Furnace Tail Weld Failure Analysis of the B35.1-Pressure Vessel

Leakage failure is caused by cracks in the weld joint between the body and flange of a pressure vessel. The container body material is graded as P460NH according to the European standard. It operates at a working temperature of 750 °C and a work pressure of 1.5 MPa. It undergone heating once every half a month. The instrument has been in service for 12 years. The crack is located at the weld joint of the flange connection at the end of the pressure vessel with a significant accumulation of black deposits on the interior surface of the reaction kettle. The medium in the production process is nitrogen, coke steam and asphalt steam mixture (including “S”). Macroscopic analysis, physical metallographic inspection, mechanical property analysis and chemical composition inspection of the failure parts are carried out to determine the cause of fracture failure of the pressure vessel B35.1 furnace tail weld. The results show that there are no detailed corrosion thinning areas in the failure parts, there are corrosion products on the surface and the corrosion cracks are distributed in a spatial network, which has the characteristics of stress corrosion. The corrosion causes should be sulfide stress corrosion caused by environment.


Introduction
Pressure vessel refers to the equipment that has a specific process function and withstands a certain pressure in industrial production.Storage and transportation vessels, reaction vessels and separation vessels are pressure vessels.Generally, the pressure vessel is composed of six parts: cylinder, head, flange, sealing element, opening and connector [1][2][3].In addition, it is equipped with safety devices, meters and internal parts to complete the function of different production processes.Due to sealing, pressure, medium and other reasons, pressure vessels are prone to explosion and combustion, endangering the safety of personnel, equipment and property, and polluting the environment [4][5][6][7].At present, all countries in the world have listed it as an important supervision and inspection product, and the special agency designated by the state has implemented supervision and inspection and technical inspection in accordance with the laws and standards stipulated by the state.
In this paper, the annular failure sample (broken into two parts) is inspected and detected.Through macroscopic observation, fracture analysis, metallograph analysis and energy spectrum detection, the causes of fracture of B35.1 furnace tail weld are discussed, which provides technical reference for the corrosion and protection of related pressure vessels.

Macro Detection
The outer wall of the failure part is bronzed, and no obvious corrosion thinning area is found (figure 1a).The inner wall has a layer of reddish-brown corrosion products and the thickness is less than 1mm.After the corrosion products are scraped off, the inner wall is black (see figure 1b) and there are hard residues on the inner wall in some areas (see figure 1c), and no obvious corrosion thinning area has been invented on the inner wall.The short failure part (figure 1c) exhibits a penetrating crack, while the long failure part shows non-penetrating cracks in certain areas.The crack originates from the inner wall and extends to the outer wall, with a depth of approximately 4mm and a length of about 120mm.(figure 1d).

Chemical Composition Analysis
The chemical composition analysis is carried out using the pass-type alloy analyzer and the test site is the section of the failure part, as shown in figure 2. The test results of the main elements are shown in table 1 below.Base metal and weld are in accordance with the requirements of P460NH standard in EN 10028-3:2009 and the requirements of welding material standard (the measured Mn content of base metal and weld is 1.05 and 1.06 respectively, and the lower limit is 1.10, which is within the allowable deviation of chemical composition in the standard).

Appearance of Fracture
The short failure part is opened along the main crack and the fracture morphology is observed, as shown in figure 3. Figure 3 shows obvious radial pattern, which indicates that is fast and stable crack propagation.A is the convergence of the radial grain that is the crack source.After ultrasonic cleaning of the fracture, the morphology of the fracture is observed under the electron microscope.The fracture is "rock sugar", there are reflective facet planes in the section and many steps between parallel cleavage planes of different heights on the fracture surface, so it is inferred that the fracture form of the sample is mainly brittle fracture.It is inferred that the fracture form of the sample is mainly brittle fracture, as shown in region B in the figure.The uncleaned and cleaned sections are detected by energy spectrum respectively and the detected parts are regions B and C in the figure, as shown in figure 3 and table 2.  The results of energy spectrum analysis shows that there is "S" element in the surface attachment before and after fracture cleaning, which should be the reaction of asphalt vapor carrying with the material to form a compound

Crack Morphology
The section of the failure part is sampled and the crack morphology is observed: the crack is located in the base metal area of the core, that is starting from the inner wall and expanding to the outer wall and the overall extension is "dendritic".Local enlargement of region A in figure 4 shows that multiple cracks are disjoint from each other and the cracks are covered with corrosion products.The cracks expand in a dendritic manner and show a network structure in space with obvious stress corrosion crack characteristics.After corrosion, the cracks are mainly intergranular cracking (figure 4B).

Metallographic Structure
The metallographic structure observation of the section of the failure part is shown in figure 5.The core of the failed part is the base material (figure 5A area) and both sides are welds.The microstructure of the base material is ferrite + pearlite.The microstructure of the heat affected zone is ferrite + pearlite with a width of about 1.3mm (figure 5B).The microstructure of the weld is ferrite + Vickers structure + a small amount of pearlite.The white phase in the weld structure is ferrite, and the coarse needle-like and block-like preeutectic ferrite are distributed along the grain boundary of the columnar crystal.The coarse needle-like Vickers structure grows into the crystal, and there is a large amount of fine needle-like ferrite and a small amount of pearlite in the crystal (figure 5C).

Energy Spectrum Analysis (EDS)
The cracks are detected by energy spectrum plane scanning, as shown in figure 6.From the surface scanning results can obviously observed that the two elements "O" and "S" are distributed along the crack, and the other elements have no obvious anomalies.

Summaries
(1) Macroscopic observation: the outer wall of the failure part is bronzed and no obvious corrosion thinning area is found.There is a layer of reddish-brown corrosion products on the inner wall and it is black after scraping the corrosion products.
(2) Chemical composition: the chemical compositions of the core base material and weld conform to the standard EN 10028-3:2009.
(3) Fracture analysis: under the optical microscope, the fracture shows obvious radial patterns.Under the electron microscope we can find the fracture is "rock candy" with many small reflective planes.The fracture surface shows many steps between parallel cleavage planes of different heights and the fracture is mainly brittle fracture.The cracks are mainly located in the base metal area of the inner wall, and the overall extension of the cracks is "dendritic" which is the intergranular cracking.The cracks show a network structure in space and there are many disjoint cracks in space.The cracks are full of corrosion products.
(4) Metallographic structure analysis: the middle of the failure part is the base metal and both sides are the weld.The weld-zone microstructure is ferrite + widmanstatten structure + a small amount of pearlite.There is no abnormality in base metal, heat affected zone (HAZ) and weld metallography.Energy spectrum detection: two elements "O" and "S" can be obviously observed along the crack in the scanning results of the crack section and the other elements have no obvious abnormality.The main chemical compositions of hard residues are iron and iron oxides.

Failure Analysis
According to GB/T 30579-2014 Damage Pattern Recognition of Pressure Equipment, the damage of carbon steel and low alloy steel in water and hydrogen sulfide environment includes hydrogen bulging, hydrogen induced cracking, stress guided hydrogen induced cracking and sulfide stress corrosion cracking.The failure parts belong to sulfide stress corrosion cracking.
(1) Alloying elements The P460NH failure parts are low carbon steel with poor sulfide corrosion resistance.The corrosion products detect by EDS and XRD are mainly FeSO4 + FeS2 + Fe, which is presumed to be caused by H2S stress corrosion.The air in the cooling zone is humid, and P460NH is more likely to occur in the humid H2S stress corrosion environment.
Fe+RS→FeS+R Sulfur can form sulfide with the iron in the steel and it forming a brittle phase at the grain boundaries of the steel.It is also reduced the plasticity and toughness of the steel.
(2) Temperature Sulfide stress corrosion cracking is usually below 82℃.Although the working temperature of the failure part is 750℃, the cracking area is located in the condensation area and the container is heated once every half a month, which has the sulfide stress corrosion cracking temperature.
(3) CO2 The behavior of carbon dioxide corrosion is different at the cathode and the anode.The continuous dissolution of iron at the anode leads to uniform corrosion or local corrosion, which is manifested by increasing wall thickness thinning of metal facilities or local corrosion damage such as pitting perforation.At the cathode, the carbon dioxide dissolves in water to form carbonic acid, which releases hydrogen ions, mainly of the following types: Anode: Fe→Fe 2+ +2e -; cathode：H2CO3→H + +HCO3 -,2H+2e -→H2.In summary, the reason for the fracture failure of the pressure container B35.1 furnace tail weld is that under the complex environment of bituminous steam and coke steam and the P460NH low carbon steel has poor sulfide corrosion resistance.At higher working temperature and longer service time, the sulfide stress corrosion cracking occurs at the grain boundary of the material and CO2 plays an auxiliary or accelerated role, and finally fracture.The cracks existing on the surface can be completely removed by grinding and observed in the subsequent use process.If the cracks are found to exceed a certain depth, it is not recommended to use.It is necessary to increase the inspection of the surface quality and find surface cracks, bubbles and other defects, which should be cleaned and evaluated in time to avoid greater losses.It is recommended to replace the corrosion resistance of good steel, such as 300 series stainless steel.

Figure 6 .
Figure 6.EDS detection of corrosion products.

Table 1 .
Chemical composition of materials in Region d (wt%).

Table 2 .
Analysis results of main elements of surface energy spectrum of failure effect (Wt%).