Finite Element (FE) analysis is also performed for SAW square butt single and double sided single through welds of P91 steel plate. The quasi-static region of weld fusion zone revealed compressive longitudinal residual stresses in square butt single-sided single-pass welds.
Preface
Therefore, the microstructural and mechanical behavior of SAW joints of P91 steel sheet is also discussed for the effect of welding and mitigation heat treatments.
Background and motivation
Experimental and numerical or finite element methods are equally applicable in determining the distribution of residual stresses with their respective limitations. It facilitates the establishment of appropriate mitigation practices that control the magnitude, nature and distribution of residual stresses.
Creep strength enhanced ferritic steels
Consequently, awareness and proper mitigation of the “as-welded” residual stresses in P91 steel welds are extremely important fields of interest. Mo and W also enhance the solid solution strengthening and act as ferrite stabilizers in P91 steel.
Welding induced residual stresses and distortions
In sections 3-3', the already solidified weld metal begins to shrink and is supported by areas close to the seam. In section 4-4', the tensile residual stress state is recognized in the welding area at room temperature, as shown in figure 1.1.
Scope of the thesis
In addition, the thesis also includes the mitigation experiment based on the predicted results of the proposed techniques. The thesis also explores the microstructural development and mechanical behavior of P91 steel weldment after submerged arc welding and heat treatments related to tempering experiments.
Thesis objectives
Therefore, remedial and corrective steps known as mitigation techniques are also explored in this thesis. Therefore, these mitigation techniques are proposed, FE modeled and simulated for different welding stages viz.
Organization of thesis
Following this introductory chapter 1, a comprehensive literature survey focusing on important areas related to P91 steel welds and thermo-mechanical modeling of the arc welding process is presented in chapter 2. It discusses comparative studies on arc single pass sunk with square butt and double. welded joints of P91 steel plate.
Introduction
Studies on P91 steel weldment
They included this for effective P91 steel welded pipe modeling, which resulted in relatively fair prediction results for residual stress distribution. However, submerged arc welding was considered for P91 steel welding by the researchers in Jiangsu Suyuan Power Equipment Co. in 2005.
FE modelling of submerged arc welding for residual stresses and distortions
52] carried out the complete experimental analysis of the submerged arc welding of 11 mm thick Cr-Mo-V steel plate. However, literature on FE modeling and simulation of submerged arc welding of P91 steel sheet is rarely found.
Phase transformation effect in welding
The schematic of volumetric changes during solid state phase transformation (SSPT) is shown in Figure 2.1. A number of research articles are reviewed on the modeling of the phase transformation in the solid and solid-liquid state during thermal welding cycles.
Mitigation techniques for welding induced residual stresses and distortions
The motto of most of the in-process techniques is limited to the generation of a tension effect in the longitudinal direction during welding. Yang & Dong [93] studied the effect of the in-process roll and drag heat sink method on the mitigation of welding-induced buckling deformations.
Summary
Finally, literature available on thermal load-based pre-, in- and post-weld softening techniques discovered that all the studies are limited to thin plates only.
Research gaps
Detailed thesis objectives
Introduction
Submerged arc welding of P91 steel plate
- SAW experiment setup
- Materials selection
- Fixture and clamps
- SAW experiment
- Temperature profile measurement
The chemical composition of the material mentioned by the manufacturer is as shown in table 3.1 below. The range of welding parameters for the test welding of square butt, single-sided and double-sided, single submerged arc welding of P91 steel is given in Table 3.4.
Measurement of welding induced distortion
Type K thermocouples are selected to record the thermal history of a point on the surface of the weld joint. In this case, the angular distortion and bending of the edge of the base plate on both sides of the weld are measured to analyze the distortions caused by welding.
Measurement of welding induced residual stresses
Deep Hole Drilling (DHD) measurement
Releases the residual stresses in the cylindrical volume, whose corresponding deformation is measured on the inner surface of the hole. Finally, the biaxial stress components can be obtained based on the constitutive relations as given by Eq.
Contour measurement
One of the planes that resembles the deformed surface of the cut part is taken to reduce the deformation results (which are applied in the negative direction). The non-uniformity in the cutting process and the misalignment in the positioning of the sample on the CMM bed cause incorrect surface deformation data of the cutting surface.
Thermo-mechanical elastic plastic FE modelling of SAW butt joints of P91 steel
- Modelling and meshing
- Thermal modelling
- Formulation of heat transfer model
- Structural modelling
- Modelling SSPT phenomenon
- Materials properties
According to Fourier's law, the heat flux vector in terms of the thermal gradient is given by Eq. Zero shear loads are applied to the nodes at the corner of the weld joint model, as shown in Figure 3.13.
Mitigation techniques
FE modelling of preheating
HL =Hs+.C T. s−T, (At solidus temperature) (3.43) Eq.(3.44) gives the expression that includes the enthalpy value at solidus temperature and the enthalpy value obtained for the temperature difference of solidus and liquidus temperatures in the temperature range between solidus to liquidus, i.e. The enthalpy values are calculated at different temperatures in their respective phase change states, given in Table 3.5 below.
FE modelling of in-process mitigation
A 10 mm wide area is selected as a side heating area on both sides of the weld line. However, few pieces of research have carried out the thermal stress mitigation technique with combined side heating and heat dissipation methods.
FE modelling of post weld heat treatment
Ac and nc values can be obtained by performing a stress relaxation test on the base or weld metal. Therefore, the proportionality limit is considered as the maximum initial stress for the stress relaxation test for constant modulus of elasticity.
Mechanical and microstructural characterisation
- Sample preparation
- Macro and micrograph study
- Microhardness Test
- Universal tensile test & fractography
- XRD and EDS analysis
Characterization of the microstructure is carried out in an optical microscope (Make: Carl Zeiss) as shown in Figure 3.22 (b) in different areas, i.e. Microhardness measurement is performed on a Vickers microhardness tester (Make: Omni Tech), as shown in Figure 3.23.
Summary
UTS, YS and ductility of the welds are characterized by performing a tensile test on a universal hydraulic servo-controlled universal tensile testing machine (brand: Instron). The test is also performed for the electrode wire and base metal to assess and compare the strength of welded joints.
Introduction
Weld preparation
This is reduced to 4.9 mm for square butt double-sided single-pass welding due to less heat input in each weld pass. A solid penetration depth is observed, especially in square butt single side single pass welding without any edge preparation.
Thermal history
Similarly, a peak temperature of 570 oC is observed in the 2nd weld pass with the thermocouple attached at a point 16 mm away from the center line of the weld on the bottom surface. The lower side weld shows a sudden rise in temperature from the cooling rate of the upper side weld in double sided welding.
Welding induced residual distortion
The heat profile for a single-pass square butt weld shows a peak temperature of about 723 oC at a point 12 mm away from the centerline of the weld on the top surface. The maximum value of angular distortion for a square face single-sided DC joint is observed around ~0.12 mm centered on the top surface of the base plate edge.
Welding induced residual stresses
Square butt single side single pass weld joint
The compressive residual stress dropped sharply to -3.5 MPa towards the bottom side at 11 mm thickness. Generally, volumetric shrinkage in the weld develops tensile stress during the cooling process.
Square butt double side single pass weld joint
The tensile residual stress distribution is observed in the center of the weld plate. However, the square double-sided single pass weld shows tensile residual stress in the medium thickness area, as shown in Figure 4.13.
Summary
It is also observed that the residual tensile stress reaches its peak in the lower half of the plate thickness at 9 mm from the top surface with a value of 32.4 MPa. The upper and lower weld bead crown areas of a square butt single-sided single-pass joint may also exhibit compressive residual stresses, which is assessed by FE results explained in the next chapter.
Introduction
Thermal model fitting and temperature distribution
The peak temperature of 1893 oC is observed at FZ in single-pass, single-sided welding. Similarly, Figure 5.2 (a) and (b) show that molten weld metal is present during the upper and lower weld passes in double-sided single pass welding.
Welding induced distortion
The contour diagram of vertical deformation in the square butt double sided single pass weld is also shown in Figure 5.10 below. Double-sided butt-welding reduced the weld-induced strain values, which is also observed in Figure 5.11 and Figure 5.12.
Welding induced residual stress
Effect of SSPT phenomenon in residual stress distribution
In contrast, the contour plot in Figure 5.13 (b) shows that the entire quasi-static weld region developed compressive residual stresses in the P91 steel weld. P91 steel weld shows compressive residual stress in the weld area and tensile stress in the nearby area between weld and base metal.
Residual stresses in square butt single and double-side welded joint models
The residual stress values in the FZ area are close enough to the measured results. It reveals residual tensile stress in the middle of the thickness range of the weld fusion zone, corresponding to measured results.
Summary
However, the residual stress values within the 2-9 mm thickness zone were in good agreement with the experimental results. The minimum prediction error for longitudinal residual stresses in the FZ of a single pass square butt weld joint is 4.2% at 4.7 mm thickness.
Introduction
Effect of preheating
Angular deformation and edge bending in a preheated square butt one-sided one-pass welded joint are shown in Figure 6.3 and Figure 6.4, respectively. The effect of implementing the semiconductor phase transformation can also be seen in Figure 6.3 and Figure 6.4.
Effect of in-process mitigation heat treatments
Thermal results
The heat sink effect dominates the side heating effect for cooling rate in combined side heating and heat sink heat treatments. In contrast, side heating dominates the heat sink effect to reduce the thermal gradient between the side and weld regions.
Welding induced distortion
Two observation points are selected in the heat sink area near the FZ and the side heating area to track the change in temperature with time. Both side heating and heat sinking have been found to be less effective in reducing the distortion values.
Welding induced residual stress
Consequently, the change in the heating location does not significantly affect the longitudinal residual stress distribution in the weld area. The combined softening heat treatment develops a crown shape longitudinal residual stress distribution over the weld in the transverse direction.
Effect of post-weld heat treatment
Therefore, the compressive residual stress value decreased for the weld centerline by increasing the side heating temperature. The stress relaxation pattern during PWHT cycles for a point within the weld bead and base metal region is shown in Figure 6.20.
Summary
Finally, the post-weld heat treatment also shows a significant stress-relaxation effect on SAW-welded P91 steel. 7 EFFECT OF PREHEATING AND PWHT ON THE RESIDUAL STRESSES AND DISTORTION OF SAW WELDED BUTT JOINTS OF P91 STEEL.
Introduction
Preheating and PWHT experiments
Mitigation effect on square butt single side single pass welded joint
The residual stress distribution (disregarding the upper and lower weld crowns) in the HAZ and FZ of the post-weld heat-treated and as-welded cases is shown in Figure 7.3 (a) and (b) below. Both longitudinal and transverse maximum values of residual stress (tensile or compressive) are observed in the welded specimen.
Mitigation effect on square butt double side single pass welded joint
In the HAZ region, preheating slightly reduces the longitudinal residual stress in the upper and lower regions, such as. On the other hand, post-welding heat treatment successfully reduces the total residual stress values (both compressive and tensile values) in both the weld bead crown and the center region, as observed in Figure 7.7 (c).
Summary
The post-weld heat treatment alleviated the highest compressive residual stresses, especially in the lower spindle area from -194 MPa (as welded) to -41 MPa (PWHT). 8 MICROSTRUCTURE AND MECHANICAL CHARACTERIZATION OF SAW WELDED FUSION JOINTS OF P91 STEEL WITH PREHEATING AND POST-WELD HEAT TREATMENT.
Introduction
Macro and microstructural characterization
A high thermal gradient in the weld zone increases the cooling rate, which transforms the weld into a martensitic structure during cooling. The microstructures of different zones in the heat-treated weld after welding are shown in Figure 8.4.
Micro-hardness across the weld
It is observed that preheating alone reduces the hardness value from 437.2 HV (as welded) to 401 HV (preheated), which means that together they reduce the total hardness values in the HAZ and FZ for an even distribution across the FZ.
Weld strength and ductility
It can be seen from Table 8.2 and Table 8.3 that preheating and PWHT significantly improved the ductility compared to the as-welded specimen. Therefore, PWHT caused a decrease in the UTS value of the as-welded and preheated welds from 740 MPa (as-welded weld) and 721 MPa (preheated weld) to 627 MPa (PWHT on as-welded weld) and 618 MPa ( PWHT) on preheated welding).
Fracture surface analysis
This indicates that preheating and PWHT improve the malleability of the weld joints individually as well as combined. In preheating, the cooling rate is slowed down, which increases the fraction of the ferrite phase with a coarse dendrite structure, which causes a better ductility of the weld.
XRD and EDS analysis
The enlargement of cementite particles and equiaxed ferrite grains reduces the dislocation density in the post-weld heat-treated welds, which reduces the sizes of YS and UTS. The abundance of Mn and Mo in the as-welded welds indicates the higher weld strength than preheated and post-weld heat-treated welds [21].
Summary
Preheating also reduced UTS and YS by 2.6% and 4.87%, respectively, compared to the as-welded condition. The UTS and YS of the heat-treated welds, preheated and after welding, are lower than the base metal by 5% and 11%, respectively.
Conclusions of the present work
The high thermal gradient and high heat intensity cause compressive residual stress in the FZ of SAW-welded square butt single side single door joint of P91 steel. PWHT effectively mitigated the residual stresses in both FZ and HAZ of both square butt single and double sided single door joints of P91 steel.
Future scope
Sun, Finite element simulation of welding and residual stresses in a P91 steel tube with solid-state phase transformation and post-weld heat treatment, J. Withers, The effect of weld fusion zone shape on residual stress in submerged arc welding, Int.
