Inspection of casting

Different quality demands for different castings, then the inspection methods will be different.

1.  Visual inspection　 The visual inspection means inspect the casting surface appearance based on vision and some simple inspection tools like magnifying glass and tapeline, for example, inspect the surface roughness, surface defects, shape deformation, size deviation, weight deviation etc.

2.  Nondestructive inspection1) Radiographic inspection　 　 The radiographic inspection can be divided into X-ray detection and γ-ray detection, which can detect the inner defects of casting.　 　 The principle of radiographic inspection is that as the X-ray or γ-ray through the casting, the ray energy will be absorbed by casting atom to be reduced, then the ray intensity will be different, which will show different black ranges on the photographic film after the sensitization, and the stronger ray intensity will bring larger black range. If there are some holes in the casting, the ray intensity through the casting will be stronger, and then we can know the inner defects, even know the section shape.  　 For steel casting, the X-ray through thickness is 180mm, and its 300mm for γ-ray.

2) Magnetic inspection　 　 The magnetic inspection is used for detecting the defects near the casting surface, such as cracks, holes, slag inclusions etc. On inspection, put the casting in the middle of anode and cathode of electromagnet to let magnetic line through the casting, and spread magnetic powder or its suspension on the inspected surface. If there’s a defect in the casting, which will cause great reluctance for the magnetic line, then the magnetic line will leave the casting and detour around the defect, then go back into the casting again. These magnetic lines will hold the magnetic powder on leaving casting, and then form a pattern similar with defect. The location of pattern is the location of defect.　 　 The magnetic inspection demands smooth casting surface, so the casting need to be polished.

3) Ultrasonic inspection　 　 The ultrasonic inspection can detect the defects like hole, cracks, slag inclusions etc, and its detective thickness can up to 1000mm for steel casting.　 　 The principle of ultrasonic inspection is that as the ultrasonic propagates from one media to another media, it will be reflected from the interface, especially between air and metal, almost 99% will be reflected.　 　 For ultrasonic inspection, it demands the surface roughness Ra≤0.01mm for machined and Ra≤0.012 for not machined. Then brush couplant on this surface such as engine oil, water, paste etc, which will let the probe of ultrasonic defectoscope fit good with casting surface, and let most of ultrasonic though the casting. On inspection, move the probe slowly and adjust the sensitivity, then notice the graphic on the screen, which will show the size and depth of the defect. If there’s a defect, the ultrasonic will be reflected on shown on the screen. The distance form surface reflection to defect reflection shows the depth, and the higher crest of reflected wave shows bigger defect. 　 　 The frequency of ultrasonic is 1.5-4.5MHz. It has big detective range and good sensitivity. But it’s only used for simple shape casting, and its request for roughness is high, so, its use is limited.   

4) Fluorescent inspection　 　 For fluorescent inspection, it will use fluorescent liquid. This liquid is a mixture of 85% kerosene and 15% aviation gasoline on mass fraction. It has the characteristic that intense fluorescence will shown under the ultraviolet irradiation.　 　 On inspection, brush above fluorescent liquid on the casting surface. This liquid will

penetrate into the cracks. Eliminate the superfluous liquid, and then spread showing powder such as talcum powder on the surface. These powders will absorb the liquid penetrating into the cracks for capillarity. Under the ultraviolet irradiation, it will show bright white if there’s a crack.　 　 The fluorescent inspection is easy and with good sensitivity. But it’s only used for inspect the surface cracks, not for the inner cracks.

5) Leakage test　 　 The leakage test is used for testing the compactness of casting. Some parts demand to withstand middle or high pressure, such as valve body, pump body, cylinder body etc. They all need leakage test.　 　 On testing, press air or water under certain pressure into the casting cavity. If there’s any defects like dispersed shrinkage, cracks etc, the air or water will leak out from the casting wall, then the defect is detected. The testing pressure should be 30%-50% more than the working pressure.　 　 It’s a little difficult to find air leakage, so, the small casting part can be put into the water, and the big casting’s surface can be brushed with suds. For using water, it’s easier to find the defects. If the casting part is without airtight cavity, we can add the kerosene in the cavity and spread white powders on the outside surface. After some time, usually 24 hours, if there’s kerosene in the white powder, that’s the leakage.