| Up to now, the welding theory for high carbon alloy steel has not been well established.With the rapid development of the manufacturing industry in the world, the consumption of high carbon alloy steel, which is widely used to make tools and dies, has been increasing more rapidly than ever before. But because of the high manufacturing cost and high damaging rate of the high carbon alloy steel tools, it is urgent to introduce an effective welding technology to prolong their servicing time.At present, the deposited metal with high carbon Cr-W-Mo alloy system is generally used to meet the demand for repairing or surface hardening of high carbon alloy steel tools. Due to the brittleness of this kind of deposited metal, the preheat at about 350-500 ℃ and the post-welding heat treatment are necessary to improve the cracking resistance. The compositions and requirements for hot welding of American product MG700 and homemade D317A type coated electrodes represent the current state in the welding field of high carbon alloy steel.In the academic field, much attention was paid to twin martensite, which is the essential strengthening phase of the deposited metal with high carbon Cr-W-Mo alloy system, and retained austenite which is a toughening phase. Since twin martensite results in the high hardness and brittleness of the deposited metal, the effect of austenite on toughness is restricted. The current research on the welding of high carbon alloy steel is still unsophisticated, therefore little information about it can be found in textbooks and manuals.After plenty of tentative tests, high carbon Ti-Nb-V alloy system was finally selected as the basic alloy system of the deposited metal, which facilitates making creative design with respect to strengthening mechanism, controlling of carbon distribution and the microstructure.Electrode coating was used as carrier of alloy elements, so that the composition of the deposited metal could be adjusted conveniently. The specimens were deposited by shielded arc cold welding. By means of SEM, TEM, X-ray diffraction and EPMA, different kinds of deposited metals with C-Ti, C-Nb, C-V, C-Ti-V and C-Ti-Nb-Valloy system were investigated respectively. Main topics involved in the research were as follows:(1) Thermodynamic analysis for the formation of carbidesWith duly processing the activity interaction parameters and other thermodynamic parameters at high temperature, Visual Basic program was developed to calculate the standard Gibbs free energy and determine the forming condition of carbides in Fe-based solutions.ZrC, TiC and NbC could precipitate through in-situ reaction between pure metal and graphite even when the temperature exceeds 3000K, and ZrC is most easy to form. Although the thermodynamic condition for the formation of VC at 2200~3000K could also be satisfied, the thermodynamic driving force is low.In the multicomponent Fe-based solution, TiC and NbC are apter to precipitate from liquid solution than ZrC, and almost all the Ti and Nb are combined with carbon into carbides above 800 °C. The formation of VC in liquid solution requires high concentration of V and carbon, VC tends to precipitate secondly under 800 °C.(2) Second phase strengthening theory and carbon depletion in solid solution matrixHere the second phase includes pure carbides and complex carbides which nucleated on oxides in the deposited metal.The forming mechanism of the second phase in the deposited metal with C-Ti-Nb-V alloy system were proposed on the basis of the x-ray diffraction pattern of slug and the microstructure investigation of the welding droplets and the deposited metal. The first is forming through in-situ reaction between carbon and alloy elements in the welding arc condition, and then the producing in alloy congregated micro-region in the welding droplets, and finally the precipitating from the welding pool during cooling.By adjusting the content of C, Ti and Nb, the distribution, morphology and quantity of the second phases could be controlled. If the ratio between the carbide forming elements and the carbon content was approximate to the ideal chemical ratio, more dispersed second phases could precipitate, thus the second phase strengthening was maximated.The allocation of carbon and alloy elements between the second phase and thematrix can be controlled in the deposited metal with C-Ti-Nb-V alloy system. With their strong carbide-forming tendency, Ti and Nb were combined with most of the carbon into carbides, then the solution strengthening effect of carbon was lowered. Additionally, the formation of TiC and NbC reduced the precipitation of carbides of V and Cr, so most of V and Cr dissolved in the matrix, which was necessary to ensure the strength of the matrix.(3) Non-twin toughening of solid solution matrixThe early formation of plenty of the dispersed second phases in liquid not only supplied nucleating core for austenite and made the microstructure finer, but also avoided twin martensite and reduced the content of retained austenite through depleting carbon in the matrix. By adjusting the ratio between Ti, Nb and C, the carbon depleting degree of the austenite can be modified to achieve different kinds of solid solution matrix, such as ferrite, low carbon martensite and so on.The suitable composition of the deposited metal to get low carbon martensite matrix with a little film-shaped retained austenite and the dispersed second phases was presented as following:(0.8~1.0)C, (0.3~0.65)Ti, (l~1.8)Nb, (1.4~2.3)V, (1.3~1.5)Cr, ( 1.3~ 1.5)Ni, (0.1~0.3)Mo, (0.015~0.05)RE(4) Interface between matrix and second phaseThe results of TEM and SAD indicated that no coherent interface exists between the matrix and the second phase, which may be resulted from the great difference in the lattice parameters. However, the interface is straight and free of any oxides, and there is certain orientation relationship between the two phases, namely, (002) ? -Fe//(220)(NbTi)c or ( Oil) <■-Fe //( Oil )(NbTi)c. The parallel orientation relationship indicates the strong combination between the second phases and the matrix, which is very important to prevent the second phase from departing from the matrix during wearing process and improve the strengthening effect of the second phases.(5) Test of mechanical performance of the deposited metalsThe investigation of impact toughness and fracture face of the deposited metals reveals that the toughness of the deposited metal with low carbon martensite matrix was increased, which was the overall effect of the second phase dispersing, the...
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