Abrasive wear of Hilong BoTN hardfacings

The spread of steels, which are used to produce locks of steel drill pipes, adversely affects their wear resistance, which, in combination with low hardness of HV 2400 ... 2800 MPa as well as of the thread of screw, results in low wear resistance and the need for their reconstruction at the pipe control shop. An efficient way of improving the quality of drill pipe jonts is to hard-face them by the outside diameter with wear-resistant materials (hardbanding). One of the companies engaged in the development of hardfacing materials and hardbanding is Hilong (China) with weld seams of the brand BoTn. According to the results of the studies the following conclusion can be made: hardfacing increases the durability of the hardware, contributing to an increase in wear resistance of locks of DP under the conditions of abrasive action of aggressive geological formations; the usage of DP without wear-resistant weld seams is impermissible, because their further operation, as part of the drill-stem, can lead to emergency consequences; application of the pipes with the hardfacing collars together with the collars without hardfacing, due to varying degree of wear of jonts in the drill-stem, is also impermissible.


Introduction
One of the causes of low durability of drill pipes (DP) is an abrasive wear of joints [1]. More than 60 % of pipes are rejected due to the wear of outer joint diameter, while other parameters meet the acceptable norms. The main reason of drill pipe joint wear in an open wellbore is presence of hard layers in aggressive geological bodies. During drilling the lower part of drill stem consists of heavyduty drill pipe (HDDP), heavyweight drill pipe (HWDP) and steel drill pipe (SDP). HDDP and HWDP are made of single structural alloy steel workpieces 40ХГМА (1.2311), 40ХН2МА (4340). The legnth of DP is 8,5…12,5 metres. During borehole drilling the lower part of drill stem is in direct contact with abrasive parts of rockbreaking layers. The intensity of wear of DP surface layers during abrasive wear hardly depends on original mechanical properties of joint material. The chemical composition of joint material [5] of steel pipes shows a significant dispersion of steels by carbon content (0,22…0,37), what, in combination with other alloying elements, matches steels 20ХГНМ…40ХГНМ. Hardness of the DP joint material is 28…36 HRC and is achieved by induction heat treatment. In order to increase the longevity of DP, hardfacings made of wear-resistant materials are welded to the outer diameter of the pipe. In most cases there are 3 of such hardfacings , each of them is 25,4 mm wide and overall width is 76,2±6,35 mm. Hardfacing are jam-welded in argon-shielded environment . Welded material stands above joint body up to 2,5 mm. Such way of increasing durability is widely used by foreign companies [2,3,6]. Thus, one of the priorities during production and full repair of drill pipes is reinforcing welding of wear-resistant materials (hardbanding) of outer joint surface [4,5]. Hardbandingwear-resistant metal coating, applied by way of electronic welding to the outer surface of drill pipe joint in order to increase the longevity of the joint and decrease the wear of casing pipe.

Research methodology
Further use of wear-resistant hardfacings for drill stems is impossible without determining their level of effectiveness . For this it is necessary to carry out a comparative test of different padding materials in relation to DP joints without hardfacings. Hilong hardfacings ВоТn 1000, ВоТn 3000, ВоТn 5000 (table 1) are welded to joints by "Technomash" of Nevyansk . For producing sample parts joints of SDP 89 were mechanically processed: turning to eliminate surface irregularities of hardfacings; inner diameter boring up to 105 mm; cutting a ring with a height of 20±0,1 mm. The ring was cut in segments 35 mm long. Each ring was cut in four segments, three of which were used for wear experiments, and one sample part for metallographic analysis. The wearing scheme consist of a Sample 1 is fixed on the base of the pendulum arm 2 with lever 3 screwed in from the side. Weight is hanged from the right part of lever 3, which ensures constant pressure in the contact are of sample 1 and turning rubber roller 6. Abrasive material with grit of 16 mm is supplied through the distribution device 5 by the track pan 4 to the contact area of sample piece 1 and rubber roller 6 . The duration of experiments, established by standarts ASTMG65 and GOST 23.208-79, equaled 30 minutes for each sample piece.
Chemical analysis was carried out with laser atomic emission spectrometer « LAES» .Cross-sections were made using stand methods with Stuers equipment. Etch processing was done in a 4% solution of nitric acid in ethanol. Etching welded hardfacings was done using the chemical agent Marble (20 g CuSO 4 , 100 ml HCl, 100 ml ethanol).
Metallographic analysis was carried out using CarlZeiss Axio Observer D1 with zooming in from 200 to 1000 times. Microhardness was measured using a durometer DuraScan 70 with press of 100 grams.

Experiment results
Diameters of joint hardfacings for SDP were measured using a micrometer MC 125 with 0,01mm accuracy. Results of measurements of joints and hardfacings are presented in table 2.
Chemical analysis of sample pieces shows that joints were made of medium carbonlow alloyed steel 37ХГМА (4041). Alloying elements chrome, manganese, molybdenum, nickel significantly increases hardenability of steels. Hardening effect is amplified when steel is alloyed with several elements. All alloying elements decrease critical hardening rate. Therefore, hardness of alloyed steels can be increased by cooling them at lower rates, than for carbon steels. Besides, hardening alloyed steels can be carried out using slower cooling agents, what can decrease the possibility of hardening defects, first of all hardening cracks. This is especially important for DP body and joint, that are assembled by friction welding. Hardfacings were found to have carbon content up to 0,7% and carbide forming chemical elements: chromeup to 8%, manganeseup to 0,8%, siliconup to 0,4%, molybdenum -0,13% (ВоТn 5000). Main alloying elements increase ferrites hardness and durability. Silicon, manganese and nickel increase durability of ferrite the most. Chrome is known to improve mechanical properties of structural steel and special properties of steels, for example, resistance to aggressive environments, that are present during borehole drilling. Molybdenum is a strong carbide that are forming element. It forms carbides and alloyed ferric carbide, promotes grain size reduction, increases durability and high temperature strength. At the same time, there was no molybdenum found in BoTn 1000 and BoTn 3000 hardfacings. Vanadium and titanium are also used to reduce grain size, but the content of these elements should be limited (up to 0,3…0,5%), as they can form carbides at grain boundaries, which can lead to brittle fracture. Among tested paddings titanium was only found in ВоТn 5000, less than 1%. Nickel gives steels corrosion resistance, high durability and ductility, increases hardenability, impact toughness, decreases coldshortness threshold, thus decreasing the potential of brittle fracture of welded metal. The largest content of nickel (up to 3%) was found in ВоТn 3000 hardfacing. Niobium improves acid resistance and promotes the decrease of corrosion in welded constructions. Niobium in ВоТn 3000 hardfacing is dispersed irregularly in the cross-section totaling 44 % in spectrum 2. Microstructure of sample pieces is presented in fig. 1. Thickness of the hardfacing after mechanical processing equaled: about 2 mm for BoTn 1000 padding; up to 3,5 mm for BoTn 3000 and BoTn 5000 hardfacings. Welded metal has dendrite structure. In contact are of hardfacing and base material dendrite axes are structured perpendicularly to the plate surface. This is especially distinct for BoTn 5000 hardfacing. Hardfacing has an area of thermal influence with changed structure close to itlarge-needled martensite near the hardfacing, small-needled martensite further. Structure of the main metalsecondary sorbite. Segregation banding can be visible in the main metal, which is typical for hot forming. Dispersion of microhardness in welded layers on sample pieces is given in figure 3. Hardfacing hardness decreases with distance from the surface due to dendrite structure enlargement and is within the range: HV7300…8000 MPa for BoTn 1000, HV 8100…9600 MPa for BoTn 3000 и HV 9400…11200 MPa for BoTn 5000.  figure 5. Tests of welded materials in relation to joint material used in batch production tell about the effectiveness of each tested sample piece. BoTn 5000 hardfacing provide the best wear resistance. Overall, there is a correspondence between hardfacing hardness and wear resistance. In order to determine the effectiveness of wear resistant hardfacings for SDP 86, metrological test were carried out on the premises of « BIS-Service» in Nizhnevartovsk, Russia. SDP 86 were supplied for repair due to them having been rejected because of inner and outer joint threads. Measurements of outer joint diameter were carried out using a beam trammel BT 250 with 0,05 mm precision. It was found out that one drill stem contained both SDP 86 with outer welded hardfacings (BoTn 3000), and joints without hardfacing. minor wear of welded material (diameter 110, 8…112,0 mm) and no joint wear. There was also no roughening and detachment of material. According to metrological analysis of SDP 86, supplied for repair, following conclusions can be made : hardfacings increase durability of drill pipe parts, promoting increased abrasive wear resistance for DP joints in the aggressive geological environments ; using drill pipes without hardfacings is unacceptable, as their further use as parts of drill stems may lead to emergencies; -application of the pipes with the hardfacing collars together with the collars without hardfacing, due to varying degree of wear of locks in the drill-stem, is also impermissible.