Failure Analysis of Surface Stellite Alloy Overlayer on Disc of Wedge Gate Valve in a Nuclear Power Plant

Failure Analysis of Surface Stellite Alloy Overlayer on Disc of Wedge Gate Valve in a Nuclear Power Plant

Stellite alloy, commonly known as cobalt-based alloy, is a kind of cemented carbide with Co, Cr and W as the main elements. It can be sprayed on the surface of vulnerable parts by various welding processes, and can also be made into castings [1_4]. Due to its characteristics of wear resistance, corrosion resistance, oxidation resistance and high temperature resistance, it is widely used in various hard coatings or contact sealing surfaces [5 7].
Visual inspection in a nuclear power plant found that the surfacing layer of Stellite alloy on the sealing surface of both sides of a wedge gate valve disc was suspected to fall off locally, and the surfacing layer on the surface of the check valve was in the high temperature and high pressure water environment with B and Li in the primary circuit during its service. The diameter of the valve is about 90 mm, the matrix is cast austenitic stainless steel Z3 CND 19-10M, and the surfacing layer is AWS A5.21ERCoCr-A, which is similar to Stellite 6 alloy. Tungsten Argon Arc welding is used in surfacing.
(GTAW), the thickness of surfacing layer is about 2_3 mm. Based on the observation of the micro-structure of surfacing layer and the surface and cross-section morphology of the suspected dropping area, combined with the detection of local chemical composition, the fundamental reasons for the failure of surfacing layer are analyzed and discussed.

1. Experimental methods

Fig. 1a is a macroscopic photograph of one side of the wedge gate valve disc from the nuclear power plant site. The outer edge of the circular valve disc is the Stellite alloy surfacing layer. Visual inspection shows that the surface surfacing layer is suspected to fall off, as shown by the black arrow position in the figure. The structure of the other side of the disc and the failure characteristics of the surfacing layer are basically the same as that of the other side of the disc. Fig. 1b and C are further morphological observations of the suspected dropping areas of surfacing layer on the front and rear sealing surfaces of the valve, respectively. Clear suspected local dropping areas are observed near the radial middle of the sealing surface. According to the black line in the figure, several samples of surfacing layer microstructure observation, cracking area surface morphology observation and local chemical composition were obtained.
Samples cut from typical parts were cleaned by ultrasonic in anhydrous ethanol and dried by air drum, then energy spectrum analyzer was used.

Ultra 55 scanning electron microscopy (SEM) of EDS was used to analyze the surface morphology and local chemical composition of the suspected exfoliation zone on the surfacing layer surface. In order to observe the metallographic structure, local chemical composition and cross-sectional morphology of the surfacing layer, the cross-section of the sample was eroded by aqua regia after grinding with sandpaper, mechanical polishing and ultrasonic cleaning in anhydrous ethanol, and then observed and analyzed by SEM.

Fig.1 Macroscopic morphology (a) and schematic of sampling (b), (c) for analysis of surface welding overlayer on valve disc

2 Results and discussion

2.1 Macroscopic Inspection
Fig. 2a and B are low-power macro-topographic photographs of the samples cut on the two sealing surfaces of the valve disc, respectively. In the near-middle part of the radial direction of the surfacing layer, i.e. the suspected dropping area in the visual inspection mentioned above, obvious cracks were observed. After the use of the check valve,
Local cracking occurred mainly in surfacing layer, and shedding occurred only in the place where surface cracking was serious. Preliminary inspection showed that there were three cracks in the two sealing surfaces (number 1 3). There were many cracks in each cracking area.

Fig.2 Macroscopic morphology of surface welding overlayer on valvedisc:(a) one side, (b) anotherside

2.2 Observation and analysis of metallographic structure of surfacing layer
Fig. 3a and B are metallographic observations of the cross section of the surfacing layer on the valve surface far from the surface crack area. The structure of surfacing layer is mainly coarse dendrite and equiaxed dendrite. In dendrites, the matrix is mainly gamma-Co, and the eutectic structure between dendrites is composed of gamma-Co and carbides of Fe and Cr. The eutectic carbides between dendrites are characterized by parallel short rods, spheres or petals. This is basically consistent with the metallographic structure of the Stellite alloy surfacing layer reported in the relevant literature [8_10].
EDS was used to analyze the chemical composition at different locations (number 1 4) in Fig. 3b. The results are shown in Table 1. The position of number 1 is the matrix of gamma-Co in dendrite, the content of Co is the highest, followed by Cr and Fe. Number 2 is the eutectic structure of interdendritic carbides of gamma-C O and C R and W, with the highest content of Cr, followed by Co, C and W, and a small amount of Mo. Number 3 is also located between dendrites, but it is mainly rich in W and relatively high in Mo. It should be mainly W carbides. The difference of local chemical composition leads to the difference of contrast between this position and Number 2, which is also located between dendrites. Besides Co, Cr and W, the content of S and Mn in position 4 is obviously higher than that in other locations. It should be a welding inclusion rich in S and Mn.
In the solidification process of surfacing layer, the formation of Cr-rich carbides between dendrites requires a large number of Cr atoms to diffuse from dendrite to dendrite. As a result, poor Cr zone may be formed in the matrix of dendrite near the dendrite. The formation of poor Cr zone is very harmful to the corrosion of surfacing layer and may lead to local preferential corrosion [11] 。 EDS was used to analyze the local chemical composition of the black line in Fig. 4a. The position crossed the complete interdendritic structure and contained the adjacent intradendritic structures on both sides. The results were shown in Fig. 4b. The interdendrite is rich in chromium. At the interface between dendrites and within dendrites, the content of chromium decreases steadily, and there is no obvious chromium-poor zone.

Fig.3 Metallurgical structure of surface Stellite alloy overlayer on valve disc: (a) low magnification, (b) high magnification

2	17.9	54.0	5.7		0.5	8.6	0.2	1.8	/	/
3	30.7	17.3	6.5		0.8	30.0	1.4	6.9	/	/
4	18.1	25.6	4.2		0.7	10.3	0.8	3.5	16.4	15.1

Fig. 4 Chemical composition by line scanning of surface Stellite alloy overlayer on valve disc: (a) SEM image of the surface Stellite alloy overlayer, (b) element distribution along the black line in Fig. 4a

2.3 SEM Surface Morphology Observation and Analysis of Crack Zone in Surfacing Layer
Fig. 5a-c is the SEM low-power morphology observation of two sealing surfaces and three crack areas of the valve disc. Fig. 5B and C are on the same side of the sealing surface (corresponding to positions 2 and 3 in Fig. 2b). The size of the three crack areas are larger, which are 4.5mm x 7.2mm, 5.2mm x 5.5mm and 2.7mm x 3.0mm, respectively. Several obvious cracks can be observed in each region. The propagation path of cracks is approximately continuous and the bifurcation phenomenon is obvious. Although the length of cracks varies, they are all larger than 1 mm, and the longest crack is about 5 mm. The direction of crack propagation in the same crack region is different, approximate vertical distribution, approximate 45 degrees to the radial direction of the valve disc, while the cracks in the same direction of crack propagation are approximate parallel arrangement. In Fig. 5a, the crack propagation path is relatively wide, and the shedding phenomenon can be observed only locally; in Fig. 5B and C, the crack propagation path is relatively narrow and the shedding is not obvious. Fig. 6a-d is a high-power morphological observation of crack propagation paths on valve surface at different locations. Its morphological characteristics and the morphologies of stress corrosion cracking and intergranular or transgranular crack propagation of corrosion fatigue reported in literature are obviously different [128_15]: The crack propagation paths here are locally tortuous, and one or both sides of the crack path are extruded by external forces. The characteristics of downward depression or cliff-like staggering are very obvious (Fig. 6a and b). In addition, there are obvious holes in the crack propagation path, and the dendrite structure of surfacing layer formed during solidification can be observed around it. Similar structures can also be observed on the crack surface along the thickness direction of surfacing layer (Fig. 6C and d). The edges of different spherical dendrites are smooth and dendrite. The interdendritic structure is not obvious, and the crack propagation is mainly along the interdendritic structure. However, the characteristics of interdendritic structure breaking or cracking due to external force or corrosion are not observed in the interdendritic structure. Slip bands due to stress and deformation were observed on the surface of dendrite, as shown by the position of black arrow in figure 6d, which coincided with the working process of the globe valve disc pressing against each other to form a seal during the use of the sealing surface.

Fig.5 SEM morphologies at low magnification of the cracking area in surface welding overlayer on valve disc: (a) surface cracks on one side, (b) and (c): surface cracks on another side

Fig.6 SEM morphologies at high magnification of 4 different cracking areas (a) – (d) in surface welding overlayer on valve disc

In figs. 6B and c, there are also scattered dots with darker colors near the crack propagation paths. Figure 7 is a locally enlarged morphology observation. It can be seen that these dots are actually particles on the surface of surfacing layer. According to the morphology, these particles may be welding inclusions or precipitates in surfacing layer. Although the weld inclusions were observed in the metallographic structure of the surfacing layer in Fig. 3 above, the number of these inclusions was much less than that of the granular substances shown in Fig. 7, so the possibility of these particles being weld inclusions was low. The positions of numbers 1 and 2 are intradendritic and interdendritic respectively, so these particles are distributed in both interdendritic (digital 3) and intradendritic (digital 4 and 5). The chemical composition of the positions indicated by different figures in the figure is analyzed. The results are shown in Table 2. The chemical composition of the digital 1 position is similar to that of the dendritic intra-dendritic structure analyzed in Table 1, but the results of the digital 2 position are different from those of the dendritic inter-dendritic structure analyzed in Table 1. The Co content is higher than that of the Cr content. This may be due to the contact of the high temperature and high pressure water in the primary circuit of the nuclear power plant during the use of the check valve and the dendritic inter-dendritic structure in the surfacing Corrosion occurs in the part. In fact, the morphology shown in Figure 7 is the direct observation of SEM after cutting and cleaning. The surface has not been treated by other methods. It can be seen that the surface of surfacing layer has indeed been corroded between dendrites. The chemical composition of the granular material shown in Figure 3-5 is similar: the content of O is very high, mainly the oxides of Co, Cr, Fe and W. The chemical composition of these locations and the positions between dendrites after corrosion (number 1 and 2) are obviously different, and these oxide particles have the characteristics of embedding in the surfacing layer surface. It is inferred that these oxide particles in solution should be formed by embedding the corrosion surfacing layer surface under the action of extrusion force on the sealing surface. It is further confirmed that the sealing surface of the check valve is greatly extruded by external force in the course of use.

Fig.7 SEM images at high magnification of the cracking region on surface welding overlayer

Table 1 Chemical composition of surface Stellite alloy overlayer on the
valve disc(mass fraction /%)

Position	Co	Cr	Fe	Ni	W	Si
1	53.7	21.1	12.2	1.9	3.1	1.0
2	42.4	29.0	10.7	1.4	5.0	0.8
3	19.6	19.9	11.0	0.7	9.8	2.8
4	25.3	22.5	14.8	1.0	8.5	1.9
5	23.4	18.6	14.3	0.9	7.1	1.3

2.4 Observation and Analysis of SEM Section Morphology of Crack Zone in Surfacing Layer
In order to clarify the causes of surface cracks in surfacing layer, the specimens were further cut according to the cutting method in Figure 8a, and the propagation characteristics of cracks in the thickness direction of surfacing layer indicated by arrows in the figure were observed. Fig. 8b is a macroscopic picture of the left sample section in Fig. 8A after pre-grinding and polishing. The crack propagation path of the section can be observed. Fig. 8c, 9a and B are the SEM morphologies of the crack path, respectively. Similar to the observation of crack surface morphology (Fig. 6): The crack propagation path in the cross section is wider, nearly perpendicular to the surface of surfacing layer, and the depth of propagation in the thickness direction of surfacing layer is about 1.1 mm. Around the propagation path, it can be observed that the surfacing layer formed approximately spherical dendrite structure during solidification, with different dendrite inner groups. The edge of the weave is smooth, the interdendritic structure is not obvious, and the crack propagation is mainly along the interdendrite. However, the characteristics of interdendrite breaking or cracking due to external force or corrosion are not observed in the interdendrite, and the characteristics of downward depression are observed again at the left side of the crack surface in Fig. 9A. Fig. 9b is the front of crack propagation, which mainly consists of semi-continuous or nearly continuous voids, and locally forms continuous banded voids. In addition, at a distance of about 0.3 0.4 mm from the surface, a band-like region with a length of about 2.8 mm and a width of about 0.4 mm, which is approximately perpendicular to the propagation path of the crack cross-section and approximately parallel to the surface, is observed.
Therefore, the crack propagates along the vertical surface and seems to propagate along the direction parallel to the surface, which may be related to the wedge-shaped globe valve and the friction shear action parallel to the surface of the surfacing layer while sealing and pressing. Fig. 10a and B are high magnification topographic observations of the zonal region. The zonal area is also mainly composed of holes. The morphological characteristics of these holes are basically the same as those observed in Fig. 6 and Fig. 9. Compared with the normal structure of the surfacing layer in Fig. 3, it can be seen that the front of the voids is normal interdendritic structure, so the voids are mainly distributed between dendrites and should be the undeveloped interdendritic structure.
Therefore, based on the above analysis, it can be determined that the banded voids observed at present should be due to the formation of crystalline shrinkage voids in the local area of surfacing layer on the surface of globe valve disc [16]. The formation of such welding defects is usually caused by improper welding process or unreasonable selection of welding parameters, such as too fast welding speed or too low welding current, phase transformation shrinkage when liquid phase changes into solid phase in welding pool, or high temperature liquid metal caused by fast arc closing speed at welding end. Rapid cooling and shrinkage eventually lead to the formation of semi-continuous or nearly continuous long shrinkage voids in the local area between dendrites. The strip shrinkage hole is not only perpendicular to the surfacing layer surface, but also parallel to the surfacing layer surface, which is mainly related to the welding process. The serious banded shrinkage hole can not withstand large compression or shear stress due to the lack of internal structure, so when the valve closing seal face is obliquely compressed, the two sides of shrinkage hole stagger and local collapse appear on the surfacing layer surface, resulting in obvious surface cracks. The different width of strip shrinkage holes results in different width crack paths on the surface of surfacing layer (Fig. 5).
In addition, the lack of interdendritic structure leads to poor connection in adjacent dendrites, and local shedding occurs under the oblique compressive force on the sealing surface. Therefore, the crack path observed in the thickness direction of surfacing layer is actually a relatively wide banded shrinkage, which is not a real crack caused by the cracking and propagation of interdendritic structure. Therefore, no obvious cracking characteristics are observed on the surface of the inner dendritic structure of the crack propagation path. Deformation slip bands were observed on the surface of dendrites (Fig. 6d).
Fig. 11 is also a welding defect observed at a distance of 0.4 mm from the cross-section of the surfacing layer to the surface. It should be a welding pore with a size of 80 micron x 130 micron. Wire-like welding inclusions are also distributed in the surfacing layer near the surfacing layer, and both within and between dendrites. The chemical composition of the area shown in Fig. 11b was analyzed by scanning electron microscopy (SEM). The results are shown in Fig. 12. The main inclusions in welding are oxides rich in W and Si.

Fig.8 Cross-sectional sample preparation (a), (b) and (c) SEM images of the cracking area in surface welding overlayer on valve disc

Fig.9 Cross-sectional SEM images at low magnification of the cracking area in surface welding overlayer on valve disc: (a) near the overlyer surface, (b) near the front of cracks

Fig.10 Cross-sectional SEM images at high magnification of the cracking area in surface welding overlayer on valve disc: (a) one area, (b) another area

Fig.11 SEM images of welding defects in surface welding overlayer on valve disc: (a) low magnification, (b) high magnification

Fig.12 Chemical composition of welding defects in surface welding overlayer on valve disc: (a) Cr, (b) Co, (c) Si, (d) W, (e) O, (f) Fe

3 Conclusions And Suggestions

• (1) Metallographic observation and local chemical composition analysis showed that there was no abnormality in the metallographic structure and chemical composition of the Stellite alloy surfacing layer on the valve surface.
• (2) The main reason for the cracking of the Stellite alloy surfacing layer on the surface of the wedge gate valve disc is the improper welding process during surfacing operation, which results in the formation of crystalline shrinkage defects between dendrites in the weld structure, more serious banded shrinkage holes, dislocation and collapse under the oblique compressive force on the sealing surface, and the surfacing layer surface. The obvious surface crack is formed, which belongs to unqualified manufacturing.
• (3) It is suggested that a non-destructive testing method should be used to systematically inspect the surfacing layer of Stellite alloy discs of the same manufacturer or process being used in nuclear power plants, and to replace or repair the abnormal discs, so as to eliminate the potential safety hazards caused by the failure of surfacing layer on the sealing surface in the later period.

Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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