| In the current research an attempt has been made to obtain high strength low alloy steel with high ductility and good weldability. With this purpose, samples with different compositions, ZG18MnCrMoRE and ZG18MnCrMoNiRE, were designed and taken into three different specialized heat treatment processes - annealing+ quenching and tempering, normalizing+quenching and tempering, annealing+ normalizing+quenching and tempering. Optical microscope, Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometer (EDS), Impact Testing Machine and Universal Testing Machine were employed to describe the effect of composition and heat treatment on microstructure and performance of samples in this research. Welding performance of ZG18MnCrMoNiRE was also investigated in this current research.The research results show that as-cast microstructure of ZG18MnCrMoRE consists of acicular ferrite, thin lamellar pearlite and a small amount of bulk ferrite. Thin lamellar pearlite shows accumulation and obviously segregated distribution. Microstructure after three different heat treatment processes are all composed of tempered sorbite remaining martensitic orientation. As-cast microstructure of ZG18MnCrMoNiRE is made up of bulk ferrite, pearlite and acicular ferrite. Tempered sorbite remaining martensitic orientation with lath-shaped ferrite is observed in the samples after annealing+quenching and tempering, normalizing+ quenching and tempering. After annealing+normalizing+quenching and tempering, microstructure of ZG18MnCrMoNiRE is tempered sorbite with equiaxed ferrite.Tensile strength of ZG18MnCrMoRE after annealing+normalizing+quenching and tempering is 990MPa which is 70 MPa higher than the samples after the other two heat treatment processes, and the yield strength shows a 70 MPa higher than the counterparts with the value of 885MPa. Impact toughness and elongation after annealing+normalizing+quenching and tempering are 35.2 J and12%, and show slight improvements,12 J and 1% respectively, than the other two heat treatment procedures. Tensile strength of ZG18MnCrMoNiRE after annealing+normalizing+ quenching and tempering is 1035 MPa which is 100 MPa higher than the samples after the other two heat treatment processes, and the yield strength shows a 70 MPa higher than the counterparts with the value of 885MPa. Impact toughness and elongation after annealing+normalizing+quenching and tempering are 56 J and16.5%, and show slight improvements,8.5J and 1.5% respectively, than the other two heat treatment processes.Results of EDS show that lath-shaped MnS-FeS inclusion in the center of dimples is refined and almost completely changed into spherical inclusion after inoculations with 0.2% Fe-Si-RE master alloys and 0.02% Ti. As a result, impact of inclusion to the toughness of samples significantly reduced and ductility of samples is 30% higher at ambient temperature.ZG18MnCrMoRE and ZG18MnCrMoNiRE both showed typical ductile fractures after three different heat treatments and mainly consisted of vast dimples and a small amount of tearing lath, and a lot of circular-shaped inclusions were observed in the center of dimples. Fracture morphology of samples after annealing+ quenching and tempering, normalizing+quenching and tempering contained a lot of large-scale dimples, and the dimples showed obvious non-uniform in size and distribution. In contrast, refinements and notably homogeneous distributions of dimples are observed after annealing+normalizing+quenching and tempering.MK·GHS90solid welding wire of 1.2mm diameter was employed to weld the ZG18MnCrMoNiRE under 250A welding current and 34V electronic pressure with welding speed of 350mm/min.150℃preheating was conducted before welding and heat input of welding was 15-25kJ/cm, after welding the samples were kept 250℃for 2h. Results of maximum hardness test of heat-affected zone and Y-block groove welding test showed that cold crack ratios of surface and cross-section are both less than 20% in certain experiment condition which is proved to effectively prevent cold crack in practical welding.In conclusion, the high strength low alloy steel with high ductility in this current research performed excellent combination properties and could be widely used in mining and mineral industries, especially in production of parts to make hydraulic support socket post. |
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