Laser Technologies of Gas Powder Surfacing and Heat Treatment of Drilling Equipment for Arctica Project Tasks. Part II
There are a number of adventages of laser surfacing over traditional surfacing methods. High energy concentration in the hot spot makes it possible to conduct the process at higher processing speeds [1–7].
Since tungsten carbides are one of the most important materials used for manufacturing of wear-resistant coatings for drilling equipment, the results of the research carried out by the author of the article will be shown below . It is obvious that development of state-of-the-art technology puts forward increasingly stringent requirements for materials, and the increase in parts wear resistance is an urgent task for many industries, in particular, oil and gas industry. A promising alternate solution to this problem is composite coatings on heavily worn-out parts.
Composite material (CM) is a non-uniform void-free material made of two or more components, among which there are hardening or reinforcing elements serving to provide the material with required mechanical characteristics, and a matrix serving to ensure joint action of reinforcing elements. The properties of these materials mainly depend on physical and mechanical properties of the components and bonding strength between them. A distinctive feature of the materials is that they show benefits of the components, not their drawbacks. At the same time, composite materials possess the properties extrinsic to individual constituent components. In order to optimize properties the components exhibiting starkly different but complementary properties are selected. Composite materials show high specific and fatigue endurance rates and increased wear resistance, they provide dimensional stability of the structure. Nowadays manufacturing and implementation of innovative structural materials exhibiting high physical and mechanical properties is of vital importance. Among structural materials the use of carbide steels, composite materials made of alloy steel and carbides with a mass fraction ranging from 20 to 70%, is getting widespread. By their properties carbide steels are intermediate between steels and hard alloys. The use of tungsten carbide as the hardening phase in the steel matrix makes it possible to increase hardness, strength and wear resistance.
Composite material can be manufactured either by matrix smelting and adding strengthening particles, or by resmelting of all the components followed by isolation of the required structures. In the first case, it is necessary to smelt only the matrix retaining the original structure in the carbide. In the second case, it is necessary to smelt the entire material without significant evaporation, and thereafter, get from the total liquid bath strengthening phases dissociated during crystallization. In case of the use of non-concentrated energy sources, high linear heat output of the process results in significant heating of the back support and its deformation. Concentrated energy sources make it possible to heat the back support and pad by spot heating with a minimum heat input. Concentrated heat sources include electron-beam and laser sources.
Laser surfacing is carried out by layer-by-layer local melting of powders and their alloying with the surface of metal being processed, as a result, linear heat output of the process is lower than when arc and plasma methods are used, respectively, the heat effect on back support is minimal. Due to the local nature of the heat effect and the flexibility of monitoring the variables of the composite coating process laser surfacing method makes it possible to apply a wide variety of pads and their combinations.
The authors of the article  have tested the laser surfacing process on the robotic laser station using a fiber-optics laser (ABB IRB2400 robot + МТС‑250 rotary head, ЛС‑4К fiber-optics laser). A laser disintegrating nozzle with a multi-jet powder supply (YC‑50 head for surfacing) made it possible to apply rollers about 4 mm wide and 1–1.2 mm high in one pass. 50% of the overlap layer amounted to 2–2.5 mm thick for experimental compositions. Thus, fewer thermal cycles of circumjacent metal heating to high temperatures, over 800 °C, are needed to achieve the required layer thickness. The powder material was supplied by a 2-vessel dispenser.
Metal powder derived by melt gas atomization method was used as an additive. Granulometric composition of the particles corresponds to the requirements set in the requirements specification: fraction size of all the powders used ranged from 40 to 150 µm. Chemical composition of the powders used is shown in Table 1 and thermal properties are shown in Table 2. Nickel-based powders are high corrosion resistant and abrasion wear resistant self-fluxing alloys.
Two types of tungsten carbides were used as the strengthening phase: chipped and sintered carbides. Sintered cobalt-bound tungsten carbides are small chipped carbides integrated into spherical particles by a cobalt base. Such carbides get melt into the matrix and form a void-free uniform transition without lattice distortion and microdefects formation.
Experimental samples, 50Ч30Ч20 mm in size, were surfaced. Various modes and percentage of tungsten carbides were used. In order to prevent cracks preforms were heated up to 450 °C.
Type ПГСР‑4 nickel alloy. Surfacing of experimental samples, 50Ч30Ч20 mm in size, resulted in no cracks and pores, carbides remained non-dissolved (Fig. 9). The photo on the top shows the macrostructure of the alloying area with the back support. Alloyage is smooth; carbides are distributed evenly across and along the whole coating. The alloy was surfaced in one pass; the coating thickness amounted to 2.2 mm. Dependent on different values of tungsten carbide content the coating hardness ranged from 61 to 67 HRC (Table 3). As the content of the carbide phase in the pad increases, hardness and brittleness increase, so the optimal content of WC / Co (40% by volume) was detected in sample No. 1 due to the most even distribution of carbides across and along the coating.
The results of preliminary samples researches have made to come to a decision to surface this alloy on experimental parts. However, mechanical processing of the alloy with such high hardness has led to its destruction.
Type PR-НХ16СР3 nickel self-fluxing alloy. In comparison with the first case the nickel alloy used is self-fluxing with lower hardness (up to 50 HRC). While analyzing the preliminary research it was decided to surface this alloy with additives of chipped tungsten carbides on the experimental parts. 4 alternate alloy compositions in various modes were applied on the samples (Table 4).
The process of material coating was stable; layering was even in the broad window of modes. Ground coatings are shown on Fig. 10.
The researches have shown the following result. Samples No.1 and 3 showed high wear resistance in comparison with the required value. Samples applied in modes No. 2 and No. 4 happened to be relatively less wear resistant. Coating No. 1 has been recognized to be the most suitable; however, a defect – porosity – has been detected in it. This defect had to be eliminated by taking manufacturing process measures:
• Thorough preform surface preparation: degreasing, decontamination and preheating.
• Powder blend preparation: sieving, dehumidifying.
• Increase of the process linear heat output aimed to replenish heat dispersion in a massive part.
Within the scope of the research work-size samples have also been tested for the process surfacing operation. In order to improve wear resistance of cleansing knives they have been surfaced. Heating pattern of real-size parts and small preforms differed from each other during surfacing. That is why surfacing modes derived at the preliminary stage had to be adjusted for large parts. The surfaced composition was a type ПР-НХ16СР3 nickel self-fluxing alloy with chipped tungsten carbide additives. A number of experiments on ПР-НХ16СР3 type alloy with tungsten carbide additives and cobalt as a binding medium resulted in failure to achieve stability of the application process, even distribution of carbides; however, wear resistance and high crack resistance of such coatings failed to have been achieved simultaneously. Therefore, it was decided to use chipped tungsten carbide as strengthening particles. The fractions were 50–150 µm in size, the particles were irregular in shape and held more hard in a ductile matrix.
Powder blend composed of ПР-НХ16СР3 and chipped tungsten carbides was surfaced on the portion of a work-size knife in the mode specified in Table 5.
DOMESTIC INSTALLATIONS FOR THE USE OF ADDITIVE TECHNOLOGIES DURING RECONDITIONING OF DRILLING EQUIPMENT
Activities of ILIT RAS (the Institute on Laser and Information Technologies of the Russian Academy of Sciences)
Fig. 11 shows drilling equipment strengthening facilities supplied in Okha, Sakhalin region, in 1997. Moreover, the strengthening facilities were supplied to Bulgaria, a number of Russian enterprises. As an example, Fig. 12 shows drilling equipment bearings reinforced by 2 kW МТЛ‑2 laser.
Experimental laser station of ILIT RAS for SLS-technology
Detailed description of the experimental laser station SLS is shown in . It is based on a waveguide multi-channel CO2 laser with emission power amounting up to 1.5 kW, single-mode operation and 3–6 kW power with a super-Gaussian (shelf) distribution. Operation of Hybrid‑1 technological laser is a single-mode which makes it possible to focus emission into a spot, less than 100 µm in diameter. Availability of appropriate software and hardware makes it possible to grow parts with high space resolution. Hybrid‑2 laser differing from the previous version only by the cavity mirrors provides a unique uniform distribution of power density in the processed field, from 0.5 to 10 mm in diameter.
Laser complexes for additive technologies of Russian universities
The most sophisticated domestic installation has been manufactured in Bauman MSTU. Fiber-optics laser power is 0.1–5 kW, spot diameter is 0.2–5 mm, grown layer thickness is 0.2–2 mm. The third domestic selective laser melting plant ПТК-ПС has been manufactured in Stankin. The power of the fiber laser used is 0.5 kW.
Thus, all technological prerequisites which make it possible to start rapidly development and sophistication of the technologies for reconditioning, repair and enhancement of performance characteristics of the drilling equipment used for extraction of mineral deposits, particularly, in the Arctic zone, have been set uporiginated.