| ZG40Cr25Ni20Si2 steel has the characteristics of good high-temperature mechanical properties,oxidation resistance,corrosion resistance and organizational stability.Therefore,it has been widely applied in parts such as automobile exhaust manifold,turbocharger housing,steam turbine shell.However,it is found that the coarse matrix and more interdendrite carbides easily reduce the thermal fatigue performance of casting in manufacture,lowering service life.In this paper,the effects of pouring temperature and alloying treatment on microstructure and mechanical properties of the steel are explored to reveal the strengthening mechanism of the steel,providing theoretical support for the production of high quality heat resistant stainless steel castings.The results show that:The microstructure of as-cast ZG40Cr25N20Si2 steel consists of austenite dendritic crystals and interdendrutic carbides.The important ways of improving high-temperature strength and thermal fatigue property of the alloy include refining and strengthening the austenitic matrix,reducing the number of interdendritic carbides and improving the morphology and distribution of interdendritic carbides.In the case of multi-addition of Al,Ti,V,W elements,the Al and Ti elements can significantly refine the austenite grains,and the V and W elements will promote the carbide precipitation,moreover the V elements can obviously improve the morphology and distribution of intergranular carbide.With increasing the contents of Al,Ti,W elements,the hardness of the alloy significantly rises.When the alloys multi-alloyed with the element of V,Ti,W elements,the microstructure of the matrix will be refined,while the content of carbide dramatically will increase,it demonstrates that the addition of A1 element will forcefully promote the formation of interdendritic carbides when the alloy contains the promoting carbide elements.Under the fixed composition of the alloy,with the increase of Al content,on one hand,the austenite grains of ZG40Cr25Ni20Si2 steel will be obviously refined,and matrix morphology transforms from developed dendritic into equiaxed morphology,on the other hand,the content of carbide will increase and be refined,it will change from long rod-shaped to discrete blocky or globular morphology.Accordingly,the hardness will increase up to 175HB and 181HB.To some degrees,the addition of alloy elements can play the effect of fine-grain strengthening,dispersion strengthening and solid solution strengthening.Among them,the element of A1 has the distinct effect of fine-grain strengthening and solid solution strengthening.However,when the content of A1 is more than 1.5wt%,Al-riched phase distributed in the inner of austenite grain will weak the mechanical properties of the alloy.Thus,the content of Al should not be more than 1.5wt%in ZG40Cr25Ni20Si2 steel.The element of Ti can significantly refine matrix grain and has the effect of fine-grain strengthening,but when the content of Ti is more than 1.0%,the content of carbide will increase,and the element of Ti will reduce the mechanical performance of the steel.The element of V,which can refine the carbide and greatly improve the morphology and distribution of carbide,has the effect of dispersion strengthening.While the element of W can not only solutionize in austenite,but be easy to form carbide and increase the content of carbide.Therefore,to some extent the element of W also has the effect of dispersion strengthening.With the increase of pouring temperature,the size of austenitic secondary arm spacing gradually increases,and the dendritic morphology of grows stronger.The intergranular carbide gradually reduces and be refined,which distributes in grain boundary and innergrain.The tensile strength and elongation of the alloy will increase obviously,especially when pouring temperature is 1628?,the tensile strength reaches 495MPa and the elongation is up to 16%. |
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