Yukihiko Horii

Effect of Chromium Content on Stress Corrosion Cracking Susceptibility of Shielded Metal Arc Weld Metals for 600 Type Alloy in High Temperature Pressurized Pure Water

The stress corrosion cracking (SCC) susceptibility of SMAW metals for Inconel alloy 600 to which Cr was added to 14.8–21.4 mass% has been investigated on the basis of a creviced bent beam test in pressurized hot water (corresponding to the service condition of boiling water reactor nuclear power plant), since the TIG weld metal of alloy 82 involving 18–22 mass% Cr possesses much better resistance to SCC than the SMAW metal of alloy 182 (Cr content = 3–17 mass%). When their Cr contents were increased to the same level as those of the alloy 82, the weld metals of alloy 182 sustained only slight SCCs in the as-welded state, and no crack was detected after the post-weld heat treatment stress relief+low-temperature ageing (SR+LTA) of stress relief annealing at 893 K followed by ageing at 673 K. These results suggest that the higher Cr content of the alloy 82 is responsible for its higher resistance to SCC than that of the alloy 182. The Cr carbides precipitated at the grain boundary during the welding, and the SR+LTA treatment was also changed from M7C3 type to M23C6 type with the increase in the Cr content. Although the Cr content at the grain boundary in weld metal containing 14.8 mass% Cr subjected to the SR+LTA treatment was 3 mass%, the Cr content of weld metal containing 18.5 mass% Cr was not less than 10 mass%. The addition of the Cr to the alloy 182 increased the Cr content in the grain boundary region, suggesting that the inter-granular SCC can be suppressed when the Cr content at the grain boundary is not less than 10 mass%. In addition to the carbide, Ni16(Mn, Cr)6Si7 (G phase) was precipitated at the grain boundary in the alloy 182 containing 18.5 mass% Cr when the SR+LTA treatment was applied. transmission electron microscope-EDS analyses suggested that the G phase was enriched in P, and therefore could decrease the P content in the grain boundary region. Probably, the decreased P content at the grain boundary due to the precipitation of G phase contributed to the enhancement of the SCC resistance of the Cr-added alloy 182 by the SR+LTA treatment.

The Influence of structures of Nickel Base Alloy Weld Metals on Flaw Detectability by Ultrasonic Test

Ultrasonic Test (UT) method is the most appropriate method in many kinds of non-destructive test methods because of capabilities for detection and depth sizing of flaw. But it is generally said that the flaw detection in nickel base alloy welds is more difficult than that in ferritic steel welds because of dispersion and attenuation of the ultrasonic wave. Therefore, influences of welding conditions on flaw detectability and signal to noise ratio (SN ratio) of UT in nickel base alloy welds were investigated. Tested weld joints were prepared with multi-pass gas tungsten arc method. Partial weld joints were applied to weaving or magnetic stirring when that welded. Specimens having an electrical discharge machining slit in weld metal or in heat affected zone were tested by a longitudinal wave of 5 MHz in frequency and 45 degree in incident angle. Obtained results are as follows. Welded joints having columnar structures with the same growth direction showed high SN ratio that means good flaw detectability. Effective welding conditions, which make the columnar structure to be the same growth direction, were found to be the application of weaving or magnetic stirring. Same growth direction in columnar structures means same angle between the columnar structure growth direction and the ultrasonic wave propagating direction. Then, ultrasonic wave has almost same velocity in the weld metal, and the acoustic impedance will be almost same. High SN ratio may be caused by above phenomena. The SN ratio of weld joints by different base metal can be well explained by the difference of acoustic impedance at the boundary columnar structure and base metal.

Toughness recovery of modified 9Cr–1Mo steel weld metals after long-term thermal ageing and its evaluation by electrochemical measurement

Recently, thermal power plants have tended to operate under higher temperature and pressure steam conditions for CO2 reduction. Modified 9Cr–1Mo steels are used in the ultra-super critical power plant because of their excellent creep properties. However, the toughness of base metal decreases after long-term thermal ageing at operation temperature. Moreover, toughness of weld metal is lowest in the MIG weldment. In this study, metallurgical factors of toughness of the modified 9Cr–1Mo steel weld metals by thermal ageing at operation temperature were investigated. And the evaluation method of toughness by the electrochemical measurement used with 5% sulphuric acid aqueous solution was investigated. The weld metal that received post-welding heat treatment (PWHT: 1023 K and 5.4 ks) decreased toughness by thermal ageing in 873 K and 31.5 Ms. However, toughness of the welds after thermal ageing was recovered to the same level as PWHT welds when the thermal aged welds received the same thermal history as PWHT. A lot of the large Laves phase was observed in the weld metal after the thermal ageing. However, almost all of the Laves phase dissolved in parent phase by the same thermal history as PWHT. Therefore, it seemed that the decrease and the recovery of toughness were mainly determined by the behaviour of precipitation of the Laves phase. The peak of the current density (Ip) in the electrochemical measurement appeared in the weld metals that precipitated a lot of large Laves phase. It seemed that appearance of Ip was caused by the dissolution of the Laves phase. A good correlation between toughness and Ip was observed. Therefore, it can be said that toughness of the weld metals is supposed to be predicted using an Ip of the electrochemical measurement.

Effects of impurities and alloying elements on reheat cracking sensitivity of 720 N/mm2 class high‐strength steel

Summary This paper describes an investigation of the reheat cracking sensitivity of 720 N/mm2 class high‐strength steel from the viewpoint of the chemical composition. The impurities and alloying elements were varied within a specific range from the basic chemical composition of this steel (2.8%Ni, 1.5%Cr, 0.5%Mo, 0.10%C, 0.005%P, 0.001 %S, 0.006%N), and the change in the cracking sensitivity was examined. The following results were obtained in the implant cracking test: This steel is basically insensitive to reheat cracking. This is due to the fact that its contents of phosphorus and sulphur are extremely low (0.005%P, 0.001%S) and that it contains no alloying elements such as vanadium and niobium. At the normal nitrogen content, it is found that, if the contents of phosphorus and sulphur exceed specific critical values (Pcrit for phosphorus and Scrit for sulphur), the deleterious effects of these elements appear. For this steel, Pcrit is 0.011% and Scrit 0.006%. At an ultra‐low nitrogen content, the del...

Application of AC‐MIG narrow‐gap welding to butt joints in 980 M Pa high‐strength steels (5th report). Development of AC‐MIG welding

Summary Previous papers have shown the high‐current AC‐MIG welding process to be superior to the conventional DC‐MIG process in terms of welding efficiency and welded joint performance. The authors have used the AC‐MIG process to weld heavy‐gauge steel plates at a tensile strength level of 980 MPa. Weld metal with a very low hydrogen content is obtained by the newly developed process, resulting in a reduced preheating temperature during welding of high‐strength steels. The AC‐MIG process also produces an arc climbing effect with electrode negative polarity ‐ a desirable effect in narrow‐gap welding. AC‐MIG weld metal further contains low oxygen, conferring high toughness. As a result, a narrow‐gap AC‐MIG welding process has been developed to ensure high efficiency in processing and metallurgically high‐quality welds in welding of 980 MPa high‐strength steels for penstocks (pressure shafts) in hydroelectric power stations. Welding wires with chemical compositions satisfying suitable strength and toughness ...