APPENDIX A Yield Strength from Hardness Additional Background

Transcription

1 REFERENCES 1. Burgoon, D. A., Chang, O. C., Francini, R. B., Leis, B. N., and Rust, S. W., Determining The Yield Stress of In-Service Pipe, for ASME Gas Pipeline Safety Research Committee, Battelle, December, 1999 and published by ASME as CRTD Vol O Neill, H., Hardness Measurement of Metals and Alloys, Second Edition, Chapman and Hall Ltd, Tabor, D., The Hardness of Metals, Oxford University Press, UK, 1951 (Reprinted by Redwood Books Ltd, UK). 4. Boklen, R., A Simple Method for Obtaining the Ductility From a 100o-Cone Impression, The Science of Hardness Testing And Its Research Applications, Chapter 8, American Society for Metals, Boyer, H. E., ed.; Hardness Testing, ASM International, Mott, B. W., Micro-Indentation Hardness Testing, Butterworths Scientific Publications, London, Small, L., Hardness Theory and Practice Part 1 Practice, Service Diamond Tool Co., Ferndale, MI., Revankar, G., Hardness Testing, in ASM Handbook Volume 8, Mechanical Testing and Evaluation, ASM International, Anon, Wilson-Instron portable hardness tester data sheet. 10. Leeb, D., Definition of the hardness value L in the Equotip dynamic measuring method, VDI Berichte Nr. 583, 1986/ Leeb, D., Dynamic hardness testing of metallic materials, NDT International, December 1979, pp Private communication, Teleweld Corp, February Anon, Advanced Indentation System 3000, Prakash, R. V. and Shin, C. S., An Evaluation of Stress Strain Property Prediction by Automated Ball Indentation (ABI) Testing, Journal of Testing and Evaluation, Vol. 35, No. 3, ASTM, Choi, Y., et. al.; Applications of Advanced Indentation Technique to Pre-Qualification and Periodic Monitoring of Strength Performance of Industrial Structures, Key Engineering Materials, Trans Tech Publications, Ahn, J-H. and Kwon, D., Derivation of Plastic Stress-strain Relationship from Ball Indentions: Examination of Strain Definition and Pileup Effect, Journal of Materials Research, Vol. 16, No. 11, November 2001, Materials Research Society. 17. Frank, S., Mobile Hardness Testing Application Guide for Hardness Testers, GE Inspection Technologies. 18. Borggreen, K. and Auerkari, P., Performance of some portable hardness testers, Sommer, J., The Possibilities of Mobile Hardness Testing A User Related Hardness Testing Comparison of the Static UCI Method and the Dynamic Rebound Method, October 1998, Vol. 3, No. 10, Frank, S., Portable Hardness Testing Principles and Applications, October 2002, Vol. 7, No. 10, Anon, Krautkramer MIC 10 Portable Hardness Tester product literature, Anon, Sonohard Ultrasonic Hardness Tester product literature, Microphotonics, 67

2 23. Anon, Ernst Portable Hardness Tester product literature; Anon, Equostat Hardness Tester product literature; Boyer, H. E., ed., Hardness Testing, ASM International, Metals Park, OH, Fultz, J., Hard and Fast Rules for Portable Hardness Testing, Inspection Trends, Frank, S., Portable Hardness Testing Principles and Applications, Frank, S., TIV (Through Indenter Viewing) New Possibilities of Mobile Hardness Testing, Anon, Krautkramer TIV Optical Hardness Tester- Mobile and Direct, Shlegel, V. et. al., The Use of MET-UD Combined Portable Hardness Testers For Technological Control, Centre for Physical and Mechanical Measurements, FSU, Borggreen, K. and Auerkari, P., Performance of some portable hardness testers. Paper authors are Swedish and Finnish. No additional details available. 32. Anon, Krautkramer MIC 10 Portable Hardness Tester product literature, Nelson, P.R., Coffin, M., Copeland, K.A.F., Introductory Statistics for Engineering Experimentation, Elsevier Academic Press, Bhattacharyya, G.K., Johnson, R.A., Statistical Concepts and Methods, John Wiley & Sons, Anon, Guidelines on the Estimation of Uncertainty in Hardness Measurements, European Association of National Metrology Institutes (EURAMET), EURAMET/cg-16/v.01, July Low, S. R., Rockwell Hardness Measurement of Metallic Materials, NIST Recommended Practice Guide, Special Publication 960-5, National Institute of Standards and Technology, January Brice, L., Davis, F., and Crawshaw, A., Uncertainty in Hardness Measurement, NPL Report CMAM 87, National Physics Laboratory, UK, April Polzin, T., Determination of Uncertainty for Hardness Measurement: Proposal of the Standard, Available Software, Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement, Springer. Volume 8, Number 12, December Adams, T. M., G104-A2LA Guide for Estimation of Measurement Uncertainty In Testing, July ASM Metals Handbook, Volume Auerkari, P., On the Correlation of Hardness with Tensile and Yield Strength, Technical Research Center of Finland, Research Report 416, July

3 APPENDIX A Yield Strength from Hardness Additional Background This Appendix provides additional details pertaining to relationships that have been established that allow yield strength predictions from hardness measurements. In addition to the basic concepts covered in Section 3 of this Guide, other specific applications and methods that have been described in the literature are included below. Source A: The Welding Institute (UK). In 1975, The Welding Institute (UK) published data showing the relationship of Vickers hardness test results and yield strengths for weld metals, heat affected zones, and base metals. Data was collected that covered a wide range of steel types over a period of time. Their primary motivation was to develop hardness-yield strength correlations that could be used to estimate the yield strength in areas such as weld metals and heat affected zones where input for maximum defect size calculations could be simply obtained without the need to produce coupons suitable for tensile testing. Yield strength estimates from hardness testing of base metals were also considered to be useful where rapid evaluations were required. Figure A-1 illustrates the relationship between Vickers hardness (HV) and yield strength (ksi) for base metal including the 95% confidence limits. The original graph included higher strength materials that were outside the scope of the steels of interest in this Guide. Although a wide range of materials were represented, the relationship indicated in Figure A-1 suggests a reasonable correlation was still obtained. Yield Strength (ksi) Hardness (HV) Figure A-1. TWI Yield Strength Hardness Relationship Source B: Hart, P. H. M., Yield Strength from Hardness Data, The Welding Institute Research Bulletin, June 1975, p 176. In 1978, additional yield strength-hardness data developed by The Welding Institute was added and the prior correlation was re-evaluated for two pass and multi-pass welds. This revised A-1

4 correlation was considered to improve yield strength predictions especially, for lower strength materials. Source C: Pargeter, R. J., Yield Strength from Hardness-A Reappraisal For Weld Metal, The Welding Institute Research Bulletin, November 1978, pp Hardness-yield strength correlations were applied in 1966 to determine rolling schedules for sheet or strip. Yield strengths were required over a range of reductions that could be obtained by typical tensile or compression testing. A method was derived that allowed Vickers hardness test indentation estimates of compressive yield strength as alternative estimation method on rolled samples. The methodology was validated for a range of alloys including copper, aluminum, and other materials and compared well with tensile and plane compression test data. The accuracy achieved was considered to be sufficient to allow hardness testing in lieu of tensile/compression testing and also served as a quality control tool. Source D: Oliver, B. R. and Bowers, J. E., The Determination of Yield Stress from Hardness Measurements, Journal of the Institute of Metals, 1966, pp A field evaluation of the hardness yield strength relationship of high strength, quenched and tempered downhole tubular materials was conducted in the 1990 s. In this application, a portable Brinell tester was used to develop a relationship from tests conducted on API Grades C75 through V140 materials. Even though the tubular yield strength could be predicted within a ± 10,000 psi range, it was found that the prediction was sufficient to identify tubulars that failed due to low strengths and those that had been improperly heat treated resulting in low strength levels. Source E: Fehr, G. and Long, R., Determining Metal Yield Strength in the Field, ASME PD- Vol. 56, Drilling Technology, The objective of this work was to establish a relationship between the 0.2% offset yield strength of a material and the Vickers hardness and also consider the strain hardening coefficient. The methodology was derived from earlier work and relationships developed by Tabor and Meyer (3) from analyses were conducted on an aluminum alloy and 1040 steel. A correlation was developed between the experimental yield strength (σ y ) and the value determined from the following expression resulting from this work: σ y H = B 3 m 2 where: H = Vickers hardness B = Constant (0.1 for steel) m = Meyer s hardness coefficient n = strain hardening coefficient (=m-2) Comparison of the yield strengths estimated using the above equation with experimental tensile data showed a good correlation was achieved. The validity of the equation was also evaluated A-2

5 versus unknown aluminum, brass and steel tensile data. Good agreement between the above equation and the tensile data was achieved for all three materials. Source F: Cahoon, J. R., Broughton, W. H., and Kutzak, A. R., The Determination of Yield Strength From Hardness Measurements, ASME Intl, Metallurgical Transactions, Volume 2, July An automotive sheet metal application prompted the development of a hardness based yield strength estimation method for in-process material evaluations prior to and after forming operations. This method made use of the earlier relationships developed by Tabor and Meyer in addition to the results attributed by Cahoon that are described in the previous paragraph. A series of nomographs was constructed to permit yield strength measurements from the results of three Rockwell hardness tests conducted at different loads (i.e., HRF, HRB, HRG). Source G: George, R. A., Dinda, S., and Kasper, A. S., Estimating Yield Strength From Hardness Data, Metals Progress, ASM International, May This project was primarily an application of earlier work done by Tabor and the methodology reported by Cahoon (see synopsis F above). In this case, some modifications of Cahoon s work were suggested and difficulties with estimating the strain hardening coefficient (n) were also reported. It was concluded that a reasonable estimate of both yield and tensile strength could be obtained from the Vickers hardness if n is known or can be determined. Source H: Auerkari, P., On the Correlation of Hardness with Tensile and Yield Strength, Technical Research Center of Finland, Research Report 416, July This work considered the relation between hardness and strength properties of metals was reviewed and compared with experimental data. It was found that the methods proposed by Cahoon resulted in a good correlation with tensile strength and provided a reasonable correlation with yield strength provided the strain hardening exponent (n) remains essentially constant. It was concluded that the available expressions correlating hardness with yield and tensile strength are very useful. Figure A-2 illustrates a yield strength hardness relationship developed from experimental data. A-3

6 Figure A-2. Hardness Yield Strength Relationship (From Auerkari) A-4

7 APPENDIX B Field Hardness Tester -- Detailed Information This Appendix provides additional details concerning the design and operational criteria necessary to properly select and apply the portable hardness testers covered in this Guide. All references in this Appendix refer to the Guide main body reference list. B-1 Leeb Rebound Hardness Testers B-1.1. General Information Leeb scale hardness testers are quite similar to the older Shore hardness testing method that consisted of a ball that was dropped through a glass tube and accelerated due to gravitational force onto the material surface to be tested. The Shore test represented the first attempt to measure hardness by observing the energy loss after the indenter rebounded from the test piece. In contrast, Leeb scale hardness testers use a spring force to accelerate the indenter into the pipe surface and represents a significant advancement of the dynamic hardness testing method (2,3). Rebound hardness testers based on the Leeb method have been available for about 30 years. Following expiration of Leeb s original patents, the number of available Leeb scale rebound testers expanded considerably since their initial introduction. In 1995, a modified Leeb tester was introduced that contained multiple indenter velocity measuring coils that automatically compensated for gravity effects on the test. In 1997, the first edition of ASTM A956 was issued which provided standardized test criteria for this method. Additional Leeb hardness test standardization efforts by DIN in Europe are ongoing. Leeb scale rebound hardness testers are offered by several manufacturers and are available in different styles and configurations plus two different methods of compensating for the test position. All Leeb scale testers consist of a spring loaded impact device with a tungsten carbide (WC) ball (i.e., D type ), an induction coil for indenter velocity measurements, a support ring, and an electronic display indicating the hardness plus other functions including hardness scale conversions. Some of these testers consist of one piece integral units with the body containing the impact device built into the electronic display while others have a separate impact body that is connected to the electronic package. In some cases, the impact body is offset to one side of the integral unit. Figures B-1 and B-2 are typical examples of the two types of Leeb hardness testers that are currently available. B-1

8 Figure B-1. Integral Leeb Hardness Tester Figure B-2. Separate Indenter Type Leeb (23, 24) Tester The Leeb scale (L) hardness value is simply defined as the ratio of the indenter impact velocity (V i ) as it is accelerated toward the test piece and the rebound velocity (V r ) as follows (10,11). Vr L = 1000 Vi The indenter impact creates localized plastic deformation of the test piece which causes the indenter to lose a part of its initial velocity and the rebound is driven by the elastic recovery. Lower hardness materials result in a greater velocity loss. Leeb rebound hardness testers can be used in all positions but the single coil indenter velocity measurement method, common to most available testers, is sensitive to the effects of gravity on the indenter during the test. As a result, such Leeb hardness testers require the application of a hardness correction factor when an indention is made in other than the vertical down position. In general, this correction factor is applied by programming the electronics package to automatically apply the required factor each time tests are conducted in other positions. If this capability is not included in the electronics package, the correction factors can be found in ASTM A956 for the different available indenter types. A different implementation of the Leeb tester produced by Krautkramer employs a dual coil indenter velocity measuring system that automatically applies the proper position correction factor (10,11,21). Different indenters are available for many of the Leeb testers shown in Table B-1(with different impact energies and tips) that are suitable for different materials and wall thicknesses. For steels used in pipelines, the most useful has been the D indenter with a 3 mm diameter tungsten carbide ball. Typically, the Leeb hardness (HL) scale result for pipeline steel materials with a D style indenter ranges from about In itself, the Leeb hardness value is typically not used. Therefore, conversion to the more commonly used hardness scales such as Rockwell B (HRB), HRC, and others is typically part of test process. B-2

9 B-1.2. Leeb Hardness Tester Thickness Considerations As the indenter impacts the material surface, the impact energy (900 N or 202 lbf for a D indenter) creates surface plastic deformation and the rebound is driven by the elastic recovery. The impact energy imparted also creates a vibration in the test piece which can also further reduce the indenter rebound velocity. This vibration results in an artificial hardness reduction and also increases test scatter with decreasing pipe wall thickness. In addition to the wall thickness alone, the hardness reduction magnitude is also related to the pipe elastic ring stiffness as indicated by the diameter/thickness ratio or other measures (41). One example of the thickness effect on Leeb rebound hardness ( D indenter) testing results and Vickers hardness testing (98 N) is shown in Figure B-3. It can be seen the Leeb (converted to Vickers hardness) and the standard Vickers hardness values are essentially equivalent above a wall thickness of about 0.79-inch (20 mm). For lower wall thicknesses, the actual Vickers hardness is greater than the converted Leeb hardness by the factor shown on the y-axis of Figure B-3 (20) Normalized Hardness Thickness (in) Figure B-3. Comparison of Vickers and Leeb Hardness vs. Wall Thickness Another example of a similar relationship is illustrated by the results of pipe hardness tests conducted by the authors. Figure B-4 illustrates the difference between the hardness values made with a Leeb rebound hardness tester (converted to HRB) on pipe sections with different diameters and a range of wall thicknesses compared to direct HRB test data. The vertical axis indicates the HRB scale ratio of the lab and field test data. For the range of wall thicknesses shown, Figure B-4 indicates the HRB correction factor that should be multiplied by field test result to achieve equivalency with a standard HRB test result and should be included in any hardness-yield strength correlations. These data are very similar to that shown in Figure B-3. B-3

10 2 Normalized Hardness (HRB/HRB Converted) 1.8 1/Correction Factor = /wt^ Pipe Wall Thickness (in) Figure B-4. Comparison of HRB and Leeb Hardness vs. Pipe Wall Thickness An evaluation of other similar data from the literature was also conducted. The relative elastic (compliance) ratio for two circular flat plates with the same diameter and different thicknesses under point loading at the center and supported at the rim is shown in Figure B-5. Again, this relationship is similar to Figures B-3 and B-4 except that the correction factor does not tend to begin rising rapidly until the wall thickness is less than shown in Figure B-3 and B-4. Compared to Figure B-4, this relationship indicates that the required Leeb hardness correction factor is a function of both the wall thickness (i.e., local mass) and the pipe stiffness (41). Correction Factor Plate Thickness (in) Figure B-5. Calculated Pipe Stiffness Correction Factor for Leeb Hardness Testing (41) A similar graph reported in the same reference is shown in Figure B-6 except that it based on pipe compliance for pipe with different diameter/thickness ratios (D/t) that have been referenced to the compliance of a solid round bar. This relationship was derived from basic compliance expressions with some finite element analyses (FEA) validation up to a D/t ratio of 8.3. According to this reference, Figures B-5 and B-6 are used together depending on the pipe D/t ratio. For pipe with low D/t ratios, Figure B-6 provides the appropriate correction factor but the correction factor can never exceed that shown in Figure B-5 for the same wall thickness. Therefore, for larger diameter, high D/t pipe, Figure B-5 provides the appropriate correction factor according to Reference 41. It was also stated that the largest correction factor component is the pipe cross sectional dimensions (41). B-4

11 Correction Factor Pipe D/t Ratio Figure B-6. Calculated Pipe Stiffness Correction Factor for Leeb Hardness Testing Referenced To Solid Round Bar (41) When using a Leeb rebound hardness tester, the need for a hardness correction due to wall thickness can also be qualitatively evaluated by the audible sound of the impact. When a large, solid mass (such as the test block) is impacted, the audible sound can be described as a click which indicates surface vibrations are not a factor. However, when a thinner wall section is impacted by a Leeb indenter, the audible sound emitted is similar to a bong or like a bell ringing. This sound indicates that the recorded Leeb hardness will probably underestimate the actual material hardness and an appropriate correction factor should be applied to obtain the proper result. ASTM A states that the minimum test piece weight and thickness should be 15 lb and inch respectively. Test pieces weighing less than 15 lb or with thicknesses less than inch of any weight should be rigidly supported during a Leeb rebound test. However, considering the data from the literature and developed by the authors as shown in Figures B-3 through B-6, it does not appear possible to apply the ASTM stated limits to line pipe testing without applying a correction factor. Some specific considerations applied to Leeb rebound testers are: Tests by the authors indicate that for testing pipeline materials, a group of at least 10 tests should be used at each location. The HLD value standard deviation of each group of tests should be a maximum of about 10. If it is above 10, the test area should be re-prepared and re-tested. This requirement is similar to ASTM A956 repeatability criteria. It is recommended that the correction factor relationship with pipe wall thickness be used where Leeb testers are used on wall thicknesses less than about inch. Each test area should be prepared by sanding in multiple steps through at least 180 grit abrasives. Excessively rough surface finishes can artificially lower the hardness value. Leeb testers are battery powered, portable, and suitable for testing in all positions with minimum clearance. Only small areas of coating must be removed and testing can be done without access to the full pipe circumference. B-5

12 Ambient temperature extremes must be considered that typically range from about 32 to 120 F. The specific temperature range recommended by each manufacturer should be considered. The typical Leeb scale hardness test block furnished with a D type impact body has a HLD hardness of about 750. It would be desirable to have a test block more consistent with the material hardness range to be tested (i.e., HLD ). Other Leeb scale test blocks are available such as with the G type impact body with a HLD of about 600 that exceeds the likely pipe test range. An alternative would be to attach a standard steel Rockwell B test block to surface of the standard Leeb test block (or a large, smooth steel section) with the coupling paste (furnished with Leeb testers) and calibrate directly using the HRB conversion contained in the Leeb hardness tester. When testing is conducted on individual pipe segments or coupons, the proximity of an open end will artificially lower the hardness value. It is recommended that hardness testing should not be attempted within 12 inches of an open pipe end. Due to the design of multiple coil Leeb tester impact bodies that compensate for position, the required indenter rebound travel distance is greater. Therefore, restrictions on the minimum hardness that can be accurately measured may exist since more rebound energy is lost during the impact. This should be evaluated prior to field test applications where lower hardness materials may be involved. The internal hardness scale conversion algorithms in all brands of Leeb rebound testers may not be equivalent to each and should be compared using a standard test block. Due to the impact body and support shoe design, precise placement of an indentation can be difficult to achieve. Therefore, if precise indentation locations are needed, an alternative hardness tester is recommended. It is extremely important that the Leeb tester impact body is firmly held on and is maintained perpendicular to the test piece surface during the test. The manufacturer or supplier of any Leeb scale hardness testers should state that their equipment is certified to be in compliance with the requirements of ASTM A956. Some available testers could contain design deficiencies that could lead to erroneous data. For Leeb testers with the impact body connected to the electronics package by a special cable, continued cable integrity under field conditions has been found to be problematic. It is recommended that at least one spare cable be immediately available during field testing. B-2 Ultrasonic Contact Impedance (UCI) This is an application of Vickers hardness testing except that the hardness is not determined from the hardness impression dimensions first used in A Vickers diamond indenter is attached to the end of a metal rod that is being resonated into a longitudinal oscillation mode at frequency of about 70 khz by piezoelectric transducers. A typical UCI hardness tester is shown in Figure B-7 (20,21). Such hardness testers are commercially available from several manufacturers. B-6

13 Figure B-7. UCI Hardness Tester (21) When the load is applied to the rod by a spring, the indenter penetrates the test piece and the rod oscillation frequency changes in proportion to the indention contact area. This frequency shift will increase as the indentation area becomes larger in lower hardness materials. Since the frequency shift is proportional to the indention size and Young s modulus, the hardness can be determined as described by the following equation where (C) is constant, (P) is the applied load, and (A f ) is the measured frequency shift under load (20,21). P HV = C Af During the test, the UCI instrument monitors the oscillation frequency, performs the required calculations and displays the hardness level. Internal algorithms permit automatic conversions to other common hardness scales. In some cases, motorized probes are used for lower loads to minimize operator influence. UCI hardness testers have been used for field hardness testing on pipelines and components. Since the UCI analysis method is a function of Young s modulus, a recalibration is required when testing materials with a different modulus value. UCI hardness testers have six available loads ranging from 0.1 N (0.1 kgf) to 98 N (10 kgf) depending on the application. For pipeline applications and particularly where conversions to material strength are anticipated, the 98 N load is required as stated in ASTM (19). Since UCI testing may also create test piece flexural vibrations (similar to Leeb rebound testers) resulting from the indenter rod oscillations, hardness data scatter can become an increasing issue with thicknesses less than about 0.6-inch (15 mm). In any case, the test piece wall thickness should not be less than about 0.12-inch (~3 mm). A comparison between UCI data (HRB scale conversion) and laboratory HRB test data on pipe materials was conducted by a pipeline operator. Measurements were made over a wide range of pipe diameters (2 34 inch nominal OD) with wall thicknesses between 0.15 and 0.75 inch. Test surfaces were prepared using sanding discs with decreasing grit sizes down to 180. Figure B-8 B-7

14 illustrates the comparison of the laboratory direct HRB hardness data with the UCI data converted to HRB from unpublished test results conducted by the authors. The linear regression line shown (through the origin) indicates a reasonable correspondence between field and laboratory data with significant scatter in some cases. Although some significant scatter is evident, the overall correlation is reasonable Lab - HRB y = x UCI - HRB Scale Figure B-8. Comparison of Lab HRB to UCI Data Converted to HRB Additional comparisons between UCI data converted to HRB and direct HRB hardness measurements were made using the pipe wall thickness and diameter versus the difference between the field and laboratory test data as field HRB minus lab HRB. Figures B-9 and B-10 illustrate the hardness difference (field HRB minus lab HRB) or error plotted versus the pipe wall thickness and diameter respectively. With respect to pipe wall thickness, the most extreme scatter is evident when testing the thinner wall pipe which is consistent with the minimum wall thickness requirements discussed above. Similarly, Figure B-7 indicates more scatter when testing smaller pipe diameters. It should also be noted that data points located well below the regression line shown in Figure B-8 are primarily from 6-inch outside diameter (OD) and smaller pipe with half the data from 2-inch OD pipe. This suggests that smaller pipe diameters are more difficult to properly test in the field with a UCI tester and most likely related to the indenter probe not being held perpendicular to the pipe surface. B-8

15 Delta(UCI-Lab HRB) Pipe Wall Thickness(in) Figure B-9. Effect of Wall Thickness on UCI Test Error Delta(UCI-Lab HRB) Pipe Diameter(in) Figure B-10. Effect of Pipe Diameter on UCI Test Error From these data, some specific considerations for UCI hardness testers are: Small diameter pipe is more difficult to accurately test. This implies that the indenter alignment with the pipe surface is an important essential test variable. Lower pipe wall thicknesses increase hardness data scatter. For thin wall pipe materials, loads less than 98 N would be preferable but this may impact the accuracy of conversions to strength. Surface preparation and finish requirements are more demanding to minimize test scatter especially with lower loads. Compared to Leeb rebound hardness testing, a UCI tester would be more appropriate for field testing pipe with wall thicknesses less than inch although increased scatter would be expected. B-9

16 Data in the literature confirms the author s experience that it is critical that test probe should remain firmly held and perpendicular to the pipe surface for the test duration. B-3 Direct Reading Rockwell Hardness Testers Several versions of a direct reading Rockwell scale hardness tester that can be used on pipe are currently available from Wilson-Instron specified as Models M-0 through M-9. Each of these testers has a range of available test loads from 15 to 150 kgf, allowing measurement in regular Rockwell (HRB, HRC) or Rockwell superficial scales. The testing force is applied by a pre-loaded spring mechanism. All of these hardness testers must be mounted on the pipe and utilize different mounting methods. The M-2 model shown in Figure B-12 has a C-Frame clamping system that would be suitable for small diameter pipe. Others are equipped with a chain or steel band wrapped around the pipe circumference with measurement head similar to that shown in Figure B-12. The M-8 model, shown in Figure B-11, is equipped with electromagnetic base and is reported to be applicable to 1.5 inch OD and larger pipe. No external power source is required for these portable testers except that 120V AC electrical power is required for the magnetic mount on the M-8 model (9). For pipeline testing, considering the yield strength estimation objective, it is recommended that HRB scale should be used to eliminate a required scale conversion. Therefore, the minimum wall thickness is inch considering the test 100 kgf load required for HRB testing. Lighter loads using the Rockwell superficial hardness scales may permit their use on thinner wall pipe. Another reason for the inch minimum wall thickness is to provide for an adequate magnetic coupling force for the M-8 model magnetic mount to prevent lift-off during the test. The magnetic mounted version, although considered portable, weighs about 42 lbs and could be difficult to transport and operate in difficult terrain. The chain mounted version (15-35 lbs) can be used in different positions while the magnetic mounted tester would be applicable primarily at or close to the top of a pipeline. Also, unless 120V AC power is locally available, an enginegenerator would also be needed for the magnetic mounted tester. The portable models that are mounted with a chain or strap require access to the complete pipe circumference thus increasing the amount of excavation required at each test location. In order to properly attach such testers to a pipeline, the coating in the test area as well as completely around the pipe, in most cases, would require removal. Very little coating deformation could be allowed to maintain a tight mount that would resist lift-off as the test load is applied. Figures B-11 and B- 12 illustrate the magnetic mounted and C-clamp tester models. B-10

17 Figure B-11. Magnetic Mounted Direct Figure B-12. Clamp Type Direct Rockwell Tester (9) Rockwell Tester One of the major benefits in using these portable testers is that Rockwell B hardness values can be directly converted to published equivalent strength values without the need for intermediate conversions. Ultimately, this could benefit the accuracy of strength estimates from hardness test results since intermediate scale conversions are not required. Also, for Rockwell B testing (100 kg load, 1/16 diameter ball), surface preparation requirements are less stringent and reliable results with minimal scatter can be obtained without a high quality surface finish. A recent significant change in ASTM E 18 is that Rockwell B testing now must be conducted with 1/16-inch diameter tungsten carbide (WC) rather than steel ball indenters. Steel indenter balls may still be specified in product specification or hardness test procedure. Preliminary data indicates that WC balls tend to read about 1 HRB unit less than a steel ball. When HRB results are reported, it should be indicated which type of indenter ball was used. Some specific considerations applied to direct reading Rockwell testers are: The magnetic mounted version may not be useable where surface corrosion or roughness exists that could decrease the magnetic coupling force. Coating would need to be removed over about a length of 14 inches along the pipe axis to accommodate the mounting. The chain or band mounted version can be difficult to properly align and set up for testing especially where the pipe to be tested is not straight. Application to bends or elbows can be difficult. Complete access to the full pipe circumference is required for mounting. Testing cannot be done within restricted pipe access locations. For other than the top of the pipe, more clearance would be required as compared to other hardness testing methods. Except for the Model M-8 magnetic mounted tester, the others can reportedly be used in all positions. However, the author s experience has indicated that testing in positions other than the top quadrant can be difficult to accomplish in practice. B-4 Manual Indention Testers Manual indentation hardness testers, sometimes described as Rockwell like hardness testers require the test technician to apply the test load. They consist of an electronics and data processing package that is coupled to the indenter device as shown in Figures B-13 and B-14. B-11

18 Figure B-15 illustrates a test technician applying the test load. They are typically battery powered with some having a 120V AC power alternative. These testers use either conical diamond or ball indenters with hardness determined from calculations based on impression depth. Conversion to most common hardness scales is provided with internal algorithms. These testers are light and portable and may be used in any position assuming that sufficient space exists for the operator to properly apply the load (23,24). Figure B-13. Manual Indentation Tester Unit (24) Figure B-14. Manual Indentation Indenter Body (24) Figure B-15. Manual Indentation Load Application (23) As the test technician applies a load to the indenter device, a preload is first applied that is controlled by spring loading followed by the total test load. The maximum load magnitude is also controlled by the spring loading within the indenter device. As load is removed, the indention depth difference between the preload and maximum load is determined and the hardness value is calculated. Since the indenter device body rests directly on the test piece, any test piece deflection does not affect the hardness measurement. Therefore, this method is more suited to thinner materials than other hardness testing methods. One of these instruments uses a unique hardness scale described as HRZ which is then converted into the desired standard scale including HRB, HRC or others (24). Some specific considerations applied to manual indentation testers are: Load must be applied by the test technician while holding the tester squarely on the test surface. Therefore, access must be sufficient to accommodate the operator and test instrument. Although these hardness testers can be used in all positions, only the top and B-12

19 sides of a pipeline could be realistically considered. In cases where the operator cannot evenly apply the test load, their usefulness would be limited. Testing can be done on thinner wall material without the need for correction factors as with the Leeb rebound testers. As with Leeb rebound and UCI hardness testers, only a minimal amount of coating must be removed. The role of the test technician is significant, requiring training to properly operate the test equipment. B-5 Brinell Hardness Testing The Brinell hardness test method relates metal hardness to the diameter of the test impression made by a ball indenter. For a given load, the hardness increases as the impression diameter decreases. Several types of portable Brinell testers are available that could be applied to pipeline field hardness testing. They include a clamp loading device and comparative testers that are typically impact loaded with a hammer down to a minimum of 1-inch OD. The clamp loading type would only be applicable to small diameter pipe (1-inch minimum OD) while the comparative pin type and Telebrineler are applicable on larger pipe diameters. The pin type incorporates a calibrated shear pin with a known shear load that is inserted into the indenter device against the indenter and driven into the test piece by a hammer impact or static load. The indenter is forced into the test material only as far as it takes for the shear pin to fail. The shear pins are calibrated which provides the basis for tester calibration. Excess applied load is absorbed by the indenter device body as the indenter moves into an internal cavity. The indentation diameter is then optically measured to determine the hardness. Figure B-16 shows a pin type Brinell tester (12, 23). Figure B-16. Pin Brinell Tester (23) With pin Brinell testing, if the hammer blow does not break the shear pin, the load on the indenter will be less than the target load. Otherwise, if the hammer blow is sufficient to break the B-13

20 shear pin, then the ball indenter applied load is sufficient. Shear pin failure prevents excessively high loads from being applied to the pipe surface. The Telebrinell tester is somewhat similar in that the indention force is provided by a hammer impact. For this method, the force is used to simultaneously drive two indenters; one into the test piece surface and the other into a calibrated reference bar that was selected to have a similar hardness level as the test piece. The indentation diameters in the reference bar and the test piece are then optically measured and compared with the ratio of the two indention diameters used to determine the pipe hardness value. Since the Telebrinell tester is based on a comparison of the hardness impressions on a known and unknown material, it is a self calibrating method. With Telebrinell testing, the applied pipe and test bar load is dependent upon the force of the hammer blow. Since the measured hardness value is affected by the applied load, the Telebrineller may be less accurate. However, the Telebrineller can produce usable results on relatively thin wall or large D/t pipe, whereas the same pipe may elastically deform sufficiently to prevent enough load from being applied to break the shear pin if using a pin Brinell test. In such a case, the load from the hammer blow causes the pipe to elastically deform before sufficient load is applied to break the shear pin. Any pipe on which the shear pin can be broken during the pin Brinell test without producing a dent on the pipe (other than the ball indenter impression) can be tested using the pin Brinell method. No hardness measurement is possible with the pin Brinell method if the pin does not break. However, a useable, hardness measurement can be obtained by the Telebrineller at loads that would be insufficient to break the shear pin in the pin Brinell method, although the pin Brinell test procedure specifies that the applied load should be sufficient to produce a dent of a specified size range in the calibrated test bar. Telebrinell tests that do not produce a large enough impression are invalid as a result of the applied load being outside the target range. Brinell hardness testing is typically performed with 10 mm diameter hardened steel or WC ball indenter with a maximum load of 3000 kgf load although 1500 kgf, 500 kgf, and lower loads can be applied. Smaller ball type indenters are also permitted by ASTM E10. The test piece minimum thickness should be such that no bulging or through wall deformation is evident on the opposite side of the pipe. Minimum thickness requirements for Brinnell testing are sometimes given as at least 10 times the impression depth but such guidelines typically apply to a supported test piece and not an unsupported pipe. The remaining impression is comparatively large compared to other hardness testing methods and may not be suitable for all pipeline applications, especially in cases where ductility at the service temperature and fracture toughness may be marginal. Although, the pipe surface finish is not critical considering the indenter ball size and loading, it should be adequately prepared to facilitate an accurate indention diameter measurement. The ultimate accuracy of this method is largely dependent on the test technician s ability and judgment. Therefore, sufficient training is an essential element in the success of field Brinell hardness testing. B-14

21 Some specific considerations applied to portable Brinnell hardness testers are: Hammer impact type Brinell testing would be difficult to use in limited access areas or in all positions. It would be feasible only on the pipe sides provided sufficient excavation was performed and on the top of the pipe. One advantage of the method is that it samples a larger volume of the metal than either the ball rebound or UCI method. As such, it is less sensitive to large pipe grain size issues and the hardness reading is not affected by minimal surface preparation. However, the large remaining impression may not be suitable for all applications. Even through good surface preparation is not significant for creating a valid hardness indention, it does facilitate a more accurate indention diameter measurement. According to manufacturers, the minimum wall thickness for hammer impact testers that depend on a reference bar comparison is about 3/16-inch. However, if any test piece deflection results from the test, such a comparison would be invalid. The applicability of this method on pipe should be evaluated prior to testing. B-6 TIV Vickers Method A more relative and recently developed static field hardness testing method is Transpyramidal Indenter Viewing or Through Indenter Viewing (TIV) and is also known as the Through Diamond Technique (TDT). This is an application of standard Vickers hardness testing except that indention dimensions are measured under load. The TIV system consists of two components including the indenter body and Vickers diamond indenter and the computer/electronics package as shown in Figure B-17. The computer within the electronics package allows data accumulation and some statistical calculations (28, 29). Figure B-17. Vickers TIV Hardness Tester (29) The indenter body shown in Figure B17 contains the loading mechanics and associated electronics, optics, and a CCD (charge-coupled device) digital camera that can create high resolution digital images under a variety of lighting conditions and allows the user to see through the indenter. The hardness is measured during the loading process and is immediately sent for B-15

22 evaluation as the required test load is attained. At that point, the image of indenter is captured and an automatic hardness evaluation is made. Also, the operator can evaluate the indention and measurement quality plus observe the condition of the diamond indenter (28, 29). Two test loads are available including 5 kgf (50 N) and 1 kgf (10 N). Load is manually applied to the top of the indenter body by the operator (similar to the previously described manual indentation testers). This method is reportedly applicable to thin materials and mass does not affect the results. Internal conversion to other hardness scales including HRB per ASTM E140 is included. It can be powered by rechargeable batteries, standard c-cells, or a line current adapter (29). Some specific considerations applied to the TIV hardness testers are: The indenter body is comparatively large which would make precise location of indentions difficult and may also impact access. It is a self calibrating test method. Preparation and pipe surface issues such as grain size would tend to affect this test method similarly to the UCI method and more so than others like Leeb rebound and manual indentation testers. B-7 Automated Ball Indentation (ABI) Method Early work by Meyer and others covered in Section 3 of this Guide established that a true stress-- true strain relationship could be determined from successively higher indentation loads in metals by a spherical indenter. This concept was advanced considerably within the past 20 years by development of the ABI method (13-16). ABI is an indentation depth sensing testing method that continuously records and processes indentation information and have been designed for both lab and field applications. The field applicable version typically consists of a laptop computer which controls the indentation system that is mounted on a pipe with straps or a magnetic shoe. Figures B-18 and B-19 show the two main components and an example of the portable version mounted on a pipeline (13). Figure B-18. ABI System and Computer Control (13) Figure B-19. ABI System Mounted On Pipeline (13) B-16

23 The computer continuously controls the load and displacement of an indenter as it is loaded and unloaded from the pipe surface. Based on the load-displacement data collected after several load/unload cycles, information such as a true stress true strain curve can be generated thereby allowing yield and tensile strength estimates. Other information such as residual stress, basic hardness values, and fracture toughness (Kc) reportedly can also be estimated (14, 15, 16). ABI systems use different types of indenters including small diameter TC spherical (0.5 and 1.0 mm) and Vickers diamonds. Both battery and 120V AC power can be used in the field. Due to the size of the test head, additional coating removal would be required when compared to other field hardness testers such as the Leeb rebound or UCI. In cases where additional pipe material information is needed other than the basic conversion from hardness to strength, the ABI method is the only indentation test method capable of generating such data that is currently available (14-16). Some specific considerations applied to ABI hardness testers are: ABI systems can generate more data and can directly estimate yield and tensile strength without a need for conversion tables. Such testing would likely be performed by a qualified contractor rather than directly by a pipeline operator. Compared to typical field applicable hardness testers, ABI systems are expensive. Testing rates would be somewhat slower than typical field hardness testers. Compared to smaller, more compact hardness testers, ABI system components and the need for high quality 120V AC power may reduce field portability. Table B-1 summarizes some of the detailed operating parameters for numerous commercially available hardness testers that could be used for pipeline testing projects. In many cases, other manufacturers produce similar equipment that could also be used. It should be noted that Table B-1 applies to hardness tester availability at the time the data was compiled and is subject to change. B-17