Premature failure of work-rolls in tandem mill: Some microstructural revelations

The microstructural features of prematurely spalled tandem mill work-rolls were examined in an attempt to correlate microstructure with spalling behavior and roll performance. Spalled samples were collected from work-rolls that had shown variations in roll life under similar conditions of mill usage. Optical microscopy revealed that a fine dispersion of spheroidal carbides in a matrix of tempered martensite was conducive to superior performance in terms of roll life (i.e., tonnage rolled), and that coarse angular and irregular shape carbides were detrimental to roll life. Image analysis of roll microstructures indicated that small carbide size, large carbide volume fraction, and high carbide count were characteristic of higher-life rolls, and that large carbide size, low carbide volume fraction, and less carbide density were typical of lower-life rolls. The carbides in both types of microstructure were M₇C₃ type.     

Fracture toughness of steels with nickel content in respect of carbide morphology

Abstract

This paper is focused on the influence of Ni addition on the microstructure and fracture toughness of structural steels after tempering. Nickel is known to increase the resistance to cleavage fracture of steel and decrease a ductile–brittle transition temperature. The medium carbon, low alloy martensitic steels attain the best combination of properties in low tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. In the present research, four model alloys of different Ni contents (from 0·35 to 4·00%) were used. All samples were in as quenched and tempered condition. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200°C for 2?h. An increase in nickel content in the investigated model structural steels causes a decrease in ε carbide volume fraction in their microstructure. Cementite nucleates independently in the boundaries of martensite laths and in the twin boundaries in the areas where the ε carbide has been dissolved. It was stated that stress intensity factor K_Ic significantly decreases in the case of the presence of dispersive elongated cementite precipitations at the boundaries of the prior austenite grains.     

Fracture behaviour in tempered martensite embrittlement of medium-carbon alloy steels

The effect of alloying additions (nickel and silicon) on the fracture behaviour in tempered martensite embrittlement (TME) has been studied in commercial alloy steels. The fracture behaviour is analysed using the fractographs of the impact specimens tested at various temperatures. In 4140-Ni(4340) steel, where nickel-addition increases the intrinsic matrix toughness, the intergranular brittle type of THE is observed. In 4140-Si(4140 + 2Si) steel, where silicon-addition decreases the intrinsic matrix toughness, the intergranular brittle type of TME is also observed. The occurrence of the intergranular brittle type of TME is attributed to the activation of coarse grain-boundary carbides at the grain boundaries which the relatively high impurity content of commercial alloy steel renders impurer (i.e. weaker), despite relatively low intrinsic matrix toughness in 4140-Si steel.     

A study of heat treatment of high‐boron high‐speed steel roll

High‐boron high‐speed steel (HSS) is a cheap roll material. In the paper, the authors research the effect of heat treatment on the microstructure and properties of high‐boron high‐speed steel HSS roll containing 0.54% C, 1.96% B, 3.82% W, 7.06% Mo, 5.23% Cr and 2.62% Al by means of the optical microscopy (OM), the scanning electron microscopy (SEM), X‐ray diffraction (XRD) and hardness test. The results showed that as‐cast structure of boron‐bearing high‐speed steel HSS consisted of martensite, pearlite, M₂(B, C), M₃(B, C) and M₂₃(B, C)₆ type borocarbides. After quenching, the matrix transformed into the lath martensite, and M₃(B, C) dissolved into the matrix. When quenching temperature is lower than 1050°C, the hardness is increased with the increase of quenching temperature under oil cooling, while quenching temperature excels 1100°C, the hardness will decrease with the increase of quenching temperature. Under the condition of salt bath and air cooling, the effect of quenching temperature on the hardness is similar to the above law, but the quenching temperature obtaining the highest hardness is higher than that of oil cooling. The highest hardness is obtained while tempering at 525°C. The hardness of high‐boron high‐speed steel HSS roll is 66.5 HRC, and its impact toughness excels 13.1 J/cm². Using in pre‐finishing stands of high‐speed hot wire‐rod rolling mill, the wear rate of high‐boron HSS rolls is 0.26 mm/one thousand tons steel. However the manufacturing cost of high‐boron HSS rolls is obviously lower than that of powder metallurgy hard alloy rolls, it is only 28% of that of powder metallurgy (PM) hard alloy rolls.     

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

热处理对激光选区熔化成型高速钢组织和力学性能的影响EI北大核心CSCD

基于激光选区熔化(selective laser melting,SLM)技术,采用加热打印基板和低功率慢扫描的打印策略,制备了近全致密、低缺陷的高速钢样品;对比分析了固溶淬火及1~4次高温回火等热处理工艺对高速钢显微组织及力学性能的影响。结果表明:SLM极高的熔融/冷却速率产生了细晶奥氏体组织,解决了高速钢中常见的粗大莱氏体组织和网状碳化物问题。固溶淬火处理后高速钢组织由马氏体和残余奥氏体组成。随后在数次高温回火过程中,高速钢逐渐向回火马氏体转变,并析出大量微米级和纳米级MC型碳化物。在马氏体相变强化和MC型碳化物沉淀强化作用下,固溶淬火+3次回火的Tempered-Ⅲ样品硬度60HRC,抗弯强度3621 MPa,弯曲断裂应变为10.1%,获得硬度、强度和韧性匹配较佳的综合性能。继续增加回火次数则导致部分碳化物长大,使得高速钢弯曲断裂应变有所降低。通过SLM技术结合固溶淬火+高温回火,能够充分发挥细晶强化、相变强化和沉淀强化效果,为高强高韧复杂形状高速钢零件的快速制备提供了新途径。     

Effect of K/Na on microstructure of high-speed steel used for rolls

In the present work, the effect of K/Na on microstructure of high-speed steel (HSS) used for rolls was investigated utilizing Hi-scope video microscope (HSVM) and electron probe microanalyser (EPMA). As-cast microstructure of the alloy is mainly composed of pearlite matrix, M₇C₃, M₂C and MC eutectic carbides. The carbides are connected or placed next to each other to form a network along grain boundaries. After K/Na modification, the morphology, size and distribution of carbides change greatly. The carbide network tends to break, and all carbides are refined and distributed homogeneously in the matrix. The mechanism of K/Na modification on microstructure of the alloy is also discussed.     

The effect of the reduction of carbon content on the toughness of high chromium white irons in the as-cast state

Three high chromium white cast irons were examined in the as-cast state to determine the effect of the carbon content on the fracture toughness. The plane strain fracture toughness K _Ic and the fracture strength were measured for each alloy. X-ray mapping was used to identify the phases on the fracture surfaces. Scanning electron fractography and optical microscopy were used to determine the volume fraction of each phase on the fracture surfaces. It was found that most fracture occurred in the eutectic carbides, but that for the alloys with a reduced volume fraction of eutectic carbides, a small amount of crack propagation occurred in the austenitic dendrites. This change in crack path correlated with an increase in fracture toughness. The Ritchie-Knott-Rice model of brittle fracture was applied. It was found to sensibly predict the critical length for fracture for each alloy. Deep etching was employed to examine the distribution of eutectic carbides. It was found that the eutectic carbides formed a continuous network in each case.     

Metallurgical investigation of prematurely failed hot-strip mill work-rolls: Some microstructural observations

A metallurgical investigation of failed samples of hot-strip mill work-rolls used in an integrated steel plant was made to determine the influence of microstructural characteristics on failure susceptibility and roll life. The samples investigated pertained to prematurely failed indefinite chill double-poured (ICDP) iron work-rolls, which exhibited varying roll lives under similar mill operating environments. Although microstructures of all the investigated rolls showed similar graphite morphologies irrespective of their mill performance, discernible differences in carbide characteristics could be observed between high and low life rolls. Microstructural observation of nital-etched roll specimens revealed that lower life rolls were characterized by carbide microcracking. The propensity for cracking was particularly high in carbides exhibiting microhardness greater than 1020 VPN. Electron-probe microanalysis (EPMA) indicated that carbides in the spalled rolls were mostly of M₃C type, where M was Fe and Cr. Quantitative image analysis of phases in the investigated rolls revealed that while graphite volume fraction in the range of 4.0 to 6.4% did not significantly affect roll life, carbide content higher than 28.5 vol% was found detrimental. In fact, a carbide content in the range of 24.0 to 28.50 vol% was found to be desirable for higher roll life. The study thus revealed that although carbides are indispensable for high hardness, resistance to wear, and thermal cracking, an excessive volume fraction (>30 vol%) of high hardness (microhardness > 1020 VPN) carbides accentuated microcracking, which ultimately induced premature spalling of hot-strip mill work-rolls.     

Powder metallurgy aluminium alloys: characteristics of an Al-Cr-Fe rapidly solidified alloy

An Al-4Cr-1 Fe alloy has been evolved utilizing the advantages of rapid solidification technology. The paper describes the formation of the as-atomized (inert gas) powder microstructure and its decomposition duringin situ heating. It was observed that the most typical powder microstructure had a cellular morphology with a fine intercellular network consisting of ironrich phases. Decomposition of the powder duringin situ heating commenced from the intercellular network, finally resulting in a matrix with a high volume fraction of chromium-rich globular-like precipitates. Consolidation was achieved through cold compaction and hot extrusion; the alloy being easily extrudable. The room-temperature mechanical properties of the alloy were also assessed. The 0.2% proof stress and the tensile strength were below the target limits for dispersion-strengthened alloys, but the elongation and fracture toughness values were very promising. Finally, the extruded microstructure was related to mechanical properties.     

Analyses of the failures on shear cutting blades after trimming of ultra high-strength steel

Damages on shear cutting blades were analyzed after 50,000 strokes of trimming on an ultra high-strength steel sheet. Traditional D2 alloy and an advanced one (Cr08H) based on the composition of 1C-8Cr were quenched from 1030 °C, tempered at 180 °C and submitted to the shear cutting test. Cr08H had lower hardness, a smaller volume fraction of M₇C₃ carbides while it contained a larger volume fraction of retained austenite. And these resulted in more scratches and rounded edges because of degraded resistance to wear and local plastic deformation. In spite of higher impact toughness, Cr08H exhibited inferior resistance to chipping which was the consequence of localized brittle fracture. It could be concluded that this was caused by more transformation of austenite as well as by insufficiently hardened matrix, both of which were attributed to inappropriate conditions of the heat treatment.     

轧辊用高速钢组织与性能研究

研究了四种不同成分的轧辊用高速钢淬火和回火后的显微组织与性能。结果表明,轧辊用高速钢合适的淬火温度为1050℃～1100℃,回火温度为500℃～550℃,热处理后组织由回火马氏体、少量残余奥氏体以及各种碳化物组成,大量细小碳化物呈弥撒分布,铸态组织中网状碳化物基本消除。回火后高速钢硬度可达HRC62,冲击韧性在6J／cm^2以上,可以满足制作高速钢轧辊的要求。     

Oxidation behaviour of ledeburitic steels for hot rolls

The oxidation behaviour of two high speed steels (HSS) employed for the production of hot rolls was studied. The steels have slightly different chromium contents and volume fractions of primary carbides. Because oxidation nucleates at the matrix–carbide interfaces and propagates in the matrix without involving the carbides, the oxide scale grows less uniform. All the primary carbides have a higher Cr content than the matrix; therefore they tend to reduce the oxidation resistance. The slight differences in chromium content and in carbide volume fraction are responsible for the different oxidation resistance at 600 and 700°C, whilst at 500°C the two steels have almost the same resistance.     

Reinforcement of Hadfield steel matrix by oriented high‐chromium cast iron bars

To attain a wear‐resistant material compatible with high hardness and high toughness, Hadfield steel matrix was reinforced by oriented high chromium cast iron bars, through inserting high chromium alloys flux‐cored welding wires into Hadfield steel melt at 1500 ± 10 °C. The obtained composites were investigated by XRD, SEM, micro‐hardness, three‐body abrasion wear and impact toughness testers. The results show that the alloy powders inside the flux‐cored welding wires can be melted by the heat capacity of Hadfield steel melt and in situ solidified into high chromium cast iron bar reinforcements tightly embedded in the matrix. The micro‐hardness of reinforcements of the water‐quenched composite is about four times higher than that of the matrix. The impact toughness of the water‐quenched composite is higher than that of the as‐cast composite and lower than that of Hadfield steel, and its fracture mechanism is very complicated and refers to brittle and ductile mixture fracture mode. The excellent impact toughness and better wear resistance of the water‐quenched composite are attributed to combine fully the advantages and avoid the drawbacks of both Hadfield steel and high chromium cast iron. Additionally, in industrial application, the pulverizer plate produced by this composite, has also better wear resistance compared to the reference Hadfield steel pulverizer plate.     

Fracture toughness of M2 and H13 alloy tool steels

Abstract

The influence of microstructural variations on the fracture toughness of two tool steels having compositions (wt-%) lC–4Cr–5Mo–2V–6W (AISI M2 high-speed steel) and 0·35C–5Cr–1·5Mo;amp;#x2013;1V (AISI H13 hot-work steel) was investigated. In the as-hardened condition, the H13 steel has a higher fracture toughness than M2 steel, and the latter steel is harder. In the tempered condition, the H13 steel is again softer and has a higher fracture toughness than M2. There is a decrease in fracture toughness and an increase in hardness when the austenitizing temperature is above I050°C for M2 steel and above 1100°C for H13 steel, in both the as hardened and hardened and tempered conditions. The fracture toughness of both steels was enhanced by reducing the grain size and increasing the overall carbide volume in the matrix. The steel samples of average grain diameter ≥40μm exhibit 2–3 MN m ^?3/2 lower fracture toughness than samples of average grain diameter ≤15 μm. A high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. Tempering improved the fracture toughness of M2 and H13 steels. The present results are explained using observations of changes in the microstructure and the modes of fracture.

MST/468     

M2铸造高速钢的变质处理

用Re Al N复合变质剂对M 2铸造高速钢进行变质处理 ,消除了钢中网状共晶碳化物 ,细化了基体组织 ,减轻了W、Mo元素偏析 ,在不降低M 2高速钢硬度的情况下 ,韧性大幅度提高 ,经 1180～ 12 0 0℃淬火 ,5 6 0℃三次回火后 ,硬度保持在HRC6 5～ 6 6 ,冲击韧性由 8.5J提高到 17.0J。变质处理M 2铸造高速钢具有优异的抗热疲劳性能和抗高温磨损性能     

Hot compression and fracture toughness of HSS composed hot strip work rolls

The rolls in the finishing stands must have good resistance to both wear and fire cracks. The use of high-speed steel at the finishing stands has shown satisfactory results. The composition of this high-alloyed steel is an important parameter as the most significant change lays on the type, morphology, and volume fraction of the eutectics carbides. The heat treatment of these products consists of high temperature austenization followed by quenching and two temperings, as required in order to increase their overall hardness and to completely eliminate residual austenite. The influence of tempering temperatures on the mechanical properties of these products, determined using tensile, hot compression and fracture toughness tests, was studied in this research work. Their corresponding failure micromechanisms were defined by means of the analysis of fracture surfaces.     

Influence of microstructure and chromium content on oxidation behaviour of spin cast high speed steels

Abstract

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.     

Influence of heat treatment and particle shape on mechanical properties of infiltrated Al2O3 particle reinforced Al-2 wt-%Cu

Abstract

Al-2 wt-%Cu composites were produced by gas pressure infiltration of powder beds with a high volume fraction (45 to 60 vol.-%) of angular or polygonal alumina particles. The tensile behaviour and fracture toughness of the composites were characterised in as cast, solutionised and peak aged (T6) conditions. It was shown that coarse intermetallics that are formed during solidification and located preferentially at the particle/matrix interface lead to lower toughness compared with the same composites in solutionised and T6 conditions. The particle nature and shape exert a strong influence on the properties of the composites: polygonal particles are intrinsically stronger than angular particles and yield stronger, tougher, and more ductile composites. Composite toughness variations are explained in terms of fracture micromechanisms.     

The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron

The fracture toughness of a high chromium, reduced carbon white cast iron was measured using the K_Ic fracture toughness test. The toughness was found to increase with increasing heat treatment temperature for the temperature range of 1273–1423 K. Increases in the fracture toughness were due to crack deflection into the dendritic phase. Cracking in the dendrites was promoted by the presence of secondary carbides which formed during the high temperature heat treatment employed. The characteristic distance for brittle fracture as calculated by the Ritchie–Knott–Rice model correlated well with the centre to centre mean free path of the secondary carbides on the fracture plane.