1. INTRODUCTION:

Surface hardening processes widely used to improve wear resistance and fatigue strength of the most moving mechanical parts. Hence, estimation of case hardened depth is very important and useful particularly from the perspective of the manufacturers in practice. Ultrasonic testing probably is one of the operative, precise, and fast non-destructive methods for evaluating induction surface hardened parts.

Standard destructive hardness tests require taking samples, so they are time consuming and expensive during an in-line production evaluate. If a high accuracy is needed; the instruments used for this kind of test are generally bulky and heavy. Light hand-held testers are also available but they have generally much lower accuracy or they need to be calibrated for each material separately.

The use of non-destructive test methods is desirable to the surface hardening structures especially for large and expensive components which cannot be destroyed. One of the non-destructive solutions to estimate hardness level is based on the correlation between ultrasound wave velocity and attenuation with the real hardness values measured by traditional methods [1]. In this paper, the experimental evidence is presented that ultrasound wave velocity measurements can be used to determine the hardness of different types of martens tic steel, without the need for adjusting the fitting curve in each case. To establish the correlation with hardness, precise ultrasonic velocity must be measured. Although attenuation measurements may be more sensitive to changes in material microstructure, due to the simplicity of measurement, sound velocity and chemical analysis were used in this study to predict hardness. Another result, presented in [1], proves that material hardness can be determined only by the measurements of ultrasound wave velocity and attenuation. The relationship between steel hardness and ultrasound velocity and attenuation were also presented in the literature [1].

The Barkhausen noise is a magnetic method that also used for evaluating case hardening layers [2,3,4]. Ferroelectric materials consist of domains magnetized along preferential crystallo-graphic direction. Their walls move under the influence of an applied changing magnetic fields resulting in an abrupt small change of the magnetic flux that emits an electrical impulse which can be received by a detection coil. A Barkhausen noise emission spectrum that depends on microstructure and stress in the material was detected. Since the movement of the domain walls was constrained. Although the Barkhausen noise technique has gained industrial acceptance, the magnetic parameters are influenced by many micro structural properties which interactions are not completely understood so it is somehow a drawback of this method.

The paper [5] demonstrates the capabilities of the photo-thermal radiometric interferometric phase (PTR) technique for measuring the effective case depth in case-hardened industrial steels for the first time. Curves show that it is possible to measure the unknown effective case depth of each type of screw, based on the established calibration curve for the given geometry accurately and non-destructively. This paper demonstrated a quantitative non-destructive technique for evaluating effective case depth in heat treated case-hardened steel products using laser PTR phase minima.

Another result, presented in papers, proves that case hardened depth can be determined by eddy current method [6], current potential-drop measurement in steel rod [7], and Laser ultrasonic method [8,9].

Depth of hardness is estimated for hardened shaft, by ultrasonic backscatter technique and produced A-scan images, presented in [10].

This paper is focused on detection hardening profile of VCN100 plate steel which was induction hardened. After sample preparation, ultrasound waves were applied to plates and reflected echoes extracted from marten site transition zone. Fine microstructure in the hardened zone backscatters less ultrasonic energy than the coarser features in the untreated zone, this deference is exploitable. Inherently, interpretation of ultrasonic backscatter signals is complicated, because of low signal to noise ratio (SNR) and overlapping echoes. Therefore, some special signal processing methods applied for the signals accumulated from the surface hardened specimens. By some special signal processing methods and filtering in MATLAB software, after deniosing some equipment noises, we could interpret echoes and obtain correlation diagram between TOF (Time of Flight) of backscattered echoes from transition zone and hardened case depth.

The main objective of this research is to develop backscatter technique for measurement of the hardened layer thickness and produce C-scan images and show the profile of the hardened layer.

2. EXPERIMENTAL:

The sample used in present work is a plate of VCN 100 steel, and was machined into test specimen of 150×90×30 mm³ size and was induction hardened. To avoid heat affected zone and marten site transition in cut off line, sample was cut by water jet machine (Fig.1).

Figure 1: Cross section of induction hardened VCN 100 plate.

C-scans acquire ultrasonic data over an area, usually in a raster pattern. At each data points a parameter (or more than one) is extracted from the time-versus-amplitude signal; common parameters are time-of-flight, amplitude, phase, or attenuation. Then the parameter is plotted versus position to build a bird’s-eye view of the feature or flaw within the specimen, i.e., a slice of the object normal to the ultrasonic test beam is imaged [11].

The main parts of the C-scan equipment are mechanical, ultrasonic, and software parts. The mechanical part consists of servo motors and encoders, supports automatic motions precisely. Ultrasonic hardware part consists of pulser/receiver, Analog to Digital (A/D) card, probes, and cables. Sample frequency on software adjusted in 100 Mega Samples per Second. The experiment includes an immersion ultrasonic test where both the sample and the ultrasonic probe are immersed in a water tank. This experiment carried out by 25MHz ultrasonic immersion focal point probe (fig.2), which positioned 12mm above the plate with the holder angle 18 degree (Eqn.1). In the probe positioning, the mode conversion of ultrasonic waves transmitted from water to solid considered. With this in mind and According to the Snell’s Law (Eqn.2) (fig.3) [11] and Scattering energy, the optimum position to sense maximum energy rate of backscattered ultrasonic waves from transition zone of hardened portion of the plate calculated.

Figure 2: Ultrasonic immersion focal point probe.

Eqn. 1 WP = F – MP (c_tm/c_W)

Where, WP is Water Path, MP is Material Depth, F as Focal Length in water, c_tm is Sound Velocity in the Test Material, and c_W is Sound Velocity in Water.

Figure 3: The Snell’s Law

Eqn. 2 (c₁/c₂)=(sinθ₁/sinθ₂)

Where c defined as sound velocity in a media.

In test procedure, the gain of pulser/receiver set on 45 dB and its energy was in maximum value. In fact, attenuation occurred in echoes because of incessant reflection and scattering in ultrasonic transmitted waves in hardened case zone, but by adjusting special setup of pulser/receiver and probe, we could gain appropriate signals practically. Figure 4 shows an ultrasonic backscattered sample echo of hardened plate.

Figure 4: sample signal.

3. RESULTS AND DISCUSSION:

The ultrasonic backscatters result from the grains which are obstacles for ultrasonic waves in the material. The grains scatter them in all directions and also back to the emitting transducer where they are received as noise. The backscattering amplitude reflected from the fine-grained martens tic structure of the case hardened region is smaller than those from the coarse-grained core material. In the transition zone there are variations in grain boundaries, grain size, and therefore the acoustic impedance value. If these solid-state properties in adjacent are increase, different backscattering signals in the hardened and bulk material occur. These amplitude characteristics can be used to evaluate the case depth by using time-of-flight measurements. For inductive hardened parts, the ultrasonic backscattering method uses this fact that the hardened layers (marten site) are almost transparent to ultrasonic waves (in the frequency range of 20 MHz) while the bulk material (ferriteperlite) scatters ultrasonic waves very strongly (fig.5).

Figure 5: backscattering method.

The backscattering signal appears as noise on the screen of an ultrasonic test device. The boundary between the hardened layer and the core material can be identified by a step in the noise signal. To apply this method, the grains in the core material have to be distinctly larger than in the hardened material. Moreover, there is an echo reflected at the surface of the test sample, which interferes with grain noise up to a depth of some wavelength. Therefore, we used some special signal processing and filtering in MATLAB software to interpret the signals. Signal processing methods have evolved in algorithmic complexity aiming for optimal utilization of the information in order to achieve the best performance. In the first step, by a wavelet method in MATLAB software, we denoised the noises of scanning device (Figure.6).

Figure 6: Sample denoised signal by wavelet method.

Figure 7 presents the 95% confidence level correlation between destructive case hardened depth and TOF of reference samples. The ultrasonic wave velocity in martensite structures is approximately constant therefore we assumed the linear correlation between case hardened depth and TOF. Using some signal processing methods such as Moving Average Filter and Gaussian Smoothing Filter, the TOF value for each points of hardened plate was measured.

Figure 7: correlation between destructive measurements of case hardened depth and TOF of Reference samples.

Finally, the profile of the hardened layer produced as C-scan images for portion of the VCN 100 plate in 20×20×10 mm³ size, which is showing the distribution of hardening depth (Figure 8). The signal recorded step was 0.5 mm in x and y dimensions. In this test, more than 20000 data point signals were recorded. To verifying the results, Samples tested by micro hardness tester.

Figure 8: C-scan monitoring of the case depth of sample and sample cross section view.

4. CONCLUSIONS:

The ultrasonic backscatter technique is developed in this study to predict for measurement of the thickness of the hardened layer (case depth) on inductions-hardened VCN 100 steel plates. The developed technique scans the whole steel plate and the hardening profile demonstrated. After some efficacious signal processing methods and filtering system noises in each echoes, the C-scan of hardened case depth of VCN steel plate monitored clearly. The results confirm that the presented technique is capable for measuring and monitoring C-scan depth profile of hardness very precisely.

5. REFERENCES:

1. T. Lukomski and T. Stepinski (2010), Steel hardness evaluation based on ultrasound velocity measurements, Materials Characterisation, Insight Vol. 52, No 11.

2. M. Dubios and M. Fiset (1995), evaluation of case depth on steels by Barkhausen noise measurement, Materials science and technology, vol.11, 264-267.

3. S. Santa-aho, M. Vippola, A. Sorsa, K. Leiviska, M. Lindgren, T. Lepisto (2012), Ultilization of Barkhausen noise magnetizing sweeps for case-depth detection from hardened steel, NDT&E International.

4. Satrou Kobayashi (2013), Evaluation of case depth in induction-hardened steels: Magnetic hysteresis measurements and hardness-depth profiling by differential permeability analysis, Journal of Magnetism and Magnetic Materials, Elsevier.

5. C. Wang and A. Mandelis (2007), Case depth determination in heat-treated industrial steel products using photothermal radiometric interferometric phase minima, NDT&E International 40, 158-167.

6. Theiner, W., R. Kern, and M. Stroh (2002), Process Integrated Nondestructive Testing of Ground and Case Depth Hardened Parts, European Conference on Non-Destructive Testing, Barcelona, Spain, June, 17-21.

7. J. R. Bowler, Y. Huang, H. Sun, J. Brown and N. Bowler (2008), Alternating current potential-drop measurement of the depth of hardening in steel rod, Meas. Sci. Technol. 19.

8. D. Schneider, R. Hofmann, T. Schwarz, T. Grosser, E. Hensel (2012), Evaluating surface hardened steels by laser-acoustics, surface and coating technology 206, 2079-2088.

9. M. R. Chehrezad, F. Honarvar (2013), Laser Ultrasonic Method in Nondestructive Evaluations, ISME, No. 88.

10. R. Baqeri, F. Honarvar, R. Mehdizad (2012), Case Depth Profile Measurement of Hardened Components Using Ultrasonic Backscattering Method, 18^th World Conference on Nondestructive Testing, Durban, South Africa.

11. Peter J. Shull (2002), Nondestructive Evaluation Theory Techniques and Applications, Marcel Dekker.

Received on 10.11.2020 Modified on 19.12.2020

Research J. Science and Tech. 2021; 13(1):49-54.

DOI: 10.5958/2349-2988.2021.00009.7