Effect of Mn addition on microstructure and mechanical properties of GX40CrNiSi25-12 austenitic heat resistant steel

Three types of steels were designed on the basis of GX40CrNiSi25-12 austenitic heat resistant steel by adding different Mn contents(2wt.%,6wt.%,and 12wt.%).Thermodynamic calculation,microstructure characterization and mechanical property tests were conducted to investigate the effect of Mn addition on the microstructure and mechanical properties of the austenitic heat resistant steel.Results show that the matrix structure in all the three types of steels at room temperature is completely austenite.Carbides NbC and M_(23)C_(6)precipitate at grain boundaries of austenite matrix.With the increase of Mn content,the number of carbides increases and their distribution becomes more uniform.With the Mn content increases from 1.99%to 12.06%,the ultimate tensile strength,yield strength and elongation increase by 14.6%,8.0%and 46.3%,respectively.The improvement of the mechanical properties of austenitic steels can be explained by utilizing classic theories of alloy strengthening,including solid solution strengthening,precipitation strengthening,and grain refinement.The increase in alloy strength can be attributed to solid solution strengthening and precipitation strengthening caused by the addition of Mn.The improvement of the plasticity of austenitic steels can be explained from two aspects:grain refinement and homogenization of precipitated phases.

Coarsening behavior of MX carbonitrides in type 347H heat-resistant austenitic steel during thermal aging

In this work, the growth kinetics of MX （M - metal, X - C/N） nanoprecipitates in type 347H austenitic steel was systematically studied. To investigate the coarsening behavior and the growth mechanism of MX carbonitrides during long-term aging, experiments were performed at 700, 800, 850, and 900℃ for different periods （1, 24, 70, and 100 h）. The precipitation behavior of carbonitrides in specimens subjected to various aging conditions was explored using carbon replicas and transmission electron microscopy （TEM） observations. The corresponding sizes ofMX carbonitrides were measured. The results demonstrates that MX carbonitrides precipitate in type 347H austenitic steel as Nb（C,N）. The coarsening rate constant is time-independent; however, an increase in aging temperature results in an increase in coarsening rate of Nb（C,N）. The coarsening process was analyzed according to the calculated diffusion activation energy of Nb（C,N）. When the aging temperature was 800-900℃, the mean activation energy was 294 kJ·mol -1, and the coarsening behavior was controlled primarily by the diffusion of Nb atoms.

Hot deformation behavior of Super304H austenitic heat resistant steel

The hot compression tests of Super304H austenitic heat resistant steel were carried out at 800-1200℃and 0.005-5 s^-1 using a Gleeble 3500 thermal-mechanical simulator,and its deformation behavior was analyzed.The results show that the flow stress of Super304H steel decreases with the decrease of strain rate and the increase of deformation temperature; the hot deformation activation energy of the steel is 485 kJ/mol.The hot deformation equation and the relationship between the peak stress and the deformation temperature and strain rate is obtained.The softening caused by deformation heating cannot be neglected when both the deformation temperature and strain rate are higher.

Effects of Austenite Stabilization on the Onset of Martensite Transformation in T91 Steel

The influences of thermal stabilization of austenitic on the onset temperature for a martensite transformation in T91 ferritic heat-resistant steel were studied by high-resolution differential dilatometer. The phase transformation kinetic information was obtained by adopting lever rule from the recorded dilatometric curves. The results show that an inverse stabilization, featured by the damage of ＂the atmosphere of carbon atoms＂ and the increase of the starting temperature for martensite transformation takes place when the T91 ferritic steel is isothermally treated above the Ms point, and it becomes strong with increasing the holding time. While the continued temperature for martensite transformation decreases gradually when isothermally holding at a temperature below Ms point. The observed inverse stabilization behavior could be attributed to the relatively high temperature of Ms point in the explored T91 ferritic heat-resistant steel.

Microstructures and Mechanical Properties of a Newly Developed Austenitic Heat Resistant Steel

The effect of heat treatment on the microstructures and mechanical properties of a newly developed austenitic heat resistant steel(named as T8 alloy) for ultra-supercritical applications have been studied. Results show that the main phases in the alloy after solution treatment are γ and primary MX. Subsequent aging treatment causes the precipitation of M_(23)C_6 carbides along the grain boundaries and a small number of nanoscale MX inside the grains. In addition, with increasing the aging temperature and time, the morphology of M_(23)C_6 carbides changes from semi-continuous chain to continuous network.Compared with a commercial HR3C alloy, T8 alloy has comparable tensile strength, but higher stress rupture strength. The dominant cracking mechanism of the alloy during tensile test at room temperature is transgranular, while at high temperature, intergranular cracking becomes the main cracking mode, which may be caused by the precipitation of continuous M_(23)C_6 carbides along the grain boundaries. Typical intergranular cracking is the dominant cracking mode of the alloy at all stress rupture tests.

Effects of Al on Microstructure and High-Temperature Wear Properties of Austenitic Heat-Resistant Steel

Microstructure and high-temperature dry sliding wear at 600 ~C in ambient air of austenitic heat-resistant steel ZG40Cr25Ni20 with different contents （mass percent） of AI （0 to 7.10~） have been investigated. The results show that microstructures of 4.68% and 7.10% A1 addition content consist of the matrix and reinforcement of inter- metallic compound y＇ and carbide, while microstructures of ZG40Cr25Ni20 without A1 and with A1 of 1.68% are ab- sent of y＇. Higher wear resistance than the original ZG40Cr25Ni20 alloy is achieved in alloys with higher content of A1 under the same high-temperature wear test condition. The wear rates of Fe-25Cr-20Ni-7.10A1 and Fe-25Cr-20Ni- 4.68A1 are only 20.83% and 45.83% of that of Fe-25Cr-20Ni, respectively. Heat-resistant steels with higher con- tents of AI （4.72% and 7.10%） have higher hardness than those with lower contents of AI （1.68% and 0）. Wear mechanisms of ZG40Cr25Ni20 are considered as severe plough plastic deformation and slight adhesive. However, wear mechanisms of Fe-25Cr-20Ni 4.68A1 are light micro-cutting and oxidation-wear, while that of Fe-25Cr-20Ni- 7. 10A1 are severe adhesive transfer and oxidation-wear_

Effect of rare earth alloying on creep rupture of economical 21Cr-11Ni-N heat-resistant austenitic steel at 650 ℃

The effect of rare earth（RE） on creep rupture of economical 21Cr-11Ni-N heat-resistant austenitic steel was investigated at 650 °C under different stress levels. It was found that RE could increase the time to creep rupture, especially at long-term creep duration. The logarithm of the time to creep rupture（lgtr） was a linear function of the applied stress（σ）. RE addition was favorable to generating a high fraction of low-coincidence site lattice（CSL） boundaries which was a possible cause for improving the creep rupture resistance. The fracture surface of RE-added steel exhibited less intergranular cracks suggesting the alteration on the nature of grain boundaries due to the presence of RE. RE addition changed the morphology of the intergranular chromium carbides from continuous network shape to fragmentary distribution which was another cause for longer creep duration. These results strongly suggested that the effect of RE alloying played a crucial role in improving the creep rupture resistance.

Precipitate Behavior in Fe–20Cr–30Ni–2Nb Austenitic Heat-Resistant Steel

In this study, the precipitation behavior of a new austenitic heat-resistant steel （Fe-20Cr-30Ni-2Nb, in at%） was investigated. The effects of alloying addition of boron （B） and lanthanum （La） on the microstructure of the austenitic steel were scrutinized using SEM, EPMA, TEM, and XRD. The results showed that the addition of B enhanced the precipitation of bar-type Laves phase. A small precipitate with high La concentration was observed at the grain boundary in the alloy without aging; similar precipitates without La also presented in region adjacent to the La single phase. This result indicates that La can exist independently and does not contribute to the formation of new compounds. However, in both B- and La-modified alloy, B appeared in the precipitate free zone. In the alloy containing both B and La, only Fe2Nb Laves- phase precipitates, as indicated by the XRD result.

Strengthening effect of Cu-rich phase precipitation in 18Cr9Ni3CuNbN austenitic heat-resisting steel

The Cu-containing austenitic heat-resistant steel 18Cr9Ni3CuNbN, which is being used as superheater and reheater tube material for modern ultra-super-critical （USC） power plants all over the world, has been investigated at 650 ℃ long time aging till 10 000 h. SEM, TEM and 3DAP （three dimensional atom probe） have been used to follow microstructural changes with mechanical property variations. Experimental results show that Cu-rich phase and MX precipitate in the grains as well as M 23 C 6 precipitates at grain boundaries are the main precipitation strengthening phases in this steel. Among them Cu-rich phase is the most important strengthening phase. Homogeneous distribution of very fine nano-size Cu-rich phase has been formed at very early stage of 650 ℃ aging （less than 1 h）. Cu atoms gradually concentrate to Cu-rich particles and the other elements （such as Fe, Cr, Ni etc） diffuse away from Curich particles to γ-matrix with the increasing of aging time at 650 ？ C. The growth rate of Cu-rich phase at 650 ℃ long time aging is very slow and the average diameters of Cu-rich phase have been determined by TEM method. Cu-rich phase keeps in about 30 nm till 650 ℃ aging for 10 000 h. It shows that nano-size Cu-rich phase precipitation strengthening can be kept for long time aging at 650 ℃ because of its excellent stability at high temperatures. According to structure stability study and mechanical properties determination results the Cu-rich phase precipitation sequence and its strengthening mechanism model have been suggested and discussed.

HR3C奥氏体耐热钢晶界蠕变损伤的电子背散射衍射分析

对不同条件下的HR3C奥氏体耐热钢蠕变试样进行性能测试和微观结构观察,研究奥氏体耐热不锈钢蠕变沿晶破坏行为和损伤特征。电子背散射衍射(EBSD)分析结果表明,不同蠕变速度下材料的晶粒尺寸基本不变,未产生择优取向,但高蠕变速度条件下部分孪晶界演变成一般晶界,而低蠕变速度下原始孪晶结构基本保持不变。EBSD分析结果清楚地反映了微观蠕变应变的不均匀性。沿晶微裂纹和蠕变空洞的产生和扩展是最显著的损伤特征,这些损伤现象与不同蠕变速度下的蠕变机制及晶界性质密切相关。

Embrittlement Mechanism due to Slow Cooling During Quenching for M152 Martensitic Heat Resistant Steel

The mechanism of brittleness of M152 martensitic heat resistant steel due to slow cooling during quenching was experimentally investigated. The mechanical property tests and microstructure observation were conducted by TEM and XRD. The results showed that the presence of irreversible brittleness during slow cooling of quenching for M152 steel is attributed to the continuous M23C6 precipitation along prior austenite grain boundaries and M2C along prior residual austenite film. The residual austenite in the steel was unstable and decomposed after the precipitation of second phase during the process of slow cooling of quenching. The low cooling rate within the temperature range from 820 ℃ to 660 ℃ plays a key role in impact toughness, and the precipitation of second phase in the same temperature range results in irreversible brittleness.

精密铸造锅炉管件用耐热奥氏体钢的研究

为了优化耐热奥氏体钢在锅炉管件精密铸造中的工艺,选取典型焊接接头进行分析,预热温度≥5℃,电压13~14V,焊层数3层,电流190~200 A,层间温度50~230℃。结果显示,600~1200℃温度环境下,AFA25-15为单相奥氏体基体,析出相为NiAl、σ相、M_(23)C_(6)、MC碳化物。通过适当冷变形处理之后,强度明显提升,AFA-50%与AFA-20%抗拉强度均有所降低,断后伸长率变动差异表现不明显。材料氧化腐蚀程度与温度呈正比关系,S31042材料抗氧化腐蚀能力最强,S31035与C-HRA-5材料抗氧化腐蚀能力基本相当。通过本文的研究与探讨,有利于改善锅炉管件用耐热奥氏体钢的综合性能,提升拉伸性能。

锅炉高温再热器长时服役TP310HCbN钢管失效分析

针对某660 MW超超临界机组服役4.6万h后的锅炉高温再热器,通过金相检测、扫描电镜、拉伸试验和冲击试验等方法对泄漏的TP310HCbN钢管的组织形貌和力学性能进行研究。结果表明:长时高温服役使TP310HCbN钢管的金相组织老化至3~4级,晶界析出大量碳化物,管材严重脆化,导致其韧性储备不足,受外力撞击后外壁表面形成大量微裂纹,随着服役时间的延长,管样裂纹逐渐扩展并最终导致爆管泄漏。

热处理工艺对AFA耐热钢组织和力学性能的影响

为考察热处理工艺对AFA耐热钢组织和力学性能的影响,本文利用光学显微镜、扫描电镜、X射线衍射等技术,研究了新型含铝奥氏体耐热钢(AFA)在不同保温温度和冷却方式下的组织演变规律及力学性能的变化。结果表明:加热温度和冷却方式对试样的显微组织均产生影响。随着保温温度的升高,晶粒的平均尺寸逐渐增大。在1150和1200℃保温冷却后,空冷试样的晶粒尺寸略大于炉冷试样。不同温度下保温冷却后,析出相在晶内和晶界均有分布,晶内析出相形状变为圆形或颗粒状,晶界处变为长条状,但种类并未发生改变。析出相的平均尺寸随着保温温度的升高而增大。在1200℃下炉冷后,析出相的平均尺寸、面密度和晶界覆盖率均高于空冷,此时炉冷试样的硬度值达到170 HV,高于空冷,但抗拉强度和延伸率较低,分别为680 MPa和13.04%。

时效粗、细晶Super304H钢的第二相析出行为及力学性能

采用预变形后固溶处理的方法制备了粗晶Super304H钢试样,对比研究了粗、细晶Super304H钢试样在700℃时效过程中的第二相析出行为及力学性能。结果表明:时效过程中,细小MX相与富Cu相颗粒主要分布于奥氏体晶内,奥氏体晶粒尺寸对其析出行为影响不大。粗晶Super304H钢中的M23C6相颗粒择优沿奥氏体晶界析出,长大速率大,时效1200 h后,呈连续网络状分布。随着时效时间的延长,粗、细晶Super304H钢试样的室温及高温拉伸强度先上升后下降,最终趋于稳定,断后伸长率单调下降。时效态粗晶Super304H钢试样的室温、高温拉伸力学性能,尤其是塑性,均明显小于时效态细晶Super304H钢试样。

多种高温金属材料在超临界二氧化碳中的腐蚀行为

超临界二氧化碳布雷顿循环发电是一种新兴的发电技术,保证其中高温材料的抗腐蚀性能满足要求是设备安全运行的必要前提。该文研究3种耐高温材料P92、Sanicro25、Inconel625在600℃、25MPa超临界二氧化碳中的腐蚀行为。采用扫描电镜、能谱分析、X射线衍射、X射线光电子能谱、原子力显微镜以及辉光放电光谱仪对腐蚀产物进行表征。结果表明:P92形成了较为疏松、多孔洞的双层氧化膜结构;Sanicro25和Inconel625则形成致密的Cr_(2)O_(3)氧化膜。P92的氧化层有明显的孔洞形成,而氧化层下方基体发生渗碳;Sanicro25和Inconel625氧化层下方则有明显的贫Cr现象发生。Sanicro25和Inconel625抗腐蚀性能和抗渗碳能力明显优于P92。同时,基于“空洞诱导双层氧化膜形成机理”,指出合理的Cr元素含量是预防渗碳、保证氧化膜致密性的关键因素之一。结果可为超临界二氧化碳循环机组的选材和腐蚀防护提供一定参考和借鉴。

时效时间对SP2215同种钢焊接接头微观组织和力学性能的影响

采用手工钨极氩弧焊(manual-gas tungsten arc welding,M-GTAW)和ERNiCr-3焊丝制备了SP2215小径管焊接接头,分析了650℃时效不同时间后接头的微观组织、力学性能及拉伸断裂机理.结果表明:焊态下SP2215母材组织由奥氏体和少量孪晶及一次NbN和Z相组成,焊缝金属以完全奥氏体组织凝固形成柱状晶,Nb元素在枝晶间偏析并形成呈链状分布的一次NbC相;时效50 h后SP2215母材奥氏体晶界处开始析出M23C6,并随时效时间延长逐渐粗化,同时焊缝枝晶间逐渐析出NbC相,并随时效时间延长逐渐形成团簇状;母材和焊缝显微硬度均随时效时间延长而升高,均在时效500 h时达到最大值且母材硬度始终高于焊缝硬度;不同时效时间接头室温拉伸时均以韧性方式在焊缝处断裂,但起裂位置和断裂方向随时效时间的延长发生改变,时效0~114 h时以横向拉断柱状晶的方式断裂,起裂位置为枝晶间一次NbC粒子,时效时间为500~2012 h时以平行于柱状晶方向断裂,起裂位置为枝晶核心;650℃高温拉伸时接头断裂位置随时效时间延长逐渐从焊缝向SP2215侧转变,时效0~114 h时以韧性方式在焊缝处断裂,时效2012 h时以准解理方式在SP2215侧断裂.

新型奥氏体耐热钢SP2215的开发与研究

通过设计合理化学成分,采用常规高压锅炉用耐热钢管管坯生产工艺,成功生产出新型奥氏体耐热钢SP2215管坯。相比耐热钢HR3C,SP2215合金元素含量低,生产成本相对较低,具有更优秀的热塑性,能更好适应二辊斜轧穿孔工艺;SP2215在650℃、700℃下的高温持久强度显著优于HR3C,高温抗蒸汽氧化腐蚀和焊接性能方面与之相当。因此,SP2215的综合性能明显优于HR3C,可作为620~650℃超超临界电站锅炉用不锈耐热钢材料。

奥氏体化温度对超强耐热齿轮轴承钢显微组织和强韧性的影响

新型超强耐热齿轮轴承钢具有优越的强韧性。通过改变钢的淬火加热温度,结合拉伸、冲击、断裂韧度等力学性能测试以及TEM,SEM,EDS等微观分析技术,研究不同奥氏体化温度下钢的显微组织与力学性能。结果表明:1060℃奥氏体化后,钢中存在未溶碳化物M_(6)C,冲击功和断裂韧度较低;1080~1100℃奥氏体化后,M_(6)C碳化物固溶,冲击功和断裂韧度显著增加。在1060~1100℃奥氏体化后,抗拉强度和塑性变化不大,规定塑性延伸强度随奥氏体化温度的增加略有降低。M_(6)C碳化物加速裂纹的萌生与扩展,导致韧性下降。在1080~1100℃奥氏体化后,超强耐热齿轮轴承钢可获得超高强度和高韧性,抗拉强度不小于2000 MPa,规定塑性延伸强度不小于1800 MPa,断裂韧度不小于100 MPa·m^(1/2)。

一种适用于气阀用耐热钢显微组织的腐蚀方法

采用不同腐蚀剂对热轧态和时效态5Cr21Mn9Ni4N耐热钢进行腐蚀,研究了不同腐蚀剂对5Cr21Mn9Ni4N钢腐蚀效果的影响。结果表明:传统的硝酸乙醇溶液无法腐蚀试样,试样表面无任何变化;三氯化铁盐酸水溶液无法使试样显示清晰完整的组织及晶界,且存在过度腐蚀现象;采用5 g FeCl_(3)+50 mL HCl+100 mL H_(2)O腐蚀剂与10 g CuSO_(4)+15 mL H_(2)SO_(4)+100 mL HCl+5 mL C_(3)H_(8)O_(3)腐蚀剂配合使用,在室温下对热轧态及时效态5Cr21Mn9Ni4N钢进行腐蚀,能够清晰显示材料的奥氏体组织。