Decline in utility boilers, boiler condenser of a petrochemical company pipeline throttling gate valve bonnet the fastening bolts sudden fracture, resulting in a large number of steam leaks caused the emergency shutdown, replace the valve. The medium of the condenser is 108 ° C high-temperature water, and 200 ° C steam mixture, a pressure of about 36kg/cm2. The failure valves model JZ46W, the specification DN50 put into use in 2006, the life cycle of close to four and a half years. No abnormal valve bonnet parts, only the bonnet the fastening bolts connected to the valve body fracture. Valve fastening bolts hex flange bolts, three of the four fastening bolts fracture, a break. Figure 1.
Macroscopic examination
Remove the four bolts from the valve cover cleaning and Macroscopic examination. The bolt fracture site is not in the thread at the root of the end cap, but the bolt. Figure 2. As can be seen from the broken bolt fracture surface, gray color larger areas, a relatively new area smaller. The dark color area should be pre-cracked fracture by oxidation corrosion, a relatively new area should be this sudden fracture fracture [Figure 2 (a)]. The end cap portion of the bolt should be forging forming. Close to the fracture of the outer edge of the end cap portion of the obvious existence of multiple forging crack. See Figure 2 (b).
Bolt fracture micro detection analysis
Observed three breaking bolts and a fracture end caps repeatedly using an ultrasonic cleaner to clean using imported by scanning electron microscopy and energy dispersive spectroscopy analysis of with four fracture surface morphology, analysis of the corrosion product ingredients. As can be seen from the fracture morphology, fracture most of the morphology of the region are the characteristics of intergranular crack along the grain boundary cracking is typical of brittle fracture. Fracture gray color area 200 times the scanning electron microscope, you can see that the intergranular fracture surface is covered with a layer of corrosion products, by component analysis, corrosion products are mainly iron oxide, the presence of the the Cl element and S elements of the corrosion products. Figure 3 and Figure 4.
200 times the scanning electron microscope, the morphology of the fracture fresh small part of the area is not intergranular fracture fresh regional characteristics, but the characteristics of the dimple dimple fracture characteristics of normal plastic fracture. This bolt fracture belongs to delayed fracture, that first crack initiation, followed by crack propagation, and finally a sudden rupture.
As can be seen from the bolt morphology of the outer edge of the end cap area, bolt forging process is not standardized, forging temperature lower bolt end cap parts of upsetting a large amount of deformation, resulting in more forging crack, and after forging No secondary processing to remove these cracks.
The bolt Material Composition detection analysis
Sampled from a fracture on the bolt, the use of direct-reading spectrometer and carbon and sulfur analyzer detection analysis bolts Material Composition. Table 1.
Bolts commonly used in stainless steel 1Cr17Ni7 and 1Cr18Ni9, but standard in accordance with the national standard GB/T1220-1992 (stainless steel grades and chemical composition), breaking the bolt material components do not meet the 1Cr17Ni7 and 1Cr18Ni9 close to lCrl8Mn8Ni5N. However, in accordance with the requirements of GB lCrl8Mn8Ni5N, the material composition of the fracture bolt, containing high amount of C, Cr content is low, which greatly reduces its corrosion resistance.
Bolt microstructure detection analysis
Two samples taken from the broken bolt, water sandpaper to its pre-grinding, fine grinding, grinding, polishing and Wang aqueous solution etched metallographic sample prepared. Detection analysis on a sample microstructure using optical microscopy; were observed and analyzed by scanning electron microscopy of a sample imports forging defects. After testing, the bolt metallographic organization is austenitic grain size graded 6, the organization is relatively coarse.
Forging defects microscopic morphology by bolts can be seen that the surface and interior of the bolts were found larger forging cracks and inclusions, and the fracture and cracks were found in the vicinity of the form of intergranular cracking, which bolts intergranular cracking primarily with forging defects.
Bolt Fracture Analysis and Countermeasures
After the detection analysis shows that the the valve bolt failure parts in the root of the end cap, the failure of the nature belonging brittle delayed fracture, fracture type along the grain characteristics.
(1) bolt breaking the main reason for the bolt material defect exists, the concrete manifestation: ① The bolt material composition does not comply with the requirements of the GB, the high C content, the Cr content is low, reducing its resistance to intergranular corrosion; ② the end cap of the bolt forging forming process is not standardized, low forging temperature, resulting in the roots of the end cap forging cracks and other defects; 3 bolt forging, heat treatment, the root of the end cap to the presence of residual stress, and cooling the grain boundary precipitation of chromium carbonation material, causing grain boundary weakening prone to intergranular cracking.
The bolt fracture secondary factors for ambient temperature fluctuations and medium corrosion factors, specific performance: the valve withstand the temperature fluctuations caused by bolts produce thermal fatigue stress accelerate the crack initiation and propagation.
(2) After the detection analysis shows that for the the valve original manufacturing process, manufacturers of accessories gatekeeper lax pays little attention to quality accessories, valve cover bolts in the process of using a fracture, leading to downtime. Valve bolt fracture immediately carry out investigation, replacement of the valves of the same manufacturer, and lessons learned at the same time, strengthen the arrival of the quality inspection of the valve, the sampling of the key parts of the valve accessories, to ensure that the use of the process is safe and reliable.
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