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众所周知,磁化场垂直于缺陷时,磁粉显现的缺陷最清楚;磁化场倾斜于缺陷时,磁粉显现缺陷的能力变差了;而磁化场平行于缺陷时,则磁粉根本不显现缺陷。这在磁粉探伤实践和理论上都十分重要,可是作者至今尚未见到对此问题全面、精确的论述。另外,作者1979年还遇到过下述疑难:某热交换器低合金钢管壁上肉眼易见的巨大折叠状裂缝不论磁化电流增加到多大(近9000A)也丝毫不吸附磁粉。现有的磁粉探伤理论,不论是磁力线“折射”说,还是磁偶极子密度变化说,都无法解释上述现象。为解决这个难题,首先要弄清磁化场方向对磁粉显现缺陷的影响,故作者对此进行了定量研究。
It is well-known that when the magnetizing field is perpendicular to the defect, the defects of the magnetic powder appear most clearly. When the magnetizing field is inclined to the defect, the ability of the magnetic powder to show defects becomes worse. When the magnetizing field is parallel to the defect, the magnetic powder does not show defects at all. This is very important in the practice and theory of magnetic particle inspection, but the author has not yet seen a comprehensive and accurate discussion of this issue. In addition, the author also encountered the following difficulties in 1979: The large folded cracks visible to the naked eye in the heat exchanger low-alloy steel pipe did not adsorb magnetic powder no matter how much the magnetizing current (up to 9000A) increased. The existing theory of magnetic particle inspection, whether it is the “refraction” of magnetic lines or the change of density of magnetic dipoles, can not explain the above phenomenon. In order to solve this problem, we must first understand the influence of magnetization field on the defects of magnetic particles, so the author has carried on the quantitative research.