Evaluation of structural stability and creep resistance of 9-12% Cr steels

During creep exposure of modified chromium steels lowering of solid solution strengthening due to precipitation of Laves phase as well as coarsening of all precipitates causes degradation of creep resistance. Two distinct domains of the stress dependence of creep rate and time to rupture have been observed in precipitation strengthened modified chromium steels. The stress characterizing the transition between these domains was found to be closely related to the Orowan stress. This stress consists in these steels of the contribution from large particles on subgrain boundaries (mainly M₂₃C₆ and during the limited time also Laves phase) and from small precipitates (Nb(C,N) and VN) inside subgrains. This has to be considered when measuring the interparticle spacing and calculating Orowan stress. Larson-Miller parametric equation is used to elucidate the necessity of long-term creep testing. By means of two heats of CrMoVNbN steel it is shown that reliable extrapolation of creep properties is possible only in a stress and temperature domain in which only one creep creep rupture mechanism operate. In the high stress domain Larson-Miller constant C_LM is well above 30 while in the low stress domain this constant does not exceed 25. When the extrapolation is based mainly on short-term creep tests, the C_LM constant is close to that valid in the high stress domain and therefore it overestimates long-term creep strength.     

Effect of titanium,niobium and zirconium on creep rupture strength and ductility of cobalt base superalloys

An attempt has been made to develop a cobalt base casting superalloy (30Cr-10Ni-7W-Co) having high creep rupture strength and ductility for first stage nozzles of gas turbines. In cobalt base superalloys, there was found to exist a close correlation between the creep rupture strength and MC type carbide forming elements such as Ti, Nb and Zr. In cobalt base alloys with 0.25 wt pct C, precipitation and coarsening of carbides can be reduced by addition of Ti, Nb and Zr. Therefore, by adding the optimum amount of Ti, Nb and Zr, precipitation of carbides in the alloy reaches such an amount as to give the highest creep rupture strength. Excess addition of Ti, Nb and Zr does not improve the creep rupture strength. By adding C, creep rupture strength of the cobalt alloy with Ti, Nb and Zr can be improved and becomes the highest at 0.40 wt pct. C. According to the experimental results, the creep rupture strength becomes the highest at a value of (Ti + Nb + Zr)/C (atomic ratio) of about 0.3. Contrary to the expectation, it was found in this experiment that the ductility in creep rupture tests increases with increasing carbon content up to 0.6 wt pct.     

Investigation on the strengthening mechanism of S30432 austenitic heat-resistant steel

From the viewpoint of energy-saving and environment protection,it is necessary to develop Ultra Super Critical（USC） fossil-fired power plants.In order to ensure the reliable operation of power plants under high steam conditions,good mechanical properties（particularly high creep strength）,corrosion resistance and fabricability are generally required for the heat resistant steels used in USC boilers.Among these heat-resistant steels,S30432 austenitic heat-resistant steels are of interest due to high creep strength,excellent oxidation and corrosion resistance at temperatures up to 650 -700℃.In this paper,the strengthening mechanism of S30432 austenitic heat-resistant steel was investigated based on the precipitation behavior of S30432 during aging and creep at 650℃.Results show that the microstructure of as-supplied S30432 steel is austenite,the main precipitation consists of only Nb（C,N）.After aged for 10 000 h or crept for 10 712 h,there is a slight increase in the size of fine Nb（C,N）,but the transformation from Nb（C,N） to NbCrN does not occur.Aging and creep results in the precipitation ofε-Cu and M₂₃C₆.The coarsening velocity ofε-Cu particles diminishes greatly and they are still very fine in the long-term creep range.With the increase of aging and creep time M₂₃C₆ carbides tend to coarsen gradually.The size of M₂₃C₆ is larger and the coarsening is easier in contrast toε-Cu and Nb（C,N）.Nb（C,N） precipitates in the as-supplied microstructure,while aging and creep result in the precipitation ofε-Cu and M₂₃C₆.High creep rupture strength of S30432 steel is attributed to the precipitation hardening ofε-Cu,Nb（C,N） and M₂₃C₆.Extremely,ε-Cu plays an important role in improving the creep rupture strength of S30432,and at least 61%of the creep rupture strength of S30432 at 650℃results from the precipitation hardening ofε-Cu particles.     

Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb

The new ferritic heat-resisting steels of 0.05C-10Cr-2Mo-0.10V-0.05Nb (Cb) composition with high creep rupture strength and good ductility have already been reported. The optimum amounts of V and Nb that can be added to the 0.05C-10Cr-2Mo steels and their effects on the creep rupture strength and microstructure of the steels have been studied in this experiment. The optimum amounts of V and Nb are about 0.10 pct V and 0.05 pct Nb at 600 °C for 10,000 h, but shift to 0.18 pct V and 0.05 pct Nb at 650 °C. Nb-bearing steels are preferred to other grades on the short-time side, because NbC precipitation during initial tempering stages delays recovery of martensite. On the long-time side, however, V-bearing steels have higher creep rupture strength. By adding V to the steels, electron microscopic examination reveals a stable microstructure, retardation during creep of the softening of tempered martensite, fine and uniform distribution of precipitates, and promotion of the precipitation of Fe₂Mo.     

Al对T91耐热钢蠕变性能的影响

沪产T91钢脱氧后残留在钢中的铝含量相对较高，它优先与氮结合，影响了起析出强化作用的（Nb,V)(C,N)析出数量，是引起持久强度降低的原因之一。沿晶界分布的AlN是蠕变空洞的形核核心，它还促进在其上形核的M23C6开裂形成空洞，对沪产T91钢蠕变脆性的产生有重要影响。为提高国产T91钢的性能，必须选择适宜的脱氧方法，限制脱氧时加入的铝含量。     

The effect of fine precipitate particles on the creep behaviour of ferritic model steels -I. Experiments

In view of efforts to develop ferritic creep resistant steels for applications above 600°C the effect of fine precipitate particles on the creep behaviour of ferritic model steels was studied as a function of stress, temperature and particle distribution. The chosen model steels contained 20% Cr (by mass), up to 0.9% Nb and up to 0.1 % C to produce NbC volume fractions up to 0.8% with particle sizes of about 0.1 μm (order of magnitude). The alloys and structures are briefly described (NbC solubility, precipitation and ageing behaviour, recrystallization and grain growth, oxidation resistance) as well as the mechanical short-term behaviour. The creep behaviour was studied between 600°C and 800°C (with emphasis on 700°C) at strain rates between 10^?11 and 10^?6 s^?1 with times to rupture up to 20000 h. The creep resistance of the model steels at 700°C (for a strain rate of 10^?8s^?1) increases with increasing NbC content from about 5 MN/m² for the alloy without NbC to about 50 MN/m² for the alloys with 0.6% or 0.8% NbC. The analysis of the obtained results is the subject of the second part of this report.     

Suppression of creep cavitation in precipitation-hardened austenitic stainless steel to enhance creep rupture strength

Extensive creep cavitation in Ti, Nb and Cu containing precipitation hardened austenitic steels was found to limit the usefulness of deformation resistance to increase long-term creep rupture strength. The steels were microalloyed with boron and cerium that resulted in increase in creep rupture strength and ductility of the steels significantly. Grain boundary sliding and creep cavity nucleation and growth in the steels were suppressed greatly on microalloying. Auger spectroscopic analysis revealed the segregation of boron instead of sulfur on cavity surface and the absence of sulfur contamination of grain boundary upon the microalloying. Suppression of creep cavitation through the control of trace elements segregation along with the precipitation hardening increased the creep rupture strength of austenitic stainless steels.     

Effects of some carbide stabilizing elements on creep-rupture strength and microstructural changes of 18-10 austenitic steel

The creep rupture test has been carried out for 18Cr-10Ni-0.1 wt pct C stainless steels bearing individually Ti, Nb(Cb), and V, followed by the microstructural study. The highest value of 700°C-10⁴ h rupture strength in a titanium and niobium series (the steel containing various amounts of titanium and niobium, respectively) has been obtained at Ti/C and Nb/C atomic ratio of 0.8 and 0.2 to 0.4, respectively. On the other hand, in a vanadium series, the creep rupture strength of the steel showed its maximum at V/C atomic ratio of about unity in the testing at the temperature of 700° and 800°C, but at 600°C, the strength increases monotonically with vanadium content up to 1.53 wt pct. Such high strength in the steels con-taining proper amount of Ti, Nb, and V is related mainly with the fine distribution of M₂₃C₆ precipitates which is caused by the acceleration of nucleation due to the foregoing precipi-tation of a MC type carbide within the austenite grains. And it has been deduced that the solid solution strengthening effect of the vanadium contributes also to the remarkable in-crease in the rupture strength of the vanadium steel at 600°C.     

Effects of aluminum content on the mechanical properties of a 9Cr-0.5Mo-1.8W steel

The effects of aluminum content on mechanical properties of a 9Cr-0.5Mo-1.8W steel have been investigated. It was found that aluminum addition had a beneficial effect on toughness, but significantly reduced the creep resistance of the steel, especially on the long-term side. Examination of the microstructure and precipitation characteristics revealed that almost all of the aluminum added existed as AIN-type nitrides after normalizing and tempering. The undissolved AIN in high aluminum steels resulted in a dramatic refinement of prior austenite grains, which contributed to the improvement of toughness and was also partially responsible for the decreased creep rupture strength. The formation of AIN suppressed the precipitation of VN-type nitride; AIN also provided formation sites for Nb(C, N) and M₂₃C₆ type, which had an equivalent effect to the coalescence of these precipitates on AIN, resulting in the reduction of precipitate density and, therefore, decreased creep resistance.     

钒对T122铁素体耐热钢组织和性能的影响

研究了(%):0.11C- 11.89～12. 19Cr- 1.86～1.90W-0.37～0.38Mo-0.9Cu铁素体耐热钢T122 中 0.14%～0.31%V-0.05%Nb复合强化添加剂中V 含量对钢的组织和性能的影响。结果发现,随V 含量增加, 钢中δ铁素体量增加,室温拉伸强度、650℃拉伸强度和650℃持久强度逐渐降低;当V 含量为0.19%时T122 钢的室温拉伸强度、650℃拉伸强度和持久性能较高。 V 含量变化影响钢中M₂C、MX 和Laves相的析出, 0.19%V-0.05%Nb析出强化效果最好     

Nb(C，N)作为取向硅钢中抑制剂的可行性

在铌微合金高强度钢中,主要利用铌来提高奥氏体的再结晶温度,从而实现控制轧制和控制冷却;同时,利用铌的析出进行强化来提高铌微合金钢的强度和韧性。结合取向硅钢抑制剂的特点和要求,分析了Nb(C,N)作为抑制剂的可行性。由于Nb(C,N)在钢中的固溶温度低,析出颗粒尺寸小,Nb(C,N)具备作为取向硅钢抑制剂的基本特征和优势,故其可作为取向硅钢中抑制剂。     

Influence of Chromium on Mechanical Properties and CO2/H2S Corrosion Behavior of P110 Grade Tube Steel

Four kinds of P110 grade tube steels containing different chromium contents were designed to probe the in- fluence of Cr on the properties of tube steel. The microstrueture, mechanical properties and corrosion behavior of the four kinds of P110 grade tube steels were studied deeply and thoroughly. The analysis of the mechanical properties indicated that tensile strength and yield strength of the steel plates were improved, while transverse and longitudinal impact energy and elongation first increased and then decreased when Cr content rose. The impact energy and elonga tion reached the peak when Cr content was 1 ~. Cr precipitates were found only in 3 ~ Cr steel, with （Nb, Ti）（C, N） or Nb（C,N） as the core of precipitation and then grew up. The corrosion experiments demonstrated that the scales on the four steels had a two layer structure under the corrosion of CO2 and H2 S. The outer layer was mainly com- posed of FeS or FeS1 x and the inner layer consisted of FeCOa and Cr compounds. Cr was rich in the inner layer and the Cr content of the inner layer increased with the Cr content in matrix. The enriched Cr enhanced the compactness of the scales, further hindering the diffusion of ions from liquid to the surface of steel, thus reducing corrosion rate.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture and creep strengths were considerably increased. Formerly with Lawrence Berkeley Laboratory.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through the α→γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta-7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base Fe-Ta-Cr alloy, the rupture and creep strengths were considerably increased. M. Dilip Bhandarkar, formerly with Lawrence Berkeley Laboratory.     

Creep and stress rupture of a mechanically alloyed oxide dispersion and precipitation strengthened nickel-base superalloy

The creep and stress rupture behavior of a mechanically alloyed oxide dispersion strengthened (ODS) and γ′ precipitation strengthened nickel-base alloy (alloy MA 6000E) was studied at intermediate and elevated temperatures. At 760 °C, MA 6000E exhibits the high creep strength characteristic of nickel-base superalloys and at 1093 °C the creep strength is superior to other ODS nickel-base alloys. The stress dependence of the creep rate is very sharp at both test temperatures and the apparent creep activation energy measured around 760 °C is high, much larger in magnitude than the self-diffusion energy. Stress rupture in this large grain size material is transgranular and crystallographic cracking is observed. The rupture ductility is dependent on creep strain rate, but usually is low. These and accompanying microstructural results are discussed with respect to other ODS alloys and superalloys and the creep behavior is rationalized by invoking a recently-developed resisting stress model of creep in materials strengthened by second phase particles. The analysis indicates that at the intermediate temperature the creep strength is controlled by the high volume fraction of γ′ precipitates and the contribution to the creep strength from the oxide dispersion is small. At the elevated temperature, the creep strength is derived mainly from the inert oxide dispersoids. Formerly at Columbia University.     

高Ti-Q550钢中Nb、Ti第二相的析出行为

 采用Thermo-Calc软件、热模拟及扫描电镜研究高Ti-Q550钢中微合金的析出规律。采用Thermo-Calc软件计算不同温度下Nb、Ti的析出规律,钛含量对钢中Nb、Ti析出规律及A3的影响。采用热模拟和扫描电镜研究钢中铌相的析出温度。计算结果表明,钛相的析出温度为1498℃,铌相析出温度为1251℃;随着钢中钛含量的增加,(Nb,Ti)C相析出温度和A3温度升高,但铌在钢中的固溶量降低;当钛的质量分数小于0. 08%时,Ti(N,C)相析出温度随钛含量增加而升高,但当钛的质量分数大于0. 08%时,相析出温度基本不变,钛在钢中的固溶量随钛含量增加而增加。     

Numerical Analysis of the Nb(C,N) Precipitation Kinetics in Microalloyed Steels

Precipitation kinetics of Nb(C,N) in microalloyed steels is crucial for the achievement of favoured steel properties. Therefore, numerous experimental studies have been performed in the past and various theoretical models have been developed to describe Nb(C,N) precipitation. However, the experimental data is sometimes contradictory and even the thermodynamic data for NbC solubility in austenite have a large scatter. In this paper, experimental results on the Nb(C,N) and NbV(C,N) precipitation kinetics in deformed and undeformed austenite are reviewed. Based on these data and with the precipitation kinetics module of the software package MatCalc, computer simulations are performed. The predicted interfacial energy of precipitates is adjusted to match the observed kinetics. A comparison between experimental information and simulation, i.e. time ‐ temperature ‐ precipitation (TTP) diagrams, is drawn and discussed. The results of the computer simulations using modified interfacial energies are in good agreement with the experiments.     

Creep rupture strength of tungsten-alloyed 9-12 % Cr steels for piping in power plants

The creep rupture strengths of the three tungsten-alloyed 9-12% Cr-steels E 911, NF 616 and HCM 12A are discussed. It is shown that Larson-Miller assessments may lead to an overestimation of the long-term creep rupture strength because of microstructural changes. The presented assessments indicate that all three alloys will have similar creep rupture strengths in the range of 110-120 MPa for 10⁵ h and 600°C. To obtain reliable estimates for creep rupture strength, however, further long-term tests are needed. In any case, the tungsten-alloyed steels will enable the main steam temperature to be raised by about 20°C over that possible with P 91.     

合金元素对Cr-Mo系锅炉钢性能的影响

叙述了不同工作温度下使用的锅炉钢种类，分析了Ｃｒ、Ｍｏ、Ｖ、Ｎｂ、Ｎ等合金元素对提高锅炉钢蠕变强度和焊接性能的影响。