The railroad shunting engine that my factory produces uses diesel engine, those who use is crankshaft of founding of 42crMo report broken bits. The technological process of crankshaft of founding of electric broken bits is: Go stress anneal → 2 times normalizing → rough machining → moves normalizing → to handle → semifinishing machining → to go character finish machining → of stress anneal → goes essence of → of processing of nitriding of → of kibble of stress anneal → grinds → to pledge check → assembles crankshaft → to load experiment of function of diesel engine → . Come a few years, crankshaft uses a case all the time better, but in installing machine experiment process, produce 050615 crankshaft diameter of a connecting rod is transverse and abrupt break off accident. For this, we diesel engine disintegrate hind, undertook respectively to crankshaft macroscopical examine, metallographic organization examines, inspection of quality of chemical composition analysis, objective and analysis of heat treatment craft, found the reason that rupture finally, put forward to improve measure. 1. Manage is changed examine (1) is macroscopical examine the panorama of macroscopical fracture surface of crankshaft is shown 1 times like the graph. Clarity of hill fracture surface is visible: Section and matrix of line of center of music abduct axis are perpendicular, section is made the same score together; Fracture surface shows one locate defect of apparent indentation shape; Aperture of oil duct of connecting rod diameter () of part of fracture surface black, as it happens and indentation shape blemish are linked together. From the graph the 2 form appearance that can see indentation shape drawback, its position and oil duct aperture intersect, dimension is 40mm × 20mm, hole bottom evenness is smooth. From the graph 3 can see, fracture surface is fracture surface of fiber shape plasticity, it is thus clear that apparent radiative shape tears off mountain, break a division finally lesser. (2) metallographic examines to be in sampling in indentation shape blemish, classics is times taller microscope observation, central place and brim place all did not see the metalloid field trash that distributings centrally, press GB 10561 1989 A, B kind assess, all 1 class of ≤ . Microstructure shows have brilliant of branch of apparent casting state, crankshaft core and brilliant of brim place branch all are brilliant axis with pearlite spot (see a picture 4, graph 5) , matrix brilliant granuality can be judged for 7 class. Because crankshaft diameter is bigger, heat treatment attune pledges when quenching, be appeared without complete temper by dipping in water, crankshaft brim organization and core organization are so different (see the 4 5) that reach a figure. Stand by crackle source indentation to be in microstructure to did not see unusual (see a picture the 6 7) that reach a figure. (3) chemical composition analyses a course to undertake to blemish place chemical composition is analysed, the chemical composition of this crankshaft accords with GB ／ T3077, the 42CrMo in 1999 standards the technology of steel asks (if express,show) 1 times. (4) crankshaft objective detects detect from the outside and afferent ministry of crankshaft hardness, detect the circumstance is shown 2 times like the watch. The mechanical function result that in crankshaft 2/3 place sampling detects sees a table 3. Hardness of layer of crankshaft surface nitriding detects the result sees a table 4. From objective examination data looks, the moves qualitative, surface nitriding of heat treatment handled working procedure to basically satisfy technical requirement. 2. Case study (1) looks from fracture surface for fiber shape fracture surface, crackle source (indentation shape blemish) the central place that is located in axial diameter, the plasticity that should belong to thermal stress to cause ruptures. Those who rupture is patulous happen in workpiece to heat when difference in temperature of the inside and outside in the process is bigger, bear when central place bigger pull stress, because temperature is inferior and cannot rely on plasticity to be out of shape will try universal time coordinated, expand along indentation shape blemish crackle. Well-known, heat rate is influential to the size of thermal stress. Heat when rate is rapid, make difference in temperature of 17 inside and outside increases, bring about workpiece thermal stress to increase. Organize to the metallographic that forms in founding process especially, to heating the influence of speed is more sensitive. Set in craft of crankshaft heat treatment heat 120 ℃ of speed ≤ / H. Heat heating furnace to 845 in be operated actually. C, load crankshaft next the classics in hot stove 2. 5h arrives at 845 ℃ (add up to 338 ℃ about / H) . To large work, heat treatment heats rate is too rapid, will bring about all difference in temperature of inside and outside of Wen Qian workpiece is big, place of difference in temperature arises pull blemish of shape of stress edge indentation to release expanded original crackle. (It is normal that the microstructure of each place all belongs to 2) material interior. The indentation shape blemish that is located in 20mm of × of 40mm of place of axial diameter center is the stress with material severe interior to center an area, it is the apparent crackle source that crankshaft ruptures. Bear when crankshaft center place when pulling stress, indentation brim produces tremendous pointed horn effect, crackle can expand quickly here, make carrying capacity of crankshaft unit area drops quickly. Crankshaft loads diesel engine to do function test, crankshaft wants those who light baric force and reciprocate quality force to bear hand in those who become action, intense to bend and turn round, hand in become fatigue stress and shafting are turned round shake etc. Regard eligible crankshaft as completely OK and contented use requirement, and to been put in the crankshaft of crackle, mean those who reduced crankshaft is effective bear the weight of sectional, once the outside force that in diesel engine experiment process produces exceeded crankshaft allowable stress, will bring about crackle instant to expand the whole cross section of crankshaft, make crankshaft happening breaks off. This says palpability cuts the account with lesser area finally adequately. (The indentation that 3) regards crackle as the source is in crankshaft founding process not the blemish of seam. According to the configuration of indentation, can affirming is the mark that future of the gas in caky process must reach transgression and leaves in founding of connecting rod diameter. Central indentation is the upside in process of steel ingot remelting fills shrink shrinkage cavity of the existence in the part, be should the craft surplus of purify, curiuming when cutting way of connecting rod of the crankshaft that make, fail complete purify and in staying in diameter of crankshaft connecting rod. Because gas exists, so the herein when founding forms blemish of not complete fusion. (4) crankshaft should use CEW-300 crankshaft every after finished product machine of special flaw detection makes flaw detection inspection, but can check the surface and close surface to whether be put in blemish only, only ability of use ultrasonic flaw detection checks interior to whether be put in blemish. To crankshaft, as a result of appearance complex, inlet port is much, because this does ultrasonic flaw detection,the examination is put in certain difficulty. By above the analysis can conclude: The indentation shape blemish of crankshaft center place is the crackle source that rupture, in heat treatment process, the thermal stress that because heat,with cooling rate fast place produces, those who brought about crackle is patulous, cause crankshaft to bear the weight of effectively sectional decrease, in process of diesel engine experiment, outside force exceeded an unit sectional allowable stress, make crankshaft instant happens rupture. 3. Adopted step (1) the stoma that because fill,arises to upside of founding steel ingot, excision dimension turns into by original 50 ～ 60mm be more than 100mm, in order to ensure shrinkage cavity complete purify. (2) sets according to heat treatment craft, strict control heats and cooling rate. (3) revises operating rules of original flaw detection of crankshaft of founding of electric broken bits, execute the to steel ingot flaw detection examination before founding, and crankshaft is after finish machining, the flaw detection examination before assembling. CNC Milling
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