| Marine engineering equipment has long service time,high maintenance cost and harsh service environment.Therefore,the requirements for steel for marine engineering equipment are also getting higher and higher.It is required to have high strength,good low temperature toughness,corrosion resistance,large thickness,large specifications,etc.With the continuous improvement of requirements,the welding problem of marine engineering steel has become increasingly prominent.The welded joint becomes a weak zone in the welded structure.For the medium plates in marine engineering steel,how to eliminate the welding defects in the welding process under the premise of ensuring the welding efficiency,and obtain excellent and stable low temperature toughness and good corrosion resistance in the welding area after welding is the core problem to be solved urgently.Therefore,it is necessary to study the composition,process,microstructure and performance of welded joint in weak zones that affect service safety in marine equipment welded structures.In this paper,the austenite grain growth behavior of two kinds of marine steels with different compositions is explored from the austenite grain growth law of marine steel.The results show that in the two different composition systems of marine steel AH36 and EH36.The low carbon composition system of steel EH36 has more obvious inhibition effect on austenite grain growth at higher austenitizing temperature.At the same time,austenitizing temperature and holding time are positively correlated with grain size,but the effect of austenitizing temperature on grain size is more significant.The prior austenite grain size has an effect on the microstructure after quenching.The austenite with smaller grains after quenching contains a higher proportion of large angle grain boundaries that hinder crack propagation,and has better toughness.Through the experimental analysis and calculation,the austenite grain growth kinetic models of these two test steels were obtained,which can be used to predict the austenite grain size under different welding processes.The multi-pass laser-arc hybrid welding of DH36 marine steel plate was carried out and the microstructure of the joint was carefully partitioned.The microstructure of the top and bottom of the joint and the low temperature impact toughness at-20°C were studied.The results show that the microstructure of the FCWM in the top part of the joint is widmnstatten and acicular ferrite,and the microstructure of the BWM in the bottom of the joint and the top and bottom heat affected zones of the joint are bainite.The impact energy of the joints is greater than 110 J at different Charpy impact test positions(weld metal center,equivalent fusion line,0.5,1.0,1.5 mm outside the equivalent fusion line).The impact toughness at the weld metal center(WM-C)is poor.The main reason for the low toughness of WM-C in the top part of the joint is that V-notch penetrates a large number of welds reheated zone(ferrite coarsening).The main reason for the low toughness of WM-C at the bottom of the joint is that V-notch penetrates a large number of brittle zones containing upper bainite and most of the FCWM zone are reheated weld.Overall,the impact toughness at the bottom of the joint is more excellent.The reason for the different toughness is related to the proportion and distribution of the microstructure at the V-notch of the impact specimen.Through the observation of the fracture surface of the impact specimen,it is also found that there are obvious differences in the fracture morphology of the fracture surface corresponding to different microstructures.The effects of heat input(16 k J/cm,25 k J/cm,36 k J/cm,50 k J/cm)on the microstructure and low temperature impact toughness of coarse grain heat affected zone(CGHAZ)of welded joint were studied for EH550 marine steel.The results show that the microstructure of CGHAZ is bainite under different heat input.With the increase of heat input,the CGHAZ grains gradually coarsen,the lath bainite characteristics weaken,and the granular bainite characteristics enhance.When the heat input is 36 k J/cm,the CGHAZ grains are significantly coarsened and the width is about 55 % higher than that of 25 KJ/cm.The joint toughness was the best when the heat input was 25 k J/cm,mainly because V-notch penetrated a large number of fine grain heat affected zones.When the heat input is 50 k J/cm,the worst toughness is caused by V-notch penetrating a large number of CGHAZ with severely coarse grain.The proportion of high angle grain boundary in CGHAZ under different heat input is 28 %,27 %,19 % and 18 % respectively.The heat input in actual welding should be controlled at about 25 k J/cm.At the same time,the secondary cracks in the fracture of the impact specimen corresponding to CGHAZ were observed.It was found that no secondary cracks were found when the heat input was 16 k J/cm and 25 k J/cm.The average number of secondary cracks under 36 k J/cm and 50 k J/cm heat input is 38/mm and 20/mm respectively,and the average deflection times of secondary cracks are 23/mm and 5/mm respectively.The obtained statistical results can establish a certain relationship between the microstructure and toughness of welded joints.Salt spray corrosion experiments were carried out on EH550 Marine steel welded joint,and the influence of heat input on corrosion resistance of CGHAZ was studied.It was found that with the increase of heat input,the corrosion resistance of CGHAZ gradually decreased,and the thickness of the rust layer gradually increased.The decrease of corrosion resistance of CGHAZ is related to the coarsening of CGHAZ microstructure and the large and aggregated M-A constituents.After EDS analysis of the CGHAZ rust layer,it is found that the enrichment degree of Cr,Ni and Cu elements in the rust layer is different.Cr element has the most obvious enrichment and has a stronger protective effect on the matrix. |
