Heat treatment effect of 65G and 60C2A steels on strength characteristics of the hydrodynamic oscillator built-in drill head

. The ways of improving the physical and mechanical properties of the components of a hydromonitor drill head with an integrated generator of hydrodynamic oscillations are reviewed. On the basis of the results of this consideration, an improvement of the working body of the HDD technique is proposed. On the example of 60S2A and 65G steels, the influence of different modes of heat treatment of the improved drilling head on the ultimate strength, relative lengthening, and surface hardness of the detail petal, which is the basic loaded element of the generator of hydrodynamic vibrations, is examined. The analysis of physical and mechanical properties has shown a significant increase of parameters influencing the increase of rigidity properties of the investigated hydrodynamic oscillator part, which allows us to make a conclusion about the effectiveness of its application for the increase of drill bit operational life. An experimental and analytical method was carried out to describe the increase of physical-mechanical properties of the part - petal, which is the main loaded element of the generator of hydrodynamic oscillations. Increase of durability of rock destruction tool for pilot hole laying by increasing of physical-mechanical properties of used material for individual parts of hydrodynamic oscillator.


Introduction
The Russian national economy has changed greatly in the new century; there is a need to develop and ensure the performance of the socioeconomic complexes of the northern regions. The general and specific structural changes in the interaction between the federal and territorial level of state administration and the regional economy have also become more acute. Today, innovative technology is evolving toward longer and more productive machinery, more reliable, and safer to operate. The growth of automation and robotisation is particularly necessary when there is a shortage of skilled workers. Northern manufacturers operate in a more challenging environment. Costs of industrial enterprises in the northern regions, higher than the average indicator in the sector by 20-30 %, are explained by the necessity to increase the share of investments and also material and labour costs [1,2].
During the operation of hydrodynamic oscillations generator, which is a part of waterjet drilling head [3,4], its parts are subject to mechanical wear due to friction forces, especially cavitation that destroys surface layer, thereby reducing service life. At the same time, some of the parts are exposed to dynamic loads resulting from frequency vibrations [5,6]. The peculiarity of construction application does not allow the cross-section, overall dimensions, and weight of parts. Therefore, the materials used and the protection methods used to protect the parts must guarantee high resistance to wear, corrosion, static and dynamic strength [7,8].
Alloy steels have to meet these requirements. Heat treatment followed by quenching and low-temperature tempering gives the required strength, high hardness, and wear resistance to the working surface of the parts with high specific loads [9,10].
The mechanical, physical and technological properties of metals are highly influenced by their chemical composition. The properties of steels depend on the percentage of carbon in them. With increasing carbon content in steel, time resistance, yield strength, hardness increases and relative elongation and impact toughness decrease [11].
High demands are made on the parameters and properties of materials when it is possible to change either only the parameters or only the properties. The simplest solution is to choose a stronger material, but it should be considered that the higher the strength, the higher the resistance to plastic deformation. Reducing the maximum stresses induced by the stress concentration in the notch, due to fabrication techniques or metal heterogeneity, during plastic deformation is a very important factor in reliability. A ductile and deformable material, under the influence of high stress peaks, can, through flowability, reduce the stress at the base of the notch and create a hardening zone around the notch [12].
In high-strength materials, defects form a high stress concentration at the root of the notch, and it is only possible to reduce the occurring stresses by crack propagation in the material. Studying the resistance to cracking and brittle fracture hazards is becoming increasingly important for component reliability [13]. Therefore, there is a limit to the materials that can be used -only from the strength point of view. The ability of stress at the root of the crack to induce deformation reduces the uncontrolled propagation of the crack. If stresses are concentrated around nonmetallic inclusions and the root of the crack, in the absence of ductility, it can lead to a brittle fracture of the part before the elastic limit is exceeded [14].
Also, at low temperatures, the yield strength of steel increases considerably due to the braking effect of dislocation motion. When a certain temperature is reached (the so-called critical brittle temperature or cold brittleness threshold), brittle fracture occurs before the state of plastic fluidity. Attention should be paid to the cold brittleness of iron alloys as the main structural material used. Fragility in structural steels is mainly caused by phosphorus and a group of base metals (lead, tin, arsenic, and antimony); to a lesser extent by impurities of sulphur, copper, and zinc. Of these impurities, phosphorus has the greatest effect and its concentration is strictly limited in steelmaking codes. With increasing carbon in steel, the influence of these elements increases, and with decreasing temperature, they compete with each other in grain boundaries and mutually enhance their embrittlement effect on steel [15][16][17].
The lobe of the hydrodynamic oscillation generator, built into the drilling head, operates under conditions of cavitation and repeated alternating loads. The cavitation processes encountered with the integrated hydrodynamic oscillator in the conductor drilling head negatively impact the deformation surface and cause damage, thus reducing the service life of the drilling head. To increase tool life and cost of ownership, methods are being considered to increase material attributes such as static, dynamic, and cyclic load ratings, high ductility, high tensile strength, and toughness, high relaxation resistance, and corrosion resistance. The not less important property of the hydrodynamic oscillation generator metal is its low susceptibility to tempering embrittlement, low critical speed of hardening, high hardenability, low tendency to grain growth, and decarburization in heat treatment [18][19][20][21].
The aim of the research was the modes of heat treatment and their influence on hardness increase. Chemical composition of investigated steels is shown in Table 1.
Steel 60C2A was subjected to hardening with heating to the temperature of 860-870 ° C and holding at this temperature for 2 minutes. The planting in the furnace was carried out at a temperature of 860 C°. Cooling was carried out in oil. Then tempering was carried out with heating to 400-410 C° and soaking at this temperature for 4 hours. Setting in the furnace was carried out at a temperature of 300 C° with cooling in the air.
Steel 65G was subjected to hardening with heating to a temperature of 830-840 C° and holding at this temperature for 20 minutes. Setting in the furnace was carried out at a temperature of 800 C°. Cooling was carried out in oil. Then a tempering was carried out with heating to 400-410 C° and soaking at this temperature for 3 hours. Setting in the furnace was carried out at a temperature of 300 C° with cooling in the air.
Hardness measurements were made on the device for measuring the hardness of metals and alloys by the Vickers method, Brinell and Rockwell with an electronic computer system UT-5011A. The device is a stationary measuring instrument, consisting of a device for load application and measuring unit. The principle of operation is to embed a diamond cone or spherical tip into the surface of the sample under the action of successively applied forces of preliminary as well as primary forces in determining the penetration depth of the tip after the force has been removed ( Fig. 1).

Fig. 1. Rockwell hardness measurement diagram.
A PMT-3M micro-hardness tester was also measured. The micro-hardness tester leaves an imprint when the diamond tip (pyramid) is pressed into the metal under a certain load. The micro-hardness number can be determined by dividing the normal load F applied to the diamond tip by the conditional surface area of the imprint S. (1) here is a diamond tip load; is the lateral surface area of the resulting print. The Vickers micro-hardness number is calculated according to the formula:  (2) here P defines normal load applied to the diamond tip, N; N is force (kgf); dis the arithmetic mean of the lengths of both diagonals of the square print, μm. The tensile testing of material specimens is carried out using an I2143M tensile testing machine. The load applied by the machines deforms the test specimen and the value of this load and the corresponding deformation of the specimen are measured.
To determine the tensile strength, the specimens are loaded at a constant rate of expansion until failure occurs.
The tensile strength is calculated by the formula: (3) here is maximum tensile force.

Experimental data
We will consider values of microhardness parameters presented in Table 2, steels 60S2A-TSh-S-3.0x34 GOST 2283-79 melting No. 1T46911 and 65G-S-2.4x50 GOST 2283-79 melting No. 014929 before heat treatment then we observe that at identical values of steels hardness on НRC, at steel 60S2A with lower tensile strength has slightly higher microhardness than at steel 65G. A comparative microhardness characterisation is shown in Figure 2 and 3. The decarburised layer thickness was also investigated before the heat treatment in Table  3.  After heat treatment, the values of mechanical properties of 60S2A and 65G steel increase significantly. The increase in tensile strength of both steels increases by a factor of 2.4, while the relative elongation decreases, which characterises a reduction in ductility. The values of mechanical properties are given in Table 4. The increase in microhardness of samples No. 16,17 and No. 12, 13 after heat treatment shows the increase in local plastic deformation resistance, which enables us to improve the durability of hydrodynamic oscillator plates built into hydraulic conductor drilling head.
The analysis of chemical composition shows that steel 65G contains more phosphorus than steel 60C2A, which increases embrittlement effect on steel at lower temperature and increases its cold-brittleness. To reduce the influence of the decarburized layer on the reduction of surface hardness, machining is carried out after heat treatment. This allows the micro-hardness to be uniform throughout the entire surface layer of the part, reducing the effect of cavitation fracture during the drilling head process.

Conclusions
As a result of investigations, the dependence of increasing mechanical properties, microhardness of steel 60S2A GOST 2283-79 melting No. 1T46911 and 65G GOST 2283-79 melting No. 014929 was determined. The given dependence allows us to determine the ways of increase of service life of the oscillator plate of the hydrodynamic oscillator, builtin the hydro-monitoring drill head.