control the mold temperature and injection temperature.
- When a flat ingot longitudinally cracked ingot shell is subjected to the static pressure of molten steel, each side of its cross-section looks like a beam that is fixed at both ends and bears the static pressure uniform load. The span of the two fulcrum points of the beam on its wide surface is the largest, so the tensile stress at the center of the wide surface is the largest, which can easily cause longitudinal cracks. In order to reduce the tensile stress there, it is necessary to try to reduce the distance between the static pressure bearing points on the wide surface of the ingot. For this reason, for general flat ingots, the width-to-thickness ratio should be controlled not greater than 3. For large flat ingots, several longitudinal ribs can be designed on the wide surface to increase the fulcrum of the mold wall to the shell and reduce the span between the fulcrum. .
- When the axial intergranular crack is solidified in the ingot core, the outer layer of the steel ingot restrains the solidification shrinkage of the core part, which generates a tensile force, which causes the ingot core to generate radial cracks. At this time, if the fluidity of the ingot molten steel is not good enough to make up for the existing cracks, the cracks will remain and become axial intergranular cracks. Steels with high chromium content, such as Cr5Mo, 1Crl3, Crl7, Cr25, Cr27, 18CrNiW, etc., are poor in thermal conductivity, poor in high temperature plasticity, and poor in fluidity, and are therefore prone to such cracks. The prevention method is to increase the taper of the ingot mold to improve the conditions for the molten steel replenishment when the ingot core is solidified, so as to fill and eliminate the axial intergranular cracks. In addition, increasing the processing compression ratio of steel ingots can also promote the healing of axial intergranular cracks.
- Cold crack prevention measures The structural stress that occurs due to phase transition is the main cause of cold cracks. Therefore, in the cooling process, the steel grade with a smaller volume change in phase transition has a lower tendency to cold crack. Therefore, the cold cracking tendency of the austenitic steel and martensitic steel is the largest, the pearlite steel is the second, and the ferritic steel is smaller; the austenitic steel has no phase change when cooling, and basically does not cause cold cracking. In order to prevent cold cracking, the condensed steel ingot can be cooled slowly and kept at a high temperature for a long time to ensure that austenite can be fully decomposed into pearlite to reduce the phase change structure stress. In actual operation, depending on the type of steel and the size of the ingot, that is, according to the severity of the thermal stress and the phase change structure stress, different slow cooling measures can be adopted.
- There are three commonly used slow cooling measures:
- (1) Mold cooling, that is, leaving the steel ingot in the heated ingot mold to cool it slowly before demolding it;
- (2) Pit cold, the red hot steel ingot that is about to be demolded is piled into a heat-resistant pit made of refractory material and slowly cooled;
- (3) Annealing, that is, the red hot steel ingot that is demolded is sent to a special annealing furnace for long-term annealing to ensure that the austenite structure is completely decomposed into pearlite.
- Generally, for different steel types and ingot sizes, the measures selected are:
- (1) Pearlite steel (less alloying elements and lower carbon content) uses die cooling.
- (2) Pearlite steel (more alloying elements and higher carbon content), small steel ingots use die cooling, and large steel ingots use pit cooling.
- (3) Pearlite-martensitic steel, pit cooling or annealing.
- (4) Semi-ferrite-semi-martensite steel, martensitic steel with less carbon, small steel ingots are pit-cooled, and large steel ingots are annealed.
- (5) Martensitic steel (higher carbon content) and bainite steel should be annealed. [1]