Carbon in the alloy steel shaft failure analysis

In the 2018 - carbon alloy steel shaft failure analysis 07 - 18 XinYongMu introduction carbon alloy steel after tempering has good plastic toughness and impact resistance to cooperate, because of their excellent comprehensive mechanical properties, so widely used in the engineering construction. My company used 42 crmo forgings as indispensable axial structure in the construction of a project, the workpiece size as Ф 130 long and 500 mm. Heat treatment after cutting the workpiece conjoined test bar, it found that the crack. 1. Materials and process crack shaft machining process for: forging round steel cutting normalizing rough machining quenched and tempered finishing ultrasonic flaw detection. Heat treatment process diagram as shown in figure 1, normalizing 880 ℃ heat preservation 3 small cold of time and space. Conditioning 860 ℃ heat preservation 3 hours when PAG aqueous solution control cooling, 580 ℃ heat preservation. 5 hours from air cooling, to fulfill the requirements of hardness 241 ~ 286 HBS and mechanical performance requirements. After heat treatment on the workpiece with separation test bar, sawing test bar, as shown in figure 2 crack were found after sawing. To observe the crack the overall appearance, will be shown in figure 2 vertical cracks after sawing the crack source as shown in figure 3. In the crack source and take block near the surface of workpiece is analyzed. 2. The analysis process to find out the reasons of artifacts appear in crack, the technological process of the product, crack macro morphology, chemical composition and microstructure were analyzed. （ 1) First analyze the technological process, the forging shaft, a total of four parts, including a joint test bar. The four axis of ultrasonic flaw detection are qualified before heat treatment, heat treatment after sawing conjoined was found during test bar as shown in figure 2 inside crack, which can be judgment is cracks occurring in the process of heat treatment. Ultrasonic testing for the other three pieces of shaft, found no crack. （ 2) Observation form of crack, crack in the first place in the center of the workpiece to form, and then out to the sides form inside crack extension. In general, the workpiece during quenching owing to the comprehensive influence of thermal stress and organization stress, axial parts are mostly formed in the surface tensile stress, heart form a compressive stress, so most of axial parts during quenching crack is formed in the surface of the first crack expand to the center again. The crack source at the center of the workpiece, extend outward to near surface condition is rare, so we take the crack source in the center of the workpiece block for testing, we also reference blocks in workpiece surface take a comparison test. （ 3) First detection of chemical composition, the results are shown in table 1. Excess carbon content of workpiece center crack source, close to the surface of carbon content in the normal range. Other chemical composition are within the normal range. （ 4) Metallographic analysis from artifacts taking longitudinal crack parts of metallographic sample, after grinding and polishing, erosion, with 4% nitric acid alcohol solution under optical microscope to observe the microstructure. Figure 4 a is the crack source 500 times metallographic specimen. Sample in the center of workpiece, the workpiece diameter is 130 mm. We observed in figure 4 a metallographic organization, found that in addition to part of the blocky ferrite, and retain lath martensite position to sorbite and retention of plume on bayesian position to temper that is not a normal ferrite and pearlite mixed group, the heart of the workpiece in disarray, the mixture of brittleness is big, bulky organization fracture resistance is low, if in these area formation during quenching hardening tensile stress, easy to form the quenching crack. Figure 4 b for nearly 500 times metallographic specimen surface. Near the surface of uniform fine tempered sorbite organization, this is normal and tempering treatment group, shows that the heat treatment process is reasonable. 3. Conclusion combined with chemical analysis and metallographic analysis results, because the heart of the local regional carbon content exceeds bid, lead to these areas had a big brittleness when quenching bainite organization, resulting from a heart of abnormal tissue in the quenching process of organizational stress, should have in mind the formation of compressive stress into tensile stress, coupled with the core of the mixture of bulky groups brittleness is big, the fracture resistance is poor, so when the workpiece quenching cracks formed in these parts first source, in quenching process, to the near surface crack extension, thus forming crack in shown in figure 2. Local materials center the harm of the carbon content exceeds bid. Chemicals raw materials into the factory inspection, sampling location will not take to heart, as stipulated in the national standard of such material defects will not be found, so buried a hidden danger for subsequent processing and use, such material defects need to be in the process of steel production.