RU2156683C1 - Method for strengthening steel plates - Google Patents

Abstract

FIELD: processes for surface plastic deforming of steel plates by shots. SUBSTANCE: method comprises steps of successively deforming steel plate by means of small- and large-grain shots; realizing initial stage of strengthening with use of large diameter (2mm-3mm) shots in order to provide predetermined intensity of plastic deformation in centers of impressions equal to allowable uniform deformation of strengthened material; subjecting plate to uniform plastic deformation by stretching it until residual deformation value equal to (0.5-1.0)% in order to release residual stresses; performing secondary stage of strengthening with use of small diameter shots with diameter equal to (1-1,5)mm until receiving in center of impressions intensity of stress equal to yielding strength of plate material subjected to initial strengthening. EFFECT: increased useful life of steel plates in conditions of applying variable loads to them due to provision of strengthened layer with optimal distribution of residual stresses. 2 cl, 1 tbl

Description

 The invention relates to mechanical engineering, in particular to methods of surface plastic deformation (PPD) of parts with a shot to increase their durability when working under conditions of exposure to variable loads.

 There is a method of hardening parts with a shot (Odintsov L.G. Hardening and finishing of parts by surface plastic deformation: Handbook. - M.: Mashinostroenie, 1987. - 328 p., P. 249 - 250), which consists in preliminarily hardening with a given shot diameters of 5 ... 6 batches of auxiliary plates (4 plates in each batch) at different flight speeds of the shot and processing time, measure the deflection arrow of the plates, determine the optimal processing time, after which the deflection practically does not increase, process the shot with installed op The maximum time of the main plate.

 The disadvantage of this method is that the experimentally found hardening mode is valid only for a particular plate material and cannot be extended to plates with other strength characteristics of the material. In addition, obtaining the maximum deflection of the plate after processing leads to the appearance of large tensile residual stresses under the surface hardened layer, which contribute to a decrease in the durability of the plate during its operation under the influence of variable loads.

 The closest in technical essence is the method of surface hardening of flat torsion bars (as.with. 1488328, C 21 D 7/06, application. 03.12.86, publ. 23.06.89), including bead-blasting processing of the plane, while the torsion plate is packaged using gaskets with a width of 0.25-0.5 of the width of the part and carry out bead-blasting processing of the plane with a gasket and ribs free from overlap.

 The disadvantage of this method is that it does not take into account the strength properties of the processed material of the torsion plate, which does not allow to increase the durability of the part under conditions of exposure to variable loads.

 Known methods have a low technical level, since they do not take into account the strength properties of the hardened material of the steel plate, which does not allow to obtain optimal values of the intensity of plastic deformation and the intensity of stresses on the surface of the plate; in addition, as a result of hardening by known methods in the cross section of the plate under the hardened layer, significant tensile residual stresses arise, which cause a significant decrease in the durability of the plates.

 In this regard, the most important task is to create a new method of hardening steel plates by processing with a shot, which allows you to assign processing modes taking into account the strength properties of the hardened material, while at the initial stage of hardening, a deformation on the surface of the plate equal to the ultimate uniform deformation of the hardened material is achieved, followed by uniform plastic deformation by stretching to relieve residual stresses, while maintaining the hardened surface layer, and then on W of the original stage of hardening, the stress intensity on the surface of the plate is achieved in accordance with the initial stage, which is equal to the yield strength of the hardened material of the plate, which can significantly reduce the harmful residual tensile stresses arising under the hardened layer and significantly reduce the durability of the plate; this provides the greatest favorable residual compressive stresses in the hardened surface layer, which significantly increases the durability of the hardened plate.

 The technical result of the claimed method is the creation of a new technology of hardening by shot steel plates, ensuring the creation of a hardened layer in the plate with a strain rate on the surface equal to the ultimate uniform deformation of the plate material, while the stress intensity on the plate surface is reached, equal to the yield strength of the hardened plate material, and significantly reduced residual tensile stresses acting under the hardened layer, thereby significantly increasing The durability of a hardened plate operating under conditions of exposure to variable loads is maintained.

 The specified technical result is achieved by the fact that the steel plate is successively treated with coarse and fine shots, while the initial stage of hardening is carried out by treating with a large shot with a diameter of 2-3 mm to obtain a given intensity of plastic deformation in the center of the prints equal to the ultimate uniform deformation of the hardened material, and subject the plate to uniform plastic deformation by stretching to a value of residual deformation of 0.5-1.0% to ensure the removal of residual stress And then carried out secondary hardening step of fine fraction with a diameter of 1.0-1.5 mm to obtain a stress intensity fingerprint center equal to the yield strength of the hardened in the initial stage of the plate material, the plate is subjected to processing, working in conditions of variable loads.

 A significant difference of the proposed method is that at the initial stage of hardening with a coarse shot, a predetermined intensity of plastic deformation in the center of the prints of the grains is obtained, equal to the ultimate uniform deformation of the hardened plate material. The fact is that, from the point of view of strengthening favorable hardening factors (residual compressive stresses, degree of hardening of the material, thickness of the hardened layer), it is necessary that the intensity of plastic deformation in the center of the prints be the highest. However, with an increase in the intensity of plastic deformation, the process of softening of the material also increases, since dangerous microcracks appear in the surface layer and particles of peeling metal form. Thus, when processing with a shot, when the intensity of plastic deformation is equal to the maximum uniform deformation of the hardened material, the compressive residual stresses in the surface layer of the plate are greatest, and various kinds of defects do not yet receive a dangerous degree of development.

 A significant difference of the proposed method is that after the end of the initial stage of hardening, the plate is subjected to uniform plastic deformation by stretching to a value of residual deformation of 0.5-1.0% with the release of residual stresses, which allows, retaining the hardened surface layer formed after the initial stage remove harmful (i.e., reducing the durability of the plate) residual tensile stresses acting under the hardened layer, the value of which may be weight Lime is significant, since when hardening with coarse shot, the depth of the hardened layer is also significant. Obviously, this also removes the favorable (from the point of view of increasing the durability of the plate) residual compressive stresses acting in the hardened layer, since the residual stresses are a system balanced in the volume of the plate.

 A significant difference of the proposed method is the fact that they carry out the secondary stage of hardening with a fine fraction with a stress intensity in the center of the print equal to the yield strength of the hardened plate material, which allows to obtain maximum favorable residual compressive stresses equal to the yield strength of the hardened material on the surface of the hardened plate plates; in this case, since the diameter of the residual imprints from the fine fraction is noticeably smaller than in the first stage of hardening by large fraction, the penetration depth of secondary plastic deformation is also less than in the first stage. For this reason, harmful residual tensile stresses are also reduced in size, and the transition of compressive residual stresses to tensile stresses occurs inside the layer hardened in the first stage, which also contributes to an increase in the durability of the plate operating under variable loads.

 An analysis of the technique carried out by the applicant, including a search by patent and scientific and technical sources of information and identification of sources containing information about analogues of the claimed invention, made it possible to establish that the applicant did not find an analogue characterized by features identical to all the essential features of the claimed invention, and a definition from the list of identified analogues of the prototype as the closest in the totality of the features of the analogue revealed a set of essential (in relation to the seen Ithel technical result) distinguishing features in the claimed object set forth in the claims.

 Therefore, the claimed invention meets the requirement of "novelty" under the current legislation.

 To verify the conformity of the claimed invention to the requirements of the inventive step, the applicant conducted an additional search for known solutions in order to identify features that match the distinctive features of the prototype of the claimed invention, the results of which show that the claimed invention does not follow explicitly from the prior art, since the prior art determined by the applicant, the effect of the actions provided for by the essential features of the claimed invention is not revealed the achievement of a technical result.

 Therefore, the claimed invention meets the requirement of "inventive step" under applicable law.

 The method of hardening steel plates is implemented as follows.

 The initial stage of hardening is carried out by treatment with coarse shot with a diameter of 2-3 mm to obtain a given intensity of plastic deformation in the center of the prints equal to the ultimate uniform deformation of the hardened material.

The value of the ultimate uniform deformation ε _{p of the} plate material can be determined both from the results of standard tensile tests of the samples and from the value of the plastic hardness ND
ε _p = 245 / HD, (1)
where ND (MPa) is determined according to GOST 18835-73.

The value of the intensity of plastic deformation in the center of the residual imprint ε _{i, o} on the surface of the hardened plate is determined by the formula proposed by the authors
ε _{i, o} = 0.424 (1-2μ ₂ ) d _c / D _k , (2)
where D _to - the diameter of the coarse fraction; d _to - the diameter of the residual imprint during hardening by large fraction; μ ₂ is the Poisson's ratio of the plate material.

Thus, at the initial stage of hardening, one should seek to obtain a diameter of the residual imprint equal to
d _k = ε _p • D _k / 0.424 (1-2μ ₂ ). (3)
After the initial stage of hardening, the plate is subjected to uniform plastic deformation by stretching to a value of residual deformation of 0.5-1.0% to ensure the removal of residual stresses. This operation is carried out using known tensile testing machines.

 Then, the secondary stage of plate hardening is carried out with a fine shot with a diameter of 1.0-1.5 mm with a stress intensity in the center of the print equal to the yield strength of the plate material hardened at the initial stage.

где σ_т - предел текучести исходного материала пластины;
γ - коэффициент упрочнения, равный
γ = (S_в-σ_т)/ε_p, (5)
где S_b - истинный предел прочности исходного материала пластины

- предел прочности и предельная равномерная деформация исходного материала пластины;
ε_i,o - интенсивность пластической деформации в центре остаточного отпечатка (см. формулу (2)), которая при использовании мелкой дроби определяется как
ε_i,o = 0,424(1-2μ₂)d_м/D_м (6)
где D_м - диаметр мелкой дроби;
d_м - диаметр остаточного отпечатка при упрочнении мелкой дробью.Since the intensity of plastic deformation was equal to the ultimate uniform deformation at the first stage of hardening, according to the material deformation diagram, the yield strength of the plate material strengthened at the first stage is equal to the true tensile strength S _{b of the} initial (unstrengthened) plate material. The value of the stress intensity in the center of the print is determined by the formula

where σ _t - yield strength of the source material of the plate;
γ is the hardening coefficient equal to
γ = (S _in -σ _t ) / ε _p , (5)
where S _b is the true tensile strength of the source material of the plate

- tensile strength and ultimate uniform deformation of the source material of the plate;
ε _{i, o} is the intensity of plastic deformation in the center of the residual imprint (see formula (2)), which when using a fine fraction is defined as
ε _{i, o} = 0.424 (1-2μ ₂ ) d _m / D _m (6)
where D _m is the diameter of the fine fraction;
d _m - the diameter of the residual imprint during hardening by a fine fraction.

Для реализации предложенного способа приводятся экспериментальные данные для конструкционных сталей, на примере стали 30ХГСА, с получением показателя долговечности пластины по числу циклов до разрушения (см. таблицу). Каждая партия образцов состояла из 5-6 пластин.Thus, assuming that the stress intensity in the center of the indent σ _{i, o} is set equal to the true tensile strength s _{b of the} initial material of the plate, from formulas (4) - (6) we find the required diameter d _{m of the} residual indentation under hardening by a fine fraction

To implement the proposed method, experimental data are presented for structural steels, using the example of 30KhGSA steel, to obtain a plate durability index by the number of cycles to failure (see table). Each batch of samples consisted of 5-6 plates.

An experimental verification of the method. The samples were flat plate thickness of 4 mm and a width of 20 mm in accordance with GOST 25502-79, made of steel as delivered 30KhGSA: yield stress σ _m = 570 MPa, tensile strength σ _B = 780 MPa, hardness of plastic ND 2700 MPa. Fractional reinforced both sides of the plate.

Hardening in the primary and secondary stages was carried out with a different combination of fraction diameters:
for coarse shot, diameters of 2 mm, 2.5 mm and 3 mm were used;
for small fractions - 1 mm, 1.25 mm and 1.5 mm.

Preliminarily, the ultimate uniform strain ε _{p of the} plate material was determined by the formula (1)
ε _p = 245 / HD = 245/2700 = 0.0907.
Then, using the formula (3), we determined the diameter d _{to the} residual imprint, which should be obtained at the initial stage when hardening by large fraction.

коэффициент Пуанссона для стали 30ХГСА принят равным μ₂ = 0,3.
Для диаметра дроби D_к = 2,5 мм:
d_к = 0,0907•2,5/424(1-2•0,3) = 1,34 мм.For the diameter of the fraction D _to = 2 mm:

Poinson's ratio for steel 30KhGSA is taken equal to μ ₂ = 0.3.
For the diameter of the fraction D _to = 2.5 mm:
d _to = 0.0907 • 2.5 / 424 (1-2 • 0.3) = 1.34 mm.

For the diameter of the fraction D _to = 3.0 mm:
d _to = 0.0907 • 3.0 / 0.424 / (1-2 • 0.3) = 1.60 mm.

 Hardening by a fraction at the primary stage was carried out using a rotary type shot blast gun with ensuring the diameters of the residual prints found above.

 After the initial stage of hardening, the plates were subjected to uniform plastic deformation using a UMM-10 tensile testing machine to ensure the removal of residual stresses. Moreover, for different batches of plates, the values of residual strains were 0.5, 0.75, and 1.0%.

Before the secondary hardening stage of the fine shot was carried out, the required diameters d _{m of the} residual prints were preliminarily determined to obtain the stress intensity in the center of the prints equal to the yield strength of the plate material hardened at the initial stage.

а также определяли по формуле (5) коэффициент упрочнения материала пластины
γ = (S_в-σ_т)/ε_p = (854-570)/0,0907 = 3131.
Затем по формуле (7) определяли диаметры остаточных отпечатков, которые необходимо задавать при упрочнении мелкой дробью.For this, the true ultimate strength of the initial material of the plate was previously determined

and also determined by the formula (5) the coefficient of hardening of the plate material
γ = (S _in -σ _t ) / ε _p = (854-570) / 0.0907 = 3131.
Then, using the formula (7), we determined the diameters of the residual prints, which must be set during hardening with a fine fraction.

Для диаметра дроби D_м = 1,25 мм:
d_м=0,395•D_м=0,395•1,25=0,493 мм.For the diameter of the fraction D _m = 1 mm:

For the diameter of the fraction D _m = 1.25 mm:
d _m = 0.395 • D _m = 0.395 • 1.25 = 0.493 mm.

For the diameter of the fraction D _m = 1.5 mm:
d _m = 0.395 • D _m = 0395 • 1.5 mm = 0.592 mm.

 After completion of the hardening of the plates, the durability of the plate material under the influence of variable loads was determined.

 Fatigue tests were carried out according to the symmetrical cantilever bending scheme, according to GOST 25502-79, on a resonant type machine with a rigid loading cycle, which was controlled by the deflection of the plate. As a criterion of fatigue strength, the number of cycles N to complete destruction of the plate from fatigue fracture was chosen. The amplitude value of the bending stresses corresponded to the yield strength of the unstrengthened plate material. The results of comparative fatigue tests of the plates are shown in the table.

 As can be seen from the table, when using the proposed method of hardening with various combinations of diameters of coarse and fine fractions, as well as the values of residual tensile strain in the mentioned ranges of variation of these parameters (series of plates n 1-27), the average number of cycles to failure is N = 1492000; in this case, the values of the number of cycles N with a different combination of parameters deviate from the indicated average value, as a rule, by no more than ± 5%. The number of cycles before the destruction of the plates, hardened by the proposed method, is more than 9 times the number of cycles before the destruction of an unstressed plate (series of plates N 28) and 3.58 times the number of cycles before the destruction of the plates, strengthened by the prototype method (series of plates N 29).

 Using the proposed method in comparison with the known provides the following advantages.

 The method can significantly increase the durability (number of cycles to failure) of steel plates working under conditions of exposure to variable loads: for example, 3.58 times compared with the prototype.

 The method allows you to assign hardening modes depending on the mechanical characteristics of the source material of the plate, as well as on the mechanical characteristics of the hardened layer formed after the initial stage of hardening by coarse shot.

 The proposed method can be directly used in industry using existing shot blasting machines and explosive machines.

Thus, the above information indicates the fulfillment of the following conditions when using the claimed invention:
- the method embodying the claimed invention is a new technology of hardening by shot steel plates, which provides the creation of a hardened layer in the plate with a strain rate on the surface equal to the ultimate uniform deformation of the plate material and with an optimal distribution of residual stresses, which can significantly increase the durability of the products when using method in industry for hardening plates, springs, sheet products, etc. in the process of their production, operation and repair;
- for the claimed invention in the form described in the independent clause of the claims below, the possibility of its implementation using the means and methods described above or known prior to the priority date is confirmed;
- the method embodying the claimed invention, when implemented, is able to ensure the achievement of the technical result perceived by the applicant.

 Therefore, the claimed invention meets the requirement of "industrial applicability" under applicable law.

Claims (2)

 1. The method of hardening steel plates, in which the steel plate is treated with a shot, characterized in that the steel plate is successively treated with coarse and fine shots, while the initial stage of hardening is carried out by treating with a large shot with a diameter of 2.0 - 3.0 mm to obtain the specified the intensity of plastic deformation in the center of the prints, equal to the ultimate uniform deformation of the hardened material, and subject the plate to uniform plastic deformation by stretching to the residual deformation of 0.5 - 1.0% to ensure the removal of residual stresses, and then carry out the secondary stage of hardening with a fine fraction with a diameter of 1.0 - 1.5 mm to obtain a stress intensity in the center of the print equal to the yield strength of the plate material hardened at the initial stage .  2. The method according to claim 1, characterized in that the plate is subjected to processing, working under conditions of exposure to variable loads.

Images