In this study, a boron-alloyed experimental steel has been rolled to form a sheet product, and an austenization treatment has been performed, followed by rapid quenching and then tempering. By austenization, a homogeneous austenite phase is obtained and a martensitic microstructure is formed after rapid quenching. This kind of microstructure exhibits high hardness, but low toughness. Thus, a tempering heat treatment is applied at a proper temperature (200 °C and 600 °C) and various times, in order to provide optimum toughness and to develop an ideal combination of strength and toughness.After this process, at first the phases formed during the equilibrium state have been studied for the alloy system using ThermoCalc data, and also their amounts have been evaluated. The microstructures acquired after the heat treatments have been examined by light microscopy. The effects of the formed microstructures on mechanical properties are comparatively given by their strength, toughness, and elongation values. Furthermore, the failure mechanism with fractografical examinations has been determined by using a Scanning Electron Microscope (SEM).

Fractographical examinations present a good relationship between frac ture surface topography and basic rupture mechanism. Fracture consists of crack formation (crack nucleation) and crack growth (crack propagation). Many parametres such as type of loading, sensitivity of materials to crack play an important role on failure. Fracture in engineering alloys can occur by a transgranular (through the grains) or an intergranular (along the grain boundaries) fracture path. Scanning Electron Microscopy (SEM) is used in fractographical examinations presented in this study, due to its high resolution. In general SEM examinations are an important part of failure analysis.In this study newly developed boron added armor steel has been rolled to form sheet product. Heat treatment series including austenitization, quenching and then tempering have been applied on boron added armor steel respectively. The effects of formed microstructures on mechanical properties have been studied extensively. Furthermore failure mechanism has been determined through fractografical examinations by using SEM . Failure analysis and type of fracture provide also a good approach to understand the ballistic behaviour of armor steel. When the material is exposed to any kind of impact loading as performed by kinetic penetrator etc., it is obvious that the characteri stic of failure will give information about its ballistic performance.

In the study for this contribution, good mechanical properties have been obtained from Mo (and Cu) alloyed PM-steels after suitable heat treatment. The influence of two to four weight percent nickel additions in these Mo (and Cu) alloyed PM-steels on the mechanical properties have been studied. The influence of such an alloying modification on the defect characteristics has also been investigated. The heat treatment of the steel material has been realized via a novel way, i.e. with the sinter-hardening method. The mechanical properties like tensile strength, matrix microhardness, and toughness have been measured. Transverse rupture tests have been implemented for toughness measurement while the transverse rupture strength data has been obtained by using the Weibull statistics. Microstructural studies have been performed with various microscopy techniques, where the general steel matrix was observed to exist of tempered martensite, bainite, residual austenite, and micropores. The amount of pores was measured with image analysis technique, while the amount of retained austenite was measured with the magnetic saturation method. The results show that the values of the transverse rupture test are determined primarily by pores/pore agglomerates that fit big size defect criteria. On the other hand, a positive effect of increasing Nickel content has been observed on the resistance against fracture.

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The performance of steels depends on the properties associated with their microstructures, that is type, volume fraction, form factor, size and the area distribution of the various phases involved. Because all the phases in steels are crystalline, steel microstructures are made up of various crystals, sometimes as many as three or four different types which are physically blended by solidification, solid-state phase changes, hot deformation, cold deformation and heat treatment. Each type of microstructure is developed to characteristic property ranges by specific processing routes that control the microstructural changes. Thus processing technologies are used to tailor the final microstructure.