Determining role of microstructure on crack arrest and propagation phenomenon in low-carbon microalloyed steel

Herein, it is aimed to correlate the crystallographic structure, crack propagation, and toughness scatter with the prior austenite grain size (PAGS) in a low‐carbon low‐alloy (LCLA) steel from the perspective of crystallography. Two average austenite grain sizes, 16.1 ± 13.4 μm and 30.1 ± 24.9 μm, are obtained by reheating the experimental steel to 950 °C (S950) and 1100 °C (S1100), respectively. Overall, the weaker variant selection and higher density of high‐angle grain boundaries (HAGBs) are observed in the as‐quenched microstructure of S1100. However, coarser blocks and Bain groups (BPs) also appear in S1100 due to some oversized PAGs, which cause a more uneven microstructure in S1100 than that in S950. The ductile‐to‐brittle transition temperature (DBTT) is estimated to be −60 and −88 °C for S1100 and S950, respectively. In addition, a larger scatter of impact toughness occurs in S1100 in the DBTT range from −35 to −80 °C. The inhomogeneous microstructure inherited from oversized PAGs explains higher DBTT and large scatter in impact toughness of S1100. Therefore, to improve low‐temperature toughness, it is more important to balance the blocks and BPs’ size refinement and uniformity rather than only consider the fraction of HAGBs in quenched LALC steel.

σ_{normalF} = {[\frac{{π E γ}_{p}}{(1 - ν^{2}) d}]}^{1 / 2}

Section: Discussionmentioning

confidence: 99%

“…In addition, DBTT can also be evaluated as a function of effective grain size by the following equation. [ 38 ]

DBTT = \frac{1}{β} {ln B - ln [σ_{0} + (k_{normalc} - k_{normaly}) d^{- 1 / 2}]}

Section: Discussionmentioning

confidence: 99%

Effect of Prior Austenite Grain Size on Crystallographic Characteristics and Low‐Temperature Toughness of a Quenched Low‐Carbon Low‐Alloy Steel

Tian

Misra

et al. 2021

“…According to Wang et al, [24] the fracture stress σ c can be described by the Hall-Petch relationship…”

Section: Toughening Mechanisms With Different Treatmentsmentioning

confidence: 99%

Process Routes of Low‐Ni Liquefied Natural Gas Tank Steel with Excellent Cryogenic Toughness

Chen

Zhang

Ren

et al. 2021

Herein, several treatments, including modified quenching and tempering (QT) treatment, ultrafast cooling and tempering (UFC‐T) treatment, and ultrafast cooling, intercritical quenching, and tempering (UFC‐LT) treatment, are used to prepare low‐Ni steels and compared with conventional QT treatment. All the three treatments enable low‐Ni steel to achieve equivalent or better cryogenic toughness than 9% Ni steel. The toughening effect of modified QT treatment is better than that of UFC‐T treatment, which is attributed to the increase in density of high‐angle grain boundaries (HAGBs). For modified QT treatment, fine equiaxed prior austenite grains and weak variant selection of martensite transformation lead to higher density HAGBs. Although the coarse elongated prior austenite grains in UFC‐T treatment are beneficial to Bain grouping and not conducive to matrix refinement, the intercritical quenching step in UFC‐LT treatment further increases the density of HAGBs due to the jagged prior austenite grain boundaries (PAGBs) and fresh martensite formed along PAGBs. Therefore, UFC‐LT treatment achieves high‐density HAGBs and excellent cryogenic toughness similar to modified QT treatment. Due to the wider tempering temperature process window, UFC‐LT treatment is the optimal process route for low‐Ni liquefied natural gas tank steel.

“…So, the propagation of the initiated cleavage cracks is limited, which can raise fracture toughness in the transition region in the form of a lower ductile–brittle transition temperature. [ 3 ] The existing delamination will transform the global plane‐strain fracture into a series of local plane‐stress failures, which cause a triaxial stress relaxation at the crack tip to blunt the crack. Thus, multidelamination cracks act like a sum of plane‐stress subunits, and the plastic deformation is enhanced, also known as delamination toughening.…”

Section: Introductionmentioning

confidence: 99%

A Two‐Step Thermomechanical Process for Improving High‐Angle Grain Boundary Proportion and Achieving Delamination Toughening in Hot‐Rolled Steel with a Limited Slab‐to‐Plate Ratio

Wang

Tian

et al. 2022

A two-step thermomechanical process is utilized to increase high-angle grain boundary (HAGB) proportion and achieve delamination cracks in hot-rolled steel produced by a limited slab-to-plate ratio. Three plates are austenitized at 900 °C and hot rolled with 65%, 57%, and 50% reduction at 880-800 °C after rolling at 1100-1050 °C followed by water cooling. Water quenching tests are carried out to prove the advantages of the two-step thermomechanical process. The steels having 69.9-71.8% HAGB proportion and delamination cracks are fabricated with a final thickness of 48 mm and a total rolling reduction ratio of 3.33. The high HAGB proportion is attributed to the abnormally refined prior austenite grain of 18.5 AE 10.5 μm caused by the heating temperature of 900 °C. Thus, 93-129% more effective interfacial area (193-229 mm À1 ) can be provided than the conventional process (100 mm À1 ) during rolling below the nonrecrystallization temperature of austenite. Abundant ferrite grains that form and impinge from the different prior austenite grains obtain 33.6-34.5% frequency distribution of the grain boundary misorientation angles, which fall into 20°-47°. The premises of delamination toughening are that the delamination cracks appear at the opposite side of the V-notch with a reduced number.