The type 440 c stainless steel ring forgings scrap case analysis

The American brand stainless steel ring forgings scrap case analysis of 2019-440 c 12 - 10 Fan Qianwei Xu Zhoujin ( Guizhou aerospace science and technology co. , LTD. ) Approximately 440 c for the brand, the Chinese 9 cr18mo, is a kind of high carbon high chromium martensitic stainless steel, the carbon content, The mass fraction) About 1%. The steel is suitable for manufacturing in corrosion environment and no lubrication strong oxidizing atmosphere of bearings, bushings, valve of fuel attachment bushing and other wear resistant and corrosion resistant parts. Because of its characteristics of high carbon high chromium carbide quantity after heat treatment, good abrasion resistance, good corrosion resistance. In the atmosphere, water, sea water and some acids and salts in aqueous solution has good corrosion resistance. Owing to the high carbon content, the steel production is easy to appear the decarburization, quenching crack and residual austenite, is more difficult to heat treatment of steel. 440 c high carbon high chromium martensitic stainless steel forgings is company accepts that a company with the bearing stress of ring forgings. Two batches of forging the physical and chemical quenching specimen after annealing for a long time, in the quenching process and cracking phenomena have appeared in the process of cryogenic treatment. To find out the fracture failure reason, analyses the quenching crack sample detection. 1. Experiment material and process of the forging forming, heat treatment process system for annealing, quenching, cryogenic treatment, tempering. Annealing: 857 ℃ & plusmn; 14 ℃ heat preservation at 20 ℃ after 6 h/h with the oven to 593 ℃ cold out cold, after annealing hardness under 255 HBW. Quenching: 30 min at 800 ℃ preheat, heating up the second period of preheat to 1000 ℃ for 30 min, up to 1052 ℃ & plusmn; 10 ℃ heat preservation 50 min oil cooled to room temperature. Cryogenic treatment: - 73℃± 11 ℃ cold air cooling to room temperature after treatment. Tempering treatment: ( 232± 8) ℃报; 1. 5 h air cooling. 2. Experimental results and analysis ( 1) Microstructure examination and macro analysis of quenching crack the macroscopic and microscopic morphology of samples as shown in figure 1 and figure 1 a to macroscopic crack images, figure 1 b for the depth of decarburization layer on the surface of the sample pictures, figure 1 c for decarburization layer crack in microscopic images. From figure 1 a shows, crack extending along a longitudinal, micro cracks did not appear around the main branch, as a crack, the crack origin position in the sample surface. From figure 1 b micro-crack metallographic images can be seen that the surface in deep decarburization layer. You can see from figure 1 c, the decarburization layer crack did not appear on both sides, to show that the crack is cooling cracks formed in the process of heat treatment. Organized by figure 2 shows, for hidden acicular martensite + retained austenite + granular carbides + block eutectic carbides. Since 440 c after quenching organization in the presence of large amounts of residual austenite, the retained austenite if not change completely during tempering, in use is slow decomposition, transform into martensite, make the workpiece size expansion, at the same time, the martensitic precipitation carbides can appear shrinkage, make workpiece size change again and again, for the precision equipment and forging is very serious problem, so must be cold processing after quenching, the transformation of retained austenite fully, for the high precision requirement of should also be on the first tempering after cryogenic treatment again. Due to cryogenic treatment retained austenite to martensite transformation, further organization transformation stress caused further increase of tensile stress, the crack of making sample after cryogenic treatment. This is another cause of the 440 c material quenching crack. （ 2) Chemical composition analysis sampling from the quenching crack samples, measured by wet method on the chemical composition analysis, the results are shown in table 1. From table 1 can see, chemical composition AMS5630L standard requirements. （ 3) Mechanical properties analysis of vickers hardness test results are shown in table 2. According to the ASTME384 - test method 11 five numerical average, according to ASTME140 - 12 b conversion calibrate schelometers. it hardness by crackle hardness test result conforms to the technical requirements. 3. The discussion and analysis of the causes of crack is due to surface decarburized layer, organization transformation, first produced in the process of hardening cooling by surface decarburization layer of the decarburization layer of Ms points less Ms point is high, so in the process of quenching martensite structure change in the first place. And quenched martensite is brittle organization, feature is hard and brittle, this group increases with the increase of carbon content brittleness, equivalent to form a layer on the surface of the shell. Beware of begins to cool down the following points to Ms martensite microstructure transformation, due to the change of organization size, make the surface organization produces tensile stress, resulting in surface macroscopic crack. Produce surface decarburization is the result of air cooling after forging and subsequent annealing. For annealing system: 857 ℃ heat preservation 6 ~ 6. 5 to 20 ℃ after h/h oven to 593 ℃ cold air cooling oven, achieve approximately 24 h due to heat treatment for a long time, cause the forging of the decarburization layer of the surface reaches 1 ~ 2 mm. And anatomy of the sample is made by forging a sampling, the decarburization layer forgings without removing or remove incomplete sample cracking in the process of quenching tempering and cryogenic treatment. Therefore, in the forgings anneal after adding a rough machining to remove decarburization layer, forgings and the sample after removal of the decarburization layer again after quenching heat treatment, cold treatment, have not been cracking phenomenon. Due to the high carbon content, carbide material quantity, therefore preheating can reduce the thermal stress and to reduce the deformation and cracking tendency of spare parts. For aviation requirements of high precision and complicated shape, such as forging, to prevent cracking, after quenching can be the first to deal with the stress of about 100 ℃. 4. Conclusions and recommendations, 1) Forging surface decarburization is forging quenching or cryogenic treatment produced when the main cause of cracking. （ 2) Suggest remove decarburization layer before the forging heat treatment, and ensure that don't appear in the process of heat treatment decarburization or carbonization phenomenon, for precision parts can be used in a vacuum or in a protective atmosphere heat treatment. Source: the metal processing ( Hot working) Magazine,