Microstructure Evolution and Mechanical Properties of HR3C Steel during Long-term Aging at High Temperature

Microstructure evolution and the changes in mechanical properties of HR3 Csteel during long-term aging at650,700 and 750℃ were investigated.The precipitated phases of the aging steel included M23C6 carbides,Z-phase and a trace amount of Nb(C,N).The M23C6 carbides were distributed mainly at the grain boundary,while Z-phase was mainly inside the grains.Amounts of both M23C6 carbides and Z-phase during the aging process increased with increasing aging period and temperature.Coarsening of M23C6 carbides was influenced significantly by aging time and temperature,while the size of the Z-phase was relatively less affected by the aging time and temperature,which had a steady strengthening effect.Coarsening of the M23C6 carbides was the main reason for the decline in high temperature yield strength during long-term aging at 750℃.The M23C6 carbides were linked into a continuous chain along the grain boundary which accounted for the decrease of toughness during aging.     

钼钨钒合金化热作模具钢高温回火组织演变

为适应热冲压技术的发展需求,开发了一种新型高热导率高耐磨性能热冲压用模具钢材料。采用扫描电镜（SEM）、透射电镜（TEM）等检测手段对钼钨钒合金化新型模具钢的高温回火性能与组织特征进行了研究。阐明了新型热冲压模具钢回火过程碳化物析出与演变规律。实验结果表明:实验用钼钨钒合金化模具钢材料具有良好的回火二次硬化性能,在500～600 ℃温度区间回火时,回火组织硬度上升;在600 ℃回火出现二次硬化峰值;当回火温度超过600 ℃后,组织软化程度明显,回火硬度开始下降。实验模具钢在高温回火过程中的硬度变化与其合金碳化物的偏聚、析出和聚集长大密切相关。当在560 ℃以下回火时,实验钢组织中未有合金碳化物析出;当回火温度大于560 ℃时,回火组织中开始析出M₂C型碳化物;当回火温度高于600 ℃后开始析出MC型碳化物;当在620 ℃长时间回火后M₂C型碳化物转化为M₆C型碳化物,此时实验钢硬度开始明显下降;而当回火温度高于660 ℃时,新型实验钢组织中主要为M₆C和MC型合金碳化物。      

Transformation and Precipitation Kinetics in 30Cr10Ni Duplex Stainless Steel

To improve the microstructure during casting, hot forming, and heat treatment of 30Cr10Ni duplex stainless steel, accurate data on the precipitation and transformation processes at high temperatures are needed. In this article, the precipitation and transformation processes at various aging times in the temperature range 873 K to 1573 K (600 °C to 1300 °C) were studied. The 30Cr10Ni ferrous alloy contains a relatively large amount of Cr, Ni, and C, which results in a complex microstructure. In addition to the ferrite, austenite, and sigma phase, the M₂₃C₆ and MC carbides were also observed in the microstructure. The precipitation of the sigma phase was observed after just 3 minutes of aging, and after 30 minutes of aging at approximately 1053 K (780 °C), its fraction exceeded 40 pct. An intensive austenite-to-ferrite transformation was observed above 1423 K (1150 °C). Optical microscopy, energy-dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD), as well as micro-indentation hardness, hardness, impact toughness, and tensile tests, were carried out to evaluate the obtained microstructures of aged samples.     

Carbide precipitation in 12Cr1MoV power plant steel

The chemistry and other characteristics of carbide precipitates in 12CrlMoV steel of the type used in the power generation industry were studied using energy dispersive X-ray spectroscopy and electron and X-ray diffraction techniques, and the results have been compared against thermodynamic calculations. As a result of the much larger substitutional solute concentrations present in the alloy, unlike the carbides that occur in steels containing smaller concentrations of chromium and molybdenum, it is found that the equilibrium M₂₃C₆ carbide precipitates very rapidly during heat treatments of the kind used routinely for stress-relief purposes. The chemical compositions of carbides therefore do not change much during subsequent service at elevated temperatures.     

Carbon migration in 5Cr-0.5Mo/21Cr-12Ni dissimilar metal welds

The carbon migration between a ferritic steel and an austenitic steel was studied in submerged arc-welded 5Cr-0.5Mo/21Cr-12Ni dissimilar metal welds (DMWs) after aging at 500°C for various times and after long-term service in technical practice. The distribution of carbon, chromium, nickel, and iron in the areas around the weld interface was determined by electron probe microanalysis, and the microstructural aspect in the carbon-depleted/enriched zone was characterized by optical microscopy and transmission electron microscopy (TEM). Furthermore, the precipitation sequences and composition characteristics of the carbides were identified by diffraction pattern microanalysis and energy-dispersive X-ray (EDX) microanalysis. It was found (1) that there exists a coherent relationship between intracrystalline M₂₃C₆ and the austenitic matrix; (2) that the composition of M₂₃C₆ in the carbon-enriched zone is independent of the duration of aging and service; (3) that the maximum carbon concentration is determined by the carbide type, the composition characteristic of precipitated carbides, and the concentration of carbide-forming Cr adjacent to the weld interface in the carbon-enriched zone; and (4) that the carbon migration in the 5Cr-0.5Mo/21Cr-12Ni DMWs can be described by a diffusion model.     

高温过热器12Cr2MoWVTib（102）钢管的显微组织和相结构分析

本文对原始状态和在火力发电厂使用不同时间的高这热器１０２钢管进行了显微组织和相结构变化规律的探讨和研究。显微组织研究表明，１０２钢管在高温高压下长期运行过程中，使用温度对显微组织和碳化物相有明显影响，而使用时间对其影响不如温度影响强烈。碳化物颗粒图象分析表明，碳化物随着使用时间缓慢长大，而新的碳化物不沉淀，使颗粒总数不断增加，颗粒，间距逐渐缩小。Ｘ射线衍射数据证明，钢管在高温长期使用过程中，ＭＣ相     

Precipitate Redistribution during Creep of Alloy 617

Nickel-based superalloys are being considered for applications within advanced nuclear power generation systems due to their high-temperature strength and corrosion resistance. Alloy 617, a candidate for use in heat exchangers, derives its strength from both solid solution strengthening and the precipitation of carbide particles. However, during creep, carbides that are supposed to retard grain boundary motion are found to dissolve and reprecipitate on boundaries in tension. To quantify the redistribution, we have used electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS) to analyze the microstructure of 617 after creep testing at 900 °C and 1000 °C. The data were analyzed with respect to the location of the carbides (e.g., intergranular vs intragranular), grain boundary character, and precipitate type (i.e., Cr rich or Mo rich). We find that grain boundary character is the most important factor in carbide distribution; some evidence of preferential distribution to boundaries in tension is also observed at higher applied stresses. Finally, the results suggest that the observed redistribution is due to the migration of carbides to the boundaries and not the migration of boundaries to the precipitates.     

Microstructure Evolution of a Simulated Coarse-Grained Heat-Affected Zone of T23 Steel During Aging

In this work, simulated CGHAZ of T23 steel was produced via a thermomechanical simulator, and then the CGHAZ specimens were aged at 650 °C for 0 to 240 hours to simulate the microstructure evolution of as-welded CGHAZ during service. Microstructure change and carbide precipitation were observed by OM, SEM, EBSD and TEM + EDS. Carbide precipitation kinetics in T23 steel at 650 °C was calculated for comparison with the experiment results. The hardness change of CGHAZ during aging was detected, and the effect of microstructure evolution on hardness was analyzed. The results showed that the CGHAZ of T23 steel exhibited a mixed microstructure of martensite and bainite with high hardness in as-welded condition. After aging at 650 °C, the microstructure recovered, recrystallization occurred, the dislocation density decreased, and the lath width increased. Consequently, the hardness dropped, the drop depending on the aging time. In the early stage of aging (before 24 hours), the precipitations inside the grain were mainly M3C, M7C3 and a small number of M23C6 carbides, while the precipitation at the grain boundaries was M23C6. The precipitation of M23C6 caused the hardness to drop rapidly. When aged for 24 to 48 hours, MX precipitated inside grains extensively. The precipitation hardening produced by MX could slow down the decline of hardness. As the aging proceeded, carbide precipitated and transformed as follows: M3C → M3C + M7C3 + M23C6 → M3C + M7C3 + M23C6 + MX → M23C6 + MX + M6C. W-rich carbides precipitated in some grain boundaries of CGHAZ during aging, which may be related to the W segregation at those grain boundaries.     

700℃时效过程中HR100合金的碳化物析出行为

对经700℃时效不同时间后的镍基HR100合金中的析出相进行了定性与定量分析,研究了HR100合金中的碳化物在长期时效过程中的析出行为。结果表明,HR100合金中的M_6C型碳化物为亚稳相。在长期时效过程中,随着合金元素的扩散,HR100合金中的M_6C会逐渐转化为M_(12)C。而且在700℃时效1万h过程中,HR100合金中的碳化物的析出行为主要分3个阶段:快速析出与M_6C→M_(12)C转化阶段,相对稳定阶段,M_(12)C和M23C6进一步析出、长大阶段。另外,对不同类型碳化物中的合金元素配比在长期时效过程中的变化进行了分析,发现在(M_6C+M_(12)C)的粗化过程中,Cr元素和Mo元素起主要作用;而在M_(23)C_6的粗化过程中,Cr元素起主要作用。     

Modeling M6C precipitation in niobium-alloyed ferritic stainless steel

Niobium is used as an important alloying element in the design of heat-resistant stainless steels for automotive exhaust systems. When in solid solution, the niobium improves both the high-temperature strength and the resistance to thermal fatigue life. However, it also forms coarse Fe₃Nb₃C carbides during service at elevated temperatures, making it important to understand the kinetics of carbide precipitation and coarsening. In the present work, the kinetics of M₆C precipitation in ferrite have been modeled, taking into account the multicomponent nature of the diffusion process while at the same time allowing for capillarity effects. The lack of appropriate thermodynamic data has been dealt with using a solubility product based on new experiments on a 19Cr-0.8Nb mass pct steel.     

Characterization of medium- and low-temperature carbides in a low-carbon steel by internal friction

The formation of carbides during the aging of a quenched steel with mass contents of 0.04% carbon was investigated by means of internal friction by measuring the carbon Snoek peak height. The aging was carried out at different temperatures ranging from 40 to 350°C so that different types of carbides were formed. Low-temperature carbides develop after aging below 100°C. The Johnson-Mehl equation is applied to evaluate the kinetics of precipitation and re-dissolution of the different carbides. The solubility of carbon in ferrite is influenced by the pre-existing carbides, which is, in part, the reason for the variation of thermodynamic data in previous work.     

Effects of Cr Reduction on High-Temperature Strength of High-Ni Austenitic Cast Steels Used for High-Performance Turbo-chargers

Since greater high-temperature strength is required for maintaining high-performance turbo-chargers at higher exhaust gas temperatures, e.g., 1323 K (1050 °C), high-Ni (20 wt pct) austenitic steel (ASTM HK40 steel) is presented as an excellent turbo-charger candidate material. To enhance the strength, three types of austenitic cast steel were fabricated in this study by controlling the Cr content in HK40 steel, and high-temperature strength improvement was achieved by detailed microstructural evolution including carbide formation and matrix strengthening. Room temperature and high-temperature strengths were expected to be proportional to the carbide volume fraction, but revealed an opposite trend because the steel containing more Cr (having more carbides) revealed lower strength than the steel containing less Cr (having fewer carbides). This result was associated mainly with the M₇C₃ to M₂₃C₆ decomposition occurring at high temperatures in the less-Cr-steel that beneficially strengthened the austenite matrix and reduced the hardness difference between the carbide and matrix, consequently improving the high-temperature strength. In considering the alloying prices (14 pct cost saving of alloying elements) as well as the high-temperature strength, the steel containing less Cr is promising for new high-performance turbo-charger applications.

     

Carbide precipitation and high-temperature strength of hot-rolled high-strength,low-alloy steels containing Nb and Mo

The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass hot rolling and coiling at 650°C, 600°C, and 550°C. Microstructural analysis of the carbides has been performed using field-emission gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both Nb and Mo exhibited a higher strength at high temperatures (∼600 °C) in comparison to the steel containing only Nb. The addition of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides had one variant of the B-N relationship ((100)_MC//(100)_ferrite, [011]_MC//[010]_ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus, believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles.     

H13钢中一次碳化物的特征及控制进展

首先结合H13钢的成分特点肯定了H13钢优异的材料性能,随后总结了一次碳化物与H13钢使用寿命之间的关系。进一步系统地论述和研究了H13钢中一次碳化物的特征,包括二维和三维形貌、热稳定性、析出机理等。最后对比了4种H13钢中一次碳化物的控制手段,包括成分优化、冷速控制、Mg处理和稀土处理。相关论述和研究工作能够对钢中一次碳化物的合理优化起到一定的启发作用。      

Influence of long-term aging and superimposed creep stress on the microstructure of 2.25cr-1Mo steel

To understand the influence of high-temperature aging and superimposed creep stress on the microstructural variations in a 2.25Cr-1Mo steel, the shoulder and gage portions of the specimens subjected to stress-rupture tests at 540 °C and 580 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the steel exhibited a tempered bainitic structure and the carbides were present as M₃C globules, M₂C platelets, and M₂₃C₆ rectangular parallelepipeds. Aging the steel at 540 °C for 7022 hours or 17,946 hours resulted in considerable coarsening of M₂C and caused precipitation of M₆C carbides. The superimposed creep stress enhanced the M₂C precipitation. The ferrite matrix exhibited some recovery in the specimens exposed for 17,946 hours. While M₂C platelets were observed in a few areas after 14,836 hours of aging at 580 °C, this carbide was virtually nonexistent when a stress of 78 MPa was superimposed. Amounts of M₂₃C₆ persisted throughout the tests at both 540 °C and 580 °C. The M₆C carbide became more predominant after long exposure at 580 °C. The ferrite matrix recovered considerably in specimens subjected to creep stress at 580 °C for 14,836 hours.     

Effects of Pulsed Current and Pulsed Magnetic Field Complex Pretreatment on Martensite Transformation of Cr5 Steel during Continuous Cooling

Carbide precipitation and martensite transformation in Cr5 steel have been observed in situ by high-temperature confocal laser-scanning microscopy.In this way,the influences of pulsed current and pulsed magnetic field complex pretreatment on carbide precipitation and martensite transformation during continuous cooling have been studied.The results show that the electropulsing complex pretreatment promotes the precipitation of M_7C_3-type carbides at high temperature,increases the start and finish temperatures of martensite transformation,and extends the phase transformation time.Martensite prefers to nucleate in the austenite with less precipitation of carbides due to the chemically homogeneous distribution of solute atoms.     

Carbon Redistribution and Carbide Precipitation in a High-Strength Low-Carbon HSLA-115 Steel Studied on a Nanoscale by Atom Probe Tomography

HSLA-115 is a newly developed Cu-bearing high-strength low-carbon martensitic steel for use in Naval structural applications. This research provides, for the first time, a comprehensive compositional analysis of carbon redistribution and associated complex phase transformations in an isothermal aging study of HSLA-115 at 823 K (550 °C). Specifically, we characterize carbon segregation at lath boundaries, grain-refining niobium carbonitrides, cementite, and secondary hardening M₂C carbides, in addition to copper precipitation, by 3D atom probe tomography (APT). Segregation of carbon (3 to 6 at. pct C) is observed at martensitic lath boundaries in the as-quenched and 0.12-hour aged microstructures. On further aging, carbon redistributes itself forming cementite and M₂C carbides. Niobium carbonitride precipitates do not dissolve during the austenitizing treatment and are inherited in the as-quenched and aged microstructures; these are characterized along with cementite by synchrotron X-ray diffraction and APT. Sub-nanometer-sized M₂C carbide precipitates are observed after the formation of Cu precipitates, co-located with the latter, indicating heterogeneous nucleation of M₂C. The temporal evolution of the composition and morphology of M₂C carbides at 823 K (550 °C) is described using APT; their precipitation kinetics is intertwined with Cu precipitates, affecting the bulk mechanical properties of HSLA-115. Phase compositions determined by APT are compared with computed compositions at thermodynamic equilibrium using ThermoCalc.     

喷射成形M3型高速钢碳化物组织特征与加热过程演化

采用常规铸造和喷射成形工艺分别制备了M3型高速钢铸坯和沉积坯.利用扫描电子显微镜、X射线能谱和X射线衍射等分析方法对冷却速度对合金的显微组织的影响,加热温度对M3高速钢中M₂C共晶碳化物分解行为的影响,以及热加工变形后铸态和沉积态组织的变化进行了研究.结果表明:铸态合金含有粗大的一次枝晶和M₂C共晶碳化物,而喷射成形沉积坯主要为等轴晶且碳化物细小均匀;冷却速度的提高极大地抑制了碳化物的析出和晶粒长大;加热温度的提高有利于M₂C共晶碳化物分解,过高的温度使得分解后的M₆C长大,不利于合金性能的提高;沉积坯经恰当的预热处理和热变形可以获得理想的变形组织.      

15CrMoR钢的显微组织与时效冲击性能

 为探索原始组织形态对15CrMoR钢时效过程低温冲击性能的影响,明确15CrMoR钢具有高时效冲击性能稳定性的原始组织形态,通过控制奥氏体化后的冷却方式获得了15CrMoR钢的3种原始组织,使用OM、SEM、EPMA和EBSD等材料结构表征方法和低温冲击测试研究了15CrMoR钢的显微组织和时效态低温冲击性能。结果表明,15CrMoR钢奥氏体化后分别以炉冷、空冷和风冷的方式冷却至室温,分别获得了粗大铁素体+片状珠光体组织、铁素体+退化珠光体组织和粒状贝氏体组织。片状珠光体组织中碳化物主要呈层片状,退化珠光体中的碳化物主要呈断续短杆状和颗粒状,粒状贝氏体中的富碳M-A岛主要沿晶界分布。3种原始组织形态的15CrMoR钢在循环时效过程中均发生了晶界碳化物析出和长大,导致低温冲击性能不断恶化。当晶界碳化物呈链状分布时,15CrMoR钢的低温冲击性能较差。粗大的铁素体+片状珠光体组织晶界面积较少,导致晶界碳化物容易呈链状分布;粒状贝氏体中主要沿晶界分布的富碳M-A岛也容易导致晶界碳化物呈链状分布。因此,原始组织为铁素体+退化珠光体的15CrMoR钢在循环时效过程中具有较好的冲击性能稳定性,经历6次循环时效后,-10 ℃平均冲击吸收功仍高达196 J;而原始组织为铁素体+片状珠光体和原始组织为粒状珠光体的15CrMoR钢,经历4次循环时效后,晶界处已形成呈链状分布的碳化物,-10 ℃平均冲击吸收功均仅为18 J。     

高碳铬不锈轴承钢孪晶碳化物研究

研究了高碳铬不锈轴承钢“孪晶碳化物”（直线状和链状碳化物）的影响因素及形成原因,结果表明：加热温度达到1140℃,退火后开始出现沿晶界分布的链状碳化物;加热温度≥1160℃,退火后出现大量直线状和链状两种形态的碳化物。材料从高温直接冷却时,温度≥1080℃并且冷却速度≤80℃／h可能析出链状碳化物,并且温度越高冷却速度越慢析出的可能性就越大。直线状碳化物形成原因为：材料加热温度过高．晶粒长大的过程中晶界迁移时偶然发生堆垛错误形成了生长孪晶,在随后的退火过程中碳化物向奥氏体挛晶界面沉淀而形成,是真正意义上的孪晶碳化物。链状碳化物是由于材料过热或者局部过热,在随后冷却过程中碳化物沿奥氏体晶界析出而形成的,本质上是一种网状碳化物。