Research On Hardenability Of CM690 Anchor Steels

As a main part of the ships as well as ocean engineering,the mechanical properties of anchor play an important role on the loading capacity and speed of ships,which also determines the security of the whole system.Considering the extremely hard working conditions and large self-weight,the anchor is supposed to possess not only high strength and good toughness but also a good corrosion resistance and fatigue resistance.CM690,an outstanding representative of three-stage anchor steels,was widely used in practical production.The mechanical properties of CM690 depend on the chemical components of the steel as well as the heat treatment conditions.The large size of CM690,which can be up to φ280mm,makes it difficult to be hardened during thermal refining.The formation of non-martensite microstructure will degrade the fatigue resistance performance and reduce the strength and hardness after tempering,and the mechanical properties of CM690 also go down with it.Therefore,having a systemic study on the hardenability of CM690 anchor steels is of great importance in both theoretical analysis and the development of processing technology.In this research,two types of quenching test were designed.One was Jominy test,which consists of step-up Jominy test and step-down Jominy test both at 800℃,840℃,880℃,920℃ and 960℃.The other was direct quenching test,including step-down direct quenching test at 960℃,920℃,880℃,840℃,800℃and quenching at 800℃for 1h,2h,4h.The hardenability of CM690 anchor steels was investigated by means of optical microstructure analysis,Rockwell hardness tests and SEM analysis,and the influence of different austenitizing temperature,austenitic grain size and the law of microstructure evolution of CM690 anchor steels in quenching process was mainly discussed,respectively.With the austenitizing temperature increasing from 800℃ to 960℃,the hardenability of CM690 anchor steels increased as well.Except good hardenability,the substructure of steels and other factors also have an important impact on mechanical properties.When quenching at 960℃,the martensite became coarsening compared to that of lower quenching temperature,which resulted in decreased martensite property and degradation of overall performance.In conclusion,880℃-920℃ are the suitable range of quenching,during which fine lath martensite and acicular martensite were obtained,leading to outstanding hardenability as well as mechanical properties.The microstructure and hardness change of CM690 anchor steels is more obvious in Jominy test,as the height rises,the cooling rate decreasing leads to the hardness gradually decreasing,martensitic transformation,bainite transformation,ferrite eduction and pearlite transformation occur in sequence,making the hardness descends gradually.It was found that the sensitive area of CM690 end quenched samples was at the height of 7-15 mm,where ferrite eduction will occur.With the decrease of cooling rate,both microstructure and hardness show obvious changes,so the cooling rate of this area is supposed to be carefully controlled in the practical production application.Therefore,multiphase structure with different component proportion could be obtained by controlling different cooling rate in quenching process.Hardenability and hardening capacity are both key factors in improving the mechanical properties of steels.With the increase of austenite grain size,the hardenability of CM690 steels increases while the hardening capacity decreases,so it’s unscientific to improve hardenability by only increasing austenitic grain size.In practical production,reasonably performing comprehensive analysis on the influence of austenite grain size between hardenability and hardening capacity is the necessary guarantee of selecting suitable heat treatment conditions as well as obtaining optimal performance.