Analysis of failure cases caused by manufacturing and installation of austenitic stainless steel flanges

Analysis of failure cases caused by manufacturing and installation of austenitic stainless steel flanges

Two typical cases in chemical plants were cited, which caused the failure of stainless steel flanges due to manufacturing and installation factors respectively. After analyzing and studying the failed parts, it is found that the surface defects in the forging process, as well as the improper selection of welding materials or processes, are important factors leading to the later failure of stainless steel flanges.

1. Research background

Austenitic stainless steel flanges are widely used in the chemical energy field due to their outstanding mechanical properties and corrosion resistance. In the reality of chemical production, flange interface is often the most prone to leakage in the pipeline connection, if the pipeline transport medium is toxic and harmful or flammable and explosive media, the failure of these flanges may bring incalculable losses. A considerable part of these failures is due to the hidden problems left in the flange manufacturing or installation, so the study of austenitic stainless steel flange in the manufacturing, installation process improperly caused by the failure of austenitic stainless steel flange manufacturing and installation, long-term stable operation is of great significance.

2. Flange forging process and failure factor analysis

Compared with ordinary carbon steel, austenitic stainless steel has higher requirements for forging performance and process, mainly due to the low heat conductivity of austenitic stainless steel, high forging temperature requirements, strong overheating sensitivity, high high temperature deformation resistance, etc., all of which bring difficulties to the forging of stainless steel. In order to ensure product quality, the forging process of stainless steel flange must be strictly controlled [1].
Intergranular corrosion is an important cause of metal material failure of austenitic stainless steel, which is due to the precipitation of carbon from the supersaturated solid solution and the formation of the compound chromium carbide with the important corrosion-resistant element chromium in the intergranular aggregation, and the increase in carbon content will enhance the susceptibility to intergranular corrosion.
Austenitic stainless steel in the initial heating stage of poor thermal conductivity, if the beginning of the forging temperature is too high heating too fast is easy to high-temperature ferrite, grain growth, at this time, the austenitic stainless steel corrosion resistance, impact toughness, creep properties will be produced by the decline. Forging temperature should be 1150°C – 1180°C. The final forging temperature can not be too low, the final forging temperature is too low will lead to a decline in material plasticity, continue to forging may lead to cracking, so the final forging temperature of 850°C – 900°C. For materials larger than 120mm in diameter, should be fully preheated to ensure uniform heating of the material, and then heated to the initial forging temperature.
For high neck flange, the neck taper leads to abrupt change in shape, forging cracks, loosening and other defects are easy to occur in this part. Due to the abrupt change in cross-section flange neck tapered outer surface may appear in the forging process folding defects. According to the characteristics of the production process and ASTM A961 [2] standard requirements analysis, austenitic stainless steel forging flange blanks in the forging process there may be the formation of oxide or other inclusions on its surface, if the surface of the blanks oxide and other impurities are not cleaned in a timely manner, the surface of its blanks attached to impurities such as impurities and oxide tend to mosaic into the metal matrix inside, and easy to form folding defects [3]. As the forging process produces a relatively high melting point of metal oxides, and the metal matrix is not easily combined together, these inclusions and oxides will destroy the continuity of the austenitic stainless steel forging material, usually this defect around the material is extruded will appear a certain plastic deformation. Folded defects near the organization because of the direct exposure of large areas at high temperatures in the air atmosphere and further oxidation corrosion, such material discontinuity is also an important cause of failure of stainless steel flanges [4].

Figure.1 Flange forging defects failure physical diagram
Figure 1a shows the failure of a chemical plant flange, which was found to have cracks on the surface after PT inspection. The flange is made of 304 austenitic stainless steel. To determine the cause of the crack will be flange along the axis of the direction of half cut, as shown in Figure 1b. After dissection found a series of loose holes near the flange welds as shown in Figure 1c. These loose holes are in the stainless steel casting organization in the process of solidification, liquid stainless steel volume in shrinkage is not effectively replenished and the formation of scattered holes. According to ASTM A961 for chemical analysis and mechanical properties testing, the flange’s mechanical properties, chemical composition are in line with the requirements of ASTM A182 standard. Check the flange quality certificate shows that the flange production quality control process although in line with the requirements of the ASTM A961 standard, however, the flange factory surface penetration flaw detection ratio shows: PT inspection is in accordance with the same furnace batch flange 10% random sampling, flange surface crack type defects there is a risk of missing inspection.
In summary, the case of austenitic stainless steel flange neck taper transverse cracks belong to forging defects, the manufacturer of the flange factory surface quality before the omission of inspection and acceptance of the pipe fittings before installation is not in place, which is also the cause of defects containing sparse was not inspected and put into the construction site [3,4].

3. Flange welding process and failure factor analysis

In order to improve the reliability of flange and pipe connection, the general use of welding method of connection. Fusion welding under the action of high-temperature heat source, the weld on both sides of a certain range of organization and performance changes in the region called the heat-affected zone. For austenitic stainless steel forgings, after welding treatment is equivalent to a slow cooling after high temperature treatment, chromium carbide precipitates from austenite and aggregates in the intergranular. This process also enhances the susceptibility of the material to intergranular corrosion.

Figure.2 Flange welding defects failure physical diagram
Figure 2 is a coastal chemical plant failure flange physical diagram, staff inspection found that the flange has leakage. The flange will be cut off the naked eye can be seen in the weld heat-affected zone near the side of the forgings produce cracks. After cutting the flange along the axis, the crack was found to run through the entire thickness of the section. After testing the material at the crack of the forging, it was found that there were obvious signs of carbide precipitation at the lattice edge of the heat-affected zone of the flange, observed under magnification, the crack extended along the grain boundary, there were dendritic cracks around the weld, and chloride ions were found on the fracture. It is inferred that the austenitic stainless steel flange for stress corrosion cracking. Stress corrosion cracking has three important influencing factors: tensile stress environment, corrosive media and sensitive materials.
From the stress point of view, the flange is subject to stress mainly: flange forging and residual stress generated by the weld, the pipeline medium working pressure generated by the radial tensile stress, pipe insulation conditions and media factors generated by the temperature difference stress. The working pressure of the gas in the pipeline is small, only 5.5 kg, and it produces negligible tensile stress in the axial direction of the pipeline. And the operating temperature of the pipeline has been stable at about 37 degrees Celsius, and there is no great temperature difference between various parts, so the effect of temperature stress can also be ignored. Judging from this, the flange is mainly affected by the residual stress caused by welding, which is an important condition causing stress corrosion cracking.
Flange cracking from the outside, so it is mainly from the external corrosive medium to the flange cracking influence. The plant is located in the seaside, due to rainfall and air convection factors lead to the flange more vulnerable to the influence of chloride ions. Due to evaporation aggregation, the concentration of chloride ions on the flange surface will rise, forming a stress corrosion environment [5]. Numerous experiments have shown [6] that the critical chloride ion concentration varies more significantly with the influence of material and medium temperature.Birchon et al. concluded that chloride ion concentrations in the range of 0.2-10 ppm can cause stress corrosion cracking. When chloride ions for some reason produce the phenomenon of enrichment or concentration, even if the ambient chloride ion concentration is very low, there is also the possibility of stress corrosion fracture accident [7].
Pressure pipeline and austenitic stainless steel forging flange welding process, certain areas of the heat-affected zone will inevitably stay in the austenitic stainless steel sensitization temperature range for a longer period of time, while the weld near the forging side of the heat diffusion conditions are relatively poor, making the degree of material sensitization relative to the pipe side of the sensitization more serious. Although stainless steel has a certain corrosion resistance to chloride ions, but if not solid solution treated, or sensitized by welding, it will precipitate carbide at grain boundaries, resulting in grain boundary poor chromium, sensitive to intergranular corrosion, and prone to stress corrosion cracking along the crystal under the action of residual stress and chloride ion corrosion in welding. [8]

4. Conclusion

In the field of welding operations on austenitic stainless steel flanges may lead to local sensitization of austenitic stainless steel materials, precipitation of a certain amount of grain boundary compounds, resulting in grain boundary poor chromium, making the material grain boundary strength and corrosion resistance is reduced. The precipitation of carbide on the grain boundary makes the chromium-poor area near the grain boundary in the weld residual stress, pipe system stress, and external corrosive media under the combined influence of stress corrosion cracking. It is recommended to use ultra-low carbon 304L welding rod for welding, can avoid reducing the formation of intermetallic compounds. Welding as far as possible to take argon arc welding, to avoid the generation of rework mouth. Do a good job of surface quality control before installation to ensure the long-term stable and safe operation of this type of vessel.
Authors: Dong Liang, Chen Yi, Liu Chongyang

Source: China Stainless Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

Reference:

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