SU1762190A1 - Method of diagnosis of parts having corrosion and fatigue defects - Google Patents

Abstract

The invention relates to parts (mainly power plants) that are subjected during operation to variable loads and a corrosive environment. It allows, by a one-time diagnosis of several parts of a specific purpose, differing in the level of operating voltages, to determine the residual life of the entire set of such parts. This is achieved by extrapolating the dependence of the depth of defects in parts on a parameter that takes into account simultaneously the production of parts at the time of diagnosis and the magnitude of the stresses in the part. 2 Il.

Description

The proposed method relates primarily to the elements of power plants subjected to variable loads and a corrosive environment during operation. The resource of such elements depends on the intensity of the development of defects of corrosion-fatigue origin (pits, cracks).

Known methods for predicting the development of defects such as cracks under cyclic loading are based on the formula of Rice, which relates the rate of increase

crack depths - with a scale factor-dN

voltage intensity ratio

And with (L kg0)

The stress intensity factor depends on the level and type of load, configuration and geometrical features of the part, location, size and number of cracks.

The formulas for calculating the residual life of parts with cracks, determined by the number of cycles for which the cracks reach the maximum permissible depth, are obtained by integrating expression (1) for each specific case. In this case, one has to resort to substantial simplifying assumptions (and, accordingly, a strong devaluation of the calculation results), since real cases do not reduce, as a rule, to computational schemes for which formulas for the coefficient K are given in the literature or available procedures for calculating it are given. .

Xi

About N)

Yu O

The known method for determining the residual resource based on the expression (1) allows one to dispense with the calculation of the K coefficient, which is a significant advantage of this method.

The disadvantage of this method is the need to diagnose the condition of the object three times during operation (three-fold measurement of the depth of cracks after some intervals of work). As applied to parts of heat and power equipment, the diagnostics of which is possible, as a rule, only within the framework of scheduled repair service, this means that the procedure for obtaining information on the residual resource will take a long period, measured in years.

The second disadvantage of this method is the difficulty of obtaining reliable values of constant C and p in formula (1) under the conditions of corrosive environmental conditions characteristic of the elements of heat power equipment (boilers and pipelines). The specificity of their work lies in the alternation of periods of operation at a high temperature sufficient to form on the inner surface of the tubular elements of a protective oxide film, with periods of standing, during which activation of corrosion processes is possible. In addition, the tests are carried out with a frequency much higher than the loading frequency of the equipment, and this leads to an additional deviation of the parameters of equation (1) from the values corresponding to reality,

The disadvantages of all methods based on the use of expression (1) are the limited scope of its existence: the initial stage of development of defects of corrosion-fatigue origin in the elements of steam boilers, which is often very long, cannot be described with its help. Expression (1) is only suitable for describing that stage of survivability, which is characterized by steady and progressive, (up to destruction) increase in the size of cracks.

The closest technical solution is the method of predicting the residual service life consisting in measuring the dimensions of the corrosion center (pit), building the results of this measurement of the dependence (in logarithmic coordinates — a straight line) of the size of pitting on the operation time and depending on the duration of the period of pitting development to crack. This period and proposed to take as a residual resource details.

The disadvantage of this method is that it is based on a specific analytical relationship between pitting sizes and test time (pitting diameter and depth proportional to time in degree -). Such an addiction

However, according to the given data (3), it does not take place in the entire time domain of pitting. Moreover, practice shows that the development of defects of corrosion and fatigue origin may also have horizontal sections. The definition of the limiting size of a defect in the method based on a fixed value of the stress intensity factor found

through testing, is correct in the framework of a certain design scheme that can be applied to a real situation, as a rule, only with substantial assumptions.

Limiting the service life of a part to the moment of cracking is also a disadvantage of the method, since the duration of work at the stage of the existence of a crack-like defect for many

elements are very long and replacing them according to the aforementioned attribute is premature with corresponding economic damage.

The aim of the invention is to improve the reliability and simplify the method for diagnosing parts used in the same environment, but at different operating voltages.

This goal is achieved by the fact that

The method of diagnosing parts with defects of corrosion-fatigue origin, which consists in comparing the depth of defects with the operating time and level of operating stresses, determines the maximum depth of defects in several parts of this group, builds the dependence of the maximum depth on the parameter characterizing the operating voltage, number of cycles loading and time

work details, determine the value of this parameter for the maximum allowable depth of the defect, which is used to calculate the residual resource.

As the aforementioned parameter, you can use the cotangent of the angle of inclination of the corrosion fatigue lines passing through a fixed point:

oh dpr

S - Ig 73

0)

where P is the time of the part at the time of diagnosis, determined by the number of loading cycles and the time of its operation;

(T3 amplitude of voltage variation per cycle of change in workload;

P and S are constants determined by testing parts for corrosion fatigue with varying aggressiveness of the corrosive environment.

Curves (in logarithmic coordinates - direct) of corrosion fatigue for materials of the same class under similar loading conditions, but with different aggressiveness of the environment with a decrease in the test base, approach each other, intersect at one point (Fig. 1). The constants P and S in the formula for the parameter Q are the coordinates of this point.

The parameter Q allows even a single measurement of the maximum depth of defects in several parts with different magnitudes to predict the growth of defects over time and by extrapolation to the allowable value of the maximum depth of the defect (h) to determine the residual life including all stages of development of the defect from pitting to cracks of a given depth - in contrast to the prototype method, which takes into account only the Ulcer stage.

FIG. 1 explains the essence of the parameter Q; Figure 2 illustrates an example of using the method.

To determine the constant P and S coordinates of the point of intersection of the corrosion fatigue curves (in logarithmic coordinates — direct, FIG. 1), it is necessary to carry out tests of the details (field or models) for corrosion fatigue in aggressive environments. The more aggressive the environment, the greater the inclination direct to the abscissa.

An example of using the method.

The method was used to determine the residual life of the bends of the PK-47 bypass pipes in the section between the water economizer and the middle radiation part. These bends (27 pieces) are made of pipes 245x22 and 273x25 mm, steel 20, working pressure 17.5-18.2 MPa, temperature 305-360 ° C. The operating time of the boiler at the time of diagnosis is 171 including. 513 starts. The parametric operating time, corresponding to the operating time in time and by the number of beams, taking into account the pressure tests, the coagula was P 7090, d P 3,852. According to the ultrasound testing, confirmed by cutting, the bends were hit by cracks of corrosion-resistant origin with a depth of up to h 8 mm.

The residual life was determined from the results of a study of four dismantled bends among those mentioned above. The amplitude of the stresses from the internal pressure Od, acting in the studied bends, calculated from their actual ovality and wall thickness, was 0. 113.8-160.4 MPa. For these values and the above magnitude, the operating time, using formula (2), calculated the values of the parameter Q. With ego, the values of P and S were taken according to the processing data of the published test results of gibo and their models for corrosion fatigue.

The maximum depth of cracks h in each of the four bends was determined by metallographic examination of templates cut from damaged zones. Dependency graph

between Q and q - (s - wall thickness), built

According to the data obtained, it is shown in 5 of Fig. 2. The maximum allowable depth of cracks in the considered bends is 12 mm for bends of pipes 273x25 mm (h / s 0.48; Ig h / s -0.318) corresponds (in the extrapolated section of the graph) Odop 2.745, maximum to 0 deep depth of existing cracks (8

mm 0.32: Ig- -0.494) corresponding to s 2os

Q. 2.605 (figure 2).

Residual life is determined by 5 differences

0DopO 2.745-2.605 0.14 According to expression (1), the indicated difference can be represented by the formula

0

p - p - ig PD ° P 9 P

idiop u--

(2)

S - Ig da

Based on the analysis of the available experimental data, s 2.8 can be accepted. As aa in the formula (2), the highest value should be taken for bends, 5 of which it is planned to continue operating, in the case under consideration, 166 MPa, ig s / a 2.22. For the specified values included in the formula (2) values

1DPDop - DP 0.14 (2.8-2.22) 0.081; 0 .1 dPDop 3.852+ 0.081 3.933.

The obtained value of the limiting parametric operating time while maintaining the same frequency of boiler start-ups corresponds to the maximum operating time, equal to $ 204 thousand hours. Thus, the residual life of the considered bends is 204-171 33 thousand hours.

The proposed method allows, according to one-time diagnostics, carried out, for example, during the routine shutdown of the unit, to estimate the residual service life of parts for a specific purpose.

Applied, for example, to bends of unheated boiler tubes, this makes it possible to avoid a long overdraft of equipment.

Claims (1)

The invention of the method of diagnosing parts with corrosion-fatigue defects, by which the depths of defects are measured at details with preliminary operating time, the levels of stress in the parts are determined and with their help the residual life is determined tgda parts, characterized in that. in order to increase the reliability and simplify the method for diagnosing parts used in the same environment, but with different levels of operating stresses, the depth of defects is measured on the parts at least once, the maximum depth of the defect for several samples is determined, the dependence of the maximum depth on the parameter characterizing the operating voltage, the number of loading cycles and the operation time of the parts, the value of this parameter is determined for the maximum allowable depth of the defect and this value is used to determine the residual resource. ffffl 1 O h h -0.5