Effect of carbides size and distribution on creep rate

One of the most influential microstructure constituents in creep resistant steels are carbide particles. Carbide particles act as obstacles to dislocations movement, therefore the creep rate strongly depends on their size and distribution. At elevated temperatures to which creep resistant steels are exposed, carbide morphology is altered by the coarsening mechanism, consequently deteriorating the creep resistance of these steels. The aim of this work is to study the role of size and distribution of carbide particles on creep rate. Different distributions and size of carbides were obtained by different heat treatment conditions. The effect of different carbide morphology on the creep resistance was evaluated by uniaxial constant load creep tests

Primjena novo razvijenih testova sa grijanjem i unutarnjim hlađenjem uzoraka od alatnog čelika za različite primjene

In this study two new tests were developed, i.e., with continuous internal water cooling as well as discontinuous internal water and air cooling. It was proved that the first type of testing is appropriate for simulating the time course of the temperature at a selected depth of a thermally loaded, hot-working die surface layer, i.e., the temperature field on the die surface layer. The second type of testing is appropriate for a study of the thermal fatigue resistance of a tool material.U ovom istraživanju razvijeni su dva nova testa, tj. test sa kontinuiranom unutarnjim vodenim hlađenjem, kao i test sa diskontinuiranom unutarnjim vodenim i zračnim hlađenjem. Dokazano je da je prvi tip ispitivanja primjeran za simulaciju vremenske raspodjele temperature na odabranoj dubini termalno opterećene površine alata za vruću preradu. Druga vrsta ispitivanja je prikladno za proučavanje otpora termalnog zamora alatnog materijala

Effect of segregations on mechanical properties and crack propagation in spring steel

Considerable efforts have been made over the last decades to improve performance of springsteels, which would increase the service time of springs and also allow vehicles weight reduction. There aredifferent possibilities of improving properties of spring steels, from modifying the chemical composition ofsteels to optimizing the deformation process and changing the heat treatment parameters. Another way ofimproving steel properties is through refining the microstructure and reducing amount of inclusions. Therefore,the focus of the current investigation was to determine the effect of more uniform and cleaner microstructureobtained through electro-slag remelting (ESR) of steel on the mechanical and dynamic properties of spring steel, with special focus on the resistance to fatigue crack propagation. Effect of the microstructure refinement was evaluated in terms of tensile strength, elongation, fracture and impact toughness, and fatigue resistance under bending and tensile loading. After the mechanical tests the fracture surfaces of samples were analyzed using scanning electron microscope (SEM) and the influence of microstructure properties on the crack propagation and crack propagation resistance was studied. Investigation was performed on hot rolled, soft annealed and vacuum heat treated 51CrV4 spring steel produced by conventional continuous casting and compared with steel additional refined through ESR. Results shows that elimination of segregations and microstructure refinement using additional ESR process gives some improvement in terms of better repeatability and reduced scattering, but on the other hand it has negative effect on crack propagation resistance and fatigue properties of the spring steel

Grain boundaries of M23C6 particles in high chromium creep resistant steels, stability and effect on creep rate

Specimens of high chromium creep resistant steel were tempered at 800 °C for different times and examined in SEM. After short tempering stringers of cementite particles are formed at ferrite grain boundaries. By longer tempering, the content of chromium and molybdenum increase up to Cr18Fe3Mo2C6 and the number of stringers decreases what gradually increase the creep rate

Effect of segregations on mechanical properties and crack propagation in spring steel

Considerable efforts have been made over the last decades to improve performance of spring steels, which would increase the service time of springs and also allow vehicles weight reduction. There are different possibilities of improving properties of spring steels, from modifying the chemical composition of steels to optimizing the deformation process and changing the heat treatment parameters. Another way of improving steel properties is through refining the microstructure and reducing amount of inclusions. Therefore, the focus of the current investigation was to determine the effect of more uniform and cleaner microstructure obtained through electro-slag remelting (ESR) of steel on the mechanical and dynamic properties of spring steel, with special focus on the resistance to fatigue crack propagation. Effect of the microstructure refinement was evaluated in terms of tensile strength, elongation, fracture and impact toughness, and fatigue resistance under bending and tensile loading. After the mechanical tests the fracture surfaces of samples were analyzed using scanning electron microscope (SEM) and the influence of microstructure properties on the crack propagation and crack propagation resistance was studied. Investigation was performed on hot rolled, soft annealed and vacuum heat treated 51CrV4 spring steel produced by conventional continuous casting and compared with steel additional refined through ESR. Results shows that elimination of segregations and microstructure refinement using additional ESR process gives some improvement in terms of better repeatability and reduced scattering, but on the other hand it has negative effect on crack propagation resistance and fatigue properties of the spring steel

Implementation of newly developed tests with heated and internally cooled tool steel samples for different applications

In this study two new tests were developed, i.e., with continuous internal water cooling as well as discontinuous internal water and air cooling. It was proved that the first type of testing is appropriate for simulating the time course of the temperature at a selected depth of a thermally loaded, hot-working die surface layer, i.e., the temperature field on the die surface layer. The second type of testing is appropriate for a study of the thermal fatigue resistance of a tool material

Grain boundaries of M₂₃C₆ particles in high chromium creep resistant steels, stability and effect on creep rate

Specimens of high chromium creep resistant steel were tempered at 800 °C for different times and examined in SEM. After short tempering stringers of cementite particles are formed at ferrite grain boundaries. By longer tempering, the content of chromium and molybdenum increase up to Cr₁₈Fe₃Mo₂C₆ and the number of stringers decreases what gradually increase the creep rate

Suppression of acoustic emission during superelastic tensile cycling of polycrystalline Ni50.4Ti49.6

We investigate acoustic emission (AE) that arises during the martensitic transition in a polycrystalline specimen of the prototypical superelastic/elastocaloric alloy Ni$_{50.4}$Ti$_{49.6}$ (at. %) driven using tensile strain. We use two independent AE sensors in order to locate AE events, and focus on contributions to the AE that arise away from the grips of the mechanical testing machine. Significant AE activity is present during the first mechanical loading primarily due to nucleation and growth of wide L\"uders-like bands during the forward martensitic transition (imaged using visible light and infrared (IR) radiation) that lead to persistent changes in intergranular interactions. AE activity is suppressed during the subsequent reverse martensitic transition on unloading, and in successive loading/unloading cycles, for which the L\"uders-like bands narrow and modify intergranular interactions to much less extent. After the first loading, we find that the AE activity associated with the martensitic transition is weak, and we suggest that this is because the elastic anisotropy and strain incompatibility in Ni-Ti are low. We also find that the AE activity becomes weaker on mechanically cycling due to increased retained martensite

Nanopatterning surfaces by grazing incidence swift heavy ion irradiation

International audienceNanopatterned surfaces play a key role for many applications exploiting unique features such as an enhanced surface area, long- and short-ranged morphology modulations or a spatial variation of electronic and chemical properties. Ion beam irradiation has been frequently used for nanostructuring bulk materials because it is efficient, fast, and cost-effective. In this paper we show that ion irradiation under extremely grazing incidence in conjunction with other scalable processing methods such as wet etching and thermal annealing, is a perfect tool for nanopatterning of dielectric surfaces. We demonstrate that by tuning ion energy and fluence, one can select different surface nanopattern morphologies like individual chains of nanohillocks, nanostripes, or nanoscaled ripples. Furthermore, chemical etching of the irradiated surface can be used to create a negative replica of the nanopattern as only the material making up the surface track is susceptible to the etching process and is thus removed. Also, a removal of the surface track can be achieved by thermal annealing in vacuum. All these presented strategies open up new ways for achieving control over nanoscale surface modifications using swift heavy ion beams