High Temperature Performance of Ferritic Stainless Steels Manufactured by Powder-Metallurgy

Abstract

Two different ferritic stainless steels (AISI 409Nb and 434L) with different Cr level were manufactured through powder metallurgical techniques. These sintered steels are typically used for exhaust pipes and turbochargers. Both materials were uniaxially compacted and then vacuum sintered at 1225 and 1275 ºC respectively. Microstructural features and physical properties (density, porosity) of as sintered materials were examined. Oxidation performance was evaluated through isothermal experiments in air and thermogravimetry techniques. Global weight gains are very similar for both steels in isothermal tests carried out at 700, 800 and 900 ºC for 240 h. However, oxidation at 1000 ºC is different for both steels. X-ray diffraction and scanning electron microscopy allow to explain the differences of kinetics in base of composition of scale layers. Finally, thermogravimetry gives information regarding the kinetics of oxidation processes.

Oxidation of Micro-Sized Aluminium Particles: Hollow Alumina Spheres

“…Those are examples where exposed area changes during oxidative attack. This has been also demonstrated for powder metallurgy stainless exposed at high-temperature exposure [17][18][19]. Initially, oxidation affects the whole surface of the porous material.…”

Mechanical and oxidation properties of high density sintered duplex stainless steels obtained from mix of water and gas atomised powders

“…It is true that the bibliography published up to now about high temperature oxidation resistance of PM stainless steels is based on gravimetric results. 3,15,22,24,[32][33][34] However, the study of PM materials with very different porosity levels, as those considered in this study, stresses that the mass gains are very dependent on the extent of the oxidation that occurs in the inner pores and can not be related to the amount of oxide formed on the surface. The decrease in the mass gain caused by the addition of small quantities of GA powder to the 434L is very remarkable after exposures to 800 and 900uC (Fig.…”

“…Powder metallurgy (PM) stainless steels are used for pieces that require good aqueous corrosion resistance or oxidation resistance at high temperatures. Numerous works have been focused on the study of the mechanical properties and corrosion behaviour of austenitic [1][2][3][4][5][6][7][8][9] and ferritic [1][2][3][10][11][12][13][14][15] PM stainless steels. The well known superior mechanical and corrosion behaviour of wrought duplex stainless steels (as compared with ferritic and austenitic stainless steels) [16][17][18] make PM duplex stainless steels a promising group of materials today.…”

Oxidation Behavior of Highly Porous Metallic Components

“…The decrease on the oxidation rate is not due to the formation of increasing amounts of more-protective oxides ( Table 5). The logarithmic kinetics have proven to be adequate for simulation of oxidation kinetics in PM ferritic stainless steels as of air exposure temperatures of 800 or 900°C, depending on its content in Cr [25]. In non-porous materials with thick scales, these logarithmic kinetics have been related to the presence of cavities or precipitates that hinder diffusion, in such a way that only part of the oxide is available for diffusion.…”

“…Also, formation of oxides during exposure to high temperature tends to block the small pores and reduce the irregularities of the large pores, to the point that it could significantly reduce the real area that is exposed to the attack throughout the test [24]. Logarithmic or asymptotic laws are often found when porous metals are exposed to temperatures that caused formation of amounts of oxides enough to block in short time a meaningful part of their porosity [25]. That is the case of Ni20Cr, where meaningful amount of very poorly protective oxides are formed on (Table 5) and on the surface of the inner pores (Fig.…”