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Possibility of applying X-ray methods to control the surface quality of a shaft line after finishing

The purpose of the paper is to analyze the application limits of X-ray methods of non-destructive testing of loaded parts; to compare the results of microstresses and deformations of the details’ surface layer by methods and by the method of X-ray diffraction analysis for various modes of processing the detail surface layer. The studies are carried out on a “Dron” diffractometer. The technique and algorithm of X-ray structural studies, namely, “sin2v|/”-method are represented. Residual macro σφ and micro stresses, as well as the sizes of the areas of coherent scattering (D) on the samples surfaces processed in various modes, and their distribution in the near-surface layer are designated. Phase analysis is conducted and the presence of residual austenite. The research object is the operating surface of the 46-19-186 gear tooth after various treatments: after HFC hardening; after HFC hardening, grinding and blasting in depressions; after HFC hardening and fine-finish cutting. The X-ray structural analysis (XRD) technique is presented to determine the residual macro-σφ and microstresses, the sizes of the coherent scattering regions (D) on the surfaces of the samples processed in different modes. The outcomes of X-ray structural analysis are compared with the outcomes of metallographic studiesmaking. It was determined that the stress relaxation during the manufacture of the sample is no more than 10%, and the total instrumental error of the X-ray spectral analysis method is about 1%.


Introduction
The task of increasing the operational reliability and durability of machine parts cannot be solved without an effective system for technical diagnostics of the destruction causes. The rational use of non-destructive testing methods ensures the quality of materials and details, defines the stages of the process with defects and, therefore, identifies and eliminates the causes of defects in the production process, and also monitors the development of defects during the equipment operation.Non-destructive testing is based on the use of fields, radiation and substances to obtain information about the quality of materials and objects. X-ray methods differ in the use of a set of high-tech equipment from all existing methods of non-destructive testing and allow obtaining versatile information about almost all the most important parameters of a substance, such as its composition, crystalline perfection, atomic structure, energy and electronic structures, etc.
Technological parameters designate the macro-and microstructure of the material, which the mechanical properties depend on (strength, hardness, plasticity, viscosity, elasticity, endurance, heat resistance, wear resistance, corrosion resistance, etc.), as well as their physical properties (thermal conductivity, density, thermal expansion, conductivity, magnetic, etc.). Various factors affect the X-ray method sensitivity: − constant ambient temperature; − studied material state: blurring of lines associated with the sample material dispersion, distortion of the crystal lattice, as well as the shift in the maxima positions in monolithic samples caused by residual stresses as a result of elastic deformation, has a significant effect on the accuracy of determining the interplanar distances; − diffraction angle: the larger the diffraction angle is, the smaller is the relative error in determining the interplanar distance ∆ =ctgθ; − accuracy of determining interplanar distances and lattice parameters depends on the choice of indices (hkl) of reflecting planes for materials with a non-cubic lattice. − methodological negligence in the experiment, misalignment of instruments, inaccurate sample placement and other factors affecting the recording and measurement of the diffraction angle. It is advisable to supplement the procedure for defining the phase composition with the X-ray spectral method to designate the sample chemical composition. Moreover, it is expedient to apply rotation of the sample during X-ray photography in order to eliminate the effect of mutual screening of particles in a polycrystalline specimen.
Zonal macrostresses in specimens and metal structures that stably exist in a physical object for a sufficiently long time without the application of external loads, namely, residual stresses affect the increase in the operational reliability and durability of machine details. The physical nature of their occurrence is associated with the technology of manufacturing and processing of details and can be determined by metallographic methods [2,5].

Materials and research methods
The research object is the operating surface of a gear tooth of 46-19-186 industrial tractor after various treatments: after HFC hardening; after HFC hardening, grinding and blasting in depressions; after HFC hardening and fine-finish cutting.
The metallographic studies of thin sections of the working surface of gears 46-19-186 after various technological processing modes are described in the papers [3,4]. Thin sections of the working surface were made for metallographic studies [3]. Investigations of the microhardness distribution were carried out on a V-Testor 2 hardness tester according to the Vickers method at loads of 9.8 H. The microstructure for metallographic studies was revealed by etching in a 4% alcohol solution of HNO3 after grinding and polishing the surface. The microstructure was viewed using a Neofot-21 microscope at magnifications of 400, 1000 times. The values of the martensite and pearlite points were determined visually according to the tables of GOST 8233-56.
X-ray structural studies were carried out on a "Dron-2" diffractometer. Residual macro-ᵩand microstresses, as well as the sizes of coherent scattering regions (D), were determined on the surfaces of the samples processed in different modes and their distribution in the near-surface layer. Phase analysis was carried out in parallel, and the presence of residual austenite was designated in the surface layer.
The study of the stress state and fine structure was conducted layer by layer from the surface to a depth of 0.4 mm. Metal layers were sequentially removed from the samples by electrolytic etching in an electrolyte of composition (Table 2).  [3,4]. It is essential to estimate the amount of relaxation, since the macrostresses relaxation occurs when cutting the detail. According to the literature [2,5,6,8], deformation occurs along the direction of cutting and grinding. The stress level is assessed before and after cutting. The sample is mounted on the diffractometer so that the direction of grinding is parallel to the axis of the goniometer. The obtained stress values before cutting σφ=-99kgf/mm 2 and after cutting σφ=-91kgf/mm 2 differ by the amount of permissible measurement error (~10%).
Theerror in calculating stresses σφis associated with the inaccurate finding of θ angle, since the diffraction maxima are strongly blurred due to various structural and physical factors.
To clarify the values of θ, the Kukol method -a method for identifying the position of the profile maximum is the most efficient. Kα is the doublet constituent over a relatively narrow section of the doublet profile, within which the asymmetry from stacking faults is small and does not practically appear. This method does not require measurement of the profile tails and does not involve modeling the profile with analytical expressions. The function is built without highlighting the component profiles Β-veritable physical broadening of the line θ -diffraction angle λ -X-ray wavelength The veritable physical broadening of (hkl) line is caused by both the refining of blocks and the presence of microstresses and is measured by the formula:

Results and discussion
Gear tooth material after annealing at С Т 0 900 = , τ =2 hours and cooling in the oven was used in X-ray photography as a reference.
Determination of Ϭφ microstresses of one of the finishing turning samples at a depth of 30 microns is considered as an example. Diffraction reflections from family of (211) planes are used to compute by observing the plane at different angles: x , 2 x , 3 x close to the diffraction maximum, for example, for The graph intersection point of this function and the diffraction curve corresponds to the S point, which lies in the middle between the maxima of the doublet components. The gravity center of a line with an inseparable doublet at a distance of 1/3 0 x from the component maximum with higher intensity and is computed by the formula: As an example of calculating microstresses and blocks of coherent scattering, the same sample at a depth of 30 microns is considered. For the calculation, diffraction reflections from the family of planes (211) and (110) of the sample under study and the reference were used. Annealed tooth material was used as a reference. According to the above-mentioned method, we designate α α ∆ and D by (1) and (2)   Bexp., mm-half-width of the diffraction maximum measured in mm , rad is determined due to the counter speed and belt diagram -interdoublet distance  According to the corrections graph of geometric broadening ( Figure 3): We find the ratio of   (2) an (1). The outcomes of D and ∆ at a depth of 30 microns of the surface layer of the sample after finishing turning are presented in Table 5. X-ray diffraction investigations of the studied sample of the surface layer were carried out to a depth of about 400 microns. The investigation outcomes of the surface and surface layers after the final serial processing -grinding and shot hardening are shown in Figure 5. The level of compressive stresses φreaches about 90 kgf/mm 2 on the teeth working surface. The greatest value φreaches at a depth of 30-40 microns -115 kgf/mm 2 . This level remains up to a depth of about 90-100 microns and then φdecreases to values of 30-40 kgf / mm2, without significant changing. The change nature φin the second sample after grinding is somewhat different. The maximum φ is observed on the order surface of 100 kgf/mm 2 . Further, the curve character is the same as for the first sample in the depth interval of 100-400 microns. Considering metallographic studies [8,9,10], it can be assumed that this difference is due to various processing modes.
The block sizes of coherent scattering increase after grinding from values of 650-700 Aº at a depth of 150 microns, then they monotonically decrease to values of 1500-2000 Aº order ( Figure 6).
In this case, microstresses vary from values of 1.0*10 -3 on the surface to 2.0-2.2*10 -3 at a depth of 150 microns, and then the level ∆ remains approximately the same (Figure 7). After finishing turning, the level of compressive stresses on the surface is much lower and amounts to about 15 kgf/mm 2 . Maximum value of φis reached at depths of 40-100 microns -35-45 kgf/mm 2 . Further, the stress level decline to values of 20-30 kgf/mm 2 . The block sizes of D coherent scattering in the depth interval of 0-150 microns enhance from the order values of 100 Aº to 3000-3500 Aº, and then monotonically reduce. Microstresses ∆ on the surface are much higherthan after grinding -1.60-1.70*10 -3 . They reach a maximum of (2.40-2.30 10 -3 ) at a depth of 150 microns and do not change anymore.
After hardening with HFC heating and tempering, the level of compressive stresses on the surface is minimal and reaches 20 kgf/mm 2 . At a depth of 60 microns φ grow to values of 50 kgf/mm 2 and then become lower.