Combination techniques to improve accuracy and reliability of portable hardness tests

Transcription

1 Combination techniques to improve accuracy and reliability of portable hardness tests Tom Ott, Manager, Technical Services and Operations, Proceq USA NDTMA 2016 Annual Conference, Las Vegas, NV Thursday, February 18, 2016

2 Hardness contributes to hydrogenassisted stress corrosion cracking Inadequate PWHT or environmental conditions allow atomic hydrogen to diffuse into the steel. Hydrogen combines with carbon to form methane and becomes trapped. Internal methane pressure increases(increased residual stress) to cause cracking. Susceptibility increases with hardness (less ductility, more martensite). Higher hardness indicates susceptibility to HSCC. Large, complicated structures present challenges to hardness test HAZ. Slide Proceq Picture: Chemical Safety Board

3 Overcome hardness testing challenges with combined techniques The default standard for hardness testing is laboratory based bench top instruments that cannot be easily used for field work or on large objects with access to only one surface. Portable instruments are non-destructive and designed for on-site work, but along with the advantages come some trade-offs, with each instrument (portable test method) having unique difficulties to overcome. The Combination Method optimizes the efficiency by bringing the best properties of portable test methods together with minimum disturbance to the test piece. Slide 3

4 Hardness is a process parameter of various methods Hardness is usually described rather vaguely as resistance of the material against penetration. Mohs (1812): Hardness is the ability to resist scratching. Leeb (1986): Hardness is the response of a material to stressing by penetration exerted by a harder body. Defined by energy lost through an impact. Obtain standard hardness values through precisely defined physical tests. Traditional methods à static indentation tests: 1. Under a defined force, a very hard indenter (hard metal, ceramic, or diamond) of a defined geometry and size is steadily pressed into the sample and released. 2. A deformation parameter is measured (e.g. indenter displacement, indentation area, etc.). 3. The hardness value is calculated. Slide 4

5 We all know Brinell, Vickers & Rockwell Brinell, ball indenter Vickers, pyramid indenter Rockwell, conical indenter Slide 5

6 Evolution of mobile hardness testing Bench top: Must bring the test piece to the instrument (often not possible) or mount the instrument in a reproducible way. Portable: Need for a portable and easy to use device. Impractical with adapted traditional testers High efficiency portable instrument Slide 6

7 Modify bench top methods to accommodate portable hardness testing Poldi-Hammer Tele Brineller Ernst-Method Pin Brineller 1. Die or Hammer 2. Reference bar, Shear pin 3. Indenter 4. Sample Take care with the hammer! Are the indents measured correctly? Slide 7

8 Leeb is one of the most popular test methods for many industries Leeb rebound hardness method is based on the loss of energy of an impact, a direct result of mechanical deformation during the impact. Impact body is propelled against the test surface by a spring. During the impact, kinetic energy is transformed into elastic and plastic deformation of the test object. Velocity is higher during impact than the rebound stage. Both impact and rebound velocities are measured in terms of induction voltage by the coil inside the impact device housing. Slide 8

9 UCI test principle is heavily utilized in Oil & Gas industry UCI (Ultrasonic Contact Impedance) is based on conventional Vickers indenter geometries and loads. A resonating rod attached to the indenter monitors the vibration. Frequency shift is measured during the indention process. Unlike Vickers hardness testing, diagonal lengths of the indentation area are not visually measured. The frequency shift is converted into a hardness value that is pre defined and correlated. Various test loads available either with different probes or in more advanced models with an adjustable trigger load. Slide 9

10 Portable Rockwell is now a preferable method for many industries On site NDT hardness testing based on a true indentation principle. A direct hardness measurement method as it relies on indentation (material/plastic) deformation. Less material dependent results the static principle is closely related to traditional test methods. Test piece geometries are not a major issue anymore. One can even take advantage of support accessories, clamp etc. for stability. Verification of other hardness testing tools and correlating comparator devices to a direct hardness value. Slide 10

11 Portable Rockwell complements other hardness test methods Minimum material deformation using the maximum force possible. Ability to mechanically maintain the force applied. More direct comparison to indentation hardness testers is possible. Effectively material independent conversions in comparison to UCI and Leeb, which are more affected by Young s Modulus. Sample thickness is less of an issue in comparison to other portable methods. Portable Rockwell is verified using standard traceable HRC test blocks. Leeb G Leeb D UCI UCI UCI Portable Rockwell Rockwell C Brinell 5 mm 3 mm 98 N 50 N 10 N 50 N 10/ N/mm HRC ~ HRC ~ HRC ~ Indentation Diameter (µm) Slide 11

12 Portable Rockwell method is based on traditional indentation principles Like the bench top Rockwell method, the Portable Rockwell hardness test principle is based on the penetration depth difference: A very hard (diamond) indenter of a defined shape (truncated diamond cone with an opening angle of 100 ) is forced into the surface of the test object with a pre-load of 10N. à Penetration depth measurement 1. Followed by additional load of 40N (50N total load) which is applied to the indenter. When the additional load is removed and back to pre-load. à Penetration depth measurement 2. The penetration depth difference is a direct metric of the test object s hardness. Slide 12

13 Conversion to other hardness scales is a common practice There are no direct mathematical relationships between different hardness scales. Empirical conversions have been determined based on numerous comparative measurements, therefore carrying a certain amount of imported uncertainty. Conversion tables are standardized e.g. in ASTM E140 and ISO Stipulations (limitations) of ASTM E140 and DIN EN ISO shall in all cases be followed. Slide 13

14 Confirming Portable Rockwell s material independence Monel 400A Inconel HX Monel K-500 Inconel 718 Inconel 600L NIM PE16 Waspaloy NIM PK33 NIM 80A Incaloy 909 NIM 75 NIM 90 Inconel C-276 User experience indicated that Portable Rockwell conversions are not as dependent on alloy composition as other portable methods, so a quick evaluation was performed. The samples were prepared and sent to a lab for reference testing. Portable Rockwell measurements were taken in close proximity to the laboratory tests without violating indentation separation rules. The results were charted with error bars for easy comparison. Slide Proceq

15 Rockwell verse Portable Rockwell 1.2 HRC error assigned to Rockwell according to ASTM E18 precision and bias. 5% error assigned to Portable Rockwell. Slide 15

16 Brinell verse Portable Rockwell 6.8% error assigned to Brinell according to ASTM E10 precision and bias. 5% error assigned to Portable Rockwell. Slide 16

17 Vickers verse Portable Rockwell 4% error assigned to Vickers, as per ASTM E92. 5% error assigned to Portable Rockwell. Slide 17

18 Vickers verse Portable Rockwell Most data points agree within the margins of error for the methods. NIM 75 does not agree within error, more data must be taken. Slide 18

19 Data supports the claim of material independence for Portable Rockwell Testing of more Nickel alloy samples should bring better agreement. Overlapping error bands between Portable Rockwell and bench top methods allow us to judge the test results equivalent across HB, HRC, and HV. Results consistent regardless of sample thickness from 2mm to 30 mm. Important to confirm results for each alloy and hardness level. Next step to verify performance on Titanium and other exotic alloys. Slide 19

20 Studies conducted by Nordtest* & ASME + confirm the main contributing factors Material Mass Thickness A correction factor can be applied to compensate for the bias introduced by any of the three main geometrical properties of the test piece. *Nordtest Technical Reports (99.12, 99.13, 99.36) + ASME CRTD-91 Slide 20

21 Compliance with codes requires thorough weld inspection Insufficient preheating and cooling control can result in brittleness of the HAZ, which potentially can lead into component failure. Verification of the heat-treatment status, helps identify any discrimination of alloys. Successful systematic inspection of HAZ can be achieved cost effectively with portable hardness testers of Leeb, Portable Rockwell or UCI principle. Critical sample parameters should be considered and guidelines have to be followed. Combining various test methods enables the user to overcome difficult sample geometries. Slide 21

22 Combining test methods The absolute, material independent results from Portable Rockwell can be used to provide the baseline - no more need to section a piece and send to a laboratory. Increase efficiency and reduce costs by performing most testing with the faster Leeb or UCI test methods. Select the optimum probe based on weld geometry and access. Ease of use increases the reliability of the results. Portability makes the combination method easy to implement on site. Slide 22

23 Take baseline measurement data 1. Take measurements with Leeb or UCI test method (Leeb shown on right). Measurement Slide 23 HLD Uncorrected (HBW Portable Rockwell (HBW 10/3000) 10/3000) Average Take measurements with Portable Rockwell.

24 Calculate bias and apply correction 3. Calculate bias. Measurement HLD Uncorrected (HBW 10/3000) = + 22 HBW Portable Rockwell (HBW 10/3000) Average Apply the correction to HLD measurements on the same material and geometry. Slide 24

25 Advantages of combination method Increased efficiency UCI and Leeb are faster than other portable test methods. Optimize access to difficult locations small indentation of Leeb and UCI can be placed in ideal area of HAZ with minimum disturbance to the weld. Thorough inspection with reduced costs. Verified accuracy Portable Rockwell provides a reference equal to a bench top test without the need to section a test piece. Leverage on board data storage to create more detailed reports. Slide Proceq

26 Combination techniques to improve accuracy and reliability of portable hardness tests Tom Ott, Manager, Technical Services and Operations, Proceq USA NDTMA 2016 Annual Conference, Las Vegas, NV Thursday, February 18, 2016

27 Portable hardness tester conversions Leeb 9 material groups 1. Steel / Cast Steel 2. Tool Steel 3. Stainless Steel 4. Grey Cast Iron 5. Nodular Cast Iron 6. Aluminum 6. Brass 7. Bronze 8. Copper UCI Calibration is required on samples of known hardness for each alloy a sample library must be maintained! Portable Rockwell Effectively material independent on most known alloys when compared to Rockwell, Brinell and Vickers due to the test method and its similarity to the bench-top methods. Slide 27

28 Most common portable hardness testing methods and relevant standards Leeb (Equotip): ISO new! DIN ASTM A956 ASTM A370 ASTM E140 (Conversion) Potable Rockwell (Equostat): DIN ASTM in progress UCI: DIN ASTM A1038 Slide 28

29 And a perfect solution for many applications Slide 29

30 ...and specially used for HAZ inspections and quality control Useful in restricted spaces, targeted testing and for quick tests, but requires thorough calibration on the specific sample and is known for its operator dependency. HV UCI = F E2 eff (f f 0 ) 2 Slide 30

31 Portable Rockwell has minimal thickness requirements Minimum thickness required is150 µm when penetration depth is 15 µm Slide 31

32 Nordtest & ASME cover ideal and nonideal hardness testing scenarios Shape of the test piece plays a key factor in getting consistent result, however this can be managed in many cases when all the factors are taken into consideration and kept constant. Mass of the actual sample is critical, if the mass is below the specified amount a custom correlation is required in order to achieve acceptable result. Thickness, again if the required thickness is not met additional custom adjustment of the existing curves is required. Surface finish is just as important as other factors, therefore any surface finish discrepancy should be accounted for and considered accordingly. Homogeneity or perhaps in-homogeneity of samples are critical for hardness conversions due to the affect of elasticity we should always consider any variation if a hardness conversion is required from the native scale. Slide 32

33 Testing welds and HAZ helps to prevent fatal system failure Welding à Heat treatment à Hardening process à Brittleness à Failure In fact it is very common to see the HAZ failure when weld is still intact. Weld Cost reduction in commercial insurances Avoiding down times Crack Assurance in production consistency Compliance to directives & standards Health & Safety assurance and peace of mind Environmental benefits Slide 33