Application Of Phase Equilibrium Thermodynamic Method In Alloy Design For High Carbon Alloy Steel With Ultra Fine Carbides

The type,amount and size distribution of carbides have important effect on the process,microstructure and properties of the steel.Fine carbides in the steel are very important to improve the performance of strength-toughness and fatigue crack and so on. This kind of tool and die steel is divided two classes,one is high carbon high alloy steel, and another is high carbon low alloy steel.In order to improve the performance of wear-resistance,increasing the content of alloy elements in the low alloy steel,which makes the low alloy steel become medium alloy steel.Because of the amount of retained austenite after quenching of medium alloy steel is larger than that of low alloy steel,medium alloy steel should be tempered at higher temperature,which may make the tempering hardness decrease.At the same time,due to the large amount and high distribution density of carbides that easily gather in high carbon alloy steel,it is difficult to make carbides refine by normal forging and heat treatment.But for complicated forging and heat treatment,it increases the difficulty on the process and consume more energy source.Therefore,as far as development of new tool and die steel is concerned, it is theoretical and practical significance for researching alloy design method for high carbon medium and high alloy steel with fine carbides.In this paper,the new complete method of the alloy design for high carbon medium and high alloy steel with ultra-fine carbides is put forward through the phase equilibrium thermodynamic calculation and experiment results.For practical application, new medium alloy steel DM8B and high alloy steel DM7S are developed.The results of calculation and experiment of high alloy steel show,when the content of the Cr,W and Mo elements in the steel are rational,the amount of M₆C carbides increase and the M₂₃C₆ and M₇C₃ carbides are restricted,which makes the annealing carbides refine.The V content in the steel should be controlled in order to avoid appearing eutectic MC carbides.Through the metastable equilibrium calculation, the tempering carbide phases contain M₂₃C₆,M₆C and MC stable carbides and M₇C₃, M₃C and M₂C metastable carbide,which is agreement with experimental results. The phase equilibrium thermodynamic calculation should be corrected for the medium alloy steel deviating equilibrium states in the practical annealing treatment.The Cr/(W+2Mo)ratio lower,the amount of M₆C carbides larger.The rational partition of M₆C,M₂₃C₆ and MC is advantaged to carbides refinement.So according to the amount of M₆C carbides calculated by thermodynamics,the medium alloy steel with fine carbides can be obtained.The balanced carbon formula Cp=0.016W+0.063Mo+0.099Cr+0.19V of high alloy steel is put forward.The tempering hardness value can be obtained from HRC=a(1+b)/(0.0127a+0.00267),in which a is the saturation level of carbon in the matrix and b is correction factor.The quenching hardness value of the medium alloy steel can be calculated from HRC=α(1+β)/(0.00915α+0.00586),in whichαis the square root of carbon content in the matrix andβis correction coefficient of solid solution strengthening.This formula implies the relationship of hardness measurement and martin site strengthening.High carbon high alloy steel DM7S with ultra-fine carbides is developed.This steel's annealing carbides contain M₆C,M₂₃C₆ and MC.The annealing carbides average size is 0.42μm,undissolving carbides are primarily M₆C and MC and about 0.485μm in diameter.This steel has the value of hardness HRC62.8 and other excellent mechanical properties.High carbon medium alloy steel DM8B with ultra-fine carbides is developed.This steel has M₆C,M₂₃C₆ and MC three types of annealing carbides which size is primarily in the range of 0.2～0.5μm.Undissolving carbides is M₆C,M₂₃C₆ and MC and the average size is 0.33μm.The steel has the hardness of HRC65.8 after quenching and tempering hardness is HRC62.4～63.8 after tempering at 240℃.The microstructure and properties are agreement with calculation results.The high carbon medium and high alloy steel with ultra-fine carbides and excellent mechanical properties are obtained by rational alloy design which is significant for the development of the high carbon alloy steel with ultra-fine carbides.The new complete research method is provided by the alloy design for high carbon alloy steel.