T.O. 33B-1-13-31b. Quantitative to some extent.c. Ultra-high permeability. Virtually no rententivity.d. Can bend in one direction to conform to very tightly curved surfaces. The 0.002” thick QQIs canconform to radii down to about 1/8”.e. Can be re-used with careful application and removal practice.3.3.9.3.2.3 DisadvantagesoftheQQI.a. Parts must be clean and dry for application.b. Usefulness readily destroyed with careless handling.c. Not well adapted to dry powder applications.d. Physical size limits application to some areas.3.3.9.3.2.4 ApplicationoftheQQI.To be effective, the QQI must be placed flaw side down and in intimate contact with the part surface. Also, it must beemphasized that since the QQI responds to the field in its immediate vicinity, indications can be produced in the QQIwhen no other ferromagnetic material is present. Obviously, the primary rule of assuring that the part is ferromagneticbefore attempting an inspection applies with the use of QQIs.3.3.10 SensitivityLevel.Any factor that affects the formation of magnetic indications at a discontinuity affects the sensitivity of that magneticparticle inspection. Three of the most important factors are the field direction, current level and control of themagnetic particle inspection media.3.3.10.1 CurrentLevel.The formation of magnetic particle indications at discontinuities depends upon the strength of the correspondingleakage fields. Since the leakage fields result from the field generated by the magnetizing current, the greater themagnetizing current, the greater will be the strength of the leakage fields. Thus, the sensitivity of a magnetic particleinspection is directly related to the exciting current. A current level that is too low produces leakage fields too weak toform readily discernible indications; and a current level that is too high creates a heavy background accumulation ofparticles which masks an indication. In circular magnetization, a high current level may also burn the contact points ofa part.3.3.10.2 InspectionMedia.Sensitivity level is affected not only by the current amperage, but also by the kind of magnetic particle inspectionmedia, its control and its applications.3.3.10.2.1Liquid suspensions, because of the smaller particle sizes, are the most sensitive for the detection of surfacediscontinuities. Dry powders can be better for detecting subsurface defects. Fluorescent materials have a higherapparent sensitivity than do those used with visible light, such as the black and red particles.3.3.10.2.2Inspection of parts made of materials only moderately retentive requires careful control of the way the inspection mediais applied. Usually, maximum sensitivity is obtained by applying the media while a part is being magnetized andending it before the magnetizing field is removed. This is also true in the case of automatic wet-method inspection inwhich the main bath stream is shut off shortly before the magnetizing current is ended; to avoid washing off indicationsalready formed.
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