11.04.2019 | Ausgabe 4/2019 Open Access

# Effect of Annealing on the Impact Resistance and Fracture Mechanism of PNC-60 Sinters After Cold Plastic Deformation

- Zeitschrift:
- Journal of Materials Engineering and Performance > Ausgabe 4/2019

## Publisher's Note

## Introduction

_{m}< 0) is fulfilled both globally and locally. Otherwise, in areas where σ

_{m}> 0, the porosity increases, which can impair the cohesion during forming. Therefore, in the plastic forming of sinters, processes characterized by large negative values of mean stresses are preferred. These processes include different types of isostatic pressing, closed die forging and extrusion. All of these processes ensure a significant increase in density.

_{a}[°C] and the equivalent matrix strain (∈). This approach is not found in other works, where the effect of plastic deformation on the properties of sintered products is described only in terms of the degree of deformation, neglecting the sintered matrix hardening parameter. With fixed values of the test parameters (T

_{a}and ∈), any comparison of the properties and structure of sinters subjected to plastic deformation and heat treatment requires the same final porosity (Θ

_{k}= const). To achieve this goal, the test methodology and sample preparation technique described in section 2 are used.

## Research Methodology and Procedures

_{0}= 10 mm, length l

_{0}= 60 mm and height h

_{0}) depending on the initial required porosity determined in accordance with the methodology given in section 2.1.

### Calculation Methodology

_{2}= (1/6) [(σ

_{1}− σ

_{2})

^{2}+ (σ

_{2}− σ

_{3})

^{2}− (σ

_{3}− σ

_{1})

^{2}] and J

_{1}= σ

_{1}+ σ

_{2}+ σ

_{3}are the second invariant of the stress deviator tensor and the first invariant of the stress tensor, respectively, α(Θ) and β(Θ) are porosity functions (yield criterion parameters), and σ

_{p}(∈) is the hardening curve of the sintered matrix material, where σ

_{p}is the yield stress and ∈ is the sintered matrix hardening parameter.The porosity Θ gives the proportional content of the voids in the sintered volume and is calculated from Eq 2:

_{L}is the matrix material density.

_{0}and h

_{1}are the compact initial height and final height, respectively.

_{1}. First, using the method of successive approximations, the lower limit of integration Θ

_{0}at which integral (5) reaches the assumed value ∈ for the assumed final porosity Θ

_{1}is determined. Then, from Eq 6, the cold work ratio γ and the true strain of the sinter ε = lnγ are calculated. For the set value of cold work and the assumed final height h

_{1}of the compact, the required initial height h

_{0}of the preform is derived.

_{1}= 0.12, and the dimensions of the preforms in the shape of bars were 10 mm × 60 mm × h

_{0}. The deformation was achieved by uniaxial compression to obtain compacts with a final height of h

_{1}≈ 10 mm. Two values of the sintered matrix hardening parameter were assumed, i.e., ∈ = 0.17 and ∈ = 0.50, with the corresponding logarithmic strain of the sinter ε = 0.24 and ε = 0.77, respectively. Preforms for the deformation where ε = 0.24 were designated as W2 and for the deformation where ε = 0.77 as W7. The samples after deformation were designated as S2x and S7x, respectively, where “x” represented the annealing temperature designated as 0 (without annealing), 1 (650 °C), 2 (750 °C), or 3 (850 °C). The designation SF denoted a sinter with porosity equal to the final porosity obtained without additional plastic forming. For example, sample S73 denotes the sinter subjected to strain ε = 0.77 and later annealed at 850 °C.

_{m}, the yield strength R

_{p0.2}and the elongation A

_{5}were determined in a static tensile test on cylindrical, fivefold samples with an initial diameter of ϕ = 5 mm that were machined by turning from pressed and compacted specimens.

### Mechanical Properties

Product designation | Porosity Θ | Impact resistance KC, J/cm ^{2} | Tensile strength R _{m}, MPa | Yield strength R _{p0,2}, MPa |
---|---|---|---|---|

W2 | 0.187 ± 0.003 | 20.0 ± 1.1 | 293 ± 2 | 228 ± 4 |

W7 | 0.325 ± 0.013 | 11.0 ± 4.8 | 80 ± 13 | 78 ± 16 |

SF | 0.121 ± 0.009 | 23.5 ± 5.9 | 402 ± 12 | 289 ± 9 |

Final porosity and parameters of CPD | Product designation | Annealing temperature T _{w}, °C | Impact resistance KC, J/cm ^{2} | Tensile strength R _{m}, MPa | Yield strength R _{p0,2}, MPa |
---|---|---|---|---|---|

Θ _{1} = 0.127 ± 0.005ε = 0.242 ± 0.012 ∈ = 0.165 ± 0.015 | S20 | … | 4.5 ± 0.4 | 529 ± 14 | 471 ± 9 |

S21 | 650 | 25.6 ± 0.1 | 374 ± 9 | 296 ± 9 | |

S22 | 750 | 24.1 ± 5.5 | 383 ± 2 | 301 ± 4 | |

S23 | 850 | 25.1 ± 3.5 | 396 ± 10 | 308 ± 3 | |

Θ _{1} = 0.112 ± 0.004ε = 0.772 ± 0.009 ∈ = 0.494 ± 0.011 | S70 | … | 2.0 ± 1.2 | 342 ± 140 | 331 ± 136 |

S71 | 650 | 16.1 ± 4.3 | 354 ± 32 | 321 ± 31 | |

S72 | 750 | 21.3 ± 0.7 | 331 ± 1 | 248 ± 1 | |

S73 | 850 | 30.1 ± 4.5 | 370 ± 13 | 277 ± 10 |

_{m}increases from 293 to 529 MPa, and the yield strength R

_{p0.2}increases from 228 to 471 MPa. Both of these values are also higher than those for sinters with the same porosity after single pressing and sintering, where the respective values are R

_{m}= 402 MPa and R

_{p0.2}= 289 MPa. However, the improvement in mechanical properties is accompanied by a significant reduction in the impact energy, i.e., from 20.0 J/cm

^{2}for the preform to 4.5 J/cm

^{2}for the sinter after deformation. Compared to the sinters after deformation, annealing at 650 °C reduces the strength properties, and R

_{m}= 374 MPa. With increasing annealing temperature, the tensile strength also gradually increases, reaching R

_{m}= 396 MPa after annealing at 850 °C, but even this value is inferior to the tensile strength obtained in sinters not subjected to densification.

_{p02}= 308 MPa, was recorded in sinters annealed at 850 °C. Annealing at 650 °C raises the impact resistance to 25 J/cm

^{2}. This value is slightly higher than that for sinters without additional treatment, for which the reported value is 23.5 J/cm

^{2}, and does not change significantly despite the use of a higher heat treatment temperature.

^{2}for the preform before deformation to 2 J/cm

^{2}after deformation.

^{2}was obtained, i.e., higher than that in the sinters with the same final porosity but not subjected to densification. The results of the measurements of tensile strength and yield strength on samples after large deformation are very significant. These results are characterized by a very large scatter, which is the effect of the low cohesion of sinters due to upsetting. Annealing these sinters has no major effect on the average value of the strength properties, but visibly reduces the scatter in the results obtained, which may indicate a gradual recovery of cohesion impaired by deformation. Finally, after annealing the deformed material at 850 °C, tensile strength and yield strength values of R

_{m}= 370 MPa and R

_{p0.2}= 277 MPa, respectively, were obtained, and these values are lower than those in the sinters after single pressing and single sintering and after densification with a lower degree of deformation and annealing at the same temperature. The tensile curves plotted for these sinters (Fig. 5b) show that the elongation increases with increasing annealing temperature, and this result confirms the recovery of plastic properties.

### Microstructure

### Fractography

## Discussion of Results

## Conclusions

^{2}for the starting material to KC = 4.5 J/cm

^{2}for the material after deformation with a hardening parameter of ε = 0.24 and from KC = 11 J/cm

^{2}for the preform to KC = 2 J/cm

^{2}after deformation with a hardening parameter of ε = 0.77. Annealing the deformed sinter at a temperature below the sintering point improves cohesion, causing recrystallization of the matrix and beneficial changes in the porosity morphology. The nature and magnitude of structural changes occurring at a given temperature depend on the deformation rate and initial porosity.

^{2}, i.e., to a value higher than the value obtained in the preform and sinter with the same porosity made by the method of single pressing and sintering. The higher annealing temperature of these sinters improves cohesion at the cost of reduced matrix strength and grain growth with no improvement in the impact resistance.

^{2}, and it increases to 21.3 J/cm

^{2}at 750 °C and to 30.1 J/cm

^{2}at 850 °C. Finally, the impact resistance obtained in these sinters after annealing is higher than that after the application of low-value deformation, but annealing should be carried out at a higher temperature.