Influence of themo-mechanical treatments and microstructural state on the fatigue behaviour of a weld seam: case of API X60 steel

A BSTRACT . The aim of this work is the study of the fatigue behaviour of API X60 steel and the influence of thermal and mechanical treatments. The evaluation of the integrity and safety of welded structures dictates the approach taken in this research. The microstructural observations on the different zones of the weld seam indicates that the variation of heterogeneous structure is a progressive destruction of the strips of lamination which cause a new phase leading to a drop in the mechanical properties requiring treatment after welding. The fatigue cracking rate diverges beyond the threshold of  , but no deviation of the crack from its propagation axis was noticed, which confirms the correct choice of filler metal over that of the base metal with an overmatching M = 1.1, and the treatments applied to the structure. This fatigue cracking rate transversal to the welding direction initially presents an aspect similar to that of BM but registers a delay as soon as the crack tip enters the second zone (HAZ) then it progresses rapidly. This evolution is characterized by a disturbance due to the repeated change of microstructure.


INTRODUCTION
enerally, industrial structures, consist of elements assembled by welding, show geometric discontinuities near which areas of stress concentration are formed. These areas are the prime sites for the initiation and propagation of fatigue cracks which are the real causes for the damage to these structures. When designing mechanical structures or pipelines, it is essential to have tools capable of predicting their fatigue resistance under conditions close to actual operation. G Cracks in weld joints, base metal, overloads due to vibratory movements, corrosion, maintenance and rehabilitation work can also lead to nicks or scratches and indentations in structures [1][2][3] . Rupture prevention is built on an in-depth knowledge based on tangible facts and on data to support them. It starts with the engineering and design of the pipelines, during the exploitation and manufacturing of the structures. These latter will continue to deteriorate over time; they will be subjected to variable environmental conditions and to external events that their manufacturers could not, always, predict. Several studies have discussed the factors affecting the life of welded structures, including heterogeneity, welding conditions and of course the filler metal [4][5]. The right choice of this latter improves, considerably, fatigue resistance of structures [6][7][8][9]. The microstructure of the material has a considerable effect on the propagation behaviour of fatigue cracks in structures, which has become the center of attention of researches for many years. These microstructural effects are associated with welding conditions especially the position of the weld seam according to the orientation of loads and stresses [10][11]. The thermal effect of the welding cycle gives microstructural heterogeneity in this zone (HAZ) made up of several subzones with different microstructures that can cause different behaviours [12][13][14]. A ferrite-bainitic microstructure offers better resistance to crack propagation in the welded zone than the martensitic or bainitic structures [15]. Indeed, a better combination of resistance and fatigue crack growth instead of rupture was observed for steel containing more than 70% of martensite and low carbon content. This study deals with the influence of the thermal and mechanical treatment on the fatigue behaviour of a welded joint. This is a useful step in assessing the integrity and safety of welds. On the other hand, the microstructure and effects of reducing internal stresses using the advance fatigue crack ligament pre-compression method were also analysed.

Material presentation
ur study focuses on API X60 steel, used for the manufacturing of gas tanks (Liquefied petroleum gas LPG) and the construction of pipelines. Welding is performed by Submerged Arc Welding (SAW) with coated electrodes GMoSi. The basic chemical composition and mechanical properties of the two materials are given by Tab

Micrographic examinations
Micrographic examinations on polished cuts (1 μm) then attacked with 3% Nital, were carried out using a microscope. The aim is to observe the structures in very localized areas on the surface and transversal perpendicular to the welded seam in the three areas illustrated by Fig. 1 The transformations that the HAZ undergoes cannot be simulated to the heat treatments applied to steels. Indeed, after a welding operation, there is the appearance of bainite and intergranular ferrite in the junction zone and the transformation zones. O  Zone (Z3) FM (WM): In the last pass, the arrangements of the ferritic areas and of the carburized constituents similar to ferrite present a marked arrangement linked to solidification. Observation shows the presence of a dendritic structure with islets (ferrite-perlite) and fairly large grains (Fig. 2b).  Zone HAZ: This zone presents a variation of heterogeneous structure and a progressive destruction of the bands of lamination (Fig. 2 c). In the area near the fusion line, an overheated structure appears with a particular aspect and disposition of the ferrite. We notice the presence of a new phase which is the acicular ferrite (in the form of needles). This microstructure presents islets of slatted bainite separated by ferrite and has good mechanical properties. Recent works [16][17] practically detect similar observations.   The small, punctual "brown spots" distributed over the entire micrograph are localized chemical attacks. Fig. 3 shows the border junction between the three zones.

Test conditions and instrumentation
he cracking tests were carried out on 07 mm thick CT 50 test specimens compliant to the ASTM-E-647 standard.
The tests were carried out in ambient air and for the same value of load ratio R = 0.1, and we used 3 test tubes for each case studied. The sampling of the test tubes was done according to

Calculation method of the cracking rate
The smoothing of the curves is based on an incremental polynomial method that uses the smoothing of a series of successive points by a polynomial whose growth is monotonic in this interval [18]. The equation of the smoothed curve is of the form: The cracking rate at point i a is obtained from the derivative of the first expression.

Elimination of residual stresses by local pre-compression
During a fatigue cracking test on a weld seam, it is noted that the propagation of the fatigue crack is disturbed by the existing residual stress field. To remedy this problem, we used the local pre-compression method to limit the plastic deformation to a few percentages of the thickness in order to modify the progression of the crack. Fig. 5 schematically shows the principle of the method with the dimensions of the punches to be used. Fig. 6 represents the photo taken on the studied test tubes. The applied pressure and the penetration of the punch into the ligament are given respectively by relations (5) and (6):

Crack's advance ligament treatment
This operation went through two important phases, a preheating before the welding operation and a stress relieving annealing consisting of a slow lowering of the temperature. This treatment must guarantee perfect safety and reliability at the weld level depending on the thicknesses and the quality of the material while reducing internal stresses. In fact, this treatment was carried out in an oven at a maximum temperature of 580 °C for 2 hours, followed by a slow cooling to 320 °C, then an exposure to a temperature below 200 ° C, all the process took place at a temperature below AC1. Other techniques and procedures are also used [19][20].

RESULTS AND DISCUSSION
Study on the three zones he obtained results concern the different test tubes in the three zones (BM -FM -HAZ) represented by Fig. 4    For the values of > 50 MPa.m, the cracking rate presents a significant difference (15%) between the 3 cases, which means that:  In this studied area, the cracking rate presents an almost similar rate in the three zones.  Even if the speeds differ in the three zones, but no deviation of the crack from its axis of propagation has been noticed, contrary to certain authors [21][22][23] who have shown that beyond a certain value of  the cracking speed is usually accompanied by a deviation of the crack from its initial plane towards base metal. This confirms that the weld was seine and the choice of a filler metal over that of the base metal is adequate. Usually, the deviation is due to the difference in mechanical characteristics between the three zones; the crack moves from a harder microstructure to a softer one. Fig. 8 represents the cracking speed da⁄dN according to in the molten metal after relaxation by heat treatment with the aim of eliminating the internal stresses arising from the solidification of the cooled welded parts. The results show that the crack growth is initiated at a lower level of  than the state without heat treatment. And even for larger values of , a decrease in this speed was recorded with a regular growth of the fatigue crack. This means the elimination of internal stresses caused by solidification after welding.

Transversal welding position
The variation da⁄dN, in the plate containing a transversal weld joint, follows a pace similar to that of BM, but records a delay as soon as the crack tip enters the second zone (HAZ) then it progresses rapidly with a slope m close of 7. This evolution is characterized by a disturbance due to the repeated change of the microstructure. In addition, the weld seam had a higher resistance to crack growth than the base metal, especially in the low range . As the growing crack propagated through HAZ and FM this resistance dropped (Fig. 9).     Cracking on FM without pre compression Fig. 10 represents the evolution of the cracking speed before the relaxation of internal welding stresses at the pre-compression crack point. Indeed, this behaviour is in accordance with the residual stresses and the evolution of the closing of cracks. When the residual traction stresses act to fully open the crack, the test tube adopts a crack growth behaviour comparable to the base metal but with a large accumulation of number of fatigue cycles. The results show that the crack growth is very important after the delay recorded due to the closure by internal stresses and exhibits a practically constant deviation during the advance of the crack. The slope m is increased and is comparable to the slope obtained in the HAZ. Other authors have drawn the same conclusions on X70 steel [23][24][25].

CONCLUSION
he influence of local treatment and the effects of reducing internal stresses on the fatigue behaviour of a weld joint have been dealt with in this study.
The following conclusions have been drawn: (1) Appearance of bainite and intergranular ferrite in the junction zone and the transformation zones after a welding operation and before heat treatment.  Far from the weld seam, the observation shows an alternating structure of ferrite and perlite characteristic of lamination strips. But on the seam and in the last pass, the observation shows the presence of a dendritic structure with isles (ferriteperlite) and fairly large grains. This phase can be assimilated to ferrite exhibiting a marked disposition linked to solidification.  In the HAZ, we notice a gradual destruction of the lamination strips, resulting in a variation of heterogeneous structure.
This justifies the obtained mechanical properties. (2) Fatigue cracks show that for low rates of , crack growth is similar. But for  greater than 30 Mpa.m, this crack growth becomes more important respectively in the FM and the HAZ. Approaching the third stage, the crack rate shows an important difference (15%) between the 3 cases, which means that:  After the initiation and exit from the wake of the residual stresses, the crack rate presents an almost similar pattern in the three zones.  The adequate choice of the FM melted metal and the treatment of the crack tip ligament avoided deviating the crack from its propagation axis for the different configurations. Contrary to certain authors [26] showing that beyond a certain value of , the crack rate is generally accompanied by a deviation of the crack from its initial plane towards the base metal.
(3) For the position of the transversal weld, the crack propagates perpendicular to the welding direction. We remark an evolution characterized by a disturbance due to the repeated change of microstructure. This case allows to have a similar appearance to that of BM but registers a delay as soon as the crack tip enters the second zone (HAZ) then it progresses rapidly. The mechanical properties drop as the crack propagates through the HAZ and FM.