In-depth analysis of ship stern shaft fracture reasons, 'the killer' is actually a galvanic corrosion

In-depth analysis of ship stern shaft fracture reasons, 'the killer' is actually a galvanic corrosion - 2020 04 - 24 ship stern shaft for the final end of the shaft system, its tail and screw connection, is used to transfer torque and thrust. A fishing vessel at sea road 5 a ( Years) Or so, stern shaft rupture suddenly, its material of 35 steel, a diameter of 290 mm, length is 4420 mm, quality is 2491 kg, forging forming by normalizing + tempering after treatment. To find out the Marine stern shaft fracture reasons, the author carries on the inspection and analysis. Physical and chemical inspection 1 macro analysis stern shaft fracture morphology is shown in figure 1, just visible external package stern shaft steel shaft sleeve, the inspection for the surface basic non-rust steel shaft sleeve, its contact with the stern shaft outer corrosion is serious. Stern shaft fracture (central In figure 1 B) Surface is uneven, with metallic luster, did not see signs of oxidation; The rest of the fracture, In figure 1 A) Surface is smooth, no metal luster, there were signs of oxidation. For the convenience of stern shaft and welding a lug on the fracture, as shown in figure 2. According to fracture, macro: the stern shaft fracture of the area has a smooth and flat crack propagation region, Mr Wen line (can be observed Figure 2 a) ； Most of the fracture area is relatively coarse transient breaking area. According to fracture macroscopic morphology characteristics can be determined, the nature of stern shaft fracture is fatigue fracture; According to the laws of the fatigue crack propagation can be determined, the fatigue crack source is located in a region in figure 2. 2 chemical composition analysis using German Bruker company Q8 MAGELLAN direct reading spectrometer of stern shaft fracture, chemical composition analysis of the results are shown in table 1. The chemical composition of stern shaft (table 1 The mass fraction) Results show that the stern shaft material chemical composition meet China classification society 'material and welding specification' ( 2006). And GB/T 699-2015 'high quality carbon structural steel' technical requirements. By the calculation, the stern shaft material carbon equivalent to zero. 52%. 3 chemical performance test in the stern shaft fracture 1/4 longitudinal tensile specimen and impact, respectively for room temperature tensile and charpy impact test, the results are shown in table 2. Table 2, the mechanical properties of stern shaft visible fracture of stern shaft also conform to the mechanical performance of the China classification society 'material and welding specification' ( 2006). And the technical requirements of GB/T 699-2015. Metallographic examination in the stern shaft fracture edges (4 A region) After sampling, polished metallographic specimen by Zeiss Imager. M2M metallographic microscope for metallographic analysis. Observation found that there is a lot of crack, crack from the outside to the axis of stern shaft extension, and the front end is sharp, the tail is dull, the biggest flaw size about 2. 5 mm, inside the crack has a light gray oxide, as shown in figure 3 ~ 4. Stern shaft microstructure of ferrite + pearlite, crack not seen obvious decarburization on both sides, as shown in figure 5 ~ 6. 5 fracture morphology and micro area composition analysis using Zeiss Sigma field emission scanning electron microscopy ( SEM） Of stern shaft fracture morphology observation, found that the stern shaft corrosion layer surface has a large number of micro crack, as shown in figure 7. The Oxford INCA250 spectrometer ( EDS） The fracture surface corrosion products ( Figure 7) Cracks and corrosion products ( Figure 8) Energy spectrum analysis, the results are shown in figure 9. Visible in the corrosion products in addition to the main line of iron, also visible oxygen, chlorine, such as peak line, show that corrosion products mainly for chlorine and oxygen compounds. Comprehensive analysis by chemical composition analysis, mechanical property test results can be seen that the chemical composition, mechanical properties of stern shaft fracture conforms to China classification society 'material and welding specification' ( 2006). And technical requirements of GB/T 699-2015, can rule out the possibility of material causes of stern shaft fracture. By metallographic examination results can be seen: stern shaft for ferrite + pearlite microstructure, the organization is normal, no abnormal normalizing state organization, can rule out the possibility of heat treatment causes of stern shaft fracture. Stern shaft edges have a large number of cracks and crack the decarburization, both sides have no obvious crack is in use process produce and expand gradually. According to the macro fracture morphology analysis results can be determined, the shaft rupture properties for the fatigue fracture; According to the edge crack morphology and composition of corrosion product analysis results can be determined, stern shaft galvanic corrosion happened; Thus can infer the stern shaft rupture caused by galvanic corrosion fatigue. Due to the shaft sleeve material is alloy steel, stern shaft material is carbon steel, both have different potentials, when both combination immersed in seawater, Sea water as electrolyte) , will produce the battery effect, galvanic corrosion, on the edge of the two metal contact area and low potential of 35 carbon steel corrosion quickly, and the potential of high alloy steel shaft sleeve is slow down the corrosion. Once produced, the regional galvanic corrosion under certain conditions can produce different patterns of corrosion, corrosion (with metal corrosion in the round, a little After in-depth micro cracks become) 。 Stern shaft under service condition, on the alternating stress will accelerate the expansion of the micro cracks, when stern shaft section remaining on the stress is greater than the strength of the material itself, the fracture failure will occur. Conclusion and Suggestions to the Marine stern shaft fracture is caused by galvanic corrosion fatigue. Adopted the anode and cathode, or certain protection for different potential of metal fittings processing, can reduce the occurrence of such events. From: 'the physical and chemical inspection physical volume' Vol. 54, 2018. Author: 10 Hou Shuai handsome, engineers, product quality supervision and inspection institute of Qingdao city