GB7233-87 s for Ultrasonic Testing.doc

National Standard of the Peoples Republic of China Methods for Ultrasonic Testing and GB 7233-87for Specifying Quality Levels of Steel CastingsThis standard is a general standard for ultrasonic testing of steel casting.This standard stipulates the ultrasonic testing methods for castings of carbon steel and low-alloy steel whose thickness are more than or equal to 30mm; and methods for specifying quality levels of the castings based on the result of ultrasonic testing. The used ultrasonic testing is limited to type A display pulse reflection method.Supplier and buyer when ordering goods negotiate the following requirements for ultrasonic testing of steel casting:a. Area to be inspected and probe to be used;b. Testing sensitivity of longitudinal wave straight probe;c. Qualified levels of quality of steel casting. Requirements for different quality levels of planar or non-planar discontinuity are allowed. This standard is not applicable to inspection for austenitic stainless steel casting. 1 Terms1.1 Planar discontinuity: If only two-dimensional size of the discontinuity can be inspected by methods stipulated in this standard, this discontinuity is called planar discontinuity. This kind of discontinuity includes crack, cold shut, lack of fusion etc.1.2 Non-planar discontinuity: If three-dimensional size of the discontinuity can be inspected by methods stipulated in this standard, this discontinuity is called non-planar one. This kind of discontinuity includes blowhole, shrinkage porosity, shrinkage cavity, sand inclusion, slag inclusion etc.1.3 Permeability to ultrasound：Ultrasonic longitudinal wave is vertically incident upon the testing surface and cast steel material parallel to its back, the sound pressure drop caused by transmitting to and fro through is permeability to ultrasound. Its unit is dB and it is usually expressed by dB difference between secondary and first time bottom surfaces return wave amplitude inspected with longitudinal wave straight probe. 2 Instrument, Test Block and Couplant2.1 InstrumentThe instrument shall meet the regulation of ZB Y230-84 General Technical Specification for Type A Pulse Reflection Ultrasonic Testing Detector.2.2 Probe2.2.1 The diameter of subindividual of longitudinal wave straight probe is in the range of 1030mm. If the testing surface of inspected steel casting is rough, a longitudinal wave straight probe with soft protective layer is recommended. 2.2.2 Use transverse wave angle probe whose refraction angle in steel is 45, 60 and 70, or whose K value is 1, 1.5, 2, 2.5 and 3.84GB 7233-872.2.3 The sound insulation between two subindividuals of longitudinal wave double crystal probe must be fine.2.3 Performance of the instrument systemThe testing of sensitivity margin and resolution of the instrument system shall meet the stipulation of ZB J 04 001-86 Test Method of Operation Performance for Type A Pulse Reflection Model Ultrasonic Testing System, and meet following requirements:a. If used test frequency is in the range of 22.5MHz, the testing sensitivity margin of longitudinal wave straight probe shall not be less than 30dB, and that of transverse wave probe not less than 50dB.b. Within corresponding testing frequency range, the resolutions of longitudinal wave straight probe and transverse wave angle probe shall meet the regulation of Table 1. Table 1 Lower Limit Value of Resolution of Instrument System Probe TypeLongitudinal Wave Straight ProbeTransverse Wave Angle ProbeTesting frequency (MHz) 325Resolution (dB)61520122.4 Test block2.4.1 The reference block is made of cast carbon steel or low-alloy steel which ultrasound attenuation factor shall be same with or approximate to that of tested cast steel material.The material used to make reference block shall be subjected to ultrasonic testing beforehand. Discontinuity equal to or more than 2 equivalent quantity flat-bottomed hole of the same sound path is not allowed.Side of the reference block shall be marked with name, numbering, material and permeability to ultrasound of the test block.2.4.2 See Appendix A (supplementary piece) for reference block of ZGZ series used for longitudinal wave straight probe. When the thickness of tested steel casting is more than 250mm, test block whose biggest detecting distance equals to the thickness of the steel casting shall be made.The ZGZ series reference blocks are only used to adjust testing sensitivity of longitudinal wave straight probe.2.4.3 See Appendix B (supplementary piece) for ZGS contrasting test block used by longitudinal wave double crystal probe.2.5 CouplantUsually select machine oil, water soluble couplant, mixture of machine oil and yellow glycerin，water or paste as couplant.Couplant is not permitted to cause unallowable erosion on the surface of the final product of steel casting.When adjusting instrument, checking instrument and inspecting steel casting, the same couplant must be used.3 Steel Casting3.1 Testing surface3.1.1 The steel casting shall be subjected to ultrasonic testing after its appearance is inspected to be qualified. All substances on testing surface or back surface of the steel casting, which may influence the testing, shall be cleaned away.3.1.2 The surface roughness of the testing surface of steel casting shall meet the following requirements:a. For machined surface, the Ra equal to or less than 10m.b. For casting surface, the Ra equal to or less than 12.5m. 3.1.3 If the shape of machined steel casting impedes ultrasonic testing, testing shall be carried out before machining.3.2 Classification of level of steel casting wall thickness3.2.1 Based on the appearance of the steel casting when delivered, the section thickness of the steel casting can be classified to three layers: outer layer, inner layer, outer layer. For steel casting whose thickness or section less than 90mm, each layer is 1/3 of the thickness or section size; for those equal to or more than 90mm, the thickness within 30mm to any surface is the outer layer, others are inner layer.3.2.2 Through negotiation between supplier and buyer, in testing area of the steel casting, special layer classifying requirements are allowed to be stipulated, some thickness range or area can be stipulated to be strict quality area, and concrete requirements shall be brought forward.3.3 Permeability to ultrasound3.3.1 When testing the permeability to ultrasound, make the longitudinal wave straight probe coupled contact with testing surface of steel casting and back parallel part without flaw, and set the restraint value to zero. Select a frequency same with that of longitudinal wave straight probe when testing to test, adjust the instrument until the amplitude of first times bottom surface return wave reach 50% of vertical full scale, and record the reading of the attenuator. Adjust the attenuator another time, until the amplitude of second times bottom surface return wave reach 50% of vertical full scale, and record the reading of the attenuator. The difference of two readings will be the permeability to ultrasound of this testing point.Within area of steel casting with same thickness, the testing points shall not be less than three. The average value of all testing points is the permeability to ultrasound of the tested steel casting.3.3.2 If the steel casting has no back part parallel to testing surface and big enough for test, such surface can be made at proper position of the steel casting beforehand. Also, a planar test block can be made with same process of producing steel casting, and regard the permeability to ultrasound tested at this surface as that of the steel casting.3.4 Judgment of applicability of ultrasonic testing of steel castingSet the “restraint” value to zero, use one frequency within 22.5MHz to test, test with longitudinal wave straight probe at position of maximum detecting distance of the steel casting. If the return waves amplitude of reference flat-bottomed hole of testing sensibility selected for longitudinal wave straight probe is 8dB higher than amplitude of noise signal with same sound path, ultrasonic testing is applicable to the steel casting.If above requirements cannot be met, decrease the frequency to 1MHz to test. If the requirement can be met, this frequency can be used for testing, but need to be described in the testing report.If the testing result by decreasing frequency still cannot meet the requirement of ultrasonic testing applicability, methods of heat treatment shall be adopted to improve the permeability of ultrasound of the steel casting, and after the requirement of ultrasonic testing applicability is met, the ultrasonic testing can be carried out.4 Testing4.1 Adjustment of instrument4.1.1 General adjustmentSet the “restraint” value to zero, and adopt a proper value for detecting depth. Whichever probe is used, the zero point shall be adjusted so as to make the scale of oscilloscope screen time baseline on the right of time baseline origin proportional to detecting distance.4.1.2 Adjustment and check of testing sensibility4.1.2.1 Adjustment of testing sensibility of longitudinal wave straight probeWhen test with longitudinal wave straight probe, three sensibility of 3, 4, 6 equivalent flat-bottomed hole are allowed to be adopted. The supplier and buyer shall stipulate a testing sensibility for each area of the steel casting. One of the following methods can be used to adjust the testing sensibility:a. Adjust with AVG French curve. Imbed the AVG French curve (drawn out by setting the attenuation factor to zero) conforming to inspection condition in oscilloscope screen, and preset enough decrement for the attenuator. Let longitudinal wave straight probes coupled contact upper testing surface and back parallel part without discontinuity, adjust the instrument to make the amplitude of first bottom surfaces return wave reach that marked by “B-x x dB” line on AVG French curve, According to B-x x dB to decrease decrement of x x dB. Use the distance amplitude curve on French curve, which is selected as reference flat-bottomed hole of testing sensibility, for compensation of permeability of ultrasound, and then draw out the distance amplitude curve with compensated permeability of ultrasound.See Appendix C (supplement piece) for compensation method of permeability of ultrasound.b. Adjust with reference block. Use a set of test block with selected testing sensibility reference flat-bottomed hole in Appendix A to adjust. First of all, compare the steel casting with reference block whose detecting distance equal or approximate to the width of the steel casting, work out the algebraic sum of compensation values of surface roughness and permeability of ultrasound. The preset reservation of the attenuator must not be less than the above-mentioned algebraic sum. Second, test on this reference block, adjust the instrument to make the amplitude of return wave of flat-bottomed hole reach 10%20% of vertical full scale. With the parameters of the instrument unchanged, test a series of test block with less detecting distance one by one, to create a distance amplitude curve. Compensate the permeability of ultrasound on this curve to get a distance amplitude curve with compensation of permeability of ultrasound. At last, adjust the attenuator to compensate for surface roughness and permeability of ultrasound.See Appendix C for compensation method of permeability of ultrasound.4.1.2.2 Adjustment of testing sensibility of longitudinal wave double crystal probe. The preset reservation of the attenuator must not be less than compensation of the surface roughness. Test flat-bottomed holes with different detecting distance, adjust the instrument to make the largest amplitude of return waves reach 80% of vertical full scale. With the parameters of the instrument unchanged, test the flat-bottomed holes with different detecting distance one by one to create a distance amplitude curve. Adjust the attenuator to compensate the loss of surface roughness.4.1.2.3 Adjustment of testing sensitivity of transverse wave angle probe Make the transverse wave angle probe coupled contact testing surface of steel casting, adjust the instrument to make the amplitude of noise signal reach 13 mm.If the instruments sensibility reaches largest, the amplitude of noise signal still cannot reach 1mm, then test with largest sensibility.4.1.2.4 Check of testing sensibilityDuring test, the testing sensibility shall be checked regularly. If find out any change of the testing sensibility, readjust it, and redo the testing work done since last sensibility check.At the end of each testing, the testing sensibility shall be checked.4.2 Scanning detectionThe testing personnel shall select a regular scanning detection path to detect, two consecutive scanning detections shall overlap about 15% of the size of probes subindividual. During scanning detection, the moving speed of the probe shall not be more than 150mm/s.If test with transverse wave angle probe, scanning detection must be carried out at least in two mutually vertical directions on testing surface. At the same time, continuously deflect the probe, and the deflection angle must not be less than 15.Positions that cannot be scanning detected because of restraint by geometry shape must be clearly described in the testing report.4.3 Inspection of discontinuityAdopt the probe stipulated by supplier and buyer to scanning detect the inspecting area of the steel casting.When carry out scanning detection with longitudinal wave straight probe or longitudinal double crystal probe, adopt a scanning detection sensibility 6dB higher than the testing sensitivity regulated in 4.1.2.1 and 4.1.2.2.Any position appearing following displayed situation shall be marked.a. Position wherever amplitude of return wave of the discontinuity equal to or larger than distance amplitude curve;b. Position wherever amplitude of return wave of bottom surface decrease 12dB or more than 12dB;c. Regardless of the amplitude of return wave of the discontinuity, position wherever discontinuity with linear or flaky character is displayed.4.4 Measurement of the discontinuity sizeUse testing sensitivity regulated in 4.1.2 to measure the discontinuity size marked in 4.3.4.4.1 Measurement of planar discontinuity sizeFor discontinuity displayed with linear or flaky character, draw out the discontinuity range by 6dB method. Based on geometry theory, determine the discontinuitys position, size and its dimension in thickness direction of the steel casting, and calculate its area according to regulation in Table 2.4.4.2 Measurement of non-planar discontinuity size4.4.2.1 For those discontinuity whose return waves amplitude equal to or larger than distance amplitude curve, draw out the range of discontinuity on testing surface by 6dB method, and calculate the area of discontinuity according to regulation of Table 3.When tested with longitudinal wave straight probe, the boundary and depth of the discontinuity at the side near testing surface are determined by position where the amplitude of pulse leading edge of return wave of discontinuity rise up to 6dB lower than distance amplitude curve. If tested with transversal wave angle probe, the boundary of discontinuity is determined by position where the amplitude of pulse leading edge of return wave of discontinuity rise up to 6dB lower than the largest return waves amplitude simultaneously displayed.The discontinuity dimension in thickness direction of the steel casting can be achieved by upper and lower boundary of the discontinuity detected in different directions.The discontinuity cannot be detected in different directions because of restraint of geometry of steel casting must be given clear indication in test report.4.4.2.2 For those position whose bottom surface return wave decrease 12dB or more than 12dB, the bottom surface shall be checked out whether any slant or unevenness exits or the coupled contact is good or not.Wherever the bottom surface return wave decrease 12dB or more than 12dB because of existence of discontinuity, on condition of the bottom surface returning wave decreasing 12dB, draw out the range of discontinuity on testing surface, and calculate discontinuity area according to note of Table 3.The depth of discontinuity is determined by position of leftmost wave crest on a continuous discontinuity return wave. The dimension of discontinuity in thickness direction of steel casting is determined by width between leftmost wave crest and rightmost wave crest on a continuous discontinuity return wave.For those positions with neither bottom surface return wave nor discontinuity return wave, increase testing sensitivity to test, and observe if there is slant defect between reflection surface and incidence ultrasonic beam. 4.4.3 Correct the discontinuity rangeIf the testing surface is curved surface, the discontinuity range drawn out on the testing surface shall be corrected. The correcting method may be calculation method or graphical method.4.4.4 Discontinuity with doubt For the discontinuity with doubt, other nondestructive testing method proved being tried and effective could be used to test. If still there is no way to draw a conclusion, the supplier and buyer negotiate a solution.5 Specify the Quality LevelThe quality level of steel casting can be specified individually according to planar discontinuity and non-planar discontinuity. Based on the sizes of planar discontinuity and non-planar discontinuity, the quality levels of steel castings can be specified into five. For the same type of discontinuity, on same testing con