CHANGE OF STEEL’S YIELD STRESS OVER SHIP’S SERVICE LIFE AND ITS EFFECT ON THE FIRST YIELD HULL GIRDER BENDING MOMENT

The publication deals with the decrease of the yield and tensile stress of high tensile shipbuilding steel AH-32 over ship’s service life and its effect on the first yield bending moment as a representative of the hull girder capacity. An example is given for a sample 25K DWT (25 thousand tons deadweight) bulk carrier. The probability of failure is calculated as the probability of the total hull girder bending moment exceeding the first yield bending moment. The probabilistic distributions of yield and tensile stress are obtained from laboratory test of the specimen of AH-32 steel (corroded plates of a 20 year old ship). It is found that although the decrease of yield stress may not be great, the increase of the probability of failure (i.e., the probability yield bending moment) could be substantial.

Structure heterogeneity and mechanical properties studied in thickness up to 100 mm of low-alloyed shipbuilding steel sheets with a yield strength not less than 420 MPa

This paper presents a study of changes in the structure and properties in thickness of rolled sheets up to 100 mm of low-alloyed shipbuilding steel with a yield point not less than 420 MPa. The fracture surface of samples after impact bending tests at low temperatures was investigated. It was found that the combination of the parameters of lath morphology bainite (fraction, areas average size and length) and the size of structural elements at given tolerance angles of 5 and 15° (indicating the presence or absence of a developed subgrain structure of deformation origin) determine the level of impact work at low temperatures testing.

Deformation-free submerged-arc welding of butt joints of sheet shipbuilding steel A

В работе приведены результаты экспериментального исследования способа автоматической сварки под слоем флюса плоских листовых конструкций обеспечивающего существенное снижение уровня сварочных деформаций, равномерность распределения твердости, как по поверхности стыкуемых кромок, так и в поперечном сечении сварного шва, минимизацию зон термического влияния и отсутствие в них закалочных структур. Способ основан на сдерживании процесса последовательного прохождения полиморфных превращений в формируемом металле шва до момента окончания сварки и последующем отпуске всего соединения. Полученные сварные соединения, удовлетворяют требованиям нормативной документации, регламентирующей их форму и размерность. Установлено, что применение этого способа позволяет осуществлять одностороннюю однопроходную автоматическую сварку без разделки кромок и зазора на режимах традиционной двусторонней сварки, а также существенно повысить глубину проплавления. Для реализации процесса разработан лабораторный стенд, позволяющий осуществлять автоматическую сварку без установки сварочного трактора на поверхность соединяемых деталей. Показана неэффективность расчетных методов определения напряжений и деформаций при сварке под слоем флюса. The paper presents the results of an experimental study of the method of automatic welding under a layer of flux of flat sheet structures, which provides a significant reduction in the level of welding deformations, uniformity of hardness distribution, both on the surface of butting edges and in the cross section of the weld, minimization of heat-affected zones and the absence of hardening structures. The method is based on restraining the process of sequential passage of polymorphic transformations in the formed weld metal until the end of welding and subsequent tempering of the entire joint. The resulting welded joints meet the requirements of the normative documentation governing their shape and dimension. It was found that the use of this method allows one-sided single-pass automatic welding without cutting edges and a gap in the modes of traditional two-sided welding, as well as significantly increasing the penetration depth. To implement the process, a laboratory stand has been developed that allows automatic welding without installing a welding tractor on the surface of the parts to be joined. The inefficiency of calculation methods for determining stresses and strains in submerged arc welding is shown.

Long-Term Marine Environment Exposure Effect on Butt-Welded Shipbuilding Steel

Extreme environments, such as marine environments, have negative impacts on welded steel structures, causing corrosion, reduced structural integrity and, consequently, failures. That is why it is necessary to perform an experimental research sea exposure effect on such structures and materials. Research presented in this paper deals with the mechanical behavior of butt-welded specimens made of AH36 shipbuilding steel when they are exposed to a natural marine environment (water, seawater, sea splash) for prolonged periods (3, 6, 12, 24, and 36 months). The usual approach to such research is to perform accelerated tests in a simulated laboratory environment. Here, relative mass change due to corrosion over time is given along with calculated corrosion rates. Corroded surfaces of specimens were inspected using optical and scanning electron microscopy and comparison, based on the numbers and dimensions of the corrosion pits (diameter and depth) in the observed area. As a result, it can be concluded that exposure between 3 and 6 months shows significant influence on mass loss of specimens. Further, sea splash generally has the most negative impact on corrosion rate due to the combined chemical and mechanical degradation of material. Pit density is the highest at the base metal area of the specimen. The diameters of the corrosion pits grow over the time of exposure as the pits coalesce and join. Pit depths are generally greatest in the heat affected zone area of the specimen.

Optimization of strength, ductility and wear resistance of low-carbon grade A shipbuilding steel by post-ECAP annealing

Effect of equal channel angular pressing (ECAP) and subsequent annealing on the microstructure, mechanical properties and tribological behavior of grade A shipbuilding steel was investigated. Coarse-grained microstructure of initial sample is eliminated during the ECAP at 375 °C and more refined microstructure is achieved. Post-ECAP annealing at 575 °C results in some grain growth, and also the cementite lamellae in the pearlite colonies is broken and got spheroidised during the annealing treatment. ECAP increases strength and hardness of steel significantly while it decreases the ductility. On the other hand, post-ECAP annealing treatment brings about an increase in ductility and diminishes the hardness, as expected. Strain hardening capacity, ductility, hardness and oxidation rate were found to be the main factors affecting wear resistance of the grade A steel. It was found that high hardness and strength, good wear resistance with sufficiently high ductility can be achieved in the grade A steel by applying ECAP + annealing processes.