# Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

## 2. Material and Methods

#### 2.1. Material

#### 2.2. Microstructural Characterization

#### 2.3. Mechanical Tests

^{−1}, up to a maximum load of 30,000 mN. The 50 s total indentation time was divided into three segments, consisting of 20 s loading and unloading, and 10 s holding time. The tests were performed by creating two 10 × 10 grids of indents, 100 µm spaced, for a total of 200 indents for each sample. During the test, the nanoindenter continuously recorded the penetration depth and the load. More details on the nanoindentation test are discussed in [43,44] where it is explained how it is possible to calculate Indentation Hardness HI (GPa) plastic deformation work Wp (pJ) and total deformation work Wt (pJ). High Wt values mean high energy dissipated by the material during loading which is related to high toughness.

_{t}= 3 and K

_{t}= 3.1 on the basis of the findings of Noda [48] and Young [49], respectively. The fatigue strength was evaluated at 10

^{7}cycles, in accordance with the staircase method reported in the ISO 12107 [50] standard, using 15 specimens and a 50 MPa stress step.

## 3. Results

#### 3.1. Microstructure

_{6}C (white in Figure 4 and Figure 5) carbides [21,22,24,25]. The prior austenite grain size in the K890-I and K890-II is comparable and it ranges between (4 and 9 μm), regardless of the different austenitization temperature. Moreover no retained austenite (RA), segregations, inclusions, porosity, carbides larger than 2 μm or carbides clusters were detected by metallographic analyses (Figure 3 and Figure 4).

#### 3.2. Hardness and Tensile Properties

#### 3.3. Nanoindentation

#### 3.4. Fatigue Behaviour

_{t}(estimated according to Noda [48] and Young [49]), and the fatigue notched factor K

_{f}(ratio between S’e and S’ek [57]) are reported.

#### 3.5. Fracture Surface Analyses

^{6}) in high strength steels and, in particular, tool steels; (ii) develop in correspondence with carbides clusters, primary large carbides or non-metallic inclusions, due to both particle cracking and debonding; (iii) their typical features are the result of different crack growth mechanisms that take place during the early stages of crack propagation within the fish-eye.

## 4. Discussion

_{f}, for both the K890-I and K-890-II notched samples.

_{f}for metallic materials measures the reduction in the fatigue strength of a sample due to the presence of a notch, and is defined as the ratio of the fatigue strength of the unnotched specimen (S’e) to the fatigue strength of the notched specimen (S’ek) [57]:

_{f}coefficient is obtained experimentally and depends on steel chemical composition, heat treatment, type of notch and geometry of the specimen. Its value ranges between 1 (material notch- insensitive) and the theoretical stress concentration factor K

_{t}(material fully notch-sensitive).

_{t}is the ratio between the greatest elastically calculated stress in the region of the notch (or other stress concentrators) to the corresponding nominal stress. K

_{t}is a theoretical factor that depends only on the geometry of the notch and the type of stress (normal stress, bending or torsion) [57].

_{f}is less than K

_{t}because metals locally undergo stress values higher than their yield strength, and therefore a local plasticization of the material and a redistribution of the stresses occurs at the notch root. The fatigue limit of the notched components cannot therefore be evaluated solely by considering the maximum theoretical value of the tension σ

_{max}at the notch [60] but the stress gradient, the ductility and the toughness of the material also have to be considered because of their influence on both crack initiation, from defects or surface discontinuities, and their subsequent growth [68,69,70].

^{1/2}, in fact, can be approximated by a third-order polynomial fit of UTS.

_{f}coefficients (reported in Table 5) highlights that the increase of ductility and toughness induced by the K890-II compared to K890-I treatments leads to lower K

_{f}values. Furthermore, the comparison of the experimental K

_{f}coefficients with those calculated according to previous relationships, emphasises again the importance of considering not only UTS for new PM tool steels, as in the past, but also ductility and toughness, in order to correctly assess their fatigue behaviour in the presence of notches. It is worth noting that the Neuber relationship (4) cannot be applied to the K890 steel since for high UTS it yields K

_{f}> K

_{t}, while Peterson and Heywood relationships overestimate the fatigue notch sensitivity of the K890 by about 30%. As shown in Table 5, K

_{f}values for K890 can be, alternatively, more accurately predicted by the relationship (10) developed by Hu and Cao [70] for low notch sensitivity materials, which considers the deformation and strain hardening behaviour of the material rather than the UTS.

_{f}values comparable with steels for structural applications [68,70], it is possible to infer that it could provide a viable option for the production of high performance components, even those with complex geometries, such as camshafts or crankshafts.

## 5. Conclusions

- The K890-II treatment improved tensile strength without decreasing elongation to failure, probably thanks to both a finer and more homogeneous carbide distribution into the martensite matrix and a decrease in martensite carbon supersaturation.
- Nanoindentation tests confirmed the tensile test data. The K890-II increased Indentation Hardness and indentation work compared to K890-I, which resulted in a reduction in microcracking tendency.
- Compared to the K-890-I treatment, the simultaneous higher strength, ductility and toughness induced by the K890-II treatment led to an increased fatigue strength for both the unnotched and notched specimens. Fatigue strength of the K890-II samples was about 15% and 25% higher for unnotched and notched samples, respectively, in comparison with the K890-I treatment.
- Fracture surface analyses showed that in unnotched specimens the cracks mainly originate from internal inclusions, showing a classical fish-eye morphology, while in the notched ones the cracks mainly develop from the surface.
- This present work demonstrates that the fatigue notch factor K
_{f}of the K890 tool steel, after optimized heat treatment, is comparable with that of high strength steels. This makes K890 steel an highly attractive material for the production of high performance mechanical components, such as crankshafts.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**Shape and dimensions (mm) of the rotating bending fatigue test specimens: (

**a**) unnotched and (

**b**) notched (in this figure the commas has been used as decimal sign).

**Figure 3.**Optical micrographs of K890 Microclean steel after: (

**a**) conventional K890-I and (

**b**) K890-II heat treatment.

**Figure 4.**SEM-BSE representative micrographs of sample treated according to K890-I (

**a**) and K890-II (

**b**).

**Figure 5.**Representative SEM-BSE micrograph of the K890-II treated steel with MC carbides along grain boundaries and M6C carbides inside grains. The light grey carbides, M6C type, are Mo and W rich, while the dark grey carbides MC type are V rich.

**Figure 6.**Gaussian distribution of carbides size in K890-I and K890-II specimens. K890-II presents smaller carbides size (peak value) and a more homogeneous (amplitude) carbide distribution.

**Figure 7.**XRD patterns of samples subjected to K890-I and K890-II treatments. The amount of retained austenite is lower than the detection threshold of the equipment for both heat treatments; therefore, no peaks related to the γ phase can be observed.

**Figure 8.**Comparison of stress-elongation curves of the K890 steel subjected to K890-I and K890-II heat treatments.

**Figure 10.**Gaussian distribution of HI (

**a**) and Wt (

**b**). In both graphs, the peak corresponds to the mean value and the curve amplitude to the standard deviation.

**Figure 11.**Representative images of the surface finishing of the notches for the (

**a**) K890-I and (

**b**) K890-II fatigue samples.

**Figure 12.**Fatigue fracture surface of unnotched samples. (

**a**,

**b**) crack initiation sites in the surface zone due to the accumulation of dislocation in slip bands (MPa 1150, cycles 7.5·10

^{6}) and (

**c**–

**e**) presence of a flaw (MPa 1100, cycles 6.9·10

^{6}).

**Figure 13.**SEM-SE low magnification fatigue fracture surface of an unnotched sample (MPa 1100, cycles 5.4·10

^{6}) with the crack initiation site in the sub-surface zone (

**a**) and high magnification of the “fish-eye” with corresponding SEM-EDS analysis of non-metallic inclusion from which developed the “fish-eye” (

**b**).

**Figure 14.**SEM-SE micrographs of smooth propagation zone (

**a**) and rough final failure zone (

**b**) of an unnotched sample.

**Figure 15.**SEM-SE micrographs of multiple crack initiation sites in notched specimen. The outermost zone of the fracture surfaces of the specimen was partially damaged by the final stage of fracture.

**Figure 16.**SEM-SE low magnification fractography of a notched sample (

**a**) and high magnification of the crack nucleation site in correspondence of machining flaws of the notch root (

**b**,

**c**).

**Figure 17.**SEM-SE micrographs of propagation and failure zone of unnotched (

**a**,

**b**) and notched samples (

**c**,

**d**).

C | Mn | Si | Ni | Co | Mo | V | W | Fe |
---|---|---|---|---|---|---|---|---|

0.80 ± 0.02 | 0.35 ± 0.01 | 0.59 ± 0.01 | 4.14 ± 0.04 | 3.90 ± 0.02 | 2.99 ± 0.01 | 2.30 ± 0.03 | 2.43 ± 0.01 | Bal. |

**Table 2.**Hardness and tensile properties (mean values and standard deviations) of the K890 steel subjected to K890-I and K890-II heat treatments.

HV30 | YS (MPa) | UTS (MPa) | A% (%) | |
---|---|---|---|---|

K890-I | 741 ± 3 | 2153 ± 20 | 2513 ± 10 | 2.1 ± 0.1 |

K890-II | 817 ± 17 | 2381 ± 15 | 2848 ± 22 | 2.3 ± 0.1 |

**Table 3.**Indentation Hardness (HI), Total hardening Work (Wt) and plastic Work (Wp) obtained by nanoindentation tests (mean values and standard deviations).

HI (GPa) | Wt (pJ) | Wp (pJ) | |
---|---|---|---|

K890-I | 12.5 ± 1 | 21,400 ± 875 | 15,171 ± 135 |

K890-II | 14.4 ± 2 | 22,268 ± 220 | 16,635 ± 160 |

**Table 4.**Results of bending fatigue tests carried out on notched S’e

_{k}and unnotched specimens S’e; fatigue strength at 10

^{7}cycles for a failure probability of 50%. K

_{t}is the theoretical stress concentration factor; K

_{f}is the fatigue notch factor.

${\mathit{K}}_{\mathit{t}\mathit{e}\mathit{s}\mathit{t}\mathit{i}\mathit{m}\mathit{a}\mathit{t}\mathit{e}\mathit{d}}$ | $\mathit{S}{\u2019}_{\mathit{e}}\left(\mathbf{MPa}\right)$ | $\mathit{S}{\u2019}_{\mathit{e}\mathit{k}}\left(\mathbf{MPa}\right)$ | ${\mathit{K}}_{\mathit{f}}=\frac{\mathit{S}{\u2019}_{\mathit{e}}}{\mathit{S}{\u2019}_{\mathit{e}\mathit{k}}}$ | $\frac{{\mathit{S}}^{\prime}\mathit{e}}{\mathit{U}\mathit{T}\mathit{S}}$ | |
---|---|---|---|---|---|

K890-I | 3 | 1186 ± 29 | 525 ± 26 | 2.25 | 0.47 |

K890-II | 3 | 1314 ± 39 | 692 ± 31 | 1.9 | 0.46 |

Peterson (Empirical) | Heywood (Empirical) | Hu and Cao (Empirical for Low Notch Sensitivity Materials) | Experimental | |
---|---|---|---|---|

K890-I | K_{f}=2.8 | K_{f}=2.6 | K_{f}=2.0 | K_{f} = 2.2 |

K890-II | K_{f} = 1.9 |

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**MDPI and ACS Style**

Morri, A.; Ceschini, L.; Messieri, S.
Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy. *Metals* **2022**, *12*, 900.
https://doi.org/10.3390/met12060900

**AMA Style**

Morri A, Ceschini L, Messieri S.
Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy. *Metals*. 2022; 12(6):900.
https://doi.org/10.3390/met12060900

**Chicago/Turabian Style**

Morri, Alessandro, Lorella Ceschini, and Simone Messieri.
2022. "Effect of Different Heat Treatments on Tensile Properties and Unnotched and Notched Fatigue Strength of Cold Work Tool Steel Produced by Powder Metallurgy" *Metals* 12, no. 6: 900.
https://doi.org/10.3390/met12060900