Welding Metallurgy Of Ultra Low Carbon Copper-bearing Age Strengthening Steel

This thesis based on physical simulation technology and arc welding is focused on the welding metallurgy of ultra low carbon copper-bearing age strengthening steels which are used in construction of shipbuilding and oil platform.The determination of SH-CCT diagram, single and double thermal cycles simulating the behavior of HAZ during welding were performed on a dynamic thermal machine. As can be seen from SH-CCT diagram, martensite and bainite are obtained in simulated CGHAZ with t8/5from3.7s to7s. Bainite is characterized by lath. Retained austenite is found among the bainitic ferrite laths. Granular bainite begins to form when t8/5is more than15s. Ferrite starts to be observed when t8/5is more than45s. The more heat is input, the more ferrite forms. The microstructure is predominantly bainite in a wide heat input range from3.7s to2500s. The SH-CCT diagram indicates that even under the condition of fairly rapid cooling (t8/5=3.7s), entire martensite in simulated CGHAZ has not been attained and Vickers hardness is only286HV. So the copper-bearing steel possesses a small hardening quenching tendency and an excellent resistance to cold cracking under the condition of no preheating and no postheating. While t8/5is more than7s, CGHAZ starts softening; that is to say, the hardness begins to be less than the one of base metal. The higher heat input leads to the more softening. The hardness decreases to187HV when t8/5is2500s. So it is proposed to choose t8/5ranging from7s to35s during welding to prevent softening.Single thermal cycle experiments show that copper-bearing steel has a narrow range of heat input or line energy. Under the condition of higher heat input or line energy, brittlement is easy to happen in CGHAZ. Granular bainite transformed from austenite leads to brittlement. On the contrary, lath bainitic ferrite formed with lower heat input can increase toughness. Softening begins to occur in CGHAZ (t8/5>7s). The dissolution of ε-Cu and coarser lath bainitic ferrite and more ferrite cause softening in CGHAZ. The effect of M-A constituents on the impact toughness in simulated CGHAZ was studied. The dimensions and area fraction of granular M-A constituents also influence the impact toughness. There is no visible effect on the toughness when the dimensions of M-A constituents are less than1μm and the area fraction is less than5%. The toughness decreases greatly once the dimensions exceed1μm and the area fraction is more than5%. Therefore, decreasing the dimensions and area fraction of M-A constituents by controlling welding heat input will do good to improve the impact toughness of copper-bearing steel.Experiments simulating the whole HAZ show that the softening phenomena take place in the whole heat-affected zones when t8/5is15s. The hardness in intercritical (ICHAZ) decreases by50HV compared with that of the base metal and it is the most softened region due to re-dissolving of precipitate phase and ferrite compared with other regions in HAZ. The slight increase in hardness in FGHAZ is connected with microstructure refinement. The formation of lath bainitic ferrite and the enhancement of copper solid solution strengthening cause the hardness recovery for CGHAZ. Even so, the hardness value in CGHAZ is still less than that of the base metal but the softening phenomenon is not evident. CGHAZ has relatively low impact toughness owing to the growth of austenite grain size and coarse lath bainitic ferrite when t8/5is15s. FGHAZ achieves better impact toughness for its short time maintaining at higher temperature after austenization to reduce the austenite grain size. ICHAZ has also good impact toughness.Double thermal cycle experiments demonstrate that obvious brittlement happens in the intercritically reheated CGHAZ. The reason is martensite twin formed and coarse granular bainite, which reduce the impact toughness. The higher heat is input, the more serious brittlement becomes. Thus, during multilayer welding, it is proposed to control strictly heat input.Kinetic model of ε-Cu re-solution during welding was established. The volume fraction of ε-Cu re-solution keeps changing during welding. The initial size of ε-Cu and the peak temperature mainly influence re-solution kinetics of ε-Cu. The diffusion coefficient of ε-Cu in α-Fe is the indirect reason to influence re-solution of s-Cu. The smaller the initial size of ε-Cu is, the higher the peak temperature is, and the more ε-Cu re-dissolve. Calculation results of the kinetic model and simulation results in HAZ reach an agreement, that is to say, when the peak temperature is more than1000℃, ε-Cu completely re-dissolve into matrix phase.Wire for GMAW was developed by using Mn-Ni-Cr-Cu as the main reinforcing elements, and Ti and B micro alloying without Mo and less Ni. Compared with the same strength grade for other wires in the world, this developed wire has excellent low temperature impact toughness and Charpy V-notch impact energy at-40℃is over97J.GMAW, SAW and SMAW tests were investigated using patent welding wires. The tests show that the joints get good impact toughness at low temperature for three welding methods and the average impact energy (-40℃) in the welding joints is more than73J with great allowance compared with the requirement. The tensile strengths completely meet the strength requirement of base metal. Acicular ferrites (AF) and some proeutectoid ferrites exist in the weld metal. The great amount of AF in the weld metal can significantly improve both the strength and the toughness of weld metal. Bainite is attained in actual CGHAZ. Softening phenomena are not obvious in welding joints for GMAW and SMAW. There exists a slight softening phenomenon approaching the base metal, which and simulation results have a consistency. That is to say ICHAZ has the lowest hardness value.Non-isothermal stress relief test results show that susceptible temperature of reheat cracking of copper-bearing steel is669℃and relevant cracking time is63seconds. The susceptible temperature interval is from669℃to695℃. So test steels have reheat cracking susceptibility. When CGHAZ is reheated to the temperature between650℃and700℃, intracrystalline strength is high for the distribution of carbonide of Cr and Mo. Intercrystalline failure cracking takes place owing to low strength on the grain boundary.