Due to its good corrosion resistance, stainless steel flange is widely used in fluid pipeline engineering such as petroleum, chemical industry and shipbuilding. As an important part of pipe connection, it has the advantages of easy connection and use, maintaining the sealing performance of the pipe, and facilitating the inspection and replacement of a section of the pipe. In order to find out the causes of corrosion of this batch of eyelashes, prevent recurrence of product quality problems, and reduce economic losses, we performed chemical analysis and metallographic inspections of this batch of eyelash samples.
The chemical analysis samples were cut at the corrosion flange and their chemical composition was determined by the United States Belder DV - 6 spark direct reading spectrometer. The results are shown in table I. According to the technical requirements of the chemical composition of 304 stainless steel in ASTM 276 - 2013 "standard Specialization for Stainless Steel Bars and shapes", the content of chromium elements in the composition chemistry of the failed flange is lower than the standard value.
1.2 metallographic inspection
A longitudinal cross-section sample is intercepted at the corrosion location of the failure flange, after polishing, it is not corroded, observed under the Zeiss metallographic microscope, and non-metallic inclusions are evaluated according to the microscopic inspection method from the GB/T standard grading map {{0}} for the determination of the content of non-metallic inclusions in steel: sulfides are grade 1.5; The alumina is grade 0; The silicates are grade 0; The spherical oxide is 1.5 degrees.
The sample was eroded by iron chloride hydrochloric acid solution and observed under a 100 x metallographic microscope, and it was found that the austenitic grain in the material was extremely uneven, and the grain size level was evaluated according to the GB/t6394 - 2002 average metal grain size determination method, and the coarse grain zone could be graded as 1.5 (see Figure 3); The fine-grained zone can be qualified as level 4.0.
By observing the microstructure in near-surface corrosion, it can be found that the corrosion starts at the metal surface, concentrates at the austenitic grain boundary, and extends into the interior of the material. The grain boundary in this area is destroyed due to corrosion, the bond strength between the grains is almost completely lost, and the severely corroded metal even forms dust, which can be easily scraped from the surface of the material.
The high-power structure of the corroded flange was observed through a 500x metallographic microscope, and its microstructure was cophase particles precipitated at the grain boundary of Austenite + a small amount of ferrite.
2 comprehensive analysis
The results of physical and chemical tests show that the content of chromium elements in the chemical composition of stainless steel flange is slightly lower than the standard value. The chromium element is the most important element in determining the corrosion resistance of stainless steel. It can react with oxygen to produce chromium oxide, form a passive layer and play a role in corrosion prevention. And the content of non-metallic sulfides in the material is high, and the accumulation of sulfides in local areas will lead to a decrease in the concentration of chromium elements in its surrounding areas, forming a chromium-poor zone, thereby affecting the resistance to corrosion of stainless steel.
Observing the grain size of the stainless steel flange, it can be found that its grain size is extremely uneven, and the mixed grains with unequal size in the structure are prone to forming differences in the electrode potential, producing microcells, which leads to electrochemical corrosion on the surface of the material. The coarse and fine mixed grain of stainless steel flange is mainly related to the hot machining deformation process, which is due to the strong deformation of the grain during its forging.
By analyzing the corrosion microstructure near the flange surface, it can be concluded that the corrosion starts at the flange surface and extends inward along the austenitic grain boundary. The high-power microstructure of the material shows that there is more precipitation of the third phase at the austenitic grain boundary of the material. The third compatibility accumulated at the grain boundary can easily lead to its poor grain boundary chromium, causing intergranular corrosion tendency and greatly reducing its corrosion resistance.
The third phase of stainless steel is mainly fine carbide (m 23c 6), σ Concordia δ Ferrites, etc., have a greater impact on the corrosion resistance of stainless steel. The formation temperature of m23c6 precipitation is 450 degree C - 850 degree c, mainly carbide composed of chromium metal, most of which are distributed in the grain boundaries of the crystal, and some in the interior of the crystal and glass defects, since carbide is rich in chromium and can easily lead to chromium poverty in the region; σ The phase formation temperature is 500 degree - 925 degree, in this temperature zone, ferritin partially or completely decomposes. σ Phase, with a chromium content of 42% to 50% in phase 6, is a brittle phase of high hardness that can cause a decrease in the toughness and corrosive properties of the material; δ Ferritin is a high temperature ferritin formed by crystallization when liquid iron is cooled to 1538 degree C. This phase is more brittle, easy to cause cracks during processing, and prone to spot corrosion.
3 comprehensive countermeasures
Through a series of failure analysis of corroded stainless steel flange, the following conclusions can be reached:
(1) Corrosion of stainless steel flange is the result of a variety of factors, among which the first phase precipitated at the grain boundary of the material is the main cause of flange failure. It is recommended to strictly control the heating temperature during thermal processing, do not exceed the upper temperature of the material heating process specifications, while cooling quickly after solid solution, avoiding staying in the temperature range of 450 degree {{ 2}} degree for a long time and avoiding the precipitation of particles in the Third phase.
(2) Mixed grains in the material can easily cause electrochemical corrosion on the surface of the material, and the forging ratio should be strictly controlled during the forging process.
(3) The low content of CR elements and high content of sulfides in the material directly affect the corrosion resistance of the flange, and attention should be paid to the selection of materials with pure metallurgical quality.
