Cause and harm of defect diagnosis of the hottest

Causes and hazards of defects in paper cooking pots

causes and hazards of defects

1 causes and hazards of corrosion thinning

uniform corrosion thinning in vapor phase space is due to the precipitation of oxygen in black liquor and organic acid in wood chips after steam heating, forming oxygen corrosion and organic acid vapor corrosion, which causes uniform corrosion thinning of shell in vapor phase space; The insulation layer is damaged or cracked and loosened, forming local air flow, causing dew and beads in the corresponding parts of the container, causing local ulcer corrosion in this part. On the one hand, serious ulcer corrosion will weaken the bearing capacity of the shell, even perforation and leakage, on the other hand, it will cause stress concentration, causing the fire effect of container cracking

2 causes and hazards of "cross" weld and undercut

"cross" weld and undercut are manufacturing defects. On the one hand, the cross weld will increase the welding residual stress, on the other hand, because the intersection of the weld

is heated for many times, the alloy elements are burned seriously, which will reduce the strength of the weld; Stress concentration caused by weld undercut will induce cracks

3 causes and hazards of weld internal defects

weld internal defects are caused by unreasonable welding process, poor welder level, lax implementation of welding process discipline, slag inclusion, porosity, incomplete penetration and other defects in the weld. On the one hand, the internal defects of the weld will reduce the bearing capacity of the weld, on the other hand, it will cause stress concentration and induce cracks

4 causes and hazards of surface cracks

through the analysis of the production process and the macro and micro characteristics of cracks, it can be determined that the cracks are caused by stress corrosion and belong to stress corrosion cracks

(1) there is a large tensile stress, including external stress and internal residual stress. The external stress is the stress generated by the service pressure of the container, and the internal residual stress is mainly the welding residual stress. As the wall thickness of the cooking pot reaches 32mm, its welding stress is large, and the stress relief heat treatment is not carried out after welding (generally, when the wall thickness of carbon steel reaches 38ram, the stress relief heat treatment should be carried out after welding). Therefore, there are large welding residual stresses in the welds and heat affected zones of the cooking pot. It provides stress conditions for stress corrosion

(2) the cooking pot must be pointed out that the material is ct3cii (equivalent to A3), which is a stress corrosion sensitive metal. Black liquor is NaOH with a concentration of ll0 ~ 130g/1 and Na2S aqueous solution with a concentration of 30 ~ 40g/1. The combination of the two forms a specific environment for stress corrosion

(3) stress corrosion mechanism in alkali solution

the stress corrosion mechanism of carbon steel in NaOH solution is the chemical reaction between Fe in carbon steel and steam:

3fe+4h20-fe304+4h2

reaction process towel, NaOH plays a catalytic role, and the process is:

3fe+7na0h-na3fe03+2na2fe02+7h

na3fe03+2na2fe02+4 h20-fe304+7naoh+h

3fe+4h20-fe304+4h2

covers the surface of the steel, Form a protective film. However, in the weld and its heat affected zone, under the joint action of external stress and welding residual stress (in the part with welding defects or poor weld surface formation, it will also cause stress concentration), the local stress is too high, and the excessive stress destroys this protective film. At the same time, due to the enrichment of naott on the surface, Fe304 is dissolved, or their combined action, the initial corrosion - mechanical part crack is formed on the metal surface. The crack causes naott to enrich in the crack and produce electrochemical corrosion from the surface (because the dilute alkali solution can passivate the steel, plus the covering effect of Fe304, the metal surface and the crack side surface become the large cathode of corrosion, while in the crack, the concentrated alkali solution can dissolve Fe304, so the tip area of the crack becomes a small anode). Under the joint action of external stress and welding residual stress, on the one hand, it prevents the formation of protective film at the crack tip, On the other hand, it causes high stress concentration and crack propagation

(4) damage of stress corrosion

when the stress corrosion crack extends to a certain extent, instability propagation will occur, resulting in cleavage brittle fracture of equipment. This kind of brittle fracture often occurs when the working stress is far lower than the tensile strength of the material and the medium corrosion is very slight. There is no obvious macroscopic sign before brittle fracture, which is one of the biggest defects of dangerous parts in boiler pressure vessels

2. Preventive measures

the cooking pot is an important equipment of the J '. Once an accident occurs, the consequences are very serious. In order to ensure the safety of the equipment, the panel industry is expected to usher in a small spring operation, effective monitoring and use measures should be formulated, emergency plans should be made and carefully implemented

(1) all cracks found in the inspection should be polished to eliminate and smoothly transition, and stress concentration should be avoided as far as possible. For those that need to be repaired by welding after grinding, measures should be taken to eliminate the welding at the repaired part. The shortest time for the pinion to rotate for 1 week is the aging to ensure that the normal hammer in the experiment will not touch the pinion to connect the stress

(2) the parts with serious corrosion of ulcer parts should be polished and polished. After polishing, magnetic particle flaw detection should be carried out to find out whether there are stress corrosion cracks

(3) magnetic particle inspection shall be carried out on the inner surface every three to six months. If cracks are found, they shall be eliminated in time or the equipment operation shall be stopped

(4) update and transform the cooking pot as soon as possible to eliminate potential accidents

(5) when ordering cooking pots, the manufacturer should be required to avoid assembly stress or welding residual stress as far as possible in the manufacturing process. Annealing treatment shall be carried out if necessary to eliminate residual stress

cooking pot is an important equipment of paper-making enterprises. Due to the characteristics of its production process, it is easy to cause uniform corrosion and stress corrosion of the equipment. There is no precursor before the occurrence of surface stress corrosion accident. The accident occurs suddenly, and once an accident occurs, it will cause huge loss of life and property to the enterprise. In order to ensure production safety, it is necessary to strengthen the inspection of containers, appropriately shorten the inspection period, and add metallographic inspection items in the inspection to check whether there is a tendency of stress corrosion. If there is a tendency of stress corrosion, a comprehensive inspection should be carried out every year