A. Bautista

Oxidation Behavior at 900°C of Austenitic, Ferritic, and Duplex Stainless Steels Manufactured by Powder Metallurgy

This study examines the resistance to cyclic oxidation at 900°C of two commercial P/M stainless steels—one austenitic (AISI 316L), the other ferritic (AISI 434L)—and of three duplex stainless steels of different compositions. The test results show that the porosity of these materials leads to notable differences between their high-temperature oxidation behavior and that of conventional stainless steels of similar composition. In the case of P/M materials, the resistance to oxidation and the chemical composition of the oxides formed are strongly influenced not only by the degree of porosity of each material, but also by the concentration of Ni in the base metal. In the conditions adopted in the study, ferritic stainless steel was found to provide the best high-temperature oxidation resistance.

Study of the interfaces between austenite and ferrite grains in P/M duplex stainless steels

Abstract Duplex stainless steels produced by powder metallurgy (PM) can be obtained by two different methods. On one hand, a fully pre-alloyed powder with a duplex designed composition can be used as raw material; on the other hand, by mixing ferritic and austenitic powders into the desired proportion it is possible to achieve the required duplex microstructure. Through this second method, when austenite is

Oxidation resistance of sintered stainless steels: effect of yttria additions

Abstract The porosity of sintered stainless steels modifies their oxidation behavior, as compared to that of wrought stainless steels. This work studies the oxidation behavior of three sintered stainless steels: one ferritic (AISI 434L) and two austenitic (AISI 316L and 304L). 304L with yttria additions is also been studied to explore the possibility of reducing the oxidation rate of austenitic stainless steels by using this reactive element. The results demonstrate the influence of the formation of NiFe2O4 on the high-temperature behavior of sintered austenitic stainless steels and the effectiveness of yttria additions in increasing the oxidation resistance at 800 °C.

Overaging of sealed and unsealed aluminium oxide films

The implications of aging on the properties of unsealed and poorly and properly sealed anodized aluminium exposed to the atmosphere of the University Campus of Madrid for 0, 1, 2, 3, 17 and 20 years were investigated. Aging is shown to occur for many years and to lead to a sealing quality well above that required by industrial standards.

Sealing of anodic films obtained in oxalic acid baths

The response to sealing quality control tests of anodic films obtained in oxalic and sulphuric acid baths has been compared. Unsealed films obtained in oxalic acid remain virtually unaltered in humid atmospheres, contrary to unsealed films obtained in sulphuric acid which tend to autoseal. The former also appear to take a shorter time to reach the quality thresholds of all the usual control tests on traditional sealing in boiling deionized water. However, after sealing, both types of films age in a qualitatively identical way. Transmission electron microscopy revealed the presence of a complex structure of hexagonal cells, comprising three distinct zones in the films formed in oxalic acid, the pores in which were found to be about five times larger in diameter than those in the films obtained in sulphuric acid. This considerably facilitates microstructural examination and elucidation of the underlying sealing mechanism. Furthermore, the transformation of the film morphology under the electron beam is slower in films obtained in oxalic acid.

Influence of triethanolamine additions on the sealing mechanism of anodised aluminium

Triethanolamine (TEA) solutions have previously been demonstrated to favour sealing of anodic films on aluminium. This work compares the sealing process in TEA and in de-ionised water using electrochemical impedance spectroscopy (EIS), standard tests for the study of the sealing quality and weight change measurements. It is demonstrated that the catalytic effect of the TEA is due not only to its pH, but also to its specific chemical structure. The data obtained are used to propose a sealing mechanism in this medium.

Mechanical and wear behaviour of high-speed steels reinforced with TiCN particles

Metal matrix composites (MMCs), based on M3/2 high-speed steel (HSS) and reinforced with two different percentages of TiCN (2.5% and 5% by wt), were manufactured following a conventional powder metallurgy (P/M) route: mixing, compacting and sintering. The carbide and base material powders were dry mixed and uniaxially compacted at 700 MPa. After this, vacuum sintering was carried out at 1275 °C, determined as optimal sintering temperature in a previous sinterability study. Sintered materials were characterised by measuring hardness, transverse rupture strength (TRS) and wear behaviour. The study is completed with a microstructural analysis by scanning electron microscopy (SEM) along with energy dispersive X-ray analysis (EDXA).

The response of anodized materials sealed in acetate-containing baths to atmospheric exposure

Abstract Standard sealing quality control tests, hardness measurements and electrochemical impedance spectroscopy (EIS) were used to analyse the response to atmospheric exposure for 0, 1, 2 and 20 years of anodized materials sealed in boiling de-ionized water and baths containing sodium or ammonium acetate for a variable time. Contrary to the widespread belief, the results reveal that sealing increases the hardness of anodic layers. The effect is reached more rapidly in the presence of acetate in the sealing bath. Also, the EIS results reveal that anodic films continue to age for years or even decades and eventually attain a sealing quality well above that imposed by standards. Under the most unfavourable atmospheric conditions at the University Campus of Madrid, wear of anodic films was about 0.13 μm/year, so the durability of anodized materials of the typical thicknesses used in architectural applications should easily exceed one hundred years.

Changes in cold sealed aluminium oxide films during ageing

EIS in the 100 kHz–1 mHz frequency range was applied to the study of changes in cold sealed aluminium oxide films exposed to highly wet and extremely dry atmospheres. Information about these changes was obtained from evolution of the impedance diagrams and, in particular, from film resistance and capacitance values determined from them. Results show that sealing quality, assessed from EIS, increases over months and years as ageing proceeds in a natural atmosphere. The analysis was completed with the aid of XPS and EDX techniques and standard quality control tests. Measurements show that specimens aged very rapidly in wet atmospheres, so that they passed all sealing quality tests within 72h. The sealing quality improves with ageing even in highly dry atmospheres despite the fact that pores lose part of their initial filling water. XPS analysis revealed that fluorine and nickel concentrate in the outer layers of cold sealed anodic films; on the other hand, films obtained by the traditional HTS procedure exhibit a uniform composition throughout their thickness.

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

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. Introduction Wrought stainless steels are commonly used at high temperatures (up to about 900oC) due to their ability to form protective chromium-rich scales. However, powder metallurgy (PM) offers economical advantages for processing small components of complicated shapes. Hence, PM stainless steels have started to take the place of wrought stainless steels and aluminised steels for manufacturing parts of automotive exhaust pipes or turcompressors that are often exposed to high temperatures. The high-temperature behaviour of wrought stainless steels have been studied and there are many data about their oxidation resistance in the literature [1,2]. Usually, austenitic stainless steels are preferred to ferritic stainless steels for hightemperature applications because of their better high-temperature mechanical properties. Sometimes, Nb is added to ferritic stainless steels because it improves the mechanical properties at high temperature, especially the thermal fatigue resistance of the material [3-6]. On the other hand, scarce studies have been carried out about the hightemperature behaviour of PM stainless steels. The few published data [7-14] have demonstrated that the porosity and microstructural characteristics of stainless steels obtained via PM, dramatically determine their oxidation resistance and their degradation mechanism at high temperatures. Pores favour the oxidation reactions and their number and morphology determine the final weight gain and the composition of the scales formed after the oxidation tests [7-9] It has also been noticed that PM ferritic stainless steels perform better than austenitic stainless steels during high-temperature exposures: they tend to form more protective oxides [7,10,11], suffer lower weight gains [7,10,11] and are less prone to cracking due to thermal shock [9,12]. In a study considering the influence of different additions on the Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 1149-1156 doi:10.4028/www.scientific.net/MSF.461-464.1149