FRACTURE BEHAVIOUR OF P/M Cr-V LEDEBURITIC STEEL WITH DIFFERENT SURFACE ROUGHNESS

The samples made from the Vanadis 6 PM ledeburitic tool steel were surface machined to different quality and heat treated by standard regime of the processing. Three point bending tests were carried out on processed samples. It was found that the flexural strength decreased with decreasing surface quality. The lowering of flexural strength has been accompanied with the decrease of the plastic component of plastic straining preceeding to fracture initiation (work of fracture) of the material. It indicates that the surface roughness leads to the crack initiation before a larger plastic deformation of the material can be developed. Based on the results it can be suggested that to prevent the cracking of tools in the practice it is essential to make a surface machining (grinding, lapping, polishing) to as high quality as possible

Production of hydroxyapatite-titanium composite powders by high-energy ball milling suitable for spark plasma sintering processes

Hydroxyapatite-titanium (HA-Ti) composite powders were prepared by high-energy ball milling for subsequent production of samples by spark plasma sintering (SPS). Different milling times for the preparation of HA-Ti composite powders were used. The microstructural properties and phase composition of the composite powders were studied by scanning electron microscopy and X-ray analysis. The results showed no formations of new phases in the structure during high-energy ball milling and a gradual reduction of the particles size with increasing milling time. Sintered samples from HA-Ti composite powder were prepared. A cross section analyzed by SEM and phase composition obtained by X-ray diffraction was studied. The results showed changes of phase compositions in the structure.

Surface property control by electron beam deflection during hardening

The usage of the high-energy electron beam source enables repeated surface quenching of chosen locations of an engineering part surface. Different techniques of electron beam deflection allow creating hardened layers of different shapes, hardness levels and thicknesses. Experiments were carried out with 42CrMo4 (1.7225) steel. The deflection modes tested were one-point, 6-point, line, field, and meander. The influence of process speed and defocusing of the electron beam was also taken into account. The electron beam surface quenching resulted in a very fine martensitic microstructure with a hardness of over 700 HV0.5. The thickness of the hardened layers depends on the deflection mode and is affected directly (except field deflection) by process speed. The maximum hardened depth (NCHD) was 1.49 mm. Electron beam defocusing affects the width of the hardened track and can cause extension of the trace up to 40%. The hardness values continuously decrease from the surface to the material core

Fracture behaviour of weld joints made of pearlitic and bainitic steel

The paper is concerned with microstructure evaluations and the hardness and fracture behaviour of welded joints made from cast bainitic Lo8CrNiMo steel and pearlitic rail steel of the type UIC 900A. The materials mentioned are predetermined for frogs of switches. The study is based mainly on microstructural observations and hardness measurements of the base materials, weld, and heat affected zone (HAZ). Dynamic fracture toughness was evaluated based on data from pre-cracked Charpy type specimens. The pearlitic UIC 900A steel and its HAZ had the lowest dynamic fracture toughness values and therefore the highest risk of brittle fracture. At application temperature range, this steel is on the lower shelf of the ductile-to-brittle transition, and the tempering in the HAZ did not affect the toughness substantially. The cast bainitic steel in the weld joint is characterized by higher toughness values compared to the pearlitic one, and a further increase in toughness may be expected in the HAZ. The weld zone itself is characterized by high scatter of toughness data; nevertheless, all the values are above the scatter band characterizing the pearlitic steel

Fracture toughness transition of ferritic-pearlitic steel at static and dynamic loading evaluated by Master curve concept

The paper focuses on assessing the usability of fatigue pre-cracked Charpy type specimens when evaluating the resistance of steel with ferritic-pearlitic structure to the initiation of unstable fractures. The suitability of using the specimens is evaluated on the basis of comparing experimentally established values of fracture toughness on pre-cracked Charpy type specimens and the values of this characteristics determined using Compact Tension (CT) specimens. For the evaluation and comparison of the fracture toughness temperature dependences determined on individual specimen types the master curve concept quantifying fracture toughness transition was applied. In the case of the steel employed, very good agreement was found to exist between the characteristics determined on individual types of specimen. It was shown that fracture toughness determined on pre-cracked Charpy type specimens can be regarded as a representative measure of resistance of the material employed to the brittle fracture occurrence

Fracture toughness transition of ferritic-pearlitic steel at static and dynamic loading evaluated by Master curve concept

The paper focuses on assessing the usability of fatigue pre-cracked Charpy type specimens when evaluating the resistance of steel with ferritic-pearlitic structure to the initiation of unstable fractures. The suitability of using the specimens is evaluated on the basis of comparing experimentally established values of fracture toughness on pre-cracked Charpy type specimens and the values of this characteristics determined using Compact Tension (CT) specimens. For the evaluation and comparison of the fracture toughness temperature dependences determined on individual specimen types the master curve concept quantifying fracture toughness transition was applied. In the case of the steel employed, very good agreement was found to exist between the characteristics determined on individual types of specimen. It was shown that fracture toughness determined on pre-cracked Charpy type specimens can be regarded as a representative measure of resistance of the material employed to the brittle fracture occurrence

Use of Cold Gas Dynamic Spraying of bi-metallic powder mixtures as alternative to classic powder metallurgy route for producing intermetallic materials

The paper presents cold gas dynamic spraying (or Cold Spray) as a novel surface treatment technology capable not only of surface modifications but also being used as bulk creating technology. This is demonstrated on numerous samples where bi-metallic powder feedstock is deposited into bulk, self-standing pieces of material that does not need the support of substrate. Mixtures from the group of Fe, Al, Ti, Ni, Cu were used for the initial bi-metallic mixtures. The deposited samples were then subjected to annealing at temperatures ranging from 300 to 1100°C in protective atmosphere and resulting morphologies and microstructures were analysed. Generally materials with high proportion of intermetallic phase content were obtained. These are discussed as potential scaffolds for metal or polymer matrix composites or as hi temperature resistive supports for catalysts with filter functions.

Use of Cold Gas Dynamic Spraying of bi-metallic powder mixtures as alternative to classic powder metallurgy route for producing intermetallic materials

The paper presents cold gas dynamic spraying (or Cold Spray) as a novel surface treatment technology capable not only of surface modifications but also being used as bulk creating technology. This is demonstrated on numerous samples where bi-metallic powder feedstock is deposited into bulk, self-standing pieces of material that does not need the support of substrate. Mixtures from the group of Fe, Al, Ti, Ni, Cu were used for the initial bi-metallic mixtures. The deposited samples were then subjected to annealing at temperatures ranging from 300 to 1100°C in protective atmosphere and resulting morphologies and microstructures were analysed. Generally materials with high proportion of intermetallic phase content were obtained. These are discussed as potential scaffolds for metal or polymer matrix composites or as hi temperature resistive supports for catalysts with filter functions.

Crack bridging modelling in Bioglass® based scaffolds reinforced by polyvinyl alcohol / microfibrillated cellulose composite coating

This work deals with crack bridging modelling in Bioglass® based scaffolds due the presence of a special polymer coating. This includes a careful modelling of the scaffold which is based on x-ray computed micro-tomography (micro-CT) scans and identification of bridging mechanism with the aid of extensive fractographic observations of coated, broken struts. A replacement of the real structure of scaffold by a periodic model utilizing Kelvin cell whose size corresponds to the mean cell size of the real foam is discussed. The struts of the idealized foam are modelled using the beam elements. A detailed computational analysis of crack bridging due to coating film fibrils under plane strain conditions is presented and an improvement of fracture resistance of coated scaffolds is explained

Microstructure modification of CGDS and HVOF sprayed CoNiCrAlY bond coat remelted by electron beam

In the present work two techniques are combined to optimize bond coat properties before thermal barrier coating (TBC) application, the cold gas dynamic spraying (CGDS) and electron beam remelting (EB). Results of the work focused on comparison of high velocity oxygen fuel (HVOF) and CGDS CoNiCrAlY bond coats are firstly presented. Than the effect of the electron beam remelting of the CoNiCrAlY coating manufactured by HVOF and CGDS deposition techniques is deeply investigated. The CoNiCrAlY bond coat to Inconel substrate interface displayed locations with very poor bonding, in larger extent for the states prepared by HVOF comparing to CGDS. The bond coats prepared by both ways being EB remelted are typically removal of the defects on the substrate to bond coat interface. The microstructure of the bond coat after this treatment is formed by Inconel fine grain layer being followed by the surface layer consisting of elongated dendritic microstructure. An increased porosity has been observed in interdendritical space in larger extent for CGDS samples