Influence of temperature and exploitation period on fatigue crack growth parameters in different regions of welded joints

The influence of exploitation period and temperature on the fatigue crack growth parameters in different regions of a welded joint is analysed for new and exploited low-alloyed Cr-Mo steel A-387 Gr. B. The parent metal is a part of a reactor mantle which was exploited for over 40 years, and recently replaced with new material. Fatigue crack growth parameters, threshold value Kth, coefficient C and exponent m, have been determined, both at room and exploitation temperature. Based on testing results, fatigue crack growth resistance in different regions of welded joint is analysed in order to justify the selected welding procedure specification.


INTRODUCTION
he reactor analysed here has a form of a vertical pressure vessel with a cylindrical mantle and two welded lids, made of Cr-Mo steel A-387 Gr.B, [1].It is used for some of the most important processes in the motor gasoline production, including platforming in order to change the structure of hydrocarbon compounds and to achieve a higher octane rating.Long-time, high temperature exploitation of the reactor, caused siginficant damage in reactor mantle, requiring a thorough inspection and repair of damaged parts, including replacement of a part of reactor mantle.For designed exploitation parameters (p=35 bar, t=537 °C), the material is prone to decarbonization, reducing its strength as a consequence, [2].Testing of high-cycle fatigue behaviour of new and exploited parent metal (PM), weld metal (WM) and heat affected zone (HAZ), at room and service temperature (540 °C) is necessary to get detailed insight in all parameters influencing fatigue crack growth resistance of Cr-Mo steel A-387 Gr.

FATIGUE CRACK GROWTH PARAMETERS EVALUATION
atigue crack growth testing at room temperature was performed on three-point bending specimens, as defined by ASTM E399, [3], whereas tesitng at service temperature, 540 C, was performed on modified CT specimens, as defined by standard BS 7448 Part 1, [4].The high-frequency resonant pulsator was used, in force control mode, with loading ratio R = 0.1 to obtain diagrams da/dN-K for specimens with fatigue crack tip located in PM, WM and B F HAZ, both new and expoloited material, at room and service temperature.Only two diagrams are shown here, as an illustration, whereas the others can be found in [1].Influence of testing temperature and exploitation period on the fatigue threshold Kth is graphically presented in Fig. 3-5, for PM, WM and HAZ, respectively.
Fatigue threshold, K  The influence of testing temperature and exploitation period on the fatigue crack growth rate, da/dN, is graphically presented in Fig. 6-8, for PM, WM and HAZ, respectively.

DISCUSSION
alues obtained for PM fatigue threshold, K th , are in the range 5.8 MPam (20 C) to 5.1 MPam (540 C), Tab. 5. Additional reduction for 10-15% is recorded due to exploition period 10-15%, since values for fatigue threshold, K th , are in that case in the range 5.2 MPam (20 C) to 4.7 MPam (540 C), Tab. 6.Similar effects are noticed in HAZ, where values of fatigue threshold, Kth, obtained for new material, are in the range 5.5 MPam (20 C) to 4.8 MPam (540 C), i.e. from 4.6 MPam (20 C) to 4.2 MPam (540 C) for exploited material, Tabs.8 and 9.

CONCLUSION
ased on the presented results, one can conclude the following:  Influence of material heterogeneity, as well as temperature and exploation effects, on fatigue threshold, da/dN, and crack growth rate, da/dN, is significant. Fatigue threshold values are the lowest for WM, and lowest for HAZ, whereas crack growth rate values are highest for HAZ and lowest for PM.Therefore, generally speaking, the lowest fatigue crack resistance is in HAZ. Higher temperature and longer exploitation peroids increase crack growth rates and decreases fatigue thresholds for both new and exploited materials in all regions of welded joint (PM, WM, HAZ).These effects are due to microstructural changes such as carbide formation and growth at grain boundaries and inside grains.

Table 1 :
TESTING MATERIALoth new and exploited PM was steel A-387 Gr.B with thickness of 102 mm.Chemical composition and mechanical properties for both new and exploited PM are given in Tabs. 1 and 2. Chemical composition of exploited (E) and new (N) PM specimens B. welded joints.T

Table 2 :
Chemical composition of exploited (E) and new (N) PM specimens.

Table 3 :
Chemical composition of filler materials.

Table 4 :
Mechanical properties of filler materials

Table 5 :
Fatigue crack growth parameters for specimens with notches in new PM.
a, i.e. coefficient C and exponent m, fatigue threshold Kth, and fatigue crack growth rate, da/dN, for K = 10 MPam, are given in Tabs.5-9 for new and exploited PM, for new WM, and for new and exloited HAZ, respectively.

Table 6 :
Fatigue crack growth parameters for specimens with notches in exploited PM.

Table 7 :
Fatigue crack growth parameters for specimens with notches in WM.

Table 8 :
Fatigue crack growth parameters for specimens with notches in new HAZ.

Table 9 :
Fatigue crack growth parameters for specimens with notches in exploited HAZ.