Study On The Mechanism Of Reheat Cracking In2.25Cr1Mo0.25V Steel

Compared with traditional2.25Cr1Mo steel, Vanadium-modified2.25Cr1Mo exhibits higher strength and creep resistance at elevated temperature, increased resistance to hydrogen attack, which can better satisfy the development demand of modern hydro genation reactor towards higher-parameters, larger-dimensions and lighter-weight. Unfortunately, the addition of strong carbide forming elements (such as Vanadium and Titanium) usually results in increasing reheat cracking susceptibility of welded joints during postweld heat treatments, threatening the service safety of hydrogenation reactors. Hitherto, the mechanism of reheat cracking is not clearly understood, the measures for prevention of reheat cracking lack of reliable theoretical and experiment basis. These problems have been becoming one of the main bottlenecks in the manufacturing level improvement of hydrogenation reactors. In this paper, focusing on the2.25Cr1Mo0.25V steel for the heavy forge-welded hydrogenation reactors, the relationship between manufacturing process, material microstructure, and reheat cracking susceptibility were investigated, the cracking mechanism were elaborated by characterization of macroscopical mechanical properties and analysis of micro-mechanism, and some preventive measures against reheat cracking were put forward based on related study. The main research contents were concluded as follows:(1) The screening test for simulating similar materials to the actual CGHAZThe reheat cracking most commonly occurs in the welding coarse grained heat affected zones (CGHAZ) of2.25Cr1Mo0.25V steels. In the first part of this study, based on practical welding process and classical welding heat transfer modeling, a series of thermal simulation test with different parameters were carried out on a Gleeble thermal-mechanical simulator. According to the comparison of microstructure and hardness between the simulated and actual CGHAZ, the thermal cycle curve for acquiring the most similar materials to actual CGHAZ was determined.(2) Influence of welding and postweld heat treatments processes on the microstructure and mechanical properties of CGHAZThe occurence of reheat cracking require both susceptible environmental and microstructure conditions. In this section, a series of isothermal constant strain rate tensile test were carried out on the simulated CGHAZ specimens. Influnences of temperature, welding heat input, and second thermal cycle on the microstructure, hardness, and high temperature ductility of CGHAZ were characterized, and the reheat cracking susceptibility were assessed. The results show that the simulated CGHAZ exhibits a minimum ductility around675℃, corresponding to the most susceptible temperature to reheat cracking. However, the effects of temperature on the ductility weaken at lower strain rate. The reheat cracking susceptibility of CGHAZ increases with increasing heat inputs within the range of5-100kJ/cm. Under the condition of low heat input, relatively high ductility is observed corresponding to a softening phenomenon of material, associated with a mixed fracture mode of transgranular and intergranular. While under the condition of high heat input, the ductility of CGHAZ degrade obviously corresponding to a secondary hardening phenomenon, associated with a fully intergranular microvoid coalescence fracture mode. The second welding thermal cycle with peak temperatures of890℃and1020℃promote the recrystallization of prior austenite grains in CGHAZ, which results in a remarkable elevation of the ductility and a significantly reduction of the reheat cracking susceptibility. Therefore, optimizing temperature for postweld heat treatment, reducing welding heat input, and taking advantage of second welding thermal cycle all can reduce the reheat cracking susceptibility of CGHAZ in2.25CrlMo0.25V welds.(3) Study on the stress relaxation and creep rupture behaviors of2.25Cr1Mo0.25VReheat cracking is caused by the creep damage accumulation during the relaxation of welding residual stress. In this part, stress relaxation and creep rupture test on2.25CrlMo0.25V steel in the temperature range of postweld heat treatment were conducted. A prediction model for stress relaxation behavior was established. The variations of rupture time and Larson-Miller parameter with stress in simulated CGHAZ were obtained. The accumulated creep damage of CGHAZ specimens during the stress relaxation test was evaluated. The results show that the stress relaxation behavior of both base metal and simulated CGHAZ can be predicted well based on Hook-Norton constitutive equations. The stress relaxations rupture time of simulated CGHAZ can be predicted based on Robinson time-life fraction method.(4) Study on the mechanism of reheat cracking in2.25Cr1Mo0.25V steelThe root cause of reheat cracking is the insufficient creep ductility of welding CGHAZ at temperatures for postweld heat treatment, and the key factor in determining the ductility of metal is cavities nucleation rate. In this part, the evolution of microstructure and creep damage during simulating welding thermal cycle and stress relief cracking were investigated by SEM, TEM, and EDS. Combined with the micro-mechanism of cavity nucleation at grain boundaries and the variation in hardness of CGHAZ at high temperature, the effect of microstructure evolution on cavities nucleation rate were analysed, and the mechanism of reheat cracking in2.25Cr1Mo0.25V steel were elaborated. The results show that the carbides in base metal are taken into solution during austenization in simulated welding thermal cycle. Subsequently, exposure at the temperature for postweld heat treatment causes the reprecipitation of a large number of carbides. The precipitation of intergranular carbides not only provides the condition of local stress concentrations to increase the driving force for cavities nucleation, but also induces the decrease of the volume shape factor of the cavities to reduce the resistance to cavities nucleation. The precipitation of intragranular carbides causes the secondary hardening phenomenon to increase the creep resistance of material, which slows the creep relaxation of local stress concentration and results in the increase of the driving force for cavities nucleation. Precipitation behavior of carbides both in grain interiors and grain boundaries are the dominant factors that affect cavities nucleation rate. The cavities nucleation rate also increases with decreasing surface energy resulted from impurity segregation on the grain boundary, but this effect is not significant attributed to high temperature and low impurity in the steel. Both the strengthening grain interiors induced by intragranular carbides precipitation and the weakening interface caused by intergranular carbides precipitation are the leading causes of the acceleration of cavitation and creep ductility deterioration in CGHAZ, which is the micro-mechanism of reheat cracking in welding CGHAZ of2.25Cr1Mo0.25V steel.(5) Prevention of reheat cracking during2.25Cr1Mo0.25V hydrogenation reactors fabricationBased on the results from this study and the actual manufacturing process of2.25Cr1Mo0.25V hydrogenation reactors, the recommendation for optimization of welding parameters as well as chemical composition control of base metal were made for prevention of reheat cracking in CGHAZ. The method, basis and possible limitations of preventing weld metal reheat cracking were discussed as well.