Issues Related to Creep-Strength-Enhanced Ferritic (CSEF) Steels

2014 ◽  
Author(s):  
Rama S. Koripelli ◽  
David N. French
Keyword(s):  
Heat Treatment ◽  
High Temperature ◽  
Case Studies ◽  
Creep Strength ◽  
Pressure Vessels ◽  
Low Alloy Steels ◽  
Allowable Stress ◽  
Pinning Effect ◽  
Superheater Tube

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).

Effect of Pre-Strain on Mechanical Properties of Pressure Vessel Steel Grades: Assessment of Stress Relieving / Post Weld Heat Treatments Efficiency at Regenerating Properties

2014 ◽  
Author(s):  
Sylvain Pillot ◽  
Carole Baudin ◽  
Stéphanie Corre ◽  
Deborah Heritier ◽  
Cédric Chauvy ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
High Temperature ◽  
Pressure Vessels ◽  
Low Alloy Steels ◽  
Pressure Vessel Steel ◽  
Cold Forming ◽  
Stress Relieving ◽  
Steel Grades ◽  
Weld Heat

Ensuring mechanical properties of carbon and low alloy steels after deformation is of major concern since the building process of heavy (i.e. thick-walled) pressure vessels may be directly impacted. Indeed, thick plates encounter forming and welding operations that may modify as-delivered properties. From both technical and economical points of view, cold forming is usually preferred. This technique is nowadays widespread and new rolling equipments display sufficient power to handle plates up to at least 250mm thick. Current limitations are now mainly related to maximum admissible strain in materials and regulation rules resulting from construction codes. The ASME Boilers and Pressure Vessels Construction Code on the American side and the EN 13445 Unfired Pressure Vessels Construction Code on the European side, both allow the use of as-strained material up to maximum 5% plastic (i.e. permanent) strain without any subsequent heat treatment operation. Above 5% plastic deformation, on one hand the European code requires a full quality treatment (meaning high temperature austenitization treatment, then cooling in air (normalizing – N) or in accelerated conditions (quenching – Q or accelerated cooling – NAC), followed by a Tempering treatment T) and on the other hand the ASME code only requires Tempering that can even be carried out using the mandatory Post Weld Heat Treatment (PWHT) needed by welded zones. However, it is of high importance to note that thick vessels are always submitted to a final PWHT to insure sufficient toughness in welded zones. This final PWHT is performed whatever the deformation obtained during plate rolling. In practice, there are no thick vessels made out of plates in as-strained conditions. Avoiding a full quality treatment as demanded per EN 13445 rules is of major interest for fabricators as it allows to decrease the delivery time, the risk of appearance of problematic issues (uncontrolled deformations of the vessel during high temperature treatments…) and significantly reduces the overall fabrication costs. This paper focuses on the effect of strain on conventional mechanical properties for steel grades widely used for the fabrication of heavy pressure equipments (i.e. tensile properties, hardness, Charpy V toughness) for different strain levels. In particular, it points out and discusses PWHT effects on properties of various pre-strained materials, showing that there is no need for full quality heat treatment.

Effect of Carbide Content on Temper Embrittlement of 2.25Cr1Mo Steel

2016 ◽  
Author(s):  
Yian Wang ◽  
Guoshan Xie ◽  
Zheng Zhang ◽  
Xiaolong Qian ◽  
Yufeng Zhou ◽  
...  
Keyword(s):  
High Temperature ◽  
Pressure Vessel ◽  
High Stress ◽  
Temper Embrittlement ◽  
Damage Mechanism ◽  
Pressure Vessels ◽  
Nondestructive Method ◽  
Operating Conditions ◽  
Low Alloy Steels ◽  
Carbide Content

Temper embrittlement is a common damage mechanism of pressure vessels in the chemical and petrochemical industry serviced in high temperature, which results in the reduction of roughness due to metallurgical change in some low alloy steels. Pressure vessels that are temper embrittled may be susceptible to brittle fracture under certain operating conditions which cause high stress by thermal gradients, e.g., during start-up and shutdown. 2.25Cr1-Mo steel is widely used to make hydrogenation reactor due to its superior combination of high mechanical strength, good weldability, excellent high temperature hydrogen attack (HTHA) and oxidation-resistance. However, 2.25Cr-1Mo steel is particularly susceptible to temper embrittlement. In this paper, the effect of carbide on temper embrittlement of 2.25Cr-1Mo steel was investigated. Mechanical properties and the ductile-brittle transition temperature (DBTT) of 2.25Cr-1Mo steel were measured by tensile test and impact test. The tests were performed at two positions (base metal and weld metal) and three states (original, step cooling treated and in-service for a hundred thousand hours). The content and distribution of carbides were analyzed by scanning electron microscope (SEM). The content of Cr and Mo elements in carbide was measured by energy dispersive X-ray analysis (EDS). The results showed that the embrittlement could increase the strength and reduce the plasticity. Higher carbide contents appear to be responsible for the higher DBTT. The in-service 2.25Cr-1Mo steel showed the highest DBTT and carbide content, followed by step cooling treated 2.25Cr-1Mo steel, while the as-received 2.25Cr-1Mo steel has the minimum DBTT and carbide content. At the same time, the Cr and Mo contents in carbide increased with the increasing of DBTT. It is well known that the specimen analyzed by SEM is very small in size, sampling SEM specimen is convenient and nondestructive to pressure vessel. Therefore, the relationship between DBTT and the content of carbide offers a feasible nondestructive method for quantitative measuring the temper embrittlement of 2.25Cr-1Mo steel pressure vessel.

Study on Flaw Acceptance Standard of ASME Code Sec. XI Based on Failure Probability

2004 ◽  
Author(s):  
Katsuyuki Shibata ◽  
Kunio Onizawa ◽  
YinSheng Li ◽  
Yasuhiro Kanto ◽  
Shinobu Yoshimura
Keyword(s):  
Case Studies ◽  
Failure Probability ◽  
Pressure Vessels ◽  
Aspect Ratios ◽  
Pressurized Thermal Shock ◽  
Asme Code ◽  
Order Of Magnitude ◽  
The Difference ◽  
Reactor Pressure

Based on the failure probability, the flaw acceptance standard of ASME Code Sec. XI is examined with some concerns weather the failure probability is uniform for flaws with various aspect ratios and failure frequencies are small enough. In this paper, the results of preliminary case studies are described on the failure probability of reactor pressure vessels (RPVs) with a surface flaw specified in Sec. XI. PFM code PASCAL was used for case studies. A PTS (Pressurized Thermal Shock) transient prescribed by NRC/EPRI PTS Benchmark Study was used as an applied load. Analysis results showed that the conditional failure probability of a RPV with an initial flaw of acceptable depth depends on the aspect ratio. In the case flaw shapes are close to semi-circular, the failure probability are higher than that of the cases aspect ration are less than 0.6 by one order of magnitude due to the difference of fracture behavior at the surface point. A case study for determining the acceptable flaws based on failure probability was also carried out.

Failure Analysis of T91 Finishing Superheater Tube Bursting for Improper Initial Microstructure and their Life Evaluation

2012 ◽  
Vol 557-559 ◽  
pp. 187-190
Author(s):  
Jie Ouyang ◽  
Xue Xia Xu ◽  
Yan Ting Feng ◽  
Xiao Guang Niu ◽  
Guang Zong Yan ◽  
...  
Keyword(s):  
Heat Treatment ◽  
High Temperature ◽  
Failure Analysis ◽  
Standard Heat Treatment ◽  
Creep Properties ◽  
Initial Microstructure ◽  
Superheater Tube ◽  
Life Evaluation ◽  
Creep Rupture Test ◽  
Test Result

The failure analysis was performed for T91 finishing superheater bursting tube. It is concluded that improper initial microstructure resulted from non-standard heat treatment contributes to poor creep properties at high temperature and subsequent tube bursting. In addition, life evaluation was carried out on the basis of creep-rupture test result of experimental steel with similar improper microstructure. It is in accordance with the practical condition and provides significant guidance for safety and operation supervision.

Consideration on Tempering and PWHT Temperatures of C-Mn and Low Alloy Steels Used for the Fabrication of Pressure Vessels: Smart Tuning of Heat Treatment Parameters

2018 ◽  
Author(s):  
Sylvain Pillot ◽  
Mikihiro Sakata ◽  
Lionel Coudreuse ◽  
Valéry Ngomo
Keyword(s):  
Heat Treatment ◽  
Current Trend ◽  
Pressure Vessels ◽  
Low Alloy Steels ◽  
Tempering Temperature ◽  
Ni Alloys ◽  
Safety Margins ◽  
Tempering Treatment ◽  
Alloy Steels ◽  
Wall Components

For many years, process licensors and/or end-users have frequently specified that the tempering temperature of C-Mn alloys and low alloy steels (i.e. Cr-Mo, Mn-Mo-Ni alloys) should be greater than the post-weld heat treatments (PWHT). Most of the time, tempering temperature is then required as much as 30°C (54°F) above the PWHT temperature, making it very difficult for steelmakers to be able to supply compliant materials, especially for heavy wall components. Application of rules in the applicable codes often leads steelmakers to request for deviations in cases where they become not compatible with material capabilities. This report is intended to illustrate the combined effect of tempering and PWHT on materials properties and to provide recommendations on how to tune smart the tempering treatment with the aim of proposing the most efficient complete heat treatment sequence. Data provided within this paper for C-Mn steels and low alloy grades (Cr-Mo and Mn-Mo-Ni alloys) prove that tempering can be performed at temperatures below, at or above one of PWHT without any adverse effect. Data from actual mill production records show that stringent material specifications can be met by steelmakers when they are allowed to tune smartly the heat treatment parameters (tempering temperature) in accordance with applicable construction codes. The data also demonstrate that limiting the tempering temperature in the lower range of allowed temperatures may be beneficial to customers as it gives more safety margins for fabrication and maintenance (i.e. potential repairs/modifications) of pressure vessels. It permits either to consider more cycles for PWHT or to perform PWHT at higher temperatures or for longer durations, while on the opposite, current trend imposing high tempering temperatures limits flexibility.

High-Temperature Instability of A Cr2Nb-Nb(Cr) Microlaminate Composite

1996 ◽  
Vol 54 ◽  
pp. 374-375
Author(s):  
M. Larsen ◽  
R.G. Rowe ◽  
D.W. Skelly
Keyword(s):  
Heat Treatment ◽  
Solid Solution ◽  
High Temperature ◽  
Elevated Temperatures ◽  
Aircraft Engine ◽  
Lattice Parameter ◽  
Microstructural Stability ◽  
Elevated Temperature Strength ◽  
Cross Sectional ◽  
Bcc Structure

Microlaminate composites consisting of alternating layers of a high temperature intermetallic compound for elevated temperature strength and a ductile refractory metal for toughening may have uses in aircraft engine turbines. Microstructural stability at elevated temperatures is a crucial requirement for these composites. A microlaminate composite consisting of alternating layers of Cr2Nb and Nb(Cr) was produced by vapor phase deposition. The stability of the layers at elevated temperatures was investigated by cross-sectional TEM.The as-deposited composite consists of layers of a Nb(Cr) solid solution with a composition in atomic percent of 91% Nb and 9% Cr. It has a bcc structure with highly elongated grains. Alternating with this Nb(Cr) layer is the Cr2Nb layer. However, this layer has deposited as a fine grain Cr(Nb) solid solution with a metastable bcc structure and a lattice parameter about half way between that of pure Nb and pure Cr. The atomic composition of this layer is 60% Cr and 40% Nb. The interface between the layers in the as-deposited condition appears very flat (figure 1). After a two hour, 1200 °C heat treatment, the metastable Cr(Nb) layer transforms to the Cr2Nb phase with the C15 cubic structure. Grain coarsening occurs in the Nb(Cr) layer and the interface between the layers roughen. The roughening of the interface is a prelude to an instability of the interface at higher heat treatment temperatures with perturbations of the Cr2Nb grains penetrating into the Nb(Cr) layer.

Patientenbezogene Komplexität in der Pflege – Kollektive Case Studies im Akutspital

Pflege ◽  
2018 ◽  
Vol 31 (5) ◽  
pp. 237-244 ◽  
Author(s):  
Caroline Gurtner ◽  
Rebecca Spirig ◽  
Diana Staudacher ◽  
Evelyn Huber
Keyword(s):  
Case Studies ◽  
Study Designs

Zusammenfassung. Hintergrund: Die patientenbezogene Komplexität der Pflege ist durch die Merkmale „Instabilität“, „Unsicherheit“ und „Variabilität“ definiert. Aufgrund der reduzierten Aufenthaltsdauer und der steigenden Zahl chronisch und mehrfach erkrankter Personen erhöht sich die Komplexität der Pflege. Ziel: In dieser Studie untersuchten wir das Phänomen patientenbezogener Komplexität aus Sicht von Pflegefachpersonen und Pflegeexpertinnen im Akutspital. Methode: Im Rahmen eines kollektiven Case-Study-Designs schätzten Pflegefachpersonen und Pflegeexpertinnen die Komplexität von Pflegesituationen mit einem Fragebogen ein. Danach befragten wir sie in Einzelinterviews zu ihrer Einschätzung. Mittels Within-Case-Analyse verdichteten wir die Daten induktiv zu Fallgeschichten. In der Cross-Case-Analyse verglichen wir die Fallgeschichten hinsichtlich deduktiv abgeleiteter Merkmale. Ergebnisse: Die Ausprägung der Komplexität hing in den vier Cases im Wesentlichen davon ab, ob klinische Probleme kontrollierbar und prognostizierbar waren. Je nach individuellen Ressourcen der Patientinnen und Patienten stieg bzw. sank die Komplexität. Schlussfolgerungen: Komplexe Patientensituationen fordern von Pflegefachpersonen Fachwissen, Erfahrung, kommunikative Kompetenzen sowie die Fähigkeit zur Reflexion. Berufsanfänger und Berufsanfängerinnen werden zur Entwicklung dieser Fähigkeiten idealerweise durch erfahrene Berufskolleginnen oder -kollegen unterstützt und beraten.

SenCity – Evaluating Users’ Experiences of Intelligent Lighting for Well-Being in Smart Cities

2018 ◽  
pp. 60-67
Author(s):  
Henrika Pihlajaniemi ◽  
Anna Luusua ◽  
Eveliina Juntunen
Keyword(s):  
Case Studies ◽  
Smart City ◽  
Smart Cities ◽  
Well Being ◽  
Evaluation Methods ◽  
Data Analyses ◽  
Sense Of Security ◽  
System Data

This paper presents the evaluation of usersХ experiences in three intelligent lighting pilots in Finland. Two of the case studies are related to the use of intelligent lighting in different kinds of traffic areas, having emphasis on aspects of visibility, traffic and movement safety, and sense of security. The last case study presents a more complex view to the experience of intelligent lighting in smart city contexts. The evaluation methods, tailored to each pilot context, include questionnaires, an urban dashboard, in-situ interviews and observations, evaluation probes, and system data analyses. The applicability of the selected and tested methods is discussed reflecting the process and achieved results.

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