Ultrahigh Carbon Steel Research Articles

Designing, Processing and Isothermal Transformation of Al-Si Medium Carbon Ultrafine High Strength Bainitic Steel

Medium-carbon, silicon-rich steels are commonly suggested to obtain a very fine bainitic microstructure at a low temperature slightly above Ms. Thereby, the resulted microstructure consists of slender bainitic-ferritic plates interwoven with retained austenite. The advanced strength and ductility package of this steel is much dependent on the fineness of bainitic ferrite, as well as the retained austenite phase. In this article, the aluminum to silicon ratio, and the isothermal transformation temperature have been adopted to obtain ultra-high strength high carbon steel. Optical and SEM investigation of the produced steels have been performed. XRD has been used to track the retained austenite development as a result of the change in the chemical composition of developed steels and heat treatment process. Mechanical properties in terms of hardness and microhardness of obtained phases and structure were investigated. Results show that the increment of aluminum to silicon ratio has a great effect in promoting the bainitic transformation, in tandem with improving the stability and the fineness of retained austenite. Such an advanced structure leads to enhancement in the whole mechanical properties of the high carbon steel.

Grinding tool design for the manufacturing of UHC-steel

Recently developed aluminum-alloyed ultra-high carbon steels (UHC-steels) show a high potential for industrial lightweight applications due to their exceptional mechanical properties at a reduced density of 6.7–6.9 g/cm3. However, due to its low thermal conductivity and embedded carbides, the grinding of this material is challenging. In this paper, the grindability of UHC-steel with conventional grinding wheels with a resin bond is investigated. It was shown that it is possible to productively grind UHC-steel if highly porous grinding wheels are used with sintered alumina as abrasive material.

Understanding dual precipitation strengthening in ultra-high strength low carbon steel containing nano-sized copper precipitates and carbides

Low carbon ferritic steel alloyed with Ti, Mo and Cu was hot rolled and interrupt cooled to produce nano-sized precipitates of copper and (Ti,Mo)C carbides. The steel had a tensile strength of 840 MPa, an increase in yield strength of 380 MPa over that of the plain carbon steel and reasonable ductility. Transmission electron microscopy and small angle neutron scattering were used to characterize size and volume fraction of the precipitates in the steels designed to form only copper precipitates and only (Ti,Mo)C carbides. The individual and combined precipitation strengthening contributions was calculated using the size and volume fraction of precipitates and compared with the measured values.

UHCSDB: UltraHigh Carbon Steel Micrograph DataBase

We present a new microstructure dataset consisting of ultrahigh carbon steel (UHCS) micrographs taken over a range of length scales under systematically varied heat treatments. Using the UHCS dataset as a case study, we develop a set of visualization tools for interacting with and exploring large microstructure and metadata datasets. Based on generic microstructure representations adapted from the field of computer vision, these tools enable image-based microstructure retrieval, as well as spatial maps of both microstructure and related metadata, such as processing conditions or properties measurements. We provide the microstructure image data, processing metadata, and source code for these microstructure exploration tools. The UHCS dataset is intended as a community resource for development and evaluation of microstructure data science techniques and for creation of microstructure data science teaching modules.

Exploring the microstructure manifold: Image texture representations applied to ultrahigh carbon steel microstructures

We introduce a microstructure dataset focusing on complex, hierarchical structures found in a single Ultrahigh carbon steel under a range of heat treatments. Applying image representations from contemporary computer vision research to these microstructures, we discuss how both supervised and unsupervised machine learning techniques can be used to yield insight into microstructural trends and their relationship to processing conditions. We evaluate and compare keypoint-based and convolutional neural network representations by classifying microstructures according to their primary microconstituent, and by classifying a subset of the microstructures according to the annealing conditions that generated them. Using t-SNE, a nonlinear dimensionality reduction and visualization technique, we demonstrate graphical methods of exploring microstructure and processing datasets, and for understanding and interpreting high-dimensional microstructure representations.

Material removal and chip formation mechanisms of UHC-steel during grinding

The grinding process is still an important manufacturing process for the machining of automotive components. For power train components, ultra-high carbon steel (UHC-steel) is a promising new innovative alloy because of its low specific density. Results from turning of UHC-steel showed that the texture of UHC-steel significantly differs from conventional steels. Furthermore, extremely hard carbides, which are embedded into a soft ferrite matrix, result in a UHC-steel specific machining behavior and a high tool wear rate. Therefore, UHC-steel is marked as a difficult-to-cut material. So far, there are no research results available for the grinding of UHC-steel. Therefore, fundamental investigations were conducted in order to analyze the material removal and chip formation mechanisms. Scratching tests with a geometrically defined cubic boron nitride cutting edge showed ductile material removal mechanisms for a single grain chip thickness variation from hcu = 1.5 up to 14 μm. Analysis of the contact zone by means of an innovative quick stop device confirms these results.

Coarsening of Inter- and Intra-granular Proeutectoid Cementite in an Initially Pearlitic 2C-4Cr Ultrahigh Carbon Steel

We have examined spheroidization and coarsening of cementite in an initially pearlitic 2C-4Cr ultrahigh carbon steel containing a cementite network. Coarsening kinetics of spheroidized cementite and growth of denuded zones adjacent to the cementite network were investigated by analyzing particle sizes from digital micrographs of water-quenched steel etched with Nital. Denuded zones grew at a rate proportional to t 1/4–t 1/5. Spheroidization of pearlite was completed within 90 minutes at 1073 K and 1173 K (800 °C and 900 °C), and within 5 minutes at 1243 K (970 °C). Bimodal particle size distributions were identified in most of the samples and were more pronounced at higher temperatures and hold times. Peaks in the distributions were attributed to the coarsening of intragranular and grain boundary particles at different rates. A third, non-coarsening peak of particles was present at 1073 K (800 °C) only and was attributed to particles existing prior to the heat treatment. Particle sizes were plotted vs time to investigate possible coarsening mechanisms. The coarsening exponent for the growth of grain boundary carbides was closest to 4, indicating grain boundary diffusion control. The coarsening exponent was closest to 5 for intragranular carbides, indicating suppression of volumetric diffusion (possibly due to reduced effective diffusivity because of Cr alloying) and control by dislocation diffusion.

Connections: Superplasticity, Damascus Steels, Laminated Steels, and Carbon Dating

In this paper, a description is given of the connections that evolved from the initial development of a family of superplastic plain carbon steels that came to be known as Ultra-High Carbon Steels (UHCS). It was observed that their very high carbon contents were similar, if not identical, to those of Damascus steels. There followed a series of attempts to rediscover how the famous patterns found on Damascus steels blades were formed. At the same time, in order to improve the toughness at room temperature of the newly-developed UHCS, laminated composites were made of alternating layers of UHCS and mild steel (and subsequently other steels and other metals). This led to a study of ancient laminated composites, the motives for their manufacture, and the plausibility of some of the claims relating to the number of layers in the final blades. One apparently ancient laminated composite, recovered in 1837 from the great pyramid of Giza which was constructed in about 2750 B.C., stimulated a carbon dating study of ancient steels. The modern interest in “Bladesmithing” has connections back to many of these ancient weapons.

Ultrahigh-carbon “wootz” from crucible carburization of molten iron: Hypereutectoid steel from “Tamil Nadu Process” at Mel-siruvalur

ABSTRACTAs European and Mediterranean accounts indicate, India has been famed for the production of steel, apparently made by crucible processes. Late medieval traveler’s accounts record the making of “wootz” steel in several places in southern India. This material was used for the fabled Damascus swords, which were later found to be of ultrahigh-carbon steel. Whereas studies on Asian crucible steel making from India, Central Asia and Sri Lanka have discussed various processes ranging from co-fusion of cast iron and wrought iron to solid-state carburization of wrought iron, it has been difficult to find clear evidence relating to an end product of ultrahigh-carbon steel. In this light, the archeometallurgical evidence from Mel-siruvalur in Tamil Nadu, presented in this paper, is significant in that it shows unmistakable remnants in crucibles of ultrahigh-carbon, hyper-eutectoid steel, with a likely production mechanism of molten carburization of wrought iron to steel. The favorable comparison with ultrahigh-carbon steel finds dated to early historic or megalithic times in Tamil Nadu and southern India also suggest that this method of crucible steel manufacture, which may be described as the “Tamil Nadu process”, might have been earlier or more archaic than the co-fusion process.

Analytical and microstructural approach to the study of historical ultra-high carbon (UHC) steel plaques belong to the Malek National Library and Museum Institution, Iran

A number of steel plaques in Iran belong to Safavid period (17th century) were examined for their chemical composition and microstructure. The artefacts were analyzed using micro-PIXE, SEM-EDS and reflected light microscopy (OM). According to SEM-EDS and micro-PIXE analyses, it was revealed that the plaques were made of mostly pure iron (approximately 99wt%). Presence of trace amounts of manganese, sulfur, chromium, silicon, titanium and potassium were identified in the plaques. Metallographic investigation showed that the plaques were made of hypereutectoid steel with a microstructure consisting of spherical particles of carbide in the ferrite and pearlite background. Moreover, the use of ultra-high carbon (UHC) steel with microstructure of pearlite matrix and needle-like Widmanstätten cementite was identified in one plaque. These features showed that Safavid plaques have been made of crucible steel. In addition, it demonstrated that complicated forging/heating cycles have been used for manufacturing of these openwork steel plaques. It was also revealed that in addition to arms and armors, crucible steel has served as an everyday work material in Safavid period in Iran.

Digital image analysis to quantify carbide networks in ultrahigh carbon steels

A method has been developed and demonstrated to quantify the degree of carbide network connectivity in ultrahigh carbon steels through digital image processing and analysis of experimental micrographs. It was shown that the network connectivity and carbon content can be correlated to toughness for various ultrahigh carbon steel specimens. The image analysis approach first involved segmenting the carbide network and pearlite matrix into binary contrast representations via a grayscale intensity thresholding operation. Next, the carbide network pixels were skeletonized and parceled into braches and nodes, allowing the determination of a connectivity index for the carbide network. Intermediate image processing steps to remove noise and fill voids in the network are also detailed. The connectivity indexes of scanning electron micrographs were consistent in both secondary and backscattered electron imaging modes, as well as across two different (50× and 100×) magnifications. Results from ultrahigh carbon steels reported here along with other results from the literature generally showed lower connectivity indexes correlated with higher Charpy impact energy (toughness). A deviation from this trend was observed at higher connectivity indexes, consistent with a percolation threshold for crack propagation across the carbide network.

The Novel Technique of Grain Refinement in the Aluminum-Free Ultrahigh Carbon Steel

In order to achieve high strength and toughness of the aluminum-free ultrahigh carbon steel (UHCS), the effect of a novel three-parameter combination technique including heating temperature, cooling rate, and isothermal eutectoid transformation time on the phase composition, grain morphology, and mechanical properties is investigated by X-ray diffraction analysis, optical microscopy, and mechanical tests. The result shows that when the steel is cooled from 860 °C to the eutectoid transformation temperature with the rate of 1.0 °C s−1 and held for 1.5 h, the excellent mechanical properties are obtained due to the grain refinement of carbide and matrix.

Quenching and Partitioning of Ultrahigh Carbon (1.69 Mass% C) Steel

Quenching and partitioning (Q&P) or quenching and tempering (Q&T) process of modified ultrahigh carbon steel (UHCS, Fe–1.69C–1.38Si–1.84Mn–2.20Cr–0.54Mo–0.43Ni wt%) was investigated by means of scanning electronic microscopy (SEM) equipped with energy-dispersive spectrometry (EDS) and x-ray diffraction (XRD). Mechanical properties, including hardness and impact toughness, and wear resistance were also measured. After Q&P, high impact toughness (10.5 J/cm2) and wear resistance at 100 N load in dry sliding were achieved on tested samples. We attributed the optimum mechanical properties and high wear resistance to better microstructure consisting of carbon-depleted martensite, uniformly distributed fine carbides, and high volume fraction of retained austenite.

Flow stress and temperature considerations for orthogonal cutting of an aluminum-alloyed UHC-steel

Recently developed aluminum-alloyed ultra-high carbon steels (UHC-steels) show high potential for industrial lightweight applications due to their exceptional mechanical properties at reduced density of 6.7–6.9 g/cm3. However, earlier publications highlighted that machining these steels results in excessive tool wear. The wear behavior was attributed to the intricate three-phase microstructure as well as the thermal properties of the material. This article aims to give further insight into the influence of tool geometry, cutting speed and uncut chip thickness on process forces and tool temperatures. Moreover, the effects on average strains, strain rates, temperatures in the primary shear zone and flow stress are discussed. The temperature load on the tool is calculated and validated via temperature measurements. The results indicate that the flow stress in the primary shear zone is affected by the strain and strain rate rather than by the shear zone temperature resulting in substantial strain hardening. The temperature measurements as well as the used analytical temperature model show consistently that temperatures above 900 °C can be easily exceeded at the rake face in dry machining of aluminum-alloyed UHC-steels and that the temperature is mainly determined by the cutting speed. Based on the results recommendations for the tool and process design are derived.

Microstructure evolution in warm forged sintered ultrahigh carbon steel

Fe–1.4C–0.65Si–0.85Mo ultrahigh carbon steel was liquid phase sintered in 10%H2–90%N2 at 1300°C from Höganas Astaloy 85 Mo HP base iron, fine graphite and silicon carbide powders mixed with polypropylene glycol. The microstructure then comprised fine pearlite and grain boundary cementite networks and the density increased from ∼6.8gcm−3 to ∼7.7gcm−3. A group of specimens then underwent austenitisation, isothermal quenching/autotempering at M(10%) temperature, followed by cooling to room temperature. This produced a crack-free martensitic microstructure, which transformed to ferrite plus fine spheroidised carbides by annealing for 3h at 750°C. To attain full density and well-distributed submicron carbides, these specimens were warm forged at 700–750°C. To ascertain if some processing steps can be discarded, as-sintered and quenched samples were similarly thermo-mechanically processed. The required stresses and resultant microstructures depended on temperature and strain rate, with optimum microstructure, for Bähr processing at 775°C of quenched material, fully comparable with that of prior spheroidised specimens. Microstructures and hardness values are presented for all processing routes.

Technological tradition of the Mongol Empire as inferred from bloomery and cast iron objects excavated in Karakorum

Iron objects from Karakorum, the former capital of the Mongol Empire, were metallographically examined. Most were forged out of bloomery iron, particularly those requiring superior functional properties. By contrast, approximately one third were made from cast iron, with carbon levels approximating either cast iron or ultrahigh carbon steel. The carbon concentration of the bloomery products was controlled either by a carburization treatment directed at the functional parts or by the welding of a pre-carburized steel plate to a low carbon body. By comparison, cast iron-based steelmaking was achieved by subjecting pieces of solid cast iron to a combined thermal and mechanical treatment aimed at accelerating decarburization. Some anonymous cast objects were circulated as a feedstock for this unique process, naturally taking the form of thin plates. Also, the cast products examined were contaminated with substantial amounts of sulfur and silicon, suggesting that they originated from liquid iron smelted at relatively high temperatures using fossil fuel instead of charcoal. Given these findings, it can be concluded that the Mongol Empire took advantage of an effective multi-faceted iron tradition, which combined bloomery-based and cast iron-based iron technologies. It is important to note, however, that the former still remained the key technological tradition dominating the local contemporary iron industry.

A Method for Quantitative Analysis of Carbide Network Path Lengths in Ultrahigh Carbon Steel

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Solid-state bonding of superplastic ultra-high carbon steel and structure steel with an interlayer

Solid-state bonding between two superplastic steels-ultra-high carbon steel (UHCS) and a structure steel (commonly referred to as 40Cr in china; yet contains only 1 mass% Cr)- was investigated. Industry-grade pure iron sheet was used as an interlayer and the bonding was carried out at the superplastic deformation temperature T = 750 °C . For a pre-pressing stress of P = 56.6 MPa, the initial strain rate was 1.5 × 10−4s−1, and for a bonding time of t = 10min, the joint tensile strength reached up to 702 MPa a value 35% higher than what could be obtained without an interlayer. The bonding strength reached the tensile strength of 40Cr base metal through the same thermal-stress cycling. Based on the results, it was concluded that the weldability of 1.6 mass%C-UHCS / 40Cr steel was improved by using a pure iron interlayer.

Investigating the Effects of a Heat Treatment on Microstructure of an Ultrahigh Carbon Steel through SEM and In Situ CLSM studies

Investigating the Effects of a Heat Treatment on Microstructure of an Ultrahigh Carbon Steel through SEM and <i>In Situ</i> CLSM studies

Improved rolling contact fatigue life for an ultrahigh-carbon steel with nanobainitic microstructure

In the present work, the influence of nanobainite on the rolling contact fatigue (RCF) performance of an ultrahigh-carbon steel has been studied. The results show that nanobainite can effectively improve the RCF life of the steel. In particular, the L10 life of specimens with 21 ± 1 vol.% nanobainite is approximately 3.3 times longer than that of specimens without nanobainite. The improvement in the RCF life of the steel is attributed to the nanobainite, which can delay the crack initiation and propagation.