Review of Z phase precipitation in 9–12 wt-%Cr steels

For high temperature applications, 9–12?wt-%Cr steels in fossil fired power plants rely upon precipitate strengthening from (V,Nb)N MX nitrides for long term creep strength. During prolonged exposure at service temperature, another nitride precipitates: Cr(V,Nb)N Z phase. The Z phases lowly replace MX, eventually causing a breakdown in creep strength. The present paper reviews the Z phase and its behaviour in 9–12?wt-%Cr steels including thermodynamic modelling, crystal structure, nucleation process and precipitation rate as a function of chemical composition. The influence of Z phase precipitation upon long term creep strength is assessed from several different 9–12wt-%Cr steel grades and alloy design philosophies.     

Quantitative X-ray diffraction analysis of development of Z phase in 12%Cr–Nb–V–N steel

Abstract

To study the evolution of nitrides (Nb,V)N, (V,Nb)N and Cr₂N, and in particular the formation of the Z phase Cr(V, Nb)N, a model alloy with composition similar to that of 12%Cr steels for high temperature applications, microalloyed with Nb and V but with a very low carbon content, has been designed. A quantitative determination of the volume fractions of the extracted nitrides that had formed after aging treatments at 650°C for up to 10 000 h was carried out by an X-ray diffraction procedure, based on the Rietveld approach. The investigation of the Z phase evolution by the Johnson–Mehl–Avrami–Kolmogorov kinetics at 650 and 700°C revealed that, as the kinetic exponent is very close to unity, the formation mechanism of this phase is not associated with a conventional process of nucleation but hints at a gradual diffusion controlled transformation of the pre-existing V and Nb nitrides.     

Investigations of ferritic/martensitic super heat resistant 11‐12 %Cr steels for 650 °C power plants

The investigations of advanced ferritic/martensitic 11–12 %Cr steels for 650 °C power plant components focus on the improvement of high‐temperature creep properties with respect to chemical composition. The claim of the DFG research work was the development of new heat‐resistant 12 %Cr ferritic‐martensitic steels with sufficient creep and oxidation resistance for a 650 °C application by using basic principles and concepts of physical metallurgy on the basis of the state of art and to overcome the usual trial and error industrial alloy development. Efforts are focussed on a 100,000h creep strength of 100MPa at 650 °C in combination with a sufficient corrosion resistance by a Cr content of 12 % with contents 4‐5 %W, 3.4‐5,5 %Co, V, B and 1 %Cu as well as the choice of Ta or Ti instead of Nb. The results demonstrate that the aim is not to realize with the used alloying concept. In the long term range all 12 %Cr melts have a lower creep rupture strength than the advanced 9 %Cr piping steel P92. A high creep strength could be reached with a 0.06 % Ta alloyed 11 %Cr melt, which is in addition alloyed with a higher C and B content and as well as with lower W and Co portions. The results indicate in accordance with the finding of other steel researcher that a lower Cr content allows more effectiveness for the alloying partners respectively for the generation of more stable precipitates.     

Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants

Abstract

It is crucial for the carbon concentration of 9% Cr steel to be reduced to a very low level, so as to promote the formation of MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M₂₃C₆. Sub-boundary hardening, which is inversely proportional to the width of laths and blocks, is shown to be the most important strengthening mechanism for creep and is enhanced by the fine dispersion of precipitates along boundaries. The suppression of particle coarsening during creep and the maintenance of a homogeneous distribution of M₂₃C₆ carbides near prior austenite grain boundaries, which precipitate during tempering and are less fine, are effective for preventing the long-term degradation of creep strength and for improving long-term creep strength. This can be achieved by the addition of boron. The steels considered in this paper exhibit higher creep strength at 650 °C than existing high-strength steels used for thick section boiler components.     

Stainless steels suited for solution nitriding

Solution nitriding is a new heat treatment to yield a high nitrogen case on stainless steels at 1100 ± 50°C. Combining experimental results and thermodynamic calculation steels are selected to give a hard martensitic or high strength austenitic case. Especially developed steels are discussed as well as the suitability of standard grades. A martensitic case is combined with a martensitic core in steel Cr13C0.2 and with a softer ferritic‐martensitic core in steel Cr13C0.1. The nitrogen content of an austenitic case increases with the Cr/Ni ratio, e.g. in the order of Cr17Ni12Mo2, Cr18Ni10, Cr22Ni5Mo3N0.2. The duplex microstructure of the latter provides the highest yield strength in the core. It is essential to stay clear of the austenite/austenite + M₂N boundary and avoid precipitates which deteriorate the fatigue and corrosion resistance. Seventeen steels are assessed in this report.     

Ergebnisse von Zeitstandversuchen mit glatten und gekerbten Proben aus den Stählen 20 CrMoV 13 5 und X20CrMoWV 121

Results of Creep Tests With Smooth and Notched Specimens of the Steels 20 CrMoV 13 5 and X20 CrMoWV 121 Two hydrogen resistant steels, viz. 20 CrMoV 13 5 W.-Nr. 1.7779) and X20 CrMoWV 12 1 (W.-Nr. 1.4935) used for bolts and nuts as well as for pressurized components were tested with respect to their creep resistance. Using smooth and notched specimens creep data for 450°C 500°C and 550°C were established. Steel 20CrMoV 13 5 was investigated in two quenched and tempered states (225…250 HB, 285…310 HB. The tempering treatments of X20 CrMoWV 12 1 (W.-Nr. 1.4935) lead to hardness levels of 215…226 HB resp. 275… 310 HB. Creep tests at 450°C and 500°C resp. at 500°C and 550°C were undertaken with smooth and notched specimens of 20 CrMoV 13 5-resp. X20 CrMoWV 12 1 -steels. At each temperature two resp. three different hardness levels were investigated which were produced by changing the quenching and tempering procedure. The results prove the X20 CrMoWV 12 1-steel to show high ductility in all three Q + T conditions even after long creep tests. This holds true for the 20 CrMoV 13 5-steel in the soft condition and at a test temperature of 450°C only. The creep strength of both steels is about proportional to the ultimate strength of both steels is about proportional to the ultimate strength at room temperature. With regard to the allowable stresses it could be shown that in each case the creep strength for 100.000 hours divided by a safety factor of 1.5 is smaller than the 1% creep limit for the same time.     

基于S35140钢的超低碳成分设计和力学性能研究

S35140钢是一种基于25Ni-20Cr的奥氏体耐热钢,为了获得高强度,通常会提高碳含量,但碳含量较高不利于高温时效稳定性和长期耐腐蚀性能.本文在S35140钢的基础上,大幅度降低碳含量,并通过调控N和Nb等微合金元素含量,以及加入Ti元素,促使析出新的强化相,弥补减少碳含量所导致的强度降低.同时引入一定量的Al元素...     

Evolution of precipitates,in particular cruciform and cuboid particles,during simulated direct charging of thin slab cast vanadium microalloyed steels

Abstract

A study has been undertaken of four vanadium based steels which have been processed by a simulated direct charging route with processing parameters typical of thin slab casting, where the cast product has a thickness of 50 to 80 mm (in this study 50 mm) and is fed directly to a furnace to equalise the microstructure prior to rolling. In the direct charging process, cooling rates are faster, equalisation times shorter, and the amount of deformation introduced during rolling less than in conventional practice. Samples in this study were quenched after casting, after equalisation, after the fourth rolling pass, and after coiling, to follow the evolution of microstructure. The mechanical and toughness properties and the microstructural features might be expected to differ from equivalent steels which have undergone conventional processing. The four low carbon steels (~0.06 wt-%) which were studied contained 0.1 wt-%V (V – N), 0.1 wt-%V and 0.010 wt-%Ti (V – Ti), 0.1 wt-%V and 0.03 wt-%Nb (V – Nb), and 0.1 wt-%V, 0.03 wt-%Nb and 0.007 wt-%Ti (V – Nb – Ti). steels V – N and V – Ti contained around 0.02 wt-% N, while the other two contained about 0.01 wt-%N. The as cast steels were heated at three equalising temperatures of 1050°C, 1100°C, or 1200°C and held for 30 – 60 min before rolling. Optical microscopy and analytical electron microscopy, including parallel electron energy loss spectroscopy (PEELS), were used to characterise the precipitates. In the as cast condition, dendrites and plates were found. Cuboid particles were seen at this stage in steel V – Ti, but they appeared only in the other steels after equalisation. In addition, in the final product of all the steels, fine particles were seen, but it was only in the two titanium steels that cruciform precipitates were present. PEELS analysis showed that the dendrites, plates, cuboids, cruciforms, and fine precipitates were essentially nitrides. The two Ti steels had better toughness than the other steels but inferior lower yield stress values. This was thought to be, in part, due to the formation of cruciform precipitates in austenite, thereby removing nitrogen and the microalloying elements, which would have been expected to precipitate in ferrite as dispersion hardening particles.     

Dispersion strengthening in vanadium microalloyed steels processed by simulated thin slab casting and direct charging Part 1 – Processing parameters,mechanical properties and microstructure

Abstract

A study simulating thin slab continuous casting followed by direct charging into an equalisation furnace has been undertaken based on six low carbon (0·06 wt-%) vanadium microalloyed steels. Mechanical and impact test data showed that properties were similar or better than those obtained from similar microalloyed conventional thick cast as rolled slabs. The dispersion plus dislocation strengthening was estimated to be in the range 80–250 MPa. A detailed TEM/EELS analysis of the dispersion sized sub 15 nm particles showed that in all the steels, they were essentially nitrides with little crystalline carbon detected. In the steels V–Nb, V–Ti and V–Nb–Ti, mixed transition metal nitrides were present. Modelling of equilibrium precipitates in these steels, based on a modified version of ChemSage, predicted that only vanadium rich nitrides would precipitate in austenite but that the C/N ratio would increase through the two phase field and in ferrite. The experimental analytical data clearly point to the thin slab direct charging process, which has substantially higher cooling rates than conventional casting, nucleating non-equilibrium particles in ferrite which are close to stoichiometric nitrides. These did not coarsen during the final stages of processing, but retained their highly stable average size of, ～7 nm resulting in substantial dispersion strengthening. The results are considered in conjunction with pertinent published literature.     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Study of cast microalloyed steels

Abstract

Improved grades of cast steels were prepared by microalloying C–Mn–Cr steels (0.15–0.22C, wt-%) with V (0.1)+Ti (0.01) or V (0.11)+Nb (0.034)+Ti (0.01) combinations. When quenched and tempered an ultimate tensile strength (UTS) of the order of 784–1078 MPa (80–110 kg mm2) and an elongation ranging from 10 to 35% were achieved. However, the YS/UTS ratio was usually of the order of 0.9, which indicates that these steels did not undergo enough plastic deformation before failure under tensile stress. Examination by TEM of the as quenched as well as the quenched and tempered samples yielded the following information. All the as quenched steels had lath martensite morphology with retained austenite entrapped in the interlath region. After water quenching from 950°C an appreciable volume fraction of undissolved precipitates remained in the steels. Retained austenite broke down during tempering at 400°C and above producing a chain of rod like carbides. During tempering partial recrystallisation of the laths into polygonal ferrite occurred. Fine precipitates were deposited on the dislocation substructure.     

Effect of prior austenite on reversed austenite stability and mechanical properties of low carbon medium manganese steel heavy plate

The effect of prior austenite on reversed austenite stability and mechanical properties of Fe‐0.06C‐0.2Si‐5.5Mn‐0.4Cr (wt.%) annealed steels was elucidated. With the decrease of austenitizing temperature from 1250 °C to 980 °C, the prior austenite changed from complete recrystallization to partial recrystallization, and the average austenite size was reduced. The volume fraction of reversed austenite was increased from 26.32 % to 30.25 % because of high density of grain boundaries and dislocations. The martensite transformation temperature of annealed steels was increased from ～115 °C to ～150 °C, and both of thermal and mechanical stability of reversed were reduced. There was no significant different in tensile properties, however, the impact toughness was enhanced from 100 J to 180 J at ?60 °C. The excellent impact toughness in annealed steel (austenitized at 980 °C) was obtained because of higher density of high misorientation grain boundaries, more volume fraction of reversed austenite and reduced segregation at grain boundaries.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

Studies on influence of reheating temperature and cooling rate on structure–property behaviour of lean chemistry HSLA-100 steel

Abstract

Simulation studies on the influence of reheating temperature on austenite grain coarsening in lean chemistry high strength low alloy (HSLA)-100 steel were carried out to establish optimum soaking temperature before hot rolling. Experiments carried out in ‘Gleeble-3500’ dynamic thermomechanical simulator revealed that prior austenite grain sizes varied between 26 and 98 and 34 and 126 μm after soaking at 1150, 1200 and 1250°C for 1 and 5 min respectively; a soaking temperature of 1200°C was found to be optimum. Simulation experiments on the influence of cooling rate on microstructural changes and dilatometric studies indicated lowering of transformation temperature with faster cooling. Microstructural examination of dilatometric samples confirmed martensitic transformation at faster cooling rate. The martensite structure is desirable to achieve better strength and toughness. The findings of simulation studies were subsequently used for standardising thermomechanical treatments of Nb–Cu bearing lean chemistry HSLA-100 steels. One laboratory heat of Cu bearing HSLA steel containing 0·028%Nb was made. This heat was hot rolled into 12·5 mm thick plate by varying finish rolling temperature in the range of 800–1000°C. The soaking temperature was maintained at 1200°C. The rolled plates were heat treated by both conventional reheat quenching and tempering (RQT) as well as direct quenching and tempering (DQT) techniques. Evaluation of mechanical properties revealed that plates processed through DQT route were superior to those processed through RQT route. Transmission electron microscopy revealed that martensite structure and finer interlath spacing in DQT plates resulted in superior strength and impact toughness properties as compared to RQT steels.     

Effect of additions of Ti,B and Ce on microstructural stability,creep strength and creep damage in austenitic stainless steel

Abstract

Optical and transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis of bulk extracted precipitate residues were carried out on long term (more than 80 000 h) creep tested (at 1023 K) type 304 austenitic stainless steels with different levels of Ti content to assess the microstructural stability and creep strength. B and Ce were added to the steels to suppress the creep cavitation. Finer Ti(C,N) particles with higher density and narrower size distribution were observed in steels with a higher Ti content, resulting in an increase in the creep rupture strength. However, higher Ti content increased the intergranular precipitation of the σ phase on longer creep exposure, resulting in the increase in creep cavitation and in the decrease in creep rupture strength. The study indicated an optimum level of Ti and {C + (6/7)N} content with the Ti/{C + (6/7)N} ratio close to the stoichiometric value of the Ti(C,N) precipitate particles that should also be close to their solubility limit at the solution heat treatment temperature.     

Precipitation evolution and creep strength modelling of 25Cr20NiNbN austenitic steel

25Cr-20Ni-Nb-N is a high strength and oxidation-resistant austenitic stainless steel intended for Ultra-Supercritical (USC) power plants. In this work, the precipitation evolution, and creep strength at 650 and 750°C for up to 100?000?h are predicted. Six precipitates are considered in the thermokinetic calculation by MatCalc: M₂₃C₆, η (Cr₃Ni₂SiN), σ, G, Z, Nb(C,N). For the creep strength prediction, three hardening mechanisms are taken into account: dislocation, precipitation, and solid solution hardening. Both matrix composition and precipitation evolution, calculated with MatCalc, are used for modelling the precipitation and solid solution hardening. It is found that the dislocation hardening, followed by precipitation hardening gives the largest contribution to the creep strength. The most important precipitates strengthening phases are found to be Z-Phase and Nb(C,N), which are nucleated at the dislocations. The model for the creep rate can represent how the creep exponent is raised with increasing applied stress and reduced temperature.     

The effect of processing parameters on the microstructure and mechanical properties of low-Si transformation-induced plasticity steels

A base low Si, high-Al transformation-induced plasticity (TRIP) steel and one with 0.03Nb and 0.02Ti (wt%) additions were subjected to thermo-mechanical processing (TMP) and galvanising simulations. The microstructure and mechanical properties were analysed using a combination of optical and electron microscopy, X-ray diffraction and tensile testing and the results compared with those from intercritically annealed–galvanised steels. The addition of Nb and Ti results in microstructure refinement and an increase in the amount of the retained austenite after TMP which in turn, leads to increases in the tensile strength (~750 MPa) and the total elongation (TE) (~29 %). A deterioration in the volume fraction of retained austenite and the mechanical properties was noted in both steels after the additional galvanising simulation. For the base steel, all TMP and galvanised samples presented with continuous yielding during tensile testing. The Nb–Ti steel exhibited discontinuous yielding and extended Lüders banding when TMP was followed by a longer coiling time. Both steels returned discontinuous yielding after the intercritical annealing–galvanising treatment. The discontinuous yielding behaviour was associated with the much finer ferrite grain size in the intercritically annealed steels and the ageing processes that take place during galvanising.     

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.     

Creep of silicon nitride-titanium nitride composites

The effect of particulate TiN additions (0–50 wt%) on creep behaviour of hot-pressed (5 wt%Y₂O₃ + 2 wt%Al₂O₃)-doped silicon nitride (HPSN)-based ceramics was studied. Creep was measured using a four-point bending fixture in air at 1100–1340 °C. At 1100 °C, very low creep rates of HPSN with 0–30 wt% TiN are observed at nominal stresses up to 160 MPa. At 1200 °C the creep rate is slightly higher, and at 1300 °C the creep rate is increased by three orders of magnitude compared to 1100 °C and rupture occurs after a few hours under creep conditions. It was established that the formation of a TiN skeleton could detrimentally affect the creep behaviour of HPSN. An increase in TiN content leads to higher creep rates and to shorter rupture times of the samples. Activation energies of 500–1000 kJ mol^?1 in the temperature range of 1100–1340 °C at 100 MPa, and stress exponentsn?4 in the stress range 100–160 MPa at 1130–1200 °C were calculated. Possible creep mechanisms and the effect of oxidation on creep are discussed.     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382