# Assessment of the Effect of Residual Stresses Arising in the HAZ of Welds on the Fatigue Life of S700MC Steel

^{*}

## Abstract

**:**

## 1. Introduction

^{−1}. The reason for the reduction of the heat input value is the intense grain coarsening in the HAZ and, for higher-strength steels, the softening of the structure, which can occur at the interface between the HAZ and the base material. Many studies have investigated grain coarsening and its effect on mechanical properties. For example, Bayock et al. [13] investigated the effect of heat input on the microstructure and mechanical properties of dissimilar S700MC/S960QC steels. Three different heat input values (15, 7 and 10 kJ·cm

^{−1}) were used. The best microstructure formation was obtained using a heat input of 10 kJ·cm

^{−1}. The S700MC steel was investigated by Górka [14] and also by Moravec et al. [15]. The aim of these studies was to determine the properties and microstructure of the HAZ of S700MC steel. The steel with higher strength properties was investigated by Mičian et al. [16]. In the study, the effect of the cooling rate on the mechanical properties of the heat-affected zone of the steel S960MC was investigated. Furthermore, Jambor et al. [17] dealt with the development of the microstructure in the HAZ of the S960MC steel GMAW weld.

## 2. Materials and Methods

_{g}and A

_{80}were measured at room temperature—RT. The testing device TIRA Test 2300 (TIRA GmbH, Schalkau, Germany) was used. Static tensile test was performed according to standard EN ISO 6892-1 with loading rate 1 mm/min up to achieving YS and after that 15 mm/min. The mechanical properties of the annealed material at 550 °C were measured. The reason for the annealing was to eliminate the surface residual stresses in the sample that were created during the machining process. The annealing of the samples was carried out in a Reetz vacuum furnace (HTM Reetz GmbH, Berlin, Germany). The heating rate was the same for all experiments and was stepwise (0.8 °C·min

^{−1}to 80 °C; 1.5 °C·min

^{−1}to 220 °C; 2 °C·min

^{−1}to 300 °C and 4 °C·min

^{−1}in the temperature range 300 to 550 °C). The holding time at the annealing temperature was 2 h. Additionally, the cooling rate of the sample was the same for all experiments and in this case was constant (5 °C·min

^{−1}). The heat treatment was carried out under vacuum, which was 7 × 10

^{−5}mbar. The determination of the residual stresses by XRD method confirmed that only minimal residual stresses were present in the annealed samples (550 °C for 2 h). Table 2 shows the average values of the mechanical properties of the base and annealed material obtained from 5 measurements.

^{−1}. The temperature cycles in the HAZ of the welds were measured by the thermocouples type R, connected to a DiagWeld apparatus (Technical University of Liberec, Liberec, Czech Republic) with a recording frequency of 50 Hz.

_{1}= −1.25 TPa

^{−1}and ½𝑠

_{2}= 5.75 TPa

^{−1}were used to convert deformation to stress. The value of the diffraction angle of the {211} α–Fe planes of the material in the undeformed state is 156° 2θ. These values were used from the XDR Win2000 software database. The diffraction angles were determined by the Gaussian function approximation using the Absolute Peak method. The algorithm for calculating the residual stresses was sin

^{2}ψ.

## 3. Experiments and Results

- (1)
- Obtaining a real temperature cycle corresponding to MAG welding with heat input up to 10 kJ·cm
^{−1}; - (2)
- Experimental setting of the annealing conditions (T = 550 °C; t = 2 h) at which the surface residual stresses created during the machining of the test samples are reduced;
- (3)
- Determination of mechanical properties and S–N curves for the base and annealed materials;
- (4)
- Application of a temperature–stress cycle to annealed samples (produced in accordance with Figure 2) in a Gleeble 3500;
- (5)
- Determination of the residual stresses in the samples using the XRD method;
- (6)
- Annealing to reduce residual stresses after application of temperature–stress cycle in device Gleeble 3500 realized for some samples;
- (7)
- Determination of S–N curves for the state corresponding to the influence of the temperature–stress cycle (with residual stresses) and also for the state after application of the temperature–stress cycle but with subsequent annealing to reduce residual stresses.

#### 3.1. Measurement and Application of Temperature Cycles

#### 3.2. Analysis of Residual Stresses after Machining and after Annealing

#### 3.3. Determination of S–N Curve for Base Material S700MC and Annealed Material

^{7}cycles. A total of 10 samples were tested for each variation—one sample for each stress level. The base material was tested at stress levels of 580; 550; 527.5; 505; 490; 475; 468; 461; 455 and 447.5 MPa and for the annealed material, stress levels of 580; 565; 550; 546; 542.5; 535; 527.5; 520; 500 MPa were chosen. The measured S–N curves of the base and annealed material are shown in Figure 6. The achieved fatigue life of the base material was σc = 447.5 MPa and the achieved fatigue life of the heat-treated material was σc = 500 MPa. Table 3 shows the fatigue test results for the base and annealed materials.

#### 3.4. Application of Temperature Cycles in the Gleeble 3500

^{−3}Torr in a controlled deformation mode. A rigid clamping of the sample was defined with zero dilation monitored by the L-Gauge contact strain gauge. This procedure was repeated for all 20 test samples so that the distribution and magnitude of residual stresses were the same in all samples.

_{1}= 13.24 µm, d

_{2}= 13.09 µm and d

_{3}= 13.28 µm were obtained. The average mean grain size in the highly heated area is d

_{s}= 13.21 µm.

#### 3.5. Analysis of Residual Stresses after Application of the Temperature–Stress Cycle

#### 3.6. Fatigue Life Determination of Samples with Residual Stresses

^{7}cycles. The samples were loaded at stress levels of 520; 490; 460; 400; 370; 360; 350; 345; 340 and 335 MPa. Again, a total of 10 samples were tested, one for each stress level. The fatigue test results for the samples with residual stresses are shown in Table 4 and the S–N curve is shown in Figure 11.

## 4. Discussion

_{c}are basic and important material characteristics for dynamically loaded parts. The use of HSLA steels in automotive applications requires information about the fatigue life of these steels. It is easier to obtain S–N curves of base materials than to obtain S–N curves of materials affected by a technological process such as welding.

_{c}of 10%, from 447.5 MPa to 500 MPa. EBSD analysis confirmed that there was no increase in the mean grain size.

_{c}was decreased to 335 MPa, i.e., by 33% compared to the annealed material.

## 5. Conclusions

- (1)
- After annealing the S700MC steel samples (550 °C_2 h), the YS value remained unchanged but the UTS value decreased by 7.8%. At the same time, the fatigue strength increased by 10%, from 447.5 MPa to 500 Mpa;
- (2)
- Application of the temperature–stress cycle caused residual tensile stresses in the sample with a maximum value of 507 MPa and compressive residual stresses with a maximum value of 471 Mpa;
- (3)
- The application of the temperature–stress cycle also caused a decrease in the fatigue strength σ
_{c}from 500 MPa to 335 MPa, i.e., a decrease of 33%; - (4)
- Annealing at 550 °C for 2 h caused the complete elimination of compressive residual stresses and a significant 80% reduction in tensile residual stresses in the samples subjected to the temperature–stress cycle. As a result, the fatigue strength of the samples increased from 335 MPa to 450 Mpa;
- (5)
- According to the above, residual stresses contribute to 69% of the fatigue strength reduction. The residue is due to material changes;
- (6)
- The samples with residual stresses fractured at a point 3.7 mm from the center of the sample, thus up to the tensile stress peaks. This is probably related to the redistribution of residual stresses that occur during cyclic loading.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 2.**Drawing of the sample for simulations in Gleeble (in mm) [34].

**Figure 3.**Sample drawing for fatigue testing acc. to standard EN 3987 (in mm) [35].

**Figure 9.**Courses of residual stresses after application of temperature cycles, for sample 1 (red curve), for sample 2 (blue curve), for sample 3 (grey curve).

**Figure 10.**Courses of residual stresses after application of temperature cycles (red curve) and after annealing to reduce residual stresses (blue curve).

**Figure 13.**Comparison of all measured S–N curves, base material (black curve), annealed material at 550 °C for 2 h (pink curve), after TC (orange curve), after TC + HT 550 °C for 2 h (blue curve).

C | Si | Mn | P | S | Al | Nb | V | |

ČSN EN 10149-2 | max. 0.12 | max. 0.6 | max. 2.2 | max. 0.025 | max. 0.010 | min. 0.015 | max. 0.09 | max. 0.2 |

Experiment | 0.050 | 0.196 | 1.914 | 0.006 | 0.006 | 0.037 | 0.063 | 0.072 |

Ti | Mo | B | N | Ni | Cr | W | ||

ČSN EN 10149-2 | max. 0.25 | max. - | max. 0.005 | - | - | - | - | |

Experiment | 0.056 | 0.112 | 0 | 0.013 | 0.153 | 0.035 | 0.035 |

Sample No. | YS [MPa] | UTS [MPa] | A_{g} [%] | A_{30} [%] |
---|---|---|---|---|

Basic material | 748 ± 9 | 851 ± 1 | 11.08 ± 0.62 | 24.03 ± 0.62 |

Annealing material (550 °C_2 h) | 745 ± 3 | 785 ± 3 | 9.69 ± 0.12 | 21.16 ± 0.29 |

Basic material | σ_{A} [MPa] | 580 | 550 | 527.5 | 505 | 490 |

Nf [1] | 153,695 | 234,942 | 1,056,376 | 1,804,894 | 1,063,721 | |

σ_{A} [MPa] | 475 | 468 | 461 | 455 | 447.5 | |

Nf [1] | 804,513 | 1,687,452 | 2,420,822 | 7,153,698 | 10^{7} | |

550 °C 2 h | σ_{A} [MPa] | 580 | 565 | 550 | 546 | 542.5 |

Nf [1] | 89,479 | 292,247 | 288,898 | 1,964,321 | 3,083,698 | |

σ_{A} [MPa] | 535 | 535 | 527,5 | 520 | 500 | |

Nf [1] | 589,138 | 1,011,574 | 3,680,777 | 8,881,583 | 10^{7} |

After TC | σ_{A} [MPa] | 520 | 490 | 460 | 400 | 370 |

Nf [1] | 78,412 | 290,364 | 354,888 | 536,242 | 1,138,628 | |

σ_{A} [MPa] | 360 | 350 | 345 | 340 | 335 | |

Nf [1] | 1,968,348 | 5,698,176 | 3,458,265 | 6,483,620 | 10^{7} |

After TC + annealing | σ_{A} [MPa] | 600 | 570 | 540 | 500 | 490 |

Nf [1] | 81,275 | 136,846 | 436,195 | 874,693 | 1,297,314 | |

σ_{A} [MPa] | 480 | 472.5 | 465 | 457.5 | 450 | |

Nf [1] | 2,769,674 | 4,181,239 | 6,583,912 | 8,826,743 | 10^{7} |

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**MDPI and ACS Style**

Bukovská, Š.; Moravec, J.; Solfronk, P.; Pekárek, M.
Assessment of the Effect of Residual Stresses Arising in the HAZ of Welds on the Fatigue Life of S700MC Steel. *Metals* **2022**, *12*, 1890.
https://doi.org/10.3390/met12111890

**AMA Style**

Bukovská Š, Moravec J, Solfronk P, Pekárek M.
Assessment of the Effect of Residual Stresses Arising in the HAZ of Welds on the Fatigue Life of S700MC Steel. *Metals*. 2022; 12(11):1890.
https://doi.org/10.3390/met12111890

**Chicago/Turabian Style**

Bukovská, Šárka, Jaromír Moravec, Pavel Solfronk, and Milan Pekárek.
2022. "Assessment of the Effect of Residual Stresses Arising in the HAZ of Welds on the Fatigue Life of S700MC Steel" *Metals* 12, no. 11: 1890.
https://doi.org/10.3390/met12111890