Ulrasonic Test in Ferric Steel, Stainless Steel and Welded Ferric Steel, Welded Stainless Steel

Ulrasonic Test in Ferric Steel, Stainless Steel and Welded Ferric Steel, Welded Stainless Steel

1. Devakumar1

   Assistant Professor Department of Mechanical engineering

   Cheran college of Engineering K.Paramathi, India

   S. Deva2

   Student

   Department of Mechanical engineering Cheran college of Engineering K.Paramathi, India

2. Aravind3

Student

Department of Mechanical engineering Cheran college of Engineering

K. Paramathi, India

1. Sugantp

   Student

   1. Naveen Kumar4

      Student

      Department of Mechanical engineering Cheran college of Engineering K.Paramathi, India

      Department of Mechanical engineering Cheran college of Engineering K.Paramathi, India

      Abstract The aim of this paper is to experimentally comparing the factors those are affected by the grain structure which are responsible for the energy loss of ultrasonic waves and to minimizing the energy loss in the stainless steel. The factors such as, Scattering of Ultrasonic Waves, Signal to noise ratio, and Beam Skewing. The four materials selected for the experimental comparisons are namely, Ferric (Carbon) steel, Stainless steel (Austenitic) parent material, Welded Ferric steel and Welded Stainless steel material. Stainless steel materials were widely regarded as uninspectable by ultrasonic testing, because of its grain structure, which are large in size and solid as a whole. When the Ultrasonic sound waves travels in such a medium, they get reflected at each grain boundary. Reflections will not be regular but will take the form of scattering in all direction. In order to detect the defect in stainless steel, there exist a lot of limitations. Even today, when Stainless steel material is mentioned the practiced ultrasonic tester also experiences the feeling of uneasiness.

      Keywords Stainless steel, ultrasonic Testing, Beam Skewing

      1. INTRODUCTION

         Ultrasonic techniques are very widely used for the detection of internal defects in material, but they can also be used for the detection of small surface cracks. Ultrasonic are used for the quality control inspection of part processed material, such as rolled slabs, as well as for the inspection of finished components. Sound waves are elastic waves which can be transmitted through both fluid and solid media. The audible range of frequency is from about 20Hz to 20KHz, but it is possible to produce elastic waves of the same nature as sound at frequencies up to 500MHs. Elastic waves with frequencies higher than the audio range are described as ultrasonic. The range of frequency used in ultrasonic testing is from less than 0.1 to greater than 1.5MHz. For most application the frequency used below 10MHz.

         All sound waves both audible and ultrasonic are produced by vibrating bodies. A sound or ultrasonic wave propagates through a medium as waves of particle vibration. The actual particle does not move and it is the wave or the energy that displaces the particle which is moving progressively.

      2. WAVE FORMS

         If sound waves are measured from trough to trough from crest to crest, the distance is always the same and it is known as the wavelength (). The time taken for the travel a distance of one complete wavelength is the same amount for the source to execute one complete vibration. The velocity of sound (v) is given by the equation; where, f is the frequency.

         V = f

         On the basis of mode of the particle, displacement, ultrasonic waves are classified as longitudinal waves, transverse waves, surface waves and limbs waves. All these waves forms are used in non destructive inspection of metal to varying degrees.

      3. WAVE GENERATION

         When a tuning fork is struck with a mallet, it vibrates and produces sound waves by compressing the air. These waves travel through air to the ear of the listener. The tuning fork vibrations soon die out and no longer produce waves. Similarly, in ultrasonic testing, a short pulse of electrical current nits or excites a transducer (crystal) which vibrates as did the tuning fork. The sound beam from the transducer then travels through a couple, which may be water, oil, etc., to the front surface of the test piece. Fig 1.1 Shows the transducer, in contact with the test piece, with the sound, beam pulses travelling through the piece.

         Fig 1.1 Ultrasonic Wave Generation

      4. PIEZOELECTRICITY

         In actual practice, a high frequency transmitter applies electrical pulses to a PIEZO ELECTRIC crystal. The prefix PIEZO is derived from a Greek word meaning TO PRESS. Field electricity refers to a reversible phenomenon whereby a crystal, when vibrated produces an electric current, or conversely, when an electric current is applied to the crystal, the crystal vibrates.

         This crystal then transforms the electric energy in to mechanical vibrates and transmits them through as coupling medium, such as water or oil, into the test material. These pulsed vibrations propagate through the object with a speed depending on, among other factors, the density and elasticity of the test material. These pulsed vibrations propagate through the object with a speed depending, on among the factors, the density and elasticity of the test material. In many ways high frequency vibrations react in the same way as light. For example when they strike an interrupting object, they reflect most of the sound beam energy. These reflections may then be picked up by a second or, most cases by the same crystal or transducer. Ultrasonic waves are reflected as echoes from the discontinuity and the back surface of the test piece. The echo from the discontinuity is received before the back reflection is received.

         movement direction is at right angles to the wave movement direction. The velocity of shear wave is approximately half of the longitudinal waves. Note, that the right hand transducer is mounted on a plastic wedge so that the ultrasonic waves generated by the crystal enter the material at a specific angle, depending on the velocity of sound beams travel within material.

         Fig 1.3 Longitudinal & Shear Wave Modes

         VII. SNELLS LAW

         When the sound beams velocities in the couple used in lamination testing, on the wedge material used in contact testing, are different than the sound velocity in the specimen, the longitudinal (L) beams passing through the wedge or couple are refracted when the sound beam enters the test material. Incident on refracted angles are computed by a formula developed from Snells law, after Willebrord Snell or Snellius, C.1621, a Dutch mathematician. For use in ultrasonic, Snells law has been modified slightly from its original application, which was meant to explain optical refraction.

         The following formula may be used to calculate the incident angle the resultant refracted angle, and the mode of materials, including solids immersed in water, oil, or other couplants.

      5. WAVE TRAVEL MODES:

         All materials are made up of atoms (or tiny particles) lined up in straight lines to form lattices as shown in fig 1.2. If we strike the side of this lattice we find that the first column of atoms strikes the second column which in turns the third column and so on in sequence. This motion produces a wave movement in the direction shown. In this case the particle movement in the direction is the same as wave movement direction. This type of sound wave motion is called the longitudinal or compression wave mode.

         Fig 1.2 Longitudinal Wave Mode

      6. COMPARISON OF LONGITUDINAL & SHEAR WAVES MODES

         Fig 1.3 shows two transducers generating ultasonic waves in the same piece. Note that the transducer on the right is producing a different kind of wave. These waves are called shear waves are called shear waves because the particle

         Where, Incident angle from normal to the beam in the liquid or wedge

         Angle of refraction beam in the test material

         V1 Velocity of the first medium, V2 Velocity of the second medium

         1. PROPERTIES OF ULTRASONIC WAVES

            In many ways respects ultrasonic waves resembles light waves. It travels with a characteristics velocity in a given material depending upon the type of material. It is reflected at surfaces and is refracted when crossing a boundary between two substances and diffracted at edges. They are also attenuated depending upon the acoustic impedance of the material or which they travel.

         2. ACOUSTIC IMPEDENCE:

            Wave propagates in solids and depends on the resistance of the atoms of the solid to vibrate when a force is applied; that is the acoustic impedance Z. In ultrasonic testing, cracks, boundaries, or inclusions are detected by the change in Z between the different media, when scattering and reflection in

            selection of suitable material for effective transfer of acoustic energy between components.

         3. ULTRASONIC BEAM ZONES:

            An ultrasonic beam can be divided into three zones, they are Dead Zone, Near Zone, Far Zone

            FIG 1.4 ULTRASONIC BEAM SHAPE

            1. The Dead Zone:

               Fig shows that this is the distance below the surface of a material in which a defect cannot be detected .It is not possible to detect a flaw during this ringing time. If the crystal is mounted in a suitably dimensioned block of Perspex that dead zone the dead zone can be wholly contained within the probe back. However, signals from defects close to the surface may still be lost in the interface echo.

            2. The Near Zone:

               The near zone is the zone in which the beam is almost parallel sided. The length N, of the near zone is given by the approximate relationship.

               N = d2/4 Where, d is the crystal diameter, and is the

               wavelength. The detection sensitivity is not constant throughout the near zone and is greatest towards the fur end of this zone.

            3. The Far Zone:

            The far zone is the region beyond the near zone where beam spread occurs, and within this zone the sensitivity decreases with the square of the distance from the crystal.

         4. ULTRASONIC TEST

            Ultrasonic test equipment comprises of an ultrasound generator, receiver and its amplification and display systems. Each part has got its own role to play to obtain realistic representation of homogeneity of component being tested. The main types of equipment used in ultrasonic testing of metals are

            1. Pulse-echo type

            2. Resonance type

            Pulse echo type equipments are used for flaw detection

            Fig 1.5 Ultrasonic Testing Equipment

         5. TRANSDUCERS

            Transducers are made in a limitless number of sizes and shapes from extremely small to wide paint brushes types. The many shapes are the result of much experience and requirement for many applications. Size of the transducer straighter the sound beam (less beam spread) for a given frequency. The narrow beams of the small high frequency transducers have greater ability for detecting very small discontinuities. The larger transducers transmit more sound energy into test part and so are used to gain deeper penetration. The large single crystal transducers are limited to lower frequencies because the very thin high frequency transducers; are susceptible to breaking and chipping. Depending upon the construction and modes of waves transmitted, the transducers are classified as normal probe, angle beam probes, double probes, focusing probes etc.

         6. STANDARD REFERENCE BLOCKS

            The IIW (INTERNATIONAL INSTITUTE OF

            WELDING) reference block, and the miniature angle beam field calibration block, shown in fig 3.6(a,b). are reference and standard in common use. For irregularly shaped articles, it is often necessary to make one of test articles into reference standard. In other cases, a special individual technique is developed by careful study of an article ultrasonically, and then verifying the detection of discontinuities, in the article, by destructive investigation. The results of the study then become the basic for the testing standard.

            Fig 1. 6 V-I Block

            Fig 1.7 V-Ii Block

         7. STAINLESS STEEL (AUSTENITIC) WELDMENTS The properties of any metals are affected not only by the character of phase present, but also by the size of grains that are present in the structure. Practically all metals used in everyday life are polycrystalline in their structure. Polycrystalline solid are made up of very large number of grains, having microscopic dimensions of the material. Each of the grains is itself a single crystal and thus in general anisotropic. However, because the grains are oriented at random and because they exist in very large numbers, the solids as a whole, from statistical considerations, displays isotropic properties. When the sound waves travel in such a medium, they get reflected at each grain boundary. Reflections will not be regular but will take the form of scattering equally in all direction.

            Fig 1.8 Grain Structure of Ferric Parent Steel Material

            Fig 1.9 Grain Structure of Stainless Steel Parent Material

            Fig 1.10 Grain Structure of Welded Ferric Material

            Fig 1.11 Grain Structure of Welded Stainless Steel Material

            The grain size is noted as a number called ASTM (American Society for Testing Materials) number. For example ASTM no 3 represents 64 grains per mm2

            The size, the arrangement and the elastic anisotropy of the different grain result in high scattering associated with mode conversion effects, beam distortion, and a variation of ultrasonic velocity with direction and position in the weld. The scattering energy is observed as a relatively high noise level end high attenuation.

         8. OBSERVATION AND TABULATION

            In this paper three major problems associated with ultrasonic testing of austenitic stainless steel is practically studied using reference blocks and test piece. The problems selected are

            1. Scattering due to different grain size both Ferrite and stainless steel blocks.

            2. Signal to noise ratio in Ferrite and different steel blocks.

            3. Beam skewing phenomena in Ferrite and stainless steel blocks

            TABLE 1 COMPARISON OF BACK WALL HEIGHT BETWEEN THE MATERIALS (EXPERIMENT 1)

            Thickness of sample -25 mm

            Probe used – Normal probe (4MHz)

            Echo type – Back wall echo

            1. SCATTERING DUE TO DIFFERENT GRAIN STRUCTURE

               Select Ferric steel, parent stainless steel and welded stainless steel, welded ferric steel blocks of same thickness. The probe is placed on the artificial defect created in the blocks. A constant pressure was given to the probe during whole experiment. The differences in dB are noted in four materials.

               TABLE 2 SCATTERING DUE TO DIFFERENT GRAIN SIZE (EXPERIMENT 2)

            2. SIGNAL TO NOISE RATIO

               We have selected four blocks of two Ferric steel type and two Stainless steel type. The probe is placed on the blocks and the whole echo or back wall echo is identified. Then the signal is raised as 100%. The noise level is noted before the signal. This experiment is done on the parent material and welded material side and the differences are noted. The difference in signal height and noise height gives the signal to noise ratio.

               TABLE 3 SIGNALS-TO-NOISE RATIO (EXPERIMENT 2)

               Probe Used- Normal Probe (4MHz)

            3. BEAM SKEWING

            Ferric steel, parent Stainless steel material and Welded Ferric, Welded Stainless steel material are selected. We are using only angle beam probes to study the beam skewing. The probe is placed on the material after applying couplant and the signals of the oscilloscope screen are note. For particular distance of hole the beam path was noted on the basis line of the oscilloscope. We have the actual whole depth. Using this, calculated Beam path is related to the observed beam path. The differences are obtained in the whole depth for different blocks at different depths. The calculated beam path is given by the formula as beam path = t/cos.

            Where, t thickness of the specimen.

            Fig 1.12 Ferric, Stainless Steel Parent And Welded Ferric, Welded Stainless Steel Material

            TABLE 4 BEAM SKEWING (EXPERIMENT 4)

            Material : Ferric parent material Probe used : Angle Probe

            Size of crystal: 8*9 Hole diameter: 3mm

            TABLE 5 BEAM SKEWING (EXPERIMENT 4)

            Material : Stainless steel parent material Probe Used: Angle Probe

            Size of Crystal: 8*9 Hole Diameter: 3mm

            TABLE 6 BEAM SKEWING (EXPERIMENT 4)

            Material : Ferric welded material Probe used : Angle Probe

            Size of crystal: 8*9 Hole diameter: 3mm

            TABLE 7 BEAM SKEWING (EXPERIMENT 4)

            Material : Stainless steel welded material Probe used : Angle Probe

            Size of crystal: 8*9 Hole diameter: 3mm

            Probe used: Angle Longitudinal probe

            C. Experiment 4:

            The difference in hole depth that as calculated from beam path and actually in block shows noticeable beam skewing is taken place in the welded Stainless steel material. We also noticed that as the distance increases the skewing contributes to more different in depth values.

            The dB drop methods used for finding flaws of length are not suitable for austenitic welds. Because the dB drops method is used only in the principle of constant amplitude variation.

            The reference standard selection is very much important in case of Stainless steel testing. To standardize the equipment the reference block is selected as the same material as which is under test.

            1. Experiment 1:

         9. CONCLUSION

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               1. Experiment 2:

               As a result of grain structure is different in stainless steel material the scattering is studied in the experiment. The increase in grain structure in a welded stainless steel block shows that the energy loss is more in Stainless steel welded material. Therefore the energy is last in the material then that is evidence of high scattering is taking place in grain boundaries of austenitic weld materials.

               Experiment 3:

               In the third experiment it shows that there is no noticeable noise in Ferric steel. Whereas in stainless steel the noise in very much noted. This noise may completely obscure the important test indications affecting the results.

               In the observation we also saw that the ratio is different for different blocks either parent or welded material. This may be due grain orientations in the particular direction.

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