In this study progress of the wear on the bearing surfaces of nitrided dies for aluminium (AA6063) hot extrusion is demonstrated and explained. Two groups of nitrided AISI H11 tool steel dies were ...analysed, with relatively short and long bearing surfaces. It was found that bearing surface length in relation to the microstructure of the nitrided exit edge should be considered especially in the case of relatively short bearing surface. Cracking and adhesive removal of die material were identified as the main wear mechanisms leading to the formation of craters, both deep and shallow furrows and removal of the exit edge of the bearing surface. On the exit edges with relatively short bearing surface, initiation of wear on the two characteristic spots (distances) from exit edge was observed while on the relatively long bearing surface, wear observed was mainly in the central area of the bearing surface. Formation of deep furrows originates from previously formed craters which then expand in the longitudinal direction. On the other hand formation of shallow furrows is based on the high-density of adhesively removed islets on the nitrided layer that leads to its removal in longitudinal direction as a consequence of its decreased shear strength. Oxygen and aluminium ingress was detected on the grain boundaries far beyond the crack tip, which accelerated grain boundary decohesion.
•Nitrided extrusion dies with relatively short and long bearing surfaces were analysed.•Relatively shorter bearing surface die exhibited two different pathways to failure.•One degradation pathway was found on die with longer bearing surface.•Formation of deep furrows originating from craters was observed.•High density of adhesively removed islets lowers shear strength of nitrided layer.
In the paper, five procedures for the assessment of the seismic performance of low-rise RC buildings at different levels of complexity are presented and discussed. They include simple procedures ...based on methods originally developed in Japan (levels 1 and 2), the N2 method with two variants of the mathematical model (levels 3 and 4), and non-linear dynamic analysis (level 5). The procedures have been applied to seismic assessments of three RC building structures. A small difference between the N2 and NDA results was observed, whereas the results of the procedures at the first two levels are much more conservative. On the other hand, the amount of input data and of computational work increases with the increasing complexity level. Research is still needed on the definitions of capacities, especially the shear capacity of the structural members, and on the capacity of the whole structure. The initial effective stiffness of the structure proved to be the most important quantity which determines the seismic demand.
A comprehensive, though relatively simple, non‐linear method for the seismic damage analysis of reinforced concrete buildings (the N2 method) has been elaborated. The basic features of the method ...are: the use of two separate mathematical models, application of the response spectrum approach and of the non‐linear static analysis, and the choice of a damage model which includes cumulative damage. The method yields results of reasonable accuracy provided that the structure oscillates predominantly in the first mode. Three variants of a seven‐storey building have been used as illustrative examples for the application of the method. Four different types of the analysis, with different degrees of sophistication, have been performed in order to estimate the influence of several assumptions and approximations used in the N2 method.
A newly designed “block-on-cylinder” wear resistance testing rig, which allows testing at higher contact pressures than conventional testing methods, was used to elucidate the effect of an iron ...nitride compound layer (white layer) on the wear resistance of nitrided dies used for hot extrusion of aluminium (Al). The tested dies (AISI H13) with various nitrided microstructures were provided by different manufacturers of equipment for plasma and gas nitriding. The wear surfaces were analysed by SEM, BEC, micrography and roughness measurements. It was found that the iron nitride compound layer is chemically more stable against hot Al in comparison to die material. Deterioration of the compound layer begins with cracking, and as a consequence, its spalling from the nitrided surface. A high thickness of the compound layer combined with a low nitriding depth leads to its earlier spalling and vice versa. Due to the increased roughness at the removal sites, accelerated chemical attack by hot Al takes place. Comparative and simultaneous testing of two nitrided samples was carried out.
A relatively simple nonlinear method for the seismic analysis of structures (the N2 method) is presented. It combines the pushover analysis of a multi-degree-of-freedom (MDOF) model with the response ...spectrum analysis of an equivalent single-degree-of-freedom (SDOF) system. The method is formulated in the acceleration-displacement format, which enables the visual interpretation of the procedure and of the relations between the basic quantities controlling the seismic response. Inelastic spectra, rather than elastic spectra with equivalent damping and period, are applied. This feature represents the major difference with respect to the capacity spectrum method. Moreover, demand quantities can be obtained without iteration. Generally, the results of the N2 method are reasonably accurate, provided that the structure oscillates predominantly in the first mode. Some additional limitations apply. In the paper, the method is described and discussed, and it basic derivations are given. The similarities and differences between the proposed method and the FEMA 273 and ATC 40 nonlinear static analysis procedures are discussed. Application of the method is illustrated by means of an example.
The hot deformation behaviour and microstructural evolution of AISI 904L super‐austenitic steel has been investigated by means of hot compression tests. The tests were carried out on a Gleeble 1500D ...thermo‐mechanical simulator in the temperature range from 850 °C to 1150 °C and at strain rates range from 0.001 s−1 to 5 s−1. The microstructure evolution was examined by means of light optical microscopy (LOM). The results show that after an initial deformation hardening, softening mechanisms occur. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic‐sine equation with the activation energy for deformation of 463 kJ/mol. The steady state was achieved at maximum strain of 0.9 only at the lower strain rates (under 1 s−1) and the higher temperatures (above 1100 °C). Microstructural analyses showed a gradual increase in the dynamically recrystallized area with an increase of the temperature and a decrease of the strain rate. The grain size did change, as expected, correlating to the deformation conditions.
Presented study shows that considerable reserves still exist for increasing the service times (lifetimes) of CrN – coated dies for Al hot extrusion. The main reasons for the decreased service times ...are revealed and explained regarding the selected CrN - coated die for hot extrusion, i.e. why the service time of the coated-die is not in accordance with the wear resistance of the CrN - coating. The shaping of the bearing surface and presence of the scratches, size and amount of nonmetalic inclusions in the die steel, nodular defects in the CrN - coating, as well as thicknesses uniformity of CrN - coatings along the bearing surface, are relevant influential parameters.
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.
Laboratory investigation of hot workability of 1.3302 high speed steel was carried out and its improving was achieved. Hot compression tests for the determination of optimal soaking temperature as ...well as for the study of hot workability in temperature range 1150-850 degree C, strain rates range 0,001-6 s(-1) and applied strain up to 0,9 were performed. Microstructure of deformed specimen was analyzed. Apparent activation energies for hot working for upper and for lower temperature range were calculated. Onsets of dynamical recrystallization for applied deformation conditions on the basis of calculated strain hardening ratewere determined. Extending of hot working temperature range at its lower limit, i.e. down to 850 degree C, by applying optimal soaking temperature was obtained.
