GRAIN BOUNDARY PLANE DISTRIBUTIONS IN 304 STEEL ANNEALED AT HIGH TEMPERATURE AFTER A PARALLEL PROCESSING OF MULTIPLE FORGING AND DIRECT ROLLING

Abstract

It is well-recognized that low ∑-CSL boundaries are highly populated in the grain boundary character distribution(GBCD) for austenitic stainless steel(SS) processed by low strain and subsequent annealing.However,large-strain plus annealing typically tends to introducing numerous random high angle grain boundaries(RHABs) instead of producing high fraction of∑3,∑9 and∑27 boundaries.In this case,the distribution of grain boundary planes of RHABs must be very relevant to the properties of material.The current study is to explore the evolution of GBCD and grain boundary plane distribution(GBPD) in 304 austenitic SS after large strain and subsequent annealing using electron backscatter diffraction(EBSD) and five-parameter analysis(FPA).After solid solution treatment,304 steel samples were separately processed by multiple forging(MF) and direct rolling (DR) with true straine=2 followed by same annealing at 900℃for 2—120 min.Then the GBCDs and GBPDs of the two groups of samples were examined.The results show that the total∑3~n(n=1, 2,3) special boundaries in any sample as processed take a length fraction of lower than 45%out of the entire boundaries,and with annealing proceeding the incoherent∑3 boundaries tend to be tuned into coherent ones and consequently the summation fractions of∑9 and∑27 boundaries decrease accordingly.In the two samples which were separately processed by MF and DR but followed by the same annealing at 900℃for 120 min,their random boundaries or general high angle boundaries(∑3~n special boundaries filtered) mostly appeare to be the 111 twist and 110 tilt boundaries,indicating there exist grain boundary textures(GBT) in both samples.However,in the condition of some misorientations, the GBPDs of random boundaries are quite different in the two samples.For grain boundaries of 111/30—40°misorientation,more grain boundaries of twist type nearly on the exact {111} plane are found in the specimen processed by DR and annealing for 120 min(DR120) compared to that processed by MF and annealing for 120 min(MF120).For the grain boundaries of 110/50°misorientation,it was found that most of such boundaries in MF120 are tilt type and positioned on {112},{113} and {115} planes,whereas those in DR120 are tilt or mixed type positioned on {001}, {111} and {012}.It was suggested that there are distinct effects of pre-processing on the GBPDs of annealed 304 steel.