NUCLEAR POWER CORPORATION OF INDIA LIMITED (A Govt. of India Enterprise)

Transcription

1 NUCLEAR POWER CORPORATION OF INDIA LIMITED (A Govt. of India Enterprise) DIRECTORATE OF QUALITY ASSURANCE NO.: PP-P-2014 REV. NO. 2 TITLE FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE REVISION NO DATE OF ISSUE (Month/Year) TOTAL NO. OF PAGES (Including cover sheet) March 2001 May ORIGINAL PREPARED BY NAME SIGNATURE DATE S.S. Som SSE(QA), KAPS Sd/- March 2001 REVIEWED BY APPROVED BY M. Mohan Babu, ACE (ISI) C. Awasthi, ED (QA) March 2001 March 2001

2 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 1 of 17 REVISION CONTROL SHEET NO.: PP-P-2014 REV. NO. 2 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Rev. No. & date Description of Revision Revised By Reviewed By Approved By 1 May Feb Editorial changes and changes as per latest codes/ standards Editorial changes and changes as per latest codes/ standards P.V. Khambatkone CE (QA) A.K. Palit, CE(QA) A. K. Palit, CE (QA) M. Mohan Babu CE (ISIS), QA A. K. Palit CE (QA) M. Mohan Babu CE (ISIS), QA K. P. Dwivedi ED (QA)

3 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 2 of SCOPE This procedure describes the requirements for ultrasonic examination of steel forgings by pulse-echo, manual contact method and is applicable to forged flanges, bars, and pipe fittings such as Elbows, Tees and Couplings. Forgings and forged or rolled bars which are to be bored to form tubular products shall be examined after boring. 2.0 REFERENCES This procedure is based on the following: a) ASME Section-V, Article 5, (2010 Edition). b) ASME Section - III, Subsection, NB, NC (2010 Edition). c) SA-388, Recommended practice for ultrasonic examination of steel forgings, (2010 Edition). d) PP-P-1981 Rev. 2, General requirements and Formats for Ultrasonic Examination procedure. 3.0 SCAN DIRECTIONS Following scans are applicable for the forgings of following types a) Solid cylindrical sections: Radial scan (Clause 3.1) and Axial scan (Clause & as applicable). b) Hollow cylindrical sections: Circumferential scan (clause 3.3) and axial scan (clause 3.2.2). c) Disk forging: Radial scan from the circumference (clause 3.1) and axial scan from flat face (clause 3.2.1). d) Rectangular forgings: Normal beam examination in accordance to the technique mentioned in clause No Radial Scan A radial normal beam scan using longitudinal waves shall be conducted. Scan surfaces shall include all solid cylindrical surfaces of the object. This scan shall be carried out by the Distance Amplitude Correction Curve (DAC) method using reference flat bottom hole standard. 3.2 Axial scan Normal beam examination An axial normal beam scan using longitudinal waves shall be carried out, if possible, from both ends of the forging using the flat bottom hole method Angle beam examination An axial angle beam scan using shear wave shall be carried out in place of axial normal beam scan for cylindrical sections of long length due to attenuation and also for ring and hollow forgings having an outside diameter to inside diameter ratio of less than 2 to 1 which can not be examined axially using a straight beam in axial direction.

4 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 3 of Circumferential scan A circumferential angle beam scan using shear wave from the OD surface shall be carried out for ring & hollow forgings that have a ratio of outside diameter to inside diameter of less than 2 to 1. Use of 35 probe is desirable when the OD / ID ratio is 1.75 to 1 minimum. 4.0 INSTRUMENTATION 4.1 Ultrasonic instrument Pulse-echo type instruments shall be used. The amplitude linearity, amplitude control linearity and time base linearity shall meet the requirements defined in Annexure 4. These parameters shall be checked at the time of commencement of the examination unless valid records less than six months old exist proving conformance and no repair of instrument has been necessitated. Witnessing personnel may call for rechecks of these parameters at the start of examination or at any time during examination, if there is reason to doubt instrument performance in these regards. Following table is suggestive of make & model of some UT machines: Sr. Models Models 1 M/s. Roop Ultrasonix, India. 4400, 440A, 4400M 2 M/s. Electronic and Engineering Co., India ESM-2, 2M, EX-10,100 3 M/s Modsonic Einstein 4 M/s. Krautkramer, Germany, USA USIP11, USD10, USL 48 5 M/s. Karl Deutsch, Germany 1023, 1024, 1025, M/s. Panametrics, USA. Epoch-2000, Epoch-II 7 M/s. Sonic, USA. FTS MK-I, FTS MK-IV However, Head QA / Supdt. QA of applicable unit shall have the authority to permit use of any other equipment which meets the requirements of Annexure Search Units / Probes The probes recommended for particular application, are as follows: Normal Beam probe a) Size: A maximum active are of 650 mm 2 with 13 mm minimum to 30 mm maximum dimension or 19 mm diameter minimum dimension shall be used. b) Frequency: Nominally probes of 2MHz to 5 MHz shall be used. Whenever necessary, to compensate for attenuation, 0.5 MHz probes may be used.

5 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 4 of Angle beam probe a) Size: Probes equipped from 12 by 25mm to 25 by 25 mm transducers shall be used. b) Beam Angle: 45 o angle beam search unit shall be used wherever applicable. c) Frequency: The nominal test frequency shall be 2 to 5 MHz. The exit point of the sound beam and the actual refracted beam angle of angle beam search unit shall be determined on an IIW reference block. The permissible error on either side of the nominal angle shall be 2 o and the accuracy of measurement shall be 1 o 5.0 PERSONNEL QUALIFICATIONS Scanning shall be carried out by personnel qualified by the ISNT or the ASNT to UT Level I or higher. Verification of calibration, scanning, analysis of indications and reporting shall be by personnel qualified by the ISNT or the ASNT to UT Level II or higher. The Head of the QA Unit may conduct evaluation tests to check the theoretical and practical capability of the personnel performing UT. 6.0 GENERAL REQUIREMENTS 6.1 Surface Conditions The surface of the forging to be examined shall be free of loose scale, paint, dirt etc. The surface roughness of scanning surfaces shall not exceed 6.35 µm. 6.2 Couplant Oil, grease, glycerine, water or cellulose paste shall be used as couplant. Select a suitable couplant based on the surface conditions and orientation; roughness and nonhorizontal conditions require a more viscous couplant. Any requirements for protection of the material under test, such as for sulphur and halogen content, shall be met by the couplant. 6.3 Scanning Speed The maximum rate of probe (normal beam search unit) movement shall be established by moving the probe on the calibration block for which the amplitude from FBH reflector shall be at least 50% of the PLR.The maximum rate of probe (angle beam search unit) movement shall be established by moving the probe on the calibration block for which the amplitude from reflector notch shall be at least 50% of the reference amplitude when picked up at a distance of full vee path for outer notch and be not less than 60% of the reference amplitude when at a distance of ½ vee path for inside notch. However the scanning speed shall not exceed 150 mm / sec. 6.4 Probe Overlap To assure complete coverage of the forging volume, the overlap shall be at least 10% of the probe diameter. 6.5 Calibration Identify and Validity

6 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 5 of 17 Any change of instrument or control settings (other than temporary changes of controls which operate on fixed steps), probe, couplant or probe cables shall require re-calibration. Operating variables including the test instrument, its control settings, its power source, probe, probe cable and couplant shall be unchanged once calibrated throughout scanning and evaluation, except for readjustments of gain between evaluation and scan as required during examination. After initial calibration, rechecks the proper functioning of the equipment shall be checked by the use of the reference specimens, as a minimum Reject a) At the beginning of each production run of a given size and thickness of the given material. b) After each 4 hours or less during production run. c) At the end of the production run. This control is also called suppression in some instruments and shall be maintained in the minimum position (or OFF position, if available) throughout the examination. 6.7 Continuously Variable Controls Continuously variable controls such as damping and continuously variable gain shall be maintained at either of the extremes of their positions to the extent possible to avoid inadvertent changes. 7.0 RADIAL SCAN (For Solid Cylindrical Forgings and for Disc Forgings) This scan shall be carried out by normal probe at all cylindrical surfaces of the test object. 7.1 Calibration Blocks Calibration blocks shall be of the same material specification and heat treatment as applicable to the test object. However, where the actual forging material belongs to P1 (Refer ASME Sec. IX), use of calibration block made of mild steel is permitted, provided the block is made from forged material. The surface finish at the entry surface of the calibration block should be similar to that of the test object. Prior to machining the block, the raw material shall be ultrasonic examination checked and shall be adequately defect free. The hole diameter shall be 3% of the thickness and minimum 3 mm diameter. The holes shall be machined truly flat bottomed, preferably by end milling. The diameter of the calibration blocks and block length shall be as shown in the Table of Annexure Time Base Calibration Basis Mechanically measure the test object diameter or thickness as applicable within 1 mm accuracy. Select a round figure for the required range such as 0 to 50, 0 to 100 or 0 to

7 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 6 of mm etc. The range shall be such that the first back wall echo is between 60% and 90% of the time base. Calibration on reference block such as V1 block is accepted for the cases where job configuration does not give back wall echo. 7.3 Time Base Calibration Procedure Calculate the time base position for the first back wall echo. Set the time base controls such that the second back wall echo is set to this value, with the first back wall echo being set to zero. Now, lock all the time base controls other than delay. From this setting, use the delay control to shift the first back wall echo to the calculated position. The time base is now calibrated, lock delay also. Confirm calibration from the position of echoes of other smaller diameter / thickness area wherever possible and correct any errors if necessary. 7.4 Sensitivity Calibration Setting the primary reference level At a suitable gain setting find out which hole provides the highest response. Set the amplitude of this echo to between 70% FSH and 80% FSH using the gain control. This is the reference gain and should not be changed Drawing the DAC curve Use the instrument screen as a graph. At the metal path corresponding to the first hole (which gives highest response), mark the maximized amplitude of the echo. Repeat this for all the other holes. Draw a smooth line through the points so obtained. This is the distance amplitude correction (DAC) curve. Use a suitable marking pen which should not damage the screen protection sheet. 7.5 Scan, Recording and Evaluation Scan at a gain value higher than the above reference gain by 6 db. Recording & Evaluation shall be done at reference gain for - a) Any indication whose amplitude equals or exceeds 50% of the established DAC curve,. b) Travelling, clustered or continuous indications that have amplitudes equal to or greater than 25% of the DAC curve. c) any reduction in back reflection (associated with a discontinuity indication) exceeding 20% of the calibration back reflection. Evaluate the amplitude of each indication by comparison with the height of the DAC curve at the same metal path as the indication and report it as a percentage of DAC. 7.6 Acceptance Level Indications of following nature shall cause for rejection - a) Any indication whose amplitude exceeds 100% of the established DAC curve, b) Travelling, clustered or continuous indications that have amplitudes equal to or greater than 50% of the DAC curve,

8 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 7 of 17 c) Reduction in black reflection exceeding 50% not associated with geometric configuration are unacceptable. 8.0 CIRCUMFERENTIAL SCAN (For Ring Forgings and Hollow Forgings) This scan shall be carried out by an angle probe on the OD surface of the test object. 8.1 Calibration Block Calibration blocks shall be of the same material specification and heat treatment as applicable to the test object. However, where the actual forging material belongs to P1 (Refer ASME Sec. IX), use of calibration block made of mild steel is permitted, provided the block is made from forged material. The surface finish at the contact surface of the calibration block should be similar to that of the test object. Prior to machining the block, the raw material shall be ultrasonic examination checked and shall be adequately defect-free. The block shall contain rectangular notches on the ID and OD surface in the axial direction and parallel to the axis of forging. The notch depth shall be 3% maximum of the thickness or 6 mm, whichever is smaller and its length shall be 25 mm. The width of the notch shall not be greater than twice its depth. A sketch of the notch is provided as Annexure Time Base Calibration Basis The screen range chosen shall be the shortest metal path distance to include at least 1½ skip distance for the material to be examined. Select a round figure for the required range such as 0 to 20, 0 to 25, 0 to 50 etc. Calibration on reference block such as the V1 block is accepted for cases where the job configuration does not produce a back wall echo. 8.3 Time Base Calibration Procedure Calculate the time base position for the ID notch and OD notch. Using delay control and time-base control, set the echoes from ID and OD notch to their respective position on the screen. 8.4 Sensitivity Calibration At the metal path corresponding to ID notch, obtain an indication amplitude of approximately 75% FSH at a suitable gain setting and mark it on the screen. This is the reference gain and should not be changed with above gain setting. Mark the maximized echo amplitude from OD notch corresponding to its metal path on the screen. Draw a smooth line through the points so obtained. This is the distance amplitude correction (DAC) curve. 8.5 Scan, Recording & Evaluation Scan at a gain value higher than the above reference gain by 6 db. Perform the examination by scanning over the entire surface area circumferentially in both clockwise and counter clock wise directions from the OD surface.

9 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 8 of 17 Recording & Evaluation shall be done at reference gain for any indication that has amplitude equal to or greater than 50% of the DAC curve. 8.6 Acceptance Level Any indication above 100 % DAC shall cause for rejection. 9.0 AXIAL SCANNING (for Solid Cylindrical, Disk and Hollow Forgings) This scanning shall be carried out by a normal probe by flat bottom hole technique or by an angle probe as per applicability mentioned in the clause No Angle Beam Scanning (for Hollow Forgings) Calibration block Calibration blocks shall be of the same material specification and heat treatment as applicable to the test object. However, where the actual forging material belongs to P1 (Refer ASME Sec. IX), use of calibration block made of mild steel is permitted, provided the block is made from forged material. The surface finish at the entry surface of the calibration block should be similar to that of the test object. Prior to machining the block, the raw material shall be ultrasonic examination checked and shall be adequately defect free. The block shall contain rectangular notch on the ID and OD surface in the direction perpendicular to the axis of forging. The notch depth shall be 3% maximum of the thickness or 6 mm, whichever is smaller and its length 25 mm. The width shall not be greater than twice its depth Time base calibration basis The screen range chosen shall be the shortest metal path distance to include at least 1½ skip distance for the material to be examined. Select a round figure for the required range such as 0 to 20, 0 to 25, 0 to 50 etc. Calibration on reference block such as V1 block is accepted for the cases where job configuration does not give back wall echo Time base calibration procedure Calculate the time base position for the ID notch and OD notch. Using delay control and time-base control, set the echoes from ID and OD notch to their respective position on the screen Sensitivity calibration At the metal path corresponding to ID notch, obtain an indication amplitude of approximately 75% FSH at a suitable gain setting and mark it on the screen. This is the reference gain and should not be changed with above gain setting. Mark the maximized echo amplitude from OD notch corresponding to its metal path on the screen. Draw a smooth line through the points so obtained. This is the distance amplitude correction (DAC) curve Scanning, Recording & Evaluation

10 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 9 of 17 Scanning shall be done axially from both end of the test object. Scan at a gain value higher than the above reference gain by 6 db. Perform the examination by scanning over the entire surface area circumferentially in both clockwise and counter clock wise directions from the OD surface. Evaluation shall be done at reference gain for any indication that has amplitude equal to or greater than 50% of the DAC curve Acceptance level Any indication above 100 % DAC shall be the cause for rejection. 9.2 Normal Beam Scanning (for Cylindrical Forgings and Disk Forgings) Calibration Block Calibration blocks shall be of the same material specification and heat treatment as applicable to the test object. However, where the actual forging material belongs to P1 (Refer ASME Sec. IX), use of calibration block made of mild steel is permitted, provided the block is made from forged material. The surface finish at the entry surface of the calibration block should be similar to that of the test object. Prior to machining the block, the raw material shall be ultrasonic examination checked and shall be adequately defect free. Prior to machining the block, the raw material shall be ultrasonic examination checked and shall be adequately defect free. The hole diameter shall be 3% of the thickness and minimum 3 mm diameter. The holes shall be machined truly flat bottomed, preferably by end milling. The diameter of the calibration blocks and block length shall be as shown in the Annexure Time base calibration basis Mechanically measure the test object diameter or thickness as applicable within 1 mm accuracy. Select a round figure for the required range such as 0 to 50, 0 to 100 or 0 to 200 mm etc. the range shall be such that the first back wall echo is between 60% and 90% of the time base. Calibration on reference block such as V1 block is accepted for the cases where job configuration does not give back wall echo Time base calibration procedure Calculate the time base position for the first back wall echo. Set the time base controls such that the second back wall echo is set to this value, with the first back wall echo being set to zero. Now, lock all the time base controls other than delay. From this setting, use the delay control to shift the first back wall echo to the calculated position. The time base is now calibrated, lock delay also confirm calibration from the position of echoes of other smaller diameter / thickness area wherever possible and correct any errors if necessary Sensitivity Setting the primary reference level

11 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 10 of 17 At a suitable gain setting find out which hole provides the highest response. Set the amplitude of this echo to between 70% FSH and 80% FSH using the gain control. This is the reference gain and should not be changed Drawing the DAC curve Use the instrument screen as a graph. At the metal path corresponding to the first hole (which gives highest response), mark the maximized amplitude of the echo. Repeat this for all the other holes. Draw a smooth line through the points so obtained. This is the distance amplitude correction (DAC) curve. Use a suitable marking pen which should not damage the screen protection sheet Scanning, Recording & Evaluation Scan at a gain value higher than the above reference gain by 6 db. Evaluation shall be done at reference gain for - Any indication whose amplitude equals or exceeds 50% of the established DAC curve Acceptance level Indications of following nature shall cause for rejection- Any indication whose amplitude exceeds 100% of the established DAC curve REPORTING All relevant data of the product, Ref document details, Calibration data and examination results shall be entered in the Proforma enclosed in Annexure POST EXAMINATION CLEANING After the examination the test surface shall be thoroughly cleaned free from grease, oil etc using dry cloth, acetone etc.

12 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 11 of 17 ULTRASONIC EXAMINATATION REPORT ANNEXURE 1 REPORT NO.:. DATE OF TESTING : REFERENCE NO.: REFERENCE CODE NO.: MATERIAL DESCRIPTION WORK ORDER NO. : MATERIAL : IDENTIFICATION: INSTRUMENT DESCRIPTION DIMENSIONS: SURFACE : MACHINED/ROLLED/WELDED CONDITION INSTRUMENT USED : MACHINE No.: MAKE: SEARCH UNIT : COUPLANT : OIL / WATER CALIBRATION RANGE CALIBRATION: CALIBRATION : STANDARD METHOD & TECHNIQUE: SENSITIVITY: EXAMINATION SCANNING: EXAMINATION STARTED AT : HRS. CLOSED AT : HRS CALIBRATION 1. Hrs 2. Hrs 3. Hrs RECHECKED AT OBSERVATION OF RECALIBRATION CHECK OBSERVATION : EVALUATION :

13 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 12 of 17 ANNEXURE 2 B C D STANDARD REFERENCE BLOCK (For Axial Scanning) Note: For Radial Scanning, the hole should be drilled radially on the circumference of the Reference Block.

14 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 13 of 17 DIMENSIONS OF REFERENCE BLOCKS Hole diameter (mm) Metal distance B (mm) Overall length C (mm) Diameter D (mm) B B NOTE B FOR ADDITIONAL SUPPLEMENTAL BLOCKS FOR THICKNESS > 250mm.

15 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 14 of 17 Notch Details ANNEXURE 3 Depth Width Length

16 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 15 of 17 ANNEXURE 4 (Appendix 1 to PP-P-1981, Performance Checks of UT Instrument) A. SCOPE This Appendix describes the procedures to be followed and provides the requirements to be met to check and confirm that an ultrasonic test instrument is performing adequately, if such a check is required by a Specification or Procedure. In using the following directions, indications may need to be maintained on the screen over a length of time, with assurance that there is no change in received echo energy. An immersion fixture with an immersion probe can be very convenient for maintaining this constancy over this period. However, use of such a fixture is not mandatory, though then, care has to be taken that changes do not take place and a recheck shall particularly be done with the old control settings. If the performance of the instrument does not meet the requirements, it shall be serviced and all performance parameters re-evaluated. Such servicing is not within the purview of UT personnel, but must be conducted by the manufacturer, their authorized agents or other electronic servicing personnel of adequate electronic capability. B. AMPLITUDE LINEARITY This parameter is also called Screen Height Linearity. Use any convenient angle beam probe. Set the Suppression or Reject control to minimum (or OFF, if available) which value shall be maintained undisturbed further. Pick up two simultaneous indications from ½ and 3/4T side drilled holes in any convenient test piece or block which can adjusted such as to provide a ratio of 1:2 between indication heights. That is, one echo must be twice the height of the other. It is immaterial which echo appears first on the time-base. In order to obtain this condition, one method is to obtain a stepped block (i.e. one with two distinct thicknesses) and scan it from the un-stepped flat side. On scanning the zone of thickness change, two echoes, backwall echoes from the two thicknesses, will be obtained. By selecting an appropriate probe position, the two echoes can be set at any amplitude ratio to each other, in this case, 1:2 or 2:1. Using Gain, Damping and Filter or any combination of these controls, set the amplitude of the bigger of the two echoes to 80 % full-screen-height. Without permitting probe movement, re-adjust any of the above controls to change the amplitude of the larger of the two echoes successively from 100 % to 20 % in steps of 10 % full-screen-height. Record the amplitude of the smaller echo in each case. This value must be within 5 % full-screen-height of half the bigger echo, rounded off to the nearest 1 %. For example, for the case of the bigger echo being at 60 %, the smaller echo must be within the range of 30 % full-screen-height, ± 5 %. Values outside these call for instrument servicing. C. AMPLITUDE CONTROL LINEARITY Use any convenient probe. Set the Suppression or Reject control to minimum (or OFF, if available) which value shall be maintained undisturbed further. Pick up

17 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 16 of 17 indication from from ½T side drilled in any convenient test piece or block which can readily be maintained stable. Using Gain, Damping and Filter or any combination of these controls, set the amplitude of this echo to 80 % full-screen-height. Only the Gain control or controls shall be adjusted further. Without permitting probe movement, change the Gain by ± 6 db and ± 12 db and record the amplitudes. The table below gives the requirements: Initial amplitude set at % of full screen Gain Control Change Acceptance Limits of Amplitude % of full Screen 80 % - 6 db 32 to 48 % 80 % - 12 db 16 to 24 % 40 % + 6 db 64 to 96 % 20 % + 12 db 64 to 96 % The settings and readings shall be estimated to the nearest 1% of full screen This check shall be done starting with many representative values of Gain, such that the performance of the Gain control system is evaluated over the entire range of gain control. Where more than one calibrated Gain control is available, such as two separate knobs, each of them shall be checked over their range of control. In order to check the gain controls at various ranges, other controls such as Damping and Filter may be set at different combinations at the start of each test sequence of varying the Gain by ± 6 and ± 12 db. The values of 6 and 12 db may be obtained by combining two of the controls. When not using the DGS method, depending on the application, it may be adequate to limit the check to the general range of values in use during actual scans. D. INSTRUMENT TIME -BASE LINEARITY This parameter shall be checked at or near range values corresponding to those normally in use. In the following descriptions, the percentage values refer to the time-base, 0 % and 100 % being the left and right hand extreme graduations. Select a block of uniform thickness of approximately one-fifth of the range to be checked. Use any convenient normal beam probe, and suitable settings for control settings other than those related to the time-base. Calibrate the instrument time-base to five times this thickness, by positioning the second back-wall echo to 40 % and third back-wall echo to 60 % of the screen. This would require adjustments to the Range, Material Calibration and Delay controls or their equivalent. Lock all of these controls other than Delay.

18 FOR ULTRASONIC EXAMINATION OF STEEL FORGINGS FOR GENERAL PURPOSE Page 17 of 17 Using Delay, shift the second back-wall echo to a selected screen division and record the position of the next echo, i.e. the third back-wall echo. Selected screen divisions, in turn, shall be zero, 20 %, 40 % and 80 %. The expected reading shall, in each case be obviously 20 % larger than the selected screen division. In each case the permissible error from expected reading shall be within 2 % of the time-base. In instruments with a total of 50 divisions for the time-base, this will correspond to one such small division. In many instruments, the display does not extend beyond the final time-base mark. In such cases, the check of the last point shall be done in reverse, by shifting the third back-wall echo to the last mark and reading the position of the second back-wall echo. Specifically in the case of the initial or last portions of the time-base twice the error of 2 %, i.e. 4 % may be permitted. This however, is conditional on checks of the 5 to 25 % or the 75 to 95 % or both respectively, additionally being done and the results being within 2 %, (in instruments with 2% as the small division size, these values may be changed to 6 to 26 % and 76 to 96 % respectively).