If the cylinder head is cracked, we need to analyze and study its location, cause and how to repair it, because if it is not handled in time or improperly handled, it is likely to cause serious consequences, so it is also necessary. The following is a discussion and analysis of the location, cause and repair of the cylinder head crack, so that readers can have an in-depth understanding. The specific content is as follows.
1. Cracked Parts of the Cylinder Head
(1) Cracks on the Bottom Surface of the Cylinder Head
Cracks generally occur at the edge transition fillet of the valve hole on the bottom surface of the cylinder head and between the valve hole, that is, the part with stress concentration. The exact crack location will vary depending on the random pattern, cylinder head structure and material.
The structure of the cylinder head of the marine four-stroke diesel engine is complex, with intake and exhaust valve holes, injector holes and indicator valve holes distributed on the bottom surface. Evenly, radial cracks are prone to occur between the holes on the bottom surface and on the valve seat surface, and most of the cracks extend from the central injector hole to other surrounding holes.
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(2) Cracks on the Cooling Side of the Cylinder Head
For the old-fashioned cylinder head, because there is an annular cooling water channel on the cooling side, cracks are generally generated at the root of the annular rib of the cooling water channel where the stress concentration is concentrated, and it expands to the depth (that is, to the bottom surface) in the circumferential direction, and even cracks the cylinder head. Water leaks or cracks in the valve hole wall.
For the new bored-cooled cylinder head, cracks were generated at the borehole on the cooling water side and propagated to the bottom surface. Such cracks are caused by improper corrosion inhibitor concentration and poor combustion in fresh water or by microbial corrosion in the cooling zone of the borehole.
2. Causes of Cylinder Head Cracks
The root cause of cracks in the cylinder head is thermal fatigue, mechanical fatigue or high-temperature fatigue caused by the cyclic action of thermal stress and mechanical stress, or comprehensive fatigue failure. The direct cause of cracks in the cylinder head during the operation of the diesel engine is the improper operation and management of the engineer and the poor maintenance.
(1) Improper Operation and Management
Improper operation and management by the engineer will cause overheating of parts, and excessive mechanical stress or thermal stress will cause mechanical or thermal fatigue. Diesel engine cold starts or accelerates too fast after starting, so that the temperature difference between the fire contact surface and the cooling surface of the cylinder head and other parts is too large, and thermal stress increases; frequent starting, stopping or long-term overload operation of the diesel engine increases mechanical stress and thermal stress; Poor or interrupted cooling and lubrication, cutting off cooling water immediately after shutdown, etc., can overheat parts and increase thermal stress.
(2) Poor Maintenance
The engineer failed to perform regular cylinder maintenance in accordance with the requirements of the maintenance program in the manual, and could not find problems and strengthen maintenance in time; the diesel engine was running for a long time, and the cooling water was not treated with medicine or improperly handled, resulting in serious scaling in the cooling water cavity, affecting the heat dissipation of parts Excessive thermal stress is generated; when the cylinder head is installed, the cylinder head bolts are not tightened according to the requirements of the manual or the bolts are unevenly stressed, resulting in excessive additional stress on the cylinder head.
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3. Repair of cylinder head cracks
(1) Penetrating cracks on the cylinder head and serious cracks in key parts must be treated with new methods. If there are no spare parts on the ship, only the method of sealing the cylinder can be used, and emergency measures for reducing the cylinder sailing are implemented. In order to prolong the service life of the cylinder head, it is necessary to repair the cracks on the cylinder head. Before repairing, carry out non-destructive inspection to find out the location, size and depth of the crack, and then choose the following different repair methods according to the material and structure of the cylinder head.
(2) When the cracks are small, use tools such as files, whetstones and wind grinding wheels to grind the cracks to eliminate them, and continue to use after passing the non-destructive testing or hydrostatic test. Otherwise, continue grinding and inspection. If the crack is deeper than 3% of the wall thickness, stop grinding and use other methods to repair or scrap it and replace it with a new one.
(3) Metal buckle law. Cracks on the bottom surface of the cylinder head and other parts are repaired by the metal buckle method, which not only ensures the strength requirements of the parts, but also meets the sealing requirements.
(4) Welding repair. When the crack is small, first shovel the crack and then repair it by welding. In order to obtain good welding repair quality, strict welding repair process should be formulated and appropriate welding repair method should be selected.
(5) Inlay repair. For the cracks on the hole wall, such as the inlet and exhaust valve hole walls on the cylinder head and the cracks on the injector hole wall, use inserts to repair them. The effect of this method is good, and the parts can continue to be used for more than 2 years. The material of the bushing is generally stainless steel or bronze, and a purple steel gasket is placed between the end of the bushing and the bottom of the valve hole to enhance sealing.
(6) Adhesive repair. For cracks or casting defects (trachoma) on the cylinder head and cylinder liner, use organic or inorganic adhesives to repair them according to their location and working conditions.
(7) Repair of cladding. Cracks on the outer surface of the cylinder head can be repaired with cladding. When repairing, first drill anti-cracking holes at both ends of the crack, apply adhesive (such as epoxy resin) to cover the steel plate, and fasten the steel plate to the cylinder head with screws.