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.     

Genesis of spalling in tandem mill work-rolls: Some observations in microstructural degeneration

Macroscopic investigations of spalled 3% Cr-variety forged steel work-rolls used in the tandem mill of an integrated steel plant showed steel strip welding on roll surfaces. Microstructural observations of roll samples at regions away from strip-welded zones showed the desired uniform dispersion of fine globular carbides in tempered martensite. Quantitative image analysis of all investigated rolls also showed desirable carbide characteristics, with >4 vol.% carbides and >200,000 individual carbides/mm². The carbide sizes ranged from 0.69 to 0.83 μm. In contrast, optical and scanning electron microscopy (SEM) observations of the strip-welded regions showed microstructural degeneration thought to have occurred from surface and/or subsurface damage caused by localized thermal shock and intense pressure. This possibly resulted in the formation of a rehardened and heavily retempered zone at the strip-welded region. Cracks originated in the heavily retempered zone due to residual tensile stresses and propagated under the applied rolling stresses to produce spalling.     

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.     

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.     

On the microstructure and mechanical properties of high-speed steels

The microstructure of high-speed steels consists of a martensitic matrix with a dispersion of two sets of carbides. These carbides are usually known as primary and secondary carbides. The role of the primary carbides has been reported to be of no importance in strengthening the steels, due to their large size and large interparticle spacing. The present authors have studied the role of the primary carbides on the wear of high-speed steels and found them to be of no importance, and under certain conditions contributing to higher wear rates. It has been shown analytically and experimentally that in quenched and tempered high-speed steels, the precipitation of the secondary hardening carbide (cubic M₂C type) is the main reason for the improved strength and wear resistance. This shows that the secondary hardening phenomenon of high-speed steels is a direct result of the hardening caused by the precipitation of the cubic M_2C-type carbide. The present study has estimated that at peak hardness the volume fraction of secondary hardening carbides is approximately 20%. The measured strength of high-speed steels was found to be lower than the theoretically calculated strength due to non-homogeneous precipitation of the secondary hardening carbides. Areas which were observed to be free from secondary hardening carbides are real and are not artefacts. It has been shown that the strength of high-speed steel in the region of peak hardness depends primarily on the precipitation of the secondary hardening carbide and secondarily on martensitic strengthening.     

Correlation of microstructure and fracture properties of five centrifugal cast high speed steel rolls

Abstract

The present study is concerned with effects of microstructural factors such as distribution and fraction of coarse carbides located along solidification cell boundaries and characteristics of tempered martensitic matrix on fracture properties of five high speed steel (HSS) rolls manufactured by a centrifugal casting method. In situ microfracture observation, fracture toughness measurement and fractographic observation were conducted on these rolls to clarify fracture mechanisms. The in situ observation results indicated that coarse carbides located along cell boundaries provided easy intercellular fracture sites under a low stress intensity factor level. In the rolls whose intercellular carbide fraction and matrix hardness were high, fracture easily occurred under a low stress intensity factor. On the contrary, in the rolls where a small amount of intercellular carbides was distributed on the relatively ductile matrix of lath tempered martensite, the fracture path was accompanied by a considerable amount of plastic deformation including shear band formation, thereby resulting in high fracture toughness. In order to obtain better microstructure, hardness and fracture toughness of the HSS rolls, the minimisation of intercellular carbides, the refinement of carbides and the improvement of the matrix characteristics by controlling alloying elements and heat treatment conditions were suggested.     

Effect of electromagnetic centrifugal casting on solidification microstructure of cast high speed steel roll

Using self‐made electromagnetic centrifugal casting machine, optical microscopy (OM) and D/max2200pc X‐ray diffraction, the solidification microstructure and phases of as‐cast high speed steel(HSS) roll made by sand casting, centrifugal casting and electromagnetic centrifugal casting were investigated. The experiment results show that the phases of as‐cast high speed steel (HSS) roll are alloy carbide (such as W₂C, VC, Cr₇C₃), martensite and austenite. The centrifugal casting and electromagnetic centrifugal casting can apparently improve the solidification structure of HSS roll. With the increase of electromagnetic field intensity (B), the volume fraction of austenite in the HSS solidification structure increased obviously and eutectic ledeburite decreased, the secondary carbide precipitated from the austenite is more fine and distribution of secondary carbide is more even.     

Effect of Cold Rolling on Morphology of Carbides and Properties of 7Cr17MoV Stainless Steel

The evolution of the carbides and mechanical properties of 7Cr17MoV stainless steel used as cutlery material during cold rolling was investigated in this study. The results demonstrate that the microstructure of the rolled steel is composed of pearlite and spheroidal carbides. After cold rolling, the aggregation of carbides was no longer evident, and the carbides appeared to be small in size and uniformly distributed, whereas carbides in hot-rolled strips are clearly aggregated. Decreasing the thickness of the cold-rolled strips decreased the size of the carbide particles while increasing their number, which improved the particle distribution. A large number of fine, even nano-scaled carbides were observed in the material. The carbide phase, which primarily consisted of M₂₃C₆, did not change during cold rolling and annealing. The tensile strength and yield strength first decreased and then increased, and the elongation increased as the thickness of the cold-rolled strips decreased. Dimples and inclusion particles were readily observed upon tensile fracture. The inclusions are identified to be primarily carbides and oxide inclusions.     

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M₇C₃ carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M₇C₃ and V₆C₅ carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M₂₃C₆ carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M₇C₃ carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.     

Failure of Tungsten Carbide Rolls from a Wire and Rod Mill

The paper presents the failure analysis of tungsten carbide rolls for the Wire and Rod Mill (WRM) and establishes the failure reasons and the corrective and preventive actions taken to improve roll performance. A sharp continuous increase in failure rate and decrease in average time between failure (ATBF) was observed during 2001 to 2004. The increase in premature failure of these rolls (16 rolls in 2003 and 38 rolls in 2004) raised the cost of operations. Due to unscheduled stoppage of the mill after roll failures, the unplanned mill delays also increased, causing production losses. The analysis revealed significant increase in premature failure of rolls in the finishing stands 19, 22, 23, and 24 and prefinishing stand 14. The primary causes of such failures were faulty roll cooling and roll mounting and accumulation of cobbles. Implementation of modified cooling headers, a calibrated gage to monitor hydraulic pressure during roll mounting, and eddy current inspection resulted in decreasing the trend in roll failure and increasing the trend in ATBF.     

Prediction of Performance of an Industrial Poittemill in Production of Limestone Powders

Experimental data were collected from tests using an industrial-scale high-pressure roller mill named the Poittemill grinding limestone materials for a range of parameters (such as force pressure, circumferential speed of roll, and feed size) in a Nordkalk AB plant located in Ignaberga, Sweden. These data were used to develop models of throughput, size reduction, and energy utilization with the parameters. A performance model with a correction coefficient, which has been developed, is able to describe the Poittemill throughput at various force pressures and circumferential speeds of rolls. The materials leaked beside the rolls are found to be empirically related to the circumferential speed of roll in a given force pressure. It is shown that two major parameters, force pressure and circumferential speed of roll, have an influence on the median size (d₅₀) of the ground product. Product fineness is decreased at a higher circumferential speed of roll or at a lower force pressure. The force pressure is the most dominant effect on energy utilization in the mill. The feed size used has a slight influence on the grinding results. Energy to the Poittemill for the comminution is utilized more efficiently at a lower force pressure or a higher circumferential speed of roll. Empirical models can predict the comminution characteristics with respect to the major parameters in the Poittemill system in dry mode. Product size-energy input relations have been also established, independent of the operating parameters used.     

Size reduction performance evaluation of HPGR/ball mill and HPGR/stirred mill for PGE bearing chromite ore

In the present study, size reduction experiments were performed on High-Pressure Grinding Rolls (HPGR), ball mill and stirred mill of PGE bearing chromite ore. The performance of HPGR was evaluated in two stages of size reduction to reduce energy consumption. In the first stage of HPGR, the effect of operating variables such as the gap between the rolls, roll speed, and specific pressing force on product size (P₈₀) and energy consumption (E_CS) was investigated. The process to get the smallest product size was optimized within the experimental range of investigation. The crushed product of HPGR was subjected to grinding in the second stage in a ball mill and stirred mill. The effect of mill speed, grinding time, and ball size on the performance of the ball mill was investigated and the product was further investigated in the second stage. A comparative analysis of the ball mill and stirred mill performance and energy consumption at different grinding time intervals was also performed. It was found that the ball mill consumed 54.67 kWh/t energy to reduce the F₈₀ feed size of 722.2 µm to P₈₀ product size of 275.4 µm while stirred mill consumed 32.45 kWh/t of energy to produce the product size of 235.6 µm. It also showed that stirred mill produced finer product than the ball mill at around 40% lesser consumption of energy. It can be concluded that the HPGR-Stirred mill combination was a more energy-efficient grinding circuit than the HPGR-Ball mill combination for PGE bearing chromite ore.     

Life of rolls in a cold rolling mill in a steel plant-operation versus manufacture

Failures of rolls occur due to improper manufacturing and operational parameters. Prematurely failed (spalled) roll samples collected from a reputed steel plant were examined for their chemistry, inclusion content, microstructures, carbide characteristics, hardness and retained austenite content. The residual stresses were also measured on the inner and outer surfaces of the spalled roll pieces. The higher content of retained austenite was primarily responsible for the spalling of indigenous rolls for which subzero treatment has been recommended. Several suggestions have also been made for smooth operation of the mill and consequently for the life extension of work rolls.     

Effect of cold drawing ratio on γ′ precipitation in Inconel X-750

Inconel X-750 is a Ni-based precipitation-hardened superalloy having large tensile and fracture strengths. In the study, X-750 wires were cold drawn to different extents. Small angle neutron scattering was employed to quantitatively measure the size and volume fraction of the γ′ phase as a function of the cold drawing ratio (DR) and aging temperature. The presence and size of γ′ precipitates were confirmed by transmission electron microscopy. The drawing ratio had an important effect on the volume fraction of the γ′ precipitates. However, the size of the precipitates was independent on the drawing ratio. The specimen with the minimum drawing ratio (DR0) produced the largest volume fraction of γ′ as compared with large drawing ratio (DR) specimens such as DR17 and DR42. The small volume fraction of the γ′ phase for a sizeable drawing ratio was associated with the large amount of nucleation sites for secondary carbides, M₂₃C₆, and the fast diffusion path, i.e., dislocation, needed to form M₂₃C₆. A Cr depletion zone around the secondary carbides raised the solubility of γ′. Therefore, the significant drawing ratio contributing to the large volume fraction of the secondary carbides decreased the volume fraction of the γ′ precipitates in Inconel X-750.     

Effect of microstructure on tensile fracture of quenched and lightly tempered high carbon and low chromium steel containing undissolved spheroidal carbides

High carbon and low alloy chromium steels have been studied to determine the effect of the microstructure on tensile fracture of quenched and lightly tempered low alloy steel containing undissolved spheroidal carbides. The steels with a volume fraction of 8 and 13 vol % and containing particle sizes from 0.32 to 1.14m were investigated. In the case of steel containing 8 vol % undissolved carbides, many twinned plates were observed in the matrix martensite and microtwinning was observed in the carbide/matrix interfaces. The steel failed in a macroscopically brittle manner and the true fracture stress of the steel was independent of the carbide particle size, while the data exhibited a large scatter. In the case of steel containing 13 vol % of undissolved carbides, the matrix martensite consisted predominantly of lath martensite and a well-defined forest of dislocations was observed around the carbides. Failure of the steel occurred in the relatively early stage of plastic deformation and the true fracture stress of the steel increased with decreasing carbide particle size.     

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.     

Microstructural characteristics of forged and heat treated Inconel-718 disks

Microstructure evolution from center to edge of the as-forged and heat treated Inconel-718 disks was investigated. Specifically, the evolution of primary carbides, grain size, γ″, γ′, δ, and secondary carbide particles was the focus of the current study. In fact, characterization of these microstructure features is essential for models predicting the creep and fatigue lives of the alloy. Accurate and reliable revealing of the grain boundaries in as-forged and heat treated Inconel-718 was made possible in this study by development of a new method. From microstructure investigations, nonuniformities in grain size, volume fraction, size and inter particle spacing of precipitates from center to edge were observed in both as-forged and heat treated disks. The microstructure nonuniformities resulted in significant variation in hardness from center to edge of the disks.     

Effect of cerium addition on microstructures of carbon-alloyed iron aluminides

The effect of Ce addition on the microstructure of carbon-alloyed Fe₃Al-based intermetallic has been studied. Three different alloys of composition, Fe-18.5Al-3.6C, Fe-20.0Al-20C and Fe-19.2Al-3.3C-0.07Ce (in at%), were prepared by electroslag remelting process. Their microstructures were characterized using optical and scanning electron microscopies. Stereological methods were utilized to understand the observed microstructures. All the alloys exhibited a typical two-phase microstructure consisting of Fe₃AlC carbides in an iron aluminide matrix. In the alloy without Ce addition, large bulky carbides were equally distributed throughout the matrix with many smaller precipitates interspersed in between. In the alloy with Ce addition, the carbide grain sizes were finer and uniformly distributed throughout the matrix. The effect of Ce addition on the carbide morphology has been explained based on the known effect of Ce in modifying carbide morphology in cast irons.     

MC5冷轧辊钢奥氏体化过程碳化物演变的研究

为了深入研究MC5冷轧辊钢奥氏体化过程中碳化物的演变规律,采用扫描电镜、X射线衍射分析和硬度测试技术,研究了不同奥氏体化时间对MC5钢组织和硬度的影响.结果表明,奥氏体化时间从0增加到2 h时,随着奥氏体化时间的增加,试样的淬火硬度先陡然增加,后趋于平缓;当奥氏体化时间增加到4 h时,随着奥氏体化时间的增加,试样的淬火硬度有减小的趋势.对奥氏体化时间为30 min、1和2 h的试样进行微观统计和XRD分析,发现奥氏体化时间为30 min时,试样的碳化物分布最为弥散,数量最多,尺寸最为细小,其类型主要为M7C3型.     

Characterization of the carbides and the martensite phase in powder-metallurgy high-speed steel

A microstructural characterization of the powder-metallurgy high-speed-steel S390 Microclean was performed based on an elemental distribution of the carbide phase as well as crystallographic analyses. The results showed that there were two types of carbides present: vanadium-rich carbides, which were not chemically homogeneous and exhibited a tungsten-enriched or tungsten-depleted central area; and chemically homogeneous tungsten-rich M₆C-type carbides. Despite the possibility of chemical inhomogenities, the crystallographic orientation of each of the carbides was shown to be uniform. Using electron backscatter diffraction the vanadium-rich carbides were determined to be either cubic VC or hexagonal V₆C₅, while the tungsten-rich carbides were M₆C. The electron backscatter diffraction results were also verified using X-ray diffraction. Several electron backscatter diffraction pattern maps were acquired in order to define the fraction of each carbide phase as well as the amount of martensite phase. The fraction of martensite was estimated using band-contrast images, while the fraction of carbides was calculated using the crystallographic data.