Preparation And Properties Of High Chromium Cast Iron/Low Carbon Steel Bimetals

As widely used wear resistant materials,high chromium cast irons(HCCIs)has the advantages of high specific strength and good wear resistance.The high Cr content in HCCI and the volume fraction of carbides lead to its obvious brittle characteristics together.At present,the common carbon steel or low alloy steel with good toughness and plasticity is combined with HCCIs by means of surfacing,composite casting and other technologies,which has become important methods to reduce the brittleness of HCCI.However,there are also a series of problems in practical applications,such as poor bonding force and surface cracking.In this work,HCCI/low carbon steel(LCS)bimetals were prepared by different processes.The bonding properties of the bimetals were studied and the HCCIs as wear resistant layers were strengthened by alloying and heat treatment,which improved wear resistance of the bimetal further.The microstructure of the materials was analyzed by metallographic microscope,scanning electron microscope(SEM),electron probe micro-analysis(EPMA),X-ray diffractometer(XRD)and transmission electron microscope(TEM).The properties of the bimetals were tested by means of hardness,shear strength,impact toughness and tensile strength.The abrasive wear properties of materials before and after alloying and heat treatments were studied.The main results are as follows:A Cr16 HCCI as the high hard and wear-resistant layer was bonded on the surface of low carbon steel(LCS)at 1360℃.The results show that the Cr16 HCCI and LCS were bonded well,the shear strength value is 540 MPa.There are no defects observed on the interface in terms of unbonded regions,microvoids and microcracks.C atoms diffused from the region of low concentration(LCS)to the region of high concentration(Cr16 HCCI)across the interface,being the uphill diffusion phenomenon.The thickness of the diffusion zone was about 55 μm.In the diffusion zone,the troostite consisting of M23C6 and ferrite showed up instead of M7C3 in the diffusion zone.The tensile strength of the bimetal was 448 MPa,the cleavage fracture appeared in the Cr16 HCCI side.The bending strength and impact strength of the bometal was 1208 MPa and 7.5 J·cm-2,respectively,both enhanced obviously compared with the Cr16 HCCI.6 wt%Mo was added into the Cr16 HCCI.A ledeburite matrix and Mo-rich M2C were obtained besides M7C3 in the as-cast alloyed HCCI.During the soaking process of the destabilization treatment,amounts of secondary carbides(M23C6)precipitated out of austenite.Martenstic transformation of matrix occurred in the forced air cooling process.Most of the M2C decomposed into M6C and punctate MC.The overall hardness of alloyed HCCI increased was attributed to the presence of Mo-rich carbides and martensite matrix with secondary carbides.Comparing with the destabilized sample,the toughness of the alloyed HCCI improved,growing with the increase of tempering temperature,while the hardness decreased after tempering treatment,the hardness of the QT600 sample was 47.8 HRC.The abrasive wear resistance of the HCCI,with the addition of Mo,improved after heat treatment.The abrasive wear resistance decreased with the increase of tempering temperature,due to the exsolution of C atoms in the martensitic matrix.The QT500 sample has the best wear resistance.The alloyed HCCI with excellent wear resistance was bonded to the LCS.and a heat treatment(quenching+tempering)was subjected to the bimetal.Diffusion behavior of atoms between alloyed HCCI and LCS was analyzed.The results show that the bimetal has good bonding quality because of the uphill diffusion of C atoms.A diffusion zone with width of about 37 μm formed in the alloyed HCCI side by the interface obtaining a structure of troostite.With different bonding time,there was no obvious change in the width of diffusion zone.The shear strength of the bimetal reduced markedly after heat treatments,specifically,which was decreased with the increase of tempering temperature.The impact toughness of the bimetal had a significant improvement by comparision with the alloyed HCCI.After quenching,the impact toughness of bimetal declined and the microhardness of the diffusion zone improved.The impact energy of the QT600 sample is as high as 7.3 J/cm2.Cladding HCCI/LCS bimetals,owning different Cr contents,were prepared.The micro structure of the cladding HCCI was compact,and no defect such as shrinkage,shrinkage and crack were observed.The carbide styles are mainly conposed of Cr-rich M7C3,M23C6.M3C and Mo-rich M2C,leading to the increase of wear resistance.The matrix was based on martensite at the edge of the carbides and pearlite inside the grains.The as-cast HCCI cladding layers exhibited high hardness and good wear resistant.HCCI cladding layers with different Cr contents and LCS have had built up good metallurgical bonding,the interface was clear and complete,and no micro-defects such as holes and cracks were observed.The diffusion of C atoms occurred on the interface between the HCCI cladding layers and LCS,and the diffusion direction was from the HCCI cladding layers to LCS,forming pearlite zones on the LCS side near the interface.There was a decrease in the thickness of the pearlite zones with the increase of Cr content.Diffusion layers with thickness of about 30 μm were constitued on the HCCI cladding layers side,and the microstructure in the diffusion zones was troostite.Compared with the HCCI cladding layers,the impact toughness of the HCCI/LCS bimetals was improved obviously.The bimetals with different Cr contents showed the excellent shear strength that all over 300 MPa,and reduced with the increasing Cr content.