A review of failure of sintered steels: fractography of static and dynamic crack nucleation, coalescence, growth and propagation

Abstract

The failure of sintered steels differs from the behaviour of wrought steels because of factors such as porosity, remnants of previous particle surfaces and generally more complex microstructures. All these factors influence initiation, growth and propagation of microcracks when the sintered microstructure is mechanically loaded. Fracture paths and fracture resistance are shown to be related to details of the microstructures comprising ferrite, austenite, bainite, martensite, pores and weak interfaces. All these have characteristic fracture resistance properties resulting in, frequently combinations of, dimple rupture, cleavage, intergranular and interparticle failure micromechanisms. Results are presented of systematic studies, enabling identification of relevant stresses, in static and dynamic three-point loading, as the cracking process progresses. In static loading, microcracking has been detected in some steels below the macroscopic yield stress and in the first 100 cycles in fatigue. Microcracks nucleate, grow and coalesce, in a step-wise manner, before achieving a catastrophic sizefor which conventional fracture mechanics holds. Thus, application of Paris-type analysis to Stage II fatigue is therefore inappropriate. The review focuses on failure micromechanisms and interpretation of fracture surface composition of sintered steels, particularly of those based on Distaloy AE and Astaloy CrL powders. The relationships between microstructure and mechanical properties are discussed.

Effect of the addition of niobium and aluminium on the microstructures and mechanical properties of micro-alloyed PM steels

“…As the strain increases, the microvoids grow, coalesce, which involves plastic deformation, and eventually form a continuous fracture surface. 57 However, Alloy 3 showed dimples and cleavage facets (Figure 8b) indicating that the fracture is of the mixed type. Large voids were also observed in the alloy.…”

Microdeformation Processes in Soft Magnetic Compounds

“…5. As underlined by Dudrova and Kabatova [15], well-developed voluminous necks are mostly represented by ductile dimple facets with localized plastic flow. Therefore, higher mechanical properties are corresponding to the microdeformation processes, because the value of plain porosity and elongation cannot reveal the possibility for understanding higher the value of both mechanical properties, TRS and Rm, 856 MPa and 428 MPa, respectively.…”

The Influence of Preparation Methods on Magnetic Properties of Fe/SiO_2 Soft Magnetic Composites

“…4. More details concerning the fracture surfaces were published in works [8][9][10]. Table I shows a comparison of specific electrical resistivity and coercivity of the prepared samples.…”