Recent studies have shown that the graphitisation process in a medium‑carbon steel during high temperature annealing can be accelerated by alloying; the key alloying additions are Si and Al. In addition, the nucleation, morphology and dispersion of graphite particles or nodules are observed to be affected by the pre-anneal microstructure, for example, either ferrite-pearlite, bainite or martensite, as well as the annealing time. The accelerated graphitisation kinetics and microstructural effects are revealed by light optical and electron microscopy, and in parallel with this, are also reflected by the hardness values as dictated by the normal tempering reactions and balance between the graphite and remaining carbides. Thus the extent of graphitisation is shown to influence the machinability of the experimental carbon steel. Indicative machinability was measured by tool wear during drill testing. The drill tests also indicated that optimisation of steel composition and heat treatment could potentially develop promising machinability comparable to existing commercial free-machining steel SAE 1117. Under the conditions of the drill tests conducted, graphitising from the starting bainitic microstructure also resulted in measured tool wear not too dissimilar to that of leaded free-machining steel SAE 12L14.

Appropriate graphitisation of a carbon steel may provide an alternative route to developing more simply and inexpensively alloyed machining steels which are more recyclable than those which are currently available. The extent of crystallinity of graphite particles formed during a graphitisation anneal of an experimental carbon steel was studied by Raman spectroscopy. It is demonstrated that it was possible to record the progress of graphitisation with annealing time as well as to determine which starting microstructural conditions, either ferrite-pearlite, bainite or martensite, formed the most highly crystalline graphite particles during annealing. During a machining operation the graphite should act as an internal lubricant at the chip/tool interface and thus the extent of crystallinity may influence the final machinability of the steel, or effectively, optimisation of the heat treatment. It was revealed by the Raman spectra that the crystallinity of the graphite particles formed was strongly affected by the starting microstructures as well as the graphitising anneal periods. For a similar annealing time, a better degree of crystallinity was observed in graphite particles formed from a ferrite-pearlite starting microstructure rather than bainite and martensite starting microstructures. However, crystallinity could be gradually improved with increasing annealing time from all three of the starting microstructures examined.

Alloying a medium-carbon steel has accelerated graphite formation during annealing, for example, during high-temperature tempering. Electron microscopy and related techniques have been used to characterise the graphite particle structure and dispersion from pre-anneal starting microstructures of ferrite-pearlite, bainite and martensite. Graphitisation in the solid state in steels (and also, for useful comparison, in cast irons) has previously received little attention. However, FIB/FEGSEM has permitted specimen preparation of the relatively coarse microstructures, thus enabling high-resolution observation previously limited for graphite formation in cast irons and steels. The study has importance to the potential development of more economic and user-friendly machining steels, which would rely upon internal lubrication by graphite nodules within the microstructure. Many recently developed advanced steels are reduced in carbon but expensively alloyed to produce the desired properties, whereas an aim in this project is simply to use carbon, which is a very cheap and abundant alloying element.

The machinability of an experimental medium-carbon steel with a composition designed to promote rapid graphitisation during a high temperature anneal has been studied. The goal has been to explore alternative routes to a competitive free-cutting composition enabling less expensive steelmaking, manufacturing and recycling. Three starting microstructures prior to annealing have been considered; martensite, bainite and ferrite/pearlite. The microstructures and graphite dispersions formed have been characterised by optical and electron microscopy and the performance of the steel during machining compared with commercial free-cutting steel grades. A bench-top drill rig and metallographic techniques were used to evaluate relative machinability parameters, including surface roughness, tool wear and chip morphology. Thus it proved possible to rank the experimental steel graphitised from the three starting microstructural conditions and also against the commercial free-cutting steels.

Novel advanced steel developments in recent years have resulted in many steels with complex multiphase microstructures. Foremost currently are steels that contain significant fractions of untransformed parent austenite phase. "Quenched and partitioned" steels, commonly referred to as Q&P steels, important as potential advanced high-strength (AHS) automotive steels, are an example in which the austenite is chemically stabilized by carbon partitioning from martensite following an interrupted quench above the martensite finish temperature, Mf. It is evident that the progress of microstructural change during this novel quenching and partitioning treatment, the volume of untransformed austenite stabilized during partitioning and the behavior of this austenite according to its carbon content should be understood. In relation to these requirements, recent analysis of X-ray powder diffraction (XRD) and neutron diffraction experiments will be considered. Initially, the effect of sample preparation between the two measurement techniques is considered, followed by observation of the partitioning process in real time allowed by specially prepared Mn-containing experimental alloys, with and without a crucial silicon addition required to enable austenite retention during Q&P treatment.

The equilibrium form of carbon in iron and steel is graphite. In higher carbon cast irons graphite is normally formed from the liquid state. In lower carbon steels, carbon invariably exists in the form of metastable cementite, due to slow graphitization kinetics in the solid, but in fact, graphite formation is also more likely to be prevented by alloying to stabilize the microstructure for service. Relatively few high-resolution studies of graphite formation have thus been made in the past; this is also because of probable difficulties associated with specimen preparation of a relatively coarse aggregate microstructure. More recently, new investigative techniques and methods have allowed closer examination of graphite formation in steels. Examples are given of HRTEM, EELS, ELNES, EFTEM, and FIB/FEGSEM recently applied to observations of graphite nucleation and growth in the solid state in steel. This has importance, for example, in the potential development of more economic and user-friendly machining steels, which would rely upon internal lubrication by graphite particles in the microstructure.

The potential for using graphite particles as an internal lubricant during machining is considered. Graphite particles were found to form during graphitisation of experimental medium-carbon steel alloyed with Si and Al. The graphite nucleation sites were strongly influenced by the starting microstructure, whether ferrite–pearlite, bainite or martensite, as revealed by light and electron microscopy. Favourable nucleation sites in the ferrite–pearlite starting microstructure were, not unexpectedly, found to be located within pearlite colonies, no doubt due to the presence of abundant cementite as a source of carbon. In consequence, the final distribution of graphite nodules in ferrite–pearlite microstructures was less uniform than for the bainite microstructure studied. In the case of martensite, this study found a predominance of nucleation at grain boundaries, again leading to less uniform graphite dispersions.

The addition of a third element to hypereutectoid Fe-C alloys may cause inclusions of a minor constituent in the precipitate of cementite from austenite even if the initial austenite is supersaturated only with cementite. Cementite will often become the major constituent of this kind of microstructure. For Fe-C-Cu alloys this has been explained as the result of precipitation from supersaturated cementite. An alternative mechanism could be that the mixture of cementite and a minor constituent forms by simultaneous and cooperative growth of the two phases, i.e., by a reaction that may be regarded as eutectoid. This mechanism has already been applied to explain the occurrence of eutectoid colonies with cementite as the major constituent and a minor constituent for which there was no supersaturation initially.This phenomenon has been observed in hypereutectoid ternary Fe-C alloys with Al, Mn or Si. The necessary requirements on the ordinary isothermal phase diagram are now examined with a graphical method based on the slopes of tie-lines. It predicts the phenomenon in all cases where it has been observed, including Fe-C-Cu and not in the Fe-C-Ni and Fe-C-Cr systems where it has not been observed. The requirements become more evident when the calculated phase equilibria are plotted as an isothermal phase diagram with the alloy content as a function of the carbon activity instead of carbon content. Finally, a comparison is made with bainite in Fe-C alloys where ferrite is the major and cementite the minor constituent. The same two explanations have been proposed for that case. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Designing a competitivefree-machining steel composition enabling less expensive steelmaking, manufacturing and recycling has long been a desirable objective. Foremost amongst a new approach has been simply to promote graphite formation to act as an internal lubricant, thus reducing or eliminating the need for special alloying additions(e.g. Pb, S, P, Bi, Se, Te) which can make the steels difficult to process or re-cycle, and, as more stringent health and safety legislation is introduced might eventually lead to restrictions or total prohibition from certain manufactured products. However, the sluggish kinetics of graphite formation in steel does not lend itself to the normal requirement of rapid manufacturing in thehigh tonnage steel industry. In consequence, this paper reports the machining characteristics of experimental carbon steel with a composition which accelerates graphite formation during a high temperature anneal. Three starting microstructures prior to annealing have been considered; martensite, bainite and ferrite/pearlite. These influence the eventual graphite dispersion and hence the machinability characteristics. The machining characteristics have been measured and also compared with commercial free-cutting steel grades.

This chapter introduces the quenching and partition process, a thermal treatment that is designed to employ carbon partitioning from martensite into untransformed austenite, with the purpose of generating austenite-containing microstructures in low-alloy steels with attractive performance characteristics. Some physical metallurgy fundamentals of the process and associated mechanisms are introduced. Applications for a variety of steel product forms are presented, with selected data from property measurements available to date.

•We present an innovative method of studying the Q&P heat treatment process.•A specially engineered steel was used to study carbon partitioning in real-time.•Measurements were obtained using a neutron diffractometer with an in situ furnace.•Real-time evidence of partitioning was observed in the form of lattice dilatation.•Carbon ‘trapping’ is hypothesised to reduce the carbon available for partitioning.

The graphitic form of carbon is a second phase mostly common to grey cast irons and mainly exists as flakes or with a spheroidal or nodular morphology [1]. It develops either during solidification or can be induced by heat treatment in e.g. white irons [2,3]. In carbon steels it is rarely observed because the formation time is extensive compared with the kinetically favoured iron carbide, Fe3C, cementite. When graphite occurs in steel after long periods during high temperature service it is generally considered detrimental to properties and so the steel microstructure is stabilised by alloying with strong carbide formers such as chromium [4]. Nonetheless, the formation of graphite in steel destined for extensive machining, or designed to possess good machinability or cuttability, perhaps combined with good workability, could be advantageous, in that a graphitic second phase might be expected to act as an ‘internal lubricant’ during the very severe deformation of machining, where external lubrication is relatively ineffective. However, with such high tonnage product the times required for graphitisation are extensive and therefore uneconomic for high volume production. In consequence, free-machining steels are instead alloyed with Pb, Mn, S, P, Te or Bi to improve their machinability, but these compositions can create difficulties during manufacturing and recycling [5]. Thus, attempts have been made to enhance the graphitisation kinetics in carbon steels, which can be divided into two approaches: (i) Introduce heterogeneous nucleation sites, generally carbides, nitrides or oxides; (ii) Destabilise carbide formation by alloying with appropriate graphitising elements whilst minimising carbide forming elements [5,6]. The present paper examines the heterogeneous nucleation process, and, as it can occur during both approaches, is carried out in steel designed to follow mainly the second approach i.e. with a composition lean in carbide formers but rich in graphitisers. A strong graphitiser is Al but this element can also combine with N in the steel to form a potential nitride nucleant. A medium carbon experimental steel similar to that studied in [6] with composition 0.39C, 1.86Si, 1.38Al, 0.11Mn, 0.010P, 0.002S 0.002N (wt.%) was provided by Tata Steel, UK. Samples were austenitised at 1150°C and normalised to a ferrite/pearlite structure before annealing at 680C for a period of 20 minutes. This treatment promoted a coarse dispersion of graphite particles mainly of irregular shape and a number were observed to have a nucleating particle at their core. These particles were examined by high resolution TEM after preparing suitable lamellae by FIB SEM. A typical graphite particle, apparently forming heterogeneously on a precipitate identified as AlN, is shown in the SEM micrograph of Figure 1(a) and the FIB lamella prepared from this complex is shown in Figure 1(b). An HRTEM lattice image from the graphite/precipitate interface region is shown in Figure 2 (a) and a magnified image of the square area indicated is shown in Figure 2 (b). The interface appears quite planar with a potential degree of coherency between the two lattices, indicating the preference for heterogeneous nucleation at AlN. It is also noticed that the initial growth direction varies from different sections of the interface, eventually rotating towards a more common growth direction and creating imperfect regions at the intervening sections.

The novel heat treatment concept of Quenching and Partitioning (Q&P) offers exciting prospects for the production of higher strength steel products with enhanced formability from a microstructure containing retained austenite and martensite. The Q&P process hinges on an interrupted quench and partitioning step at intermediate temperatures whereby the untransformed austenite can be thermodynamically stabilised by enrichment of carbon from the supersaturated martensite. Although the concept is similar to that producing carbide-free bainite in TRIP-assisted steel, Q&P offers the advantage of separating the ferrite formation and austenite enrichment stages of the process. While the concept is readily understood, the details of microstructural evolution during interrupted quenching and partitioning steps are difficult to study and are generally inferred from dilatometry or metallographic examination after a final quench back to room temperature. Consequently, in this study, alloying has been used to develop a model alloy in which the sequential steps of heat treatment can be separated for closer, more direct inspection by neutron diffraction techniques.

Extensive research efforts are underway globally to develop new steel microstructure concepts for high-strength sheet products, driven largely by the need for lightweight automotive structures in support of designs to enhance occupant safety and energy efficiency. One promising approach, involving the quenching and partitioning (Q&P) process, was introduced in the predecessor to this paper series, Austenite Formation and Decomposition, 2003. [ 1 ] Development of the Q&P process has continued through to the present, and the current status is highlighted in this article, along with some alternative approaches that are also receiving attention. Special emphasis is placed on the synthesis and interpretation of the fundamental phase transformation responses, perspectives related to alloying and processing, and the resulting microstructure and properties. Key mechanistic issues are discussed, including carbide formation and suppression, migration of the martensite/austenite interface, carbon partitioning, and partitioning kinetics.

One driver for steel development over recent decades has been the engineering requirements of improved strength and toughness,combined with weldability,for the safe and cost effective recovery and transmission of oil and gas.This has been achieved through refinement of grain size by microalloy precipitation and thermomechanical processing.However,little attention has been paid to these significant changes in steel chemistry and processing on corrosion resistance,despite the dominance of economic carbon steel for construction in the oil and gas industries.The more common forms of corrosion are associated with the presence of H 2 S or CO 2.CO 2 in aqueous solution forms a weak acid sufficient to promote significant localized corrosion in transmission gas/oil pipelines and in well-head applications (’down-hole’).Systematic study has identified the influence of a wide range of alloying elements and different processing conditions on the resistance of low-carbon steels to CO 2 corrosion;strong carbide-forming microalloying elements such as Ti,Nb and V,along with Cr additions,and different levels of Mn,Si,Cu,Mo and Ni,have been explored,along with treatments simulating different processing conditions.The present study also emphasizes the role of V and Ti microalloying in improving the resistance of Cr-containing carbon steel to corrosion in carbonic acid and how this is influenced by microstructure and the metallurgical condition of the microalloying addition,in particular,the extent of precipitation.It is noted that some commercially available corrosion inhibitors contain V as a vanadate compound to interfere with the corrosion process and so it is suggested that V microalloying may also be beneficial if present in an appropriate form in the steel.That Ti also seems to play a role in corrosion in the steels studied is judged to be compatible with the thermodynamics of transition metal anions in the Ti-V-Cr group.