Precipitation Of Carbonitrides Research Articles

Simulation of Precipitates Evolution in Steels with V and Nb at Annealing

Based on the mean field approximation, a model has been worked out for the description of evolution of carbonitride precipitate ensemble with various composition in steels at the stages of their growth, dissolution and coarsening. Based on the numerical realization of this model, the calculations of growth and dissolution kinetics of carbonitrides in a Fe-Nb-V-C-N system have been carried out.

Experimental research on the effect of induction reheating on the microstructure and mechanical properties of hot-rolled low-alloy steel plate

Electric induction reheating is used to control the temperature of hot-rolled steel products. To investigate the effect of this on microstructure and mechanical properties, hot-rolled low-alloy steel plate was cooled, reheated, and then either quenched or normalized. Analysis of the microstructure and mechanical properties showed that this process had a greater effect on quenched steel plate than on normalized steel plate. Reheating was found to affect the precipitation of alloy carbonitrides, and the recovery and recrystallization of deformed austenite. In the single-phase region, the lower cooling temperature resulted in better mechanical properties. In normalized steel plate, the interlamellar spacing of pearlite (ISP) increased from 118nm to 165nm after induction reheating. In steel plate quenched with a 70oC temperature difference, induction reheating reduced the strength difference from 80MPa to 23MPa. Our results provide useful references for optimizing electric induction and improving the uniformity of the mechanical properties of low-alloy steel.

Modelling of austenite grain growth kinetics in a microalloyed steel (30MSV6) in the presence of carbonitride precipitates

Austenite grain growth kinetics in the presence of secondary precipitates of a microalloyed steel (30MSV6) was studied employing quantitative metallographic techniques. Austenitizing experiments were carried out at 1,000, 1,100 and 1,200 °C. According to the experimental data, abnormal grain growth behaviour is observed at 1,100 °C while it is normal at 1,000 and 1,200 °C. TEM observation represents multicomponent carbonitride precipitate, (Ti,V)(C,N), in the as-received steel with a mean radius of 35 nm. A mathematical model is proposed considering austenite grain growth along with dissolution and coarsening kinetics of the multicomponent precipitates. The austenite grain growth model for short-term non-isothermal and subsequent long-term isothermal heating stages was developed using a statistical approach. The model includes an algorithm to estimate the size distribution of austenite grains. Precipitate mean field dissolution and Lifshitz–Slyozov–Wagner coarsening models were integrated in the proposed model to calculate the pinning pressure retarding the grain boundary movement. The mean austenite grain size and the grain size distribution (normal and abnormal) calculated by the model are in good agreement with experimental data.

Evolution of Laves-Phase Particles in a Low Carbon 9%Cr Martensitic Steel during Creep at 650°C

The evolution of microstructure in low carbon heat resistant steel of P92-type modified with 3%Co was examined during creep tests at 923K for 500, 1500, 6000 and 16000 hours. After tempering at 1023K (750°C), the steel was composed of martensite lath structure with numerous precipitates of MX-type carbonitrides and rare M23C6-type carbides. The structural changes during creep tests are characterized by an increase in the sizes of laths and second phase particles. Moreover, the Fe2W Laves-phase precipitates during long-term creep. The Laves-phase particles grow accordingly to power-law relationship of creep time.

Precipitation behavior and martensite lath coarsening during tempering of T/P92 ferritic heat-resistant steel

Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facilitates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the formation of M3C (cementite) precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides (MX) along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener’s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro-structural evolution and hardness variation, the process of tempering can be separated into three steps.

Ferritic Stainless Steels for High-Temperature Applications: Stabilization of the Microstructure by Solid State Precipitation of MX Carbonitrides

AbstractFerritic heat-resistant steels are commonly used for automotive exhaust systems and have replaced cast iron, the traditional material for this application. Efforts to improve the efficiency of engines, reduce weight, and minimize toxic ingredients by increasing the gas temperature have shifted the requirement for ferritic heat-resistant steels to a higher hot strength. Methods of improving the high-temperature strength are solid-solution strengthening, precipitation hardening, and grain refinement. In this work, the influence of MX precipitates on the high-temperature mechanical properties of three different ferritic Fe-Cr stainless steels was investigated and compared to a reference material. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900 °C for up to 1440 h. The most effective method of increasing the high-temperature strength is to alloy the steel with 2 mass% tungsten. Grain growth during annealing at 900 °C was decelerated by solid-state formation of MX carbonitrides. Microstructural investigations also revealed a slow coarsening rate of the MX precipitates.

Reasonable Temperature Schedules for Cold or Hot Charging of Continuously Cast Steel Slabs

Some continuously cast steel slabs are sensitive to transverse fracture problems during transportation or handling away from their storage state, while some steel slabs are sensitive to surface transverse cracks during the following rolling process in a certain hot charging temperature range. It is revealed that the investigated steel slabs with high fracture tendency under room cooling condition always contain pearlite transformation delayed elements, which lead to the internal brittle bainitic structure formation, while some microalloyed steels exhibit high surface crack susceptibility to hot charging temperatures due to carbonitride precipitation. According to the calculated internal cooling rates and CCT diagrams, the slabs with high fracture tendency during cold charging should be slowly cooled after cutting to length from hot strand or charged to the reheating furnace directly above their bainite formation temperatures. Based on a thermodynamic calculation for carbonitride precipitation in austenite, the sensitive hot charging temperature range of related steels was revealed for the determination of reasonable temperature schedules.

OPTIMIZING CONTROL OF PRECIPITATES IN T91FERRITIC HEAT-RESISTAN STEEL

T91 steel is one representative of (9%—12%)Cr (mass fraction) ferritic heat–resistant steel, in which MX carbonitrides and M23C6 carbides are two major strengthened precipitates for long–term creep under high temperature. This work attempted to control the precipitation of MX carbonitrides and M23C6 carbides by applying new heat treatment procedures. With the assist of Thermal–Calc software calculation, two new heat treatment procedures have been designed for T91 steel based on its traditional normalized–tempered treatment, in which an isothermal treatment at 850 was introduced between normalized treatment and tempered treatment. The mean size of M23C6 carbides decreased from 350 nm to 250 nm and the number density of MX carbonitrides increased due to the new heat treatment procedures. The calculated results of Thermal–Calc software showed that the nucleation rate of M23C6 carbides at 750 increased with the decrease of carbon content in the matrix, which might be the major reason why the mean size ofM23C6 carbides decreased.

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

AbstractThermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature.

Microstructural Evolution and Mechanical Properties of Short-Term Thermally Exposed 9/12Cr Heat-Resistant Steels

The microstructural evolution during short-term (up to 3000 hours) thermal exposure of three 9/12Cr heat-resistant steels was studied, as well as the mechanical properties after exposure. The tempered martensitic lath structure, as well as the precipitation of carbide and MX type carbonitrides in the steel matrix, was stable after 3000 hours of exposure at 873 K (600 °C). A microstructure observation showed that during the short-term thermal exposure process, the change of mechanical properties was caused mainly by the formation and growth of Laves-phase precipitates in the steels. On thermal exposure, with an increase of cobalt and tungsten contents, cobalt could promote the segregation of tungsten along the martensite lath to form Laves phase, and a large size and high density of Laves-phase precipitates along the grain boundaries could lead to the brittle intergranular fracture of the steels.

Microstructure Evolution in an Advanced 9 pct Cr Martensitic Steel during Creep at 923 K (650 °C)

Crept microstructures were examined in a 9 pct Cr martensitic steel with low carbon content. The steel was hot forged at 1323 K (1050 °C) followed by air cooling and then tempered at 1023 K (750 °C) for 3 hours. The tempered microstructure included numerous precipitates of MX-type carbonitrides and a small amount of M23C6-type carbides. Two groups of the MX-type particles were observed. The nanoscale MX-type precipitates appeared in the form of plate- and round-shaped particles. The plate-shaped MX-type particles were approximately 15 nm in the longitudinal direction and approximately 3 nm in thickness, and the round-shaped MX-type particles were 10 nm in diameter. In addition to the fine particles, the tempered martensite contained relatively coarse MX-type particles, which have a size of approximately 90 nm. The structural changes that occurred during the creep test were associated with an increase in the sizes of the lath and second-phase particles. Moreover, the creep was accompanied by appearance of Laves and Z-phase particles.

High Strength Low Alloy Steel Weldments with Accommodated Qualities to the Base Metal

In the development of high-strength and ultra-high-strength low alloy steels (HSLA), boron alloyed quenched and tempered steels, dual phase and multi phase steels, all metallurgical strengthening mechanisms like grain refinement, precipitation hardening, solid solution hardening and transformation hardening are applied to achieve the required steel properties regarding strength level and toughness. Microalloying elements like Nb, Ti, V, and B play thereby also an important role. The extent of activation of these strengthening mechanisms in the steel is dominated by the alloying concept and the milling capability of the steel plant which results in different chemical steel composition for similar strength levels and plate thicknesses depending on the steel manufacturer. If high-strength and ultra-high-strength low alloy steels are welded, changes within the optimized strip microstructure caused by the weld heat input take place in the vicinity of the weld. This area is called heat affected zone (HAZ). Close to the weld the heating of the base metal caused by the weld pool is strong: temperatures clearly above Ac3 are achieved. In this high temperature region often an undesired grain coarsening and an increase in hardness can be found. Somewhat away from the fusion line the temperatures are lower. If they are beyond Ac1 soft-zones within the microstructure can be found. Furthermore, the weld heat input can cause carbide and/or carbonitride precipitation which can embrittle the HAZ but also reheated weld areas. The selection of an appropriate filler metal is often difficult due to different alloying concepts of the steel producers. In general, a dilution with the base metal of about 25 % (depending on welding process and technology) has to be taken into account. Therefore the filler metal has to be chemically adjusted regarding the alloying concept of the steel strip to establish optimized welds. For this reason Voestalpine has developed own filler metals (solid wires, cored wires, submerged arc wire and powders) which take into account the chemical composition of its high strength and ultra high strength hot-rolled steel strips (alform® strip, alform® strip x-treme). In combination with various welding parameters, welding procedures and strip thicknesses "alform®-welding-system" supports customers with a huge set of mechanical test data of alform®-joints to give the best support.

Study on Behavior of Carbonitride in Nb Micro-Alloyed Steel in CC-HR Region

As for the techno -interface of CC-HR region, the precipitation and dissolution of carbonitride in Nb micro-alloyed steel E36 were studied during continuous casting, roller conveying, stack cooling, heat-preserved and heating units in typical slab process flow by hot simulation experiment based on study of heat transfer. The results are as follows: Ti(C,N), and (Ti,Nb)(C,N) precipitate in turn in continuous casting and conveying. A few (Ti,Nb)(C,N) particles are not solid solute completely after heating. The precipitation of carbonitride is effected significantly by different cooling rate in conveying. When slab conveied at the rate of 6.00 °C/min and then cooled down to 600°C, the distribution density of carbonitride particles is higher, and the average equivalent diameter of carbonitride particles is smaller, some carbonitride particles distribute along the austenitic grain boundaries and the generation elements of carbonitride particles are under over-saturation state. When slab conveyed at the rate of 0.30 °C/min or 0.15 °C/min and then cooled down to 400°C, the distribution density of carbonitride particles is lower, and the average equivalent diameter of carbonitride particles is bigger, the carbonitride particles precipitate fully. The behavior of carbonitride in slab are effected by different thermal history in heating. The status of carbonitride is effected insignificantly by different thermal history after heating.

Thermodynamic Calculation and Experimental Investigation of Second Phase Particles in Low Carbon Nb Microalloyed Steels

Many experimental methods such as transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy were used to study the precipitates of low carbon steel with niobium addition, the austenite/carbonitride equilibrium in FeNbCN system was performed in this paper. The equilibrium volume fraction, chemical driving force of carbonitride precipitation fraction of each element in austenite, and carbonitride at different temperature were calculated. The results show that the equilibrium mole fraction of carbonitride increases, mole fraction of each element dissolved in austenite are basically precipitated fully at low temperature, the chemical driving force is 2.072 × 103 J · mol−1 at 1050°C. It is found that Nb(CN) precipitated on grain boundaries by using TEM. The volume fraction of Nb(CN) precipitates increased with the temperature decrease, which is in good agreement with the value calculated by the model.

A Review of the Stress-Relaxation Method for Following the Kinetics of Precipitation, Recovery and Recrystallization

Stress-relaxation method was first used by Liu and Jonas in 1980s for following the kinetics of strain induced precipitation of carbonitrides in austenite. Since then, the method has been widely employed for studying not only the precipitation kinetics but also the recovery and recrystallization processes in steels and other alloys. In the present paper, the principle and the worldwide usage of the method were reviewed. The dependence of stress relaxation rate on the occurrence of precipitation, recovery and recrystallization was interpreted based on a dislocation model. The capabilities and the limitations of the method were discussed and ways to overcome the existing limitations were also proposed.

Improving creep strength of 316L stainless steel by alloying with nitrogen

► Nitrogen was found to be beneficial to creep properties at all the stress levels. ► Creep rupture life increased almost 10 times by increasing nitrogen content from 0.07 wt.% to 0.22 wt.%. ► The extent of internal creep damage and surface creep damage decreased with increasing nitrogen content. The influence of nitrogen on the creep behaviour of 316L(N) SS has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt.%. Creep tests were carried out at 923 K at stress levels of 140, 175, 200 and 225 MPa with rupture life up to 16,000 h. Creep rupture strength was found to increase substantially with increase in nitrogen content; rupture life increased almost 10 times by increasing nitrogen content from 0.07 wt.% to 0.22 wt.%. Steady state creep rate decreased significantly with increasing nitrogen content. The extent of internal creep damage and surface creep damage decreased remarkably with increasing nitrogen content, resulting in increased creep rupture strength. Solid solution strengthening, increase in Young's modulus, decrease in stacking fault energy and matrix precipitation of carbonitrides have contributed to the increase in creep strength with increasing nitrogen content.

Study of Carbonitride Precipitates in the Fe-Cr-Mn-N Hardfacing Alloy

The Fe-Cr-Mn-N hardfacing alloy was deposited on a low carbon steel substrate by the weld hardfacing technique. Ti and Nb as the most effective nitrogen-fixing elements were added in the hardfacing alloy. Carbonitride precipitates were systematically studied by canning electron microscopy and electron probe microanalyser. The thermodynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that carbonitride precipitates were complex carbonitride of Ti and Nb distributing on grain boundary and matrix of the hardfacing alloy. In as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. In heat-treated condition, a large number of secondary carbonitrides can precipitate-out with very fine size and make a great secondary hardening effect on the matrix. As a result, precipitation of carbonitride in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and increase the wear resistance of the hardfacing alloy. Keywords: carbonitride precipitate, hardfacing alloy, deposit, thermodynamics

Microstructure and Deformation Behavior of a Hot Forged 9%Cr Creep Resistant Steel

The tempered microstructure and the creep behaviour were studied in an ultra low carbon 9%Cr martensitic creep resistant steel. The starting material was forged at 1050°C followed by air cooling and then tempered at a temperature of 750°C for 3 hours. This treatment resulted in the mean transverse lath size of about 240 nm; the dislocation density in lath interiors comprised 4 × 1014 m-2. The tempered martensite lath structure (TMLS) is characterised by homogeneous precipitation of numerous MX-type carbonitrides and a small amount of relatively coarse M23C6–type carbides. Three kinds of MX carbonitrides were observed in the tempered lath martensite structure. Those were plate-shaped particles with longitudinal size of about 15 nm and thickness of 3 nm; round-shaped particles of about 10 nm in diameter; and relatively large almost equiaxed particles with mean size of about 90 nm. The large MX particles were resulted from incomplete dissolution of such carbonitrides at 1050°C, while the nanoscale particles homogeneously precipitated during the tempering. The creep tests conducted at 650°C showed that the studied steel demonstrated superior creep resistance. Namely, the rupture time was about an order as long as that for P92-type creep resistant steel.

9%Cr heat resistant steels: Alloy design, microstructure evolution and creep response at 650 °C

In this work 9%Cr alloys were designed supported by computational thermodynamic methods. Two sets of alloys were produced: 9%Cr alloys with 0.1%C and 0.05%C and 9%Cr alloys containing ∼0.03% Ti with 0.1%C and 0.05%C (always wt%). Microstructure investigations showed good agreement with the predicted phases of the thermodynamic modeling. The volume fraction of precipitated M 23C 6 carbides is directly related to the carbon content of the alloys. For Ti-containing alloys the precipitation of nano-sized Ti-rich MX carbonitrides was observed. The microstructure evolution (sub-grain and particle size) during creep at 650 °C/100 MPa was investigated by STEM-HAADF. The sub-grain size evolution and the coarsening of precipitates (MX carbonitrides, M 23C 6 and Laves phase) were more pronounced for Ti-containing alloys. 9Cr alloys without Ti and with low carbon content presented the highest creep strength of all investigated alloys.

On the Precipitation Sequence in a 10%Cr Steel under Tempering

Precipitation sequence under tempering in a 10%Cr steel has been considered in details. Two different precipitation processes have been identified, concurrently. First, at 350°C, a dispersion of M2C phase with orthorhombic lattice appears. At 525°C, the M2C phase is displaced by the formation of M23C6 carbide. However, this process remains uncompleted. Dissolution of M2C phase occurs completely due to the formation and growth of more stable M6C phase as a result of tempering in the range of 650 to 770°C. Second, Nb-rich and V-rich M(C,N) carbonitrides precipitate in the temperature interval of 500–770°C. These dispersoids play a role of heterogeneous nucleation sites for precipitations of M23C6 and M6C carbides.