·Table of Contents
Potentialities of ultrasonics for evaluating residual stresses: Influence of microstructureH. WALASZEK, Y. ABDALLAHOUI, H.P. LIEURADE
Centre Technique des Industries Mécaniques
52, avenue Félix Louat -B.P. 80067
The good mastership of residual stresses level in mechanical components is an important factor, particularly for a good fatigue-strenght of these components. This paper presents advances obtained at CETIM in the field of development of an ultrasonic method for stress measurements. This method is potentialy advandageous by its non destructivity, its good portability on site, and its relative easiness for operation.
In the paper are discussed the results obtained with ultrasonics on steel welded plate, and a comparison is made with stress measurement obtained by incremental hole drilling method and X Ray diffraction. These results are also validated by thermal relaxation of the plates.
The paper discusses also the microstructure influence on ultrasonic measurements and the precision obtained.
In conclusion is outlined the interest for studying the ability of the ultrasonic residual stress measurement method in different industrial cases.
Keywords: Non destructive measurement - ultrasonic measurement - subsurface wave - acoustoelastic coefficient - residual stresses - microstructure - welded plate.
Some technical means enabling evaluation of the stresses already exist but are generaly destructive or semi-destructive, costly and complexe to use. The currenty known methodology are following :
To evaluate experimentally residual stresses, we need the value of acoustoelastic coefficient. This can be done through a calibration (tensile test for example). These coefficients are determinated for a given material, by measuring propagation speed (or propagation time) of ultrasonic waves versus stress to which the material is submitted. The value of this ratio can be obtained experimentaly by calibrating.
In this study, we define an acoustoelastic coefficient Ki which represents the value of the curve of the relative speed variation versus stress. As the distance between ultrasonic transmitter and ultrasonic receiver is maintained constant, measurements are not carried out in speed itself, but on ultrasonic travel-time which is directly measurable :
The welding operation is characterized by a great and localized heat flux on welded component. It appears that, if we move our point of view on the weld from the fusion line to the parent-metal, we meet typical microstructures which nature depends on the maximum temperature reached and on the cooling rate at this point. We may distinguish, from parent metal to weld axis, a microstructure evolution from ferrito-perlitic structure, in parent metal, to a bainitic or a martensitic-bainitic microstructures, respectively, in the heat affected zone (HAZ) and the melted zone.
The micrographs obtained in the different zones after heat treatment show that structures remain stable before and after heat treatment. X Ray diffraction were made in these three zones. These measurements showed than stresses are close to zero state after heat treatment. This confirms that this heat treatment leads to relaxation of stresses, without affecting microstructure.
The values of acoustoelastic constant estimated in the three zones are given in table 1.
4.1 Sample used
The involved assemblies are butt welded plates structures (S355 steel grade : 8 and 30 mm thick), with a X chamfer.(figure1)
The parent metal yield strength, measured in the longitudinal or transverse direction is 400 MPa. It is the same for the deposit metal.
|Fig 1: Butt welded joint-steel S355 (thickness 8 or 30 mm)||Fig 2: refracted wave at the surface of the propagation medium|
|Zone||Acoustoelastic coefficient K1 x 10-5 [MPa]-1|
|Parent Metal (P.M.)||K1PM (measured) = - 1.25 ± 5 10-2|
|Heat Affected Zone H.A.Z||K1HAZ (estimated) # 90% K1PM|
|Melted Metal (M.M)||K1MM (estimated) # 80% K1PM|
|Table1 : Acoustoelastic coefficient measured in parent metal and estimated in H.A.Z and in deposit metal|
4.2 Measurement method
Measurements are made with a transmitter-receiver transducer generating longitudinal subsurface waves (figure 2). The transducer axis is parallel to weld axis.
Ultrasonic travel time is taken on 2 mm spaced points, on melted zone and heat affected zone, and
5 mm spaced points, in parent metal zone.
The residual stresses measured by this ultrasonic method are localized in near surface zone. Measurement is integrated on a 10 mm length. Integration depth of measurement is estimated at 1,5 mm for 10 MHz and 2,5 mm for 5 MHz. The metallurgical texture is taken into account through a double calibration i.e in longitudinal and transverse direction, when acoustoelastic constants are determinated.
So, the influence of texture was estimated to 10 %. This effect can be substracted from the measurement when the rolling direction is known.
5.1 Evaluation of welding stresses gradient
We give in figure 3 the evolution of longitudinal stress (in zone C, parallel to weld axis), evaluated by ultrasonic method and using the acoustoelastic constant determined in parent metal. This constant is given in table 1.
The stress is evaluated by measurement of ultrasonic travel time on each point. One of this point defined by the mean value of ultrasonic travel time measured from the plate end is considered as the reference point, and « zero stressed ». Therefore the given values of stresses are relative. Moreover, the stresses obtained do not take into account the effect of the microstructure variations induced by welding, on the measurement.
If we consider the instrumental error and the error due to non taking into account of the biaxiality of stresses, and the reproductibility error, the precision on ultrasonic stress evaluation method is estimated to ± 50 MPa.
5.2 Comparaison of measurements obtained with different methods, before and after microstructure correction
For first validation of ultrasonic measurements, we performed stress measurement by incremental hole drilling. The error on hole drilling is also here estimated to ± 50 MPa. Ultrasonic and hole drilling measurements are integrated on a 2 mm depth.
Figure 3 gives the residual stress profile obtained by the ultrasonic method, and results of stress measurements by incremental hole drilling obtained in parent metal (2 points) and weld (3 points).
|Fig 3: 5 MHz longitudinal stress profile obtained by ultrasonic and hole drilling method|
|Fig 4: effect of progressive thermal stress relief on ultrasonic measurements - materiel S355 steel - 8 mm thick|
5.3 Influence of thermal relaxation
|Fig 5: comparison of stress profiles obtained by ultrasonics and hole drilling method after complete thermal stress relief processing - Material S355 steel - 8 mm thick|
|Fig 6: Comparison of stress profiles obtained by ultrasonic and X Ray diffraction, before stress relief - Material S355 stell - 30 mm thick|
|Fig 7: Same as fig. 6 but after thermal stress relief|
5.4 Microstructure effect
The ultrasonic stress measurement results presented in the paper give a relative stress level, because we are not able to define precisely the reference zero-stress points on the sample which is investigated.
Apparently these results display stress level by ultrasonics higher than the yield point of the parent metal wich is surprising at a first glance.
To improve the validity of the comparison between ultrasonic method and others, we have to take into account on the microstructure effect from figure 6 (measurement on 30 mm thick plate). So, figure 8 is derived from figure 6 by the following process applied to the ultrasonic measurement:
|Fig 8: Idem fig. 6 after offset and "microstructure" correction|
The ultrasonic stress measurement method described in this paper enabled in laboratory experimental conditions, the evaluation of residual stresses on welded plates.
The measurements have been validated by two reference methods (hole drilling and X Ray diffraction) and confirmed by thermal stress relief, in the following cases :
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