Ken Yamashita

Effects of PWHT Temperature on Mechanical Properties of High-Cr Ferritic Heat-Resistant Steel Weld Metals

Welded joints of ASME Gr. 91 steel are subject to post-weld heat treatment (PWHT) to obtain sufficient mechanical properties. However, when the PWHT temperature is higher than the lower critical transformation temperature (AC1) of both the base metal and the weld metal, the mechanical properties of the weld may be degraded. To avoid this, it is important to know the AC1 of the weld. The authors have investigated several methods of measuring the AC1 of Gr. 91 steel weld metals to select the most credible method and examined the effects of the PWHT temperature on the mechanical properties of the weld metal. In addition, the proper PWHT temperature was discussed in consideration of the measurements of AC1, test results of creep-rupture performance, and observation results of precipitates. In the case of a low Mn+Ni weld metal, the upper PWHT temperature limit should be lower than AC1. On the other hand, in the case of the high Mn+Ni weld metal, remarkable negative effects on the mechanical properties of the weld metal were not found, even though the PWHT temperature exceeds AC1 by about 30 °C.

Development of Pressure Vessels Made of 9Cr-1Mo-V Steels for High-Temperature Processes in Refining Industries

Due to the increasing demands for light oil in newly industrializing countries and depletion of conventional oil resources, upgrading of heavy oil and coal-to-liquid processes have been a focus in recent years. The efficiency of these processes depends on temperature and pressure conditions, where a higher temperature, around 500°C, is likely to be used. However, 2¼Cr-1Mo-V steels which have been widely used for heavy-wall pressure vessels for many years cannot be applied to a high temperature process around 500°C since the design temperature of this material is limited to 482°C by ASME Code Section VIII, Division 2 [1].On the other hand, 9Cr-1Mo-V steels (Grade 91), which has an excellent performance at high temperature in mechanical properties and hydrogen resistance, has been used for tubing and piping materials in power industries and it can be a candidate material for the high temperature processes. However it has not been used for pressure vessels in refining industries.In order to manufacture heavy-wall pressure vessels using 9Cr-1Mo-V steels, essential techniques including manufacture of large forged shell rings, thick wall welding and overlay welding have been developed.Copyright

Existence Form of Boron in Dissimilar Weld Metals of Low Alloy Steel with Boron Bearing High Chromium Steel during Post Weld Heat Treatment

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 during PWHT was observed. From these results, the existing form of boron in boron-bearing Cr–Mo steel weld metals is discussed in this study. Reheat cracking susceptibility clearly increased with boron addition, whereas it decreased with increasing chromium content. It was shown by thermo-dynamic calculation that possible existing forms of boron were of four types, namely BN, M2B, M23(C, B)6 and dissolved boron. The amount of M23C6-type carbides was increased with increasing chromium content. On the basis of these results, it was presumed that the amount of dissolved boron in boron-bearing low-chromium steel weld metals was larger than in those with high-Cr. It was suggested that the larger amount of dissolved boron enhanced the strength difference between the matrix and the grain boundary in low-chromium steels, hence it led to higher susceptibility to reheat cracking.

Existence Form of Boron in Dissimilar Weld Metals of Low Alloy Steel With Boron Bearing High Chromium Steel

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