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Powder Metallurgy (P/M) offers a cost-effective and design-friendly option for the manufacture of stainless steel hot exhaust gas oxygen sensor (HEGO) bosses for automotive exhaust systems. The critical requirements for this application include high temperature mechanical strength, good corrosion resistance, weldability, machine-ability for thread tapping, and resistance to thread galling when exposed to temperatures as high as 1000°C (1832°F). Optimally processed P/M 400-series stainless steel oxygen sensor (HEGO) bosses meet these critical requirements, and are comparable to those made from wrought stainless steel, either cold headed or machined.

Progress in the understanding of corrosion and corrosion resistance properties of sintered stainless steels has lead to new applications that benefit from net shape processing and more efficient material utilization. Powder Metallurgy Stainless Steels includes extensive graphics, and tabular summaries provide a comprehensive reference to obtain good corrosion resistance properties of sintered stainless steels with careful processing, starting with powder selection, avoidance of contamination, efficient delubrication, and controlled sintering and cooling. Contents cover: Metallurgy and Alloy Compositions, Manufacture and Characteristics of Stainless Steel Powders, Compacting and Shaping, Sintering and Corrosion Resistance Alloying Elements, Optimal Sintering, and Surface Modification in PM Stainless Steels Mechanical Properties, Magnetic and Physical Properties, Corrosion Testing and Performance, Secondary Operations, and Applications.

Evaluation of commercial high temperature H2-sintered 410L ABS sensor rings, after use for over 10 years and/or over 100,000 miles in various automobiles and in various locations across the USA and Canada, showed only small amounts of rusting; less than which results in a 100-hour Salt Spray Test (ASTM B117); and no loss of their mechanical strength and ductility. Although sensor rings made of 434L and 434L-Modified (18 Cr, 2 Mo) show even lesser amounts of rusting and pitting, 410L sensor rings made by the above process are deemed adequate for this application. The 1000-hour Salt Spray Test is found to be much too severe for this application. A 100-hour test is considered to be more suitable in this regard.

Powder metal (P/M) 400 series stainless steel exhaust components (flanges and oxygen sensor bosses) are being increasingly used in US made automobiles. These components are shown to have superior leak-tightness and resistance to corrosion and oxidation, when compared to wrought stainless steel exhaust components. This paper compares the yield, tensile, impact, and fatigue strengths of two popular exhaust component materials, namely 409LE and 409LNi (a nickel-modified version of 409L), in the as-sintered condition. Data presented are based on samples sintered in 100% hydrogen using a commercial sintering practice, exhibiting sintered densities in the range of 7.2 to 7.3 g/cm3. Fatigue strengths were determined using the rotating beam test method, per MPIF Standard 56.

Ferritic stainless steel 409L and ferritic-martensitic stainless steel 409LNi, made via powder metallurgy (P/M) process, are being increasingly used for the manufacture of automotive exhaust flanges and oxygen sensor bosses. High temperature, hydrogen sintering is most commonly utilized for the manufacture of these components, with an aim to achieve a minimum sintered density of 7.25 g/cm3. Alloy 409L is a fully ferritic material, exhibiting a hardness in the range of 55 to 62 HRB. Alloy 409LNi, on the other hand, possesses a 50/50 ferritic-martensitic microstructure, and exhibits a hardness in the range of 85 to 92 HRB. Some amount of machining, such as thread cutting, drilling and turning, is often required in the processing of these components. This study evaluates the effects of adding a machinability enhancing agent, namely manganese sulfide, on the machinability, as well as on the corrosion resistance and mechanical properties of these alloys.

Requirements for automotive exhaust system components include room temperature and elevated temperature mechanical integrity, oxidation resistance, corrosion resistance, and weldability. Powder metallurgy offers sufficient flexibility with material and process selection to meet these challenges. This presentation will cover the effects of alloy composition, density, and process parameters on the performance of P/M S.S. components in O.E.M. specified thermal, mechanical, and corrosion tests.