Prediction of the Residual Creep Life of SA335-P22 Steel Main Steam Pipelines

Abstract

In this study, the comprehensive properties of a P22 high-temperature steam pipeline with moderate spheroidization were evaluated after more than 11,700 h of operation through a series of physico-chemical properties testing, especially for a creep test. The remaining life of the P22 high-temperature steam pipeline was analyzed and predicted by the θ parameter method, to guide the normal operation of the P22 high-temperature steam pipeline.

1. Introduction

Sa335-P22 steel pipes [1,2,3,4,5] consist of 2.25Cr-1Mo low alloy steel, as this material exhibits good weldability and persistent plasticity, and is mainly used in the pressure bearing system of main-steam pipelines in thermal power plants. A comprehensive inspection of a powerplant (Location: Nanjing, Jiangsu Province) in 2015 found that the metallographic structure of a section of SA335-P22 pipeline was moderately spheroidized. The pipeline was operated at 7800 h per year, and this batch of SA335-P22 pipeline had been in service for about 117,00 h. The dimensions and metallographic examination results of the SA335-P22 material are shown in Table 1. To ensure the safe and continuous operation of SA335-P22 main steam pipelines, the material properties of the SA335-P22 main steam pipeline were comprehensively evaluated through on-site sampling and a series of physical and chemical tests. Compared to the traditional extrapolation prediction method [6,7,8,9,10], the θ parameter method [11,12,13] better addresses the entire creep process. Therefore, based on the creep test results, residual creep life evaluation of a SA335-P22 main steam pipeline was conducted by the θ parameter method.

Table 1. The specifications and metallographic examination results of the SA335-P22.

Material	Medium	Dimension/Thickness (mm)	Design Temperature (°C)	Design Pressure (MPa)	Working Temperature (°C)	Working Pressure (MPa)	Spheroidization Degree
P22	Steam	219/20.6	524	8.5	510	7.0	Moderate

2. Physical and Chemical Tests and Result Analysis

The physical and chemical tests consisted of chemical composition analysis [14], mechanical property [15], metallographic analysis [16], hardness, and creep rupture tests [17].

2.1. Chemical Composition Analysis

The chemical composition of the SA335-P22 steel pipe was analyzed by energy spectrum analysis. As shown in Table 2, its chemical composition met the ASME SA335 [18] requirements for P22 steel pipes.

Table 2. Chemical composition (wt).

Material	C	Mn	p	S	Si	Cr	Mo	Other
ASME SA335-P22	0.05–0.15	0.30–0.60	≤0.025	≤0.025	≤0.50	1.90–2.60	0.87–1.13	-
Result	0.10	0.43	0.014	0.005	0.24	2.09	0.94
Conclusion	Meets ASME SA335 requirements for new P22 pipes

2.2. Mechanical Properties

2.2.1. Tension Test at Room Temperature

The test was carried out according to GB/T 228.1-2010 “Metallic materials tensile test part1 room temperature test method” [19]. The size of the sample is shown in Figure 1, and the test was conducted on a MTS809 tension torsion testing machine. The longitudinal tensile test results are presented in Table 3, and the transverse tensile test results are provided in Table 4. The tensile performance results obtained at room temperature demonstrated that the P22 pipe had poor plastic properties at room temperature, although all indicators met the relevant AEM SA335 requirements.

Figure 1. The size of the sample at room temperature (mm).

Table 3. The results of the longitudinal tensile test.

Sampling Position	Sample	Rp0.2/MPa	Rm/MPa	A/%
SA335/SA335M	-	≥205	≥415	≥22 (portrait)
Near the outer wall	1	322	538	31.5
	2	326	542	28.5
	3	316	532	26.5
Near the middle wall	1	344	553	35.0
	2	350	559	30.5
	3	330	534	29.5
Near the inner wall	1	334	536	25.0
	2	349	557	26.5
	3	351	557	29.5
Conclusion	Room temperature longitudinal tension meets the requirements of ASME SA335/SA335M for the P22 new pipe

Table 4. The transverse tensile test results.

Sampling Position	Sample	Rp0.2/MPa	Rm/MPa	A/%
SA335/SA335M	-	≥205	≥415	≥14 (portrait)
Near the middle wall	1	343	553	25.5
	2	346	554	28.0
	3	346	556	28.0
Near the inner wall	1	345	544	26.0
	2	348	546	26.0
	3	331	541	29.0
Conclusion	Room temperature longitudinal tension meets the requirements of ASME SA335/SA335M for P22 new pipe

2.2.2. Tensile Test at High Temperatures

Testing was carried out according to GB/T 228.2-2015 “Metallic materials tensile test part 2 high-temperature test method” [20], and the size of the sample is shown in Figure 2. The test temperatures of the pipe were 540 °C and 580 °C, according to the actual operating conditions and creep test requirements of the pipe. The sample under the high-temperature condition was only selected in the longitudinal direction, and the test was conducted on an MTS809 tension torsion testing machine. Table 5 shows the tensile performance test results at 540 °C, while the tensile performance test results at 580 °C are shown in Table 6.

Figure 2. The size of the sample at a high temperature (mm).

Table 5. The results of the tensile test at 540 °C.

Sampling Position	Sample	Rp0.2/MPa	Rm/MPa	A/%
12Cr2MoG (500 °C)	-	≥173	-	-
12Cr2MoG (550 °C)	-	≥159	-	-
10CrMo910 (500 °C)	-	≥180	-	-
Near the outer wall	1	286	442	21.5
	2	268	427	22.5
	3	276	432	24.0
Near the middle wall	1	301	451	23.0
	2	278	437	21.5
	3	286	443	23.0
Near the inner wall	1	281	440	-
	2	301	452	23.0
	3	305	457	22.0
Conclusion	The yield strength at 540 °C was higher than 12Cr2MOG at 500 °C in GB/T5310-2017 and 10CrMo910 at 500°C in DL/T999-2006.

Table 6. The tensile test results at 580 °C.

Sampling Position	Sample	Rp0.2/MPa	Rm/MPa	A/%
12Cr2MoG (500 °C)	-	≥173	-	-
12Cr2MoG (550 °C)	-	≥159	-	-
10CrMo910 (500 °C)	-	≥180	-	-
Near the outer wall	1	276	431	24.0
	2	278	424	22.5
Near the middle wall	1	281	441	22.0
	2	275	433	24.5
	3	279	421	24.0
Near the inner wall	1	272	435	23.0
	2	264	413	25.5
	3	290	447	20.5
Conclusion	The yield strength at 580 °C was higher than 12Cr2MOG at 500 °C in GB/T5310-2017 and 10CrMo910 at 500°C in DL/T999-2006.

As shown in Table 5 and Table 6, the performance of the sample at 540 °C and 580 °C met the requirements for the yield strength of 12Cr2MoG at 500 °C in the seamless steel tubes for high-pressure boilers (GB/T 5310-2017) [21], as well as the requirements for the yield strength of 10CrMo910 at 500 °C in EN10216-2:2002 provided in Appendix C of DL/T 999-2006.

2.3. Metallographic Test and Result Analysis

2.3.1. Metallographic Tests

The sample was 20 mm (length) × original wall thickness (width) × 15 mm (height) in size. The position of the utilized sample is shown in Figure 3, which consisted of four directions perpendicular to each other in the cross-section of the steel pipe (0°, 90, 180, 270°). The sample was prepared by grinding, polishing, and corrosion in accordance with the GB/T 13298-2015 [22] methods for the testing of metal microstructures.

Figure 3. Schematic diagram of metallographic sampling position.

2.3.2. Analysis of the Results

According to the previous metallographic tests of the cross-section and longitudinal section, we found no obvious differences in the microstructures of the samples in the four directions (0°, 90, 180, 270°). Therefore, the samples at 0° and 270° were selected for metallographic analysis of the internal and external surfaces.

(1)

Metallographic analysis of the external surface

Figure 4 and Figure 5 show the metallographic structure of the outer surface in the 270° and 0°. Through analysis, we could easily determine that the structure was ferrite and bainite. The bainite area had started to disperse, there was no obvious carbide aggregation at the grain boundary, and no creep holes were found in the leaves.

Figure 4. Metallographic structure of external surface in the 270° direction. (a) 500×, (b) 1000×.

Figure 5. Metallographic structure of inner surface in the 0° direction. (a) 500×, (b) 1000×.

(2)

Metallographic analysis of the inner surface

Figure 6 and Figure 7 show the metallographic structure of the inner surface in the 270° and 0° directions. Through analysis, we could easily determine that the structure was ferrite and bainite. The bainite area had started to disperse, there was no obvious carbide aggregation at the grain boundary, and there were no creep holes in the leaves.

Figure 6. Metallographic structure of inner surface in the 270° direction. (a) 500×, (b) 1000×.

Figure 7. Metallographic structure of inner surface in the 0°direction. (a) 500×, (b) 1000×.

3. Life Assessment

3.1. Creep Test

According to GB/T 2039-2012 (uniaxial tensile creep test method for metallic materials) [23], constant load creep tests at five stress levels were carried out at 540 °C and 580 °C. The creep test results showed that the longest fracture time was 6948 h at 540 °C and 148 MPa, and the endurance test lasted 11,012 h at 540 °C and 125 MPa. After the creep test, fracture of the sample was achieved and its micro morphology was observed under a scanning electron microscope, as shown in Figure 8 and Figure 9. We observed that the macro-fracture morphology was cup-shaped, and the micro-fracture morphology covered the fiber area and shear lip area, and no radiation area was observed. Significant plastic deformation occurred throughout the entire creep process.

Figure 8. Fracture morphology of the 540 °C creep specimen.

Figure 9. Fracture morphology of the 580 °C creep specimen.

3.2. Calculation of the Working Stress

CAESAR II software was used to analyze the stress of the P22 steel pipe. According to the data provided by the service manufacturer, a pipe system model was established. Following the relevant basic parameters of the P22 pipe, the maximum stress was calculated to be 112 MPa (Figure 10, A point). From the high-temperature tensile test and data, Rp0.2 = 268 MPa, Rm = 427 MPa, and E = 173,500,000 Pa were achieved at 540 °C, and E = 210,980,000 Pa, coupled with a Poisson’s ratio of 0.3, was achieved at room temperature.

Figure 10. The location diagram of the maximum stress point.

3.3. Assessment of Residual Life

Compared to the traditional extrapolation prediction method, the θ parameter prediction method more fully considered the entire creep process and more accurately reflected the deformation characteristics of the materials, so that the residual creep life of the P22 steel pipe was predicted by the θ parameter prediction method in this paper.

3.3.1. Basic Principles of the θ Parameter Method

(1)

A group of samples was tested at different temperatures and stress levels, and the creep fracture curves of each sample at a certain temperature and stress were obtained.

(2)

We used Equation (1) to fit the creep fracture curve of each sample under different temperatures and stresses, and solved ${\theta }_i$ (i = 1, 2, 3) in the creep equation of each sample:

$$\mathsf{\epsilon }={\theta }_{1}t+{\theta }_2\left(e^{{\theta }_{3}t}-1\right)$$

(1)

where and are the rate parameters of the second and third stages of creep, is the deformation parameter of the third stage of creep, and t is the time of creep.

(1)

We used ${\theta }_i$ from Equation (1), the temperature of the test (T), and the stress (σ) to obtain $a_i$, $b_i$, $c_i$, and $d_i$ in Equation (2). On this foundation, the relationship between ${\theta }_i$, T, and σ was established.

$$l{g}^{{\theta }_i}=a_i+b_i\sigma +c_{i}T+d_i\sigma T$$

(2)

where coefficients $a_i$, $b_i$, $c_i$, and $d_i$ are independent with stress and temperature.

(2)

We used $a_i$, $b_i$, $c_i$, and $d_i$ achieved in Equation (2) to allocate ${\theta }_i$ under the condition of specified temperature and pressure. Then, ${\theta }_i$ was substituted into Equation (1) to obtain the material creep curve.

(3)

As stipulated in DL/T 940-2005 (technical guidelines for life assessment of steam pipes in thermal power plants) [24], the creep life can be determined by determining the creep strain at the transition tangent point from the second stage (approximate straight line) to the third stage as the failure point on the creep deformation curve of materials under the service conditions of steam pipes.

3.3.2. Extrapolation Analysis

According to the creep fracture curves of the creep tests at 540 °C and 580 °C, the ${\theta }_i$ values were obtained by the quasi-Newton method (BFGS) and general global optimization method, as shown in Table 7.

Table 7. The values of θ_i at different stress levels at 540 °C and 580 °C.

Temperature (°C)	Stress (MPa)	θ1	θ2	θ3
540	218	1.3458 × 10−3	9.9846 × 10−6	0.0733
	208	3.1977 × 10−4	2.3522 × 10−6	0.02743
	198	2.3577 × 10−4	1.3341 × 10−6	0.0201
	178	3.4904 × 10−5	9.5889 × 10−6	0.0035
	168	4.7631 × 10−5	2.5901 × 10−6	0.0041
580	170	7.6631 × 10−4	3.4986 × 10−6	0.0596
	160	2.8969 × 10−4	2.0798 × 10−6	0.0294
	150	1.6092 × 10−4	4.2577 × 10−7	0.0186
	140	1.0447 × 10−4	2.1470 × 10−6	0.0103

When the temperature was constant, it had a linear relationship between $\lg {\theta }_i$ with ${\sigma }_i$. Using the relevant data in Table 7, the values of $a_i$, $b_i$, $c_i$, $d_i$ were obtained by linear fitting, as shown in Table 8.

Table 8. The fitting coefficient of θ_i.

θi	a	b	c	d
θ1	−74.6496	0.2222	0.07810	−0.0002
θ2	55.1724	−0.3212	−0.0739	0.0004
θ3	−61.4492	0.1739	0.0656	−0.0002

After T = 813 k and σ = 112 MPa were input into Equation (2), the results were incorporated into Equation (1) to obtain Equation (3):

$$\mathsf{\epsilon }=\left(1.1480E-7\right)t+\left(6.8346E-6\right)(e^{\left(2.9546E-5\right)t}-1)$$

(3)

Using Equation (2), the creep fracture curve of P22 pine under the working conditions of 540 °C and 9.58 MPa was drawn, as shown in Figure 11. According to DL/T940-2005 “Technical guidelines for the life assessment of steam pipelines in thermal power plants”, the residual creep life was determined to be 226,205 h, and the specific location is shown in Figure 11. The P22 steel pipe could operate for another 29 years if operated for 7800 h per year.

Figure 11. The creep fracture curve of the P22 pine.

4. Conclusions

In this study, a series of comprehensive physical and chemical tests were carried out for a replaced P22 steam pipeline, and the comprehensive properties of the P22 pipeline materials under the current state were analyzed. On this basis, high-temperature tensile and creep tests were carried out. Based on the θ parameter extrapolation method and the creep test results, the creep residual life of the P22 steam pipeline was scientifically predicted. The results showed that the P22 steam pipeline could still operate for more than 200,000 h and nearly 30 years.

Considering the evaluation results, several suggestions were put forward.

(a)

During actual operation, the operating parameters of the P22 steam pipeline should be strictly monitored to allow it to operate under allowable operating conditions, to avoid safety events.

(b)

In the next periodic inspection and shutdown maintenance, the P22 steam pipeline should be the main inspection object to receive attention. If necessary, the service cycle of the P22 steam pipeline could be shortened.