Study On Microstructure And Mechanical Properties Of 2100 MPa Secondary-hardening Ultra-high Strength Steel With High Toughness

Based on the current research and development of secondary hardening ultra-high strength steel, an advanced and economically steel contained W with high strength and toughness level of 2100 MPa has been smelted by VIM+VAR combined with the composition of A310 steel through rational design of alloy system. The relationship between mechanical properties and microstructure of tested steel-A315 has been studied during different heat treatment processes by means of mechanical testing, optical microscopy, TEM, SEM and X-ray diffraction in the paper. The tempering curve and optimal heat treatment have been established. The secondary hardening behavior, the law of the growth of austenite grain, the process of tempering and the mechanism of strengthening and toughening have been also investigated. The main results obtained are as follows:At the range of 950~1200℃, the prior-austenite grain of A315 steel increases exponentially with the increasing of heating temperature, and the relationship of its growth with the extending of time is in accordance with composite function model. The prior-austenite grain of A315 steel begins to coarsen and the rate of grain growth speeds up when the heating temperature is over 1100℃. The rate of grain growth decreases when the time is over 120 min. The growing can be divided into two stages: the inhibition growing up stage and the free growing up stage. By comparative analysis of the Beck, Hillert and Sellars models, it can be concluded that Sellars model has high accuracy to predict the austenite grain size of A315 steel.The tensile and yield strength which has the highest value at 1050℃, and impact toughness of the A315 steel rise firstly then drop with the temperature rising when the steel has been quenched at 950~1150℃.When the solution temperature is under 1050℃, the impact toughness of the steel is low. And a few of undissolved carbides exist on the impact fracture, such as M6 C. The main ingredients of the undissolved carbides are Fe, Cr, Mo and W. When the solution temperature is at 1050℃,the undissolved phases practically disappear. When the solution temperature is above 1050℃, the strength of the A315 steel drops and the prior-austenite grain of A315 steel increases. The optimum quenching temperature of A315 tested steel is about 1050℃.When tempered at 200℃ for 8h, the steel has high toughness since low yield strength, few coarse particles and the addition of impurity elements rarely at the range of 200~600℃. When tempered at 300℃,a large number of ε-carbides are precipitated in the martensite, the toughness falls and the strength of the steel rises. When tempered at 440℃, much coarse cementite lamina is precipitated in the martensitic laths and grains, which causes the strength constantly rising and the impact toughness down to the minimum value. The cementites are dissolved and the M2 C alloy carbides and reverted austenite begin to precipitate. The yield strength and tensile strength reach the peak value when tempered at 490℃ and 500℃ respectively. The impact energy reaches the highest value when tempered at 510℃. If the tempering temperature is over 560℃, the M2 C Alloy carbides and reverted austenite begin to coarsen.The strength of steel rises firstly then drop, which is attributed to coarsening of M2 C carbides and decreasing of its coherency with matrix of steel with increasing tempering time. The yield strength and tensile strength reach the peak value 2114 MPa and 1721 MPa when tempered for 5h and 8h respectively. With the extension of tempering time, AKU of the overall trend is decreasing. When tempering for 8h and 20 h, there are two peaks, 44 J and 38 J respectively.The optimum heat treatment system of A315 steel can be formulated as: oil quenching at 1050℃ for 1h; deep frozen at-73℃ for 1h, up to room temperature; tempering at 510℃ for 8h, air cooling.The A315 steel is strengthened mainly by lath martensite matrix with high dislocation and precipitation of M2 C carbides in the matrix and grain boundaries. The reverted austenite of thin-film promotes the toughness of the steel, which is precipitated among the boundaries of lath martensite.