Precipitation in V bearing microalloyed steel containing low concentrations of Ti and Nb

Abstract

The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0·007–0·008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V, Nb, Ti)(C, N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45–70 nm), spherical (20–45 nm), irregular (20–45 nm), and fine (10–20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite–ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti, Nb, V)N) in the core and carbides ((Ti, Nb, V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti_0·53Nb_0·35V_0·12 and Ti_0·6V_0·4, Nb_0·51V_0·49 and Ti_0·64Nb_0·36. Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1₂ structure and exhibited a Baker–Nutting orientation relationship with the ferrite matrix. M₄C₃ types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.     

Effect of thermal aging on pitting corrosion resistance of 16Cr – 5Ni – 1Mo precipitation hardening stainless steel

Abstract

Specimens of precipitation hardening 16-5-1 stainless steel were solution treated at 1050°C for 1 h followed by aging at temperatures in the range 400 – 750°C for various holding times (1 – 16 h). After heat treatment, two types of corrosion test (accelerated and immersion testing) were conducted in 6% ferric chloride solution. The results showed that the pitting corrosion resistance was affected by austenite content, δ ferrite and precipitation of molybdenum and chromium carbides. Three critical temperature ranges were identified, which were related to the phases formed: (a) high corrosion rate at 475°C (δ ferrite and Mo₂ C); (b) low corrosion rate at 550 – 625°C (reversed austenite and Laves phase); (c) intermediate corrosion rate at 750°C (Cr₂₃ C₆ and TiC). The morphology of the pitting was dependent on the form of the δ ferrite and carbides.     

Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

Abstract

For a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min^?1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be ≦600°C to produce fine (≦0·2 μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.

MST/1858     

Surface cracking mechanism of continuously cast low carbon low alloy steel slabs

Abstract

The present state of understanding of surface cracking in low C low alloy steel slabs in the continuous casting (CC) and direct rolling (DR) processes is outlined. Hot cracking of the CC slab surface can be explained in terms of carbide and/or nitride precipitation behaviour. In addition to γ grain boundary precipitation, the matrix strengthening owing to dynamic precipitation and the existence of softer layers along the boundaries such as grain boundary allotriomorphs of ferrite or precipitate free zones play animportant role in intergranular ductile fracture. The origin of hot cracking during the DR process lies also in the precipitation of carbides and/or nitrides, and is not related to the severe embrittlement caused by a similar mechanism with dynamic precipitation of sulphides, which is observed usually in the high strain rate deformation after reheating at higher temperatures. Furthermore, a well known effect of C on hot cracking susceptibility in both CC and DR processes, attaining a maximum in the range 0·10–0·15 wt.–%C, is found to arise mainly from γ grain growth during solidification in the mould. Some methods to prevent surface cracking are also discussed.

MST/1226     

Microstructural examination of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

A detailed microstructural characterization of two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~ 400–250°C), has been carried out using principally electron microscopy, X-ray diffraction, and dilatometry. Upper bainite in these silicon-containing steels consists of bainitic ferrite laths and interwoven thin films of retained austenite instead of cementite. Coarser granular regions of retained austenite may also be obtained. The bainitic ferrite laths (or plates) in lower bainitic structures contain intralath carbides, but the interlath morphology of retained austenite still occurs. The variations in these microstructures with isothermal transformation temperature, and the thermal stability of the retained austenite phase is described and discussed.

MST/526     

Effect of cerium sulphide particle dispersions on acicular ferrite microstructure development in steels

Abstract

Of the phases found in wrought steels, cerium sulphide particles are notable in that they can be both stable at liquid metal temperatures and exhibit good lattice coherency with α iron. An investigation has been carried out to determine the effectiveness of cerium sulphide particle dispersions in nucleating intragranular acicular ferrite microstructures in steels. Vacuum melts of 50 kg have been manufactured of appropriate base steel compositions with varying additions of cerium (0·04–0·18%) and sulphur (0·01–0·04%). The work has shown that 0·02–0·12% cerium and 70–340 ppm sulphur may be retained in steels after deoxidation and desulphurisation reactions while oxygen can be reduced to <20 ppm. Resulting inclusions are largely spheroidal in shape and consist of several crystalline compounds, notably CeS, Ce₃S₄ and Ce₂O₂S. The inclusion numbers are of the order of 0·68–6·12 × 10⁶ mm^?3 with mean diameters of 0·63–1·70 μm. The densities of these inclusion dispersions are approaching those in weld metals where acicular ferrite is the dominant microstructure constituent. Significant volume fractions of acicular ferrite (up to 65%) have been obtained in steels after thermally cycling in a dilatometer and cooling at rates simulating transformation conditions ranging from high heat input welding to air cooling of forgings and water cooling of plate. A potential beneficial effect of acicular ferrite on mechanical properties in high heat input welding (heat affected zone grain refinement) and in thermo-mechanically processed steels (relaxed schedules) has been highlighted. A pilot plant billet cast of steel has shown the feasibility of achieving the required particle dispersions and acicular ferrite microstructure in tonnage steelmaking.     

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains d_α formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size d_γ. In the cooling rate range studied (q= 0·05–5 K s^?1), d_α was found to be proportional to q^?0·26d_γ^0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q^?0·17d_γ^0·33. This was in close agreement with the dependence obtained in the present experiment.

MST/466     

Modelling thermal and microstructural evolution on runout table of hot strip mill

Abstract

A mathematical model has been developed to predict the thermal and phase transformation response of a 0·34 and a 0·05 wt-%C steel during cooling on the runout table of a hot strip mill. The model incorporates the cooling characteristics of laminar water bar sprays, the austenite–ferrite plus pearlite phase transformation kinetics as a function of the austenite grain size, and the heat of transformation. Overall heat transfer coefficients for the laminar water banks were determined from data obtained from in-plant trials carried out at the Stelco Lake Erie Works (LEW) hot strip mill. Isothermal and continuous cooling diametral dilatometer tests were performed on a Gleeble 1500 thermomechanical simulator at temperatures and cooling rates that simulate LEW hot strip mill conditions. The isothermal data were used to establish the phase transformation kinetics as afunction of austenite grain size and temperature. The continuous cooling results were used to obtain the relationship between cooling rate, transformation start temperature, and fraction of ferrite formed. The model was tested and validated by simulating the LEW cooling conditions while monitoring the phase transformation behaviour and by comparison of predicted and measured microstructural detail.

MST/1331     

Josefsson

Abstract

An investigation of 2·25Cr–1Mo–0·1C weld metal has been carried out using atom probe field ion microscopy. The weld metal had a microstructure consisting of bainitic ferrite and retained austenite, but no cementite. The carbon concentration at the austenite/ferrite interface was found to change abruptly, whereas the concentration of substitutional alloying elements was the same in both phases. No enrichment could be found at the interface.

MST/1444     

Age hardening behaviour during reverse transformation from martensite to austenite in Fe–31·1wt-%Ni

Abstract

The effects of thermomechanical treatments on the reverse transformation behaviour from twinned plate martensite to austenite in Fe–31·1%Ni have been studied. The variation of both diffusion controlled and diffusionless reverse transformation with temperature and time was examined. Diffusional reversion was dominant at lower reheating temperatures and led to a fine martensite–austenite duplex microstructure with a grain size of 0·01–0·1 μm, which caused a remarkable hardening ?H_v of 170–230 HV during aging. Cold working of the martensite promoted diffusional reversion and enhanced age hardening. X-ray analysis indicates that the age hardening is caused mainly by elastic strain resulting from coherent precipitation of austenite in martensite.

MST/1414     

Structure–property relationships for quenched and tempered flanges made to ASTM A350 LF2 specification

Abstract

The structure–property relationships for quenched and tempered flanges made to the ASTM A350 LF2 specification have been determined. Samples from Al containing and Al free forged flanges have been heated to temperatures in the range 900–1000°C to produce a wide range in γ grain size and quenched in oil or iced water to produce a variety of commercially obtainable microstructures from fine grained ferrite–pearlite to fully bainitic. After quenching, the samples were tempered at 600°C, and the Charpy V-notch impact values at ?46°C and room temperature hardness values were obtained. Raising the quenching temperature reduced the impact energy values and increased the hardness. Increasing the cooling rate also increased hardness, but there was little change in impact behaviour. Impact behaviour was found to be mainly dependent on the γ grain size: the coarser the grain size, i.e. the higher the quench temperature, the lower the impact value. The facet size was also found to be related to γ grain size, indicating that the high angle grain boundaries, i.e. γ, bainite colony, and ferrite boundaries, were the main obstacles to crack propagation. Increasing the cooling rate from the austenitising temperature increased hardness without significantly affecting the impact behaviour. It is believed that the expected deterioration in impact behaviour associated with an increase in hardness was offset by a refinement of the carbides and the removal of the long carbides situated at the interlath ferrite boundaries. The impact energy in joules at ?46°C was given by the equation: impact energy absorbed at ?46°C=82+18·6d^?1/2?0·8H+0·05CR, where d is austenite grain size in millimetres, H is hardness HV10, and CR is cooling rate in K min^?. To meet the ASTM A350 LF2 impact requirement, the γ grain size should be below 40 μm, and this necessitates a grain refining addition.

MST/771     

Transformation of (Cr,M)7C3-type carbides during nitriding of chromium alloyed steels

Thein situ rearrangement of (Cr, M)₇C₃-type carbides has been observed during ion nitriding of a commercial chromium-carbon alloyed steel (Z160CDV12). The mass balance concerning nitrogen, carbon and substitutional elements proves that carbides undergo a total transformation into substitutional (Cr, M) N-type nitrides with a simultaneous release ofα-iron from the very nitride phase. A detailed transmission electron microscopy microstructural study confirms the analytical results. There are no crystallographic relations between the carbides and their corresponding transformation products. The rejection of carbon from the carbides into the ferrite matrix leads to the precipitation of a network of cementite along the prior austenite grain boundaries.     

XRD and TEM analysis of microstructure in the welding zone of 9Cr-1Mo-V-Nb heat-resisting steel

Under the condition of tungsten inert gas shielded welding (TIG) + shielded metal arc welding (SMAW) technology, the microstructure in the welding zone of 9Cr-1Mo-V-Nb (P91) heat-resisting steel is studied by means of X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The test results indicate that when the weld heat input (E) of TIG is 8.5 ∼ 11.7 kJ/cm and the weld heat input of SMAW is 13.3 ∼ 210 kJ/cm, the microstructure in the weld metal is composed of austenite and a little amount of δ ferrite. The substructure of austenite is crypto-crystal martensite, which included angle. There are some spot precipitates in the martensite base. TEM analysis indicates that the fine structure in the heat-affected zone is lath martensite. There are some carbides (lattice constant, 1.064 nm) at the boundary of grain as well as inside the grain, most of which are Cr₂₃C₆ and a little amount of (Fe, Me)₂₃C₆.     

Influence of manganese and sulphur on overheating of some low-alloy steels

Abstract

An assessment has been made of the overheating behaviour of three low-alloy steels used in the electric power generating industries. The steels, 1Cr–Mo–V, 2·25Cr–1Mo, and 3·5Ni–Cr–Mo–V have been prepared as high-purity versions with low tramp element contents, sulphur contents of 0·001%, and manganese contents of 0·02 and 0·2%. For comparison, commercial steels produced by good practice and containing 0·006–0·011%S and 0·17–0·21%Mn have also been examined (all compositions in wt-%). The upper shelf energies of the high-purity versions of the steels in the fully heat treated condition indicate that these steels do not overheat after treatment at temperatures up to 1400°C, whereas the commercial versions do overheat and, in some cases, show a severe reduction in their impact energy levels. In some cases, the high-purity steels show an unusually low tendency to austenite grain growth after reheating at temperatures up to 1400°C. The results obtained show that new specifications for low–alloy steels could be developed which would give freedom from overheating during forging and greatly improved upper shelf energies after heat treatment.

MST/362     

Austenite–ferrite transformation in enhanced niobium,low carbon steel

The austenite to ferrite transformation characteristics of a commercial high strength line pipe steel containing 0·05 wt-% carbon and 0·095 wt-% niobium have been rigorously studied by continuous cooling experiments in the range between 960 and 1260°C. A significant delay in the austenite to allotriomorphic ferrite transformation has been demonstrated to occur under practically relevant thermal processing conditions. The effects of prior austenite grain size and soluble niobium have been carefully evaluated and isolated and it has been concluded that the amount of niobium in solution in the austenite is primarily responsible for the retardation. Alternative hypotheses to explain the mechanism whereby niobium exerts this effect on the hardenability of steel are discussed in detail. Soluble niobium reducing the austenite grain boundary energy is argued to be the most convincing explanation of the phenomenon and a reduction of grain boundary energy of 0·286 J m^?2 per wt-% of soluble niobium content has been proposed.     

Microstructure of copper-bearing C---Mn weld metal: As-welded and stress-relieved states

The effect of copper concentration in the range from 0.02 wt.% to 1.4 wt.% on the microstructure and hardness of multipass C---Mn welds has been studied both in the as-welded state and after a stress relief treatment of 2 h at 590°C. The microstructure has been investigated in terms of the proportions of the various ferrite morphologies, grain size, second-phase (carbides, martensite, retained austenite, …) volume fraction, inclusion distribution, and the extent of copper precipitation.

The main effects due to copper are (1) increase in hardness, (2) refinement of the microstructure, (3) increase in the second-phase volume fraction and (4) local precipitation of ε-Cu in the as-welded state. These results are discussed with respect to copper as a γ-stabilizing element. The local precipitation of copper is discussed in terms of composition variations due to segregation effects in the welds.

The stress relief treatment leads to (i) precipitation and spheroidization of carbides, and (ii) precipitation of ε-Cu, including grain boundary precipitation, in the 0.66% Cu and 1.4% Cu welds.     

齿轮钢20CrMoNb奥氏体晶粒尺寸及相变行为

在Gleeble1500热模拟试验机上对20CrMoNb齿轮钢进行了不同温度处理以及连续冷却相变实验,使用光学显微镜、透射电子显微镜以及膨胀曲线方法研究了齿轮钢20CrMoNb加热时奥氏体晶粒尺寸变化及连续冷却相变行为.实验结果表明,加热温度在1050℃以下时,奥氏体晶粒细小;超过此温度,NbC粒子数量因溶解而大大降低,对晶界的钉扎作用消失,奥氏体晶粒急剧粗化.20CrMoNb齿轮钢含有一定量的Mo、Nb元素,奥氏体比较稳定,出现先共析铁素体与珠光体的冷速很小.     

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.     

The effect of the solution treatment onto the microstructure and mechanical properties of MAG pulsed welded joints from X2CrNiMoN22‐5‐3 Duplex stainless steels

The metallurgical behaviour by Duplex stainless steels welding is affected by reducing the austenite proportion in weld and in the area adjacent to the fusion line of the molten metal bath and also by the precipitation of nitrides Cr₂N, carbides M₂₃C₆ and intermetallic phases, σ, χ, Laves. The modalities for obtaining a quantitative ratio of the two phases (Austenite/Ferrite) close to that of the base metal (～50 % Austenite and 50 % Ferrite) aims to adjust the chemical composition of the weld by selecting a filler material with a higher nickel content (the element which beside nitrogen promotes the austenite formation), the heat cycle control of the welding process and the application of a post‐welding solution treatment. The present paper explores the effect of such heat treatment on balance restoring between austenite and ferrite and the reduction of the alloying elements segregation phenomena. By optical and scanning electron microscopy examinations and also X‐ray diffraction analyses the microstructural changes induced by the applied treatment are highlighted and by impact toughness and static tensile tests is demonstrated the positive effect of the heat treatment onto the ensuring of the welded joints quality.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 1 – Microstructural analysis

Abstract

Specimens of 16Cr - 5Ni - 1Mo stainless steel were solution treated at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy, and X-ray diffraction examinations were conducted. The microstructure of all aged specimens was found to consist of martensite with variable fractions of δ ferrite and reversed austenite. Very fine precipitates of Mo carbides were revealed in the specimens aged at 475 ° C. The specimens aged at 625 ° C showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe₂Mo Laves phase was detected by X-ray analysis. Above 625 ° C, Cr₂₃C₆ and TiC became the predominate carbides heterogeneously precipitated in the martensitic matrix. Partial transformation of reversed austenite to unaged martensite was observed at temperatures above 625 ° C.