T-91 and P-91 are the oldest of a new class of creep-strength-enhanced ferritic steels (CSEF) approved for use in boilers and pressure vessels. These newer alloys develop high strength through heat treatment, a rapid cooling or quenching to form martensite, followed by a temper to improve ductility. As a result, these alloys offer a much higher allowable stress which means thinner sections provide adequate strength for high-temperature service. Most of the applications thus far have been a substitute for P-22/T-22. The primary advantages of T91 materials over conventional low-alloy steels are: higher allowable stresses for a given temperature, improved oxidation, corrosion, creep and fatigue resistance. T23 is also considered as a member of the family of CSEF steels. The alloying elements such as tungsten, vanadium, boron, titanium and niobium and heat treatment separate this alloy from the well defined T22 steel. Although, T23 is designated for tubing application, its piping counterpart P23 has a strong potential in header applications due to superior strength compared to P22 headers. Now that T-91 and P-91 have been in service for nearly 30 years, some shortcomings have become apparent. A perusal of the allowable stress values for T-91 shows a drop off in tensile strength above about 1150°F. Thus, start-up conditions where superheaters, and especially reheaters, may experience metal temperatures above 1200°F, lead to over-tempering and loss of creep strength. During welding, the temperature varies from above the melting point of the steel to room temperature. The heat-affected zone (HAZ) is defined as the zone next to the fusion line at the edge of the weld metal that has been heated high enough to form austenite, i.e., above the lower critical transformation temperature. On cooling, the austenite transforms to martensite. Next to this region of microstructural transformation, there is an area heated to just below the austenite formation temperature, but above the tempering temperature of the tube/pipe when manufactured. This region has been, in effect, over-tempered by the welding and subsequent post-weld heat treatment (PWHT). Over-tempering softens the tempered martensite with the associated loss of both tensile and creep strength. This region of low strength is subject to failure during service. Creep strength of T91 steel is obtained via a quenching process followed by controlled tempering treatment. Elements such as niobium and vanadium in the steel precipitate at defect sites as carbides; this is known as the ‘pinning effect’. Any subsequent welding/cold working requires a precise PWHT. Inappropriate and/or lack of PWHT can destroy the ‘pinning effect’ resulting in loss of creep strength and premature failures. Several case studies will be presented with the problems associated with T91/T23 materials. Case studies will be presented, with the results of optical microscopy, scanning electron microscopy, hardness measurements and energy dispersive spectroscopy analysis. One case study will discuss how the over-tempering caused a reduced creep strength, resulting in premature creep failure in a finishing superheater tube. A second case presents the carburization of a heat recovery steam generator (HRSG) superheater tube, resulting in reduced corrosion/oxidation resistance. A case study demonstrates how a short-term overheating excursion led to reheat cracking in T23 tubing. Another case will present creep degradation in T91 reheater steel tube due to high temperature exposures (over-tempering).
Value Stream Mapping (VSM) is the one of the techniques used in lean manufacturing; it is a method of visually mapping a product’s production path (including materials and information) from door-to-door. Value Stream Mapping can serve as a starting point to help managers, engineers, suppliers and customers recognize waste and identify its causes. Traditionally VSM tool is used in manufacturing environment. In this study it has been applied for the distribution system. This study is conducted on a distribution company delivering automotive parts to the customers in and around Las Vegas. The primary objective of this system engineering approach is to create the present and future state Value Stream Mapping for a small sampling of three different categories of parts based on average monthly usage from the time they arrive on the receiving dock to the time they are placed into the delivery vehicle. In this work the value added and non-value added activities in the distribution process are identified. The future state map has been suggested with a set of short-term and long-term recommendations.
Significant efforts are in progress to identify and characterize the tensile properties of structural materials for application in hydrogen generation using a thermochemical process known as the sulfur-iodine cycle. Austenitic Alloy-22 has been evaluated for its tensile properties at temperatures ranging from ambient to 1000°C. As expected, the tensile strength was gradually decreased with increasing temperature due to the ease of plastic deformation at these temperatures. However, the failure strain gradually decreased from room temperature to 600°C, possibly due to the occurrence of a phenomenon known as dynamic strain aging. Transmission electron microscopy revealed maximum dislocation density at 600°C. Fractographic evaluation of the tested specimens by scanning electron microscopy showed dimpled microstructures at the primary fracture surface indicating ductile failures. However, at higher temperatures intergranular brittle failures were observed.
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