Wyniki 1-2 spośród 2 dla zapytania: authorDesc:"Artur Bajwoluk"

Inicjacja i propagacja mikropęknięć w stabilnym staliwie austenitycznym DOI:10.15199/28.2015.6.10

  Initiation and propagation of mini-cracks in stable austenitic cast steel Carburizing furnaces tooling is made of stable austenitic cast steel. During the service life of the tooling damages occur, which disqualify it from further usage. This paper focuses on the issue of mini-cracks formation in the tooling, during the cooling process. The literature shows, that a great difference in thermal expansion between cast structural components has a pivotal meaning for a problem of the cracks creating. A model which is presented in the literature shows, that the presence of carbides near the alloy surface can cause strong tensile stresses, which may initiate cracks formation. However the model does not include an oxide layer which forms on the alloy surface. It also does not explain why the cracks arise in practice, only at grains boundaries and not in other places where carbides are located. In the article computational model for finite element analysis of grains boundary located near the alloy surface which is covered with oxide layer was presented. The proposed model was used for the numerical analysis of the influence of the carbide layer (the depth of the layer) and oxide layer geometry on the stress distribution in the surface layer, during the cooling process. The results show a clear relationship between the carbide layer and the stresses occurring at the surface. They also show that, despite the presence of the oxide layer, tensile stresses may occur at the alloy surface. The necessary calculations were made in the Midas NFX 2014 software. Key words: stable austenitic cast steel, stresses, finite elements method, cracking. Osprzęt pieców do nawęglania jest wykonywany ze stabilnego staliwa austenitycznego. W czasie jego użytkowania dochodzi do zniszczeń, które dyskwalifikują go z dalszej eksploatacji. Praca skupia się na zagadnieniu powstawania mikropęknięć w osprzęcie podczas jego chłodzenia. Literatura przedmiotu wskazuje, że kluczowe znac[...]

Thermal micro- and macrostresses in the surface layer of cast steel accessories for carburizing furnaces DOI:10.15199/28.2018.5.3

  1. INTRODUCTION High heat resistance and creep resistance make the austenitic Fe-Ni-Cr alloys widely used as a material for accessories of the furnaces for heat and thermochemical treatment. Due to the specific character of furnace operation, its tooling is exposed to the effect of numerous highly unfavourable factors, including high temperature and its rapid changes, strongly carburizing atmosphere as well as mechanical and thermal loads originating from the processed batch of items. The furnace operation in cycles puts castings under the conditions conducive to the process of thermal fatigue. The austenitic nickel-chromium alloys are characterized by relatively good strength properties at high temperature. However, as a result of interaction of the above mentioned factors, in components made from these alloys develop cracks and deformations eliminating them from further use [1÷7]. Both cracks and deformations are initiated by stresses arising during temperature changes, which effects accumulate in subsequent cycles of operation. It is generally believed that the main sources of stresses arising in furnace accessories are the difference in the thermal expansion of the structural constituents of cast steel from which these accessories are made and temperature gradient generated on the cross-section of the cooled or heated part. Analysis of microstresses caused by different thermal expansion of phases, precipitates and inclusions present in cast austenitic Fe-Ni-Cr steel was carried out, among others, in [8, 9], while macrostresses generated on the cross-section of cooled components (cast pallets on which the charge is placed during heat treatment) were examined in [10, 11]. In the above-mentioned articles, however, the described sources of stresses were considered as independent, individually operating factors responsible for the destruction of furnace accessories. The aim of this article is to discuss the results of analysis[...]

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