RESIDUAL LIFE ASSESSMENT OF A PIPELINE BEND BY USING SMALL PUNCH CREEP TESTS

Small punch creep test (SPC test) is an alternative and still developing method to conventional uniaxial creep test and as such can be used for obtaining information about the creep properties of steels and alloys even when only a limited amount of test material is available. SPC method has been also implemented into EN 10371 "Metallic materials – Small punch test method". SPC testing can also be used with advantage also for assessment of the residual life of critical components of fossil fueled power plants. The biaxial nature of the SPC tests differs considerably from the uniaxial conventional creep tests but it is possible to convert the equivalent SPC loading to the uniaxial creep stress on the basis of the equal test durations. Such an approach is demonstrated on the material of a pipeline bend after long-term exposition at 490 °C and the results are used for the assessment of the residual lifetime of this bend made of a 0.5Cr-0.5Mo-0.3V steel.


INTRODUCTION
The pressure parts of fossil fueled boilers are exposed during long-term operation to high temperature and pressure, which lead to the material degradation, i. e. changes in physical and mechanical properties and/or the damage of microstructure.Determination of the current material state and its properties and the associated estimation of residual life of a component using standardized testing procedures requires a large amount of test material and time-consuming shutdown associated not only with material sampling but also subsequent repair of the sampling point.Often, these places become the place of subsequent damage and/or cracks, during further operation.The small punch test method, which largely eliminates these hazards while maintaining an adequate level of accuracy and reliability, is becoming an increasingly exploited method for evaluating the degradation of material properties and determining the current mechanical properties of individual components of critical power plant components (yield strength, tensile strength, transition temperatures, fracture toughness, creep characteristics, etc.).National standards or binding guidelines already exist in Japan and the USA [1][2].The European standard "Metallic materials -Small punch test method" was also issued in 2021 and covers the evaluation of mechanical properties by tensile testing from cryogenic to high temperatures and also deals with the estimation of creep characteristics based on the results of small punch tests [3].
The small punch test method was exploited in the case of determining material characteristics and residual lifetime assessment of a steam pipe bend made of 0.5Cr-0.5Mo-0.3Vsteel and operated in a 110 MW fossil fueled boiler at a temperature of 490-500 °C for more than 270,000 hours.Although the operating temperature of this steam pipeline is relatively low for the use of a given steel make (the maximum permissible temperature of this steel is up to 580 °C), a very long operating time does not exclude the possibility of appreciable degradation of its microstructure as well as the material properties.The evaluation of the actual material characteristic and mechanical properties and their influence on lifetime assessment of power plant components performed earlier [4] was supplemented by small punch creep testing.

TESTED MATERIAL AND PROCEDURES
The test material was scooped out at the extrados of the pipe bend in the form of three small samples using the SSam sampling device.After sampling, the edges of the sampling points were smoothed by grinding to avoid any structural notch.Thus, three disc-shaped samples were at disposal, on which it was possible to: • perform control chemical analysis and to confirm the make of used steel, • determinate the yield stress and tensile strength by small punch tests (SPT), • perform hardness measurement, • analyse the degradation of the microstructure (cavitation damage, degradation of the microstructure) including its evaluation according to VGB TW 507 [4], • perform SPC tests at various loads and to calculate the residual lifetime.

RESULTS OF ANALYSES OF CHEMICAL COMPOSITION AND MECHANICAL PROPERTIES
The analysis of the chemical composition was performed on one of the small samples by X-ray spectrometry, the contents of carbon, sulfur and nitrogen by the combustion method on the LECO analyzer.The results are stated in Table 1 together with the nominal composition of the steel 15 128, the national equivalent steel grade in the Czech Republic [5].It is well known that the atoms of the substitution trace elements migrate to the grain boundaries, which results in embrittlement of the material.In order to evaluate the metallurgical quality of steel, a criterion of embrittlement susceptibility during long-term exposure, so-called CEF (Creep Embrittlement Factor), was formulated based on the concentration of phosphorus (P), antimony (Sb), tin (Sn) and arsenic (As) [6]: Steels with values CEF<0.15 are considered to have a very good metallurgical quality.In the respective case, the value of CEF was as low as 0.034 (see Table 1) and the steel thus has very low embrittlement susceptibility.
The mechanical properties of the bend were tested by small punch tests at room temperature.Disc-shaped test specimens with 8 mm in diameter and 0.5 mm thick were tested on INOVA 250 testing machine.The results are stated in Table 2, including the standardized values specified in the material standard for normalized and tempered state (grade 15 128.5) and after accelerated cooling and tempering (grade 15 128.9).Hardness measurements were also performed on one of the small samples using the HV 10 method and the results of hardness measurements are summarized also in Table 2, altogether with the informative hardness of this steel stated in the material standard.It is evident that the mechanical strength of the pipe bend is high and lies above the standardized range for the steel grade 15 128.5 and close to the maximum tensile strength valid for the grade 15 128.9.The same is true for the results of the hardness measurements.Also, this information confirms the assumptions of a very good condition of the bend material.

RESULTS OF METALLOGRAPHIC ANALYSIS OF THE STEAM PIPELINE BEND
Metallographic analysis was performed on a sample parallel to the sampling plane as well as to the pipe surface at the bend extrados and was concentrated in analysis of microstructure, evaluation of structural changes due to long-term exposure at high temperature and especially analysis of creep cavitation damage, including its evaluation according to VGB TW 507.
The microstructure of the steel consisted of ferrite with fine precipitates, blocks of tempered bainite and smaller dark islands, originally probably pearlitic, see Figure 1.The spheroidization of carbide particles in bainite or pearlitic lamellae was just starting and the secondary precipitates along the grain boundaries, which is the typical sign of structural degradation due to long-term high temperature exposure, were observed only very rarely, see Figure 2.

RESULTS OF SPC TESTS
The method of SPC testing and evaluation of the test results are based on their correlation with the results of creep tests.Due to the complex stress state in SPC it is not possible to exactly transfer loading in SPC testing where: tr is time to rupture (h) F is the applied load (N) ASP, nSP are temperature dependent constants For the extrapolation of the results of creep as well as SPC tests, the Larson-Miller parameter PLM is often exploited in the form: where: CLM is Larson-Miller constant, in this case implicitly chosen at C = 20.
This parameterization of temperature and rupture time allows to influence the influence of both of these variables on one axis and thus perform make a direct comparison of creep tests performed at different temperatures.
The load dependence of timer to rupture calculated by Norton´s equation is stated in Figure 4 and the relation of load versus Larson-Miller parameter in Figure 5.
Figure 4 Correlation between loading and time to rupture

APPLICATION OF RESULTS FOR RESIDUAL LIFE ASSESSMENT
The hoop stress, which was determined for this bend by strength calculation of the pipeline taking into account the relaxation due to creep, is 142 MPa and, when using the safety factor k = 1.25, the limit hoop stress is 177.5 MPa.When using the conversion coefficient Ψ = 2.1 (see Equation ( 1)) determined in our creep laboratory for the conventional creep and SPC tests of this group of steels (low-alloy Cr-Mo-V steels), then the equivalent load to 177 MPa for SPC test is 373 N. Assuming the validity of the Larson-Miller equation compiled from the results of SPC tests in the whole considered temperature range, it is possible to convert the results obtained at a temperature of 600 °C to a working temperature, i.e. 490 °C.The results of this conversion are given in Table 5 for the load corresponding to the above operating stress.
Although the calculated time to rupture is very high, in principle it confirms the results of the other analyses, where both the strength properties as well as the microstructure corresponded to the as-received material without any creep exposition.This may be due to the fact that the operating temperature of the steam pipeline

Figure 1 Figure 2
Figure 1 Microstructure of pipeline elbow Figure 2 Detail of microstructure of pipeline elbow

3 Figure 3
Figure 3 Results of SPC tests of steampipe bendThe Norton´s creep power law was applied on the results of SPC tests in the form of:

Figure 5
Figure 5 Correlation between time to rupture and L-M parameter

Table 1
Chemical composition of pipeline elbow, mass%

Table 2
Yield and tensile strength and hardness HV 10 at room temperature determined from SPT

Table 3
Description of Neubauer damage classes in VGB TW-507