RU2231057C2 - Process of nondestructive test of degree of damage of metals in used elements of thermal power equipment - Google Patents

Abstract

FIELD: methods of study and analysis of material with use of acoustic waves.

SUBSTANCE: proposed process of nondestructive test of degree of damage in elements of thermal power equipment is based on establishment of criterion of degree of damage of metals in elements and on its usage to find time dependence from moment of test to moment of probable destruction of element of thermal power equipment. Delay of surface wave of ultrasonic oscillations on surface of new element, in zone of emergency destruction of metal of element and on surface of metal in tested zone of used element is measured.

EFFECT: capability to give both quantitative estimation of degree of damage to surface metal in element in process of its working and to relate it to time of maximum permissible usage.

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Description

The invention relates to methods for researching or analyzing materials using acoustic waves, in particular to non-destructive testing methods by examining the propagation speed of acoustic waves and measuring their attenuation rate.

During the long-term operation of power equipment at high temperatures and pressures in the metal, complex physical and chemical processes occur, primarily associated with the decomposition of the perlite component of the microstructure, coagulation and spheroidization of carbides, the formation of micropores or wedge-shaped cracks.

The greatest danger from the point of view of the reliability of power equipment is represented by bends (bends) of steam and steam bypass pipes, in which, despite the same operating temperature of straight pipe sections, the rate of accumulation of microdamage is much higher, which can be explained by the high level of additional effective stresses, depending on the shape of the bend, compensating stresses, stresses from weight loads, etc.

Measurement of the degree of metal damage due to changes in its microstructure during operation directly at the facility is a rather difficult and time-consuming task.

In the energy sector, there is a metal control system, the so-called non-destructive testing system using the acoustic emission method (see Gosgortekhnadzor of Russia, State Unitary Enterprise and Scientific and Technical Center “Industrial Safety”. Collection of documents. Series 28. Issue 2001).

The method of acoustic emission is based, in particular, on the assessment of the reliability of metals in steam pipelines (see Smirnov A.N. Performance of heat-resistant steels depending on structural changes and its evaluation by the method of acoustic emission: Abstract of thesis ... Candidate of Technical Science . - Novokuznetsk, 1985).

The acoustic emission method is based on the emission of elastic waves by a material caused by a dynamic local restructuring of its structure.

The disadvantages of the method of acoustic emission as a method of non-destructive testing of the degree of damage to the metal include:

- the complexity and high cost of control equipment;

- not an explicit relationship between damage to the surface of the metal and the temporal parameters of the destruction process during equipment operation.

Closest to the invention should include the method of ultrasonic control of surface roughness (see Vasiliev AG, Muravyev VV, Smirnov AN Ultrasonic method for monitoring surface roughness / RAS. - Defectoscopy, No. 2. Separate issue, 1994) .

The method is based on the effect of surface roughness on the propagation of a surface acoustic wave.

The process of measuring the speed of ultrasonic vibrations is as follows: sensors with a fixed base are installed along the generatrix of the power equipment element in the initial state and on the generatrix of the reference cylindrical sample, the surface roughness of which is measured and must meet the requirements for heat and power equipment components prepared for operation.

It was noted that the delay in the surface wave of ultrasonic vibrations also depends on surface damage not related to its processing.

The disadvantages of the method of ultrasonic control of surface roughness (damage) should include the fact that even assuming that the influence of the surface roughness of the metal and its structural changes on the speed of ultrasonic vibrations were identical, only quantitative changes in the state of the element metal could be estimated in an indirect way.

The essence of the invention is to determine the criterion of the degree of damage to metal elements of power equipment. For this, the delay of the surface wave of ultrasonic vibrations on the metal surface of the new element is measured, the delay of the surface wave in the zone of emergency destruction of the metal of the element and the delay of the surface wave on the metal surface in the controlled zone of the element in operation.

Then, the criterion K _{n of the} degree of damage to the surface of the metal of the element is determined from the relation

where W ₀ - the average delay of the surface wave on the metal surface of new elements;

W _p - the average delay of the surface wave on the metal surface in the zone of destruction of the element;

W _t is the delay of the surface wave on the metal surface in the controlled area of the element during operation.

The criterion K _{n the} degree of damage to the surface of the metal of the element, expressed in relative units, allows you to judge the quantitative value of the damage and stop the use of heat and power equipment to replace the controlled element, based on the ratio of K _n = 0.7-0.9.

For steel grade 12X1MF under normal operating conditions, W ₀ = 6440 ns, W _p = 6540 ns.

Measurements of the delay of the surface wave for a lifetime of 0.25; 0.5; 0.75 from the operating time of the heat and power equipment element to failure showed values of W _t respectively 6454, 6473, 6500 ns, and the criterion for the state of damage had values of K _n respectively 0.139; 0.328 and 0.602. Since the values of the criterion Кп of the state of metal damage for the new (W _t = W ₀ ) and destroyed (W _t = W _p ) items of equipment have values of 0 and 1, respectively, its disproportionate dependence on the operating time was noted, according to which the time before the destruction of the element is determined from the ratio

where T is the average operating time of an element of a particular metal until it is destroyed, h;

γ = 1-1,2 - coefficient taking into account operating conditions.

The success of using the proposed approach to the tasks of forecasting and assessing the degree of damage to metal of power equipment is largely determined by the level of hardware and software for supporting the spectral-acoustic method.

We used the ASTRON automated spectral-acoustic system. The basis of the operation of the hardware of the system is a method for detailed registration of the entire series of reflected acoustic pulses for its subsequent processing by the software of the system. In the processing part of the system (NOTEBOOK type computer), the oscillograms of reflected pulses are sequentially converted with a certain discrediting step from the moment of sounding the metal until the arrival of the nth reflected pulse.

By the value of the criterion K _{n the} degree of damage to the metal surface, it is possible to set the time to the probable moment of destruction of the metal of the element, as well as a rational period of time until the next control measurement or to determine quite accurately the time of replacement of the element of equipment before its emergency destruction.

Thus, the proposed method for the first time allows not only to quantify the degree of damage to the surface of the metal element during its operation, but also to relate it to the time of possible (permissible) operation.

Claims (2)

1. The method of non-destructive testing of the degree of damage to metals of operated elements of thermal power equipment, including determining the delay time of an ultrasonic surface wave, characterized in that the determination of the delay time of a surface wave is performed on the surface of a new element, in the zone of destruction of the element and in the controlled area of the operated element, and then determine criterion for the degree of damage to the operated element from the ratio

where K _p - the criterion of the degree of damage to the metal in relative units; W ₀ is the average delay time of an ultrasonic surface wave on the surface of new elements, ns; W _p - the average delay time of an ultrasonic surface wave on a metal surface in the zone of destruction of the element, ns; W ₁ - delay time of an ultrasonic surface wave on the surface of the operated element, ns, moreover, the replacement of the operated element is carried out under the condition K _p = 0.7-0.9. 2. The method according to claim 1, characterized in that the operating time of the element is determined from the ratio

where Δt is the operating time of the operated element until its probable destruction from the moment of control, h; T - the average life of the element to its destruction, h; γ - coefficient taking into account the operating conditions of the element, γ = 1 ÷ 1.2.