Steel Casting Heat Treatment
Steel casting is made of cast steel, similar to cast iron, but better than cast iron. In the process of casting, the defects of stomata defect and Angle positioning are not accurate. In the long term, it is possible that the casing fracture will occur.
Steel used for casting casting. A kind of casting alloy. Cast steel is divided into casting carbon steel, casting low alloy steel and casting special steel 3.
Cast carbon steel
Carbon is a major alloy element with a small number of other elements. The carbon steel containing less than 0.2 percent of carbon is cast low-carbon steel, containing 0.2 percent to 0.5 percent of carbon steel, which is more than 0.5 percent carbon steel. With the increase of carbon content, the strength of cast carbon steel increases and the hardness increases. The casting carbon steel has higher strength, plasticity and toughness, and the cost is lower. It is used in heavy machinery to manufacture the parts that bear the heavy load, such as the frame of the rolling mill and the base of the hydraulic press. Part of a railway vehicle used for making large and bearing shocks such as a rocking pillow, side frame, wheel and car hook.
Cast low alloy steel
Cast steel containing manganese, chromium, copper and other alloy elements. The total amount of alloy element is less than 5%, has greater impact toughness, and can obtain better mechanical properties through heat treatment. Casting low alloy steel has better performance than carbon steel, which can reduce the quality of parts and improve service life.
Special steel is cast
Alloy cast steel, which is refined for special needs, has a variety of varieties, usually containing one or more of the high alloy elements to obtain some special performance. For example, high manganese steel with an 11% to 14% manganese steel can withstand the impact wear, and is used for wear-resisting parts of mining machinery and engineering machinery. Is mainly made of chromium or nickel chromium alloy elements of all kinds of stainless steel, used in corrosion or high temperature above 650 ℃ under the condition of working parts, such as chemical in the body, pump, the container or the large capacity power plant steam turbine casing, etc. Inspection of casting surface and near surface defects of cast steel casting
Liquid penetrant detection is used to check the various opening defects on the surface of the casting, such as surface cracks, surface pinholes and other undetectable defects. Commonly used penetrant testing is shading detection, it is capable of high permeability colored (red) liquid (penetrant) soaking or spraying on the casting surface, penetrating agent into the open defect, a quick wipe surface infiltration liquid layer, and then will be easy to do agent (also called imaging agent) sprayed on the surface, the casting stay will remain in the open defect penetrant after coming in, according to agent is dyed, which can reflect the shape, size and distribution of defects. It should be pointed out that, the accuracy of the penetrant testing decreased with increasing surface roughness checked material, namely the light detection, the better the results, the surface grinding machine for grinding the surface of the highest detection accuracy, can even detect the intergranular crack. In addition to colouring detection, fluorescence penetration detection is also a commonly used liquid penetrant detection method. It needs to be equipped with ultraviolet lamp to conduct irradiation observation, and the detection sensitivity is higher than that of stain detection.
Eddy current testing for eddy current testing is suitable for defects that are not more than 6 ~ 7MM below surface. Eddy current testing is divided into two types: the placement coil and the passing coil method. : when the specimen is placed near the coil of alternating current on, into the alternating magnetic field of the specimens can be induced in the specimen perpendicular to the direction of the magnetic field phase, the shape of vortex flow of current (eddy current), eddy current will produce a and motivation in the opposite direction from the magnetic field magnetic field, reduce part of the original magnetic field in coil, thus cause the coil impedance change. If the casting defects in the surface, the electrical characteristics of the eddy current distortion happens, to detect the presence of defects, the main disadvantage of eddy current testing is not intuitive display detected defect size and shape, normally only determine the defect on the surface of the position and depth, and it small openings on the workpiece surface defect detection sensitivity than penetrant testing.
3 magnetic particle testing magnetic particle testing is suitable for testing surface defect and number of mm deep beneath the surface defects, it needs a dc or ac magnetization equipment and magnetic powder can (or magnetic suspension) testing operation. Magnetizing equipment is used to generate magnetic field, magnetic powder or magnetic suspension on the inside and outside of the casting. When a magnetic field is generated within a certain range of the casting, the defect in the magnetized area will produce an leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic particle is sucked in so that the defect can be displayed. The defects of this show are basically crosscutting defects of the magnetic field lines in parallel to the magnetic field lines are defects showed not to come out, therefore, the operation need to change the direction of magnetization, in order to make sure to check out the direction of the unknown various defects.
2. The detection of internal defects in castings is an internal defect, and the commonly used nondestructive testing methods are ray detection and ultrasonic testing. Ray detection effect is best, it can be reflect the internal defect type, shape, size and distribution of the visual image, but for large casting large thickness, ultrasonic testing is very effective, can more accurately measure the position of the internal defects, equivalent size and distribution.
1 ray detection (microfocus XRAY), radiographic testing, in general use x-rays or gamma rays as a source of radiation, so you need to produce radiation equipment and other ancillary facilities, and when the workpiece on ray irradiation field, ray radiation intensity will be affected by the internal defects of castings. Through the casting of radiation intensity as the defect size and nature of the different with the change of the local, ray image formation defects, recorded by X-ray film imaging, or through the screen to observe real-time detection, or counter by radiation detection. Which record by ray film imaging method is the most commonly used method, known as the radiographic inspection, radiographic reflected defect image is intuitive, defect shape, size, quantity, plane position and distribution can be presented, only defect depth generally cannot be reflected, need to take special measures and calculations to determine. Now appear ray computer tomographic photographic method, because the equipment is expensive, the cost is high, it is not popular, but the new technology represents the high-resolution X-ray testing technology for the future development direction. In addition, the micro focus X-ray system with approximate point source can actually eliminate the fuzzy edge generated by the larger focus device, so as to make the image clear. The digital image system can improve the image's signal-to-noise ratio and improve the image definition.
Heat treatment of cast steel parts
Steel casting shall be used after heat treatment. Because of as-cast steel internal porosity, crack, shrinkage cavity and shrinkage, coarse grains, organization and uneven residual stresses in the casting defects, such as make the strength, especially the plasticity and toughness of steel is reduced greatly.
In order to refine grain, uniform tissue and eliminate internal stress, steel casting must be treated with normal fire or annealing. The mechanical properties of the steel after the fire treatment are high and the cost is lower, so the application is more. However, due to the positive heat treatment, the internal stress of the lower annealing can only be applied to steel casting parts with carbon content less than 0.35%. Because the low carbon cast steel plastic is good, it is not easy to crack when cooling. In order to reduce the internal stress, the cast steel parts should be heated after the positive fire. For steel parts with complex structure and easy to produce cracks with carbon content of more than 0.35%, the annealing treatment can only be carried out. Cast steel should not be hardened, otherwise easy to crack.