Effect of Restraint Intensity of Joint on Lamellar Tearing (2nd Report)

Assessment by H-type Restraint Cracking Test

Abstract

In the previous paper, multirun RRC (Rigid Restraint Cracking) test and Modified Cranfield test were carried out for evaluating the susceptibility of lamellar tearing of SM41 and HT50. As the result, the phenomena of lamellar tearing was deeply dependent on the distribution of non-metallic inclusion (mainly Mn-S of A-type inclusion) and the restraint condition of joint, with discussing the relation between the susceptibility of lamellar tearing in the both testing methods and through-thickness tensile properties of steels.
In this report, for the application to an actual structure, it is examined to get a standard for selection of plate material in the point of view of the prevention of lamellar tearing by a H-type restraint cracking test with an interposed plate.
Conclusions obtained are as follows.
(1) Higher sulphur content and restraint intensity (K) increase crack susceptibility. Also, good correlation between the susceptibility of lamellar tearing and through-thickness properties, such as the reduction of area at Z-direction, the ratio of transverse shrinkage by Brodeau-type test, or Inclusion Shape Factor in microstructure. Therefore, these may be available as an acceptance criteria for lamellar tearing.
(2) The changes of leg length and groove geometry influence to the critical value of crack initiation in the above tests.
(3) As for the susceptibility for lamellar tearing, good correlation between crack sensitivity parameters in Modified Cranfield test and H-type restraint cracking test is obtained. So, the susceptibility of lamellar tearing may be evaluated with the parameter obtained by Modified Cranfield test, which is simple and effective in reproducible, by clarifying the above correlation.
(4) The susceptibility of lamellar tearing at interposed structure, as an example of the application of an actual structure in shipbuilding, is determined by calculating the value of restraint intensity of joint. As a result, grade of steel plate adopted may be selected by the above mentioned criteria for the prevention of lamellar tearing.