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涡流检测中磁性夹杂的困惑

A competent eddy current Inspector would know how to distinguish between deep internal pits and magnetic inclusions by using a multi-frequency test。

一个资深的涡流检测员应该知道如何用多频涡流来区分较深的内部缺陷和磁性夹杂

To show how easily this confusion can be made,Figure 1 shows the signal from the through wall hole in an ASME Calibration Tube and Figure 2 shows the signal of a magnetic inclusion in the same tube(or is it the other way around?).It can be seen that in Figure 1,the through wall hole is correctly analyzed as 100 per cent through the wall(6 th line of text in the menu),and in Figure 2,the signal from the magnetic inclusion where there is little or no wall loss has been analyzed as 98 per cent through the wall.

在实际检测过程中,很容易将深部的内部缺陷和磁性夹杂相混淆,图1显示了通孔缺陷的结果,图2是同一根管子内的磁性夹杂的涡流信号(或则从反方向来看)。从图中可以看出,图1为一个通孔缺陷的信号显示,其结果毫无疑问。图2中,为一个磁性夹杂的涡流信号,这里几乎没有或者没有壁厚损失,但是将98%定义为通孔缺陷。

An Inspector may choose not to investigate any further and just report this as a 98 per cent I.D.pit,recommending that the tube be removed from service,either by plugging or by replacement.

The way to distinguish between a serious internal pit and a mundane magnetic inclusion in the tube is with a multi-frequency test.

检测员可能只是认为其为一个98%的缺陷,而没有去进一步调查,而做出报废,更换等结论,造成成本浪费。

区分严重的内部缺陷和一般的磁性夹杂的方法可以选择多频涡流检测手段

First,I would like to review the signals that you get from a multi frequency test of an ASME calibration tube.In Figure 3,the eddy current instrument is set somewhat differently than I have used in previous articles.As before,Channel 1 is in the upper left hand corner and is 2SDF(two times the Skin Depth Frequency).This is considered the primary inspection frequency,and it was used in Figures 1 and 2 above.Channel 2 is set to 1SDF and is in the upper right hand corner of the screen.Channel 3 is in the lower left hand signal.This is an absolute channel,and it is not important for this discussion.Channel 4 is set to one half SDF,and is in the lower right hand corner of the screen.

首先,我们再了解一下通过多频涡流从ASME标准试块上测得的涡流信号,在图3中,左上角为通道1,采用2SDF(2倍集肤深度频率)。该频率作为检测主要频率,以上图1和图2也是这个频率。右上角为通道2,采用1SDF,左下角微通道3,其为绝对通道,其对本次讨论无关紧要。右下角为通道4,采用0.5SDF,

In past articles,the output from the Mixer Channel was normally in this location.In the screens for Channels 1,2,and 3,you can see the defect signals from the 100,80,60,40,and 20 per cent O.D.defects in the ASME Calibration Tube.In the highest frequency channel where the skin effect is the greatest,there is a large phase angle spread between the through wall defect(approximately 45 degrees clockwise from horizontal),and the shallowest defect(because the skin effect is more predominant at this higher frequency).At the lowest frequency in the lower right hand corner of the screen,the phase angle spread between the through wall hole and the 20 per cent defect is much less.

根据经验,如图3所示,通道1,2,3中,你可以看出,100%,80%,60%,40%,20%人工缺陷的信号。频率最高时,集肤效应最明显。通孔缺陷和最浅的缺陷间有最大的相位差(从水平位置顺时针45度左右),因为频率越高,集肤效应越强,此时该频率为检测的主要频率。在频率最小即0.5SDF时,两者的相位差要小很多。

Figure 4 shows the signal from the through wall hole in all four channels.

图4显示了4个通道的通孔缺陷的涡流信号

Figure 5 shows the signal from the magnetic inclusion in all four channels.In Channel 1,which is the primary inspection frequency,the magnetic inclusion is analyzed as 98 per cent through the wall from the inside.The real clue that this is a magnetic inclusion and not a through wall defect is in Channel 4,the low frequency or confirmation channel,where it can be seen that the magnetic inclusion signal has been rotated to look like a shallow O.D.defect in that channel.

图5显示了4个通道的磁性夹杂的涡流信号。通道1,测试频率为主要频率,磁性夹杂被定义为98%厚(从内部算起),实际结果是这是一个磁性夹杂,而不是一个通孔缺陷,可以通过通道4得出结论,低频率或者确认通道,从结果可以看出,磁性夹杂的信号显示看起来很像外表面的浅表面缺陷。

In Figures 4 and 5,the eddy current instrument has been set to give the per cent wall loss of the defect signal for all available channels.This information appears just below the word Channel 1A in the menu,where indications are given for Channels 1 through 4(C1 through C4)and Mixers 1 and 2(M1 and M2).Note that the analysis for Channels 1,2,4 and M1 are 100,99,96 per cent outside and 98 per cent inside.The maximum error is 4 per cent.With such good agreement between the analysis in the various channels,the Inspector can be confident that this is truly a defect.By comparison,in Figure 5 the analysis in these Channels varies from 98 per cent inside for Channel 1 to 13 per cent outside for Channel 4,the confirmation channel.Since the confirmation channel is so far from agreeing with the primary channel,it is concluded that this is not a defect and in fact,this is the signature from a magnetic inclusion.

通过图4和图5,仪器设置为通过壁厚损失来显示缺陷信号,其信息显示于单词Channel 1A下面,给出了通道1到通道4和Mixer1和2。注意到通道1,2,4和M1的分析结果是100%,99%,96%OD和98%ID,最大误差4%。根据这些数据的分析,可以肯定这是一个真实的缺陷。对比于图5,从通道1和通道4类比,其结果从98%ID降到13%OD,因为确认通道的结果与主要通道的结果相差太大,多以得出结论是其不是真正的缺陷,而是磁性夹杂。

CONCLUSION结论

Real tube wall loss,such as an I.D.pit,will analyze with relatively similar per cent wall loss indications in all frequency channels,whereas signals from another source,such as internal magnetic inclusions,will analyze at significantly different per cent wall loss values in the various frequency channels.An eddy current Inspector must analyze all eddy current signals at all available frequencies in order to confirm whether or not the signal originates from a wall loss defect.

真实的壁厚损失,例如内壁缺陷,在所有通道内使用相对百分比来分析,然而来源于其他的信号,例如内壁的磁性夹杂,在不同频率状态下,其壁厚损失的百分比结果会相差太大。所以涡流检测人员必须在不同的频率下分析所有涡流信号,来确认是否是壁厚损失产生的涡流信号

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