Influence of Warm Tempforming on Microstructure and Mechanical Properties in an Ultrahigh-Strength Medium-Carbon Low-All
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STRONGER and tougher structural materials are always needed to reduce weight and improve safety in transportation, enhance architectural flexibility in construction, and improve performance in heavy industry. Low-alloy steels are workhorse materials in most industries due to their low cost and high performance. However, ultrahigh-strength low-alloy steels with yield strength (ry) exceeding 1400 MPa typically exhibit low impact toughness; their V-notch Charpy absorbed energy (vE) is 10 to 40 J at room temperature,[1,2] which limits their structural applications. There are two important aspects to improve the notch toughness of structural materials: (1) increase of the intrinsic fracture resistance in solid materials, and (2) relaxation of stress concentration at crack tips.[3] The well-known techniques for raising the intrinsic fracture resistance of steels are (1) the reduction of impurity elements such as P[4] and S,[5] and inclusions causing embrittlement,[6–8] (2) the reduction of C,[9] (3) the addition of alloying elements such as Ni,[9,10] and (4) grain refinement.[10–12] A much better combination of ry and vE was obtained in a high-purity 18 wt pct Ni maraging steel strengthened by precipitating YUUJI KIMURA and TADANOBU INOUE, Senior Researchers, are with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Contact email Kimura.Yuuji@ nims.go.jp Manuscript submitted May 25, 2012. Article published online September 7, 2012 560—VOLUME 44A, JANUARY 2013
nanometer-size intermetallic compounds on a matrix of tough lath martensite; however the vE decreased to 60 J at the ry of 1700 MPa.[13] In the latter aspect to improve the notch toughness of structural materials, delaminations are known to relax the triaxial tension generated by the localized plastic constrain at the notch and/or the crack tip ahead of advancing crack tips. In certain Al-Li alloys[14,15] and thermomechanical treated steels,[16–20] the notch toughness at low temperatures was reported to be improved by controlling the occurrence of delamination cracks (i.e., delamination toughening). For these techniques, grain refinement and delamination toughening are considered to be effective in lowering the ductile-to-brittle transition temperature (DBTT) in low-alloy steels.[19,20] We recently discovered that delaminations in ultrafine elongated grain (UFEG) structures resulted in the remarkable enhancement of the impact toughness of ultrahigh-strength low-alloy steel bars.[21–23] In a 0.4 pct C-2 pct Si-1 pct Cr-1 pct Mo steel with a UFEG structure,[22] the ry of 1840 MPa and the vE of 226 J were obtained at room temperature. In addition, the developed UFEG structure steel showed a significant inverse temperature dependence of the vE in response to the delamination at the temperature range from 333 K to 213 K (60 C to 60 C), where ultrahigh-strength steels typically undergo ductile-to-brittle transition (DBT).[1,2] A static three-point bending test also demonstrated that the static fracture toughness of the de
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