Effects Of Heat Treatment On The Structure And Properties Of Low Carbon Alloy Steel For Marine Engineering

As the key structural material of marine engineering equipment,alloy steel is widely used in offshore drilling platform,production platform and submarine pipeline.The alloy steel is used in ocean engineering with long service time,poor working environment and the high maintenance cost,so it need to have high strength,high toughness,weldability,characteristics of strong corrosion resistance and to fit the high thickness,normalized production requirements.Due to the particularity of working environment,some of the key components in marine structure often need to adopt high-performance casting steel and forging steel.At present,the field of casting steel used in ocean engineering research in China is relatively weak and still need to rely on imports.Therefore,more and more research need to improve this situation.In this paper,the low carbon alloy steel is cast and forged which is independent designed and mainly applied in the field of marine engineering.Metallographic microscope,scanning electron microscope(SEM),energy spectrum analyzer,X-ray diffraction(XRD),universal tensile tester,electrochemical workstation and simulated seawater immersion corrosion analysis method were used to study the different austenitizing temperature,austenitizing heat preservation time,quenching and tempering temperature on the microstructure of low carbon alloy steel casting and forging state.The influence law of mechanical properties and corrosion resistance and mechanism of the best heat treatment process parameters were obtained in order to improve the comprehensive mechanics properties of alloy steel.The results show that the cast and forging annealing tissues of low carbon alloy steel are both ferritic and pearlite.The hardness of the cast alloy steel is HRC29,the tensile strength,yield strength,elongation and reduction of area are 835MPa,676MPa,8%and 33%,respectively.After forging annealing,the hardness of alloy steel is HRC28,and the tensile strength,yield strength,elongation and reduction of area at indoor temperature are 728MPa,614MPa,15.3%and 55.4%,respectively.In summary,the strength of alloy steel after annealing is reduced,while the toughness is increased compared to the cast.Between 840~920?,with the increase of austenitizing temperature and heat preservation time,the number of lath martensite in alloy increases after quenching,plate spacing increases and the hardness decreased after rising first.In 900?and 30 min after quenching,organization medium plate martensite reaches the most homogeneous tiny and the highest hardness.By using different quenching medium,the structure and hardness of alloy steel are the best when water quenching is adopted,the oil quenching is the lower and the air cooling is the lowest.Between 200~650?tempering,with the increase of tempering temperature,the alloy steel quenching organization gradually transform into tempered martensite,tempered troostite and tempered sorbite,strength gradually decline,plastic toughness,impact toughness increases after fall first.Temper brittleness happens during300~500?and occurs tempering secondary hardening phenomenon at 500?.In conclusion,the optimum heat treatment process of alloy steel is as follows:900?and 30min oil quenching and 600?tempering.After the heat treatment using the optimized process,the tensile strength,yield strength,elongation and reduction of area of the cast alloy steel are725MPa,682MPa,13.3%and 64.5%,respectively,and the impact toughness is 124J/cm~2.The tensile strength,yield strength,elongation,and reduction of area of alloy steel forging state are 706 Mpa,658 Mpa,15.5%and 66.5%respectively,and the impact toughness is 366 J/cm~2,In summary,the forged alloy steel has good comprehensive mechanical properties,which can be widely used in marine cable and offshore platform structure.The electrochemical test results show that with the increase of tempering temperature,corrosion resistance of the cast alloy steel and forged alloy steel in simulated seawater are increased and at 600?tempering corrosion resistance is the best.The state of forging is better than the cast alloy steel.The soak experimental results show that alloy steel forging state at the beginning of the simulation in the seawater corrosion rate decreases significantly at 600?tempering process.With the extension of soaking time,the corrosion rate performances tend to steady and corrosion products increase thickening.The corrosion products mainly consists of three phases including?-FeOOH,?-FeOOH and CrOOH.