Life extension technology for steam pipe systems—II. Development of life prediction methodology

Part II of this paper addresses the development of a fracture mechanics based life prediction methodology of steam pipes which operate at elevated temperatures but in the sub-creep temperature range. Elastic-plastic fracture mechanics concepts were employed to establish the remaining life prediction methodology and inspection criteria of steam pipes. Leak-before-break analyses were utilized to determine the flaw inspection criteria. Both tension and bending type loading conditions were considered in the life prediction analysis. The life assessment technology is concerned with the fatigue crack growth life of circumferential cracks in a pipe. The material properties of the A106B steam pipe steel reported in Part I of this paper were used to predict the fatigue life of steam pipes. The effects of operating parameters (e.g. stress and temperature), pipe size, and material properties on the remaining life and inspection intervals of steam pipes can be quantitatively evaluated.     

Fatigue, monotonic and fracture toughness properties of a Cr–Mn–N steel

The low-cycle fatigue, monotonic and fracture toughness behaviour of E3949, a Cr–Mn–N austenitic stainless steel, used for drillcollar connections was studied. Low-cycle fatigue tests were carried out at room temperature under total strain control in the range of 0.40 to 1.50% using Companion Specimens Test (CST) and Incremental Step Test (IST) methods. Cyclic softening without saturation was observed in all tests. Massing cyclic stress–strain behaviour was observed only with the IST method. The fatigue life behaviour obeyed Basquin and Coffin–Manson relationships and the high value obtained for ′_f imparts a significant improvement in fatigue resistance of this alloy compared to AISI 304LN. The J–R curves and J_IC values were obtained at room temperature and at 150°C by using single specimens and the elastic compliance technique for crack length measurement. The observed decrease in crack initiation fracture toughness at 150°C is proposed to be due to a dynamic strain ageing effect, which impairs ductility.     

Effect of surface roughness on low-cycle fatigue life of Cr---Mo---V steel at 550 °C

Low-cycle fatigue tests have been carried out on Cr---Mo---V steel specimens with two different modes of surface roughness at 550 °C. The fatigue life of specimens with a rough surface is approximately half of that of specimens with a smooth surface over the plastic strain range investigated. From interrupted tests, it is seen that low-cycle fatigue behaviour is largely influenced by the crack initiation process even for a high strain range and the decrease of fatigue life in specimens with a rough surface is mainly due to a reduction in the number of cycles for crack initiation.     

Effect of pre-charged hydrogen on fatigue crack growth of low alloy steel at 288 °C

Hydrogen effects on mechanical strength and crack growth were studied at high temperatures. The study was motivated by the fact that the environmentally assisted cracking (EAC) of pressure vessel steel SA508 Cl.3 in 288 °C water was suspected to be related to hydrogen embrittlement. Fatigue crack growth rate and tensile tests were performed with hydrogen pre-charged specimens at high temperatures. At 288 °C the fatigue crack growth rate of the hydrogen pre-charged specimen was faster than that of as-received; the fatigue fracture surface of hydrogen pre-charged specimen correspondingly showed EAC like feature. Meanwhile, ductile striation was evident for the case of as-received in both air and argon gas environments. In the dynamic strain aging (DSA) loading condition at 288 °C during tensile tests, the pre-charged hydrogen induced a marked softening (decrease in ultimate tensile strength; UTS) as well as a little ductility loss; this was accompanied by the macrocracks grown from microvoids/microcracks promoted by DSA and hydrogen. These experiments showed that hydrogen embrittlement is an effective mechanism of EAC not only at low temperature but also at the high temperature.     

Low frequency fatigue crack growth behavior of a470 class 8 rotor steel at 538°C (1000°F)

Fatigue crack growth rate data were developed at various frequencies and hold times at maximum load for A470 Class 8 steel at 538°C (1000°F) by using an accelerated test method which involves alternating test frequency and temperature. These data were consistent with fatigue crack growth rate data obtained from the same material and developed according to the ASTM specification E-647-T78. This result suggests that there is no transient effect associated with the alternating test frequency and temperature and that the accelerated testing procedure can be used to expedite the development of elevated temperature fatigue crack growth rate data at very low frequencies and long hold times. At 538°C (1000°F) fatigue crack growth properties with hold time developed from both 1T-CT and multiple-edge-craek tension specimens fall in the same scatter band on the da/dN vs ΔK plot. This result indicates the applicability of ΔK to characterize the fatigue crack growth behavior with hold time at elevated temperature. Also, the model proposed by Saxena et al. was found to successfully predict the fatigue crack growth rate properties with 28 min hold time of the A470 Class 8 rotor steel at 538°C (1000°F).     

High-temperature crack growth in low-cycle fatigue

The rates of low-cycle fatigue crack propagation in 1 Cr-1 Mo-0·25 V low alloy steel were determined under several types of loading at 1000 F. It was found that crack growth rates correlate well with the nominal crack tip stress. This method was also applied with good results to some published data where the fracture mechanics stress intensity factor had given only limited correlation. Several questions are thus raised regarding the relative importance of crack tip stress and crack length during crack growth in actual components.     

Investigation of low temperature effects on fatigue behavior of PVDF

Fatigue behavior of polyvinylidiene fluoride (PVDF) pipes is investigated under low temperatures to characterize the temperature effects. The analysis included experimental evaluation of fatigue life for test samples taken directly from the manufactured pipes used for service as opposed to compression molded compact tension samples used in previous works. In this test, short sections from an extruded pipe are used to better represent the material service conditions. A compact test chamber was designed to control the test temperature. The samples were loaded into the test rig and allowed to cool for 30 min ensuring a constant and even temperature distribution. Cooling was done in a sealed test chamber using carbon dioxide gas. Two test temperatures of −20 °C and −10 °C were chosen since they represent typical temperature during which failure occurs during actual pipe service in cold environments. Fractured surfaces were inspected and fatigue data were analyzed using a standard procedure for calculation of fatigue life with a semi-elliptical surface crack assumption was performed; from which parameters of the Paris law for fatigue fracture were obtained. Comparing the results with previous works it is found that they capture the trend of the PVDF material behavior for high temperature.     

TC4钛合金焊接接头疲劳性能试验研究

对TC钛合金焊接试样进行了低周疲劳试验，在应变控制试验条件下，得到了焊接接头疲劳裂纹萌生寿命。选取弹性模量的变化间接反映焊接接头中的损伤，对焊接结构疲功损伤变化规律进行了探讨并建立了相应的应变－寿命预测方法，研究结果为航空发动机焊接结构寿命的预测提供了参考依据。     

Cyclic stress response, strain resistance and fracture behavior of modified 1070 steel

A combined experimental and finite element study of fatigue crack closure in modified 1070 steel has been conducted. In this paper, the material property evaluations required for this study are presented. The monotonic and cyclic stress-strain properties, cyclic stress response, cyclic strain resistance, low cycle fatigue life and fracture behavior are examined. The low cycle fatigue tests were conducted using tension-compression cycling, under total strain amplitude control, over a wide range of strain levels. The material was found to possess medium strength and high ductility; while displaying a strain level dependent combination of cyclic strain softening and hardening behavior. The observed softening behavior is attributed to the rearrangement of dislocations produced by processing, formation of slip bands on the specimen surface and the formation of microcracks. The observed hardening behavior is ascribed to contributions from synergistic influences of dislocation multiplication, dislocation-dislocation interactions and dislocation-microstructural feature interactions. The material followed the strain-life relationships attributed to Basquin and Coffin-Manson. The fracture surfaces of the fatigue specimens showed distinct regions of crack initiation, microscopic-macroscopic crack growth and sudden fracture. The low-cycle fatigue characteristics and fracture behavior are discussed in the light of competing and mutually interactive influences of cyclic strain amplitude, concomitant response stress, intrinsic microstructural effects and dislocation-microstructure interactions during cyclic straining.     

0Cr16 Ni5 Mo不锈钢疲劳性能研究

通过疲劳试验对 0Cr16Ni5Mo不锈钢疲劳性能和海水中的腐蚀疲劳性能进行了研究 ,并对断口特征进行了金相及SEM分析。结果表明 :钢的σ-1 、σ-1SCC 和τ-1 分别为 5 5 0MPa、40 8MPa和 2 85MPa ,σ-1 、τ-1与σb 、σ0 .2 之间符合奥金格公式。不同条件下的疲劳断口特征各异 ,纯旋转弯曲疲劳断口属正常形貌 ,而扭转疲劳断口的扩展阶段疲劳条带不明显 ,这与受力条件复杂有关。海水介质中疲劳开裂具有表面多条裂纹源特征 ,裂纹尖端的应力集中加速裂纹扩展造成腐蚀疲劳强度降低 ,裂纹扩展至断裂符合正常疲劳断裂机理。     

AUSTEMPERING OF A SILICON MANGANESE CAST STEEL

The influence of austempering on the microstructure and mechanical properties of an alloyed cast steel containing high silicon (3.00%) and high manganese (2.00%) was studied. The influence of microstructure on the plain strain fracture toughness of this new steel was also examined. The test results show that by using a suitable austempering process, i.e., by austenitizing at 1010°C (1850°F) for 2 hr and then subsequently austempering at 316°C (600°F) for 6 hr, it is possible to produce more than 80% austenite in the matrix of the material. Such a large percentage of austenite in the matrix made the steel almost nonmagnetic. Austempering resulted in a significant improvement in mechanical properties as well as fracture toughness of the material. The potential applications of this steel are in naval structural components, aircraft, and automotive components.     

Investigation of the strength and cyclic fracture toughness of BT-16 titanium alloy subjected to thermal cycling simulating diffusion welding

The effect of the thermal cycle of diffusion welding on the service characteristics of BT-16 titanium alloy are considered. Using standard test equipment the mechanical properties of the alloy after thermal cycling to 860 or 960°C were studied at test temperatures of 960, 860, 300 and 20°C and the fatigue strength and fracture toughness were studied at room temperature.

It was shown that although specimens welded at 960°C displayed superior plastic properties, these were only partially effective in arresting propagating cracks. A greater fatigue crack resistance was seen with material welded at 860°C and it is this temperature which is recommended for diffusion bonding BT-16 titanium alloy components.     

Fatigue crack growth rates and fracture toughness in electroslag refined En 52 steel

En 52 steel has been electroslag refined and the resultant effects of refining on its mechanical properties have been assessed. It was found that refining caused a decrease in fatigue crack growth rates and increases in fatigue strength, fracture toughness, Charpy fracture energy and tensile ductility. Fatigue crack growth rates in region I and in region III were found to be considerably lower in the electroslag refined steel: they were unaffected in region II. The fracture toughness values for the electroslag refined steel are nearly twice those estimated for the unrefined steel. Measurements on heat-treated samples have shown that the electroslag refined steel has a better response to heat-treatment. The improvement in the mechanical properties is explained in terms of the removal of nonmetallic inclusions and a reduction in the sulphur content of the steel.     

Effect of laser shock processing on fatigue crack growth of duplex stainless steel

Duplex stainless steels have wide application in different fields like the ship, petrochemical and chemical industries that is due to their high strength and excellent toughness properties as well as their high corrosion resistance. In this work an investigation is performed to evaluate the effect of laser shock processing on some mechanical properties of 2205 duplex stainless steel. Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switched Nd:YAG laser, operating at 10 Hz with infrared (1064 nm) radiation. The pulses are focused to a diameter of 1.5 mm. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is determined by the contour method. It is observed that the higher the pulse density the greater the compressive residual stress. Pulse densities of 900, 1600 and 2500 pul/cm² are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness if this steel.     

Tensile,fracture toughness and crack growth properties of a roll-extruded HP 9Ni-4Co-25C steel alloy

A section of internal roll-extruded HP 9Ni-4Co-25C steel alloy was obtained from an experimental 120-in. rocket motor case for Titan III and sent to the Air Force Materials Laboratory for evaluation by the Space and Missile System Organization (SAMSO). Tensile and fracture toughness properties were obtained at room temperature and at ?65°F. Crack growth properties were obtained at room temperature in laboratory air and in distilled water. An increase in tensile strength and a decrease in ductility were observed to occur with increased cold forming reduction. Fracture toughness properties appeared to decrease with increased rolling reduction. Fatigue crack growth properties in laboratory air were compatible with other high strength steels for the lower ΔK values. Fatigue crack growth properties in distilled water showed no acceleration due to aqueous environment influences. Statically loaded stress corrosion crack (SCC) growth properties were an improvement over available D6ac steel alloy data.     

Assessing the accuracy of fatigue life in surface‐cracked straight pipes

The demonstration of leak before brake (LBB) based on fracture mechanics requires information on the initial size of a defect, initiation of crack growth from the inherent defect and subsequent crack growth rates. In the present paper the prediction methodologies have been tested for three different full scale pipes geometry experimentally tested data. The prediction accuracy of two SIF solutions available in the literature has also been judged. The effect of fatigue crack closure and corrections needed in the numerical prediction methodology using FEM have also been included. The results showed that the FEM could fairly predict the fatigue crack initiation and crack growth life of full‐scale piping components having a constant depth crack profile.     

Temperature effects on fatigue crack growth in polycarbonate

The fatigue behaviour of a high strength thermoplastic, polycarbonate, has been investigated as a function of temperature. Fatigue crack growth properties were measured in the temperature range of 100 to 373 K and were analysed using a fracture mechanics approach. Fatigue behaviour was found to be related to the fracture toughness of the material. This correlation with fracture toughness was used to develop an empirical model based on the toughness for describing the effect of temperature on fatigue crack growth, and to consider fatigue in terms of the secondary losses of the polymer.     

An evaluation of the crack growth and fracture properties of AISI 403 modified 12 Cr stainless steel

The plane strain fracture toughness, $\text{K}{}_{\mathrm{Ic}}$, and fatigue crack growth rate material properties were developed for three heats of AISI 403 modified 12 Cr stainless steel. Valid (per ASTM requirements) fracture toughness tests were conducted in the temperature range ?200°F to 175°F. In addition, both the room temperature air environment plus 520°F, 1200psi distilled water environment fatigue crack growth rate material properties are presented. Finally, a hypothetical example problem is included which demonstrates the application of fracture mechanics technology to an AISI 403 modified 12 Cr stainless steel turbine rotor.     

Fatigue crack initiation in notched specimens of 17Mn4 steel

Fatigue crack initiation life is predicted for notched bend specimens with two different notch acuities and two stress ratios using the equivalent strain energy density method. The material used was 17Mn4 steel, widely applied in pressure vessel construction. The monotonic and cyclic mechanical properties and the strain-life constants were experimental determined in this work. A comparison was made between predicted and experimental lives obtained for two different optically measured crack lengths. Good agreement was obtained for stress ratios, R, of 0 and −1, and for two notch stress concentration factor values. The observed mean stress effect on crack initiation lives is well modelled in the prediction by the method used with calculation of both strain range and mean stress at the root of the notch.     

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.