Hot rolling mill roll microstructure interpretation: A computational thermodynamics study

The microstructure of the interface of a high chromium cast iron/ductile cast iron composite hot rolling mill roll has been interpreted with the help of Fe-Cr-Si-C laboratory alloys and thermodynamic calculations of their solidification paths. It has been shown that the mill roll presents two limiting microstructures: the ductile cast iron core and the white cast iron shell. While the core is heavily contaminated with chromium, originating from the partial dissolution of the shell during core pouring, the shell shows a typical high chromium white cast iron micro-structure, since it remains solid during processing. The high chromium contents in the core originate a typical “mottled” cast iron microstructure, combining nodular graphite and ledeburite. The region between the two limiting microstructures is called interface. As the distance from the core toward the shell increases in the interface, the content of chromium increases but that of silicon decreases. The resulting microstructural gradient shows ledeburitic M₃C, hexagonal-section M₃C, duplex M₃C/M₇C₃ carbides, and M₇C₃ carbide, which could be well correlated with the calculated solidification paths of the laboratory samples.     

Microstructure and properties of high chromium cast irons: effect of heat treatments and alloying additions

Abstract

Different combinations of critical and subcritical heat treatments variously modify the initial as cast microstructure of high chromium white cast irons leading to secondary carbide precipitation of different extent and nature. Destabilisation (critical heat treatment) of austenite at 970°C for 2·5 h followed by annealing (subcritical heat treatment) at 600°C for 13 h results in massive precipitation of M₂₃C₆ carbide particles along with spheroidised M₇C₃. The reversed order of heat treatments leads to extensive precipitation of M₇C₃ secondary carbide particles. Mo has a favouring effect on the hardness of the microstructures containing pearlite by limiting pearlite formation. The gradual increase in the alloying additions, C and Cr, increases the hardness of the materials at the different treatment states by inducing carbide precipitation. The increase in the Si content leads to the opposite effect by favouring pearlite formation.     

含Cr13%高铬铸铁的试验研究

在试验的基础上研究了含Ｃｒ１３％的高铬铸铁的力学性能和组织特点,发现含硅量和变质处理对不同基体的高铬铸铁具有相似的影响规律;根据ＦｅＣＣｒ三元相图对Ｃｒ１３％高铬铸铁的凝固过程进行了分析,从一个新的角度揭示了铸造条件、含硅量和变质质处理对高铬铸铁性能和组织的影响机理。     

Effect of TiC Particle Additions on Structure and Properties of Hypereutectic High Chromium Cast Iron

The present work studies the effects of titanium carbide (TiC) particle additions on the microstructure and mechanical properties of hypereutectic high chromium cast iron containing about 20%Cr and 4.0%C by the optical microscopy (OM), the transmission electron microscopy (TEM), the X-ray diffraction (XRD) analysis, the digital image analysis system, impact tester, and hardness tester. The carbides of high chromium cast iron are refined gradually and the shape of the primary M₇C₃-type carbides became more isotropic with the increase of TiC particles. When the addition of TiC particles exceeds 1.0 wt.%, the change of microstructure is not obvious. The hardness of high chromium cast iron has a slight increase and impact toughness has a obvious increase with the increase of TiC additions. However, when the addition of TiC particles exceeds 1.0 wt.%, the impact toughness has no obvious change.     

Comparative Analysis of the Microstructural Features of 28 wt.% Cr Cast Iron Fabricated by Pulsed Plasma Deposition and Conventional Casting

The effect of pulsed plasma deposition (by an electrothermal axial plasma accelerator) followed by post-heat treatment on the structure and microhardness of a 28 wt.% Cr white cast iron is analyzed and discussed with respect to the microstructure of the conventionally cast monolithic counterpart. The cast iron (as deposited on a 14 wt.% Cr cast iron substrate) had a microhardness of 630-750 HV_0.05; it had layered light contrast/dark contrast structure where dark contrast layers contain fine carbide network. Pulsed plasma deposition followed by heat treatment resulted in a substantial refinement of the microstructure: eutectic M₇C₃ coarse acicular plates in the conventional cast iron were replaced by fine M₇C₃, M₃C₂, M₃C particles (Cr depleted in favor of Fe), while the initial carbide particle of 2-3 μm was reduced to 0.6 μm. Secondary dendrite arm spacing decreased from 15 to 1.3 μm, accordingly. The carbide volume fraction in the post-heat-treated coating remarkably increased with respect to the conventional counterpart resulting in a substantial increase in the coating hardness (1300-1750 HV_0.05). The heat-treated coating displayed higher resistance to three-body abrasion than the as-deposited coating and similar resistance with that of the conventionally cast iron.     

Enhancement of wear resistance and impact toughness of as cast hypoeutectic high chromium cast iron using niobium

The effect of niobium additions on the as-cast microstructure of a hypoeutectic high-Cr cast iron containing 2.2 wt.% C and 16.5 wt.% Cr was investigated. With increasing niobium content, the eutectic M₇C₃ carbides were refined and became less elongated as well as its volume fraction was decreased gradually. The first precipitated NbC particles could be act as heterogeneous substrate of proeutectic austenite and a barrier to M₇C₃ grain growth. The morphology of NbC carbides changed with increasing niobium content. Such NbC particles were increased with increasing Nb content and subsequently contributed to increased hardness. Optimum toughness was obtained for the irons alloyed with 2.14% Nb. The effects of applied load and Nb-additions on wear resistance of high chromium cast iron have been studied. The results showed that wear resistance increases with increasing Nb addition. Furthermore, the effectiveness of the NbC particles on the weight loss was more evident at higher loads.     

Mechanical and Tribological Properties of HVOF-Sprayed (Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-NiCr+Ni) Composite Coating on Ductile Cast Iron

The aim of the investigations was to compare the microstructure, mechanical, and wear properties of Cr₃C₂-NiCr+Ni and Cr₃C₂-NiCr coatings deposited by HVOF technique (the high-velocity oxygen fuel spray process) on ductile cast iron. The effect of nickel particles added to the chromium carbide coating on mechanical and wear behavior in the system of Cr ₃ C ₂ -NiCr+Ni/ductile cast iron was analyzed in order to improve the lifetime of coated materials. The structure with particular emphasis of characteristic of the interface in the system of composite coating (Cr ₃ C ₂ -NiCr+Ni)/ductile cast iron was studied using the optical, scanning, and transmission electron microscopes, as well as the analysis of chemical and phase composition in microareas. Experimental results show that HVOF-sprayed Cr₃C₂-NiCr+Ni composite coating exhibits low porosity, high hardness, dense structure with large, partially molten Ni particles and very fine Cr₃C₂ and Cr₇C₃ particles embedded in NiCr alloy matrix, coming to the size of nanocrystalline. The results were discussed in reference to examination of bending strength considering cracking and delamination in the system of composite coating (Cr ₃ C ₂ -NiCr+Ni)/ductile cast iron as well as hardness and wear resistance of the coating. The composite structure of the coating provides the relatively good plasticity of the coating, which in turn has a positive effect on the adhesion of coating to the substrate and cohesion of the composite coating (Cr₃C₂-NiCr+Ni) in wear conditions.     

电磁复合铸造轧辊工艺研究

利用自制的电磁复合铸造试验装置,研究了电磁复合铸造轧辊的基本工艺过程,并对高铬铸铁复合轧辊耐磨层的性能和复合情况进行了分析.结果表明:在合理的工艺条件下,可以得到界面结合良好、组织优良、耐磨层硬度分布均匀且符合要求的复合轧辊,探索了一条铸造复合轧辊的新思路.     

Effect of surface Cr/C infiltration on microstructure,mechanical properties and wear resistance of high chromium cast iron

A new type of high chromium cast iron (HCCI) was prepared, and its microstructure, mechanical properties, and abrasion resistance were investigated systematically. Results showed that after surface carburizing and chromizing, the microstructure of HCCI mainly consists of martensite, boride (M₂B), and carbide (M₇C₃), accompanied with a large amount of secondary precipitations M₂₃C₆. Moreover, the morphology and hardness of the carbide and boride in HCCI change little, while the volume fraction of carbide and boride increases from 16.23% to 23.16%. This effectively increases the surface hardness of HCCI from 64.53±0.50 HRC to 66.58±0.50 HRC, with the result that the surface of HCCI possesses a better abrasion resistance compared to the center position. Furthermore, the wear mechanism of HCCI changes from micro-plowing to micro-cutting with the increase of surface hardness.

     

The Kinetics of Phase Transformations During Tempering in Laser Melted High Chromium Cast Steel

The precipitation of secondary carbides in the laser melted high chromium cast steels during tempering at 300-650?°C for 2?h in air furnace was characterized and the present phases was identified, by using transmission electron microscopy. Laser melted high chromium cast steel consists of austenitic dendrites and interdendritic M₂₃C₆ carbides. The austenite has such a strong tempering stability that it remains unchanged at temperature below 400?°C and the secondary hardening phenomenon starts from 450?°C to the maximum value of 672 HV at 560?°C. After tempering at 450?°C fine M₂₃C₆ carbides precipitate from the supersaturated austenite preferentially. In addition, the dislocation lines and slip bands still exist inside the austenite. While tempering at temperature below 560?°C, the secondary hardening simultaneously results from the martensite phase transformation and the precipitation of carbides as well as dislocation strengthening within a refined microstructure. Moreover, the formation of the ferrite matrix and large quality of coarse lamellar M₃C carbides when the samples were tempered at 650?°C contributes to the decrease of hardness.     

Development of boron white cast iron

Abstract

With boron substituting for carbon in cast iron composition and eutectic borides substituting for eutectic carbides in microstructure as the hard wear resistant phase, a new kind of wear resistant white cast iron has been developed. The microstructure and mechanical properties of this new white cast iron both in the as cast state and after appropriate heat treatments were studied. The results show that the as cast microstructure of the boron white cast iron comprises a dendritic matrix and interdendritic eutectics, and the eutectic compound is that of M₂B or M′_0˙9Cr_1˙1B_0˙9 type, where M represents Fe, Cr or Mn and M′ represents Fe or Mn. The morphology of the eutectic borides is much like that of carbide in high chromium white cast iron, but the hardness of boride is higher than that of carbide. The matrix in as cast microstructure comprises martensite and pearlite. After austenitising and quenching, the matrix mostly changes to lath type martensite and the eutectic borides remain unchanged. In addition, two different sizes of particles, with different forming processes during heat treatment, appear in the matrix. The boron white cast iron possesses higher hardness and toughness than conventional white cast iron and nickel hard white cast iron, and has a better balance between hardness and toughness than high chromium white cast iron.     

含Nb元素堆焊层金属中碳化物的析出行为

以堆焊连铸辊为研究对象,研制三种不同合金元素Nb加入量的药芯焊丝,采用金相显微镜和扫描电镜对其显微组织、碳化物形貌进行了观察. 采用X射线衍射仪对其相结构进行了测定. 采用Thermo-Calc软件对含铌堆焊层金属中碳化物的析出行为进行分析. 结果表明,堆焊层金属显微组织为铁素体、M₂₃C₆和MC. 随着Nb元素含量增加,其显微组织得到细化,NbC沿晶界析出. 热力学计算结果表明,析出碳化物主要为MC,M₂₃C₆. 随着Nb元素含量的增加,MC析出量增多,M₂₃C₆析出量减小. MC中主要是Nb元素,并溶解了一定量的Mo,V,Cr和Fe元素;M₂₃C₆中主要是Fe,Cr元素,即Nb元素含量变化主要影响MC型碳化物.     

Effects of heat treatment on microstructure and mechanical properties of Inconel 625 alloy fabricated by wire arc additive manufacturing process

The microstructure and mechanical properties of Inconel 625 alloy fabricated by wire arc additive manufacturing process were evaluated under as-prepared and heat-treated conditions. A dendritic Ni-based solid solution phase along with (Nb, Ti)C carbide, Laves, and δ-Ni₃Nb secondary phases were developed in the microstructure of the as-prepared alloy. Solution heat treatment led to the dissolution of Laves and Ni₃Nb phases. In addition, dendrites were replaced with large columnar grains. Aging heat treatment resulted in the formation of grain boundary M₂₃C₆ carbide and nanometric γ″ precipitates. Hardness, yield and tensile strengths, as well as elongation of the as-prepared part, were close to those of the cast alloy and its fracture occurred in a transgranular ductile mode. Solution heat treatment improved hardness and yield strength and declined the elongation, but it did not have a considerable impact on the tensile strength. Furthermore, aging heat treatment caused the tensile properties to deteriorate and changed the fracture to a mixture of transgranular ductile and intergranular brittle mode.     

Crystallography of in-situ transformations of the <Emphasis Type="Italic">M</Emphasis><Subscript>7</Subscript>C<Subscript>3</Subscript> carbide in the cast Fe–Cr–Ni alloy

In the process of holding of the cast heat-resistant Fe–Cr–Ni (0.45C–25Cr–35Ni) alloy at 1150°C, the eutectic chromium carbide present in its structure undergoes a gradual transition M ₇C₃ → M ₂₃C₆. The gradual formation of domains of the M ₂₃C₆ carbide inside the particles of the M ₇C₃ carbide makes it possible to assume that the observed phase transition is the well-known carbide transformation of the in situ type. The mechanism of the in situ transformation of the crystal structure of the carbide from M ₇C₃ into M ₂₃C₆ with a change in the number of nearest metal neighbors of carbon atoms is explained within the previously developed combinatory model of polymorphic transitions in the metals.     

高铬铸钢组合立辊在热轧板带轧机中的应用

高铬铸钢组合立辊采用高铬铸钢离心复合材质辊套,锻钢辊轴组装而成.外层具有良好的耐磨性、抗热裂性,芯部具有高强度、强韧性.目前首次在某钢厂热轧板带轧机中使用,依此评述了高铬铸钢组合立辊的性能特点、组织、制造工艺以及在生产线上的使用情况.由于具有较高的耐磨性和热稳定性,不仅使得立辊使用寿命延长,换辊次数减少,轧机效率提高,而且提高了带钢边部质量,降低了边部质量缺陷发生率.     

Correlation of microstructure with the wear resistance and fracture toughness of white cast iron alloys

The objective of this investigation was to set down (on the basis of the results obtained by the examination of white cast iron alloys with different contents of alloying elements) a correlation between chemical composition and microstructure, on one hand, and the properties relevant for this group of materials, i.e., wear resistance and fracture toughness, on the other. Experimental results indicate that the volume fraction of the eutectic carbide phase (M₃C or M₇C₃) have an important influence on the wear resistance of white iron alloys under low-stress abrasion conditions. Besides, the martensitic or martensite-austenitic matrix microstructure more adequately reinforced the eutectic carbides, minimizing cracking and removal during wear, than did the austenitic matrix. The secondary carbides which precipitate in the matrix regions of high chromium iron also influence the abrasion behaviour. The results of fracture toughness tests show that the dynamic fracture toughness in white irons is determined mainly by the properties of the matrix. The high chromium iron containing 1.19 wt% V in the as-cast condition, showed the greater fracture toughness when compared to other experimental alloys. The higher toughness was attributed to strengthening during fracture, since very fine secondary carbide particles were present mainly in an austenitic matrix.     

Ta对IN617B镍基高温合金凝固组织及元素偏析的影响

IN617B镍基高温合金是700℃先进超超临界燃煤电站的主要候选材料.采用OM、SEM、TEM、EDS、EPMA和热力学计算等方法研究Ta对IN617B镍基高温合金凝固行为及组织的影响.研究结果表明:IN617B合金的凝固组织呈典型的树枝晶形貌,在枝晶间和晶界处分布着网状的M6C、板条状的M23C6和颗粒状的Ti(C,...     

Effects of high temperature and cryogenic treatment on the microstructure and abrasion resistance of a high chromium cast iron

Effects of deep cryogenic treatment on the microstructure, hardening and abrasion resistance behaviors of 16Cr1Mo1Cu cast iron subjected to destabilization treatment were investigated. The results show that the cryogenic treatment can effectively reduce the retained austenite after destabilization heat treatment, but cannot make retained austenite transform completely. Cryogenic treatment can markedly improve bulk hardness and abrasion resistance of the high chromium cast iron. In the course of destabilization treatment and then cryogenic treatment, the amount of precipitated secondary carbide, M₂₃C₆, was more than that in air cooling. The additional fine secondary carbide precipitated during the cryogenics treat after destabilization heat treatment comparing with air cooling, is the main reason for the increase of the bulk hardness and wear resistance.     

Experimental investigation of in-situ transformations of the <Emphasis Type="Italic">M</Emphasis><Subscript>7</Subscript>C<Subscript>3</Subscript> carbide in the cast Fe–Cr–Ni alloy

The microstructure and the phase composition of a heat-resistant Fe–Cr–Ni alloy (0. 45C–25Cr–35Ni) has been investigated in the cast state and after annealing at 1150°C for 2–100 h. After a 2-h high-temperature annealing, the fragmentation of the crystal structure of the eutectic M ₇C₃ carbides into domains of ~500 nm in size with a partial transition into M ₂₃C₆ carbides is observed. After a 100-h holding, the complete transition of the hexagonal M ₇C₃ carbides into M ₂₃C₆ with a face-centered cubic structure occurs. The carbide transition M ₇C₃ →M ₂₃ can be considered to be an in situ transformation.     

铬对铸态珠光体球铁显微组织的影响

采用电子探针和X-射线衍射测量铬在铸态球铁组织中的分布和形态,分析铬对铸态珠光体球铁组织的影响.结果表明,铬在渗碳体中具有严重的偏析倾向,并具有细化和促进珠光体形成的作用.