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The nanoscale structure of ferrite in a 22% Cr duplex stainless steel weld metal (fabricated with grade 2209 filler wire) aged at 400 °C was characterized via scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy through the Super-XTM detector; the results were compared with those of a corresponding grade base metal (2205). The weld metal-ferrite exhibited a more pronounced spinodal decomposition than the base metal-ferrite because of its higher Ni content. Moreover, the presence of Ni-rich solute clusters with a body-centered-cubic structure was notably evident in the weld metal-ferrite. The investigation of the precipitation hardening mechanism using the Ardell model confirmed that, besides the extended spinodal decomposition, Ni-rich solute clusters significantly contributed to the improved age hardening performance of the weld metal-ferrite in the 22% Cr duplex stainless steel.

ASME SA-533 Class 2 steel provides 30% higher tensile strength and 90% higher yield strength compared to ASME SA-516 Grade 70 steel, which is commonly used for onshore pressure vessels. Owing to the higher allowable stress, the use of the SA-533 Class 2 steel leads to significant reduction of the vessel weight by permitting thinner vessel walls, which in turn, leads to lighter supporting steel structures. This is quite beneficial for offshore applications, since the space and weight capacity are limited on an offshore platform or the structure modules on a ship. Further, when vessels are intended for use in gas processing plants, depending on the process design, not only higher tensile strength but also greater impact toughness at low temperatures [e.g., minus 29°C (minus 20°F) or lower] are required. This consequently imposes special considerations on the vessel fabrication. Especially, careful attention is needed in establishing welding and heat treatment procedures to achieve both properties in the weldment. This paper presents the use of ASME SA-533 Type C Class 2 steel for a pressure vessel intended for use in the gas inlet facility of a floating LNG plant.

The present study has demonstrated the differences in the 475 °C embrittlement behavior of the base and weld metals of a 22 mass% Cr-duplex stainless steel. A grade 2205 solution-annealed plate was used as the base metal. The corresponding Ni-rich weld metal, which comprised multiple layers, was fabricated using a grade 2209 filler wire. The base and weld metals were subjected to isothermal aging for various durations (up to 1000 h) at 400 °C. Microstructural characterization, Charpy impact toughness testing, and Vickers microhardness measurements were conducted before and after aging. Furthermore, the microstructure of the ferrite phase, after aging, was evaluated using transmission electron microscopy (TEM). There was a substantial decrease in the impact toughness in both the base and weld metals with increasing aging time, although the decrease in the impact toughness was more pronounced for the weld metal. The hardening of ferrite was more prominent in each layer of the weld metal than in the base metal. The TEM observations indicated that the ferrite phase in both the base and weld metals underwent spinodal decomposition, which was accelerated in the weld metal. Regarding the major alloying elements (i.e., Cr, Mo and Ni) contained in the ferrite phase, the content of each, Cr and Mo, was lower in each layer of the weld metal than in the base meal, while the Ni content was, conversely, higher in each layer of the weld metal than in the base metal. The higher Ni content promoted the decomposition of ferrite in each layer of the weld metal during aging. This resulted in an acceleration of the 475 °C embrittlement in the multilayered weld metal of the 22 mass% Cr duplex stainless steel compared to the base metal.

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.

Duplex and super duplex stainless steels are susceptible to thermal aging embrittlement, referred to as 475°C (885°F) embrittlement. The object of this study is to understand the difference in susceptibility to 475°C (885°F) embrittlement of the base metal and the weld metal of these steels. Isothermal aging heat treatment at 300–450 °C (570–840°F) up to 1,000 hours was performed on 22% Cr duplex stainless steel: UNS S32205 and 25% Cr super duplex stainless steel: UNS S32750 and S32760 and these weld metals made using their matching SMAW electrodes or GTAW rods. After heat treatment, the embrittlement behavior was evaluated by Charpy impact test and Vickers hardness test. The results revealed the time-temperature embrittlement curves of the weld metals were displaced to a significantly shorter period of time and extended to lower temperatures compared to those of the corresponding base metals. More importantly, these results suggested that the maximum design temperature limit on these steels currently specified in the ASME Pressure Piping Codes and Boiler and Pressure Vessel Code is not always sufficient to avoid the risk of 475°C (885°F) embrittlement in their welded components.

21/4Cr-1Mo-V steel, such as the ASTM A542/A542M type D, which has remarkable anti-hydrogen embrittlement and creep rupture properties, is widely used in main components such as pressure vessels in oil refinery plants. Submerged arc welding (SAW), shielded metal arc welding (SMAW), and gas-shielded tungsten arc welding (GTAW) consumables, equivalent to base metal, are selected for the welding of this steel. As referred to in American Society of Mechanical Engineers (ASME) Section VIII, various properties, such as tensile strength and impact toughness in addition to creep rupture, are required in 21/4Cr-1Mo-V steel weld metal. Especially in creep rupture properties, the lower limit of the design temperature, which is required in main components (mentioned above), was lowered from 470 to 440 °C, based on the revision of the ASME code in 2009. Additionally, temper embrittlement behavior, occurring under high temperature over a long period of time, should be considered as well when dealing with this deposited metal. In this study, we have discussed the validity of precipitates in order to develop the creep rupture and temper embrittlement properties of 21/4Cr-1Mo-V steel weld metal. As a result, it was found that MX in crystal grains improves creep rupture lifetime and that, in the prior γ grain boundaries, it inhibits embrittlement caused by the segration of impurities.

Hydroprocessing reactors are made of heavy wall Cr-Mo steels with stainless steel cladding and installed in the most of oil refineries in the world during past half century. After long time operation, they found several material degradation and/or damages due to high pressure and high temperature hydrogen services. This paper presents typical remediation method of flaws in stainless steel cladding of reactors including flaw sizing, FFS assessment and repair procedures.

Recently, higher steam inlet temperature and pressure has been required for fossil power plants in order to increase the power generation efficiency. Therefore, boron-bearing high-chromium (high-Cr) steels with higher long-term creep strength are applied to structure materials in these plants. Dissimilar (metal) weld joints between a boron-bearing high-Cr steel and a low-alloy steel are applied for various parts of boilers and other equipment in the plants. Caution has to be paid to maintain the quality of these weld joints because boron is diffused to the low-alloy steel weld deposits by dilution from the boron-bearing high-Cr base metal. However, the existing form of boron and the effect of boron on reheat cracking susceptibility during post-weld heat treatment (PWHT) have not been surveyed in the previous literature. Therefore, using Cr–Mo low-alloy steels with varied contents of B and Cr, reheat cracking susceptibility of the weld metal was evaluated, and the precipitation behaviour of the carbides du...

Oxidation and creep behaviors of a Ni-Mo-based alloy (Hastelloy-X) with a diffusion barrier coating consisting of duplex, inner Re-based alloy, and outer β-NiAl layers were investigated at 1243 K in air with an external tensile stress of 22.5 MPa. For comparison the alloys, as-received, heat treated, and with the Ni-aluminide coating, were oxidized under creep deformation. Creep rapture time for the diffusion barrier-coated alloy was longer than those for the bare alloy and with the β-NiAl-coated alloy. After creep deformation to a strain of 3.5% for 190 h, it was found that the Re-based alloy layer has few cracks and flaws and the β-NiAl layer has the similar structure and composition before and after the creep test. The external scale mainly consisted of θ-Al 2 O 3 at the early stage of creep, and with further oxidation the external scale became a duplex layer, inner, equi-axed α-Al 2 O 3 , and outer, plate-like θ-Al 2 O 3 , which exfoliated significantly. It was concluded that the Re-based alloy layer acts effectively as a barrier against inward Al diffusion and outward diffusion of alloy elements under creep deformation.

A diffusion-barrier-coating system with a duplex layer structure comprised of an inner Re-base alloy layer and an outer β-NiAl layer was formed on the Ni–Mo alloy, Hastelloy-X. Alloy specimens with and without the coating were oxidized at 970 °C in air for up to 200 h with an imposed tensile stress of 22.5 MPa. The oxidation behavior under the stress-free condition was also investigated for comparison purposes. Strain rates of the specimens with a diffusion-barrier-coating system decreased rapidly for about 5 h, followed by a slow creep-deformation with a strain of 3.5% and strain rates of (0.7–0.2) × 10−7/s for 200 h. There was little change in both the coating structure and the composition (at%) of the inner Re-base alloy layer. Considering the creep behavior of the uncoated alloy, as well as the fact that there were few cracks and flaws in the Re-base alloy layer, it was concluded that this inner layer was subject to creep-deformation along with the alloy substrate. The external scale on the coated alloy consisted mainly of θ-Al2O3 at the early stage of the oxidation/deformation, and with further oxidation the surface scale formed a duplex layer structure consisting of outer plate-like θ-Al2O3 and inner equi-axed Al2O3. There was exfoliation of the outer θ-Al2O3 scale during the creep deformation. After the 200 h oxidation the outer β-NiAl contained (40–50)% Al, while the alloy substrate near the inner layer had less than 1 at% Al. It was found that the Re-base alloy layer acted an effective barrier against inward Al diffusion and outward diffusion of alloying elements.

Recently, boron bearing high chromium steel is applied to structural material of a fossil power generation plant. Dissimilar weld joints between this steel and low alloy steel are applied for various parts such as boiler or other equipments in this plant. In this weld joint, boron added to low alloy steel weld metals by the dilution from boron bearing base metal. However, the existence form of boron and the effect of boron on reheat cracking susceptibility during post weld heat treatment (PWHT) have not been surveyed in previous literature. So, reheat cracking test, precipitated carbide observation of boron bearing Cr-Mo low alloy steel weld metal was carried out in this study. From these test results, the reheat cracking susceptibility clearly increased by boron addition. Moreover, it decreased according to increasing chromium content. It was estimated that the structural form of boron was recognized in four kinds, such as BN, M2B, M23(C,B)6 and dissolved boron by thermo-dynamic calculation. The amount of M23C6 type carbide was increased with increasing chromium content. On the basis of these results, it was presumed that large amounts of dissolved boron existed in boron bearing low chromium steel weld metal compared with high chromium one during PWHT.Copyright © 2013 by ASME