Xiao Guang Niu

Mechanism Analysis of a Boiler Economizer Corrosion

Reasons for boiler economizer tube corrosion were discussed based on results of macroscopic inspection, metallurgical microstructure examination, SEM and EDS fracture analysis. Results showed that the tube corrosion was due to sulfur corrosion that resulted in matrix exposure and led to further oxygen corrosion.

Analysis and Suggestions on Leakage of Boiler Low Temperature Reheater

Leakage causes of boiler low temperature reheater were discussed by means of macroscopic inspection, chemical composition analysis, metallurgical microstructure examination, SEM and EDS fracture analysis. Results showed that the reheater leakage occurred for cracks originating from the fillet seam of ear plate. The main causes lie in improper structural design of the ear plate welding directly on the reheater elbow where suffered from complicated stress condition, wall thinning and out-of-round. In addition, as a weak zone, heat effect zone of the fillet welding seam was more likely to crack under fatigue stress for frequent unit start and stop and pipeline vibration. Installation stress existed between the ear plate and support block also contributed to the cracking. Suggestions were provided to avoid similar accident based on the comprehensive analysis of leakage causes.

Transverse Vibration Testing Research for 6.8 Grades Bolted Joint Used on Transmission Tower

Five groups of vibration experiments were carried out on the 6.8 grades and M20 bolts and nuts that used on transmission tower. The test result shows that the basic way to improve the anti-loosening property was increasing the initial preload. Both increasing initial torque and appending lubrication can obviously increase the initial preload, however appending lubrication may reduce its anti-loosening property under the situation of same preload. In the vibrating process the second stage plays a decisive role for anti-loosening property, it can be expressed as reduced rate “K” of the preload.

Research on Characteristics of Joint Point of Power Transmission Tower

Based on tension tests for tower bolt, it was found that the pretension increased with the extension of torque retention time, and manifested characteristics of decreasing at early stage then increasing at later stage during the tension process, that provided guidance for bolt fastening.

Fracture Analysis on Drainage Pipe Socket of Boiler

Rupture reason of boiler drainage pipe socket was analyzed by means of chemical composition analysis, macro-observation, metallurgical and SEM microstructure examination. Results showed that the rupture of the drainage pipe socket was a result of creep cracking initiated in intercritical zone of HAZ due to long period operation under high temperature. During creep process, carbides precipitated on fine grain boundaries of intercritical zone and grew up gradually into new coarse carbides as favorite nucleation site for creep void. With the growth, coalesce and connection of creep voids carries on, micro-cracks formed on original austenite boundaries and finally led to fracture of weld joint.

Failure Analysis on Boiler Industrial Extraction Steam Pipe

Failure reason of boiler extraction steam pipe was discussed by means of macro-observation, chemical composition analysis, metallurgical and SEM microstructure examination and EDS analysis. Results showed that the crack initiated from original corrosion flaw on outer wall of the extraction pipe, propagated rapidly under large pressure caused by water-hammer due to no drainage design in the unit pipe system until final bursting.

Failure Analysis on Strain Clamp on 220kV Transmission Line

Failure analysis on a strain clamp cracking on 220kV transmission line was carried out based on results of macro-observation, mechanical property examination and EDAX analysis. It is concluded that the main reason of the clamp failure lies in incomplete surface cleaning during wire crimping that cause large current flowing through steel cores, heating them and reduced their strength. In addition, oxidation due to residue water for insufficient baking also contributes to the failure. Finally suggestions were put forward.

Research on NDT Technique for Thick-Walled Metal Parts of Power Stations

A NDT technique suited to inspecting thick-walled metal parts of power stations is comprised of time-of-flight-diffraction technique, Pulse-echo technique and magnetic particle inspection technique or dye penetrate technique. Flaw sizing accuracy and probability of detection can be improved greatly with application of the technique, and the risk of components` failure can be decreased greatly.

Research on Welding Repair for Tee Joint Crack of Final Superheater Header of 300MW Station Boiler

Through methods, such as microscope analysis, metallographic examination, the crack located nearby the fusion line between tee joint and outlet steam pipe of subcritical power station boiler final superheater header is reheat crack (the main crack occurred in the heat affected zone and extended along the fusion line and grain boundary in HAZ. There are many small cracks paralleling with main crack). The base metal material (12Cr1MoV) and excessive welding residual stress are the primary inducement of crack. In repair crack, the measures of reducing residual stress, such as lower welding heat input, reducing thickness of welding layer, hammer blowing, assigning reasonably heat treatment power ,lengthening holding time and so on, were taken. The welding repair succeeded greatly.

Failure Analysis on Tube Socket Crack of Supercritical Boiler Water Wall Header

Through methods, such as microscope analysis, mechanical analysis, chemical analysis, metallography examination and scanning electron microscope etc, the causes resulting in lots of tube socket cracks of supercritical boiler water wall header (15CrMo) have been analyzed. The result indicates failure of the supercritical boiler water wall header tube sockets are the ductile fracture and belong to thermal stress crack. The unreasonable structure, excessive thermal stress between header and sockets are the primarily inducement of crack failure.