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.     

Failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting. Part I: Material and fractographic characterization

The failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting and used in cast glass rolling was carried out by means of traditional characterization techniques (optical metallography, SEM, EDX microanalysis, tensile testing and XRD). The roll was in the as-cast condition and its microstructure featured large proportion of δ ferrite (between 20% and 27%) in a martensitic (α′) matrix, with the δ/α′ interfaces presenting an intergranular network of M₂₃C₆ carbides. The crack propagation began in the internal surface of the roll, with δ/α′ intergranular and transgranular cleavage in the “equiaxed region” of the casting, progressing to δ/α′ intergranular ductile fracture in the “columnar” and “chilled regions”. Tensile thermal stresses in the internal surface of the roll associated with microstructural embrittlement (network of interfacial carbide and microporosities) are thought to be the main causes for the premature failure of the roll. Finally, materials selection was performed to replace the CA-15M stainless steel with another class of stainless steel for centrifugal casting.     

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.     

Role of residual stresses induced by industrial fabrication on stress corrosion cracking susceptibility of austenitic stainless steel

Effect of residual stress generated during tube fabrication, roll expansion and machining of stainless steel on the stress corrosion cracking (SCC) susceptibility was studied by testing fabricated tubes, tube–tube sheet joint and heavily machined plate of austenitic stainless steel (SS) in boiling MgCl₂. U bend samples of machined plate were exposed to acidified SO₄ + Cl^? environment at room temperature to study its ambient temperature SCC behavior. The results correlate the SCC behavior of the SS tubes and roll expanded joints to the nature and magnitude of residual stresses present. The study also highlights the distinct difference in ambient temperature SCC behavior of machined vs. nonmachined surfaces.     

A new mechanism in hydrogen-enhanced fatigue crack growth behavior of a 1900-MPa-class high-strength steel

This paper presents a new mechanism controlling the acceleration of fatigue crack growth of a hydrogen-charged high-strength steel (bearing steel SAE52100, ?? _ult?>?1, 900MPa, HV =?569). Three- dimensionally complicated shape of a primary crack and secondary cracks were observed in hydrogen- charged specimens. Marked acceleration of fatigue crack growth in the presence of hydrogen was observed particularly at low test frequency, and was attributed to the initiation and successive coalescence of secondary cracks formed ahead of primary crack. These secondary cracks were produced along prior-austenite grain boundaries and carbide boundaries, or by direct cracking of carbides. Surprisingly, secondary cracks were observed outside the ordinary plastic zone ahead of the crack tip. TEM observation elucidated that the secondary cracks outside the crack tip plastic zone were produced by hydrogen-induced deformation twins impinging on grain boundaries and carbides. These results suggest a new mechanism of the acceleration of fatigue crack growth rates in high-strength steels caused by hydrogen-induced deformation twins, rather than due to hydrogen- enhanced localized plasticity. The phenomena associated with time dependent fatigue crack growth are presumed to be correlated with the initiation and coalescence of secondary cracks in the presence of hydrogen.     

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.     

Effect of magnetic field on interfacial energy and precipitation behavior of carbides in reduced activation steels

This work investigated the influence of high temperature and high magnetic field on the carbide precipitation behavior in reduced activation steels. As-quenched steels are tempered at 923 K for 3 h with and without a 10 T magnetic field. The applied field can effectively prevent the directional growth of rod-shaped M₂₃C₆ carbides along martensite packet boundaries. The aspect ratio of M₂₃C₆ carbides decreased from 2.3 to 1.2 due to an increase of the carbide/ferrite interfacial energy under the high magnetic field. Applications of the Weiss molecular field theory to calculate the difference in interfacial energy caused by the high magnetic field, and of the Langer-Schwartz theory to model metal carbide (MC) precipitation behavior under the magnetic field were described. Results indicated that the density of MC decreased by nearly an order of magnitude and its mean size increased by 40% owing to an increase of 0.03 J/m² of the carbide/ferrite interfacial energy.     

Creep rupture behaviour of low alloy ferritic steel weldments

To establish differences in rupture lives and ductilities between parent metal, weld metal and weldjoint, a commercial heat of lCrMoV cast steel welded with 21/4Cr1M0 steel electrodes was creep tested over a range of stresses at 550°C using constant load creep testing units. The results indicate that, while there is no significant variation in rupture lives, ductility in the weldjoint showed a decreasing trend over longer periods of testing. In weldjoint specimens that comprised parent metal, heat affected zone (HAZ) and weld metal, fracture occurred in the weld metal quite near to the fusion boundary over the entire range of stresses. The rupture ductility in the weldjoint was found to be lower than in parent or weld metal.

Scanning electron microscopy (SEM) revealed the fracture surfaces of parent and weld metal to be heavily dimpled, the dimples originating mostly around carbide precipitates, whereas the fracture surfaces of the weldjoint were found to be somewhat faceted.

The results of the present work suggest that weldments made with 2114CrlMo steel deposits possess comparable creep lives to the parent metal of 1CrMoV steel, and the weld metal in the weldjoint near the fusion boundary exhibits a tendency to embrittle over longer periods of testing.     

Laser surface hardening of AISI 01 tool steel and its microstructure

Abstract

The effects of laser surface hardening on AISI 01 tool steel samples were studied by changing the laser operating parameter combinations and the initial steel microstructure. Both melted and solid state transformed regions were produced, and then studied using optical microscopy, analytical electron microscopy, X-ray diffraction, and measurements of micro hardness to investigate the hardening mechanisms and the development of compressive residual stresses. The results indicate that hardened case depths up to 0·6 mm can be obtained using a laser beam operated at a power of 500 W and a scan rate of 2·1 mm s^?1, but that different amounts of retained austenite and undissolved carbides are observed for different beam powers. Quenched and tempered AISI 01 steel samples, with initial hardness values in the range 30–40 HRC, are better suited for laser surface hardening compared with the samples with initial hardness of 48–50 HRC, because the formation of an over tempered region adjacent to the hardened zone can be avoided.

MST/901     

Effects of growth rate on carbides and microporosity in DS200 + Hf superalloy

The effects of growth rate on the carbide morphology and microporosity were investigated using DS200 + Hf superalloy, between 16.7×10^–6 and 266.7×10^–6 m s^–1. The fact that the shape factor remained almost unchanged with the growth rate indicates that the shape of the carbide particles does not directly depend on the cooling rate in this alloy. The stability of carbide particles was considered in terms of the interfacial energy between the carbide and matrix interface and the fluctuation of carbide composition. It was observed that the carbide/--matrix interfacial area per unit volume as a function of growth rate remained almost unchanged (especially above 66.7×10^–6 m s^–1), indicating that the rate of coarsening of carbides during solidification is not affected by the carbide/matrix specific interface energy. One of the factors which determines the rate controlling step for the coarsening of carbide particles is suggested to be Ti in the interdendritic and grain-boundary regions, and Hf in the vicinity of the incipient melting region.     

Corrosion-mechanical resistance of pipe steel in hydrogen-sulfide containing media

We investigate the resistance to hydrogen-sulfide stress corrosion cracking and hydrogen-induced cracking of ASTM A333 steel intended for the oil and gas industry. On the basis of the results obtained for seven meltings of this steel supplied for the Tengiz gas-processing plant, we established that all meltings satisfy specifications of the NACE MR0175-96 standard in their chemical composition and strength characteristics (S < 0.005%. P < 0.025%, HRC < 22, etc.). For two of the seven investigated meltings, resistance to hydrogen-sulfide stress corrosion cracking is not high (threshold stresses < 0.8Σ^min _0.2) and one melting has a low resistance to hydrogen-induced cracking (coefficient of the length of regions of crack formation > 6% and coefficient of the width of regions of crack formation > 3%). Therefore, complete (100%) incoming control over the corrosion-mechanical resistance of all materials of equipment operating in hydrogen-sulfide-containing media must be carried out. Translated from Fizyko-Khimichna Mekhanika Materialiv. Vol. 36, No. 3, pp. 93–97, May-June. 2000.     

Austenitic stainless steel bearing Nb compositional and plastic deformation effects

Austenitic stainless steel has excellent ductility. Consequently, it has capability for heavily cold deformation, despite its high strength and high work hardening ability. Austenitic stainless steel predominantly contains high levels of chromium and nickel. Additional elements may be added to enhance performance. The target of this paper is to melt and cast several austenitic stainless steel alloys with different Nb contents. Furthermore the effects of the chemical composition on strength as well as the effect of cold rolling on the creation of induced martensite phase are also studied. The microstructural investigation shows that grain coarsening was observed on the as-cast structure accompanying with thick grain boundary carbides along with carbide agglomerations at the triple points. Hot deformation diminishes the grains as well as the carbide films surrounding the grains. Solution treatment creates austenitic grains free of grain boundary carbides. Cold deformation creates highly elongated grains associated with wavy pancaked structure. Numerical modeling extensively used to detect the proof strength at high temperatures (up to 600 °C). The detected proof strength decreases drastically by raising the deformation temperatures. Nb was found to increase the proof strength even at high temperatures. The measured mechanical properties of the alloys under investigation are higher than that of detected ones by Kimura model, where the model did not pay attention to the Nb effect. Elliason model for the flow curve of different alloys has been extensively studied and applied. The detected results have been verified by the microstructural changes during deformation.     

Carbide types in an advanced microalloyed bainitic/ferritic Cr–Mo Steel – TEM observations and thermodynamic calculations

The strength and toughness of low alloyed ferritic/bainitic steels depend on their microstructure, which evolves during thermo‐mechanical treatments along the processing chain. Chromium‐molybdenum steel microstructures are complex. Therefore, only a limited number of attempts have been made to fully characterize carbide populations in such steels. In the present work, analytical transmission electron microscopy is employed to study the microstructure of a low alloyed chromium‐molybdenum steel, which features ferritic (F, mainly α‐iron and niobium‐carbides) and bainitic (B, α‐phase, dislocation, grain/subgrain boundaries, various M_xC_y carbides) regions. The crystal structure and chemical nature of more than 200 carbides are determined and their distributions in the two microstructural regions are analyzed. The present work shows how particles can be identified in an effective manner and how the microstructural findings can be interpreted on the basis of thermodynamic calculations.     

The influence of austenisation on the kinetics of tempering—Mössbauer effect study

The Mössbauer effect in ⁵⁷Fe is used to investigate the kinetics of carbide precipitation in plain carbon steel. The investigated carbides were extracted chemically after the steel had been treated at various austenisation and tempering temperatures. The relative amount of cementite precipitated after a certain heat treatment was determined from the spectral area. The activation energy of the process is found to be 0.3 eV. A formula is suggested for the dependence of carbide formation rate on the austenisation temperature.It is found that in the process of the chemical extraction, amorphous iron gel or oxyhydroxide isomers form, rather than iron oxides. The effective internal field H_n and the isomer shift of the extracted carbides depend on the tempering temperature, while the carbide formation rate depends on the austenisation temperature as well.This research is sponsored by the USA National Bureau of Standards Grant No. G-87 (1968).In partial fulfilment of the requirements for the D.Sc. degree.     

Application of quenching–partitioning–tempering process and modification to a newly designed ultrahigh carbon steel

The applicability of quenching–partitioning–tempering (Q–P–T) process to an ultrahigh carbon steel (UHCS) has been investigated by means of optical microscopy (OM), scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and mechanical property tests. The molten steel was modified with a multi-component modifier-rare earth and a low melting point alloy (Al–Bi–Sb) before casting into iron molds. Observations showed that the carbide exists as partly isolated and fine blocky structure in as-cast microstructure, indicating good effect of modification. After the Q–P–T treatment, carbon was partitioned into austenite from martensite, creating a mixture of carbon-depleted martensite, carbon-enriched retained austenite and fine carbides. This kind of microstructure leads to a much higher impact toughness, 32 J/cm², in comparison with the value, i.e., no more than 20 J/cm², of the conventional quenching and tempering (Q–T) treatment at the same hardness level. Furthermore, wear-resisting property of the steel has also been investigated. It showed that the Q–P–T treated steel has better abrasive wear resistance, about 18% increased, compared with the Q–T treated alloy under high load conditions.     

Influence of carbides morphology on fracture toughness of cast steel G200CrMoNi4-3-3

We analyze the influence of small modification of chemical composition of G200CrMoNi4-3-3 cast steel on the morphology of carbides and on material crack resistance. Using the Termo-Calc software the volume fraction of carbide phase was determined and the results correlated with microstructure observations. Crack resistance of cast steel was determined using SENB specimens and finding critical values of stress intensity factor K_Q. Metallographic and fractographic observations of fracture surfaces allowed identifying the mechanism of cracking. __________ Translated from Problemy Prochnosti, No. 1, pp. 137–140, January–February, 2008.     

Nano-sized precipitates in 11 % Cr ferritic-martensitic steel after thermomechanical treatment

9 %–12 % Cr ferritic/martensitic steels with a good long-term creep strength at temperatures up to 650 °C and higher are being developed in order to increase steam temperature of coal-fired power plants.Thermomechanical treatment can effectively enhance the mechanical properties of high-Cr ferritic/martensitic steels mainly due to plenty of nano-sized precipitates produced by thermomechanical treatment. Nano-sized precipitates in an 11 % Cr ferritic/martensitic steel produced by a thermomechanical treatment, including warm rolling at 650 °C plus tempering at 650 °C for 1 h, were investigated by transmission electron microscopy. The average size of precipitates in the steel after the thermomechanical treatment was determined to be about 30 nm in diameter, which is only one-third of the average size of precipitates in the steel with the normalized and tempered condition. A large number of Cr-rich precipitates having an average diameter of about 25 nm in the steel produced by the thermomechanical treatment were identified as Cr-rich M₂C carbide with a hexagonal crystal structure, rather than M₂₃C₆ or MX phase. The plenty of nano-sized Cr-rich M₂C carbides were dominant phase in the steel after the thermomechanical treatment. The reason why prior precipitate phase formed in the steel during the thermomechanical treatment was Cr-rich M₂C carbide is also discussed.     

Numerical Simulation of Solidification of Work Roll in Centrifugal Casting Process

A program on the solidification process of horizontal centrifugal casting coupled with eutectic carbides segregation has been developed in this paper.Due to the geometrical features of work roll,a cylindrical coordinate system was used.The temperature field of the outer layer at the end of filling process was imported as the initial temperature condition for the solidification process.The model of eutectic carbides segregation caused by different densities between eutectic MC and the molten steel was coupled in the program.The temperature field of the outer layer of work roll during horizontal centrifugal casting process was investigated.Results show that the outer layer has a "sandwich shape" solid fraction manner.Results also indicate that the segregation of eutectic MC is quite severe during centrifugal casting process.It forms four zones of different content of carbides in radial direction.The simulated results of MC carbides segregation phenomenon agree with the experimental observations.     

热处理对粉末冶金高速钢T15碳化物的影响

碳化物是高速钢组织的重要组成部分,其种类和数量对钢的力学性能有重要的影响。本文通过采用物理化学电解法提取碳化物,用SEM对碳化物的形貌进行观察,分别用EDS、XRD测定碳化物的成分和结构,并研究了碳化物对硬度的影响。结果表明：退火态的碳化物质量百分数为28．1％,淬火态的为18．4％,而回火态的质量百分数为24．8％,回火态碳化物扫描照片有明显的二次析出相,产生了二次硬化现象,硬度明显升高。     

Effect of vanadium on cast carbide in high speed steels

Abstract

The effect of vanadium (0–4%) on the morphology and amount of eutectic and eutectoid carbides in high speed steels has been investigated using scanning electron microscopy and image analysis. It was found that vanadium promotes the formation of MC carbide and M₂C carbide, but inhibits the formation of M₆C carbide. In the vanadium free steels, the eutectic carbide consists solely of skeletal M₆C. For each steel composition, there is a critical vanadium content at which the skeletal eutectic changes to lamellar eutectic and the critical value decreases as the molybdenum content of steel increases. The effect of vanadium on the total amount of eutectic carbide differs in tungsten alloyed and molybdenum alloyed high speed steels. The δ eutectoid has a rodlike morphology in tungsten high speed steels; δ eutectoid is not present in Mo–W or molybdenum high speed steels. Increasing the vanadium content leads to an increase in the size of eutectic and eutectoid carbides.

MST/1264