# Fatigue Limit Improvement and Rendering Defects Harmless by Needle Peening for High Tensile Steel Welded Joint

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Test Material and Specimens

#### 2.2. Needle Peening and the Introduction of the Surface Defect

#### 2.3. Fatigue Test Method

^{7}cycles.

#### 2.4. Residual Stress Measurement

#### 2.5. Hardness Measurement

#### 2.6. Metal Microstructure Observation

#### 2.7. Finite Element Analysis of the Stress Concentration of the Weld Toe

#### 2.8. Non-propagating Crack Observation

## 3. Results and Discussion

#### 3.1. Fatigue Test Results

_{a}) against cycles to fracture (N

_{f}). Figure 6 summarizes all fatigue test results as the relationship between the fatigue limit (σ

_{w}) and the depth of the semicircular slit (a). All fatigue test results were shown in Table A1, Table A2, Table A3, Table A4, Table A5 and Table A6 in appendix section.

- (a)
- The fatigue limit of WNS increased to the same level as that of WN.
- (b)
- WNS fractured at a location other than the slit.

#### 3.2. Residual Stress Measurement

_{w}= 240 MPa was also measured to investigate whether the compressive residual stress changed due to cyclic loading.

#### 3.3. Hardness Measurement

_{w}= 240 MPa was also measured to investigate the effect of cyclic loading on the hardness.

#### 3.4. Metal Microstructure Observation

#### 3.5. Finite Element Analysis of the Stress Concentration of the Weld Toe

_{t}of the weld toe was calculated to compare the stress with nominal stress. We found that the K

_{t}of W was 1.6 and K

_{t}of WN was 1.8. The reason that the stress concentration increased after NP can be explained by the effect of the dent generated via NP, which is shown in Figure 12b.

_{t}of W, which was more than 2.0. However, notably, the stress concentration of the welded joint can increase after NP if a small NP is applied for the welded joint stress concentration at the weld toe.

#### 3.6. Dominant Contributing Factor in Fatigue Limit Improvement due to NP

#### 3.7. Non-propagating Crack Observation

## 4. Conclusions

- (1)
- The fatigue limit of the defect-free welded specimen increased by 9% after NP at a stress ratio of R = 0.05. The fatigue limit of the welded specimen containing a 1.0-mm-deep semicircular slit was significantly increased (200%) by NP as the slit-containing specimen reached the same fatigue limit as that of the defect-free NP-treated welded specimen. This result indicates that the fatigue limits containing surface defects with depths less than 1 mm, which are not detected through NDI, are considered to be equal to that of the NP-treated welded specimen without a defect. Therefore, the reliability of HTS-welded joints can be significantly improved by NP and the problem regarding the reliability of HTS-welded joints that restricts the industrial utilization of HTS can be solved by performing NP.
- (2)
- The dominant factor that contributed to the improvement in the fatigue limit and increase in the acceptable defect size was the introduction of large and deep compressive residual stress during NP. Furthermore, the increase in hardness at the surface due to work hardening and grain refining caused by NP also contributed to the improvement in the fatigue limit.
- (3)
- It was clarified that whether the slit affecting the fatigue limit of NP-treated welded joint was determined by the threshold condition for the crack propagation. Additionally, it was found that an acceptable fatigue crack size for the NP-treated welded joint was close to the slit size introduced in the present study, but it was slightly larger than the slit.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Appendix A

^{7}cycles.

Specimen ID | Stress Amplitude σ_{a} | Cycles to Failure N_{f} | Fracture Origin |
---|---|---|---|

7 | 280 | 353,975 | Weld toe |

4 | 260 | 4,163,922 | Weld toe |

5 | 240 | 7,816,232 | Weld toe |

3 | 220 | >10,000,000 | Run-out |

6 | 200 | >10,000,000 | Run-out |

Specimen ID | Stress Amplitude σ_{a} | Cycles to Failure N_{f} | Fracture Origin |
---|---|---|---|

21 | 100 | 2,103,232 | Slit |

23 | 90 | 2,963,700 | Slit |

16 | 80 | >10,000,000 | Run-out |

Specimen ID | Stress Amplitude σ_{a} | Cycles to Failure N_{f} | Fracture Origin |
---|---|---|---|

97 | 120 | 1,820,548 | Slit |

99 | 100 | 4,228,474 | Slit |

96 | 80 | >10,000,000 | Run-out |

Specimen ID | Stress Amplitude σ_{a} | Cycles to Failure N_{f} | Fracture Origin |
---|---|---|---|

36 | 340 | 214,915 | Bottom of dent |

27 | 280 | 530,242 | Bottom of dent |

110 | 280 | 557,058 | Bottom of dent |

37 | 260 | >10,000,000 | Run-out |

113 | 260 | 1,323,142 | Bottom of dent |

35 | 240 | >10,000,000 | Run-out |

116 | 240 | >10,000,000 | Run-out |

Specimen ID | Stress Amplitude σ_{a} | Cycles to Failure N_{f} | Fracture Origin |
---|---|---|---|

100 | 280 | 500,542 | Outside of the slit |

117 | 260 | 756,941 | Outside of the slit |

114 | 240 | >10,000,000 | Run-out |

Specimen ID | Stress Amplitude σ_{a} | Cycles to Failure N_{f} | Fracture Origin |
---|---|---|---|

106 | 220 | 1,081,373 | Slit |

108 | 200 | 1,473,980 | Slit |

112 | 180 | 1,566,177 | Slit |

107 | 160 | >10,000,000 | Run-out |

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**Figure 1.**Schematic view of needle peening (NP) at the weld toe. NP introduces beneficial compressive residual stresses in the surface layer around the weld toe by repeatedly striking the surface with a needle pin.

**Figure 2.**Shape and dimensions of (

**a**) the welded plate and (

**b**) the test specimen. The test material was supplied in the form of 7-mm-thick plates. The welded plate was cut to prepare the test specimens such that the longitudinal direction of the specimen corresponded to the rolling direction of the plate.

**Figure 3.**Flow chart of specimen preparation and types of specimens. Four groups of specimens were prepared, which were namely the welded specimen (W), NP-treated welded specimen (WN), welded specimen with a semicircular slit at the weld toe (WS) and NP-treated welded specimen with a semicircular slit at the weld toe (WNS).

**Figure 4.**Overview of the slit and loading point: (

**a**) position of the slit with respect to the weld bead; (

**b**) position of the slit with respect to the weld bead and the location of the NP treatment; (

**c**) shape and dimensions of the semi-circular slit where a is the slit depth; and (

**d**) relationship between the slit position and loading point.

**Figure 5.**S-N diagram of (

**a**) W and WN, (

**b**) WS1.0 and WNS1.0 and (

**c**) WS1.5 and WNS 1.5. The S-N diagram plots nominal stress amplitude (σ

_{a}) against cycles to fracture (N

_{f}). Three-point bending fatigue tests were performed with load control. The span length of the three-point bending was 80 mm. All tests were performed at a stress of R = 0.05 and a frequency of f = 20 Hz. The fatigue test was terminated after 1 × 10

^{7}cycles. The asterisks (*) indicate that a fatigue crack occurred at a different location compared to the slit. Fatigue limits σ

_{w}for each type of the specimen were indicated outside the frame of the graph.

**Figure 6.**Relationship between the fatigue limit and the semicircular slit depth. The fatigue limit was defined as the maximum stress amplitude at which the specimen could endure 1 × 10

^{7}cycles.

**Figure 7.**Fracture surfaces of (

**a**) WNS1.0 at σ

_{a}= 260 MPa and (

**b**) WNS1.5 at σ

_{a}= 220 MPa observed using a digital optical microscope.

**Figure 8.**Residual stress distribution at the weld toe. The longitudinal residual stress measurement of the weld toe for W and WN was obtained prior to the fatigue test. The residual stress for WN after the fatigue test at a fatigue limit of σ

_{w}= 240 MPa was also measured.

**Figure 9.**Vickers hardness profiles. In-depth hardness profiles of the weld toe at the surface layer of W and WN prior to the fatigue test were measured. The hardness profile of WN after the fatigue test at a fatigue limit of σ

_{w}= 240 MPa was also measured.

**Figure 11.**Finite element models of W created based on the shape measurements of the specimens under the fatigue limit using a laser displacement gauge. The model corresponded to symmetric half models of the welded specimen that comprised a 20-node hexahedral element with a minimum element size of 0.05 mm.

**Figure 12.**Contour of the longitudinal stress at a normal bending stress of 200 MPa for: (

**a**) W and (

**b**) WN.

**Figure 14.**Fracture surface around the slit for WNS1.0 tested under the fatigue limit of σ

_{w}= 240 MPa. Non-propagating cracks were marked with a heat tint color via heat treatment at 573 K in air. The specimen was compulsorily fractured in a brittle manner using liquid nitrogen after a fatigue crack propagated under excessive cyclic loading.

C | Si | Mn | P | S | Ni | Cr | Mo | Nb | B |
---|---|---|---|---|---|---|---|---|---|

0.14 | 0.35 | 1.18 | 0.005 | 0.001 | 0.01 | 0.09 | 0.12 | 0.02 | 0.001 |

Yield Stress (MPa) | Ultimate Tensile Stress (MPa) | Vickers Hardness HV |
---|---|---|

822 | 839 | 267 |

Parameters | Conditions |
---|---|

Number of layers | 1 |

Number of passes | 1 |

Welding position | Flat |

Diameter of the welding rod (mm) | Φ 2.4 |

Current (A) | 180 |

Voltage (V) | 9.7 |

Welding speed (cm/min) | 12 |

Parameters | Conditions |
---|---|

Air pressure (MPa) | 0.5 |

Radius of needle pin (mm) | 1.5 |

Material of needle pin | High carbon chromium bearing steel (JIS-SUJ2) |

Coverage (%) | 400 |

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Fueki, R.; Takahashi, K.; Handa, M.
Fatigue Limit Improvement and Rendering Defects Harmless by Needle Peening for High Tensile Steel Welded Joint. *Metals* **2019**, *9*, 143.
https://doi.org/10.3390/met9020143

**AMA Style**

Fueki R, Takahashi K, Handa M.
Fatigue Limit Improvement and Rendering Defects Harmless by Needle Peening for High Tensile Steel Welded Joint. *Metals*. 2019; 9(2):143.
https://doi.org/10.3390/met9020143

**Chicago/Turabian Style**

Fueki, Ryutaro, Koji Takahashi, and Mitsuru Handa.
2019. "Fatigue Limit Improvement and Rendering Defects Harmless by Needle Peening for High Tensile Steel Welded Joint" *Metals* 9, no. 2: 143.
https://doi.org/10.3390/met9020143