Keywords: spherical tank valve: austenitic stainless steel; bolt cracking; causes; measures A set of transmission station spherical tank flange valve with stainless steel bolts, the structure shown in Figure 1, the other tank also use the same stainless steel bolts . Because these connections are mostly used in the atmosphere, there is no thermal protection, from the material point of view the choice of stainless steel as a bolt material is feasible. The group of bolts a total of about one year after the discovery of five fractures, one cracking, the other a crack found flaw detection. Other similar storage tanks also use the same type of stainless steel bolts, but the type of stainless steel is unknown. The stainless steel bolts have a 37.5mm long M16 thread on each end and a smooth cylindrical 15mm long in the middle. The causes of cracking of austenitic stainless steel bolts are analyzed and discussed, and the measures to prevent bolt breakage are put forward.
A macro analysis 1.1 bolt geometry measurement. M16 bolt failure for the two ends of a length of 37.5mm, the middle 15mm long smooth cylindrical, with a tooth top area of 25.00mm2; root area of 21.71mm2; the middle of the cylinder area of 23.09mm2. From the marked area of bolt variable cross-section at the point of view, the minimum area of the root part of the bolt, the maximum force, the working stress than the middle of the bolt about 6%.
1.2 Bolt chemical composition determination. The chemical composition analysis of the bolt results in (%) 0.20C, 0.80Mn, 0.70Si, 0.015S, 0.024P, 17.10Cr, 8.20Ni and 0.18Ti, so the bolt does not belong to 1Cr18Ni9 or 1Crl8Ni9Ti. Its carbon content is higher than the national standard and the alloy element Ti is lower than the national standard. Description of the bolt material is the market of stainless steel scrap by electric furnace mixing.
1.3 Fracture macro observation of the bolt. Bolt cracks or fracture occurred in the middle of the bolt smooth cylindrical parts. Broken or cracked bolts did not occur obvious plastic deformation, fracture in different parts of the same cross-section, constitute a step-shaped fracture, the fracture is covered with yellow-brown corrosion products.
2 microscopic observation of the fracture bolt 2.1 Microstructure observation of the bolt Take the axial section of the bolt, made of metallographic sample, polished and etched microstructure. It can be seen that the bolt is austenitic forged steel with fine grains and a grain diameter of 5 to 30 μm. The grain boundaries are surrounded by fine chains of carbides. Bolt is after solution treatment, but the carbide is not completely dissolved into it, so the solution is not sufficient. Inclusions with oxides and titanium nitride based.
2.2 Determination of microhardness As the bolt is a variable cross-section components, it is necessary to determine the different parts of its microhardness, Figure 2 shows the determination of microhardness.
From the measurement results, the microhardness of the bolt gradually decreases from the outside to the inside, in which the hardness of the thread root is highest (HRC 43.2) and lowest in the center (HRC 30). The hardness of the smooth section is lower than the thread section 6.5HRC.
2.3 Crack morphological observation Fracture bolt cut along the axial direction of the fracture found on the secondary cracks. At the same time, an expanded crack is found in the same direction almost in the same direction as the fracture. The beginning of the crack is relatively straight and the tail has a secondary crack, which is a typical feature of stress corrosion cracking. Secondary cracks at the fracture are also caused by stress corrosion and the fine grains are clearly visible on polished specimens (Fig. 10). Through the microscopic observation of section and section, it can be found that the bolt fracture belongs to stress corrosion cracking, and the crack expands in the way of intergranular stress corrosion cracking under the stress. Under the electron microscope observation of the fracture features, we can see the brittle fracture morphology on different levels of the fracture, which shows that there are many fracture sources in the fracture of the bolt, and the fracture surface is not smooth, there are many secondary cracks above. At the same time can be seen fine along the crystal crack. Cross-sectional observation shows that the fracture is caused by corrosion and mainly breaks along the crystal.
2.4 Analysis of corrosion products Fracture bolt fracture surface covered with yellow-brown corrosion products, its energy spectrum analysis. It is found that the corrosion products covered by the fracture surface are mainly O, Fe, Ni, Mn, Cr, S and Cl. The content of S in the corrosion products is more than that of Cl, and the content of S in the corrosion products in most areas exceeds 5%, while the content of Cl is about 3%. The content of Cl in the local part of the fracture exceeded S.
3 Discussion According to the above macro observation and microanalysis, it can be known that the cracking or breaking of the ball valve bolt is caused by the stress and the corrosion factors such as S, Cl, that is, the stress corrosion cracking (SCC). The axial cross section of the spherical tank valve bolt found that the bottom of the bolt showed a ribbon-shaped microstructure, indicating that the thread is formed by rolling, the root of the thread with compressive stress on the cross-section of the microhardness determination The result also confirms this point. Austenitic stainless steel is not sensitive to SCC due to the large compressive stress. The smooth section in the middle of the bolt, there is no such microstructure after rolling, hardness is low, the role of stress in the stress prone to stress corrosion cracking. This is the main reason why SCC cracking occurs in the smooth center of the bolt, not in the heavily threaded part. At the same time the c bolt material content exceeded, Ni content close to the lower limit, Ti content lower than the national standard have left SCC hidden trouble. In short the presence of SO2-4 and Cl-, the tensile stress suffered by the bolt and the substandard material composition of the ball valve for the stress corrosion cracking of the valve provides an extremely favorable condition, leaving the bolt intergranular stress corrosion cracking.
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