Brittle Crack Arrestability and Inverse Fracture in DWTT

The drop-weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle crack propagation, and the shear area fraction SA% in the DWTT has been adopted in the requirement for the linepipe steels. However, recent studies have pointed out the issue of ambiguity in evaluation of the DWTT when a ductile crack initiates from the notch and then transits to a brittle crack during ductile crack propagation. This fracture behavior is termed “inverse fracture.” According to the API Recommended Practice 5L3 (API RP 5L3), a test is considered invalid when a DWTT specimen shows inverse fracture. In this case, it is difficult to examine the acceptance criterion (85% shear area transition temperature) for linepipe steels. Because the purpose of the DWTT is to evaluate the brittle crack arrestability of the steels in a pressurized linepipe, the DWTT results should be examined with a propagating brittle crack arrest test. A large-scale brittle crack arrest test called the West Jefferson test is generally conducted to reproduce the crack propagation and arrest behavior in actual linepipes. However, it is somewhat difficult to control the lower test temperature and to initiate brittle crack in recent high-toughness steels in this burst test. Although the test stress conditions of the uniaxial tension in the plate tension brittle crack arrest test and the biaxial tension in a pressurized pipe are different, the plate tension brittle crack arrest test has the advantages of accurate control of the test temperature, test stress, and brittle crack initiation in comparison with the actual pipe burst test. Therefore, in this study, the brittle crack arrestability of linepipe steel which showed inverse fracture in the DWTT was investigated by conducting plate tension brittle crack arrest tests under an isothermal condition (crack arrest temperature test (CAT test)), which simulates the condition of the actual pipelines in service. This study also investigated the local shear lip thickness fraction in the CAT tests together with the shear area fraction SA% measured in DWTTs. Based on the results, the effect of brittle crack arrestability on inverse fracture appearance in the DWTTs was discussed in comparison with the brittle crack arrest behavior in the CAT tests.

Rational Alloy Design of Niobium-Bearing HSLA Steels

In the 61 years that niobium has been used in commercial steels, it has proven to be beneficial via several properties, such as strength and toughness. Over this time, numerous studies have been performed and papers published showing that both the strength and toughness can be improved with higher Nb additions. Earlier studies have verified this trend for steels containing up to about 0.04 wt.% Nb. Basic studies have shown that the addition of Nb increases the recrystallization-stop temperature, T5% or Tnr. These same studies have shown that with up to about 0.05 wt.% of Nb, the T5% temperature increases in the range of finish rolling, which is the basis of controlled rolling. These studies also have shown that at very high Nb levels, exceeding approximately 0.06 wt.% Nb, the recrystallization-stop temperature or T5% can increase into the temperature range of rough rolling, and this could result in insufficient grain refinement and recrystallization during rough rolling. However, the question remains as to how much Nb can be added before the detriments outweigh the benefits. While the benefits are easily observed and discussed, the detriments are not. These detriments at high Nb levels include cost, undissolved Nb particles, weldability issues, higher mill loads and roll wear and the lessening of grain refinement that might otherwise occur during plate rough rolling. This loss of grain refinement is important, since coarse grained microstructures often result in failure in the drop weight tear testing of the plate and pipe. The purpose of this paper is to discuss the practical limits of Nb microalloying in controlled rolled low carbon linepipe steels of gauges ranging from 12 to 25 mm in thickness.

Inverse Fracture in DWTT and Brittle Crack Behavior in Large-Scale Brittle Crack Arrest Test

The drop weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle fracture propagation. Although there is an ambiguity in the evaluation of DWTT results if inverse fracture appears on the fracture surfaces, the cause of inverse fracture is not yet fully understood. In the present work, DWTTs were performed with X65, X70, and X80 steel linepipes. In addition to the conventional DWTT specimen with a pressed notch (PN), PN specimens with a back slot (BS) and specimens with a chevron notch (CN) or static precrack (SPC) were also examined, and the fracture appearances in different strengths and different initial notch types were compared. Although the frequency of inverse fracture in these DWTTs was different with each material and each specimen type, there was no material or specimen type that was entirely free from inverse fracture. The purpose of the DWTT is to evaluate the brittle crack arrestability of the material in a pressurized linepipe. Therefore, the DWTT results should be examined with a running brittle crack arrest (BCA) test. A large-scale BCA test with temperature gradient was also performed with the X65 mother plate, and the shear area fraction measured in the DWTT fracture surface was compared with the local shear lip thickness fraction in the BCA test. Based on the results, the count of inverse fracture in the DWTT was discussed in comparison with the long BCA behavior in the BCA test.