ELSEVIER Journal of Materials Processing Technology 42 (1994) 51-59

Journal of Materials Processing Technology

I

Study of the influence of deformation and thermal treatment on the ultrasonic behaviour of steel

R. P r a s a d a '* , S. K u m a r b

a NIFFT, P.O. HA TIA, Ranchi-834 003, India b B.LT., Mesra, Ranchi, India

(Received January 19, 1993; accepted July 20, 1993)

Industrial Summary

In this study an attempt has been made to determine the influence of the deformation and the thermal treatment given to a steel forging on the ultrasonic velocity and its attenuation. Data obtained indicates that both the amount of deformation and the type of thermal treatment given to a forging influence the longitudinal ultrasonic velocity and attenuation. It was observed that with increasing degree of deformation, the ultrasonic velocity decreases, the velocity being found to be maximum for the normalised condition and the minimum for the hardened condition, in annealed samples the velocity lying between the two former values. Further, the ultrasonic velocity was found to increase with increasing tempering temperature.

Attenuation decreases with the degree of deformation and was found to be minimum for the normalised case and maximum for the simply-forged case. In addition, attenuation was found to increase with increase in tempering temperature.

1. Introduction

A very common use of ultrasonics in the metals industry is for the detection of defects. However, in recent years it has become used quite commonly also for material characterisation, i.e. to obtain information about the microstructure, the grain size and thermal treatment of a metal. However, it was considered that the deformation and the thermal treatment to which a metal has been subjected could influence the ultrasonic velocity and its attenuation. The prime aim of the presently reported work was thus to explore the effect of the degree of deformation and of the type of heat treatment given to a steel forging on both the ultrasonic velocity and its attenuation.

* Corresponding author.

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52 R. Prasad et al. ,,'Journal of Materials Processing Technology 42 (1994) 51 59

2. Experimental procedure

2.1. Sample preparation

Cast-steel samples of initial thickness 69.3, 86.6, 130 and 208 mm were hot- upset at a temperature of 950°C using a 1600kg capacity hydraulic press, the deformation applied being 25%, 40%, 60% and 75% height-reduction, respec- tively, the final height of each of the samples being kept to within 52 ___ 0.5 mm by the use of a 'stopper' of this (52 mm) height. Samples with zero percentage reduction (as-cast) were of size 52 + 0.5 mm thickness and 52 mm diameter, these samples not being hot-upset.

The hot-upset samples were later given thermal treatments such as annealing, normalising, hardening, and hardening & tempering, as detailed below:

(i) Annealing temperature 850°C, soaking time 1.5 h, furnace cooling to room temperature.

(ii) Normalising temperature 850°C, soaking time 1.5 h, air cooling to room temperature.

(iii) Hardening temperature 820°C, soaking time 1 h, oil, quenching, harden- ing carried out after annealing.

(iv) The hardened samples were later tempered at 200 °C, 400 °C and 600 °C; soaking time 2 h, air cooling.

After thermal treatment the surfaces of the samples were ground to remove scale in order to secure good coupling with the ultrasonic probe during scanning.

2.2. Ultrasonic velocity and attenuation measurement

An ECL-make model 6255 Ultrasonic Flaw Detector was used for the purpose of the study of the ultrasonic behaviour of the samples. A normal 2.5 MHz probe of 20 mm diameter was employed, machine oil being used as a couplant.

The longitudinal ultrasonic velocity was calculated using the following equation:

DsTtYs Vt= DtT~ '

where Ds is the distance from the first back-echo to the second back-echo for the ISW reference block (100 mm); Dt is the distance from the first back-echo to the second back-echo for the different samples (mm); Tt is the final thickness of the samples (52 + 0.5 mm); V~ is the velocity of sound in the ISW reference block (taken to be 6000 m/s); Ts is the thickness of the ISW block (100 mm); and V, is the longitudinal ultrasonic velocity in the samples.

The velocity of sound obtained for different samples is shown in Table 1. For attenuation measurement, a coupling medium was applied over the surface of the sample, after which the probe was placed onto the surface and a small weight then

R. Prasad et al./ Journal of Materials Processing Technology 42 (1994) 51-59

Table 1 Longitudinal ultrasonic velocity (m/s)

53

Reduction As-forged Annealed Norm. Hardened HT HT HT (%) (200 °C) (400 °C) (600 °C)

0 (As-cast) 5800 5940 6180 5090 5280 5510 5690 25 5370 5520 5790 4880 5010 5220 5400 40 4880 5090 5220 4510 4720 4900 5210 60 4390 4570 4750 4020 4400 4570 4800 75 3870 4010 4360 3030 4090 4310 4540

Table 2 Details of back-echo heights (cm)

Reduction As- fo rged Annealed Norm. Hardened HT HT HT (%) (200 °C) (400 °C) (600 °C)

0 (As-cast) 2.8 3.40 3.80 3.70 3.60 3.30 3.15 25 3.0 3.75 4.20 3.95 3.70 3.50 3.40 40 3.25 3.95 4.30 4.20 3.95 3.85 3.60 60 3.45 4.14 4.55 4.45 4.10 4.00 3.90 75 3.7 4.35 4.90 4.70 4.25 4.15 4.10

Table 3 Details of hardness (Rc)

Reduction As- forged Annealed Norm. Hardened HT HT HT (%) (200 °C) (400 °C) (600 °C)

0 (As-cast) 32 23 29 38.5 36.5 35 32.5 25 36 27 33.5 41 38.5 37 35 40 39 31 36 45.5 41.5 39.5 37.5 60 43 35 40 48 43 41 39 75 48 41 44.5 52.5 47.5 43.5 41.5

placed onto the probe. Back-echo heights were measured only after they had become constant, details of the back-echo heights obtained being given in Table 2.

In addition, hardness readings were taken on different samples, shown in Table 3.

3. Results and discussion

A separate paper 'An Inves t iga t ion into the Ul t rason ic Behav iour of Cast and

Hea t - t rea ted structures in Steel ' has a l ready been publ ished [1], hence discussion on

these (as-cast) samples has been omi t ted here.

54 R. Prasad et aL/ Journal of Materials Processing Technology 42 (1994) 51 59

6500

60 O0

l -

< sooo E

>

~00

3000

I I I I I i I ,

~0 20 90 40 50 60 70 80

°/,REDUCTION .-

Fig. 1. Ultrasonic velocity versus percentage reduction for specimens: (1) forged and normalised; (2) forged and annealed; (3) forged; (4) forged and hardened.

3.1. Results

Results obtained, shown in Table 1, indicate that the longitudinal ultrasonic velocity decreases with increasing degree of deformation, in as-forged, forged and annealed, forged & normalised, hardened (Fig. 1), and, hardened and tempered samples, the velocity being maximum in the case of normalised samples and minimum in the case of hardened samples. The ultrasonic velocity in the case of annealed samples lies between that of the normalised and the hardened samples. Further, from Table 1, it is clear also that the ultrasonic velocity increases with increase of the tempering temperature (Fig. 2).

Table 2 indicates that the attenuation decreases with degree of deformation, whether the samples are simply forged, forged & annealed, forged & normalised, forged & hardened (Fig. 3) or hardened & tempered.

Comparing the back-echo heights of the simply forged, annealed, normalised & hardened samples, it is found that the attenuation is minimum in the case of normalised samples and maximum in case of simply forged samples (Fig. 3). Further, it is evident also from the said table that the attenuation increases with the increase of the tempering temperature (Fig. 4).

Table 3 indicates that the hardness increases with the degree of deformation for all of the samples, irrespective of the thermal treatment to which they have been

R. Prasad et al./ Journal of Materials Processing Technology 42 (1994.) 51-59 55

E

l-- (J O ..J h l >

(._J Z 6R

5800

5700 / ® j 4

56 O0

5500

53oo ®~ ~

~8oo r ~ ~®

3900 ~ i ., J , , 100 200 300 400 500 600

TEMPERING TEMR *C

Fig. 2. Ultrasonic velocity versus tempering temperature for: (1) as-cast; and for reductions of: (2) 25%; (3) 40%; (4) 60%; (5) 75%.

L~

C.) t o

5.0

4.,8

4.6

'~-'~ ~ ~ _ _ ~ _ _ ~ ~ .~

4.0 3.8

3:6 3.4- - ® ~

3.2 t 3.0 / ~ , 8 I / i i I I i i i

0 10 20 30 40 50 60 70 t~0

REDUCTION

Fig. 3. Back-echo height versus percentage reduction for specimens: (1) as-forged; (2) forged and annealed; (3) forged and hardened; (4) forged and normalised.

56 R. Prasad et aL .' Journal oJ Materials Processing Technology 42 (1994) 51 59

I 4.4

4.2 E u 4.0

T 3.8 ~ 9

z 3.G O z 3.4

~: 3.2 o

3.0

2

4

i

100 l 1 I i i

200 NO 400 500 GO0

o

TEMPERING TEMP. C ---"-"

Fig. 4. Back-echo height versus tempering temperature, for reductions of: (1) 75%; (2) 60%; (3) 40%; (4) 2 5 ° .

submitted. From metallographic examination (results not reported herein) it was found that the grain size decreases with increase of the forging reduction.

3.2. Discussion

3.2.1. Velocity changes When steel is hot worked there is breakdown of the grains into sub-grains.

The hot-deformation process is followed by the dynamic softening process of recovery & recrystallisation and a new set of grains, smaller than the original grains, is formed, the recrystallised grain size being inversely proportional to the degree of deformation, the greater the degree of deformation the smaller the recrystallised grain size I-2,3]. Research workers have assumed ,that the longitudinal ultrasonic velocity varies from grain to grain because of misorientation of grains, which is related to variation in the elastic constant from grain to grain in the same direction [4]: as the grain size decreases (i.e. as the number of grains per unit area increases) with increasing degree of deformation, this effect will become more and more pronounced. The ultrasonic velocity is further affected by dislocation, increase in the dislocation density decreasing the ultrasonic velocity [4]. The dislocation density increases with the degree of deformation, the increase in dislocation density being directly related to the degree of deformation, hence with increase in the degree of deformation, the ultrasonic velocity will decrease.

The decrease in ultrasonic velocity with increase in the degree of deformation may, therefore, be related to the increase in misorientation of grains and an increase in dislocation density.

When steel is hot forged, it develops internal stresses during the process of deforma- tion and cooling stresses during cooling from finishing temperature to room temper- ature, such internal stresses being removed by annealing. Hence in an annealed forging the ultrasonic velocity will be greater [4], as compared to that in a simply- forged forging.

R. Prasad et al./ Journal of Materials Processing Technology 42 (1994) 51-59 57

Normalising treatment removes internal stresses as in the case of annealing. Moreover, normalising produces finer ferrite and cementite lamellae [2], as a result of which the ultrasonic velocity in a normalised forging will be greater compared to that in an annealed forging.

In the hardening process steel is quenched from an elevated temperature in the austenite range. During this quenching process, the parent austenite phase (FCC lattice) transforms into martensite (body-centered tetragonal lattice) which is accom- panied by homogeneous elastic lattice deformation and by a significant increase in dislocation density [5,6]. Increase in dislocation density causes a decrease in ultra- sonic velocity [4], therefore in a steel forging which is hardened the ultrasonic velocity is lower as compared to that in an annealed/normalised forging having a lower dislocation density.

When hardened steel is tempered at 200 °C, the tetragonality of the martensite decreases and then disappears, which results in decrease in internal stresses, so that the ultrasonic velocity in a steel tempered at 200 °C is greater [4].

Tempering steel at 400 °C results in decrease in internal stresses, just as happens during tempering at 200 °C. Moreover, dislocations anneal-out at 400 °C. Therefore, there is decrease in the dislocation density [5], as a result of which the ultrasonic velocity in a steel tempered at 400°C is greater than that in a steel tempered at 200 °C.

When steel is tempered at 600°C, the following changes in the matrix take place [5]: (i) internal stresses decrease; (ii) dislocations anneal-out; and (iii) cementite particles assume a spherical shape. Because of these factors the ultrasonic velocity in a steel tempered at 600°C is much greater than that in steels tempered at 200 °C and 400 °C. Figs. 1 and 2 show the relationship between the longitudinal ultrasonic velocity, the percentage reduction and the tempering temperature.

3.2.2. Changes in attenuation Attenuation in all of the samples, irrespective of the thermal treatment given to

them (i.e. whether as-forged, forged & annealed, forged & normalised, forged & hardened and, hardened & tempered), decreases with increasing degree of deforma- tion (Fig. 3), due to grain refinement occurring with increasing degree of deformation [2,3], finer grain size causing less attenuation [7].

Attenuation is maximum in the case of simply-forged samples, which may be related to internal stresses resulting from: (i) hot deformation; and (ii) cooling from the finishing temperature to room temperature.

The attenuation is greater in case of annealed samples than in the case of nor- malised samples, the reason being that an annealed sample is soft (i.e. has a lower hardness, Table 3) and contains a larger area of ferrite than a normalised sample. Such a matrix is able to absorb (dampen) more sound energy, hence the attenuation in the case of annealed samples is greater than in the case of normalised samples.

Normalised steel contains a finer and more even distribution of ferrite and pearlite and it is also less soft than annealed steel, such a structure causing

58 R. Prasad et al. / Journal of Materials Processing Technology 42 (1994) 51- 59

less sonic damping than an annealed structure [8]. As such, attenuation in the case of normalised samples is minimum compared to that for other samples.

The hardening process introduces internal stresses into the lattice resulting from lattice deformation, this being accompanied by an increase in dislocation density, a higher value of the latter causing greater absorption of sonic energy [4,7].

When steel is tempered there is a reduction in the internal stresses and in the dislocation density, which is accompanied by reduction in the hardness and increase in the ductility. The ductility is seen to increase (i.e. the hardness decreases) with increase in the tempering temperature (Table 3). Because of the increase in ductility the damping capacity of tempered steel increases and it is able to absorb a greater amount of sonic energy. As a consequence, attenuation in the case of tempered steel is greater than that in the case of hardened steel, and increases with increase in the tempering temperature (i.e. increase in the softness) [1]. Figs. 3 and 4 show the relationship between back-echo height, the percentage reduction and the tempering temperature.

4. Conclusions

The ultrasonic velocity and attenuation in steel are influenced by the degree of deformation and by the thermal treatment to which it has been submitted, both the velocity and the attenuation decreasing with increasing of the degree of deformation. The ultrasonic velocity is maximum in the case of a normalised forging and minimum in the case of a hardened forging, for the same degree of deformation. In addition, the velocity increases with increase in the tempering temperature. The attenuation is maximum in simply-forged samples and minimum in normalized samples, the attenu- ation also increasing with increasing of the tempering temperature.

The measurement of ultrasonic velocity and attenuation in a steel forging can be used to determine the thermal condition of the forging. This technique will reduce the need for metallography and has advantage over metallography in the sense that it scans the entire surface and the entire volume of the section along its length and breadth, which is not possible in case of metallography. The technique may be useful also for sorting purposes when the hardness ranges overlap under different thermal conditions.

References

[1] R. Prasad and S. Kumar, An investigation into the ultrasonic behaviour of cast and heat treated structure in steel, Brit. J. NDT., 33(10) (Oct. 1991), 506-508.

[2] T.G. Byrer, Forging Handbook, American Society for Metals, Metals Park, Ohio, 1985. [3] J.S. Campbell, Principles of Manufacturing Materials and Processes, Koga Kusha Co. Ltd, Tokyo, 1961. [4] R.S. Sharpe, Research Techniques in Non-Destructive Testing, Vol. VI, Academic Press, London, 1982. [5] R.W. Cahn and P. Hassen, Physical Metallurgy, Part 2, 3rd Ed., North-Holland Physics Publishing,

Amsterdam, 1983.

R. Prasad et al./ Journal of Materials Processing Technology 42 (1994) 51- 59 59

[6] D. Peckner, The strenothening of Metals, Reinhold, New York, 1967. [7] J. Krautkramer and H. Krautkramer, Ultrasonic Testing of Materials, 2nd Ed., Springer, Berlin, 1983. [8] R. Prasad, An investigation into the correlation between micro-structure and ultrasonic properties of

steel, Brit. J. NDT., 32(3) (Aug. 1990) 403-404.