Casting detection including size, shape and surface inspection visual inspection, chemical composition analysis and mechanical property testing, for more important or casting process is easy to produce the casting, still need to undertake a nondestructive test, can be used for ductile iron castings quality testing nondestructive testing techniques including liquid penetrant testing, magnetic particle testing, eddy current testing, radiographic testing, ultrasonic testing, vibration testing.
1 casting surface and the near surface defect detection
1.1 liquid penetrant testing
Liquid penetrant testing is used to check casting on the surface of various openings of the defects, such as crack on the surface, the surface defects such as pinholes naked eye is difficult to find. Penetrant testing is commonly used color detection, it is with high penetration ability of colored (usually red) liquid ( osmotic agent ) soaked or sprayed on the casting surface, penetrating agent into the openings of the defects inside, quick wipe the surface of liquid permeable layer, and then easy to do display agent ( also called imaging agent ) spraying to the casting surface, to remain in an open defect penetrant in the suction out, display agent was staining, which can reflect the defects in the shape, size and distribution. What need points out is, penetrant testing accuracy with the material being the surface roughness increases, whereby the surface of the light detecting effect is better, the grinder polishing surface detection accuracy is highest, can detect even a intergranular crack. In addition to coloring detection, fluorescent penetrant testing is also commonly used liquid penetrant testing method, it need to configure the ultraviolet lamp irradiation observation, detection sensitivity than coloring detection of high.
The 1.2 eddy current detection
Eddy current testing is applied to check the surface is generally not more than 6 ~ 7MM deep defect. Eddy current testing points placed type coil method and through coil method 2. : when the specimen is placed in an alternating current to the coil passes nearby, into the specimen in the specimen in the alternating magnetic field induced magnetic field perpendicular to the direction and motivation, a eddy current flowing ( vortex ), eddy current produces a magnetic field opposite to the direction of the magnetic field and the excitation coil, so that the original magnetic field has a portion of reduced, causing the coil impedance change. If the casting surface defect, the eddy current electric characteristics will happen distortion, thereby detect defects, eddy current testing is the main drawback of not displaying the detected defect size and shape, the general can only identify the defect surface position and depth, in addition it on the surface of the workpiece on the small open defect detection sensitivity as well penetrant testing.
1.3 magnetic particle testing
Magnetic particle testing suitable for the detection of surface defects and surface following several millimeters deep defects, it needs to DC ( or AC ) magnetization device and magnetic ( or magnetic fluid) to test operation. Magnetization device used in the casting and surface to generate a magnetic field, magnetic or magnetic liquid used to display defects. When casting a certain range to generate a magnetic field, magnetization regions within the defect will occur when the leakage magnetic field, and magnetic powder or suspension, magnetic powder is sucked, so that it can show defects. It displays the defects are basically transverse magnetic flaw, for parallel to the magnetic strip defect display does not come out, therefore, when the operation requires changing the magnetization direction, in order to assure to be able to check out the unknown direction of each defect.
2 casting defect detection
For internal defects, the common nondestructive testing method is radiographic testing and ultrasonic testing. One of ray detection effect is best, it can be reflected the flaw type, shape, size and distribution of the visual image, but for large thickness of large castings, ultrasonic testing is very effective, can accurately detect the position of internal defects, equivalent size and distribution.
2.1 radiographic testing ( micro focus XRAY )
Radiographic testing, usually with X X-ray or gamma ray as ray source, therefore need to produce ray equipment and other ancillary facilities, when the workpiece is placed ray irradiation, radiation intensity will be the effect of internal defects in castings. Through the casting injection of the radiation intensity of defect size, with different properties and local changes, formation of defects in X-ray image, by ray film to video recording, or through the screen to real-time detection of observation, or by radiation analyzer. Through the X-ray film recording method is the most commonly used method, also known as the radiographic testing, radiographic reflected the image of the defect is intuitive, defect shape, size, number, location and distribution range can appear, only defect depth cannot in general be reflected, need to take special measures and calculations to determine. Now used ray computerized tomography method, because equipment is more expensive, the use of high cost, it is not popular, but this new technology represents a high-resolution X-ray detection technology development direction in the future. In addition, the use of approximate point source microfocus X ray system may actually be the removal of larger focus device for generating fuzzy edge, so that a clear outline. The use of digital image system can increase the SNR of images, further improve image clarity.
2.2 ultrasonic detection
Ultrasonic testing can also be used for examination of the internal defect, it is the use of high frequency acoustic beam in casting internal communication, meet internal surfaces or defects arise when reflection and find defects. The reflected sound energy is the size of the inner surface or defects of the directivity and properties as well as the reflector of the acoustic impedance of the function, it can be applied to various defects or inner surface reflected sound energy to detect the presence of defects position, thickness or surface defect depth. Ultrasonic detection as a broader application of NDT method, its main advantage lies in: high sensitivity, can detect tiny crack; high penetrating ability, can detect heavy section casting. The main limitation is the following: in the contour dimension and directivity of bad disconnection defect reflection waveform interpretation difficulties; for unpleasant internal structure, such as grain size, microstructure, porosity, and inclusion content or fine dispersed precipitates, also prevented waveform interpretation; in addition, when detecting the need for reference standard test block. [
How to repair the defects of castings
Casting: solve the casting shrinkage method, the most basic starting point is " heat balance ". The method is: ( 1) in the cast of machine tool structure is formed in the thick and the hot spot, the rapid solidification, artificially causing machine casting temperature field basic balance. Used both insid, e and outside the cold iron, partial use of large quantity of heat storage of zircon sand, chromite sand or special coating. ( 2) the process design. Ingate in machine tool castings relatively thin wall, number and dispersion. The earliest into the thick wall of liquid metal in the solidification, thin after solidification, which basically balanced solidification everywhere. For the uniform wall thickness of machine tool castings, adopts a plurality of ingate and outlet. Ingate, dispersed and uniformly distributed, so that the overall heat balance. Vent hole and is fine, smooth exhaust and radiating role. ( 3) changing the ingate position ( 4) selection of large quantity of heat storage modeling material, the EPC production of anti-wear products is extremely important! Chromite sand to replace the quartz sand and other heat accumulation of small other sand, can get good effect, water Bi microseismic better! ( 5) low temperature fast firing, opening and gating system. The liquid metal fast, stable, balanced with casting. This is because of their. ( 6) ductile iron casting machine strength, surface hardness ≥90, sand box rigidity, favorable to eliminate shrinkage. ( 7) need to riser, while the first move heat riser away from hot spot, and. If the riser is arranged in the thermal Festival, will increase the riser size, forming " heat heating ". Lane is bad, not only the shrinkage are difficult to remove, also will produce the concentrated shrinkage, but also reduce the process yield rate. ( 8) mold tilt display and alloying, benefit. Elimination of shrinkage defects in machine tool castings is a complex cognitive process. Should with " heat balance " as the basic principle of the Osprey body castings, scientific analysis, formulate reasonable process, choose suitable moved the modeling material, tooling and correct operation and standardization. Then any machine tool castings shrinkage defects can be solved.
Due to many factors, often appear pores, pinholes, cracks, slag, such as pit defect. Commonly used repairing device for argon arc welding, resistance welding, cold welding. For the appearance and quality requirement of casting defects can use argon arc welding machine, heat, fast welding to repair. But in the field of precision casting defect repair, due to argon welding thermal effects, repair will cause the deformation of castings, low hardness, porosity, local annealing, cracking, pinhole, abrasion and scratching, biting edge, or binding force is not enough and the internal stress of injury and other two defects. Cold welding is overcome above disadvantages, the advantages of the small heat affected zone, no need of preheating temperature of casting, cold welding repair, no deformation, undercut and residual stress, does not generate local annealing, do not change the casting metal microstructure. Thus cold welding machine is suitable for precision casting surface defect repair. Cold welding range ofΦ 1.5-Φ 1.2mm welding melting point repeatedly stacking process, in a large area defect repair process, repair efficiency is to restrict its wide application is the only factor. For large defects, recommend traditional welding process and casting defect repairing machine composite application. Sometimes we don't have a lot of defects, it is not necessary to invest more cost, we use some repair agent can be repaired, simple and convenient, such as iron, we can use (Jin et into ) JS902 repairing it, you can use aside for later use, so can we factory cost saving ah, let our casting manufacturers put more money into improving the quality of the product itself, allowing users to create more wealth.
