JPS6154179B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the grain size of steel materials using ultrasonic waves generated by pulsed laser irradiation.

A vibrator such as a crystal is brought into close contact with a sample such as a steel material, and a high frequency and high voltage is applied to the vibrator to excite ultrasonic waves (elastic waves) in the sample, and the state inside the sample is inspected from the propagation speed of the ultrasonic waves. , so-called non-destructive testing methods using ultrasonic waves are used in various fields. However, since this method is a contact method, it is virtually impossible to perform it on samples such as steel materials that are in a high temperature state or on samples that are running at high speed.

By the way, when a strong pulsed laser beam is irradiated onto a sample surface, the material in the very surface layer (about a few angstroms) of the sample instantly evaporates and scatters, and the reaction force (compressive stress)
A strong pulsed elastic wave (ultrasonic wave) is generated in the sample. This has been shown, for example, by BPFairand et al.
Literature published in 1974 (Quantitative
assessment of laser−induced stress waves
generated at confined surfaces，Applied
Physics Letters, Vol.25, No.8, 15 October
1974, P431-P433), this method makes it possible to excite pulsed elastic waves in any sample from a remote point. Additionally, laser light can be made into an extremely narrow beam using optical techniques, and the ultrasonic waves generated on the surface of steel irradiated with such laser light can be used as an ideal point sound source. The generated ultrasonic waves propagate through the sample, and during propagation, undergo reflections, attenuation, etc. depending on the properties of the sample, reach the other surface, and cause mechanical displacement on the other surface, but the sound source is a point source. If so, it is not averaged like in the case of a wide-area sound source, and is the reflection received during propagation between the waveforms of the ultrasonic waves generated on one surface of the sample and the ultrasonic waves detected on the other surface of the sample. It is possible to clearly see the deviation (deformation state) due to the above, and in turn, it is possible to estimate the physical properties of the sample. In addition, when receiving ultrasonic waves, if a normal piezoelectric element is used, it is necessary to bring the element into close contact with the sample like a transmitter, but if a displacement measurement method using optical interference or electromagnetic induction is used, it is possible to receive ultrasonic waves without contact. It is possible to receive ultrasonic waves by pulsed lasers,
Ultrasonic detection using optical interferometry or electromagnetic induction allows non-contact physical property measurements of samples running at high temperatures or high speeds.

The present invention has been made with attention to this point, and it measures the crystal grain size of a sample, especially steel, using a strong, high-output pulsed laser beam, and also uses optical interference type or electromagnetic induction type ultrasonic wave. The aim is to enable non-contact measurement through detection, and by extension, to enable on-line measurement of physical properties of high-temperature, high-speed moving steel materials, in particular grain size measurements. The present invention involves irradiating a single surface of a steel material with a high-power pulsed laser beam having a short duration and a beam diameter to generate pulsed ultrasonic waves as a point source within the steel material. A vibration displacement detector detects the mechanical displacement that occurs on the other surface when it propagates through the steel material and reaches the other surface,
The crystal grain size of the steel material is determined by measuring the pulse width expansion of an ultrasonic pulse detected by mechanical displacement of the other surface with respect to the ultrasonic wave generated by the laser beam irradiation before propagation. However, this will be explained in detail below with reference to the illustrated embodiment.

FIG. 1 shows an embodiment of the present invention, and numeral 1 indicates a steel material to be measured (hereinafter referred to as a sample). For example, one surface 1a of this sample 1 is exposed to light from the pulsed laser 2.
A pulsed laser beam _L1 having a short duration of 20 nsec and a high output of, for example, 100 MW is irradiated.
At this time, in order to set the irradiation area of the laser beam L ₁ to one point (micro area) P ₁ , a condenser lens 3 or the like is interposed as necessary. Note that it is appropriate that the pulse laser be generated by a Q-switchable laser light source such as a ruby laser using a Q-switch. The laser beam _L1 irradiated on the sample surface 1a instantly evaporates the surface layer of the surface or its protective coating, etc.
It scatters, and the reaction force generates pulsed ultrasonic waves. This ultrasonic wave propagates through the sample 1 at a predetermined speed, reaches the other sample surface 1b after a certain period of time, and causes vibrational displacement on that surface. The vibration displacement detector 4 detects this displacement in a non-contact manner, but in this example, it detects only the mechanical displacement in a minute area occurring at a point _P2 on the other surface 1b facing the point _P1 . For this reason, it is preferable that the detection end 4a of the detector 4 be as small as possible. The following method is effective for this. That is,
First, the measurement laser beam is split by a beam splitter, one is projected onto the sample surface 1b and the other is projected onto a mirror, and the reflected light from the sample surface and mirror is guided back through the beam splitter to a common receiver. This is a displacement measurement method using Michelson type optical interferometry, which performs root-mean-square detection using the optical receiver. The second is sample surface 1
A magnetic field is applied to the sample surface 1b, and when the sample surface 1b is displaced, an electromotive force is generated on the sample surface by the displacement and the magnetic field, and an eddy current is caused to flow. This is a displacement measurement method that detects The oscilloscope 5 is used to observe the output of the detector 4, and therefore the waveform of the ultrasonic pulse that has arrived at point _P2 , but if necessary, the detector 4, especially its detection end, may also be placed on the laser beam incident point _P1 side. 4a so that the ultrasonic pulses excited at the incident point and before propagation can be simultaneously displayed.
Alternatively, the waveform of the ultrasonic pulse at point P ₁ is the laser beam
It is assumed that the waveform is equal to that of _L1 , and a part of the laser light may be split by a beam splitter and then obtained by photoelectrically converting it.

Figure 2 shows the measurement principle of the present invention.
indicates steel grains. When a point _P1 is irradiated with a laser beam _L1 with a beam diameter, a point source-shaped ultrasonic pulse _U1 is generated at the point _P1 and propagates toward the other surface 1b. In this case, if the grain system of crystal grain 1c is extremely fine and can be regarded as a homogeneous medium, the ultrasonic pulse
U ₁ either travels straight or reaches the other surface 1b with very little scattering. On the other hand, as the grain size increases, the ultrasonic pulse U ₁ is scattered at the grain boundaries of each crystal grain 1c, and the amount of reflected waves U ₂ generated increases. This degree can be clearly seen at one point _P2 on the other side 1b.
That is, if the amount of reflected wave U ₂ is small, the ultrasonic wave that reaches point P ₂ is the same as the ultrasonic wave generated at point P ₁ , but if the crystal grain size is large and the amount of reflected wave U 2 is large, the ultrasonic wave that reaches point P ₂ is
The waveform of the ultrasonic pulse observed at P ₂ is deformed accordingly. There are two types of this displacement. One is the pulse half-width, which increases as the crystal grain size increases. Figure 3 shows this, with the solid line curve
IN is the waveform at point P ₁ (pulse waveform of the irradiated laser beam), and the broken line curve OUT is the waveform at point P ₂ (waveform of detected vibration displacement), with peaks PK ₁ and PK ₂ respectively.
The half-widths Δτ ₀ and Δτ ₁ at half the level become Δτ ₀ <Δτ ₁ as the crystal grain size increases. The other one is the time difference Δτ ₂ between peak PK ₁ and PK ₂ (waveform
Naturally, there is a propagation delay between IN and OUT, but this is excluded). This also increases as the crystal grain size increases. Therefore, by actually measuring any one of these (herein referred to as the spread of the pulse waveform), the crystal grain size can be determined.

4 to 6 are actually measured data showing the relationship between pulse half width and crystal grain size. That is, FIGS. 4 and 5 show the waveforms at point P ₂ observed with the oscilloscope 5 when the beam diameter of laser light L ₁ irradiated to point P ₁ was changed to 6 mmφ and 1 mmφ, respectively. a, b, c are the crystal grain sizes, respectively.
These are waveforms for 0.022mm, 0.087mm, and 0.49mm. Curve C ₁ in Figure 6 is a plot of each half-width in Figures 4 a to c, and curve C ₂ is a plot of the half width of Figure 4 a to c.
This is a plot of each half-width in Figures a to c.
By using either one of the characteristic curves C ₁ and C ₂ , the grain size of the steel material 1 can be determined from the half width of the pulse obtained at point P ₂ .

As described above, the present invention has advantages such as being able to measure the crystal grain size of a steel material using ultrasonic waves, and enabling on-line non-destructive testing of steel materials that are moving at high temperatures and high speeds.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
Figures 2 and 3 are explanatory diagrams and waveform diagrams showing the measurement principle of the present invention, and Figures 4 a to c show ultrasonic pulses propagated through steel materials with different grain sizes.
Figures 5 a to c are waveform diagrams similar to Figure 4 with different beam diameters of pulsed laser light, and Figure 6 shows the pulse half-width and crystal of Figures 4 and 5. FIG. 3 is a characteristic diagram showing the relationship with particle size. In the figure, 1 is a steel material, 2 is a pulse laser, and 4 is a vibration displacement detector.

Claims (1)

[Claims] 1. A high-power pulsed laser beam with a short time width is irradiated with a beam diameter onto one surface of a steel material to generate a pulsed ultrasonic wave as a point source within the steel material, and the ultrasonic wave is A vibration displacement detector detects the mechanical displacement that occurs on the other surface when the sound wave propagates through the steel material and reaches the other surface, and detects the mechanical displacement of the other surface in response to the ultrasonic wave generated by the laser beam irradiation before propagation. A method for measuring the crystal grain size of a steel material using pulsed laser light, characterized in that the crystal grain size of the steel material is determined by measuring the spread of the pulse width of an ultrasonic pulse detected by mechanical displacement. 2. The pulsed laser according to claim 1, wherein the vibration displacement detector is a non-contact type that uses mean square detection of the signal light and reference light by Michelson interferometry. A method for measuring steel grain size using light. 3. A method for measuring the grain size of a steel material using pulsed laser light according to claim 1, wherein the vibration displacement detector is a non-contact type that uses electromagnetic induction.