Combined portable hardness testing solution to increase the efficiency of inspection & quality control processes

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1 19 th World Conference on Non-Destructive Testing 2016 Combined portable hardness testing solution to increase the efficiency of inspection & quality control processes Alireza AKHLAGHI 1, Christoph FREHNER Proceq SA, Schwerzenbach, Switzerland Contact s: 1 alireza.akhlaghi@proceq.com, 2 christoph.frehner@proceq.com Abstract. Successful quality control by avoiding test samples, dismantling, sectioning and / or preparation can be achieved in a timely manner when taking advantage of portable hardness testing solutions. Test principles such as Leeb Rebound, and Ultrasonic Contact Impedance (UCI) provide modern, quick and easy tools that can supplement or potentially replace traditional test methods. Portability is a key aspect for all of the above methods despite the test principles being significantly different to one another. The decision making process of which instrument to utilise is directly related to geometrical and mechanical properties of the test sample and it is firmly recommended to consider and understand the limitations prior to testing. Since there is no direct mathematical relationship between different test methods it is a common practice to correlate them to one another. As an example the existing default hardness conversions in Leeb or UCI devices are based on certain minimal sample geometries unlike instruments which have almost no restrictions in regard to thickness and mass, as well as being far more material independent. A new approach is to create a simple custom correlation for exotic materials and for samples that do not meet the geometrical requirements based on the Portable Rockwell measurements. This would allow the user to broaden the capabilities into difficult to reach areas. This paper will advance into this principle by highlighting examples such as weld inspection where testing is required on the base material, HAZ (Heat Affected Zone) and the weld. Additionally, independent guidelines (ASME, Nordtest, DGZfP) and standards (ASTM, ISO, DIN) are covered in order to help select the most suitable test method for each specific application. 1 Functional principles 1.1 Material and geometrical dependencies of various test methods Unlike the most important characteristic of Leeb and UCI test principles is their dependency on the material elasticity when compared to traditional methods which is due to the nature of the test principle itself. While Leeb measures with a dynamic impact, UCI utilises ultrasonic oscillations. License: 1 More info about this article:

2 As both methods are dynamic principles, the measurement value is significantly influenced by the elastic properties of the material. Therefore for an accurate measurement the elastic properties as well as test samples geometries have to be considered. The appropriate correlations must be created in order to convert the portable hardness values to traditional hardness scales. Research and case studies published by Nordtest, ASME and DGZfP have highlighted the dependency of dynamic portable methods in the past. The Leeb method is covered under ASTM A956 and ISO 16859, the UCI method is standardised under ASTM A1038 and DIN respectively. Where it is appropriate and available, conversions from one hardness scale to another are covered by the ASTM E140 and ISO Application of combined test methods Although Leeb has a large number of conversion curves available for various materials this is slightly different in the case of UCI. In general there is only one specific conversion available for UCI, however customised correlations can be created. Additionally the test sample s mass and thickness has to be considered in compliance to the relevant standards and guidelines in order to avoid vibrations or selfoscillation of the sample. For the Leeb principle this is typically a minimum mass of 5kg and a thickness of at least 25mm on the measurement location, although these requirements vary with the different impact devices. The UCI test requires a minimum weight of g and at least 15mm of thickness for the test sample. In such cases where requirements cannot be fulfilled the combined method can be used to overcome these restrictions, where Leeb or UCI devices are correlated to the true hardness value provided by the instrument. 1.3 Similarities of the principle to traditional methods Compared to the other two common portable test methods discussed above, Portable Rockwell is significantly different. Although fully portable, it is based on a true indentation principle. First and foremost the only minor differences to stationary Rockwell C method are the reduced test loads and the geometries of the conical indenter. Hence test samples geometrical requirements are almost negligent. Most importantly material elasticity is no longer influential in the same manner as Leeb and UCI. Additionally the dwell time of a test in comparison to stationary test methods is much shorter, at approximately five seconds, resulting in a much more time efficient hardness correlation. While the measurements with are fairly simple on flat samples, it requires higher user experience for round or curved surfaces. This can of course influence the repeatability. Both other portable methods and particularly the Leeb, on the other hand are quite easy to operate, no matter how experienced the user or how complex the test surface curvature is, reliable results can be achieved. In compliance to DIN 50157, test principle follows similar approach of utilising pre and total load to Rockwell C (HRC) principle. For example, with the preload of 10N and total load of 50N on a 55 HRC test sample, the indentation is measured at 11 µm. Although we have witnessed the material independency of method over the period of many years to further support this experience a series of tests were performed on various Nickel and Titanium alloys carefully selected to be compared to the default low alloyed Steel correlation. The data is fully illustrated in chapter 3 with a full graphical analysis of the results. 2

3 1.4 Main influential factors for the combined portable hardness testing Although the combination of two portable hardness methods can validate the test results, our main goal here is to overcome two highly influential factors i.e. material elasticity and test samples geometries. It is important to highlight that although geometrical influences can be minimised significantly, they cannot be neglected completely. For example if the test piece is far too thin it can result in no measurement value at all with either Leeb or UCI test principles, in such cases no combination can be used. Prior to creating a combined correlation it is of course critical to ensure both instruments are verified in compliance to the relevant guidelines, this procedure is generally referred to as in-direct verification 1 in both ASTM and ISO standards. 2 Application examples 2.1 Hardness inspection of exotic material As the industrial demand for exotic material such as Nickel 2 and Titanium 3 alloys increases in order to achieve specific mechanical properties for components being manufactured, there is a noticeable requirement to carry out successful quality control and inspection. Hardness testing in general is one of the main aspects to verify the integrity of a compound s mechanical reflex. For both Leeb and UCI test principle an initial correlation is required prior to testing due to material dependency of these test methods. Such a procedure was done using stationary testers in the past, however by utilising portable indentation depth measurement methods such as, the required correlation is created on site without any damage to the components. It is also important to highlight that this procedure is completely non-destructive Weld and HAZ (heat affected zone) inspection When dealing with pipes and especially for weld and HAZ inspection it is common to see the components under test falling into the non ideal category also covered by the Nordtest technical report 424 as highlighted earlier. These are the scenarios where the combined method can help as the lack of thickness and mass is instantly compensated for by utilising the indentation hardness value provided by instrument. Either Leeb or UCI devices can then be utilised to carry out the measurements on the parts and areas such as base material, heat affected zone and the weld crown. 1 Validation of an instrument s performance in compliance to the manufacturer s recommended guidelines and standards using a reference hardness test block. 2 Nickel and nickel alloys are used for a wide variety of applications, the majority of which involve corrosion resistance and/or heat resistance. 3 The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material choice for many critical applications. 4 Technology used in industry to evaluate the properties of a material, component or system without causing damage that prevents it from its intended usage. 3

4 3 Influence of material properties when using different test methods 3.1 Vickers hardness (HV) The testing was carried out in compliance to ASTM E92 (E384) and ISO 6507 for HV, DIN for indentation depth hardness and ISO and ASTM A956 for Leeb. Figures 1 & 2 highlight the material independency of for Vickers hardness as Low Alloyed Steel, Nickel and Titanium are hardness conversions are compared. Leeb (HLD) default conversion for Steel is out of acceptable tolerance for Titanium as highlighted in Figure 2. An error of 5% is considered for measurement uncertainty HV Nickel and Titanium lab results compared to conversion Table 1 - Nickel alloys HV data Table 2- Titanium alloys HV data Nickel alloys µm indentation Default (HV Steel curve) MONEL 400A INCN HX NIM 80A INLY NIM NIM Nickel alloys lab results (HV) Titanium alloys µm indentation Default (Steel curve) Default HLD to HV (Steel Curve) Sample Sample Sample Sample Sample Titanium alloys lab results (HV) 350 Nickel alloys lab results Default (Steel curve) HV MONEL 400A INCN HX NIM 80A INLY 909 NIM 75 NIM 90 Figure 1 - Nickel alloys HV conversions comparison 5 In every measurement there is always a margin of doubt or error which is expressed as + /- e.g. measurement uncertainty analysis is applied to understand differences in test results and to determine sources of error. 4

5 HV Titanium alloys lab results Default (Steel curve) Default (Steel curve) HLD to HV Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Figure 2 - Titanium alloys HV conversions comparison HV polynomial curves 350 Poly. (Nickel alloys lab results) Poly. ( Default (Steel curve)) HV µm Figure 3 - Nickel alloys HV polynomial curves comparison HV µm Figure 4 - Titanium HV polynomial curves comparison 3.2 Brinell hardness (HB) Poly. (Titanium alloys lab results) Poly. (Default (Steel curve) ) The testing was carried out in compliance to ASTM E10 and ISO 6506 for HB(W), DIN for indentation depth hardness and DIN and ASTM A956 for Leeb. Figures 5 & 6 shows how all the conversions match, with the exception of the 5

6 default low alloyed Steel conversion of Leeb D (HLD) which is out similar to the HV data. The polynomial graphs on Figures 7 & 8 highlight how close portable Rockwell conversions are to traditional stationary Brinell machine. Considering the uncertainty of measurements in compliance with standards a measurement error of 5% is accounted for HB Nickel and Titanium lab results compared to conversion Table 3- Nickel alloys HB data Table 4 - Titanium alloys HB data Nickel alloys µm indentation ASTM (Steel curve) Portable Rockwell ISO (Steel curve) Nickel alloys lab results Titanium alloys µm indentation Default HV (Steel curve) Portable Rockwell HB ASTM conversion HB ISO conversion Leeb D (HLD) Default (Steel Curve) HLD to HB Titanium alloys HB lab results INCN INCN 600L NIM PE WASPAL OY NIM PK Sample Sample Sample Sample Sample Nickel alloys lab results ASTM (Steel curve) HB INCN 718 INCN 600L NIM PE16 WASPALOY NIM PK33 Figure 5 - Nickel alloys HB conversions comparison 370 Titanium alloys HBW lab results HB ASTM conversion 320 HB ISO conversion Default (Steel curve) HLD to HB HB Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Figure 6 - Titanium alloys HB conversions comparison 6

7 3.2.2 HB polynomial curves Poly. (Nickel alloys lab results) Poly. (ASTM (Steel curve) ) HB µm Figure 7 - Nickel alloys HB polynomial curves comparison HB Poly. (Titanium alloys HBW lab results) Poly. (HB ASTM conversion) Poly. (HB ISO conversion) µm Figure 8 - Titanium HB polynomial curves comparison HRC The testing was carried out in compliance to ASTM E18 and ISO 6508 for HRC, DIN for indentation depth hardness. Table 5 - Nickel Alloys HRC data 31 Nickel alloys lab results Nickel alloys MONEL K-500 INCN C-276 µm indentation Default (Steel curve) Nickel alloys lab results HRC Default (Steel curve) Portable Rockwell MONEL K-500 INCN C-276 Figure 9 - Nickel alloys HRC conversion comparison 7

8 4 Conclusion It is extremely important for the reference test method to get as little to no influence from the elastic properties of material and geometrical aspect of the test piece. The data gathered and evaluated in this paper further supports the material and geometrical independency of the test principle. This enables it to become the main reference point, as in the majority of cases hardness correlation is required to be based on a true indentation and in compliance to the strict role of material resistance against penetration 6 i.e. the actual material deformation or plastic moment 7. Combined methods or data fusion is one of the most talked about topics in recent years for integrity and quality assessments. Hardness testing is no exception and can significantly benefit when appropriate software tools are designed and developed to serve the required computability in the inspection sector. Similar to traditional principles each portable instrument has its own native scale, although well established hardness scales of the stationary methods are still widely used. This of course dictates the need for reliable hardness conversions to accommodate all of the test methods. It is important to consider that each portable hardness test principle has its own limitation and cannot serve all applications. In addition to this and in many cases, due to material itself, a new hardness conversion is required based on the test principles dependency on Modulus of Elasticity 8 and other geometrical aspects of the test piece. For such cases, the combination of two portable hardness methods can be utilised to overcome the limitations while maintaining complete portability, as far as the reference test principle is based on indentation depth measurement. What makes this possible is by identifying and considering the influential geometrical aspects as well as the mechanical properties of the material i.e. modulus of elasticity and the plastic moment (yield strength). 5 References [1] Nordtest Technical Report 424, Michael S. Lorenzten, J. Vagn Hansen and Pertti Auerkari [2] ASME, Application Guide for Determining the Yield Strength of In-Service Pipe by Hardness Evalution. Edward B. Clark and W. E. Ahmend [3] DGZfP Fachausschuss Materialcharakterisierung Arbeitsgruppe Mobile Härteprüfung. Richtlinie MC 1 [4] ASTM E92, E384, E18, E10, A956, A1038, E140 [5] DIN 50156, 50157, 50159, [6] ISO 6506, 6507, 6508, 16859, Hardness is the mechanical resistance which a material asserts against the mechanical penetration of a harder test body. The definition of hardness differs from strength, which represents the resistance of a material to deformation. Hardness is also a measure of the abrasion properties of materials. Harder materials generally demonstrate better abrasion behaviour than others. 7 Plasticity is defined as the maximum moment where material can resist without exceeding the yield stress. Also defined as the yield moment. 8 The material's ability to resist a force and deformation, meaning that when the stress load is removed from the material it will recover to its original form. A material will continue to deform elastically as the stress upon it increases until the elastic limit is reached. 8