High-speed steel rolls have good wear resistance and heat crack resistance but are sensitive to cracks caused by cooling, improper use, or mechanical damage. Surface cracks are the main cause of the failure of high-speed steel rolls. Usually, the surface of high-speed steel rolls is prone to mechanical cracks, network “hot” cracks, stress cracks, and fatigue cracks.
Therefore, good non-destructive testing methods and procedures should be used to regularly check the rolls, repair high-speed rolls with potential problems as soon as possible, and replace them to avoid rolling accidents. Let’s learn how high-speed steel rolls are inspected.
The detection methods of high-speed steel rolls mainly include grinding machine eddy automatic flaw detection, artificial ultrasonic flaw detection, magnetic particle flaw detection, and penetrant coloring flaw detection.
Eddy current automatic flaw detection
Advantages: Automatic flaw detection, with an automatic recording system, can accurately determine the location of roll axial defects. The detection sensitivity is high, and it can find defects on the small surface and near the surface that cannot be compared with ultrasonic and other flaw detection methods.
Disadvantages: It is impossible to determine the type, shape, and depth of cracks, and it is impossible to determine the true equivalent of defects. It is affected by various factors such as the distance between the probe coil and the inspected roller, the material, size, and shape of the roller, electrical conductivity, magnetic permeability, and defect type. It is an important method for the regular inspection of eddy current sensitivity and the auxiliary inspection effectiveness of other flaw detection methods.
Ultrasonic flaw detection
In recent years, roll ultrasonic automatic flaw detection system has been popularized and used together with eddy current flaw detection.
Advantages: It has good penetration and directivity, and it is easy to find defects inside and outside the roll. Ability to locate, quantify, and characterize defects. For defects such as cracks and internal layers, the sound beam can obtain highly reflective echoes perpendicular to the defect. Through waveform transformation and frequency transformation, it is easy to find various defects and judge the nature of defects.
Pros: Workers with the necessary training experience for a good coupling agent have a wealth of knowledge about defects
The ultrasonic flaw detection method can be manual or automatic. Using the manual ultrasonic flaw detection method can detect defects on the surface and second surface of the roller main body, internal defects, and voids, but the emergence of automatic flaw detection systems for the roller body means that manual ultrasonic flaw detection is currently mainly used to detect internal defects, and can also be used for Detecting Crack Depth in Roll Neck Fillets Many roll shops use pulse-echo ultrasonic flaw detectors displayed on fluorescent screens to perform flaw detection manually.
Ultrasound is often used to judge character integrity. Many roll manufacturers carry out this inspection as part of their production techniques, but as a rule of thumb, users require ultrasonic testing of internal roll defects.
Magnetic particle inspection
The magnetic particle inspection technology is mainly used to measure the cracks on the surface of the drum and the fatigue cracks in the roll neck area, excessive cyclic load, or subsequent normal corrosion.
Advantages: It is convenient and fast, can detect opening defects and subcutaneous defects on the metal surface, and can clearly display the shape of defects.
Cons: Defects cannot be quantified. In addition, the cleaning condition and glossiness of the roller surface will affect the inspection results of magnetic particles. Penetrant dyeing flaw detection
Due to its simple use, penetrant dyeing inspection is widely used in the detection of cracks on the surface of rolls.
Advantages: It can detect complex shapes without a power supply, and point out the shape and size of defects; it can display tiny defects; it can detect ferrous and non-ferrous metals.
Disadvantages: secondary surface defects cannot be detected; the surface is clean and free of oil.
Points to pay attention to in flaw detection:
1. The surface quality of a large number of rolls is tested mainly by the automatic eddy current flaw detection of the grinding machine, supplemented by other detection methods.
2. In terms of eddy current testing, when the signal is abnormal, the authenticity of the defect and the location of the defect are the main tasks for the grinding machine operator and flaw detection inspector to judge.
3. When setting the sensitivity value of the amplitude, check it visually. In this case, the defect is visible with the naked eye. If there is no visible defect, according to the position shown by the x-axis and y-axis, after degaussing treatment in this area, a secondary flaw detection re-inspection is carried out.
4. When an abnormal signal occurs, conduct magnetic particle or penetration and ultrasonic inspection in this area to distinguish the authenticity of the defect, and use ultrasonic to quantify the defect. the
5. Compared with several auxiliary methods, magnetic particle examination combined with ultrasonic examination is more practical and rapid.