After the final heat treatment, the microstructure of 12Cr1MoVG alloy seamless tube blank has changed into Tempered Microstructure with martensite orientation, and has good strength and plastic toughness. In some areas, there are some “invisible” grain boundaries around the adjacent tempering structure, which divides the field of view into several large areas, which is the reason for the coarseness and mixed crystals after final heat treatment. These “invisible” grain boundaries retain the original structure when the non diffusion transformation occurs. In other words, before the final heat treatment of seamless tube blank, there are serious defects such as coarse grain and mixed grain.
Based on the analysis of the overall macro manufacturing process characteristics, the alloy seamless tube blank of the material has the conditions to form coarse grains before heat treatment.
(1) The melting and pouring process has the characteristics of high temperature and long mold cooling time, resulting in coarse grains in the ingot itself.
(2) In extrusion forming, due to its deformation characteristics, the grains in seamless tube blank are coarse. In addition, the microstructure genetic characteristics of P91 seamless alloy tube steel lead to the phenomenon of coarse grains and mixed grains in the seamless tube blank before final heat treatment.
Stage: high temperature heating and annealing.
According to the test results and analysis, the annealing process formulated in the early stage can only play the effect of stress and hydrogen diffusion, but can not play the role of coarse grains. Therefore, on the basis of the original annealing process, t-step high-temperature heating is added, that is, 920-1070 ℃ high-temperature heating is added between 700-770 ℃ and 600-670 ℃ stage annealing, so as to re austenitize before the final heat treatment to eliminate the structural defects in the early stage of seamless tube blank.
After the high-temperature annealing process is adopted, the grain size is improved, but the process consumes energy and takes a long time. The amount of gas is basically twice that of the original process, and the execution time is twice that of the original process.
Stage: annealing with residual temperature in combination with the characteristics of extrusion manufacturing.
In order to further optimize the process, combined with the actual production, the generation of defective structure from the source, cut off the inheritance of coarseness and mixed crystal phenomenon, and anneal at residual temperature.
The key point of this process is to combine the austenitizing temperature of the extruded seamless tube blank with the extrusion temperature. After extrusion, it is immediately cooled in the furnace to replace the air cooling of the original process, and the annealing temperature is increased, so that the seamless tube blank can be fully austenitized by using the extrusion residual temperature, and the austenite can be transformed into equilibrium structure by furnace cooling to a higher temperature (730-790 ℃), So as to organize genetic phenomena.
In addition, the grain size of seamless tube billets after changing the annealing process is also improved. 10 representative seamless tube billets are selected as an example.
From the above results, it can be seen that the grain size can be qualified at one time, the mixed crystal phenomenon is, and the high-temperature microstructure genetic phenomenon is controlled.
Post time: Jan-18-2022