# Distribution and Characteristics of Residual Stresses in Super Duplex Stainless Steel Pipe Weld

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## Abstract

**:**

## 1. Introduction

## 2. Experimental Observation

#### 2.1. Tensile Test at Elevated Temperature

#### 2.2. Metallographic Observation and Hardness Test

#### 2.3. Residual Stress Measurement

## 3. FE Simulation

#### 3.1. Model Geometry and Material Properties

**×**240 mm (length)

**×**6 mm (thickness). The single-pass GTA welding process was employed to join the steel pipes, and the parameters were a voltage of 22 V, current of 230 A and speed of 1.3 mm/s, analogous to industrial practice [23]. Only one half of the weldpiece was modeled due to the symmetric condition, and the 3D FE model with mesh refinement in the weld region and its vicinity using eight-noded isoparametric solid elements is displayed in Figure 4, where the boundary constraints are indicated by arrows. In the weld and its vicinity in which a high temperature gradient exists, a more refined mesh is required to reproduce an accurate temperature field. Element size becomes incremental with distance from the weld centerline. A mesh convergence study was carried out to assess the dependence of FE mesh size on the accuracy of analysis. As a result, it was determined that the present FE mesh with the smallest element size of 0.9 mm (axial) × 1.5 mm (thickness) × 25.6 mm (circumference) produced sufficiently accurate outcomes while reducing the computational cost. In the FE simulation, the material properties depending on the temperature of the S32750 super duplex stainless steel were taken into consideration. Figure 5 shows varying physical constants (e.g., thermal conductivity, density and specific heat) with temperature [24,25]. As described earlier, the high-temperature mechanical properties were obtained by the experiment. Figure 6 shows the temperature dependency of the mechanical properties, where the yield stress, the tensile strength and Young’s modulus are smoothly reduced to the melting point to reproduce the low strength [26]. In this work, autogenous welding is assumed, which implies that the base metal, the HAZ and the weld metal share the same material properties [27].

#### 3.2. FE Formulation

#### 3.2.1. Heat Transfer Analysis

#### 3.2.2. Mechanical (Structural) Analysis

- Equilibrium equation:

- Stress–strain constitutive equation:

#### 3.2.3. Metallurgical Phase Transformation

## 4. Results and Discussion

## 5. Conclusions

- (a)
- Super duplex stainless steel undergoes martensitic phase evolution in the HAZ and the weld metal in the process of cooling during welding.
- (b)
- The martensitic phase transformation has little impact on the evolution of axial residual stresses, i.e., the axial residual stresses are mainly formed by circumferential shrinkage during the cooling process. On the other hand, a considerable release of hoop residual stresses in the weld region and its vicinity takes place owing to the volume change in the process of phase transformation. Thus, the metallurgical phase transformation cannot be disregarded in numerical simulations of the girth-welding process to provide an accurate expression of the weld-induced residual stresses.
- (c)
- A 3D FE model should be utilized to accurately simulate the distribution of residual stresses and their characteristics along the circumference in girth-welded super duplex stainless steel pipes, since the residual stresses are by no means axisymmetric, and are caused by both the spatial deposition of the weld filler and the welding start/end effect.
- (d)
- Knowledge of the distribution and characteristics of the residual stresses found in this work can assist the production of an efficient and economic design of welded super duplex stainless steel structures.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**Optical microstructure: (

**a**) base metal (

**×**500), (

**b**) HAZ (

**×**500), (

**c**) weld metal (

**×**500), (

**d**) HAZ (

**×**200) and (

**e**) weld metal (

**×**200).

**Figure 8.**Comparison of the residual stress measurements with the FE analysis results, both with and without considering the SSPT: (

**a**) longitudinal residual stresses and (

**b**) transverse residual stresses.

**Figure 9.**Axial residual stresses on the inside surface at locations with different circumferential angles from the welding start/stop position: (

**a**) 90°, (

**b**) 180°, (

**c**) 270° and (

**d**) 360°.

**Figure 10.**Axial residual stresses on the outside surface at locations with different circumferential angles from the welding start/stop position: (

**a**) 90°, (

**b**) 180°, (

**c**) 270° and (

**d**) 360°.

**Figure 11.**Hoop residual stresses on the inside surface at locations with different circumferential angles from the welding start/stop position: (

**a**) 90°, (

**b**) 180°, (

**c**) 270° and (

**d**) 360°.

**Figure 12.**Hoop residual stresses on the outside surface at locations with different circumferential angles from the welding start/stop position: (

**a**) 90°, (

**b**) 180°, (

**c**) 270° and (

**d**) 360°.

Chemical Composition (mass, %) | ||||||||
---|---|---|---|---|---|---|---|---|

C | Mn | P | S | Si | Ni | Cr | Mo | N |

0.019 | 1.848 | 0.028 | 0.0004 | 0.468 | 5.065 | 22.255 | 2.535 | 0.1535 |

Mechanical Properties | ||||||||

Yield stress (MPa) | Ultimate strength (MPa) | Elongation (%) | ||||||

678 | 839 | 35 |

PASS | Current (A) | Voltage (V) | Velocity (mm/s) |
---|---|---|---|

1 | 140 | 12 | 0.9 |

2 | 160 | 12 | 1.9 |

3 | 170 | 12 | 1.7 |

4 | 170 | 12 | 1.2 |

5 | 170 | 12 | 1.3 |

6 | 160 | 12 | 1.0 |

Base Metal | HAZ | Weld Metal | HAZ | Base Metal | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Point number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |

Averaged value | 235 | 236 | 235 | 252 | 252 | 253 | 251 | 254 | 255 | 253 | 255 | 254 | 235 | 235 | 235 |

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**MDPI and ACS Style**

Cho, C.B.; Lee, J.-H.; Lee, C.-H.
Distribution and Characteristics of Residual Stresses in Super Duplex Stainless Steel Pipe Weld. *Metals* **2024**, *14*, 136.
https://doi.org/10.3390/met14020136

**AMA Style**

Cho CB, Lee J-H, Lee C-H.
Distribution and Characteristics of Residual Stresses in Super Duplex Stainless Steel Pipe Weld. *Metals*. 2024; 14(2):136.
https://doi.org/10.3390/met14020136

**Chicago/Turabian Style**

Cho, Chang Beck, Joo-Ho Lee, and Chin-Hyung Lee.
2024. "Distribution and Characteristics of Residual Stresses in Super Duplex Stainless Steel Pipe Weld" *Metals* 14, no. 2: 136.
https://doi.org/10.3390/met14020136