Part cracks generated during the heat treatment is one of the most severe heat treatment defects, such defects usually can not be remedied, the parts can only be scrapped, thus it causes the heat treatment the special attention of workers.
1. Parts heat cracks
During the heat treatment, the parts will have a great internal stress (the stress and thermal stress), when these stresses exceed the yield strength of the steel, it will cause deformation of parts; greater when the stress exceeds the tensile strength of steel, cause cracking of parts. Stress acting on the part of two ways: compressive stress and tensile stress. The tensile stress is formed in the quenching is the main cause of the quenching cracks. But when the plasticity higher, even if there is a large tensile stress will not cause cracking of the parts, such as no occurrence of organizational changes to stress annealing, isothermal quenching Multiple residual austenite. Only when the stress is large, with a high hardness, brittleness organizations, likely to cause cracking of parts. Quenching cracks formed two conditions must be present: tensile stress exceeds the tensile strength of the steel (of course can induce Part cracks occur, such as raw materials defects, design and machining brittle organizations; defects caused by improper, etc.).
2. About the type of crack
Classification of a wide range of crack. The direction of the crack, longitudinal cracks, transverse cracks, arc-shaped cracks and the net cracks (cracking); position of the crack surface crack (or surface cracks) and internal cracks; crack occur in a different step of stars, forging cracks, soldering crack, quenching cracks, tempering cracks, cold treatment crack, pickling cracks and grinding cracks. Part the largest quenching cracks during the heat treatment.
3. Crack resolution method
How to distinguish between what is the quenching cracks, tempering cracks, forging crack or grinding crack is very important, it is easy to accurately find crack occurrence in which step is conducive to the analysis of cracks.
A note that the quenching of the different forms of cracks and grinding cracks. For quenching was not found after grinding cracks found to distinguish between the of quenching cracks or grinding cracks. Relatively easy to crack unattached pollutants Note crack morphology, in particular the development of the crack direction, grinding cracks perpendicular to the grinding direction, showing the form of parallel lines, or in the form of a tortoise shell-like crack. Very shallow depth of grinding cracks, quenching cracks. Generally deeper than, relatively large, and has nothing to do with the grinding direction, mostly straight knife-like cracking.
B, note that the site of the occurrence of cracks. Sharp irregularity of the corner at the edge of the hole, the Marker at the playing surface stenciled at the cause and machining defects and other parts, the occurrence of cracks in these parts, mostly quenching crack.
C, is distinguished by a lobed cross-section of the observation parts quenching cracks or of forging cracks or other circumstances caused by a crack before quenching. Crack section was white or dark white or light red (the water quenching caused by the water rust), can be diagnosed as quenching cracks, crack cross-section is dark brown, and even aerobic skin, it is not quenching cracks, Department of quenching the presence of cracks, is formed by the part after forging or rolling cracks, these cracks will be expanded due to quenching. Substantially due to the quenching cracks are formed below the M point, its cross section is not oxidized.
D, in the microstructure, quenching cracks along the grain boundary fracture if the fracture along the grain boundaries, but within the intergranular fracture, a fatigue crack.
E, if the cracks around the the the decarburization layer exists, it is not quenching cracks, but before quenching cracks, quenching cracks is generated when the quenching will never happen decarburization. Part blasting directly observed with the naked eye, using a magnifying glass to observe parts of the surface can be. When the eyes or a magnifying glass can not see the cracks. You can also check with the oil immersion testing method, the upcoming parts immersed in kerosene, gasoline and other oil later remove the parts with a cotton wipe clean, then cover with lime powder or other white powder, if there are cracks in the white part of the leaking oil. Experienced inspectors can also check out the percussion method for cracks, gently tapping with a hammer and other parts, if you send a clear metallic sound, long tail, you can see no crack; Conversely, if the issue muddy the sound, there have been cracks. Also can be used to check parts of the magnetic testing method and fluorescent flaw detection method for cracks.
4. Quenching overheating and quenching cracks
Part is heated to the quenching temperature is higher than a predetermined process and will cause overheating of the quenching of the workpiece in a certain heating time. The overheating temperature will be different steels. Once the workpiece overheating, the austenite grain coarsening generated martensite needles thick, prone to martensite micro-cracks. This martensitic micro-cracks quenching cracks excitation source, and develop into a quenching cracks. Overheating the steel deteriorate greatly reduce the strength and ductility. Quenching cracks quenching caused by overheating in the production site is most common in the tool steel, particularly high-speed tool steel overheat caused by the quenching of the workpiece cracks most instances, this is due to the HSS quenching temperature close to its melting point, i.e. a slight mistake can cause overheating, even burnt. Factory in heat treatment batch W18Cr4V of steel modulus for m = 12, the outer diameter of 170mm disk-shaped gear cutter, the process under the quenching temperature is 1270 ~, but due to the failure of the temperature control, the table refers to the temperature than the actual The furnace temperature low 35 ° C, thanks to the operator found that the furnace temperature changes, to discontinue production of re-temperature take immediate measures to prevent the occurrence of a large number of overheating, but there is still a small amount of gear cutter cracks caused by overheating. The metallographic examination found quenching coarse grains, cracks occur in the network carbide, we can see that the crack heating temperature is too high. Movement eutectic carbide is formed along the grain boundaries due to overheating the reticular distribution layer of crust is formed on the grain boundaries of the steel to produce a very brittle, to hinder the plastic deformation of the steel, in a great stress generated when the quenching under the action of cause quenching cracks. Prevent parts due to quenching overheating and cracking of the main measures are:
1 craft to develop proper heating temperature and heating method. Such as some large-sized carbide segregation more serious high-speed steel cutting tools, the lower limit of the quenching temperature of the high-speed steel, should be taken in order to ensure full heating, heating time may be extended to some parts with sharp corners should be far away from the furnace heating some workpiece distance from the electrode in a salt bath furnace shall be not less than 50mm, in order to avoid overheating of the sharp corners.
Thermometer and temperature control to regularly check proofing and maintenance to ensure that the instrument is always used in normal operation conditions. Instrument to ensure that the temperature, constant temperature, the accuracy of the thermostat.
(3) quenching workers should always observe the the IF furnace temperature, and found that abnormal changes in furnace temperature, time to find a craft or instrument operators, and other relevant personnel to solve, if necessary, immediately stop, re-temperature, until the furnace temperature returned to normal after production .
The ④ equipment to ensure that the process requirements.