Abstract
Decreasing the weight of heavy-duty vehicles is an ongoing concern. However, the need to deal with high temperatures in components such as manifolds imposes, by itself, some restrictions regarding material selection, being further limited when other required properties (e.g., functional, manufacturing or cost requirements) are taken into account. Cast austenitic stainless steels may represent a good choice in this context but the existence of concentrated chromium carbides can generate undesirable results. A good combination of heat treatments can be applied to cast heat-resistant austenitic stainless steels, in an effort to achieve the dispersion of fine carbides, consequently improving their microstructure, mechanical properties and creep resistance. In this work, an austenitic stainless steel usually used in high temperature applications was characterized and subjected to solution annealing and aging heat treatments. The material analyzed was the austenitic cast stainless steel HK30 and the goals of the work were to evaluate the effects of solution annealing heat treatments on the dissolution of grain boundary chromium carbides and the effects of aging treatments on creep resistance. The results show that the elimination of grain boundary chromium carbides is possible by applying a solution annealing heat treatment. Additionally, the precipitation of fine dispersed carbides is obtained after the aging treatment with an increase of hardness and, consequently, an expected improvement of creep resistance. Thus, the novelty presented by this work consists of selecting the best heat treatment combination in order to promote dispersion of carbides, thus avoiding further crack nucleation phenomena when parts are cyclically subjected to load and unload; this work also found the most adequate mechanical properties and achieved corrosion resistance regarding the application in heavy-duty vehicle components subjected to mechanical and thermal fatigue. By discovering methods of improving the properties of cast materials, large savings can be made both in terms of production costs as well as in the overall weight of the components.
Highlights
The increasing demand in the automotive industry to reduce CO2 emissions is the driving force behind improving fuel economy and enhancing combustion efficiency in the heavy-duty vehicle industry [1]
The heat-resistant cast stainless steel HK30 (ASTM A351) is a 25Cr/20Ni alloy commonly found in exhaust manifolds of high power gasoline engines, where the gas temperatures may reach 1000 ◦ C [7,8,9]
The purpose of this work is to study the possibility of eliminating the grain boundary carbides that may appear during material processing, by applying solution annealing heat treatments and the effects of aging treatments on the microstructure and hardness of the heat-resistant cast steel HK30
Summary
The increasing demand in the automotive industry to reduce CO2 emissions is the driving force behind improving fuel economy and enhancing combustion efficiency in the heavy-duty vehicle industry [1]. This will result in a change in composition of the emitted exhaust gases and an even higher exhaust temperature, bringing higher demands for both corrosion resistance and heat-resistance materials used for exhaust components. Austenitic cast stainless steels are usually found in the most demanding conditions where ferritic alloys show limitations [2,3,4,5,6] Besides chromium, they present high amounts of nickel in their composition to stabilize the austenite phase [7,8,9]. The heat-resistant cast stainless steel HK30 (ASTM A351) is a 25Cr/20Ni alloy commonly found in exhaust manifolds of high power gasoline engines, where the gas temperatures may reach 1000 ◦ C [7,8,9]
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