铸造轧辊检测和维护.docx

1、铸造轧辊检测和维护The Davy Roll Company LimitedExcellence in Forged and Cast Roll Technology Inspection and Maintenance of Cast Rolls.The following information is intended to aid roll shop personnel with the every day maintenance and inspection of static and rocast work rolls.By optimizing grinding operation

2、s and introducing targeted inspection procedures significant improvements in roll performance and reliability can be achieved.Roll Shop Inspection Equipment.Automated eddy current systems are used in a large proportion of roll shops across the world. They provide a rapid and efficient means of testi

3、ng large areas of a rolls surface allowing the operator to manage and where necessary, remove rolling generated defects. These systems are however limited to testing near surface defects. Many roll shops feel it is necessary to introduce complementary, deep inspection systems in the form of ultrason

4、ic equipment. This can be in the form of an automated system used in cooperation with the eddy current units or manual handheld units. In roll shops were eddy current equipment is not available manual ultrasonic equipment as well as visual crack detection procedures, such as magnetic particle or die

5、 penetrant inspection become essential. Ultrasonic InspectionThe Davy Roll Company inspection departments use portable ultrasonic units for the day-to-day inspection requirements of both work and back up rolls during manufacture and at the customers site. A high level of operator skill and specific

6、training is required to fully utilise these units as well as a range of tailored ultrasonic probes. The Davy Roll Company is able to offer guidance and training to suitably experienced personnel in roll inspection practices.Ultrasonic ProbesThe following probes are recommended for roll inspection du

7、ties and available from most manufacturers. Compression Probes.TypeAngle (degrees)CrystalsFrequency (Mhz)Diameter (mm)CommentsCompression0Twin2.525This is a general-purpose probe suited to the inspection of centrifugally cast work rolls especially interface bond zones and mid-deep shell defects. Alt

8、hough a relatively low frequency probe this is ideal for use on alloy indefinite chill materials were there is a large amount of free graphite in the microstructure. Use of higher frequencies for these materials will generate large amounts of grass or scatter making resolving of the bond zone diffic

9、ult. Compression0Twin515A good probe for resolving defects close to the surface of work and back up rolls. This will penetrate to the bond zone of high chromium and high-speed steel materials. Good for evaluating and positioning small defects.Compression0Single0.550Roll makers use these probes for t

10、esting longitudinally as well as transversely through castings, for example from one side of the barrel to the other or from drive neck end face to operator end face. We mainly look for manufacturing problems and fatigue problems during service. These probes can be expensive and your usage would be

11、very low.Twin Shearwave Probes.TypeAngle (degrees)CrystalsFrequency (Mhz)Size (mm)CommentsShearwave45Twin230x15Good for deep cracks in back up rolls or examining defects at work roll bond zones.Shearwave70Twin230x15Good for shallow cracks 10mm in depth, especially fire cracking.Shearwave90Single230x

12、15Also known as surface wave, is good for inspecting the full barrel of back ups, for instance in the case of eddy current equipment failure. Is very sensitive to surface condition and cleanliness.Calibration.Accurate calibration of test equipment is essential to ensure minimum stock removal and ens

13、ure the complete removal of defects, optimizing roll performance and reliability. To ensure accurate results compression probes should be calibrated with test blocks manufactured to the same material specification as that of the roll to be tested. For shearwave probes a steel angle calibration block

14、, for example a V2 type is required to ensure precise trigonometric calibration is achieved. The following values may be used for the calibration of compression probes when specific test blocks are not available. These values are approximate only as variations between material grades, for example en

15、hanced Apex grade and standard iron grade as well as between manufacturers will exist.MaterialVelocity Alloy Indefinite Chill (also Apex)5700 m/sHigh Chromium Iron (also STELLA)6350 m/sHigh Speed Steel6250 m/sAlloy Cast Steel (also V Chrome)5918 m/sAngle probes3353 m/sDye Penetrant Inspection.Die pe

16、netrant systems are useful in determining the presence of gross cracking in back up and work rolls. The system is relatively inexpensive and requires a limited degree of operator training.There is however some limitation to this method for roll inspection. The process relies on the dye being drawn i

17、nto open surface cracks by capillary action and as such anything that impedes this process can reduce or even eradicate the effectiveness of crack detection. For example; When cracks contain large amounts of oil or debris out of the mill. After grinding the cracks may be sealed by mechanical deforma

18、tion of the surface or by grinding debris.As such additional inspection methods are advised to compliment this procedure to ensure complete removal of defects.Magnetic Particle Inspection.Magnetic particle inspection is an effective and reliable system for the detection of cracks in rolling mill rol

19、ls. This system is preferred for the detection of smaller cracks especially on ground or turned surfaces.The system works by attraction of a ferrous based dye on the roll surface towards the magnetized crack faces. This attraction occurs across the full face, at the surface and at depth. As such the

20、 system does not suffer the limitations of the dye penetrant system when the cracks are closed at the surface or filled with debris.Many systems are available on the market consisting of either mains powered electro-magnets or permanent hand held magnets.Hardness Testing.Hardness testing of cast rol

21、ls is primarily carried out with EQUOTIP type instruments. These devices determine hardness by measuring the level of rebound energy from the surface of the test piece or roll.One of the many uses of harness testing is determining if work hardened material has been removed from the barrel surface du

22、ring grinding, especially of back up rolls.These devices are very sensitive to surface cleanliness and finish. Hardness measurements taken on a rolled or rough ground surface will in general give a lower result than on a polished ground surface. Accurate measurements can only be obtained on a 400-gr

23、ade grit equivalent or better surface finish.Ultrasonic Inspection Procedures.Measuring Crack Depths.When cracks are generated in the barrel of a roll they can be parallel to or perpendicular to the surface. For example, fire cracking will initially form at right angles to the surface but as they ar

24、e subjected to the forces of rolling will propagate circumferentially and turn parallel to the surface as shown below.Cracking may not always break the surface, for example a roll which has been subjected to an impact or localized overload may initiate cracks below and aligned parallel to the barrel

25、 surface.It is therefore always advisable to inspect the area surrounding a surface crack or bruise with a compression and angle probe to ensure detection of the deepest component.Care should always be taken when assessing crack depths on curved surfaces. With shearwave probes an angle should be cho

26、sen that will allow you to work as close to the surface crack as possible. Working with the probe away from the crack on a curved surface can result in the beam moving back up the crack face towards the barrel surface resulting in deeper than actual crack depths.When using a shearwave probe crack de

27、pths should be checked from both edges to ensure the maximum depth is obtained. Cracks will not always be aligned exactly perpendicular to the surface but may be entering at an angle. If the probe angle and crack angle are similar the crack may not be detected when tested from only one side.Inspecti

28、on of Centrifugally Cast Shell to Core Bond Zone (Interface).Centrifugally cast work rolls are inspected prior to delivery to ensure an interface bond strength sufficient for standard mill operating conditions to discard size.However if rolls are subjected to abuse during rolling it is advisable to

29、inspect the bond zone or interface area to ensure there has been no detrimental effect that could ultimately result in the rolls failure.The test procedure described below compares the amount of energy returned to the probe from the interface related to the amount transmitted into and dispersed with

30、in the core material. A low energy return indicates a poor reflector or well-bonded interface while a high-energy return indicates a good reflector and hence poorer bonding.The principle is shown below with echoes for a well and poor bonded interface at the same decibel level.Good Interface Poor Int

31、erfaceTest procedureThis procedure can be used for all types of ROCAST centrifugally cast rolls. Equipment Krautkramer USN 50 or similar calibrated to BS44331 part 2 or equivalent. A 2.5 MHz twin crystal compression probe.Test block (20x40x50mm) manufactured from the same grade material as the roll

32、to be examined. Procedure Calibrate the time base at 100mm using the above test block. Measure and record the gain in decibels (db) required to obtain a full screen height (100%) back wall echo from the test block. This value (A) is then used as the minimum or theoretical value for total interface separation. Test the roll