SOUND EMISSION ANALYSIS DURING THE QUENCHING OF 42CrMO4 STEEL IN VARIOUS SPECIMEN SHAPES

Franc Ravnik, Janez Grum*

University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana

franc.ravnik@fs.uni-lj.si; janez.grum@fs.uni-lj.si * ... corresponding author Abstract The hardenability of steel through the whole workpiece depends on several facts: chemical composition, the type of steel, the size and shape representing the mass of the workpiece, microstructure prior to heat treatment, the type of quenching medium applied, the quenching method and the quenching process itself. Most common difficulties encountered are caused by deviations due to a vapour film formation around the workpiece. The vapour film formed around the workpiece produces an uneven removal of heat and consequently, a different microstructure than the one expected. In order to control the hardening process, one should be able to monitor the quenching process in real time. The paper treats an experimental setup comprising detection of sound emission together with some results obtained in the course of quenching. Due to heat transfer from a specimen to a quenching medium, film boiling and nucleate boiling occur around a heated specimen, which strongly affects sound-pressure signals emitted from the surface during quenching. Different amplitudes of emitted sound-pressure signals at different frequencies therefore mostly depend on the wetting behaviour around the surface of the specimen. It has turned out that an analysis of sound emission signals can provide useful information that confirms differences occurring in quenching specimens with different shapes in different quenching media under different quenching conditions. Analyses of sound emission demonstrated that sound emission during the quenching process can be used for monitoring the hardening process. Keywords: Sound emission, Acoustic spectrograph, Cooling rate, Polymeric water solution, Quenching, Sound pressure level, Vapour film

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1. INTRODUCTION

In order to use material as much as possible, most machine parts manufacturing concluded with quenching to obtain the desired hardness profile. During the quenching process in fluids, the occurrence of three wetting phases of heat transfer to quenching media with Leidenfrost temperature between 100 and 200° C and consequently the varying of the heat transfer coefficient α, is characteristic [Tensi et all, 1992, 1996]: Film boiling: heat transfer coef. aFB=100 to 250 W/m2K when quenching in water Nucleate boiling: vapourizing of media at simultanious vertical moving of specimen

enables heat transfer coef aNB=10 to 20 kW/m2K when quenching in water Free heat convection: aconv= approx. 700 W/m2K when quenching in water

Spreading velocity of the wetting front depends on several physical properties of the specimen and the quenching medium: disposion of the temperature throught the specimen, heat transfer coeficient α throught the specimen, surface roughness, diferent coatings on the surface (oxides, organic substances, etc.), the geometry of the specimen, temperature and Leidenfrost temperature of the quenching media, dinamical viscosity, specific heat and surface tension and temperature of the quenching media and forced convection

Specimen cooling is therefore liable to a great local variation which influences the microctructure and mechanical properties of the specimen obtained (Howes, 1997). During the quenching of a workpiece with complex shapes, all three phases occur simultaneously on different workpiece surface areas which produces considerable internal stresses and also an explicit influence on microstructural stresses leading to distortion and residual stresses, or even to distortion, residual stresses and cracking of the workpiece. [J.Grum, F. Ravnik, 2006]. Formation of vapour bubbles, their oscillation and disappearing in the liquid generate noise, which is strongest in the transition layer between film boiling and nucleate boiling of the quenching medium. Detection of emitted sound signals and their analysis can therefore provide useful information on the quenching process [Grum et al, 2003]. Sound generated by bubbles in the water was examined by Leighton [Leighton 2001]. The Minnaert model of bubble formation frequency during immersion indicates a relation between the bubble size and the frequency of bubble formation (Eq.1.). In the initial phase of nucleate boiling, bubbles of smaller diameters are formed and their frequency is higher, and vice versa when nucleate boiling of the quenching medium is nearing its end. Such conditions can be achieved during quenching in water and in different concentrations of polymeric water solutions. Minnaert's frequency of oscillation bubble:

ρχ

ω 031 pRM = [1]

where: p0 [N/m2] .... is hydrostatic pressure of the liquid around the bubble

in conditions of static equilibrium, χ .................. polytrophic coefficient, R [m] .......... radius of the bubble in the equilibrium and ρ [kg/m3] .... the density of the liquid

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A measuring setup should be adapted to the expected frequency of sound phenomena. Thus it should be known which frequencies of bubble formation and decay occur predominantly in the range of hearing and which slightly is below this threshold [Čudina, 2001]. The conditions at which the signals were detected should be monitored by temperature measurement of the specimen during the quenching process. The phenomena occurring at the workpiece/medium interface should then be logically interrelated in film boiling, including additional environment sound effects [J.Grum, F. Ravnik, 2006]. Furthermore, bubble formation and sound generation phenomena should be interrelated with material properties obtained. 2. EXPERIMENTAL PROCEDURE 2.1. Experimental setup Figure 1 shows the experimental setup for detecting sound signals during quenching. The entire equipment is immersed in a glass container with soft rubber lining inside and filled with the quenching medium. The experimental setup for detecting sound signals in wetting processes is to be independent from the quenching medium type used and of the quenching mode. Although the quenching process takes some seconds or even up to several minutes, the experimental system should register individual events sensed up to 0.1 second or less. Experimental setup comprises two independent parts, one for monitoring the temperatures at the surface and in the core of the specimen and the other for monitoring the medium temperature itself during the quenching and another for detection and processing of the sound pressure level (Lp) of emitted sound.

2.2. Specimen material and form Table 1: Chemical composition and recommended heat treatment for hardening 42CrMo4

steel

SoundblasterPreamplifirer

AmplifirerB K

&

&

Digitalthermometer

Specimen PC

software&

Hydrophone

carier&

SCXISignal

ConditioningDevice

Bath

Basket

Results

Thermocouple T1

Thermocouple T2

Temperature/time T /tStart/end temperature T

W

F

Sound pressurelevel/time L /tP

Fig. 1. Experimental setup for the measurement of temperature and emitted sound

during quenching

Element C Si max Mn P max S max Cr Mo

[weight %] 0,38 – 0,45 0,40 0,60 – 0,90 0,035 0,035 0,90 – 1,20 0,15 – 0,30

Recommended heat treatment by hardening:

Austenitization temperature of 820-850, 830-860°C quenching in water or oil.

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Low-alloy Cr-Mo heat-treatment steel (EN - 42CrMo4) was selected for the making of specimens. This steel shows high hardness after heat treatment, i.e. even up to 57 HRC. It is characterized by good through-hardening and high strength after heat treatment [Grum et al, 1998]. Therefore it is widely used in the production of statically and dynamically loaded components of vehicles, motors, and machine components with a large cross section. Its chemical composition and mechanical properties are given in Table 1. Specimen form

Fig. 2. Specimen shape and the direction of immersion by quenching The results obtained should depend on the shape and direction of the immersion. Six different rotation shapes were chosen for the test specimens as shown in figure 2. The dimensions provided similar masses of the specimens. A ratio between the specimen volume and the area P/V was also taken into account, because it significantly affects the cooling rate with the same specimen mass.

Film

boilin

g

Trans

ition b

oiling

Nucle

ate bo

iling

Free

conv

ectio

n

Fig. 3. Typical sound pressure signal. Fig. 4. Spectrogram of sound pressure signal.

Imersion direction

Imersion direction

No 1 No 3 No 2

No 4 No 6 No 7

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3.1. Frequency analysis of the sound generated by the quenching process

Sample No 1 Sample No 2 Sample No 3

Tim

e se

ries

Spec

trogr

am

Sample No 4 Sample No 6 Sample No 7

Tim

e se

ries

Spec

trogr

am

Fig. 5. Acoustic signals obtained during the quenching of six specimens in 10% polymeric

water solution The sound spectrum obtained during quenching comprises the emitted sound pressure level. Comparing spectrograms and spectrums obtained indicates important characteristics shown with differentiation in sound intensity and frequency ranges. Analysis provides detailed information on individual events in the quenching medium used. Figure 5 shows sound pressure level signals detected with the hydrophone and calculated spectrograms, giving the frequency of events during the quenching of six different specimens in 10% polymeric water solution. By comparing signals of the all specimens, the following may be concluded: Two characteristic areas can be determined: an area of lower frequencies, i.e. up 1,5

kHz, where only the amplitude changes, and an area of higher frequencies, i.e. up to 20 kHz, where the signal frequency varies as well.

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-100,0

-80,0

-60,0

-40,00 1 2 3

Frequency [kHz]

Rel

ativ

e am

plitu

de [d

B]

Sample No. 6

Sample No. 7

P/V ratio has significant influence on the duration of the transition boiling and film boiling stage where the intensity of sound is the highest. Higher P/V ratio means greater surface, i.e. more intensive heat removal.

On the other side, the duration of the nucleate boiling and free convection stage is prolonged when the P/V ratio is higher (particularly samples in Figure. 5 No.6, No.7).

Drill holes cause a significant eruption of lower frequencies and a high amplitude after the first ten seconds (particularly samples in Figure 5 No.2, No.3 and No.4). Inside the holes the vapour remains longer, and wettings occurs later.

The direction of the immersion plays an important role when the shape of the specimen prevents equal vapour removal (particularly samples in Figure 5 No.6 and No.7).

Figure 6 shows the calculated average spectrum of typical samples. By comparing signals, the following may be concluded:

Occurrence of lower frequencies in the trough holed sample than in the solid one (Figure 6, samples No. 1 and No.2).

When heat removal is hindered due to graded shape, the occurrence of intensive low frequencies is observed (Figure 6, samples No. 6 and No.7).

-120,0

-100,0

-80,0

-60,0

-40,00 5 10 15 20 25

Frequency [kHz]

Rel

ativ

e am

plitu

de [d

B]

Sample No 1

Sample No 2

-120,0

-100,0

-80,0

-60,0

-40,00 5 10 15 20 25

Frequency [kHz]

Rel

ativ

e am

plitu

de [d

B]

Sample No 6

Sample No 7

Fig. 6. The average spectrum of samples No. 1 & 2, and No.6 & 7 quenched in 10% polymeric water solution

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3.2. Microhardness measurement To obtain more information on the influence of the quenching process, i.e. different shapes, the microhardness by HV was measured on different areas of the samples: outer cylinder (left and right), top and bottom face and in axial and radial section as shown in Figure 7 A. A load of 1 daN was used to obtain more accurate measuring results of microhardness variation presented (Figure 7 B). The course of measured hardness values follows the expected profile. The deviation of microhardness profile reflects a heat removal profile due to different shapes and P/V ratio of the specimens.

View A

View B

A

Sample No.3

0102030405060708090

100

0 100 200 300 400 500 600 700 800

s teps (m m )

hard

ness

(HV)

0

100

200

300

400

500

600

700

800-25 -20 -15 -10 -5 0 5 10 15 20 25

hardness (HV)

step

s (m

m)

Path I Path IIPath IV Path IIIPath V Path VI

B

Fig. 7. A Measuring areas and B hardness profile of sample No. 3 4. CONCLUSIONS The present paper describes the results captured by measuring acoustic emission caused during quenching. The sound pressure signal, which exposes as the amplitude and duration of the signal during quenching, was captured by the hydrophone. During recording of the sound pressure, a significant change of the signal shape appears, corresponding to the formation of a vapour film phase on the specimen surface and to the nucleate boiling phase in the specimen/quenching-medium interface. Those changes were connected with the shape of the specimen and with the immersion direction by quenching. The signal was audible during the entire quenching process and the capturing with hydrophone was good. On the basis of the experiment, it could be concluded that the experimental setup for the capturing of the acoustic emission was reliable and of sufficient quality when applying quenching processes with different specimens shapes. The analysis of the results offers an interesting new approach to evaluation and, more importantly, to monitoring, controlling and optimizing of the quenching process itself.

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REFERENCES

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[2] Grum J., Božič S., Lavrič R. (1998): Influence of Mass of Steel and a Quenching Agent on Mechanical Properties of Steel, Heat Treating, Proceedings of the 18th Conference, Including the Liu Dai Memorial Symposium, ASM International, The Materials Information Society, Eds.: R.A. Wallis, H.V. Walton, 645-654.

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[4] Grum J., Božič S. (2003): Acoustic emission during quenching of 42CrMo4 steel, Proceedings of the Fourth International Conference on Quenching and the Control of Distortion, Beijing

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