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·Materials Characterization and testing
Distinction of Alloy Steels by Means of the Closely Coupled Probes Potential Drop TechniqueF. Takeo
Department of Mechanical Engineering, Hachinohe College of Technology, Hachinohe 039-1192, Japan
Department of Mechanical Engineering,Tohoku University, Sendai 980-8579, Japan
Structure Improvement Department Corporate Planning Division,Aichi Steel Works, Ltd., Tokai 476-8666, Japan
By the way, d-c potential drop technique is powerful tool for quantitative NDE of cracks. The technique using four probes, which are in close proximity to each other, has recently been proposed ; that is the closely coupled probes potential drop (CCPPD) technique. It has been shown that the sensitivity of the CCPPD technique is enhanced significantly in comparison with the usual method using a uniform current flow in the region far from the crack. In addition, the sensor of the CCPPD technique is easily positioned with smaller system.
By applying the CCPPD technique at the uncracked part of the material, the electrical resistivity of the material can easily be measured. Because the resistivity depends on the composition of alloy steels, by measuring resistivity using the CCPPD technique, the distinction of alloy steels is possible.
In the present paper, the way of distinction of alloy steels by means of the CCPPD technique is proposed.
|Fig 1: The CCPPD technique|
Because of symmetry, this problem is equivalent to the case where a pair of source and sink, which has twice strength of Fig. 1, exists in an infinite material. Then potential f at the point (x, y, z) is obtained as follows:
where r is the electrical resistivity of the material. From Eq. (1), the potential drop between two points at x=±s2 is obtained as
|Table 1 Chemical composition of alloy steels (wt. %)|
Then r can be obtained from the following equation by measuring V:
Specimens were made of 12 kinds of alloy steels that are different from each other in chemical composition as shown in Table 1. The specimens have cylindrical shape and the size of 34mm in diameter and 16mm in thickness as shown in Fig. 2.
|Fig 2: Size and shape of the specimen|
The distance of two probes for current input and output was 2s1=6mm, and that of two probes for measuring potential drop was 2s2=3mm as shown in Fig. 2. All four probes were synthesized to build a pen-like sensor. The sensor is small and easy to deal with as shown in Fig. 3. The contact of every probe to the specimen surface was made constant by using springs.
|Fig 3: Closely coupled probes sensor and equipment||Fig 4: Measuring system|
Figure 4 shows the measuring system. The equipment for the CCPPD technique contains constant d-c current source, microvoltmeter, microcomputer and display. Setting the sensor at the center of specimen surface, constant d-c current in the amount of 1A, was applied to the specimen through the current input and output probes. The potential drop V was measured by the measuring probes. The resistivity r was calculated by microcomputer from Eq. (3), and was displayed in a moment. The measurement was made 10 times with rotating the sensor in circular direction.
By using the CCPPD technique, the electrical resistivity of the alloy steels in the range of 18 to 29 ´ 10-8W×m was obtained as shown in Table 2. Dispersion of resistivity in 10 times measurement was less than 4 percent.
It was found through multivariate analysis that the resistivity of the alloy steels correlates well with their chemical composition as shown in Fig. 5. The ordinate of Fig. 5 shows the linear function of Cr% and Mn% as follows:
|Sample No.||Electrical resistivity (´ 10-8W×m)||
||Table 2: Resistivity obtained by the CCPPD technique
||Fig 5: Chemical composition versus electrical resistivity|
Using this relationship, it is possible to distinguish alloy steels each other. The procedure for distinction is proposed in the following steps:
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