Failure analysis on fracture causes of stainless-steel studs in seawater system of nuclear power plants

Nine of the twelve B8M stainless steel studs on the flange of a seawater pipeline isolation valve of a nuclear power plant were encountered with premature fracture during service. To find out the cause of the fracture failure, the fracture studs were comprehensively detected and analyzed by means of appearance morphology analysis, chemical composition analysis, mechanical properties testing, microstructure testing, and fracture micro-area analysis. The results show that the main reasons are as follows: (1) The solid solution temperature of the raw materials for processing bolts is low, so the carbides are not fully dissolved and distributed in a mesh along the grain boundary. Thus, the chromium-poor near the grain boundary forms, resulting in the mechanical properties of the bolts weakening. According to the experiment, it was found that the corrosion resistance of the bolts is weak due to the low nickel content. Carbon easily precipitates at the grain boundary and forms Cr23C6 with chrome, which weakens the bonding strength of the grain boundary and causes the bolt to break along the grain.


Introduction
Nuclear power plants use seawater to cool the second-circuit condenser.During the daily operation and maintenance of the nuclear power plant, the flange connection studs of the electric isolation valve, located at the outlet of the seawater pool drainage pump on the inlet side of the condenser, were found to be broken.When the maintenance personnel used an open wrench to remove the inlet studs of the electric isolation valve studs, the outlet of the seawater pool drainage pumped on.In the meanwhile, the fractures occurred on the inlet side of the condenser, and the bolt bite, fracture, and wire tooth damage occurred, among which nine of the twelve B8M stainless steel studs, as shown in Figure 1.The flange specification of the isolation valve is PN16 DN300, and the double-headed studs are M24.The thickness of the flange is 32 mm, and the double-headed stud and the nut are made of A193M-B8M and A194M-8M, respectively.In order to prevent the recurrence of such incidents and ensure safe operation, it is necessary to analyze the cause of this failure case.

Visual inspection
The fracture morphology of the broken studs and fractures is shown in Figure 2. It can be seen that the studs are all laterally cracked in Figure 2a.The cracking site is located in the middle of the light rod of the double-headed stud.There are axial cracks of varying weight on the surface of each bolt concentrated in the light rod area.Individual axial cracks are located at the intersection of the light rod and the thread, and the direction of crack propagation in the transverse part is at an angle to the axial direction.Besides, there are a small number of dendritic bifurcated cracks on the main crack.Further observation found that there are obvious yellow-brown corrosion products near the bolt cracks.At the same time, the surface of the cracks is semi-enclosed and extends from the surface to the inside to the axis.It is also found that there are scratches or rough surfaces at the cracks.As shown in Figure 2b, it can be seen that the surface of the fracture is covered with a layer of gray-black corrosion products.The cross-section of the fracture is flush and stepped, with no obvious necking, showing a typical brittle fracture morphology.

Chemical compositions
Four of the broken studs were prepared for chemical composition analysis, and the analysis results are shown in Table 1.The chemical compositions of carbon and nickel elements do not meet the requirements of A193M-B8M in ASTM A193/A193M "Alloy Steel and Stainless-Steel Bolt Materials for High Temperature" [1].The nickel content is between 9.85 and 14.15 wt% within the composition requirements of A193M-B8M, so the nickel content is unqualified.In addition, the content of carbon in the broken studs is higher than 0.09%, where the carbides of chrome are precipitated, weakening the grain boundary bonding strength during forging, welding, or heat treatment [2][3][4][5].As listed in Table 2, the spectral analysis of the nuts matched by the broken studs.The results show that the chemical composition of nickel elements does not meet the requirements of ASTM A194/A194M standard.Thus, the samples cannot guarantee a single austenitic matrix, or the corrosion resistance affected [3].

Metallographic structure
The metallographic structure of the stud fracture was performed at the cross-section, and the sampling location of the metallographic sample was near the fracture area.The metallographic observation was carried out under the OLYMPUS GX71 optical microscope, and the morphology is shown in Figure 3.
It is found that the initial cracks on the surface of the bolt also appeared in the heart of the bolt and spread around in a dendritic shape.Besides, it can be seen that the microstructure of stud matrix is austenitic.A large number of particle precipitates are distributed in a mesh at the grain boundary, as shown in Figure 3e.At the same time, it was observed that the cracks on the surface of the stud gradually expanded along the grain boundary to the inside of the thread after the bolt was cracked.There were multiple secondary cracks, which were distributed in a dendritic shape.The metallographic organization of area b shows that there is a significant slip zone, as shown in Figure 3c.At a magnification of 500x, it can be clearly seen that the cracks spread to the termination and are distributed along the crystal, as shown in Figures 3b and 3d.
As shown in Figure 4, the size of non-metallic inclusions is high, and the types of inclusions are diverse.We refer to the GB/T 10561-2005 "Standard Rating Chart for the Determination of the Content of Non-metallic Inclusions in Steel Micro-inspection Method".The non-metallic inclusions in the bolt samples are all fine lines, and the inclusions are longitudinal cracks near the inclusions and rated as A1.5, B1, C2.5, and D1.5, respectively.

Microscopic observation of fracture
The fracture morphology and composition of the 1#broken stud are observed by Scanning Electron Microscopy (short for SEM), as shown in Figure 5. Combined with the macroscopic analysis of the fracture, it can be seen that the fracture surface is covered with a layer of gray-black corrosion products.Besides, there are secondary cracks in the source area, the extension area, and the transient fracture area, exhibiting typical brittle fracture characteristics.After cleaning, rock sugar lumps can be seen in the fracture area along the crystal cracking morphology, and secondary cracks can be seen locally, exhibiting a cleavage fracture morphology.At the same time, a large number of granular precipitated phases can be seen on the grain boundary, distributed in a mesh, as shown in Figures 5a,  5b, and 5c.

Energy dispersive spectrometry analysis
Energy dispersive spectrometry (short for EDS) analysis was performed on the microstructure of the 1# fracture, as shown in Figure 6.It can be seen that the surface of the fracture is covered with a large number of corrosion products, and the composition is mainly oxide of iron.At the same time, there are

Analysis and discussion
B8M is the performance level of austenitic stainless steel fasteners, but the resistance to chlorine corrosion is weak.At the same time, the abnormal organization caused by improper processes during the heat treatment will also have an impact on the mechanical properties and corrosion resistance of finished fasteners.Under this study, through the above experiments, the results are analyzed as follows.
 Physical and chemical testing of the nuts: the results show that the hardness of the nuts meets the requirements of ASTM A194/A194M for 8M, but the nickle content is low, which may cause the corrosion resistance of the nuts to be affected. Analysis of the composition of broken or cracked studs found that the carbon content is higher than that of studs that have not broken.In comparison, the nickel content is lower than the lower limit, and the carbon content exceeds the standard upper limit.Carbon is easy to combine with hydrogen and oxygen to form a poor carbon zone, weakening the mechanical properties.Besides, with increasing carbon, the bolt is more likely to cause brittle fracture.Observation of the metallographic organization found that the microstructure is austenitic, and a large number of granular carbides precipitated phases distributed along the grain boundary.Besides, the granular carbides are connected and distributed in a mesh at the grain boundary.It shows that the raw material of the fastener has not been treated with a solution during the heat treatment process or the solution treatment temperature is not high.As the time is short, the carbide has not been fully dissolved, and the degree of alloying is poor.On the one hand, the presence of grain boundary carbides weakens the intergranular binding strength, which will cause the mechanical strength and toughness of the bolt to decrease.Due to the lower heat treatment temperature, the diffusion rate of chromium slows down, and the precipitation of chromium carbide accelerates, which in turn forms a chromium-poor zone near the grain boundary [6][7][8][9][10]. According to the macroscopic observation results, it is found that most of the bolt cracks are located in the middle of the light rod of the double-headed stud, and a few fractures are located at the thread.There are axial cracks of different weights on the surface of each bolt.The direction of crack propagation is at an angle to the axial direction of the bolt, and a small number of bolt cracks extend axially.There are a large number of corrosion products on the cracking site.EDS analysis shows a large number of chlorine, sulfur, and other elements in the corrosion products.It exhibits that the gap between the light rod of the double-headed bolt and the flange hole has suffered chlorine erosion, resulting in corrosion at the light rod of the double-headed bolt. The metallographic analysis of the broken studs shows that the metallographic tissues of the studs are all austenitic.However, there are a large number of precipitates on the grain boundary of the austenitic, and they are distributed in a mesh shape.There is a significant slip belt in the edge tissue of the outer surface of the bolt, indicating that the bolt has experienced a large stress concentration during use.At the end of the crack, the tissue also showed obvious crystal-along characteristics.There was no crystal-penetrating crack, indicating that the bolt did not experience a large stress at the moment of fracture.In the low-magnification organization chart, it can be found that there are multiple cracks at the beginning.From the surface of the bolt to the heart of the bolt, there are crack starting points, and the cracks are distributed in a dendritic shape after spreading.Inclusions were determined near the bolt fracture, and the inclusivity rating included all grades of A/B/C/D, of which the C grade reached the highest level of 2.5.Some cracks extended forward along the inclusivity.The presence of inclusions in metal materials can cause uneven tissue, which is often the channel of crack propagation or the origin of cracks.At the same time, the decrease in intergranular binding strength caused by the precipitation of intergranular carbides and the formation of chromium-poor regions provide conditions for the formation of bolt microcracks during service.

Conclusions
 The reason for the cracking and failure of the flange-connected double-headed bolt is the fracture along the crystal.The main reason is that the solid solution temperature of the raw material for processing the bolt is low, the carbide is not fully dissolved, and it is distributed in a mesh along the grain boundary.This can result in chromium-poor near the grain boundary, which reduces the mechanical properties of the bolt.When subjected to the corresponding external stress, the fracture along the crystal occurs. Due to the smelting of bolt raw materials, the inclusions cannot be cleaned up, and the internal inclusions of the bolt have a high rating.The improper heat treatment process in the later stage causes the precipitate to be distributed in a mesh at the grain boundary, reducing the grain boundary binding strength.In addition, the presence of bar-shaped MnS inclusions also leads to crack stress concentration and expansion along the grain boundary of metal inclusions and chromium-poor, eventually leading to cracks in the bolt rod along the axis of fracture.

Suggestions
 Investigation work should be carried out for the same batch of fasteners purchased by the same manufacturer.The same batch of fasteners should be replaced to avoid equipment failures and operating incidents. 0Cr18Ni9Ti stainless steel high-strength fasteners with low carbon content shall be replaced on-site.Manufacturers are required to ensure that the chemical composition of raw materials is qualified and that the quality of solution treatment is good.At the same time, the sampling

Figure 1 .
Figure 1.Appearance of the seawater electric isolation valve:(a) Connection of the seawater pipe; (b) Connection of the isolation valve; (c) Macro photo of the entire studs.
number of chlorine elements on the cleaned sample surface, indicating that it suffered from chlorine erosion during crack propagation.Meanwhile, the chlorine atoms penetrated deep into the grain boundary resulting in serious corrosion of the stainless steel fracture.

Figure 6 .
Figure 6.EDS analysis of stud fracture surface (a) Fracture microstructure; (b) EDS analysis of area 1; (c) EDS analysis of area 2.

Table 1 .
Chemical composition of the broken studs (wt%).
⁎ASTM A194/A194M: Standard Specification for Carbon Steel, Alloy Steel, and Stainless Steel Nuts for studs for High Pressure or High-Temperature Service, or Both.2.3.Hardness testHardness samples of equal diameter and length are taken at the head of each broken bolt.A Brinell hardness test is performed on the XHBT-3000ZⅢ automatic Brinell hardness tester in accordance with GB/T 231.1-2018 "Brinell Hardness Test of Metal Materials".The test parameters (HBW 2.5/187.5)and the hold time are 15 s.The results are shown in

Table 3 .
As can be seen from the table, the Brinell hardness of the studs inspected meets the requirements of ASTM A193/A193M for not greater than 321HBW5/750.