Seung Youb Han

Effects of microstructure and pre-strain on Bauschinger effect in API X70 and X80 linepipe steels

In this study, effects of microstructure and pre-strain on the Bauschinger effect were investigated in two API X70 and two API X80 linepipe steel sheets fabricated by controlling the cooling condition, and their yield strength and Bauschinger parameters were measured by the tension-compression test with varying tension pre-strain. The fast-cooled steels had the higher fraction of acicular ferrite, granular bainite and martensite-austenite (MA) constituents and smaller grain sizes. The reduction in yield stress (ΔYS) of the steels having a higher fraction of MA and smaller grain sizes was higher than that of the steels having a lower fraction of MA and larger grain sizes. The ΔYS was smallest at the pre-strain of 1%, reached the maximum at the pre-strain of 2%, and then decreased with increasing pre-strain. This result could be explained by the amounts of mobile dislocations and back stress, which affected the Bauschinger effect and strain hardening effect simultaneously. Since these two effects affected the yield strength on a competing basis, the Bauschinger stress and hardening parameter were used to separately analyze these effects. It could be confirmed that the Bauschinger effect and strain hardening effect were activated at pre-strains of 1–2% and 3–4%, respectively.

Analysis and estimation of the yield strength of API X70 and X80 linepipe steels by double-cycle simulation tests

In this study, the spiral piping and electric resistance welding piping was conducted on API X70 and X80 linepipe steel sheets having different microstructures, and the yield strengths of the flattened sheets were measured. A double-cycle simulation test with tension-compression-tension or compression-tension-tension for the piping and flattening processes was conducted to estimate the yield strength. The simulation test results indicated that the yield strengths of the outer or inner wall of the pipe could be estimated by combination of Swift’s equation and the Bauschinger stress parameter, and that these estimated yield strengths were well matched within a small error range with the measured yield strengths. Thus, the variations in yield strength before and after the piping could be effectively estimated using the tension/compression properties of the leveled sheets because the strength differential effect was small and the reverse flow curves were expressed by a single curve. These findings suggested that the present estimation method played an important role in controlling microstructural and manufacturing process parameters to minimize the reduction in yield strength of the linepipe steel sheets.

Analysis and estimation of yield strength of API X80 linepipe steel pipe by low-cycle fatigue tests

In the present study, spiral piping was conducted on API X80 linepipe steel, and the outer and inner wall pipe yield strengths were measured. A low-cycle fatigue test was conducted on a leveled X80 steel sheet to simulate piping and flattening processes, and the strain hardening and Bauschinger effects, induced from different strain histories, were evaluated and combined using Swift’s equation and the Bauschinger stress parameter, respectively. By analyzing the stress-strain curves obtained from the low-cycle fatigue test, the yield strengths of the outer and inner walls were estimated to be 592 MPa and 492 MPa, respectively, which are lower by 20–80 MPa than that of the actual pipe used. Possible reasons for measured and estimated yield strength differences could be the simulation determining procedure of the pre-strain and Bauschinger stress parameters, preposition of same strain hardening behavior depending on strain history, and pre-strain differences depending on thickness location in the steel sheet during piping.

In Situ fracture observation and fracture toughness analysis of pearlitic graphite cast irons with different nodularity

Effects of microstructural modification and microfracture mechanisms on fracture toughness of pearlitic graphite cast irons with different nodularity were investigated by in situ observation of microfracture process. Six pearlitic graphite cast irons were fabricated by adding a small amount of Mg as a nodularizing element for graphite, and their microstructures including pearlite, ferrite, graphite, and eutectic carbide were analyzed. Most of ferrites were observed in a layer shape around graphites because of carbon-depleted zones formed near graphites. As the nodularity and nodule count increased, fracture toughness linearly increased in the cast irons except the iron containing many fine graphites. According to in situ observation of microfracture process, cracks initiated at nodular graphites and carbides even at a small load, and then propagated readily through the adjacent graphites or carbides, thereby resulting in the lowest fracture toughness. The cast iron having widely spaced graphites and ferrite layers thickly formed around graphites showed the highest fracture toughness because of the blocking of crack propagation by ductile ferrite layers and the crack blunting and deflection by graphites, which was also confirmed by the R-curve analysis.