Search results

The purpose of this paper is to present the results of a study on friction characteristics of plasma, salt‐bath and gas nitrided layers produced in AISI 304 type austenitic and AISI 420 type martensitic stainless steels.

Plasma nitriding processes were carried out with DC‐pulsed plasma in 80% N₂+20% H₂ atmosphere at 450°C and 520°C for 8 h at a pressure of 2 mbar. Salt‐bath nitriding was performed in a cyanide‐cyanate salt‐bath at 570°C for 1.5 h. Gas nitriding was also conducted in NH₃ and CO₂ atmosphere at 570°C for 13 h. Characterization of all nitrided samples has been carried out by means of microstructure, microhardness, surface roughness measurement and friction coefficient. The morphologies of the worn surfaces of the nitrided samples were also observed using a scanning electron microscope. Friction characteristics of the nitrided samples have been investigated using a ball‐on‐disc friction and wear tester with a WC‐Co ball as the counterface under dry sliding conditions.

The plasma nitrided and salt‐bath nitrided layers on the 420 steel surfaces were much thicker than on the 304 steel surfaces. However, there was no obvious and homogeneous nitrided layer on the gas nitrided samples' surface. The plasma and salt‐bath nitriding techniques significantly increased the surface hardness of the 304 and 420 samples. The highest surface hardness of the 304 nitrided samples was obtained by the plasma nitrided technique at 520°C. On the other hand, the highest surface hardness of the 420 nitrided layers was observed in the 450°C plasma nitrided layer. Experimental friction test results showed that the salt‐bath and 450°C plasma nitrided layers were more effective in reducing the friction coefficient of the 304 and 420 stainless steels, respectively.

The relatively poor hardness and hence wear resistance of austenitic and martensitic stainless steels needs to be improved. Friction characteristic is a key property of performance for various applications of austenitic and martensitic stainless steels. This work has reported a comparison of friction characteristics of austenitic 304 and martensitic 420 stainless steels, modified using plasma, salt‐bath and gas nitriding processes. The paper is of significances for improving friction characteristics, indirectly wear performances, of austenitic and martensitic stainless steels.

Friction welding is a solid state bonding process, where the joint between two metals has been established without melting the metal. The relative motion between the faying surfaces (surfaces to be joined) under the application of pressure promotes surface interaction, friction and heat generation which subsequently results in joint formation. Stainless steel is an iron based alloy, contains various combinations of other elements to give desired characteristics, and found a wider range of applications in the areas such as petro‐chemical, fertilizer, automotive, food processing, cryogenic, nuclear and beverage sectors. In order to exploit the complete advantages of stainless steels, suitable joining techniques are highly demanded. The Friction welding is an easily integrated welding method of stainless steel, which considered as non‐weldable through fusion welding. Grain coarsening, creep failure and failure at heat‐affected zone are the major limitations of fusion welding of similar stainless steels. Friction welding eliminates such pitfalls. In the present work an attempt is made to investigate experimentally, the mechanical and metallurgical properties of friction welded joints, namely, austenitic stainless steel (AISI 304) and ferritic stainless steel (AISI 430). Evaluation of the characteristics of welded similar stainless steel joints are carried out through tensile test, hardness measurement and metallurgical investigations.

To evaluate the corrosion resistance of four different stainless steels often employed in hot end exhaust components.

This paper evaluated the outcomes of the hot salt test and the cyclic oxidation test on four different stainless steels, used as hot end exhaust components. The specimens were analyzed by means of SEM for surface changes and the weight loss was considered.

The general corrosion rate and pitting resistance under all the test conditions for hot end exhaust components indicated that 434 was by far the most corrosion resistant alloy, followed by 1.4509 and 321, and lastly 304 was the least corrosion resistant. In general, the ferritic stainless steels, especially 434, outperformed the austenitic ones under all the test conditions.

The comparison of the corrosion resistance and rate, between the frequently used ferritic and austenitic stainless steels used in the exhaust system, gave a clear indication that the ferritic steels will provide prolonged service and this could be beneficial information to the manufacturers.

A new approach to summarise the materials' behaviour and their relative performance in the tests performed was developed. This proposed summary of a number of corrosion indicators could serve as a relative guide to alloy selection for use in hot end automotive exhaust systems for both manufacturers and users.

The construction of the early platforms in the North Sea often stretched materials technology to its limits and sometimes beyond them. There are many instances where major pumps or piping systems have had to be replaced either because the original materials were not sufficient for the duty or because the process fluids have become more corrosive during the life of the oilfield. The paper reviews the considerable work that has been carried out in recent years specially directed at developing stainless steels capable of withstanding a number of the harsh corrosion environments met on off‐shore platforms. The latest stainless steels are able to withstand all the standard seawater duties without suffering from localised pitting or crevice corrosion. Their resistance to hydrogen sulphide stress corrosion means they can cope with the most sour process fluids at present met in the North Sea.

The purpose of this paper is to investigate stress corrosion cracking (SCC) for 304, 316, and 321 stainless steels in petroleum‐processing environments.

Sensitized austenitic stainless steels were subjected to a microstructure investigation and electrochemical test. Stressed sensitized 304, 316, and 321 stainless steels were selected and subjected to various environments that included polythionic acid, sour solution, and chloride solution that were prepared in the laboratory to simulate service environments in the petroleum refinery.

Microstructure investigation reveals more severe SCC in polythionic acid than in the sour and chloride solutions. Type 321 SS gives better resistance to SCC than do 304 and 316 SS in the three solutions. It is concluded that acidity of solutions has a relatively minor influence in promoting cracking. However, polythionic acid is found to be the primary causative agent.

The results demonstrated that SCC is more severe in polythionic acid than in chloride and sour solutions.

Compares the corrosion behaviour of Ti6Al4V titanium alloy, a conventional duplex stainless steel (UNS 31803) and AISI 304 austenitic stainless steel in synthetic biofluids using electrochemical techniques and comments on the suitability of DSS for use in biomedical applications. Finds that the general corrosion resistance of duplex stainless steels is slightly inferior to that of austenitic stainless steel and titanium alloy; duplex stainless steel does not show any sign of pitting when exposed to synthetic biofluids and exhibits excellent resistance to localised corrosion on par with that of titanium alloy. Concludes that duplex stainless steels are one of the best alternates to titanium alloys.

The corrosion behaviour of a ferritic (alloy 1) and two austenitic stainless steel alloys (alloys 2 and 3) was studied in a molten Li₂CO₃‐Na₂CO₃‐K₂CO₃ ternary mixture in the presence of Na₂O₂ additions at temperatures of 475, 500, 525 and 550°C. The techniques of measurements were open circuit potential, galvanostatic anodic polarisation and cyclic voltametry. The addition of Na₂O₂ increased the concentration of oxide ions in the carbonate melt. There is a tendency for oxidation and passivation of the alloys to commence immediately on their immersion in the melt, and end at the passivity breakdown, where the decomposition of carbonate ions occurs with the formation of CO₂ and O₂ gases. The oxide scales of a ferritic alloy are less protective than those formed on the austenitic alloys. The oxide scales, in most cases, are multilayered, and the presence of Na₂O₂ in the carbonate melt gives rise to the formation of a more protective inner layer of oxide scales on the surface of the austenitic alloys.

Duplex stainless steels have become important competitors to austenitic stainless steels in many applications and a great deal of attention has focused on the welding aspects. The introduction of modern grades with improved properties and a competitive price level have increased their use in the offshore, petrochemical and shipbuilding industries, for example. In particular the newer grades, with their higher nitrogen content and improved weldability, have moved duplex stainless steels from a position as “interesting” materials to one of “useful in practice”. However, duplex stainless steels differ from austenitic grades in some respects, and know‐how combined with the use of appropriate welding procedures and consumables is therefore the key to successful welding.

IN ANY survey of stainless steel applications it is soon apparent that in the vast majority of cases it is the corrosion resistance of the material which is the deciding factor in its selection. Appearance, non‐toxicity, retention of strength at high and low temperatures, and ease of fabrication are all important in varying degrees, but the widespread applications of stainless steel— a material which celebrated its first half‐century only a few years ago—follow principally from its inherent ‘stainless’ properties which confer a remarkable degree of resistance to attack in a wide variety of environments.

The purpose of this paper is to investigate interface characterization of CO₂ laser welded AISI 304 austenitic stainless steel and AISI 1010 low carbon steel couple. Laser welding experiments were carried under argon and helium atmospheres at 2000, 2250 and 2500 W heat inputs and 200‐300 cm/min welding speeds.

The microstructures of the welded joints and the heat affected zones (HAZ) were examined by optical microscopy, SEM, EDS and X‐Ray analysis. The tensile strength of the welded joints was measured.

The result of this study indicated that the width of welding zone and HAZ became much thinner depending on the increased welding speed. On the other hand, this width widened depending on the increased heat input. Tensile strength values also confirmed this result. The best properties were observed at the specimens welded under helium atmosphere, at 2500 W heat input and at 200 cm/min welding speed.

There are many reports which deal with the shape and solidification structure of the fusion zone of laser beam welds in relation to different laser parameters. However, the effect of all influencing factors of laser welding has up to now not been extensively researched. Much work is required for understanding the combined effect of laser parameters on the shape and microstructure of the fusion zone. This paper, therefore, is concerned with laser power, welding speed, defocusing distance and type of shielding gas and their effects on the fusion zone shape and final solidification structure of some stainless steels.