"The use of MET-UD combined portable hardness testers for technological control"

Harteprufung 2006. Praxis, Trends and Innovationen", Berlin, November 2006

Abstract

Portable measurement instrumentation play special role in metal articles hardness testing. Portable hardness testers are widely used in industry. There are two basically different hardness testing methods in the practice of portable hardness testers utilization. They differ in the speed with which intender penetrates into the tested metal, they can be conventionally called quasi-static and dynamic penetration methods. In case of quasi-static method, the characteristic speed of penetration is fractions of millimeter a minute; in case of dynamic method this speed is about a meter a second. In both cases the speed of penetration is much less that the velocity of the sound in metals (about 5600 meters in a second).

The present paper is the continuation of a series of articles about the use of MET-UD combined portable hardness tester for technological control of metal articles. The purpose of the present papers is to investigate the field of application and determination of the measurement errors of hardness tester which combines two methods of hardness testing (quasi-static and dynamic) of articles under the condition of inherent stress in them.

Design and distinctive features of combined portable hardness tester

MET-UD combined portable hardness tester is a small-sized device and it consists of one universal electronic module and two interchangeable probes, one is for static load and the other one is of impact type (Figure 1).

Ultrasonic contact impedance method (UCI) [1] is used for hardness testing by quasi-static method. Ultrasonic probe contains a steel rod with Vickers diamond pyramid at the top, which is an acoustic vibrator of the inbuilt ultrasonic frequency automatic generator. When pyramid is penetrated into the tested article, the system own frequency which depends of the hardness of the material changes. The penetration force is equal to 9.8 N and is created by a calibrated spring. The relative change of the resonator frequency is transformed by the electronic modulus into the hardness value in accordance with the selected scale and is then show on the display. The ultrasonic probe design makes it possible to conduct testing in hard-to-get-to places (for example, gear tooth surface etc.) and also of thin-wall articles (for example, pipelines etc.) where impact methods cannot be used. It should be taken into consideration that the result of the testing by ultrasonic method depends on the tested article modulus of elasticity.

When hardness is tested by the impact method, the rebound method [2] is used. The dynamic probe contains a hammer with a hard-alloy ball at the tip which impacts a specimen surface with a certain velocity under the spring force. The signal from the probe is sent to the electronic modulus inlet where it is transformed into the selected scale hardness number and is then shown on the display. Dynamic probe design enables to conduct a big number of measurements in a time unit.

Combined hardness tester [3,4] combines all positive features of both methods and, depending on mass, configuration, structure, degree of mechanical and heat treatment of the measured article, either ultrasonic or dynamic probe shall be connected to the electronic modulus and after that the hardness tester shall implement the relevant to the transducer method of measurement.

One of the MET-UD combined hardness tester specific features which allows to improve measurements precision is individual calibration of hardness scales, i.e. every hardness scale, Rockwell (HRC), Brinell (HB), Vickers (HV) and Shore (HSD), is calibrated with reference hardness blocks only without using any conversion tables.

The results of the research show that it is practically impossible to establish a unified conversion dependence which will be suitable for all metallic materials even if measurements are carried out with very high precision. That is why all current Russian standards do not contain any recommendations concerning hardness numbers conversion from one scale into another, on the contrary, it is indicated in them that "one should avoid such conversions except separate cases when owing to comparative tests, there is a reliable basis for conversion".

Comparative study of different conversion tables [5] shows that in every case there is some systematic influence of some unknown factors on the hardness numbers under comparison. That is why any conclusions and recommendation concerning the possibility of hardness numbers conversion from one scale into another are permissible only for approximate metal properties estimation, but not for exact recalculations.

In connection with the above said, in MET-UD hardness tester, there is an independent scale calibration both, for dynamic and ultrasonic probes. The main hardness scales are calibrated for tool and structural steel. For other metals and alloys, there are also other additional scales which are calibrated with specially manufactured test specimens made of the relevant materials. The hardness value of these specimens is established on the standard hardness machines and after that hardness tester additional scales final calibration takes place.

Thus, independent calibration allows to exclude an error of the hardness scales table conversion from one into another, and a scale transfer [6] from national primary standard machines through the reference hardness blocks to portable hardness tester directly will allow to bring a transfer error to a minimum, reducing the traceability stages from national primary standard machines to commercial measurement instruments.

Experimental results and analysis

Two different methods combined in one hardness tester allow not only fast and exact hardness measurement, but make it possible to estimate inherent stress in the metal. Earlier in the article [7], it was shown that with the help of MET-UD it is possible to define the change in the hardness of the hardened layer in depth and the impact of inherent stress in it. Experiments were made on cold rolls which were under internal compression stress impact. In the present paper, we give the results of hardness measurements of a wall of a thick-wall pipe put under high pressure so that the pipe walls suffered tensile internal stress.

To confirm the possibilities of internal stress measurement by MET-UD, experiments were conducted during which various internal stresses were created within high pressure cylindrical vessels (a piece of the pipe of hardened steel 40xN2SVA (EI643) grade) filled in by oil of I20 grade. The pipe external diameter is 66 mm, the wall thickness is 8 mm, the range of pressure delivered into the pipe is 50 … 200 MPa which corresponds to stress from 200 … 830 MPa. The scheme of the experiments is shown on Figure 2.

Symbols: Uc quasi-static load probe (ultrasonic), D - dynamic operation probe (dynamic), pressure P is delivered into the cylindrical pressure vessel manufactured from steel 40xN2SVA (EI643) grade, having external diameter of 66 mm and the wall thickness of 8 mm.

Prior to the experiment, the dynamic and ultrasonic probes were calibrated on Vickers scale and all measurements were conducted on this scale, too. In connection with the fact that internal stresses impact is rather low and the vessel surface heterogeneity could give its impact on the hardness measurements results, the series of measurements were carried out in one and the same surface areas. After measuring hardness of the pipe wall under pressure, every time measurements in the same area but without pressure were carried out so as to neglect the surface hardening heterogeneity. To increase the reliability of the measurement results, averaging by 10 surface areas was used, and it is important to note that not less than 10 measurements were made in every series. A big statistical sampling allows to obtain true results with a small error. The measurements were conducted at the oil pressure in the pipe equal to 50, 100, 150 and 200 MPa, which corresponds to the stress in the pipe wall materials of 200 MPa, 410 MPa, 620 MPa and 830 MPa.

To get rid of the impact from surface hardening heterogeneity in the process of the measurement results processing, the relative value which is the difference between the hardness (HV) under stress and without it divided by the hardness (HV0) without stress was computed for every area. Averaged by 10 areas values for every pressure are given on graph 3.

Relative stress is the stress (?) reduced to tensile strength (Rm) was laid down along x -axis, and the relative hardness values for ultrasonic and dynamic probes were laid down along y-axis. The measurement errors considering the averaging are shown in every point.

As can be seen from Figure 3, the impact of internal stresses on the measured hardness value is small, but for dynamic probe it is about 4% under maximum stress in the pipe wall material, while in case of ultrasonic probe it is less than 1%; actually, it does not exceed the measurements error. For the given grade of steel (40xN2SVA) hardened up to 500 units on Vickers scale, tensile strength is about 1800 MPa, which makes it possible to conclude that in case of internal stress in the pipe wall metal comparable with the tensile strength (only two times less than the limiting value), hardness measured by dynamic probe is lowered down while hardness measured by ultrasonic probe does not practically change.

Two different measurement methods (dynamic and ultrasonic) combined in one instrument enable to estimate approximately the existence of internal stresses, since the measurement errors in case of both, ultrasonic and dynamic probes do not exceed 1-1,5% (under the condition of averaging by the surface areas). If the values by dynamic probe are lower than the error, we can speak about tensile stress, if this value tends to be bigger, we can speak about compression stresses, as in case of cold rolls [7].

Conclusion

On the basis of the experiments results, we can conclude that the existence of considerable internal stresses in the metal can be disclosed in the course of measurements by two different methods, by ultrasonic contact impedance method and by rebound method. For rapid hardness testing and internal stress estimation, MET-UD combined hardness tester is considered to be the most appropriate, since both methods are represented in this instrument together with the averaging mode which allows to enhance the measurements precision and reliability.

Literature

1. C. Kleesattel , G.M.L. Gladwell. The contact-impedance meter. Ultrasonics, 6, 1968, Part 1, p. 175-180, and Part 2, p.244-251, and Ultrasonics, 7, 1969, Part 3, p.57-62., C. Kleesattel. The contact-impedance meter. Ultrasonics, 8, 1970, Part 4, p.39-48.
2. D. Leeb. Dynamic hardness testing of metallic materials. NDT International, 1979, V12, p. 274-278
3. Non-destructive testing equipment and devices. International specialized exhibition. Official catalogue, World Trade Center, Moscow, 10-12 April 2002, p.19.
4. Pattern approval of measuring instruments. Measurements World, Moscow, 2002, 9-10, p.65.
5. A. Gudkov, Y. Slavsky. Methods of hardness measurements the metals and alloys. Moscow, "Metallurgy", 1982, 168 p.
6. E. Aslanyan. Metrological assurance of hardness measurements. Measuring Instruments, Moscow, 2005, January №1, p. 45-50.
7. E. Aslanyan, V. Pivovarov, V. Shlegel, I. Temnitsky, D. Golotvin, R. Fazliakhmetov. Testing hardness of metal items with MET-UD combined portable hardness tester. Proc. International Conference IMEKO TC5 HARDMEKO 2004, Washington, DC, USA, 11-12 Nov.2004.

© Centre "МЕТ" +7 (495) 229-75-26, 506-90-38 E-MAIL