Aiming at the early fatigue failure case of C17200 beryllium bronze logging while drilling equipment, the effects of two surface strengthening processes on the microstructure and properties of beryllium bronze substrate were studied, the failure mechanism was explored, and the appropriate strengthening process was determined. The adapter head with laser cladding and high velocity oxygen flame spraying (HVOF) ceramic coating was used to compare and analyze the microstructure and properties of the coated and uncoated parts. The microstructure of the substrate was characterized by optical microscope (OM) + scanning electron microscope (SEM), and the phase composition of the grain boundary and grain of the over-aged structure of beryllium bronze was characterized by high resolution field emission electron microscope (FESEM) + energy dispersive spectrometer (EDS); the mechanical properties were tested by hardness tester and material testing machine. The results show that the heat input of laser cladding has an adverse effect on the microstructure and mechanical properties of beryllium bronze substrate. The local temperature of the substrate reaches 350 ℃, the structure is over-aged, the grain boundary reaction amount increases, and a large number of γCu88Be12 nodules appear. The substrate softens, the minimum hardness value is only 12 HＲC, and the strength decreases. HVOF has no effect on the structure and performance of the substrate, and no over-aging structure has appeared. The use of HVOF spraying ceramic coating can improve the reliability and safety of the adapter head, and is suitable for the surface strengthening of C17200 beryllium bronze logging while drilling equipment.

A logging while drilling equipment fracture failure event occurred on an offshore platform. The fractured part was a C17200 beryllium bronze adapter head, which was only used for 18 hours. The heat treatment process was solid solution + aging, and the hardness was 37 to 44 HＲC. The finished parts were processed domestically with foreign materials. After machining, no heat treatment was performed, and only the outer cylindrical surface was laser clad with ceramic coating. After analysis and judgment, the fractured parts were caused by fatigue fracture failure due to improper operation of the laser cladding strengthening process. The fatigue source was located at the bottom of the internal thread. The heat input generated by laser cladding affected the microstructure of the beryllium bronze substrate, causing overaging. The amount of grain boundary reaction increased and a large number of nodule tissues were precipitated, affecting the mechanical properties and reducing the hardness and strength.

In the oil drilling industry, the working conditions downhole are complex and the service conditions are harsh. It is necessary to withstand torsion, bending, tension and compression alternating stress loads and high temperature and corrosive environmental media. Drilling and logging equipment is prone to wear, stress corrosion cracking, fatigue or corrosion fatigue and other failure accidents. Therefore, a material with high strength, wear resistance, fatigue resistance, corrosion resistance, and non-magnetic properties is needed as the main body or key component of drilling equipment to meet the working conditions. C17200 beryllium bronze is a precipitation hardening copper alloy material with good comprehensive properties. After solid solution + aging treatment, it has high strength, hardness, strong corrosion resistance, conductivity and non-magnetic properties. It is widely used in marine engineering, aerospace, new energy and other fields. It is suitable for oil drilling equipment and can meet the requirements of harsh downhole environments and working conditions.

Aiming at the low surface hardness and poor wear resistance of C17200 beryllium bronze adapter, implementing appropriate surface strengthening is an effective way to improve the surface hardness and wear resistance of beryllium bronze. At present, the commonly used surface strengthening processes include magnetron sputtering titanium film and plasma high temperature thermal diffusion, laser cladding, and supersonic flame spraying (HVOF). Because the wear-resistant layer obtained by magnetron sputtering titanium film technology is only a few microns to tens of microns thick, it cannot meet the wear resistance requirements of drilling equipment and its scope of use is limited. Laser cladding is a process in which the heat provided by the laser to the cladding layer is lost a lot due to the high reflectivity of the copper alloy to the laser and the strong thermal conductivity to the coating. Under the same laser parameter conditions, it is difficult for the coating to form a metallurgical bonding interface with the substrate, and cracks, pores and other defects are easily generated in the coating after cladding. Therefore, the process operation is difficult and the product quality is difficult to guarantee. HVOF combines with the substrate in the form of mechanical bite, also known as the “anchor effect”. The high-temperature molten and semi-molten particles sprayed on the surface of the substrate at high speed impact the surface and deform, and bite with the concave and convex areas of the substrate to form a tight coating.

Based on improving the surface hardness and wear resistance of beryllium bronze logging equipment while drilling, improving the reliability of use and reducing the probability of failure, this paper uses HVOF and laser cladding ceramic coatings respectively, compares the structure, performance and fracture morphology of the substrate, explores the failure mechanism, and thus determines that HVOF has a strong adaptability to beryllium bronze equipment.

1 Experimental materials and methods
A new adapter head that was clad by HVOF and laser was selected. The total length of the part was 148.5 mm and the maximum diameter was φ47.6 mm, as shown in Figure 1 (a, b). The laser cladding fractured part and the matching fracture were shown in Figure 1 (c, d). There were multiple fatigue arcs on the undamaged fracture surface, and the fracture originated from the internal thread root under the laser cladding layer. The raw material of the adapter head was C17200 beryllium bronze bar with a diameter of φ50 mm, and the domestic corresponding grade was QBE2. Table 1 shows the chemical composition measurement values ​​of the new and old adapter head substrates and bars and the technical requirements of C17200 beryllium bronze bars. There was no obvious difference between the substrate composition of the new and failed parts and the composition of the C17200 beryllium bronze bar, which confirmed that the parts were processed using C17200 beryllium bronze bars, not domestic QBE2 beryllium bronze bars.

The coating material is carbide metal ceramic powder WC + CrNi, with a powder particle size of 15 to 85 μm. The outer cylindrical surface of the adapter is degreased, sandblasted, and preheated at 150 °C before strengthening. Laser cladding uses a 2000 W fiber solid laser to synchronously feed powder, with a spot size of φ2 mm, a scanning speed of 8 mm/s, and a cladding layer thickness of not less than 500 μm. The HVOF process uses aviation kerosene as fuel, oxygen as a combustion aid, and nitrogen as a carrier gas. The process parameters are: kerosene flow rate 22 L/h, oxygen flow rate 52 m3/h, powder feeding amount 60 g/min, spraying distance 360 ​​mm, and coating thickness 300 μm.

Samples were taken from the internal threads (fracture sites) of the beryllium bronze bar and the adapter head where the heat effect was the strongest, and from the external threads where the heat effect was the weakest. The sampling sites of the adapter head are shown in Figure 2. Hardness, metallographic and 10 mm × 10 mm × 55 mm impact specimens were made, and the metallographic etchant was a cupric chloride ammonia solution. The microstructures of the internal and external threads were compared and observed by optical microscopy (OM) + scanning electron microscopy (SEM) to explore the differences in the structures at different positions from the surface of the cladding layer. High-resolution field emission electron microscopy (FESEM) + energy dispersive spectrometer (EDS) with large crystal area that can detect beryllium elements were used to explore the composition of the over-aging reaction, grain boundary nodule precipitation phase and intragranular precipitation phase of laser cladding C17200 beryllium bronze. The hardness of the coating and substrate was tested by Rockwell and Vickers hardness testers. According to ASTM E23 and ASTM E8, V-notch Charpy impact test and tensile test of bar φ12.5 mm ×50 mm specimens were carried out at room temperature using impact and universal material testing machines.

2 Results and analysis

2.1 Microstructure analysis
C17200 beryllium bronze bar is required to contain 1.8% to 2.0% beryllium and not less than 0.2% (nickel + cobalt). The corresponding domestic imitation grade is QBe2, which contains 1.9% to 2.2% beryllium and 0.2% to 0.5% nickel. The biggest difference between the two grades of beryllium bronze is that C17200 beryllium bronze is added with trace cobalt elements, while QBe2 beryllium bronze is added with trace nickel elements. Nickel and cobalt form NiBe and CoBe compounds with beryllium. Their solubility in the α phase decreases sharply with decreasing temperature. Aging treatment plays an aging strengthening role. A small amount of nickel or cobalt can delay recrystallization, prevent grain growth and delay the decomposition of solid solution, reduce the dissolution rate of grain boundaries, delay aging softening, and improve the stability of the alloy.

Figure 3 shows the microstructure of beryllium bronze bar and two strengthening process substrates. Figure 3 (a, b) shows the longitudinal and transverse structures of beryllium bronze bar, with normal grain size and grain boundary width. Figure 3 (c, d) shows the substrate structure of the inner and outer thread parts of the new adapter part after HVOF surface strengthening. There is no obvious difference in the microstructure of the two places, and the grain boundary is not widened. The crystal is α phase substrate + a small amount of granular β phase, and the grain boundary is an island chain distribution of γ phase + a small amount of nodule structure. This structure has the best strengthening effect and the highest strength and hardness. Figure 3 (e, f) shows the microstructure of the new adapter and the internal thread of the broken part after laser cladding. It can be seen that the microstructure of the new part and the broken part is exactly the same, both are over-aged structures, the grains are accompanied by growth, the grain boundary reaction is obvious, and a large number of nodules are precipitated. At this time, the beryllium bronze substrate has obviously been over-aged, which softens the substrate. According to the above analysis, it is determined that the fatigue fracture of the adapter part is related to the laser cladding surface strengthening process.

The wall thickness from the outer circle cladding layer surface of the adapter to the bottom of the thread is 8 mm. SEM observation of the changes in the substrate structure 2, 4, and 6 mm away from the cladding layer surface is shown in Figure 4. The microstructure of the beryllium bronze substrate at the internal thread is affected by the heat of laser processing. The substrate temperature is the highest near the interface of the cladding layer, the grain boundary reaction is the most obvious, and a large number of nodules are precipitated on the grain boundary. The laser processing heat input is weakened in the area far away from the cladding layer, the grain boundary becomes less visible, a few grain boundaries are widened, and the nodule structure is reduced.

There are many literatures studying the effect of aging temperature on the performance of QBe2 beryllium bronze, but few literatures study the relationship between aging temperature and organizational morphology of C17200 beryllium bronze, and no research literature on the composition and properties of beryllium bronze grain boundary nodule structure was found. In order to explore the properties and composition of the grain boundary close-packed nodule structure and the intragranular precipitation phase after laser cladding treatment, FESEM combined with EDS of a large crystal area was used to find for the first time that the grain boundary precipitation phase and the intragranular precipitation phase are two completely different phases. As shown in Figure 5, the grain boundary close-packed nodule structure is CuBe phase, that is, the stable phase γCu88Be12, while the granular intragranular precipitation phase is Co38Be8 compound β phase, which plays a role in reducing the dissolution rate of the grain boundary and delaying aging softening.

Bronze may have three phases: α, β, and γ. The microhardness of each phase varies greatly in different states. The commonly used heat treatment state is 780 ℃ quenching + 320 ℃ × 2 h aging, Rm can reach 1250 ~ 1400 MPa, and the hardness is 375 HV0.5. The changes in the microstructure and strengthening mechanism of beryllium bronze during aging are very complex. Studies have shown that the aging of beryllium bronze is a coherent dissolution process of the supersaturated solid solution α phase. The order of dissolution in the crystal is: α phase → Be atom segregation zone (G.P. zone) → transition phase γ’ → stable phase γ. The strengthening of the alloy is mainly at the moment of the generation of the transition phase γ’. At this time, the new phase forms a coherent relationship with the parent phase, and the yield strength is the highest. Once the stable phase γ is generated, the coherent relationship is destroyed and the alloy begins to soften. The dissolution of beryllium bronze starts from the grain boundary first, which is faster than the dissolution in the crystal. The nodule structure precipitates at the grain boundary. The nodule structure continues to increase and grows into the crystal. The transition phase γ’ decomposes into the stable phase γ. When the strengthening occurs in the crystal, the grain boundary is often over-aged and the hardness of beryllium bronze is reduced.

Based on the above research results, it can be seen that HVOF heat input has no effect on the microstructure of beryllium bronze, and no obvious changes are observed in the microstructure; while laser cladding heat input has a significant effect on the microstructure of beryllium bronze. Different parts of the cladding layer are affected by the heat of laser processing, forming over-aged nodule structures at the grain boundaries, which softens the substrate.

2.2 Hardness of coating and substrate
The Rockwell hardness test results of HVOF, laser cladding (broken parts and new parts) and C17200 bars along the length direction are shown in Figure 6 (a). The Rockwell hardness is tested at 5 points on the cross section of the bar and the inner and outer threads of the adapter head with different strengthening processes. The Rockwell hardness of the adapter head substrate after HVOF did not decrease compared with the bar material, while the Rockwell hardness of the adapter head substrates of the two laser cladding adapter heads decreased significantly, and the hardness of the internal thread, which was greatly affected by heat, decreased significantly compared with the external thread. The minimum hardness was 12 HRC and the average value was 14.3 HRC. The hardness value required for the adapter head was 37-44 HRC. It can be seen that the heat input of laser processing has a great influence on beryllium bronze, which is equivalent to secondary aging treatment, triggering over-aging reaction and causing the substrate to soften.

The Vickers hardness values ​​of the coating (or cladding layer) and the substrate along the wall thickness direction prepared by the two processes are shown in Figure 6 (b). The Vickers hardness test of the substrate starts from 1 mm from the coating surface and is measured every 1 mm to the root of the internal thread. The hardness of the HVOF coating is 1300 HV0.3, and the hardness of the substrate is 396-440 HV0.3. It can be seen that the coating has a high hardness and can maintain good wear resistance. The hardness of the substrate does not change due to HVOF hot processing. This inspection result is consistent with the substrate structure. The hardness of the new laser cladding part is basically the same as that of the broken part. The hardness of the WC phase in the cladding layer is as high as 2400 HV0.3, the hardness of the bonding phase is 470 HV0.3, and the hardness of the substrate is only 212 HV0.3.

2.3 Material properties and fracture analysis
V-notch Charpy impact test was performed on C17200 beryllium bronze bar and adapter head after HVOF and laser cladding. The impact absorption energy of C17200 beryllium bronze bar is 9.0 J, the HVOF adapter head is 10.2 J, and the laser cladding adapter head is 77.3 J. It can be seen that the impact absorption energy of the adapter head increases significantly after laser cladding. The higher the impact absorption energy, the better the impact performance of the material. The fracture surfaces of the beryllium bronze bar and the HVOF adapter with the same impact absorption energy are very similar. There is no obvious plastic deformation around the fracture surface. The fracture surface is straight, which is a typical brittle fracture fracture surface. The fracture surface has a fiber area, a radial area and a shear lip area. The morphology of the fiber area and the radial area of ​​the two fracture surfaces is mainly equiaxed dimples, accompanied by a large number of secondary cracks along the grain. The shear lip area is very small, which is a tear shallow dimple feature, as shown in Figure 7 (a, b). The fracture morphology, microstructure and hardness can verify that HVOF surface strengthening does not affect the mechanical properties of the substrate and is currently a strengthening process suitable for beryllium bronze equipment.

The impact fracture of the laser cladding adapter is completely different from that of the bar. The plastic deformation around the fracture is obvious, and the fracture is straight and curved. It is a typical plastic fracture fracture. The fracture only has a fiber area and a shear lip area. The shear lip area is large, and the microstructure is a tearing dimple, as shown in Figure 7 (c). This shows that the material has good plasticity. The brittle fracture of the bar is transformed into the plastic fracture of the adapter. In the middle, it has undergone mechanical cold processing and laser cladding hot processing. Analysis shows that only the heat input of laser processing can cause secondary heat treatment of the beryllium bronze substrate. At the aging temperature of 320 ℃, it takes more than 2 hours to keep the substrate warm before overaging; if the aging temperature rises to 350 ℃, overaging can occur in just a few tens of minutes; once the aging temperature reaches 380 ℃, overaging can occur in just a few minutes. At this time, the amount of grain boundary reaction increases rapidly, the hardness decreases significantly, and the material softens, which leads to a decrease in material strength and an increase in toughness and plasticity. According to the characteristics and construction process of laser cladding of beryllium bronze, laser cladding generates a large heat input to the beryllium bronze substrate, and the local temperature of the substrate can reach 350 ℃ or higher, which causes the aging of beryllium bronze and softening of the material.

Due to the hollow shape and size of the adapter, it is impossible to take samples to make tensile specimens. Only the beryllium bronze bar is tested for material strength according to the standard specimen (φ12.5 mm ×50 mm gauge length). The test results show that the tensile strength Rm is 1350 MPa, the yield strength is 1090 MPa based on Rr0.2, and the elongation after fracture is 4.5%. The standard for C17200 beryllium bronze bar material requires that the tensile strength and yield strength be no less than 1129 MPa and 992 MPa respectively, and the elongation after fracture be no less than 4%. It can be seen that C17200 beryllium bronze has high strength and high hardness, but the toughness and plasticity of the material are poor. Its tensile fracture surface is flush, without obvious plastic deformation, and is similar to a brittle fatigue fracture. Macroscopically, it is clearly divided into a point source area, a middle rapid expansion area, and a surrounding shear lip area, as shown in Figure 8. There are multiple radial ridges in the point source area pointing to the punch point notch on the surface of the sample, indicating that the notch effect of the beryllium bronze material is obvious, that is, the stress concentration sensitivity is high. Once microcracks are initiated at the stress concentration, the cracks will expand rapidly. This situation is consistent with the fact that the fatigue fracture of the adapter head strengthened by laser cladding surface originates from the stress concentrated thread bottom. The morphology near the source area is a shallow dimple + intergranular cracking morphology, and the intergranular cracking characteristics of the rapid expansion area increase.

Conclusion

1) Laser cladding surface strengthening has an adverse effect on the microstructure and mechanical properties of the beryllium bronze substrate. The local temperature of the substrate reaches 350 ℃, the microstructure is over-aged, the amount of grain boundary reaction increases, and a large number of γCu88Be12 nodules appear at the beryllium bronze grain boundaries in the heated part of the entire adapter. The closer to the cladding layer, the more obvious the microstructure change, resulting in a decrease in the hardness and strength of the substrate.

2) HVOF surface strengthening has little effect on the microstructure and mechanical properties of the beryllium bronze substrate. The microstructure and properties of the substrate are normal, and no aging microstructure is seen. The HVOF process is more suitable for strengthening beryllium bronze logging equipment while drilling, without reducing the hardness and mechanical properties of the material, and can improve the reliability and safety of use.

3) The tensile fracture of the C17200 beryllium bronze original bar has low plasticity, low toughness and high notch sensitivity, and it is recommended to use it with caution.