Análise das causas de fissuração das válvulas Incoloy 825
Introduced a company residue hydrotreating plant high-pressure water injection line Incoloy 825 corpo da válvula and welding bevel crack detection and analysis process, from the macro-inspection, chemical composition, metallurgical organization, mechanical testing, fracture, and energy spectrum analysis and other perspectives, discussed, analyzed the reasons for the failure of the Incoloy 825 valve cracking, put forward the corresponding measures to solve the problem.
1. Incoloy 825 valve main parameters
A company residual oil hydrotreating unit Ⅱ series of intermittent injection line shut-off valve (specification DN80 1500LB, casting, valve body material Incoloy 825) in December 2014 inspection found that its butt weld has a soundhole. I was adjusting the process operation and closing the relevant valves and other isolation measures to suspend the intermittent water injection line temporarily. 2015 June Ⅱ series of change agents during maintenance, maintenance units on the sandhole parts of the grinding penetration detection (PT), ready to fill the weld, observed that the valve body and welded bevel parts of the more cracks. Immediately contacted the fabricante de válvulas to send technicians to the scene, in the presence of the manufacturer’s personnel, to expand the scope of the valve inspection, similar parts of the device with the manufacturer of Incoloy 825 valves (a total of 20 units) of the valve body and welded bevels are carried out on the PT, were found to be a large number of surface cracks. At the same time, the cracked parts of the relevant Incoloy 825 valves were lightly polished (polished to a depth of about 2-3mm), and the cracks were not eliminated. The main parameters of the cracked Incoloy 825 valves are shown in Table 1.
Table.1 Main parameters of Incoloy 825 valves
|Nome||Number of valves/set||Position||Médio||Operation condition||Valve specifications|
|Y-shaped globe valve||2||Series I and II continuous water injection lines||Demineralized water||20 MPa, 50 ° C||DN100|
|Y-type stop check valve||2||Series I and II continuous water injection lines||Demineralized water||20 MPa, 50 ° C||DN100|
|One-way valve||2||I, II series circulating machine outlet anti flying line||Circulating hydrogen||20 MPa, 90 ° C||DN250|
|Y-shaped globe valve||4||Series I and II intermittent water injection lines||Demineralized water||20 MPa, 50 ° C||DN80|
|One-way valve||2||Series I and II intermittent water injection lines||Demineralized water||20 MPa, 50 ° C||DN80|
|T-type globe valve||4||E107- I/II tube pass inlet and outlet differential pressure lead||Hot high separation gas||16 MPa, 220 ° C||DN50|
|Válvula de gaveta||4||E107- I/II tube pass inlet and outlet differential pressure lead||Hot high separation gas||16 MPa, 220 ° C||DN50|
After observing the cracks, the equipment professional immediately organized maintenance units to replace the old valves were sampled and sent for inspection and commissioned a qualified third-party testing agency, one of the Y-type disconnecting valves (Ⅰ, Ⅱ series of intermittent water injection line) cracking causes were analyzed.
2. Cracking cause analysis
2.1 Macroscopic inspection and penetration testing
Through the valve body (local arbitrary points) and welding bevel parts of the PT, found that the valve body and welding bevel side have more cracks. There are single cracks and dense cracks. Dense cracks in Figure 1, A and B, A at an area of about 40mm (longitudinal) × 50mm (ring), the longest one about 18mm; B at an area of about 40mm (longitudinal) × 20mm (ring), the longest one about 7mm. in the body of the válvula is also found in a large area of pockmarks, pockmarks about 1.5mm in diameter, depth of about 1.5mm.
Figure.1 Macro morphology of valve cracks
2.2 Chemical composition analysis
A sampling of the valve body for chemical composition analysis results is shown in Table 2. Table 2 shows that the chemical composition of the valve body meets the requirements of the relevant standards.
2.3 Tensile test
From the valve body to intercept the tensile specimen, tensile test results are shown in Table 3; Table 3 shows the tensile strength of the valve body to meet the requirements of the relevant standards.
Table.2 Chemical composition analysis results w, %
|Standard value||≤0.05||≤1.00||≤1.00||≤0.030||≤0.030||19.5 -23.5||38.0 -44.0||2.5 -3.5||1.50-3.50||0.60-1.20|
Table.3 Tensile test results
|Valve body No.1||259||559||36||Normal Temperature|
|Valve body No.2||267||589||31||Normal Temperature|
|Standard value||≥240||≥520||≥30||Normal Temperature|
2.4 Metallographic analysis
From the valve body A and B were intercepted full-thickness metallographic specimens, a thickness of about 26mm, see Figure 2.2 can be seen from the metallographic specimen by the chemical reagent erosion of the surface of the obvious traces of filler weld, measured the deepest filler weld about 7mm.
Figure.2 Metallographic specimen sampling site and macro photo morphology
2.4.1 JX1 sample
JX1 specimen micro-morphology is shown in Figure 3.3 can be seen from Figure No.1 part is located in the valve body near the outer wall, for the patch weld, there are along the crystalline micro-cracks; No.2, No.3 parts of the valve body and the patch weld channel transition zone and nearby, there are along the columnar crystal grain boundaries of the expansion of the cracks; No.4 part is also located in the valve body, can be seen in the casting of the defects are evident (loosening).
2.4.2 JX2 specimen
JX2 specimen microscopic morphology is shown in Figure 4.4 can be seen, No.1 part of the filler weld; there are obvious along the columnar crystal extension cracks; cracks can also be observed in the solidification fracture characteristics; No.2, No.3 and No.4 parts of the valve body and the filler weld transition zone and nearby, there are more along the crystalline cracks; No.5 part of the valve body is located in the casting defects are obvious (loose).
Figure.3 JX1 specimen
Figure.4 JX2 specimen
2.5 Hardness test
Vickers hardness (HV) test on the metallographic specimens JX1 and JX2, test results are shown in Table 4. The hardness of the filler weld metal is 176-207.5, the filler weld and the valve body transition zone are 173.4-211.7, and the hardness of the valve body base material is 155.2-199.6. Table 4 shows that the hardness test meets the requirements.
Table.4 Hardness test results
|JX1||Repair weld metal||198.9,202.9,186.6,191.3,176.6,186.6|
|Repair welding and valve body transition zone||191.4,186.2,190.2,211.7,179.9,183.5|
|Valve body base material||167.3,171.8,167.6,167.7,166.0,155.2|
|JX2||Repair weld metal||204.7,200.4,204.1,198.9,207.5,205.0|
|Repair welding and valve body transition zone||181.8,192.0,165.6,175.1,173.4,186.0|
|Valve body base material||199.6,188.3,168.2,188.0,186.4,178.9|
2.6 Fracture analysis
A and B intercepted fracture specimens numbered XM1 and XM2 from the valve body. Crack, fracture macro morphology is shown in Fig.5. From Fig.5 can be seen, the cracked part of the dark brown fracture, silver-white for the artificial knocking off surface, crack fracture surface from the outer surface of the longest (deep) of about 6mm, the fracture surface is uneven.
Figure.5 Macroscopic part of the fracture
The microscopic morphology of the fracture is shown in Fig.6. As seen in Fig.6. However, the fracture surface has been oxidized, the structural morphology of the dendritic crystal can still be seen, and the fracture surface at this place is a typical welded thermal crack morphology. The artificial cracking surface is a tough fossa morphology.
2.7 X-ray spectral analysis
XM2 fracture specimen selected crack fracture surface parts for spectral analysis; the analysis results are shown in Table 5.5 can be seen from the table that there are more Mn elements on the fracture in the grain boundaries of the enrichment; this enrichment will reduce the melting point of the metal in the grain boundaries of the low melting point of the phase.
Figure.6 Micro-morphology of the fracture
Table.5 Energy spectrum analysis of XM2 fracture specimen w, %
2.8 Main physical and chemical analysis results
- (1) PT inspection results show that there are cracks and pits on the outer surface of the valve body, and the longest crack is about 18 mm;
- (2) The chemical composition of the valve body meets the requirements of relevant standards;
- (3) The tensile performance index of the valve body can also meet the requirements of the relevant standards;
- (4) The hardness of the valve body meets the standard requirements;
- (5) The metallographic and fracture analysis results show that the cracks on the surface of the valve body have typical welding hot crack characteristics. The defects in other parts of the valve body are typical casting defects (loose or shrinkage);
- (6) The results of energy spectrum analysis show that the Mn element is seriously enriched on the fracture, and this “enrichment” will reduce the melting point of the metal.
3. Conclusion and measures
Comprehensive the above tests, analysis results show that the Incoloy 825 valve body cracks have two kinds of nature: the casting defects and the valve body surface welding thermal cracks.
Due to high chromium-nickel corrosion-resistant alloys (including austenitic stainless steel), liquid mobility is poor, so the casting shrinkage is more difficult, prone to shrinkage and porosity casting defects. At the same time, the welding process is also prone to weld thermal cracks. The results of anatomical analysis of the valve inferred that there are many casting defects after casting the valve body; the manufacturer only for some of the more serious defects in the parts of the filler welding, resulting in both parts of the surface defects remaining, while in the filler weld repair parts of the weld thermal cracks.
Incoloy 825 high-pressure water injection valve body surface cracks are casting defects and filler weld thermal cracks, both manufacturing defects, device shutdown during the change of agent for overhaul and replacement.
Author: Zhang Hui