·Table of Contents
The Discussion & Research of the Standardizing of the NDT Personnel Qualification Examination Test Samples
(Instituty of Boiler & Pressure Vessel Inspection of HuBei Province of China)
No.182 Xudong Road,Wuhan,Hubei,China
As an important kind of applied technology,non- destructive test(NDT) has great practicality in guaranting the quality of material,components and products.The validity of its use is decided by the capacity of testers. At present , ISO and the main industrialization countries have generally carried on the qualification evaluation examination to personnel to test the knowledge,technique and practical experience they need. The examination carried by the legal state institution includes two parts,written examination and operating examination. The later provides it must be operated on the prepared "defective test samples ".Since there isn't any qualitative requirements and quantitative quotas to the samples,the regulation appears to be abstract. Therefore, there are many differences in the yardstick of qualification examination among each qualifying body, especially in practical operating examination. Influenced by the man-made factors, the technical requirements of sample in making,selecting and using seem to be alternate , which effects the quality and justice of the examination.For example, the certificate issued by one area won't vouch for the other area,people who has been qualified in one area won't suit for the same work in the other area. Of course, the serious consequences are not only limited to this ,but also leaves a hidden peril to the tested material, components and products. For the special equipments which are explosive, inflammable, toxic and aviation , spaceflight ,nuclear energy which affect the security of national property and personal safety , it will bring about potential danger so that there is no end of trouble forthe future.Consequently, we are now eager to regulate the NDT personnel test samples .First, we must make out a unite sample checking and accepting technical standard, then develop the unit standard test sample.
Being the standard of the test samples, the difficulty of formulating is how to set each typical defect and parameter on a suitable level .That's to say ,we must consider not only the specified difficulties that the examinee might face , but also the trouble of sample processing ,making ,collecting and the technological feasibility .The overall principle is,first,to be advanced and scientific , the standardize samples must tally with the stipulation of state, meanwhile, to be adaptable and rational , it also should tally with the requirements of making and testing.For example,there are four regular test methods: Radiography Testing(RT) ¡¢ Ultrasonic Testing( UT) ¡¢ Magnetic Particle Testing(MT)¡¢and Penetrate Testing(PT),which make the principle detailed,and make the main technical requirements(such as the typical defects , types ,quantity , positions of the negatives and test samples) of the different test samples quantitative as possible in order to supply the foundation for developing , checking and accepting the samples . Through steel melting butt welding seam , the main idea of this article introduces the technical requirements and developing result of the test sample .
1. Technical requirements of standard test samples
1.1 Technical requirements of RT negative
1.1.1 Type: Steel melting butt welding seam radiographic negative
1.1.2 Size: Generally 300mm¡Á80mm or 360mm¡Á80mm;negative size of liquefied petroleum gas steel bottle 300mm ¡Á 60mm; size requirements of double-wall double-projection negative can be loosed , but each negative should have only an image of butt welding seam ,and welding seam ellipse-form must be complete , length of its minor axis at 3 - 15mm.
1.1.3 Quality: Each negative should have serial number, central symbol(double-wall double-projection negative excluded) , joint symbol and penetrometer , its radio graphic density and sensitivity must be in the regulated scope.
1.1.4 Defect: Internal defects of welding seam in negative include crack ¡¢non-melt¡¢non-welding through¡¢slit-type sediment ¡¢ strap- form gas hole ¡¢ round defects ( including tungsten sediment) , others are splash ¡¢weld-tumour ¡¢bite-side¡¢ burn- through ¡¢concave hole¡¢non-weld full etc. Slit type lined sediment or strap-form gas hole and round defect must be only one part that is evaluated grade ¢ôin each negative. Defect of each negative is one or more kinds of above mentioned defects.
1.1.5 Division: Each group has ten negatives, including a negative of thick blank (T>30 mm),a negative of steel bottle,a negative of pipe ;difficulty of evaluation of each group must be appropriate. Each group must include internal defects,such as crack ¡¢ non-melting ¡¢non-welding lined sediment¡¢gas hole etc.and surface defects e.g. bite-side and internal concave , at least there must be one false defect. Each group includes¢ñ¡¢¢ò¡¢¢ó¡¢¢ô four kinds of welding seam quality grade,also includes negative of comprehensive grade evaluation or grade evaluated according to total length of slip type lined sediment. Each group must include negative of flat position weld and vertical weld;there is at least one kind of negative of horizontal weld ,overhead weld, pipe vertical- fixed weld and downland-fixed weld.Group number should be no less than 20 of examinees,i.e.in every examination,every five examinees use one group and at least ten groups.
1.2 Technical requirements of UT welding seam test samples Imagine there are two groups of defects in the sample , one is silp-type defect , the other is spot-type defect . The letters list below indicate the meaning of testing parameters of the defects.
1.2.1 Test sample must be carbon steel or low alloy steel, there is no ¦µ2 flat-bottomed hole equivalent defect in sample material.
1.2.2 Sample length is 300mm,general thickness is 8-46mm,broadth must meet the requirement of probe-moving area.
1.2.3 Internal defect of sample is divided into slit- type defect and spot-type defect.
1.2.4 Sample welding seam is arc weld,all kinds of defects of sample can be made by the method of taking technological control , which makes it similar with natural defect as possible.
1.2.5 Testing parameters of the defect
S1: distance from begin of defect to left line of the sample, mm;
S2: distance from end of defect to left line of the sample, mm;
S3¡¢S4: distance from the largest amplitude position in the defects to left line of the sample respectively of group 1 and group 2, mm;
H¡¢h : hidden depth of defects in group 1 and group 2 respectively (distance from indicated position of defect to testing area when amplitude of defect is the largest ), mm;
q: distance of defect deviated from weld central line; mm;
Amax: the largest relative amplitude of defect, dB.
1.2.6 Each slip-type defect has six testing parameters: S1¡¢S2¡¢S3¡¢h¡¢q¡¢Amax; each spot-type defect has four testing parameters : S4¡¢H¡¢q ¡¢Amax.
1.2.7 Each sample must at least have one slip-type defect, its length should be less than 80mm, and the defect testing parameters of the sample are suitable at 6-18.
1.2.8 Defects are not set in 20mm of both edges of welding seam sample.
1.2.9 No splash, rust and dirt in probe-moving area of sample surface; concave hole must be milled flat after welded again so as to assure good sound-coupling.
1.3 Technical requirements of MT sample
Size of blank-type welding seam sample is 360mm¡Á250mm¡ÁT( 8-25)mm;
1.3.1 Sample material: Use general carbon steel or low alloy steel, no oxygenic and rusting surface.
1.3.2 Sample Size
Size of pipe-type welding seam sample is ¦µ(108-159)mm, length is 300mm;
Size of pipe-blank type welding seam sample: size of pipe is ¦µ(108-159)mm, length is 250mm,blank is square, side length is 300mm, thickness is (6-12)mm.
1.3.3 Sample defect should be line-type or spot- type defect which is not visible .
1.3.4 Testing parameters of the defect .
Imagine there are two groups of defects in the sample ,one is horizontal defect which is vertical with the central line of welding seam, the other is vertical defect.
S --- distance from the center of each group defects to the end of the sample , mm;
L --- the length of the longest defect in each group defecs, mm;
n --- the total number of each group defects.
1.3.5 Defect testing parameters of pipe-type ,pipe-blank type sample is same with blank-type welding seam sample, take pipe-main line in front of sample number as recording original point, extend towards right, its parameter records should be similar with blank type sample.
1.3.6 Sample defect should be set in welding seam and hotted-area,there is 1-2 group defects in each sample,not surpass ten defects in each group, distance between two neighbour groups>50mm,each group defects is appropriately dispersed at scope of 50mm.
1.4 Technical requirements of PT sampleBlank-type welding seam is 150mm¡Á100mm¡Á(4-8) mm;
1.4.1 Sample material: Use general carbon steel or low alloy steel or stainless steel or aluminium alloy
1.4.2 Sample size;
Pipe-type welding seam is ¦µ(32-108)mm, length is 200 mm;
Pipe-blank type welding seam:
Pipe size: ¦µ(32-108)mm, length is 100 mm;
Blank size: side length 150mm, thickness is (4-8) mm;
1.4.3 Testing parameters of defects and its requirements of setting
Each pipe-type, pipe-blank sample must set 1- 2 group of defects according to pipe size ,other requirements are same with 1.3.3, 1.3.4, 1.3.5, 1.3.6.
2 Development of standard test sample
2.1.1 Although above mentioned technical requirements take account of the problems of manufacturing accuracy and technological feasibility of all kinds of test samples,we must make experiment and demonstration .For simple technology and low cost , we must look for the best manufacturing method, so as to make all indexes of defects meet the requirements,to make sure that test sample can be used to examine the factual testing ability and level of examinees. Here , it provides the developing problem of test sample.
2.2.2 To begin experiment development , it based on four kinds of test samples(steel-melt butt welding, RT negative, steel-welding seam UT sample , steel-welding seam MT and PT sample).
2.2.3 To develop above sample, we must resolve problems as follows :
(1) Sample must meet the defect requirements of all relating test standards;
(2) Sample defects should be similar with natural defects as possible;
(3) Sample difficulty must be suitable, it should be easy to operate;
(4) Manufacturing method of sample is not complicated and easy to fulfill;
(5) Low cost
To resolve these problems, development of all kinds of sample must be based on, data-collection, and take all methods into account.Through experiments and selection,finally, make out the plan which is easy to carry out and meet basic requirements under present conditions. We resolve many difficult problems in developing process and manufacture the first lot samples.
3.1 Through experiments , analysis and selection , we resolve present problems of manufacturing method and the best technology of four kinds of samples.
3.2 Method and technology is much simile and easy to spread with much practical value and low cost.
3.3 The total amount of four kinds of samples are 150 pieces (suits), experimented by nation committee of NDT personnel qualification and certification, it is considered the defect establishment is rational, the confirming method and material are complete, and all kinds of manufactured defects can examine the factual testing ability of examinees completely.
- ISO 9712: " Non -destructive Testing Qualification and Certification of Personnel", 1992;
- GB9445 of China: "Rules for Qualification and Certification of NDT Personnel";
- Ministry of Labour of China:
"Regulation of Nondestructive Testing Personnel Qualification Evaluation Examination of Boiler & Pressure Vessel", 1993.