1.Research content
1.1 Making microscopic test for the sample taken from austenitic weld made by marine electrode 395 to help choose appropriate incidence direction of ultrasonic beam and waveform.
1.2 Taking samples from the austenitic weld of marine electrode 395 for measuring ultrasonic absorption, scattering, attenuation data to help choose appropriate technical parameters of defect detection.
1.3 According to the specification requirement of ship hull's weldment, the reference blocks used for defect detection are designed and manufactured, and each kind of artificial reflectors are drilled well for defect detection test.
1.4 Combinations between special probes of variously different size and ultrasonic instrument are chosen for defect detection test.
1.5 Different kind of artificial defective test blocks are manufactured to determine different method of defect detection and capability of finding defects and signal noise ratio and then the best sensitivity.
1.6 The correctness of positioning, quantitative and qualitative of defects is studied.
1.7 The applied technique about longitudinal wave angle beam method, longitudinal wave normal beam method and longitudinal wave small angle beam method are researched in order to use different above mentioned method for inspection and to achieve the goal that ultrasonic beam can scan the whole weld, as shown by figure 1.
Fig 1: Ultrasonic beam-scanning zone of weld section for all methods |
2. Selection of ultrasonic instrument.
Through investigation and study many times and making competitive experiment for each kind of instrument, we decided to introduce the USK TD ultrasonic detector made by German K. K. company and this detector is digital control and analogue quantity display.
3.Selection of ultrasonic probe.
3.1 To choose the correct probe is the key of success or failure for detecting austenitic weld. According to the metalographic characteristic of austenitic weld made by marine electrode 395 and through reference to the reports about inspecting austenitic weld from home and abroad, we sum up our experience about inspecting chrome-nickel-molybdnum(16-25-6)steel's austenitic weld made by marine electrode 395 and consider that the ultrasonic detection for austenitic weld should take special macrocrystalline probe for longitudinal wave as main one. If using the common transverse wave probe for inspection, the reliability of its detecting result is relatively poor.
3.2 The longitudinal wave probes for detecting macrocrystalline material chosen by us is made by German K.K company and these probes has the following. types:
- Small WSY Series:
Longitudinal wave refraction angle: 60º,70º
Frequency: 2, 4MHz
Four types of probes.
- Large WSY Series:
Longitudinal wave refraction angle: 45º,60º,70º
Frequeucy:1.5MHz
Three types of probes.
- Shock wave normal probe series:
(Wide frequency, narrow pulse)K2.2KA, 5KB, CA211A, Three types of probes.
- Minitype longitudinal wave small angle seies:
The probes are made of KB-A, K2A crystal and their incidence angle are separately 8º,10º,12º.
The above-mentioned probes are all special ones for longitudinal wave austenitic macrocrystalline material. Through repeatedly inspecting the test block of austenitic weld, we think that these probes can meet the requirement of detecting different position and different form in austenitic weld of marine electrode 395 for a certain product. The preliminary test shows that the signal noise ratio is raised and the scattering, attenuation and grass echo etc between austenite grain and austenite grain boundary is decreased distinctly, so the capability of detecting defect is improved.
4. The detailed content of test and research
4.1 The test and research for basic acoustic performance of austenitic weld of marine electrode 395. The content of test and research for basic acoustic performance of austenitic weld contains: measuring omnidirectional acoustic velocity, measuring attenuation coefficient of material, measuring signal noise ratio and measuring beam direction change generated by interface weld bond of austeritic weld when it is in the high strength low alloy carbon steel. All these basic acoustic performance has close relation with ultrasonic detection technology; that is, these data are the essential condition for detecting austenitic weld, as shown by figure 2.
Fig 2: Measurement in differently crystallized direction on austenitic weld. |
4.2 The propagation direction and waveform change when ultrasonic beam is incident upon different metal's weld bond.
There is no direction change produced by weld bond when making ultrasonic detection of common carbon steel's weld. But inspecting the weld between carbon steel and austenite 395, because of the parent metal is low-alloy high strength steel, so its acoustic velocity is 5850 m/s, after thermal refining its metallurgical structure is sorbite and the crystal grain is relative fine and material attenuation is small; while the metallurgical structure of austenite 395 weld is anisotropy dedrtic crystal, its material attenuation is large and its longitudinal wave's acoustic velocity is 5750m/s and transverse wave's acoustic velocity is 3150m/s.The refracting and waveform change condition on upper and lower interface of weld bond of ultrasonic beam when measuring on the reference block is shown as figure 3
Fig 3: Refracting waveform change of ultrasonic beam on upper and lower weld bond |
4.3 Design and manufacture of the reference blocks used for detecting austenitic weld. When the reference block is used for detecting austenitic weld, the detecting sensibility shall be adjusted by choosing different type of probe and different detecting method. The common transverse hole and flat bottom hole which are often used in international are chosen for the hole type of the reference block and the hole diameter is also referred to one of reflector on the test blocks which is often used in international. The test block is designed according to he condition of real ship's weldment, the weld is austenite and the parent metal is high alloy low carbon forged steel. For suited the requirement of detecting real ship's weldments, five types of sensitivity test blocks are designed and manufactured.
4.3.1 The block ARB-1,ARB-2
These two test blocks are suitable for adjustment of detecting sensitivity of WSY and WRY series of longitudinal wave angle probes and are used separately for two thickness ranges that web thickness>=20mm~45mm and web thickness>45mm~70mm.For the test block's form, you can see the figure 4
Fig 4: Test block ARB-1,ARB-2 |
4.3.2 Test block ARB-3,ARB-4
These two test block are used for adjustment of detecting sensitivity of longitudinal wave normal single crystal probe or shock wave probe and are suitable for two thickness ranges that steel plate thickness<=45mm and steel plate thickness>45mm~70mm.For the test block's form, you can see the figure 5:
Fig 5: Test block ARB-3,ARB-4 |
4.3.3 Test block ARB-5
Fig 6: Test block ARB-5 |
This test block is used for adjustment of detecting sensitivity of longitudinal wave small angle beam method. For the form of test block, you can see the figure 6.
4.4 Design and manufacture of artificial defect test block and natural defect test block .The purpose for manufacturing test block of artificial defect and natural defect is for providing detection object of technology method research for defect detection. According to the need for topic research, we design and manufacture 7 artificial defect test blocks and 11 natural defect test blocks. The defect type is various typical welding defects, such as crack, incomplete penetration, lack of fusion, slag inclusion, gas porosity etc.
4.5 Test and research for defect detection technology. For the research of defect detection method of austenitic weld, we shall make enough number of various artificial and natural small defect test pieces and then shall make the defect detection on the test piece in deeply and repeatedly. In special, it is even more important to repeatedly detect various typical defects which may be produced in the weld. After analysing, contrasting and correcting carefully the results of defect detection, the content involved in the defect detection technology can only be solved one by one. After making a lot of detection works, we study and solve emphatically the problems of detection technology.
4.5.1 The adjustment of defect detection sensitivity is divided different detection methods. The test block ARB provided by 4.3 is chosen. For adjustment of reflecting wave height of various reflector in the test block ARB, you can see the table 1
| NO.
| Name of test block
| Suitable web thickness(mm)
| Dimension of reflector in anstenite
| Special probe's type
| Reference wave's height(%)
| Signal noise ratio(dB)
|
| 1
| ARB-1
| <=20~45
| f3*35(deep)
| Wsy60.4
| 50
| 24
|
| 2
| ARB-1
| <=20~45
| f3*35 (deep)
| WRY60
| 50
| 16
|
| 3
| ARB-2
| >45~70
| f3*50(deep)
| WRY60
| 50
| 16
|
| 4
| ARB-2
| >45~70
| f3*50(deep)
| WRY70
| 50
| 18
|
| 5
| ARB-3
| <=45
| Flat bottomed holef4
| Shock wave probe
| 100
| 24
|
| 6
| ARB-4
| >45~70
| f3*72(deep)
| K2GB2SN
| 100
| 13
|
| 7
| ARB-5
| >45~70
| f3*45/f3*65(deep)
| W10KB-A W8K5K-etc
| 100
| 22/14
|
| Table 1. Adjustment of defect defection sensitivity |
4.5.2 Test for defect length measurement: Length measurement is made with two relative sensitivity, 6dB and 12Db. When detecting artificial defect and natural defect and taking the defect length measured by these two methods compare with one measured by radiographic inspection, the indicative defect length measured by 12dB method is close to the real defect length.
4.6 Verification and dissection of detecting technology To verify the reliability of detecting technology, the actual detection in site proves:The detecting technology method researched recently is reliable, easy to operation and high capability to find defects.The repeatability of detecting result for harmful defects is good and operation is easy.
For the typical defect which has dissecting value in detection, the test of gas gouging dissection is carried out. The dissecting result is basically same as the detecting result, so it verifies that the detecting technology is relievable, the positioning quantitative and qualitative analysis of defect are basically correct.
5. Concluding remarks
In this topic research, through repeated test and research and analysis comparison, we find the correct method for detecting austenitic weld of marine electrode 395, that is, taking longitudinal wave angle beam method as main and taking longitudinal wave normal beam method and longitudinal wave small angle beam method as subsidiary and adopting different angle, different size of probe and the method of partitioning scanning in particular for whole weld section. The characteristic of this method is: Under the premise of ensuring detecting sensitivity, it solves the problem of relative low signal noise ratio in detecting austenitic weld and eliminates basically the unfavorable factors produced by grass echo for evaluating defect. The average signal noise ratio can reach above 12dB and the defect wave can be decided easily.
In the detecting method researched recently, according to the characteristic of austenitic weld, we also carry out deeply and carefully the research of positioning, quantitative and qualitative analysis for defects and also makes the dissecting test and comparison for defects. A relative wealth of experience has been accumulated, that is much benefit for us to improve the capability of finding defects and to ensure the reliability of detecting result.
Reference document
- Translator: Zhang Jia jun Check: Zeng Ke Jing
"Ultrasonic inspection manual of austenitic weld", from nondestructive inspection journal; 91.4~9
