.. This page contain the laulan documentation .. include:: isogrk1.txt .. include:: isonum.txt ========================================== **Laser Ultrasonic testing in Metallurgy** ========================================== Introduction ============ Metals and there elaboration process are constantly improved to meet the desired properties for novel technology in aeronautic, civil engineering, automotive and biomedical industries. These efforts aim to find an optimum balance between weight, strength, ductility, fracture toughness, weldability and reactivity to the environment. New investigations are therefore routinely conducted in universities, research institutes as well as in the industries using numerous characterization techniques. For more than two decades, laser ultrasonics has grown and now finds a place alongside other in-situ methods to provide quantitative information on microstructure evolution during industrial processing routes. The technology laser ultrasonics was developed in the bases of generating and detecting ultrasound remotely [1]. Among others, two important benefits of using lasers to generate and detect ultrasound are i) the measurement on parts with complex shapes and ii) the investigations at elevated temperature during processes. The focus is brought here only to the later. Numerous scientific papers, patents as well as “proof of concepts” have demonstrated the potential of such technology for the in-situ microstructure control during thermo-mechanical processes of metals. This report summarises the metallurgical applications for which laser ultrasonics is well adapted to measurements in real time and identify areas where, on the other hand, it has some limitations. Measured Quantities =================== Ultrasound properties in a polycrystalline sample, i.e. velocity and attenuation, are function of the material properties. For an ultrasound pulse propagating in a polycrystalline material, the ultrasound velocity is directly related to the temperature, elastic constants and sample texture. It can be used to evaluate phase transformation, recrystallization and other phenomena affecting the density and bulk elastic constant of the sample. The attenuation has three sources that are the diffraction (associated to the sample geometry), scattering (caused by elastic mismatch between grains) and internal friction or absorption (generated by numerous mechanisms such as dislocations movement, magnetic domains, interface migration, solute diffusion, etc …). The principle to measure velocity and attenuation consists on generating a broadband ultrasound pulse in the sample and detecting several echoes after propagation through the microstructure. The time require for the pulse to travel in a given section of the sample is the delay and the amplitude variation between two different echoes is the amplitude decay. It is in addition necessary to estimate the propagation distance between the two echoes which is related to the sample dimension (known a priori). Phase fraction, mean grain size, recrystallized fraction and other metallurgical properties such as elastic constants, plastic strain ratio, and texture component can then be estimated in a certain range of process parameters. It is not intended to go further into the description of the principle of the technology but rather to focus on the factors that allow and limit its applicability to industrial process control in metallurgy. Number of references can be found in literature where extensive details are provided with example of applications to various materials. Process Variables and sensitivity ================================= Range of applicability ---------------------- The range of applicability and methodologies are defined based on numerous studies that have been conducted in various materials for the last 30 years. The following table contains a non-exhaustive list of important process variables allowing measurements with this technology. There are correlations between the different process variables which directly affects the range of applicability indicated in each section. For sake of clarity, the comments listed below are based on the typical capabilities of the Laser Ultrasonic for Metallurgy (LUMet) sensor (TECNAR Ltee) attached to the Gleeble 3500 thermomechanical simulator (Dynamic System Inc. Poestenkill, NY). In this sensor, a frequency-doubled Q-switched Nd:YAG laser with a wavelength of 532 nm is used for the generation of a wide band compressive ultrasound pulse. The duration of the laser pulse is approximately 6 ns, it has a maximum energy of 72 mJ and up to 50 pulses can be generated per second. The laser pulse produces a broadband ultrasound pulse by vaporizing a small quantity of material at the surface (of the order of a micrometer per hundred laser pulses). Successive arrivals of the ultrasound pulse at the generation surface are detected with a frequency-stabilized Nd:YAG pulsed laser which illuminates the surface 50 times per second, with an infrared radiation at a wave length of 1064 μm and a pulse duration of 90 μs. The infrared detection laser light reflected on the specimen surface is demodulated inside a photo-refractive crystal using an active interferometer approach. The ultrasound properties measured in this technique are representative of the average properties of the material over a volume created by the surface of the laser spot (about 2 mm) multiplied by the sample diameter. Figure below depicts a typical ultrasound waveform that can be obtained up to 50 times per second on a sheet sample. .. image:: ./ctome_image/laulan/wave1.png :align: center :scale: 70 % The initial period of the signal is disturbed by the ultrasound generation and is seen as large oscillations present during the first 0.5 to 1µs of the signal. The compressive signal are identified as large amplitude echoes arriving periodically at the generation surface. The shear signal can be observed as smaller oscillations measured between two compressive signals. The properties of this ultrasound signal are correlated to the microstructure parameters. In elastically anisotropic materials like steel, attenuation is primary due to the elastic scattering by grains and can be correlated to the average grain size. Further, the time between two successive echoes or delay is used for the evaluation of the ultrasound velocity. This second parameter is related to the bulk elastic properties that can vary during recrystallization and phase transformation as a response to the change in the density and crystallographic arrangement of grains in the material. Sample thickness ---------------- + The technology LUMet can be applied to a range of sample geometry: Flat sheet with thickness between 1 to 15 mm, cylindrical sample with diameter of 10 to 12 mm and with ratio between length and diameter between 1.5 and 1, round tube, square bar, hexagonal bar + The thickness range provided here usually allows for the measurement of at least one back wall echo which is required for both attenuation and velocity measurement. + The measurements on thick samples are more challenging due to the decrease on the signal to noise ratio at elevated temperature and in sample with large grain size. + The two others dimensions of the sample (width and length) must be large compare to the thickness. At least 2 to 3 times the thickness is recommended to avoid reflection of the pulse on other surface. (For instance, measurement in the plan RD,ND or TD,ND of a thin plate are usually not possible) + The surface roughness as well as oxide layer also play a role. Measurements are becoming more challenging in surface with roughness larger than 50 to 100 μm. The effect of various type of oxide layers remains to be fully investigated. It is known to depend strongly on the bonding of the layer, i.e. a homogeneous oxide layer well bonded to the sample surface is less dramatic than a flaky oxide layer with poor adhesion developing in the course of a treatments. + Measurement on thin specimen can be problematic as a smaller number of grains are sampled through thickness during the measurement creating additional uncertainty in the measured data. + The signals measured on thin sample (1mm and less) contain a large amplitude low frequency wave which can be considered almost as a steady wave. It is developing when the ultrasound wavelength is roughly the size of the sample thickness. This wave must be filtered out reducing the effective bandwidth available for the analysis of material parameter. + Measurement on cylinder are possible as long as the alignment between the sample and the D-laser is sufficiently good to collect some signal. Faceting both side of the cylinder enhanced the signal to noise and reduces the number of echoes originated from reflection on round surface. Grain size range ---------------- + The range of grain size for which the LUMet is well adapted ranges from 1 to 200 micrometer. + Grain size measurements can be conducted in materials which are sufficiently highly anisotropic to cause measurable scattering. + The measurement of grain size was currently validated in the following materials: Austenite in low alloyed steels, Cobalt and nickel superalloys. Grain size measurement in aluminum, magnesium, are difficult due to the small scattering in these materials. Measurements of grain size in phase with magnetic properties like ferrite in steel has not been validated yet. + Grain size lower than a couple of micrometers usually cause very weak scattering even in highly anisotropic materials making measurement challenging. + According to the established methodology for grain size measurement, the smallest grain size that can be measured is usually dictated by the mean grain size in the reference sample used for the signal processing. + The mean grain size estimated from ultrasound attenuation corresponds to the average grain size through the sample thickness. For instance, the measurement conducted in a partially recrystallized sample corresponds to the mean grain size considering both the family of recrystallized grains and the recovered grains. + The calibration or analytical approach are usually constructing using homogeneous grain size distribution, i.e. maximum grain size is 2 to 3 time larger than the mean value. Large deviations from this criteria will lead to deviation from metallographic data. + Calibrations as well as analytical model are usually constructed for materials with equiaxed or spherical grains. Strong variation in grain morphology need to be integrated in the methodology to avoid deviation from metallographic data. It was observed that the attenuation becomes lower [1] (thus predicted grain size is lowered) when the ultrasound pulse propagate in the long axis of the grain. + When the grain size becomes large, the number of grain through thickness can decreases dramatically, leading to dispersion in the measurements, i.e. smaller number of grain sampled through thickness leads to bad statistics. As a rule of thumb, a good enough statistic is obtained when the ratio of the sample thickness to the grain diameter is larger than 50. + For large grain size (> 200 um) or wide grain size distribution, conventional established methodology using single scattering approaches are no longer reliable and must be improved. Temperature ----------- + The temperature range for which LUMet is well adapted is from room temperature up to 1300 degree celcius. + Temperature play a role in the development of factors that can affect the quality of the measured ultrasound waveform. High temperature can leads to the formation of an oxide layer which won’t usually stick to the surface very well and cause challenges in the generation as well as detection of the ultrasound. The surface damage caused by the ablation during generation is also a function of the temperature (This can be problematic in application where the same position on the sample is used for a large number of measurement, i.e. to follow continuously a process for instance) It is usually recommended not to conduct more than 600 pulses at the same position. + Temperature also affect the difference between the properties of different phases and therefore the ability to measure a process. The difference between the bulk elastic constant of the parent and product phase can in some material be measurable only in a given temperature range. Materials --------- + Lumet can be used in a wide range of metals and alloys. + The ability to measure a certain metallurgical process varies from metals to metals. As an analogy, the volume change between parent and product phase has to be sufficiently different to measure the phase transformation by dilatometry. In the same way, the difference between the bulk elastic properties of the parent and product phase should be sufficiently different to be measured by laser ultrasound. + The difference between elastic constants of two phases varies with temperature. For instance, almost no difference exists between the ferrite and austenite phase in steel around 900°C where the austenite formation occurs making this process rather complicated to monitor in-situ by use of velocity. However, the difference between the bulk elastic constant of ferrite and austenite is quite large below 700°C, temperature at which the austenite decomposition occurs during cooling. This phase transformation is therefore totally measurable in a certain temperature range only. + In the same idea, the monitoring of recrystallization with laser ultrasonics rely on the variation of the bulk elastic constant during recrystallization. This elastic constant varies due to the texture change associated with recrystallization. Depending on the intensity of the texture variation during a particular process or a given material initial state, the change in elastic constant can be large enough or contrary to weak for it to be measurable by velocity measurements. Application of the LUMet technology =================================== Evaluation of austenite grain size in STEEL ------------------------------------------- Concept: +++++++ LUMet can be used to examine the evolution of the mean grain size of austenite during a heat treatment cycle in Gleeble. This is possible as the broadband ultrasonic pulse generated in the sample interacts with the microstructure. After one or more back and forth in the sample, one can generally consider that the amplitude of the echoes received is inversely proportional to the volume of the grains. The larger the grains, the smaller the amplitude of the echoes or in other words, the higher the ultrasonic attenuation. This source of attenuation due to the presence of grains is called ultrasonic scattering. It is generally the primary source of attenuation in anisotropic material like austenite in steel. The important aspect to understand about the scattering by grain is that it is function of the frequency of the pulse. That is to say that the grain size is measured by calculating the frequency dependence of the attenuation spectrum. Therefore, when evaluating the ultrasonic attenuation in a sample, one must use a method where all other sources of attenuation are not frequency dependant. This method is called the single echo technique. LUMet measures a series of waveforms in the sample. The waveforms are composed of an initial bang of generation at time is zero and of successive echoes corresponding to the arrival of the pulse at the generation surface. The propagation distance for the first echo is twice the thickness of the sample; it is 4 times its thickness for the second echo and so on. The attenuation and its frequency dependence are calculated by measuring the ratio of the amplitude of two echoes in the frequency domain. If the two echoes have different propagation distances, such as the first and second echo, or the second and third echo for instance, then another contribution called diffraction modifies the frequency dependence of the attenuation. The diffraction contribution can be phenomenologically understood by the spread of the pulse in the sample due to its particular geometry. For a given geometry, the contribution from diffraction is not the same for the first, second and third echo as they have traveled a different number of time in the sample. Instead, the attenuation must be evaluated by the ratio of the echo amplitude of two echoes that have travelled the same propagation distance, say the second echo of two different waveforms. One would then speak about single echo technique, as a single echo is used in each measured waveform. In practice, one can measure a reference waveform in a sample at room temperature, and use an echo contained in this reference waveform, called the reference echo, to calculate the ultrasonic attenuation. In this condition, the frequency dependence of the attenuation is proportional to d2-d02 where d0 is the mean grain size in the reference sample and d is the mean grain size in the sample being measured, usually called the current sample. If the reference sample has small grain size compare to current sample, then the term d02 can be neglected and the measurement of grain size is absolute. In this condition, the calibration developed by the NRC can be used to evaluate the absolute value of grain size in austenite. This calibration is valid when the sample is weakly textured which is often the case for austenite in low alloy steel. Range of measurements: ++++++++++++++++++++++ + Mean grain size range from 5μm to 200 μm, + Temperature range from 900°C to 1300°C + Grain aspect ratio (long axis divided by short axis) range from 1 to 0.7 + Grain size distribution (Maximum grain size divided by mean grain size) range from 1 to 3 + Minimum number of grain through thickness must be larger than 50 + Calibration already available for the aforementioned range of parameter [2]. + Measurement can be conducted during isothermal holding and continuous heating Steel compositions: +++++++++++++++++++ + Low carbon and low alloy steel (HSLA, TRIP, DP, CP) + Not yet validated for austenitic stainless steel + Difficult in Interstitial Free steel Note: +++++ + The measurement require a reference sample with the same geometry than that of the measured sample and fine grain size (< 5 μm) + Measurement on thick samples affects the measurable maximum grain size + Measurement at very high temperature affect the measurable maximum grain size + Measurement on thin sheet cause statistical problem due to limited number of grains through thickness + Sample thickness must be known with a precision of about 0.1 mm + Temperature must be known with a precision of about 10 °C + Error estimated to be 10 to 15 % of the absolute mean grain size + References: [3]–[10] Monitoring of phase transformation in steel =========================================== Concept: ++++++++ The evaluation of phase fraction is based on the evaluation of the velocity change during the phase transformation. The ultrasonic longitudinal velocity is experimentally obtained from the ultrasound signal by the ratio of the propagation distance to the delay between two successive echoes. The sensitivity of ultrasonic velocity to phase fraction is due to the different elastic constants and densities of the different phase components. The velocity in a two phases domain is in good approximation the sum of the velocities of the parent and product phases weighted by their volume fractions. The fraction transformed can therefore be obtained by a classical rule of mixture between the velocity measured for the parent phase and the velocity measured for the product phase. When the material used has no or weak magnetic properties, the ultrasonic velocity of the parent and product phase varies linearly with temperature and can be extrapolated for the application of a classical lever rule. In steel, the velocity of ferrite varies non-linearly with temperature below the Curie temperature of 770 °C, but varies linearly above this temperature and the velocity in austenite varies linearly for all temperatures where it is present. Therefore, the classical lever rule cannot be directly applied to the measured velocity for the evaluation of the austenite fraction transformed upon cooling. Instead, the lever rule is applied between the velocity in ferrite measured upon heating and the velocity of austenite extrapolated to low temperature from its domain of stability. Range of measurements: ++++++++++++++++++++++ + Austenite decomposition when occurring below 750°C + Austenite formation when associated with texture change, i.e. when starting from a cold rolled state for instance. + Measurement can be conducted during isothermal holding and continuous heating + Can be conducted during cooling rate up to 150°C/s + Measurement is not affected by gas blowing on the sample + Cannot be conducted during water quenching Steel compositions: +++++++++++++++++++ + Low, medium and high carbon steel Note: +++++ + The measurement of the fraction transformed require the continuous evaluation of velocity from the fully austenite region to the lower temperature range + For a single shot measurement of the fraction transformed, a calibration is necessary and the accurate knowledge of the sample temperature is required + As with dilatometry, the measurements does not provide the fraction of individual phase such as ferrite, bainite and martensite. + Evaluation of phase transformation above the Curie temperature (about 750°C) are challenging due to the fact that the velocity of the parent and product phase are similar + When using a calibration, sample thickness must be known with a precision of about 0.1 mm + When using a calibration, temperature must be known with a precision of about 10 °C + Reference [11], [12], [13] Measurement of ferrite recrystallization in steel ================================================= Concept: +++++++ LUMet is optimized for the measurement of recrystallization of ferrite in cold rolled steel thanks to the measurement of changes in ultrasonic velocity. The measurement can be done under isothermal or continuous heating conditions. For sake of simplicity, one will concentrate on measurement conducted under isothermal condition. In cold rolled steel, the grains are generally highly elongated in the rolling direction and are generally full of internal defect called dislocations. The dislocations are lines of vacancies in the crystal that are created during the cold rolling steps in order to accommodate the deformation. They are strengthening the microstructure, increasing the material yielding point; they make the material stronger but also more brittle (lower fracture toughness). Moreover, the deformation process is generally associated with a characteristic evolution of the texture in the material, i.e. some crystallographic orientations tend to align in the direction of deformation. One can speak about deformed texture in cold rolled steel for instance. These are well documented in the literature and are generally a function of the steel composition. As the sample is heated up to an intermediate temperature (below the austenite formation temperature), a phenomenon called recrystallization will occurs. It corresponds to the formation of completely new family of grain within the material. They will gradually completely replace the cold rolled structure. Not only these new grains are free of dislocations, they also possess a different crystallographic orientation, one generally talk about recrystallization texture. There is therefore a change in the texture of the sample associated to the process of recrystallization, .i.e from a deformed texture to a recrystallized texture. This change in texture varies in intensity depending on the strength of the initial texture, the material composition, and some other factors. However, this change in texture is generally strong enough to affect the value of the ultrasonic velocity. Indeed, the velocity of a wave propagating in a sample is directly proportional to the sample texture, as the texture evolves during recrystallization, the velocity value will also evolve. Range of measurements: ++++++++++++++++++++++ + In the ferrite temperature range + Measurement can be conducted during isothermal holding and continuous heating Steel compositions: +++++++++++++++++++ + Low, medium and high carbon steel + Ferritic stainless steel + Other iron based alloys with a ferritic structure Note: +++++ + The measurement of the recrystallized fraction require the continuous evaluation of velocity from the fully deformed state to the fully recrystallized state + For a single shot measurement of the recrystallized fraction, a calibration is necessary and the accurate knowledge of the sample temperature is required + The measurement sensitivity depends on the intensity of texture change during the recrystallization process, i.e. the larger the texture evolution the better the sensitivity. + When using a calibration, sample thickness must be known with a precision of about 0.1 mm + When using a calibration, Temperature must be known with a precision of about 10 °C + Reference [14],[15][3] Monitoring of Austenite recrystallization in steel ================================================= Range of measurements: ++++++++++++++++++++++ + Typically conducted in Gleeble during holding at high temperature after uniaxial compression testing + Challenging for large grain austenite grain (>150 μm) with thick sample to conserve acceptable signal to noise ratio Sample geometry: ++++++++++++++++ + Cylindrical sample with diameter of 10 to 12 mm and with ratio between length and diameter between 1.5 and 1. + Can be adapted to measurement on plate Steel compositions: +++++++++++++++++++ + Low, medium and high carbon steel Note: +++++ + The measurement of the recrystallized fraction require the continuous evaluation of attenuation and velocity from the fully deformed state to the fully recrystallized state + For a single shot measurement of the recrystallized fraction, a calibration is necessary and the accurate knowledge of the sample temperature is required + The measurement sensitivity depends on the intensity of texture change during the recrystallization process, i.e. the larger the texture evolution the better the sensitivity. + When using a calibration, sample thickness must be known with a precision of about 0.1 mm + When using a calibration, temperature must be known with a precision of about 10 °C + Reference [16] Monitoring of recovery process in steel ======================================= + Few studies shows a variation of velocity during recovery which was associated to the variation in dislocation damping. + The attenuation is both a function of the grain scattering and evolution in dislocation density. + Model are needed to estimate qualitatively the change in dislocation density + Limited study where conducted overall + Measurement are challenging and usually require simultaneous stress relaxation measurement. + The user must refer to available literature + Reference [17],[18], [19] Evaluation of grain size in other metals ======================================== Range of measurements: ++++++++++++++++++++++ + Mean grain size range from 5 μm to 200 μm, + Temperature range from room temperature 1300°C + Grain aspect ratio (long axis divided by short axis) range from 1 to 0.7 + Grain size distribution (Maximum grain size divided by mean grain size) range from 1 to 3 + Minimum number of grain through thickness must be larger than 50 Materials: ++++++++++ + Any metal with anisotropy factor larger than 2.5 + Validated for Cobalt based super alloys + Validated for Nickel based super alloys Note: +++++ + The measurement require a reference sample with the same geometry than that of the measured sample. + Measurement on thick samples affects the measurable maximum grain size + Measurement at very high temperature affect the measurable maximum grain size + Measurement on thin sheet cause statistical problem due to limited number of grains through thickness + Sample thickness must be known with a precision of about 0.1 mm + Temperature must be known with a precision of about 10 °C + Error estimated to be 10 to 15 % of the absolute mean grain size + References: [20],[21] Monitoring of recrystallization in Aluminum alloys ================================================== + The user must refer to available literature to evaluate the feasibility + Reference [22], [23], [24], [25] [26] Evaluation of residual stresses in Aluminum alloys ================================================== + The user must refer to available literature to evaluate the feasibility + Reference [27] Monitoring of phase transformation in Titanium ============================================== Range of measurements: ++++++++++++++++++++++ + Measurement can be conducted during isothermal holding and continuous heating + Can be conducted during cooling rate up to 150°C/s + Measurement is not affected by gas blowing on the sample + Cannot be conducted during water quenching Compositions: +++++++++++++ + Pure Titanium and Titanium alloys Note: +++++ + The measurement of the fraction transformed require the continuous evaluation of velocity during beta to alpha or alpha to beta transition + For a single shot measurement of the fraction transformed, a calibration is necessary and the accurate knowledge of the sample temperature is required + Extensive grain growth in the beta phase can lead to lack or repeatability in the measurement. + When using a calibration, sample thickness must be known with a precision of about 0.1 mm + When using a calibration, temperature must be known with a precision of about 10 °C + Reference [28] Ageing study in beta stabilized Titanium alloys =============================================== Range of measurements: ++++++++++++++++++++++ + Measurement can be conducted during isothermal holding Steel compositions: +++++++++++++++++++ + Beta stabilized Titanium alloys Note: +++++ + The measurement of the fraction transformed may require preliminary calibration + When using a calibration, sample thickness must be known with a precision of about 0.1 mm + When using a calibration, temperature must be known with a precision of about 10 °C + Reference [29] Evaluation of texture in hexagonal material =========================================== + The user must refer to available literature to evaluate the feasibility + Reference [30], [31] Monitoring of sintering process =============================== + The laser ultrasonic technology can be used to monitor sintering processes. [32] [34] + The rational is relatively simple. In a two phase materials, the elastic properties as well as the density of the compound are a function of the property of the individual phases weighted by the volume fraction of each phase. In compacted metallic powder, the two phases are the void and the powder which have obviously very different elastic property and density. The simple manner to quantify sintering process therefore is to follow the variation of the ultrasonics velocity during the process. As the fraction of void decreases, the velocity v = sqrt(Elastic modulus/density) will increase. + Attenuation is more complicated to explain but likely provide also a way to estimate the stage of sintering as void will act as strong scattering centre and disperse the wave energy. The paper from Liu is more fundamental and explains this concept of attenuation measurement applied to the sintering processes. [33] Evaluation of plastic strain ratio in metals ============================================ + The user must refer to available literature to evaluate the feasibility + Reference [35] Elastic Moduli measurements in other compounds ============================================== + The user must refer to available literature to evaluate the feasibility + Reference [36] Additional information concerning advance LUMet testing. ======================================================== Heating rate and cooling rate ----------------------------- The maximum heating rate recommended during LUMet testing is 100°C/s. This relates to the maximum repetition rate of LUMet laser which is 50 pulses per seconds. Larger heating rate would lead to only few measurements in a large temperature domain and can influence the measurement quality. The maximum cooling rate during LUMet test is about 50°C/s. Only gas quenching can be used with LUMet to avoid damaging the LUMet window through which the laser inter and exit the gleeble tank. Maximum stroke rate ------------------- The maximum stroke rate applicable while conserving the laser spot at the center of the sample is 2mm/s. When higher stroke rate is used, the LUMet position will no longer be at the center of the sample during the deformation. However, it will reach the specimen center shortly after completion of the deformation step. LUMet divisor ------------- LUMet divisor is a coefficient used to control the repetition rate of the laser pulses during a gleeble testing involving LUMet. The repetition rate or frequency is given by 50/LUMetDivisor. For a LUMet Divisor of 50, the frequency of the laser measurement is 1 Hz and it is 50 Hz for a LUMet divisor value of 1. The maximum LUMet divisor value is 99 corresponding to a frequency of about 0.5Hz. Only integer value can be entered in the LUMet divisor.