Effects Of Grain Boundary Segregated Phosphorus On Hot Ductility And Temper Embrittlement Of 2.25Cr-1.0Mo Steel

Solute atoms segregation to grain boundaries may result in the change of grain boundaries state and influence the mechanical behavior of engineering materials. The segregation of solute atoms in grain boundaries may be classified into equilibrium and non-equilibrium segregation. The embrittlement of P-dopped 2.25Cr-1.0Mo steels during the continuous casting and temper process was investigated by Gleeble-2000 thermal simulated machine, AES, SEM and temper embrittlement etc. The mechanism of P segregation to grain boundararies was discussed.This article includes two parts.The first part: The Gleeble-2000 thermal simulated machine, AES and SEM tested The segregation behavior of P and its influence on the high temperature mechanical behavior of 2.25Cr-l .OMo steels during the continuous casting process. The degree of segregation was different with different cooling rate. It was found at cooling rate of 10癈/s, the amount of non-equlibrium was large than that of 20癈/s and 5癈/s. It showed that there exists a critical cooling rate, at this rate, a maximum in extent of embrittlement occurs. The extent of P segregation to grain boundaries will increase and the hot ductility will become worse with the austenized temperature enhanced.The second part: The segregation behavior of P to grain boundaries and its influence on the temper embrittlement during aged process were investigated by temper embrittlement test and AES. The he amount of P segregation to grain boundaries increases quickly at the beginning of aged process, then reach a peak at one time and begin to decreases slowly, there exists a critical time.During these two processes, the segregation of P to grain boundaries in 2.25Cr-1.0Mo steels is happened. This segregation behavior accords with the mechanism of non-equilibrium segregation: it is suggested that the mechanism of segregation be based on aninequilibrium in which a sufficient quantity of recombined complexes of solute atoms and vacancies exists. These three parts: solute atom, vacancy and their recombined complex, are in equilibrium with each other. A sample properly maintained at a solid dissolution treatment temperature will, when cooled to a certain lower temperature, exhibit a loss of vacancies along the grain boundaries, whereby it attains the equilibrium vacancy concentrations at low temperatures. The decrease in the vacancy concentration causes the dissociation of the recombination complexes into vacancies and solute atoms. This in turn gives rise to the decrease in the recombined complex concentration near the grain boundary, where no other vacancy traps are present, vacancies would recombine with solute atoms and so the vacancy concentration is reduced. This makes the recombination complex concentration increase in the regions remote from the grain boundary. Consequently, a concentration gradient appears between the grain boundary and regions beyond it. The gradient of complexes drives the recombination complexes to diffuse from regions remote from the grain boundary to the grain boundary. This diffusion causes excessive solute atoms to concentrate in the vicinity of grain boundary and results in non-equilibrium grain boundary segregation.It was suggested that with correctly cooling rate, the formation of surface crack of 2.25Cr-1.0Mo steels would be controlled. Based on this measure, the product quality and efficient of casting slab would be improved. When steels employed, the critical time should be avoid to prevent appearance of temper embrittlement.