**Abstract:**

The residual stress of butt joint of X80 pipeline steel was simulated by means of finite element analysis, and the distribution of residual stress was obtained.

The prediction results were verified by X-ray diffraction and blind hole method.

The results show that the overall change trend of circumferential residual stress increases first, and then the stress reaches the peak position stably after arc stabilization, and the residual stress value decreases near the arc stop position;

The overall variation trend of the axial residual stress increases first, and then decreases steadily after reaching the peak position.

The results of residual stress simulation and prediction are consistent with those of the two measurement methods.

The results of X-ray diffraction method are higher than those of blind hole method.

The large stress position is predicted by finite element simulation technology, which has certain engineering application significance for effectively preventing stress corrosion cracking.

**Preface**

During pipeline construction, the anti-corrosion design of pipeline steel is an important factor for reliability design.

Stress corrosion cracking (SCC) is a low stress brittle cracking phenomenon under tensile stress and corrosive environment.

Generally, the pipeline surface is coated to ensure its anti-corrosion performance.

Under the influence of surface contact with soil, temperature, cathodic protection current and other factors, the chemical composition, microstructure changes, mechanical factors and stress of materials will have a direct impact on stress corrosion cracking.

Stress in mechanical factors will lead to material deformation and cracking, and its size, stress fluctuation, strain rate, etc. will affect the speed of material corrosion cracking, thus affecting the fatigue life of structural members.

Stress research has been paid more and more attention in the design of reliability and life maximization.

The analysis means include destructive test and nondestructive test, but nondestructive test is not suitable for industrial production because of its inaccuracy and high cost.

All kinds of simulation software for predicting stress can only conduct qualitative analysis but not quantitative analysis when conducting nonlinear analysis.

In order to ensure quantitative analysis, in addition to continuous improvement of simulation software, users need to have rich production experience, which can not be completely separated from the accumulation of production experience.

In order to accurately predict the distribution trend and size of residual stress after welding and ensure the high accuracy and short periodicity of production requirements, researchers have conducted a large number of welding simulation stress studies.

Bai Fang established a finite element model of X80 steel multi-layer and multipass welding, obtained the thermal cycle curve within the relative error range through tests, used to simulate the equivalent heat source loading, obtained the relationship between the peak temperature and cooling time and the weld center distance, and obtained the variation law of longitudinal and transverse residual stresses.

Gu Guolin established the finite element model of the butt joint of X80 pipeline steel, and used the X-ray diffraction method to verify that the prediction accuracy of the residual stress caused by the root node decoupling is higher, and the stress prediction result can be controlled at 10.35%.

Duan Weijun established the finite element model of S355 steel based on the thermal elastoplastic theory.

By introducing the transformation model, transformation plasticity and transformation volume model, he concluded that the equivalent stress and stress value would be greatly reduced, and the residual stress was in good agreement with the measured results.

Xiong Qingren et al. analyzed the residual stress of SSAW steel pipe by means of ring cutting test and blind hole test, compared the residual stress of steel pipe produced by different manufacturers, and proposed the residual stress control index, which effectively promoted production.

Zhang Dingquan and He Jiawen, etc. have made a comprehensive and detailed exposition of the principle, method and application of the nondestructive X-ray diffraction residual stress method, which is of guiding significance to the research of this method.

However, the above research focuses more on the distribution trend and accuracy of residual stress, and there is little research on the relationship between residual stress and corrosion resistance.

Xiong Qingren et al. analyzed the residual stress of SSAW steel pipe by means of ring cutting test and blind hole test, compared the residual stress of steel pipe produced by different manufacturers, and proposed the residual stress control index, which effectively promoted production.

Zhang Dingquan and He Jiawen, etc. have made a comprehensive and detailed exposition of the principle, method and application of the nondestructive X-ray diffraction residual stress method, which is of guiding significance to the research of this method.

However, the above research focuses more on the distribution trend and accuracy of residual stress, and there is little research on the relationship between residual stress and corrosion resistance.

In this article, by means of finite element analysis, the finite element model of X80 pipeline steel butt joint is established to predict and analyze the residual stress distribution, and then the simulation results are verified by two testing methods, namely, X-ray diffraction method and blind hole method, and its impact on corrosion resistance is analyzed in combination with the macro engineering.

**1. Establish finite element model**

**1.1 Welding parameters**

The wall thickness of butt joint pipeline steel is 22 mm, and the material is X80 pipeline steel.

The chemical composition of X80 pipeline steel is shown in Table 1.

The welding method is electrode arc welding.

The basic low hydrogen sodium electrode E7016 with a diameter of 3.2 mm is used as the welding material.

The chemical composition is shown in Table 2. Preheat 100 ℃ before welding.

The filler cover material E8010-P1 is used.

The chemical composition is shown in Table 3.

The welding parameters are: arc voltage 27~28V, welding current 120~130A, welding speed 42 cm/min, groove form is single-sided U type, and 5 layers and 10 passes of welding are adopted.

Table 1 Chemical composition of X80 pipeline steel (wt. %)

Mn | Si | p | s | Mo |

1.8294 | 0.2795 | 0.01102 | 0.0006 | 0.217 |

Ti | V | Nb | Ni | Cr |

0.0161 | 0.0586 | 0.0608 | 0.0308 | 0.0337 |

Table 2 E7016 Chemical composition of welding rod (wt. %)

C | Mn | Si | S | P |

0.06 | 1.05 | 0.65 | ≤0.02 | ≤0.02 |

Table 3 Chemical composition of E8010-P1 (wt. %)

Mn | Si | P | S | Mo | V | Cr | Ni |

0.5 | 0.14 | 0.015 | 0.02 | 0.4 | 0.01 | 0.035 | 0.4 |

**1.2 Establishment of material parameters**

The material parameters of material X80 for simulation are obtained through JMATPRO software analysis.

Some physical and mechanical parameters that vary with temperature are shown in Fig. 1.

**1.3 Establishment of grid model**

In order to ensure the accuracy of stress field solution, a relatively mature double ellipsoidal heat source model that can reflect the heat source of shielded metal arc welding is established.

Due to the relatively large residual stress values near the weld and near the weld toe, and considering the temperature gradient, the grid at the weld is densified, the transition grid is used for the heat affected zone and the position far from the weld, and the grid far from the weld is sparse.

The unit size of weld position is 2 mm, the total number of units is 108 648, and the number of nodes is 121 598, as shown in Fig. 2.

**1.4 Mechanical boundary conditions**

At the weld toe position on the outer surface of the butt joint steel pipe, along the direction of the weld cross section, select the element node at the circumferential position for X direction displacement constraint, so as to ensure that the lateral shrinkage of the X80 steel wire pipe finite element model is not affected;

Select the outer surface element node of the pipeline steel pipe with the free boundary of the butt joint along the longitudinal direction of the weld and the normal plane direction of the circumference center to constrain the Y and Z displacement, so as to ensure that the longitudinal shrinkage of the model is not affected.

**2. Test and measurement methods**

**2.1 Nondestructive X-ray diffraction method**

The X-ray stress measurement is nondestructive testing.

Its basic principle is based on the Bragg law, and the basic basis is that the lattice strain of the material caused by a certain stress state is consistent with the macro strain.

Metallic materials are generally polycrystalline, containing a large number of grains with different orientations in unit volume, and any selected crystal plane can be observed from any direction in space.

According to the elastic mechanics equation, calculate the strain value of the corresponding crystal plane through the change of the crystal plane spacing, that is

Where

- ε is the strain value of crystal plane;
- d0 is the spacing between crystal planes in the stress free state.

**2.2 Blind hole method**

The blind hole method residual stress testing is a nondestructive testing method.

It is to stick a strain rosette on the surface of the workpiece to be tested.

By drilling the workpiece in the center of the strain rosette, the balance of stress is destroyed and a certain amount of strain is generated.

After measuring the elastic strain increment near the hole, the residual stress at the small hole can be calculated using the principle of elastic mechanics.

The principal stress and direction at the small hole can be calculated according to the following formula.

Where εA is the strain of strain gauge a; A. B is the strain release coefficient.

**2.3 Test method design**

In order to minimize the error, two methods are used to measure the same test piece, first the X-ray measurement method and then the blind hole method.

In order to verify the accuracy of the calculation results, zero stress and high stress calibration equipment shall be used for X-ray diffraction measurement;

When measuring with blind hole method, the spacing between measuring points shall be 30mm to effectively release the drilling stress.

After the butt joint of X80 pipeline steel is welded, the weld toe position and weld center position shall be selected along the circumferential direction for stress measurement.

**3. Analysis and verification of simulation results**

**3.1 Analysis of circumferential residual stress measurement results**

Through simulation calculation, the simulation results of circumferential stress field of butt joint of X80 steel wire tube are obtained.

The residual stress from the weld toe position on one side of the butt joint to the relatively smooth outer surface is extracted, and the X-ray and blind hole method stress tests are carried out.

The results are shown in Fig. 3.

It can be seen from Fig. 3 that from the arc starting position to the arc stopping position, the overall change trend of the residual stress increases first, and after the arc is stabilized, the stress stably reaches the peak position and then remains.

When approaching the arc stopping position, the residual stress value decreases, which is consistent with the classical butt joint rule.

When welding the starting and stopping positions, the binding effect of the weldment is relatively small.

At the same time, there is a certain intersection at the starting and stopping positions, resulting in little change in the stress value.

The starting and stopping positions have a certain preheating effect on the stopping positions.

Therefore, the welding stress is relatively small, mainly manifested as compressive stress;

In the middle part, due to the large binding force at the welding position after welding, the stress is also large, which increases first and then decreases, mainly showing tensile stress.

The trends of the two measurement methods are in good agreement with the simulation results.

The maximum error between the simulation results and the X-ray diffraction measurement results is 490 mm away from the starting position of the weld, with an error of 15.9%;

The maximum error between the simulation results and the results measured by blind hole method is 490 mm, with an error of 12.4%.

The results of residual stress measured by X-ray method fluctuate relatively large.

Due to the measurement method and the characteristics of the measurement sample, the X-ray diffraction method requires a high flatness of the measurement surface when measuring.

However, the plane of the butt joint of X80 steel wire tube has a certain radian, and the lattice size at the measurement location changes abnormally, resulting in a large stress value.

The change trends of the two measurement methods are roughly consistent, and most of the measured values are larger than the simulation values.

During the process of simulation calculation, factors such as the fluidity of the filling unit, material hardening and phase transformation are not considered, resulting in the smaller residual stress results predicted by simulation.

**3.2 Analysis of measurement results of axial residual stress**

The test and simulation results of axial residual stress are shown in Figure 4.

It can be seen from Fig. 4 that the overall change trend of residual stress from the arc starting position to the arc stopping position is to increase first, and the residual stress decreases steadily after reaching the peak position, which is consistent with the classical butt joint rule.

Near the center of the weld, the weldment is relatively restrained, resulting in large changes in stress values and relatively large welding stresses, which are mainly tensile and compressive stresses;

Near the free end, the residual stress value is relatively small because it is not constrained by the welding heat input cold and hot shrinkage strain.

The trends of the two measurement methods are in good agreement with the simulation results.

The maximum error between the simulation results and the X-ray diffraction measurement results is 68 mm, and the error is 13.9%;

The maximum error between the simulation result and the blind hole method measurement result is 110 mm, with an error of 11.4%.

The result of X-ray method measurement of stress fluctuates relatively large.

**3.3 Effect of residual stress on corrosivity in engineering application**

The results of residual stress obtained by different measurement methods verify the accuracy of the simulation results of residual stress prediction for X80 pipeline steel.

In the after-sales application case fed back by the actual engineering application, Pakistan after-sales support personnel once found that the paint at the weld toe of X80 pipeline steel is very easy to fall off under the long-term soil corrosion and illumination, releasing harmful strain under the effects of environmental factors and residual stress, causing local corrosion and damage of X80 pipeline steel, which ultimately leads to stress corrosion cracking.

In the absence of serious accidents, on-site application problems were solved through troubleshooting analysis.

**4. Conclusion**

(1) The finite element model was used to simulate the residual stress of X80 pipeline steel butt joint, and the accuracy of the simulation results was verified by X-ray diffraction and blind hole method.

The simulation results are in good agreement with the two measurement methods, and the X-ray diffraction results are higher than the blind hole method.

(2) From the arc starting position to the arc stopping position, the overall change trend of the circumferential residual stress increases first, and after the arc is stabilized, the stress stably reaches the peak position and then maintains, and when it is close to the arc stopping position, the residual stress value decreases.

From the arc starting position to the arc stopping position, the overall change trend of the axial residual stress increases first, and the residual stress decreases steadily after reaching the peak position.

(3) X80 pipeline steel has local stress corrosion cracking.

Through finite element simulation technology, predict the location of high stress, which has certain engineering application significance for effectively preventing stress corrosion cracking.