Explosive welding is a kind of solid state welding method, and a strong metallurgical bonding can be obtained with a wide range of metal combinations. Depending on metal combinations, an intermediate layer (IML) is formed at part of the joint interface by solidification of the local melting zone. When the excessive intermediate layer is formed along the joint interface, the bonding strength decreases drastically. Therefore, it is desirable that the intermediate layer is reduced at the joint interface. In order to control the formation of the intermediate layer at the joint interface, it is necessary to reveal the cooling process at the joint interface and formation process of the intermediate layer on the explosive welding. However, it is difficult to observe the phenomenon occurring at the joint interface of the explosive welded joint which is completed in a very short time, experimentally. In this study, the cooling process at the joint interface and formation process of IML were investigated by the simulation and experiment. The cooling rate at the local melting zone reached over 106K/s. This rapid cooling was caused by large temperature gradient between joint interface and base metals. The solidification of IML proceeds from the outside of the local melting region toward the center and the final solidification area of the IML becomes the central part of the local melting region. Therefore, it is considered that the void observed at the center of the IML at the actual bonding interface is due to the solidification shrinkage generated at the final solidification area. The wavy interface (wave height, wavelength) obtained by explosive welding and the composition of the IML and the position of the void were quantitatively in good agreement with the interface form obtained by numerical analysis.
Reheat cracking during post weld heat treatment (PWHT) generally occurs in the coarse grain HAZ (heat affected zone) of Cr-Mo steel and 780MPa class high strength steel. Although reheat cracking in the weld metal of Cr-Mo steel has also become well-known recently, the effect of alloy elements in the weld metal on reheat cracking had not been investigated in detail. In particular, the small amounts of alloy elements such as carbon and boron in the weld metal are affected partly by dilution components from the base metal. The purpose of this study was to clarify the effect of the carbon and boron contents on reheat cracking susceptibility in the weld metal of Cr-Mo steel. Reheat cracking tests were carried out by the C-ring and constant stress methods. Reheat cracking susceptibility and the hardness of weld metal containing up to 0.08mass% carbon clearly increased with increasing boron contents over 3ppm. However, reheat cracking of weld metal containing approximately 7ppm boron was suppressed by increasing the carbon content in spite of an increase in hardness. The constant stress reheat cracking tests showed experimentally that the high temperature strength of the grain boundary in weld metal containing boron decreased with increasing temperature during PWHT. Secondary Ion Mass Spectrometry (SIMS) and precipitate analysis revealed that the existence form of boron on the grain boundary changed from dissolved to precipitated during PWHT, and thermo-dynamic calculation showed an increase of M23C6 carbide with increasing carbon contents. Based on these results, it was estimated that the reheat cracking susceptibility of weld metal increased by intergranular embrittlement due to precipitation of boron nitride and decreased by grain boundary strengthening due to the increase of M23C6 carbide.
Recently in coal-fired boiler of Japan, a water wall tube damage such as fireside corrosion and circumferential cracking in a reducing atmosphere due to low NOx environment has become a serious issue. And the improvement of the corrosion resistance of the furnace wall is required. In order to apply the Ni-based Alloy 622 overlay welding material to the furnace wall, corrosion test, fatigue test, and long-term aging test have been conducted. And an overlay welded panel has been inserted into the actual furnace wall. This report describes these test results of the Alloy 622 overlay welding material tube. Alloy 622 overlay weld metal demonstrates good corrosion resistance to H2S gases in a high temperature and low NOx environment. It was found that the corrosion rate of Alloy 622 is one tenth (1/10) that of 1.25%Cr bare tube in corrosion testing. From the results of the high-temperature bending fatigue test, it is considered that the Alloy 622 overlay weld metal has enough durability for actual action stress due to the rapid temperature change. The Alloy 622 overlay welded panel also showed no evidence of corrosion after seven years of service. All of the field test results clearly demonstrate that the Alloy 622 has excellent against fireside corrosion and circumferential cracking in supercritical pressure boilers.