PSI - Issue 30

G.N. Sleptsov et al. / Procedia Structural Integrity 30 (2020) 154–161

155

Sleptsov G.N. et al. / Structural Integrity Procedia 00 (2020) 000–000

2

methods for studying delayed fracture are based on this principle. Meanwhile, the mechanisms of formation and development of technological cracks are significantly different. Attempts to describe these phenomena within the framework of the general theory of damage accumulation greatly facilitate the practical application of these methods. However, for each specific case, for a specific interpretation of the experimental results, it is necessary to have a clear understanding of the mechanisms and laws of fracture processes. It is worth noting here that V.P. Larionov (2005), E. L. Makarov (2014) found that the assessment of the effect of hydrogen on the development and extension of cracks by methods of fracture mechanics significantly complicate the calculation models, or such an effect can be estimated only in local areas. Therefore, in the study of such delicate mechanisms as delayed fracture, first of all, an adequate formulation of the problem is required when planning experiments. Presently, the tasks of improving the reliability of welded joints are being solved by improving welding technologies, improving methods for estimating and optimizing quality control systems for welded joints, as well as developing scientifically based comprehensive approaches to the choice of methods for estimating welding technology. The analysis of the experimental and theoretical work carried out allowed us to argue that for estimate technological strength for each specific practical case, depending on the problem to be solved, it is necessary to use a set of parameters describing the process of forming a welded joint. Non-destructive testing systems based on high-performance computers, which provide information on the processes leading to the destruction of welded joints, on the features and patterns of evolution of the material microstructure in the process of destruction, are of great importance in optimizing the quality control systems for welded joints. According to Semashko N.A. (2002), of all non-destructive testing tools, the most promising is the method of acoustic emission (AE), which allows monitoring the process of damage accumulation in materials by registering acoustic waves caused by local dynamic rearrangement of the material structure. The aim of this work is to develop a comprehensive methodology for estimating the technological strength of welded joints in welding at low temperatures, based on the parameters reflecting the welding technology, weldability, hydrogen, force and deformation conditions of welding, and by improving the existing methods for estimating the technological strength by introducing modern equipment and systems for non-destructive testing. The main regulatory document for assessing the tendency of welded joints to form cold cracks in the Russian Federation is GOST 26388-84. This standard includes test methods for the resistance of carbon and alloy steels to cold cracking in the heat-affected zone and the weld metal. The basis of the standard is a set of methods for quantitatively assessing the resistance of steels to the formation of cold cracks, developed at MVTU im. NE Bauman, as well as the Implant technique proposed by A. Grangeon and studied by Sleptsov O.I. (1989). As technological test methods in GOST 26388-84, samples of the "sample-insert" type, cross test, technological samples of VMEI-MVTU and Tekken, Lehigh are proposed.

Nomenclature AE

acoustic emission;

volume of hydrogen evolved, cm 3 ;

2 H V

spec m mass of the sample, 100 g; D H

initial concentration of diffusion hydrogen;

2. Materials and Experimental Procedure 2.1. The improved Implant machine test method

As is known, a quantitative assessment of the technological strength of welded joints to cold cracks is carried out by determining the minimum nominal tensile stress in the weakened section, at which cracks begin to appear in the welded joint. The Implant machine test method was used to determine minimum nominal tensile stress. A feature of this method is that a constant load is applied from the outside to the tested welded insert specimens, starting from temperatures of 150 - 100 °C and below.