Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-06-15
2002-09-10
Williamd, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S593000, C073S598000, C073S600000, C073S602000, C073S620000
Reexamination Certificate
active
06446509
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and an apparatus for ultrasonic testing of the surface of columnar structures of metal, such as rolls for rolling mills, rollers and others, especially to those suitable for detecting, using surface waves, flaws such as cracks or the like existing in and just below the surface of high-speed tool steel rolls for hot rolling mills, which are made of high-speed tool steel and of which the surface is thermally/mechanically damaged while they are used for rolling, and also to a method for grinding rolls using them.
BACKGROUND OF THE INVENTION
Rolls for hot rolling of metal sheets are thermally/mechanically damaged at their surface while they are used for rolling. The details of thermal/mechanical damage at the surface of a work roll (hereinafter referred to as a high-speed tool steel roll for former stands in finishing train), which is made of high-speed tool steel and is used in hot finish rolling, are described with reference to FIG.
20
. Thermal damage to the roll is caused by steel sheets being rolled at high temperatures in former stands in finishing train, whereby deep primary cracks K, which are referred to as fire cracks, are formed in the roll
100
vertically to its surface. Mechanical damage thereto is caused by the shear stress to be put on the roll being rolled against backup rolls, whereby are formed secondary cracks L starting from the above-mentioned fire cracks K in the direction nearly in parallel with the surface of the roll. A plurality of those cracks gathers to give small pits M on the surface of the roll. If such small pits M are transferred onto the sheets being rolled, the rolled sheets shall have surface flaws. In order to evade this, the cracks are removed from rolls, for example, by means of grinding by a predetermined constant grinding allowance with a grinder, and the thus-ground rolls are again used in rolling. After having been ground, the rolls are tested by surface wave technique (hereinafter referred to as surface wave testing), for example, as in JP-A-4-276547.
Concretely, a surface wave probe (search unit) is kept in contact with the surface of a rotating roll via a membrane of a coupling liquid medium such as water or the like, whereby the surface waves from said surface wave probe is propagated inside the circumferential surface of the roll toward the direction opposite to the direction in which the roll is rotating while the liquid having been used as coupling medium is removed from the path of the surface waves in the surface of the roll. In that manner, the flaws, if any, existing in and just below the surface of the roll are detected. If some flaws are detected in such surface wave testing of rolls, the rolls shall be again ground.
An ultrasonic test apparatus is disclosed in JP-A-7-294493, to which is applied the testing method of JP-A-4-276547. The ultrasonic test apparatus comprises a rotating means for rotating a cylindrical or columnar structure to be tested for surface flaws and others, in its circumferential direction; an ultrasonic probe for detecting flaws and others by use of surface waves; a holder for holding the probe above the structure to be tested at a predetermined height relative to the surface of the structure; and a couplant supply for supplying a liquid medium such as water or the like to be a coupling medium for ultrasound transmission, to the gap between the probe and the structure to be tested. The above-mentioned holder extends downward below the probe, and has a following part to be in smooth contact with the surface of the structure to be tested. While kept in contact with the rotating structure to be tested, the following part ensures the constant distance between the probe and the structure. The above-mentioned couplant supply is disposed adjacent to the probe inside the holder. The couplant supply is provided with a housing in which the liquid medium having been led near the probe from the other place is stored. The housing is positioned adjacent to the probe, and has a lot of medium outlets at its bottom and an air-discharging through-hole at its top. Each of the medium outlets is placed just in front of the probe relative to the scanning direction of probe, while the surface of the structure being rotated to be tested is scanned by the probe, and is so disposed that it intersects the above-mentioned circumferential direction. In that constitution, the liquid medium stored in the housing is fed through those medium outlets to the gap between the probe and the surface of the structure being tested, at the place just in front of the leading edge of the probe.
Recently, however, it has been clarified that, when the testing method according to JP-A-4-276547 is applied to the detection of flaws existing in and just below the surface of high-speed tool steel rolls, especially those for former stands in finishing train, there occur serious problems such as those mentioned below.
Specifically, the small pits M shown in
FIG. 20
are not formed in the absence of the secondary cracks L. Therefore, in order to prevent the formation of the small pits M, only the secondary cracks L are to be removed through grinding. However, according to the conventional ultrasonic testing method, often observed is the phenomenon of large-amplitude reflected waves appearing even after all those secondary cracks L are removed. Grinding rolls until no such reflected wave appears results in the thorough removal of the primary cracks K needless to be removed, whereby the roll consumption shall increase.
This is because, in the surface wave testing, the reflectivity of the flaws vertical to the surface of the roll being tested is high. In this, therefore, the primary cracks K remain are detected falsely even after all the secondary cracks L are removed. The depth of the primary cracks K remain is considerably shallow, the amplitude of the wave reflected on each primary crack K remains is very small. However, in the surface of the roll, there are innumerable primary cracks K remain, and there are also innumerable reflectors by which the surface waves are reflected. Where a structure (herein roll) having such innumerable small reflectors is tested by surface waves having a wavelength of &lgr;, there always exist combinations of small reflectors between which the difference in the distance from the surface wave probe
10
is &lgr;/2, as shown in FIG.
21
.
FIG. 21
shows examples of the combinations of such small reflectors, in which small reflectors K
1
to K
4
correspond to the combinations.
Where the region in which the small reflectors K
1
to K
4
exist is tested in a conventional manner using a narrow bandwidth pulse of which the length is at least 5 times larger than the wavelength of the resulting surface wave, small reflected waves from those reflectors overlap in phase with each other, owing to the large pulse length, thereby enlarging their amplitude to give a large reflected wave that may indicate the presence of just like a large flaw, as shown in FIG.
22
. Specifically, since such a narrow bandwidth pulse of which the length is at least 5 times larger than the wavelength of the resulting surface wave is used in surface wave testing of high-speed tool steel rolls for former stands in finishing train, the amplitude of the reflected waves from the primary cracks K is detected too high in the test. As a result, the rolls are to be ground until the amplitude of the reflected waves from the primary cracks, K becomes lower than a predetermined voltage. Consequently, since the primary cracks K are almost completely removed, or that is, since the rolls are too much ground, the roll consumption is to increase.
In this connection, it may be taken into consideration to elevate the threshold voltage that is settled for detecting flaws to a degree not bringing about false detection of the primary cracks K. However, elevating the threshold voltage lowers the detectabilities of the test device, and therefore, there is a danger of overlooking cracks and the like which exist alone (these
Morii Takashi
Sugimoto Ryouichi
Takada Hajime
Yarita Ikuo
Kawasaki Steel Corporation
Saint-Surin Jacques
Williamd Hezron
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