The Regulation Of Microstructure And Property Of As-cast Fe-Cr-B-Al Alloy

The main alloy element of as-cast Fe-Cr-B-Al alloy is China's rich Cr,B,Al.It has no noble elements such V,W,Mo.This material has not only low cost,high hardness,good wear resistance and good strength and toughness used as but also low as-cast hardness,good process and excellent machining properties.The Fe-Cr-B-Al alloy is a new type wear-resistant material,which can partly replace the traditional wear-resistant materials such as high chromium cast iron and high speed steel.The dissertation designs two groups of Fe-based alloy including Fe-12Cr-1.5B-xAl and Fe-10Cr-1.5B-2Al.The microstructure,hardness,wear resistance of as-cast and heat treated alloys and high temperature oxidation resistance of as-cast alloy were studied.Firstly,the effect of alloy elements on the structure and property,the solidification process,the as-cast and heat-treatment structure of the Fe-Cr-B-Al alloy have been investigated by means of optical microscopy?OM?,scanning electron microscopy?SEM?,EDS,X-ray diffraction?XRD?,high-temperature X-ray diffraction,and the Thermo-Cal phase diagram calculation.The results show that:?1?the as-cast microstructure of aluminum-free Fe-Cr-B alloy consists of the metallic and the eutectic phase distributed at the grain boundaries.The matrix is the mixture of martensite and a small amount of austenite.The eutectic phase consists of lamellar M₂?B,C?,discontinuous reticulation M₇?C,B?₃ and short rod or granular M₂₃?C,B?₆.The addition of aluminum element changes the matrix into pearlite and ferrite and has no obvious effect on the eutectic phase.When the content of aluminum is more than 6wt.%,a part of Al and Fe are combined to form iron and aluminum intermetallic compound.?2?The as-cast microstructure of Fe-10Cr-1.5B-2Al alloy consists of ferrite,pearlite and netlike eutectics which are distributed in the grain boundary.The eutectics mainly include herringbone M₂?B,C?and chrysanthemum M₇?C,B?₃.After1000¹150 ^oC austenitic quenching,the matrix gradually turns into single martenite with the increase of the quenching temperature.There is some white granular boride in fine matrix.The netlike boride almost totally fractures and transforms from the fish-bone structure to the graininess.There is some retained austenite in the quenched structures when the quenching temperature is more than 1100 ^oC,the amount of white granular precipitates in the matrix was obviously decreased,and the eutectic structure was obviously thicker.The matrix gradually turns into martenite,retained austenite and ferrite matrix after quenched at 1100 ^oC and tempered at 400⁶00 ^oC.As the tempering temperature increasing,the content of ferrite in the matrix increases gradually,the content of martensite decreases.The net structure of eutectic boron carbide is obviously broken.Secondly,by means of Rockwell hardness test,Vickers hardness test,the MM-200 block-on-ring wear testing machine under dry friction condition.The results of mechanics property of the as-cast Fe-12Cr-1.5B-x Al alloy and mechanics property and wear-ability of the Fe-10Cr-1.5B-2Al alloy after heat-treatment are:?1?The Rockwell hardness and Vickers hardness of the Fe-12Cr-1.5B-xAl alloy shows a trend of slow increase after sharp decrease with the increasing of the aluminum content.?2?Quenching treatment has a significant effect on the hardness of Fe-10Cr-1.5B-2Al alloy,and its hardness is greatly improved.The hardness of alloy reaches the maximum value?65.4 HRC?when the quenching temperature is 1100°C.As the quenching temperature continues to rise,the hardness has a slight decrease.?3?After quenched at 1100 ^oC and tempered at 400⁶00 ^oC,the hardness of Fe-Cr-B-Al alloy has no obvious change when the tempering temperature is lower than 450 ^oC.When the tempering temperature is higher than 450 ^oC,as the tempering temperature increasing,the hardness gradually decreases.And the effect of tempering temperature on the micro-hardness of alloy matrix is consistent with that of Rockwell hardness.The tempering cycles have no obvious effect on alloy hardness,however,the tempering time has obvious influence on the alloy hardness,when the tempering time is too short or too long,the alloy hardness is lower.When the tempering time is 4 h,the alloy hardness reaches the highest 62.5 HRC.?4?The wear resistance of Fe-10Cr-1.5B-2Al alloy after 1100 ^oC quenching is optimal.After quenched at 1100^oC and tempered at 400⁶00 ^oC,the wear resistance of the alloy increases first and then decreases with the increase of tempering temperature.Fe-Cr-B-Al alloy has excellent wear resistance while quenching at 1100 ^oC and tempering at 450 ^oC.At the end,this paper carried out cyclic oxidation experiment at high temperature by means of heat treatment furnace.The main results of this research are as follows:?1?In 800 ^oC/250 h cyclic oxidation experiments,Al-free Fe-12Cr-1.5B alloy has distinct oxide spallation.However,as Al content increasing,the cracking and spalling of the oxide film of Fe-12Cr-1.5B-x Al alloy gradually disappear.And they belong to total anti-oxidant level.The Anti-oxidant ability of alloys can be significantly improved with the increase of Al content.?2?Al-free Fe-12Cr-1.5B alloy formed the rich Si area in the surface,the distribution of Cr,Mn,Fe,O was more uniform.The oxides were mainly SiO₂,?Fe,Cr?₂O₃ and M₃O₄.It formed the Al-rich area in the surface of Fe-12Cr-1.5B-xAl alloy and Si-rich area disappearance,the distribution of Cr and Fe elements were more uniform.Al formed the Al₂O₃ oxide,mainly concentrated in the rich oxide film/substrate interface.As Al content increasing,the Al₂O₃ oxide layer is easier to form,which can effectively restrain the oxidation of Fe-12Cr-1.5B-x Al alloy attributing to its excellent liquidity in high temperature.Furthermore,it can form a good combination with Cr₂O₃ oxide film,so as to restrain the oxidation of the alloy effectively.