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金属学报, 2024, 60(6): 713-730 DOI: 10.11900/0412.1961.2023.00488

综述

高Cr马氏体耐热钢的协同强化机制及形变热处理应用

张竟文, 余黎明, 刘晨曦, 丁然, 刘永长,

天津大学 材料科学与工程学院 水利工程智能建设与运维全国重点实验室 天津 300354

Synergistic Strengthening of High-Cr Martensitic Heat-Resistant Steel and Application of Thermo-Mechanical Treatments

ZHANG Jingwen, YU Liming, LIU Chenxi, DING Ran, LIU Yongchang,

State Key Laboratory of Hydraulic Engineering Simulation and Safety, School of Materials Science and Engineering, Tianjin University, Tianjin 300354, China

通讯作者: 刘永长，ycliu@tju.edu.cn，主要从事金属成形与加工研究;

责任编辑: 肖素红

收稿日期: 2023-12-15   修回日期: 2024-01-23  

基金资助: 国家自然科学基金项目(52034004)
国家重点研发计划项目(2022YFB3705300)

Corresponding authors: LIU Yongchang, professor, Tel:(022)85356410, E-mail:ycliu@tju.edu.cn

Received: 2023-12-15   Revised: 2024-01-23  

Fund supported: National Natural Science Foundation of China(52034004)
National Key Research and Development Project of China(2022YFB3705300)

作者简介 About authors

张竟文，男，1993年生，博士

摘要

高Cr (9%~12%，质量分数)马氏体耐热钢因其较高的热导率、较低的热膨胀系数以及优异的高温蠕变强度等优点而被认为是超超临界火电机组关键设备升级改造的主选材料。然而，服役过程中高Cr马氏体耐热钢高温蠕变强度的不断弱化严重影响了其安全可靠性。以往提升高Cr马氏体耐热钢高温蠕变强度的主要手段是通过合金成分优化设计来促进沉淀相弥散析出，但单一析出强化效应对蠕变强度的提升效果非常有限。近年来，位错-沉淀相-界面协同强化效应在提升高Cr马氏体耐热钢高温蠕变性能方面表现出显著效果。其原理是通过形变热处理引入位错来促进多种沉淀相弥散析出，同时通过控制相变来细化板条组织，增强位错、沉淀相及界面3者之间的交互作用，从而实现多类蠕变强化效应的协同提升。本文总结了高Cr马氏体耐热钢的协同强化机制及形变热处理组织调控，从高温蠕变强度提升角度回顾了合金成分的优化历程，阐述了热处理过程中的相变行为及高温组织退化机理，对比分析了单一析出强化效应及形变热处理后位错-沉淀相-界面协同强化效应对其高温蠕变强度的影响规律，并基于焊接接头蠕变失效行为探索了形变热处理对焊接热影响区的组织调控机制，以期为高Cr马氏体耐热钢及其他火电机组用沉淀型强化耐热钢的材料设计及工程应用提供指导。

关键词： 高Cr马氏体耐热钢; 高温蠕变强度; 协同强化; 形变热处理; 组织调控

Abstract

By virtue of their high thermal conductivity, low thermal expansion coefficient, and excellent high-temperature creep strength, high-Cr (mass fraction: 9%-12%) martensitic heat-resistant steels are the putative main constituents of the key equipment in ultra-supercritical (USC) power plants. However, the harsh environment caused by enhancing the steam parameters has recently challenged the high-temperature properties and the continually deteriorating creep strength during service has seriously threatened the safety and reliability of these steels. Previously, the creep strength of high-Cr martensitic heat-resistant steels was enhanced by optimizing the alloying compositions to promote the dispersed precipitation of strengthening phases, but the enhancement effect of reinforced single-precipitate strengthening is limited. In recent years, synergistic strengthening reinforcement of dislocation-precipitate-interface has emerged as a promising solution because the introduced dislocations promote various precipitations and the phase transformation can be controlled to tailor the lath structure, thus reinforcing the dislocation-precipitate-interface interactions and synergistically enhancing various strengthening effects. This paper overviews the synergistic strengthening of dislocation-precipitate-interface and microstructure control in high-Cr martensitic heat-resistant steels subjected to thermo-mechanical treatments. The review covers alloying optimization to improve the creep strength, the phase transformations during heating treatments, and the mechanism of microstructural degradation at high temperatures. It also compares the effects of single-precipitate and synergistic strengthening processes on creep strength and introduces microstructure control in welded joints by thermo-mechanical treatments in terms of creep failure behaviors. This research aims to guide the design and engineering applications of high-Cr martensitic heat-resistant steels and other precipitate-strengthening heat-resistant steels for USC power plants.

Keywords： high-Cr martensitic heat-resistant steel; high-temperature creep strength; synergistic strengthening; thermo-mechanical treatment; microstructure control

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本文引用格式

张竟文, 余黎明, 刘晨曦, 丁然, 刘永长. 高Cr马氏体耐热钢的协同强化机制及形变热处理应用[J]. 金属学报, 2024, 60(6): 713-730 DOI:10.11900/0412.1961.2023.00488

ZHANG Jingwen, YU Liming, LIU Chenxi, DING Ran, LIU Yongchang. Synergistic Strengthening of High-Cr Martensitic Heat-Resistant Steel and Application of Thermo-Mechanical Treatments[J]. Acta Metallurgica Sinica, 2024, 60(6): 713-730 DOI:10.11900/0412.1961.2023.00488

发展高效、清洁的先进超超临界(ultra-supercritical，USC)火电机组建设是我国实现碳达峰、碳中和发展目标的重要举措之一[1,2]，而提升火电机组发电效率的主要途径是提高其蒸汽温度和压力[3,4]。随着我国燃煤发电技术的快速发展，蒸汽温度和压力的不断提高对机组关键高温部件运行的安全性和可靠性提出了新的挑战，也对其所采用耐热结构材料的使役性能提出了更高要求[5,6]。高Cr (9%~12%，质量分数，下同)马氏体耐热钢因其具有较高的热导率、较低的热膨胀系数、良好的抗蒸汽腐蚀能力以及优异的高温抗蠕变性能，被广泛应用于火电机组水冷壁、过热器、再热器及厚壁蒸汽管道等关键部件的制造[7,8]。但是长期在高温、高压环境中服役，高Cr马氏体耐热钢及其焊接接头的高温蠕变强度会不断恶化，成为限制高参数先进电力机组发展建设的“瓶颈”要素[9,10]。

高Cr马氏体耐热钢典型组织是具有高密度位错的板条结构，并且在板条内部及边界处弥散分布着细小的沉淀强化相，其中包括M23C6相、MX相、Laves相以及最新引入的富Cu相，其高温抗蠕变性能和组织热稳定性主要源于各类沉淀相与位错及界面之间的交互作用[11~15]，高Cr马氏体耐热钢微观组织特征及协同强化机制分别如图1a[11]和b所示。然而，在高Cr马氏体耐热钢长期服役过程中，其组织中的M23C6相和Laves相会不断发生粗化，MX相会逐渐溶解并伴随Z相形成，同时富Cu相也会发生粗化和部分溶解，很大程度上弱化了沉淀强化相对位错回复和边界迁移的钉扎作用，导致其高温蠕变强度明显恶化[16~18]。长期以来，为了进一步提升高Cr马氏体耐热钢的组织热强性，一方面通过优化C、N、Nb和V等微合金化元素配比，来促进高密度MX相的弥散析出；另一方面通过提高合金体系中W和Co等间隙元素含量，以提升固溶强化效应，同时降低M23C6相和Laves相的粗化速率[19~21]。由于高Cr马氏体耐热钢的高温抗蠕变性能依赖于固溶强化、析出强化、位错强化及界面强化机制之间的耦合交互作用，导致单一沉淀相析出强化效应对高温蠕变强度的提升效果非常有限。因此，如何实现多种蠕变强化效应的协同提升对大幅提高其高温蠕变性能至关重要。

图1

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图1   高Cr马氏体耐热钢微观组织特征[11]及协同强化机制

Fig.1   Microstructure characteristics and synergistic strengthening of the high-Cr martensitic heat-resistant steels

(a) schematic diagram[11] (PAGs—prior austenite grains)

(b) synergistic strengthening of lath interfaces, dislocations, and precipitates


近年来，位错-沉淀相-界面协同强化效应在提升火电机组用耐热钢高温蠕变性能方面展现出显著的效果，主要是通过形变热处理对沉淀相析出及相变行为进行精准调控，从而获得具有优异热稳定性的理想组织[22~24]。在此背景下，本文概述了高Cr马氏体耐热钢的协同强化机制及形变热处理应用，从高Cr马氏体耐热钢的发展历程及合金成分设计出发，基于其热处理相变行为及高温蠕变组织退化机理，分析了单一沉淀相析出强化及不同形变热处理技术下位错、沉淀相及界面协同强化机制；基于焊材成分设计原则及焊接接头制备，阐明了高Cr马氏体耐热钢焊接接头的高温蠕变失效行为，探索了形变热处理在其焊接接头制备中的组织调控机制；最后，对高Cr马氏体耐热钢组织调控方法和形变热处理强化技术的应用推广进行了展望。

1 高Cr马氏体耐热钢发展回顾

20世纪70年代，受全球石油危机和能源需求激增的影响，各国开始将高Cr马氏体耐热钢作为USC火电机组的主要候选材料，并且根据日益严苛的服役要求不断对其合金成分进行优化与调整，其发展历程可以分为4个阶段[25]：第一阶段主要以T/P9钢为主[26]，通过添加V、Nb和Mo来实现析出强化和固溶强化效应，受高温铁素体存在的影响其高温蠕变强度较差；第二阶段主要以T/P91钢为主[27,28]，在T/P9钢的基础上，进一步优化了Nb、V和N元素的成分配比，减少了基体组织中高温铁素体的含量，使其高温蠕变强度获得了极大提升；第三阶段主要以T/P92钢为主[29,30]，在T/P91钢成分的基础上，将Mo含量降低至0.5%，并添加了1.5%~2.0%的W和0.004%的B，进一步提升了固溶强化和析出强化效应；第四阶段以MARBN、SAVE12AD和G115钢为主[31~33]，主要是将W替代Mo，加入奥氏体稳定元素Co，并优化了Nb、N和B的含量配比，额外添加Cu进一步提升了析出强化作用[34,35]，使其高温蠕变强度显著提高，如G115钢650℃高温蠕变强度约为P92钢的1.5倍，可与MARBN钢相媲美。

2 高Cr马氏体耐热钢热处理相变行为及高温组织退化机理

2.1 热处理相变行为

高Cr马氏体耐热钢工业热处理工艺为正火+回火，如图2a所示。正火温度控制在奥氏体相变结束温度(Ac3)以上，目的在于使基体组织完全奥氏体化并充分溶解前期铸造过程中析出的沉淀相，在冷却过程中发生完全马氏体相变，形成具有高密度位错结构的板条组织；回火温度一般控制在奥氏体相变开始温度(Ac1)以下，一方面是为了促进沉淀相的充分弥散析出，另一方面可以通过降低固溶硬化作用并促进部分位错回复使基体软化，保证其强度和塑性的良好匹配[36,37]。

图2

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图2   高Cr马氏体耐热钢工业应用正火+回火热处理工艺及冷却速率对板条组织的影响[44]

Fig.2   Industrial normalizing and tempering treatments of high-Cr martensitic heat-resistant steels (Ac1—starting temperature of austenite phase transformation, Ac3—ending temperature of austenite phase transformation) (a), and effects of cooling rate on lath structure[44] (b, c)


在整个热处理工艺中，马氏体相变只发生在正火后的冷却过程，当以大于临界冷却速率过冷到马氏体相变开始温度(Ms)以下时，原始奥氏体组织便会发生无扩散切变相变，形成马氏体[38]。一般来说，材料的初始合金成分、原始奥氏体晶粒尺寸以及冷却初始温度、冷却终止温度和冷却速率均会影响相变后的马氏体特征，进而对其性能产生明显影响[39~41]。研究[42]表明，随着钢中C含量的升高，相变后马氏体形态由板条状向针状或薄膜状转变。为了保证高Cr马氏体耐热钢足够的高温蠕变强度，其合金体系中的C含量一般不高于0.15%，且相变后的马氏体组织呈现为板条状。同时，奥氏体稳定化元素包括Mn、Ni和Cu等均会降低Ms，并有利于片状马氏体的形成。此外，随着原奥氏体晶粒尺寸的减小，Ms也会明显降低并导致马氏体板条宽长比增大[40]。值得注意的是，热处理过程中的冷却速率对马氏体相变行为的影响最为显著[43]，随着冷却速率的升高，Ms和马氏体相变结束温度(Mf)均有所降低，马氏体板条组织逐渐细化，如图2b和c[44]所示。

综上可知，影响马氏体相变的因素有很多，不同相变条件对应不同的相变过程，最终影响马氏体特征。同时，马氏体板条组织提供的蠕变强化作用与板条尺寸密切相关，板条越细，强化效果越显著。因此，基于高Cr马氏体耐热钢的微观组织特征及高温蠕变强化机制，通过相关工艺改变相变条件来实现对马氏体相变及板条组织的精准调控，可以有效提高界面强化效应，是提升其高温蠕变性能的可行途径之一。

2.2 高温组织退化机理

在高温、高压长时服役环境中，高Cr马氏体耐热钢多尺度微观组织会在温度场和应力场的耦合影响作用下不断恶化，主要包括各类沉淀相的独自演变、交互影响以及亚结构的回复退化。其中，沉淀相演变主要包括M23C6相和Laves相的粗化、富Cu相的粗化和溶解以及MX相的溶解和Z相的生成；而亚结构退化主要包括位错的回复湮灭、板条的迁移宽化以及亚晶晶粒的形核长大[45~47]。2种恶化进程同时进行，相互促进，导致其高温蠕变强度逐渐变差。

Aghajani等[48]分析了600℃、120 MPa下12%Cr马氏体耐热钢的组织退化行为，蠕变过程中M23C6相和Laves相会发生Ostwald熟化而不断长大，数密度明显降低，弱化析出强化效应。同时，Xu等[49]指出，在Laves相长大过程中，除了相邻粒子之间的合并之外，Laves相也会不断吞噬其相邻的M23C6相，展现出相对较高的粗化速率，如图3a和b[49]所示。Wang等[50]指出，相比于M23C6相和Laves相，MX相具有较高的热稳定性，对蠕变强度的贡献较为持久。但是，Sawada等[51]在T122钢中观察到，650℃蠕变2000 h后其基体组织中的MX相会发生溶解并伴随着粗大Z相的形成，加速了位错回复行为，导致其高温蠕变强度突然恶化。此外，Xiao等[52]通过中断实验澄清了蠕变过程中富Cu相对G115钢组织演变的影响，指出蠕变早期富Cu相对抑制马氏体板条粗化起主导作用，此时板条组织较为稳定；而在蠕变后期马氏体板条则发生明显宽化，这主要是因为位错网和板条边界促进了富Cu相的粗化，并且位错与富Cu相之间的交割作用会导致其部分溶解，很大程度上弱化了析出强化效应，如图3c和d[52]所示。同时，Zhang等[53]和He等[54]也分析了G115钢及其焊接接头的蠕变断裂机制，发现在蠕变后期M23C6相和富Cu相与基体之间的不协调变形会诱导界面裂纹形成。上述析出相的粗化和溶解会弱化其对位错运动及界面迁移的钉扎力，位错缠结会逐渐转变为位错排列或位错线，如图4a和b所示，并且板条组织发生回复形成低硬度亚晶晶粒，如图4c和d所示。

图3

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图3   高Cr马氏体耐热钢蠕变/时效过程中的沉淀相粗化和溶解行为[49,52]

Fig.3   Swallowing behaviors of Laves phases on M23C6 during aging (Insets show the corresponding selected area electron diffraction patterns)[49] (a, b) and the coarsening and dissolution behaviors of Cu-rich phases during creep (CRPs—Cu-rich precipitates)[52] (c, d) in high-Cr martensitic heat-resistant steels


图4

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图4   高Cr马氏体耐热钢蠕变过程中的位错及板条结构回复行为

Fig.4   Recovery behaviors of dislocations (a, b) and laths (c, d) during creep in high-Cr martensitic heat-resistant steels (a, c) transient stages (b, d) accelerated stages


由以上分析可知，沉淀相粗化溶解以及亚结构回复长大引起的多种蠕变强化机制弱化，是高Cr马氏体耐热钢高温蠕变强度不断衰减的主要原因。因此，通过抑制蠕变过程中M23C6相和Laves相的粗化并促进MX相和富Cu相的弥散析出，同时提高位错密度并细化板条组织，以增强位错、沉淀相及界面3者之间的交互强化作用，有望实现高Cr马氏体耐热钢高温蠕变强度的有效提升。

3 高Cr马氏体耐热钢单一析出强化效应及形变热处理组织调控

3.1 单一析出强化效应

长期以来，基于对高Cr马氏体耐热钢高温组织退化机理的深入了解，为了进一步提升其高温蠕变强度，对其合金体系中的微量元素含量不断进行优化与调整，期望通过改善沉淀相的析出长大行为以提升析出强化效应。在各类析出相中，M23C6相和Laves相一般在回火或蠕变过程中沿原始奥氏体晶界或马氏体板条界面弥散析出，对阻碍界面迁移至关重要，但2者较高的粗化速率仍是导致蠕变断裂的主要因素[55~57]。研究[58~63]表明，通过控制高Cr马氏体耐热钢合金体系中B、W、Co和V等含量可以改变M23C6相的结构及化学组成，明显降低其粗化速率，但需同时考虑相关联元素的配比以保证其优异的综合服役性能。同时，虽然Hosoi等[64]指出降低Si含量可以改变Laves相的平均电子浓度和原子半径，从而提高其热稳定性，但其对高Cr马氏体耐热钢高温蠕变性能的提升效果并不理想。

相比于M23C6相和Laves相，MX相在高温蠕变过程中不易粗化，可以有效抑制高Cr马氏体耐热钢蠕变过程中的位错回复行为[11,21]。但因其体积分数相对较低，对高温蠕变强度的提升效果有限。近期，Mao等[65]制备了Zr微合金化低碳铁素体/马氏体钢，通过优化C/Zr元素含量，进一步提高了板条内部MX相的数密度及热稳定性，并且细化了界面处M23C6相的尺寸，使其高温蠕变强度获得大幅提升。富Cu相作为高Cr马氏体耐热钢中新引入的沉淀强化相，在提升其高温组织稳定性方面扮演着重要角色[11,52]。但是在长期蠕变过程中，位错网与富Cu相之间的交割作用会使其部分溶解，导致其蠕变贡献力明显降低。此外，最新研究[54]表明，富Cu相与其他析出相(Laves相、M23C6相等)之间存在复杂的交互作用机制，但其对高温蠕变性能的影响规律尚不清楚。

除上述常见的沉淀相以外，将化学结构稳定、熔点较高的纳米氧化物颗粒(一般为Y2O3)引入到高Cr马氏体耐热钢中，对提升其高温蠕变性能大有裨益[66~68]。Zhao等[69]和Zhou等[70]制备了Zr、Ti及Al微合金化氧化物弥散强化(oxide dispersion strengthened，ODS)铁素体/马氏体耐热钢，实现了基体组织及氧化物强化相的精准调控，有效改善了其力学性能及高温蠕变性能。但由于ODS钢制备工艺复杂，成形工艺尚不成熟，且力学性能存在各向异性，目前仍无法进行大规模批量生产及工业应用。

由此可知，通过优化高Cr马氏体耐热钢合金成分调控沉淀相的析出种类、形态分布和化学成分等特征，可以在一定程度上改善其组织热强性，实现高温蠕变性能的有效提升。然而，沉淀相作为高Cr马氏体耐热钢微观组织中的单一强化组元，需要与其他组元(比如位错、界面等)协同作用才能充分发挥其强化效应。此外，过度合金化还会导致冷热加工性能、焊接性能及塑韧性急剧下降，无法满足服役过程中的综合使役性能需求。

3.2 形变热处理组织调控

近年来，形变热处理在提升沉淀型强化耐热钢的高温蠕变性能方面表现出明显的优势，主要是通过调控沉淀相析出分布来提升析出强化效应，同时引入形变位错并细化板条组织，以提升位错强化和界面强化效应[71~73]。相比于基于成分优化的单一析出强化效应，形变热处理可实现多类蠕变强化效应的协同提升，并且工艺简单、成本较低。目前，形变热处理的强化机制主要可以分为位错-蠕变沉淀-界面协同强化效应和位错-回火沉淀-界面协同强化效应。其中，“蠕变沉淀”主要是指在蠕变过程中能够析出的沉淀相(包括大部分Laves相和少量M23C6相、富Cu相)；而“回火沉淀”则是在回火过程中析出的沉淀相(包括大部分M23C6相、MX相和富Cu相)。

3.2.1 位错-蠕变沉淀-界面协同强化效应

位错-蠕变沉淀-界面协同强化效应主要是在高Cr马氏体耐热钢正火+回火热处理后进行冷变形处理，如图5a所示，通过在基体中引入高密度形变位错，调控蠕变过程中沉淀相的分布、形态及数密度；同时，通过挤压作用细化板条，增加单位面积内的界面密度，如图5b和c所示；最终，通过增强蠕变过程中位错、沉淀相及板条界面3者之间的交互强化作用，实现组织热稳定性的有效提升。

图5

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图5   可实现位错-蠕变沉淀-界面协同强化效应的形变热处理示意图及冷变形对板条组织的影响

Fig.5   Schematic showing the deformation and heating treatments of synergistic strengthening of dislocation-precipitate-interface (a), and effects of cold rolling on lath structure (b, c)


Abe[74]分析了9Cr-1W马氏体耐热钢正火+回火后不同变形量冷轧处理下的高温蠕变性能，当变形量为20%时，其高温蠕变强度相比回火态有所提升；但是，当变形量提升至40%时，由于位错胞周围M23C6相的钉扎作用相对较弱，导致蠕变过程中亚晶晶粒快速长大，其高温蠕变强度反而恶化。Zhang等[75]也研究了冷变形对G115钢高温蠕变行为及组织退化的影响，当变形量在5%~20%之间时，随着变形量的增大，板条尺寸逐渐细化，位错密度不断提高，同时形变位错明显提高了板条内部富Cu相的数密度并增强了2者之间的交互作用，如图6a和c[75]所示，有效提升了短期蠕变强度；但是当变形量继续增大至45%或60%时，马氏体板条组织发生破碎并生成位错胞，同时长期蠕变过程中高密度M23C6相和Laves相会发生快速粗化，导致亚结构剧烈回复，如图6b和d[75]所示。Yadav等[76]也在700℃、175 MPa下研究了冷变形对含Ti奥氏体耐热钢高温蠕变性能的影响，发现随着变形量的增大，其高温蠕变强度也呈现出先提高后降低的规律。除考虑变形量的影响外，Vijayanand等[77,78]在不同温度(650和700℃)下分析了冷变形后奥氏体耐热钢14Cr-15Ni 的高温蠕变行为，发现650℃下高数密度Ti(C, N) 相可以有效抑制蠕变过程中的位错回复行为，蠕变断裂寿命随着变形量的增加而延长；但是当温度升至700℃时，形变位错会加速再结晶晶粒的形核长大并促进部分沉淀相发生溶解，导致其蠕变强度恶化且与变形量关系不大。

图6

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图6   高温蠕变过程中初始态及20%冷轧态G115钢中富Cu相与位错之间的交互作用，及45%冷轧态G115钢中的位错胞结构及蠕变过程中沉淀相的快速粗化[75]

Fig.6   Interactions between Cu-rich particles and dislocations in the G115 steel for initial state (a) and 20% cold rolling (c); and dislocation cells for initial state (b) and coarse precipitates after creep rupture (d) in the G115 steel with 45% cold rolling[75] (NT—normalizing and tempering, CR—cold rolling)


综上可知，基于正火+回火+冷变形工艺的位错-蠕变沉淀-界面协同强化效应可以实现短期蠕变过程中高Cr马氏体耐热钢及其他沉淀型强化钢高温蠕变性能的有效提升。但是在长期蠕变过程中，形变位错诱导析出的高密度不稳定沉淀相(如M23C6相和Laves相)会发生快速粗化，明显降低组织稳定性，对高温蠕变性能产生不利影响。此外，其强化效果和变形量密切相关，同时受服役温度影响较大，在工程应用中仍然存在局限性。

3.2.2 位错-回火沉淀-界面协同强化效应

位错-回火沉淀-界面协同强化效应主要是在高Cr马氏体耐热钢正火后通过形变在基体中引入复杂位错结构，调控回火过程中的沉淀相析出及亚结构转变行为，构建具有弥散分布沉淀相、高密度位错、细化板条及低比例亚晶晶粒的高热稳定性组织。相比于位错-蠕变沉淀-界面协同强化，位错-回火沉淀-界面协同强化可以通过控制变形量、变形温度、回火温度及保温时间等参数实现多尺度微观组织的精准调控，可操作性高且不存在严重的组织畸变。根据形变温度的不同，可以分为正火+冷变形+回火和正火+热变形+回火2种工艺，如图7所示。

图7

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图7   能够实现位错-回火沉淀-界面协同强化效应的形变热处理工艺示意图

Fig.7   Schematics showing the deformation and heating treatments of synergistic strengthening of dislocation-tempered precipitate-interface

(a) cold rolling (b) hot rolling


正火+冷变形+回火工艺主要是在正火冷却后进行室温形变处理，其中不涉及对马氏体相变行为的影响，但可以通过优化变形量、回火时间及温度调控回火过程中沉淀相的析出分布、形态及位错结构和板条尺寸等组织特征。Lu等[79]研究了冷变形对27Cr-4Mo-2Ni铁素体不锈钢时效过程中沉淀相析出行为及力学性能的影响，形变位错促进了时效过程中χ相、σ相和Laves相的弥散析出，有效抑制了拉伸过程中的位错运动，使其抗拉强度获得了明显提升，但硬质相与基体之间的不协调变形导致其塑性较差。同时，Manojkumar等[80]则发现虽然冷变形可以提高304HCu钢时效过程中富Cu相和Nb(C, N) 相的析出数密度，但随着时效时间的延长，位错会发生剧烈回复且沉淀相不断粗化，导致位错硬化及固溶硬化作用减弱，其硬度不断下降。Zhang等[11,81]也通过优化变形量和回火时间，实现了G115钢组织热稳定性及高温蠕变性能的有效提升，当变形量为20%、回火温度为760℃且保温时间为60 min时，板条内部MX相和富Cu相的数密度得到了明显提高，如图8a~d[11]所示，同时保留了部分形变位错和细化板条，有效延缓了高温组织劣化进程，使其在650℃、160 MPa下的高温蠕变断裂时间延长为原来的2.58倍，如图8e[11]所示。由此可知，基于正火+冷变形+回火工艺的位错-回火沉淀-界面协同强化效应具备大幅提升高Cr马氏体耐热钢力学性能及高温蠕变性能的潜力，但是其强化效果与工艺参数(温度、时间、变形量等)密切相关，需要摸索出最优工艺方案，以实现最大限度的性能提升。

图8

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图8   初始态和20%冷轧态G115钢回火不同时间后沉淀相与位错之间的交互作用及650℃、160 MPa下的蠕变应变-时间曲线[11]

Fig.8   Interactions between precipitates and dislocations of initial state and 20% cold-rolled G115 steel after tempering for different time (a-d) and creep strain versus time curves obtained at 650oC under 160 MPa (e)[11]

(a, b) MX particles (c, d) Cu-rich particles


正火+热变形+回火工艺主要是通过形变热处理(thermal-mechanical treatment，TMT)，即高温正火 + 不同相区热变形(完全奥氏化区、奥氏体 + 马氏体双相区) + 回火，将塑性变形和热处理有机结合到一起，获得多种蠕变强化效应的协同提升[82~84]。其中，高温正火的目的是使前期铸造过程中形成的沉淀相尽可能溶解，以便在后续回火过程中能最大限度地二次析出。不同相区热变形主要是指控制热轧工艺(温度、应变量和应变速率等)和冷却工艺(冷却速率、冷却初始温度和终止温度等)，一方面可以通过影响再结晶行为和马氏体相变调控原始晶粒结构和马氏体板条组织，实现界面强化效应；另一方面可以在基体中引入大量形变位错并促进沉淀相弥散析出，实现位错强化和析出强化效应。回火主要是使沉淀相充分析出并保证基体良好的塑韧性。

Vivas等[85,86]指出，相比于传统正火+回火工艺，TMT可以大幅度提升9%Cr马氏体耐热钢板条内部MX相的数密度，如图9a和b[85]所示，明显抑制蠕变过程中的位错运动，使其高温蠕变强度获得了显著提高；同时，较低的形变温度更有利于保留形变产生的复杂位错，提升位错强化作用。Sakthivel等[87]发现，在9Cr-1Mo钢中，TMT工艺不仅可以提高板条内部MX相的数密度，还可以促进细小M23C6相在晶界处弥散析出，通过同时钉扎位错运动和抑制边界迁移来提高组织热稳定性。TMT工艺除了能够改善沉淀相析出行为外，Chen等[88]还发现通过控制变形温度可以有效细化马氏体耐热钢403Nb基体中的板条组织，如图9c和d[88]所示，实现沉淀析出相和板条界面的协同强化效应，使其高温蠕变强度提升了近3倍。此外，Sunil等[89]研究了TMT对AISI 304L钢相变行为及强塑性的影响规律，通过控制相变行为获得了奥氏体+马氏体双相组织，并促进了沉淀相的高密度弥散析出，使其抗拉强度由1.7 GPa提升至2.2 GPa，同时保持了较高的塑性。目前，基于TMT工艺的位错-回火沉淀-界面协同强化效应在提升沉淀型耐热钢及相变型高强钢服役性能方面已展现出明显的优势。但需要指出的是，为进一步推动其在火电机组高Cr马氏体耐热钢关键高温部件制造成形中的应用，急需开发成熟的热加工工艺方案。

图9

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图9   初始态和形变热处理(TMT)态9Cr马氏体耐热钢中的MX相分布[85]及初始态和TMT态403Nb马氏体耐热钢中的板条组织[88]

Fig.9   MX particles of initial and TMT-9Cr martensitic heat-resistant steels[85] (a, b), and lath structures of initial and TMT-403Nb martensitic heat-resistant steels[88] (c, d) (TMT—thermal-mechanical treatment)


4 高Cr马氏体耐热钢焊接接头制备及热影响区组织演变行为控制

4.1 焊接接头制备

在火电机组建设过程中，通常采用钨极气体保护氩弧焊(gas tungsten arc welding，GTAW)和手工电弧焊(shielded metal arc welding，SMAW)相结合的熔化焊技术对高Cr马氏体耐热钢焊接构件进行连接成形[90~92]。为了预防焊接冷/热裂纹的形成，通常要选择合适的预热和层间温度[53]。同时，根据经历峰值温度的不同，焊接接头热影响区可以分为粗晶热影响区(coarse-grain heat affected zone，CGHAZ)、细晶热影响区(fine-grain heat affected zone，FGHAZ)和临界热影响区(inter-critical heat affected zone，ICHAZ)，并且各区域之间具有显著的微观组织和力学性能差异[90,93]。虽然焊后热处理能够在一定程度上消除焊接接头的组织/性能异质性，但其仍是高温蠕变过程中的高危断裂区，严重影响焊接构件的服役寿命[94]。因此，未来可发展适用于高Cr马氏体耐热钢的新型熔化焊成形技术，比如激光焊接[95]、激光-电弧混合焊接[96]、活性钨极气体保护氩弧焊接[92]、真空电子束焊接[97]及冷金属过渡-脉冲混合焊接[98]等，通过降低热输入并增加熔深来减小焊接接头热影响区的宽度，进而实现其高温蠕变性能的有效提升。

此外，以真空扩散连接(diffusion bonding，DB)和搅拌摩擦焊(fricition stir welding，FSW)为代表的固相连接技术因具有较低的连接温度、不涉及焊缝金属的熔化和凝固、焊接接头成分均匀及尺寸装配精度高等优点，被推荐应用于高Cr马氏体耐热钢焊接接头的制备成形[99~103]。Noh等[104]制备了9Cr-ODS钢的固相扩散连接接头，连接界面结合紧密无缺陷，且界面附近纳米沉淀相均匀弥散分布，接头展现出与母材相当的拉伸性能。同时，通过添加易扩散、低熔点的过渡中间层(比如镍基[105]或铁基非晶箔[106])可实现高Cr马氏体耐热钢的瞬时液相扩散连接，进一步提升界面结合质量并降低连接温度和压力[100]。此外，Zhang等[107]制备了低活化铁素体/马氏体钢(RAFM钢)的FSW焊接接头，根据晶粒结构和析出相分布可以分为搅拌区(stir zone，SZ)、热机械影响区(thermal-mechanical affected zone，TMAZ)、热影响区(heat-affected zone，HAZ)和母材(base metal，BM) 4部分，并且接头展现出优异的高温拉伸强度和韧性。值得注意的是，在FSW过程中，SZ中的M23C6相会发生完全溶解，但MX相因熔点较高则无明显变化[103]。因此，为了使M23C6相二次再析出并充分发挥其析出强化效应，焊后热处理必不可少。

相比于传统熔化焊技术，真空扩散连接工艺可以有效避免高Cr马氏体耐热钢焊接接头热影响区的形成，但目前大多数研究主要集中在接头界面组织调控及界面强度提升方面，而对其高温蠕变行为尚不清楚。同时，扩散连接工件尺寸受限于真空室的尺寸，在火电机组大型构件制备成形中存在应用局限性。此外，虽然FSW焊接接头中并没有熔化凝固区形成，但仍然会存在一定宽度的热影响区，在高温服役过程中会不断发生组织劣化而导致其提前断裂。因此，上述2种固相连接工艺只能作为高Cr马氏体耐热钢熔化焊接成形的有益补充，但无法取而代之。

4.2 焊接接头高温蠕变失效机制

相比于固相连接，传统熔化焊技术因工艺成熟、操作简单及成本较低等优点仍是现阶段高Cr马氏体耐热钢焊接接头制备成形的主要手段。但是在高温蠕变过程中，焊接接头各区域之间的组织、性能及应力状态差异会导致其提前断裂。在众多失效类型中，由晶粒细化、晶粒异质及M23C6相不均匀分布和粗化导致的发生在FGHAZ或ICHAZ中的第IV类裂纹脆性断裂最为常见和致命[108~110]。

在晶粒细化和晶粒异质方面，在焊接热循环过程中FGHAZ经历的峰值温度高于Ac3，原始马氏体会发生完全α/γ扩散型逆相变，形成均匀细晶组织，如图10a[94]所示。在蠕变过程中，FGHAZ中细晶界面上的M23C6相会发生快速粗化，成为蠕变孔洞的主要形核位点，如图10b和c[53]所示。此外，ICHAZ经历的峰值温度在Ac1~Ac3之间，原始马氏体会发生部分奥氏体化，形成包含高硬度再结晶奥氏体晶粒和低硬度过回火马氏体晶粒的异质组织，如图10d和e[111]所示。在蠕变变形过程中，不同晶粒结构之间的硬度差异会导致不协调变形，进而诱导界面裂纹形成，如图10f[111]所示，导致其提前断裂。在M23C6相不均匀分布和粗化方面，焊接热循环过程中FGHAZ中焊前回火形成的M23C6相会发生部分溶解，由于峰值温度保温时间较短，C或者Cr元素来不及扩散而在原始奥氏体晶粒或板条束边界发生偏聚，导致焊后热处理过程中在奥氏体化晶粒和板条界面处只有少量M23C6相再析出，弱化了蠕变过程中其对此类界面迁移的钉扎作用，如图11[112]所示。同时，FGHAZ和ICHAZ中未溶解的M23C6相会在焊后热处理中发生二次粗化。

图10

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图10   高Cr马氏体耐热钢焊接热循环中细晶热影响区(FGHAZ)中形成的均匀细晶结构[94]和裂纹[53]、临界热影响区(ICHAZ)中具有硬度差异的晶粒异质及由此引发的蠕变界面裂纹[111]

Fig.10   Fine-grain structure[94] (a) and creep cracks[53] (b, c) in fine-grain heat affected zone (FGHAZ), heterogeneous grains (marked by letters A-G) with different hardnesses and creep cracks[111] (d-f) in inter-critical heat affected zone (ICHAZ) of high-Cr martensitic heat-resistant steels (Inset in Fig.10c shows the corresponding EDS map of element Cr)


图11

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图11   高Cr马氏体耐热钢焊接过程中FGHAZ中元素偏聚引发的M23C6相不均匀分布及蠕变裂纹[112]

Fig.11   Element accumulation (a), uneven distribution of M23C6 (b), and creep cracks (c) in FGHAZ in high-Cr martensitic heat-resistant steels[112]


针对上述问题，Pandey 等[113]提出，可以通过焊后正火+回火代替传统的焊后直接回火工艺，不仅可促进FGHAZ中的细化晶粒长大，同时还可以缓解ICHAZ中晶粒间的硬度差异。但是在实际工业应用中，对焊后大口径厚壁管道进行高温正火处理较为困难，并且局部正火会导致焊缝边界出现二次软化区，对蠕变强度产生不利影响。此外，Khajuria等[114]和Matsunaga等[115]报道了基于成分优化的解决方案，主要是通过提高B元素含量并促进其界面择优分布，有效降低界面能，进而抑制细晶晶粒的形核。需要注意的是，在提高B元素含量的同时要严格控制N元素含量，以防止脆性BN粗大颗粒的形成。此外，Yu等[116]提出了焊前半回火工艺，通过降低回火温度来细化M23C6相，使其在焊接热循环过程中更容易发生完全固溶，以防止焊后热处理过程中未溶粒子的过分粗化。但是，此时焊接接头会表现出较低的韧性及较高的韧-脆转变温度。在诸多尝试中，虽然高Cr马氏体耐热钢焊接接头的高温蠕变性能得到了提升，但均未从根本上改善热影响区的组织异质性，迫切需要探索新的组织调控思路。

4.3 焊接热影响区组织演变行为控制

在焊接过程中，母材的初始组织状态会直接影响热影响区的组织演变，并决定焊接接头的高温蠕变性能。基于上述形变热处理工艺在实现高Cr马氏体耐热钢位错-沉淀相-界面协同强化效应方面表现出的明显优势，并结合焊接接头第IV类裂纹失效机制的分析，可将形变热处理技术拓展应用于焊接接头热影响区组织调控方面，以提升其高温蠕变性能。主要是在焊接前对高Cr马氏体耐热钢进行TMT或变形回火处理，通过改变母材微观组织特征以调控后续焊接热循环过程中的奥氏体逆相变、元素分布及沉淀相溶解和再析出行为，从根本上缓解热影响区的组织异质性，最终实现母材及焊接接头高温蠕变强度的共同提升，如图12所示。

图12

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图12   高Cr马氏体耐热钢焊接热影响区形变热处理组织调控工艺示意图

Fig.12   Schematics showing the deformation and heating treatment for microstructure control of high-Cr martensitic heat-resistant steel weld joints

(a) deformation and heating treatments before welding

(b) heating history during welding and post weld heating treatment


Sakthivel等[117]对比分析了预TMT处理和原始Grade 91钢焊接接头的高温蠕变行为，虽然2者断裂均发生于ICHAZ中，但是相比于原始接头，TMT接头展现出更为优异的高温蠕变强度，这是因为TMT有效促进了母材中MX相的弥散析出，并且在焊接完成后能继续保留在热影响区，很大程度上提升了焊接接头的析出强化效应；同时，高密度MX相与M23C6相之间存在竞争形核长大关系，有效抑制了焊后热处理过程中M23C6相的粗化。Shassere等[118]则发现不同温度的TMT形变处理会对Grade 91钢焊接过程中FGHAZ的组织演变产生不同的影响，随着形变温度的升高，FGHAZ中再析出的M23C6相分布更为均匀，但基体组织中会生成部分高温铁素体，对其高温蠕变强度产生不利影响。此外，Zhang等[119]研究了变形回火对G115钢焊接接头FGHAZ组织演变、力学性能及高温蠕变性能的影响，变形引入的形变位错能够加速焊接过程中M23C6相的固溶，并缓解C和Cr元素的界面偏聚，如图13a、b、d和e[119]所示，以促进后续热处理过程中M23C6相在再奥氏体晶粒及板条界面处的均匀弥散再析出，同时还能够提高板条内部MX相和富Cu相的数密度，如图13c和f[119]所示，使其高温蠕变强度得到了明显提升。

图13

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图13   初始态和形变热处理后G115钢Gleeble热模拟FGHAZ中的元素分布及第二相再析出行为[119]

Fig.13   Distributions of elements (a, b, d, e) and precipitation of strengthening particles (c, f) in the Gleeble simulated FGHAZ of initial and deformation-heated G115 steel[119]


目前，形变热处理工艺具备改善高Cr马氏体耐热钢焊接接头热影响区组织异质性的潜力，是未来提升其热稳定性及高温蠕变性能的重要发展方向。但是，目前大多数研究主要集中在调控焊接过程中沉淀相溶解、再析出及长大行为方面，而对相变行为的影响及晶粒结构演变的作用机制尚不清楚。未来研究应重点澄清形变处理后的母材遗传组织特征(包括B元素的界面择优分布、高密度位错及细化界面)对焊接过程中奥氏体逆相变行为的影响机制，以进一步解决FGHAZ和ICHAZ中的晶粒细化问题。此外，结合微观组织及力学性能演变进行工艺参数优化，并开发出适用于高Cr马氏体耐热钢焊接构件的集性能提升与构件成形于一体的热加工工艺，对推动形变热处理组织调控技术在火电机组关键部件成形中的应用至关重要。

5 总结与展望

(1) 高Cr马氏体耐热钢的高温抗蠕变性能源于固溶强化、析出强化、沉淀强化及界面强化之间的耦合效应。虽然基于成分优化调控沉淀相析出行为及稳定性可提高组织热强性，但单一析出强化效应对高温蠕变强度的提升效果有限。通过形变热处理可同时改善沉淀相析出分布、位错结构及马氏体板条组织，增强3者之间的交互作用，有望实现高温持久强度的大幅提升。此外，未来研究应着重探索高Cr马氏体耐热钢高温蠕变过程中沉淀相演变行为之间的相互影响机制，进一步揭示位错、界面对沉淀相粗化、溶解行为的作用机理，重点关注相变组织、沉淀相形态与高温服役性能间的关联性，开发基于多类蠕变强化效应协同提升的组织调控新方法。

(2) 基于预冷变形的位错-蠕变沉淀-界面协同强化效应可有效提升高Cr马氏体耐热钢的短时高温蠕变强度，但在长期服役过程中受高密度沉淀相快速粗化的影响，亚结构剧烈回复，从而对其高温蠕变性能产生不利影响。上述方法存在工程应用局限性，未来研究应集中在微变形量(< 10%)下高Cr马氏体耐热钢的高温蠕变行为及强化机制，关注微变形量下的位错引入及板条细化机制，从沉淀相与基体间的界面关系入手，优化设计变形量以适当降低组织畸变程度，从根本上抑制高温、长时服役过程中的位错回复、板条宽化及沉淀相粗化行为，实现高热强性微观组织的设计与构建。

(3) 采用变形回火的位错-回火沉淀-界面协同强化效应可稳定提升高Cr马氏体耐热钢的持久强度。通过控扎控冷改善马氏体板条组织，同时引入形变位错来促进沉淀相的均匀弥散析出，获得具有复杂位错、高密度细小沉淀相及细化板条的理想组织，可实现多种强化效应的协同提升。未来研究应结合高Cr马氏体耐热钢热变形行为及大口径厚壁管道等关键部件工业制造流程，建立基于相变强化和形变强化效应协同提升的蠕变强化机制，开发集性能强化和构件成形于一体的热加工工艺。

(4) 相比于传统熔化焊技术，真空扩散连接和搅拌摩擦焊工艺可实现高Cr马氏体耐热钢的固相高强连接，有效缓解了熔焊接头服役时第IV类蠕变裂纹的形成。需要指出的是，真空扩散连接构件尺寸受真空室尺寸限制，而搅拌摩擦焊对搅拌头材质要求较高且焊接过程中损耗较大，成本较高，上述2种固相连接技术目前只能作为熔化焊技术的有益补充。未来研究应聚焦于开发高能量、低热输入的新型熔化焊接技术，以尽可能减小焊接热影响区的宽度。此外，应着重解决固相连接技术的尺寸和成本控制问题，关注加工过程的高热强性组织设计与调控，拓展其在大型复杂构件成形中的广泛应用。

(5) 通过形变热处理定向调控焊前基体组织，可实现高Cr马氏体耐热钢焊接接头FGHAZ和ICHAZ中相变及沉淀析出行为的精准控制：通过诱导B元素界面择优分布来抑制奥氏体逆相变并减缓晶粒细化，通过形变位错来促进沉淀相二次均匀再析出，实现位错、沉淀相及界面之间的协同强化。未来应继续深入研究形变热处理母材遗传组织特征对其焊接热影响区组织演变及高温蠕变性能的影响，开发能够实现B元素界面均匀分布的有效调控手段，在控制工艺的前提下揭示B元素对奥氏体逆相变行为的作用机制，进一步阐明高密度MX相、界面和位错的形变引入机制及其对M23C6相演化行为的影响机理，构建具有非相变晶粒结构及弥散分布沉淀相的高热稳定性均质热影响区。

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On Laves phase in a 9Cr3W3CoB martensitic heat resistant steel when aged at high temperatures

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9Cr3W3CoB steels are developed to serve at the temperature range of 620-650 °C, and have been recognized as the most promising martensitic heat-resistant steels for supercritical power plants. Due to the high W and Co contents, the Fe2W Laves phase in such 9Cr3 W3CoB steel possesses some specialties in thermodynamics. In the present research, it was found that even when aged at 800 °C in the 9Cr3W3CoB steel, instead of dissolving, Laves phase formed after 50 h and kept on increasing in size and number density until 1000 h, indicating that the Laves phase was marching for the thermodynamic equilibrium during aging. In this thermodynamic process, the W-rich M3B2 borides in as-received steel and M23C6 carbides were revealed to dissolve, supporting the growth of Laves phase. SEM investigation indicates that Laves phase tended to form clusters, and finally grow as a unit bulk Laves phase with an irregular shape. Besides, Laves phase nucleated next to M23C6 carbides and enwrapped them inside at 800 °C. In addition, the growth processes of Laves phase and M23C6 carbides were a competitive procedure, the coarsening of M23C6 carbides was prior to the growth of Laves phase at 750 °C while the growth of Laves phase was prior to the coarsening of M23C6 carbides at 800 °C.

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Creep deformation and rupture behaviour of a reduced activation ferritic-martensitic (RAFM) steel subjected to thermo-mechanical treatment (TMT) is studied and compared with those of conventional normalized and tempered (N + T) steel. In TMT processing, the steel is warm rolled and aged in austenite phase field at 973 K before the martensite transformation on cooling and is then tempered at 1038 K. The TMT processing renders the steel with higher dislocation density, refinement in lath structure and large quantity of finer M23C6 and MX precipitates than those in the N + T steel. Creep tests are carried out at 823 K over the stress range 180-300 MPa. TMT processing of the steel decreases its minimum creep rate ((epsilon) over dot(min)) with corresponding increase in time to onset of tertiary stage of creep deformation, rupture life (t(r)) and creep rupture ductility (epsilon(f)). The stress exponent value (n), obtained from minimum creep rate vs. stress plot, increases upon TMT processing, indicating high resistance to creep deformation than in the N + T steel. Resisting stress as estimated based on the Lagneborg and Bergman method is found to increase on TMT processing and is associated with high damage tolerance parameter, defined as lambda = epsilon(f)/((epsilon) over dot(min).t(r)). Enhanced creep deformation and rupture strength of the TMT steel, compared to N + T steel, is attributed to the microstructural refinement. Post-creep microstructural investigations show higher microstructural stability of the steel on TMT processing and are in line with the observed high damage tolerance parameter (lambda), longer time to onset of tertiary creep and rupture life. (C) 2019 Elsevier B.V.

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The tungsten inert gas welded P91 steel welded joints were subjected to the two different type of heat treatments including the postweld direct tempering (PWDT) and re-austenitizing based tempering (PWNT) treatment. The microstructure of weld fusion and heat affected zone (HAZ) were characterized in different heat treatment conditions using optical microscope and scanning electron microscope. For as-welded joint, a great heterogeneity was observed in microstructure and mechanical properties across the weldments. The Charpy toughness of the as-welded joint was measured much lower than the minimum recommended value of 47 J and it was measured 8 +/- 5 J. The PWHTs have found a beneficial effect in decreasing the microstructure heterogeneity across the welded joint and improving the mechanical properties. The PWDT resulted in a drastic improvement in the Charpy impact toughness of the welded joint and it was measured 59 +/- 5 J which was higher than the minimum required value of 47 J but still inferior than the base metal. The delta ferrite still remained in overlap zone of the weld fusion zone. The PWNT treatment resulted in homogeneous microstructure and hardness variation across the welded joint in transverse direction and Charpy impact toughness (149 +/- 6 J) exceeded than that achieved in base metal. (C) 2018 Politechnika Wroclawska. Published by Elsevier B.V.

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Improving the steam temperature and the pressure of the boiler applied in the thermal power could enhance the coal-fired efficiency and reduce the emission of harmful gases. Due to the dual impact of dwindling fossil resources and an exacerbated global greenhouse effect, it is critical to develop new heat-resistant boiler materials for ultra super-critical (USC) units at temperatures of 650oC and higher. With great thermal conductivity, good fatigue resistance, and low cost, martensitic heat-resistant steel G115, based on P92 steel applied in 600oC USC units, is a promising steel to be applied to this among all candidate materials. This paper introduces the main chemical composition and the microstructure feature of G115 steel, and the research progress in the areas of microstructure stability, creep performance, fatigue resistance, steam oxidation resistance, and industrial pipe production are summarized, with a focus on the role of Cu-rich phase in G115 steel. Finally, some key points on G115 steel are proposed to provide ideas for future research.

何焕生, 余黎明, 刘晨曦 等.

新一代马氏体耐热钢G115的研究进展

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DOI      [本文引用: 1]

提高火电机组中耐热锅炉的蒸汽温度和压力参数可以有效提升燃煤效率，减少有害气体排放。受煤炭资源紧缺和温室效应的双重影响，发展650℃及更高温度超超临界(ultra super-critical，USC)机组中的耐热锅炉材料已迫在眉睫。我国在600℃ USC机组用耐热材料P92钢基础上研发的马氏体耐热钢G115有望成为优选材料之一。本文介绍了G115钢的成分特点、形貌特征，综述了其在组织稳定性、蠕变性能、抗疲劳性能、抗蒸汽氧化性能以及工业管材制备等方面的研究进展，重点归纳了G115钢中富Cu相的作用，展望了未来研究重点，以期为更深入研究G115钢提供可行思路。

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Progress and perspective in martensitic transformation are described.The definition of martensite reaction, the thermodynamics of martensitic trans-formation, the effect of austenite condition on martensitic transformation, kinetics,nucleation and growth, as well as the crystallography of martensitic transformationare presented.

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叙述马氏体相变研究的一些进展和瞻望,包括马氏体相变的定义,马氏体相变热力学,奥氏体状态对马氏体相变的影响,动力学,形核和长大,以及马氏体相变晶体学。

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采用热膨胀法测量了马氏体试验钢在20~70℃/s冷却速度下的马氏体相变起始温度(Ms点温度),并研究了其对试验钢硬度的影响。结果表明,冷却速度的提高可增加奥氏体强度、提高奥氏体的热稳定性,进而降低马氏体相变起始温度。同时,更高的冷却速度容易造成晶格畸变,增大位错密度,还会提高过冷度,细化有效晶粒尺寸,使试验钢的硬度提高。

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Effect of prior austenite grain size in medium manganese steel on kinetics of martensitic transformation was studied. The kinetics of martensitic transformation and microstructure evolution were analyzed by combining with SEM,XRD,thermal dilatometer and EBSD results.Different prior austenite grain sizes with(190 &#177;15),(36 &#177;2),(11 &#177;2) and(4.8 &#177;2) μm were achieved,respectively,by heating at different austenitizing temperatures. The results show that with the decreasing of prior austenite grain size,the martensite start temperature decrease from 289℃ to 250℃,while the kinetic of martensitic transformation increase at first then decrease. The kinetics of martensitic transformation is closely related to the number of martensite nucleation cites per unit volume,while the number of martensite nucleation cites are closed to the prior austenite grain size and the martensite lath aspect ratio. When the size of prior austenite grain decreases to 5 μm,the aspect ratio of martensite lath increases,while the increase rate of martensite nucleation cites with undercooling significantly reduces,which results in the decrease of the rate of martensite transformation.

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对中锰钢中原奥氏体晶粒尺寸对马氏体相变动力学的影响进行了分析。利用SEM、XRD、热膨胀相变仪和EBSD等检测手段，研究了不同原奥氏体晶粒尺寸下马氏体相变速率和马氏体板条组织的变化。通过不同温度的奥氏体化处理，分别得到了尺寸为（190&#177;15）、（36&#177;2）、（11&#177;2）和（4.8&#177;2）μm的原奥氏体晶粒。结果表明：随着原奥氏体晶粒尺寸的降低，马氏体相变开始温度由289℃降低到250℃，而马氏体的相变速率先升高后降低。分析表明，马氏体的相变速率与马氏体的形核点数量直接相关，而马氏体的形核点数量受原奥氏体晶粒尺寸大小和马氏体板条的宽长比影响。当原奥氏体晶粒尺寸小于5μm时，马氏体板条的宽长比明显增加，马氏体形核点数目随过冷度增加而增加的速率明显降低，从而造成马氏体相变速率降低。

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[本文引用: 1]

Vivas J, Capdevila C, Jimenez J A, et al.

Effect of ausforming temperature on the microstructure of G91 steel

[J]. Metals, 2017, 7: 236

Prakash, Vanaja J, Laha K, et al.

Influence of thermo-mechanical treatment in ferritic phase field on microstructure and mechanical properties of reduced activation ferritic-martensitic steel

[J]. IOP Conf. Ser.: Mater. Sci. Eng., 2018, 338: 012027

[本文引用: 1]

Vivas J, Capdevila C, Altstadt E, et al.

Importance of austenitization temperature and ausforming on creep strength in 9Cr ferritic/martensitic steel

[J]. Scr. Mater., 2018, 153: 14

[本文引用: 4]

Vivas J, Capdevila C, Altstadt E, et al.

Effect of ausforming temperature on creep strength of G91 investigated by means of small punch creep tests

[J]. Mater. Sci. Eng., 2018, A728: 259

[本文引用: 1]

Sakthivel T, Shruti P, Parameswaran P, et al.

Enhancement in creep strength of modified 9Cr-1Mo steel through thermo-mechanical treatment

[J]. Trans. Indian Inst. Met., 2017, 70: 1177

[本文引用: 1]

Chen L Q, Zeng Z Y, Zhao Y, et al.

Microstructures and high-temperature mechanical properties of a martensitic heat-resistant stainless steel 403Nb processed by thermo-mechanical treatment

[J]. Metall. Mater. Trans., 2014, 45A: 1498

[本文引用: 4]

Sunil S, Kapoor R, Sarkar S K, et al.

Ultra-high strength steel made from AISI 304L using a novel thermo-mechanical processing technique

[J]. Acta Mater., 2021, 221: 117379

[本文引用: 1]

Kumar S, Kumar S, Pandey C, et al.

Effect of post-weld heat treatment and dissimilar filler metal composition on the microstructural developments, and mechanical properties of gas tungsten arc welded joint of P91 steel

[J]. Int. J. Press. Vessels Pip., 2021, 191: 104373

[本文引用: 2]

Maduraimuthu V, Vasantharaja P, Vasudevan M, et al.

Microstructure and mechanical properties of 9Cr-0.5Mo-1.8W-VNb (P92) steel weld joints processed by fusion welding

[J]. Mater. Sci. Eng., 2021, A813: 141186

Pandey C, Mahapatra M M, Kumar P, et al.

Comparative study of autogenous tungsten inert gas welding and tungsten arc welding with filler wire for dissimilar P91 and P92 steel weld joint

[J]. Mater. Sci. Eng., 2018, A712: 720

[本文引用: 2]

Pandey C, Mahapatra M M, Kumar P, et al.

Effect of post weld heat treatments on microstructure evolution and type IV cracking behavior of the P91 steel welds joint

[J]. J. Mater. Process. Technol., 2019, 266: 140

[本文引用: 1]

Zhang J W, Yu L M, Gao Q Z, et al.

Development of weld filler material to match the advanced martensitic heat resistance steel G115 and tailoring the performance by tempering temperature

[J]. J. Mater. Res. Technol., 2022, 21: 2515

[本文引用: 4]

Fu J, van Slingerland J, Brouwer H, et al.

Applicability study of pulsed laser beam welding on ferritic-martensitic ODS Eurofer steel

[J]. Metals, 2020, 10: 736

[本文引用: 1]

Hao K D, Zhang C, Zeng X Y, et al.

Effect of heat input on weld microstructure and toughness of laser-arc hybrid welding of martensitic stainless steel

[J]. J. Mater. Process. Technol., 2017, 245: 7

[本文引用: 1]

Zhang Y Y, Gou G Q.

Microstructure and properties of Co3W2 heat-resistant steel by vacuum electron beam welding

[J]. Int. J. Mod. Phys., 2020, 34B: 2040058

[本文引用: 1]

Xu L Y, Pang H N, Zhao L, et al.

Microstructure and mechanical properties of CMT + Pwelding process on G115 steel

[J]. Trans. China Weld. Inst., 2020, 41(8): 1

[本文引用: 1]

徐连勇, 庞红宁, 赵 雷 等.

G115钢CMT + P焊接工艺及组织和性能

[J]. 焊接学报, 2020, 41(8): 1

DOI      [本文引用: 1]

基于冷金属过渡加脉冲(CMT + P)的焊接方法，研究了新型回火马氏体耐热钢G115的焊接性以及焊接接头组织和性能. 结果表明，焊接接头经热处理后为回火马氏体组织，焊缝晶粒呈现出等轴晶和柱状晶两种不同的形貌，而焊接热影响区和母材晶粒均为等轴晶. 与焊条电弧焊(SMAW)相比，CMT + P焊接方法有效降低了热输入，大幅度减小了热影响区宽度，提高了焊接接头的拉伸性能和热影响区冲击韧性，焊接接头焊缝冲击韧性略有降低. 焊接接头的室温和高温拉伸断裂机理均为韧性断裂，室温拉伸断口的韧窝内存在一定量的析出相.

Chen M L, Jiang B, Ding R, et al.

Microstructure evolution and tensile behaviors of dissimilar TLP joint of austenitic steel and high-Cr ferritic steel

[J]. Mater. Sci. Eng., 2023, A870: 144818

[本文引用: 1]

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Diffusion bonding of 9Cr martensitic/ferritic heat-resistant steels with an electrodeposited Ni interlayer

[J]. Metals, 2018, 8: 1012

[本文引用: 1]

Hua Y, Chen J G, Yu L M, et al.

Microstructure evolution and mechanical properties of dissimilar material diffusion-bonded joint for high Cr ferrite heat-resistant steel and austenitic heat-resistant steel

[J]. Acta Metall. Sin., 2022, 58: 141

DOI     

High Cr ferrite heat-resistant steel has excellent geometric structure stability, low radiation swelling rate, and good corrosion resistance of liquid metal. TP347H austenitic heat-resistant steel is based on the traditional 18-8 austenitic steel with the addition of a certain amount of Nb and a small amount of N to precipitate MX-type carbonitride, which results in superior high-temperature properties. Steam with high temperature and pressure flowing through supercritical thermal power units may exhibit heterogeneous connections between high Cr ferrite and austenitic heat-resistant steel components in the supercritical thermal power units. In this study, the vacuum diffusion-bonding of dissimilar materials between high Cr ferritic and TP347H austenitic heat-resistant steel was performed, the effects of diffusion-bonding time and post weld heat treatment (PWHT) process on the microstructural evolution and mechanical properties of the diffusion-affected zone was examined. The results indicated that with the extension of diffusion-bonding time, the interfacial bonding rate gradually increased. The interaction due to the difference in deformation storage energy and dislocation slips resulted in dynamic recrystallization, and the fine grains formed at the diffusion-bonding interface evolved into a serrated interface. Fine and dispersed MX and M23C6 phases were precipitated in the austenite grain boundaries and at the grain boundaries of the diffusion-bonding zone. After PWHT, the grains in the diffusion-bonding zone were further refined, dislocations were stable, dislocation density reduced, small-angle grain boundaries increased, and element diffusion was more sufficient. Tensile tests at different temperatures showed that the fractured sites were all in the matrix, which indicates that high-quality diffusion-bonding joints of dissimilar materials were achieved.

化 雨, 陈建国, 余黎明 等.

高Cr铁素体耐热钢与奥氏体耐热钢的异种材料扩散连接接头组织演变及力学性能

[J]. 金属学报, 2022, 58: 141

Han W T, Chen D S, Ha Y, et al.

Modifications of grain-boundary structure by friction stir welding in the joint of nano-structured oxide dispersion strengthened ferritic steel and reduced activation martensitic steel

[J]. Scr. Mater., 2015, 105: 2

Hua P, Moronov S, Nie C Z, et al.

Microstructure and properties in friction stir weld of 12Cr steel

[J]. Sci. Technol. Weld. Join., 2014, 19: 76

[本文引用: 2]

Noh S, Kasada R, Kimura A.

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[J]. Acta Mater., 2011, 59: 3196

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[J]. Mater. Des., 2015, 88: 1321

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Homogenization stage during TLP bonding of RAFM steel with a Fe-Si-B interlayer: Microstructure evolution and mechanical properties

[J]. Mater. Sci. Eng., 2020, A780: 139205

[本文引用: 1]

Zhang C, Cui L, Liu Y C, et al.

Microstructures and mechanical properties of friction stir welds on 9%Cr reduced activation ferritic/martensitic steel

[J]. J. Mater. Sci. Technol., 2018, 34: 756

DOI      [本文引用: 1]

In this study, the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic (RAFM) steel friction stir welded joints were investigated. When a W-Re tool is used, the recommended welding parameters are 300 rpm rotational speed, 60 mm/min welding speed and 10 kn axial force. In stir zone (SZ), austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths. The microstructure in SZ contains lath martensite with high dislocation density, a lot of nano-sized MX and M3C phase particles, but almost no M23C6 precipitates. In thermal mechanically affect zone (TMAZ) and heat affect zone (HAZ), refinement of prior austenite and martensitic laths and partial dissolution of M23C6 precipitates are obtained at relatively low rotational speed. However, with the increase of heat input, coarsening of martensitic laths, prior austenite grains, and complete dissolution of M23C6 precipitates are achieved. Impact toughness of SZ at -20 °C is slightly lower than that of base material (BM), and exhibits a decreasing trend with the increase of rotational speed.

Parker J.

Factors affecting type IV creep damage in grade 91 steel welds

[J]. Mater. Sci. Eng., 2013, A578: 430

[本文引用: 1]

Xue W, Pan Q G, Ren Y Y, et al.

Microstructure and type IV cracking behavior of HAZ in P92 steel weldment

[J]. Mater. Sci. Eng., 2012, A552: 493

Zhang Q B, Zhang J X, Zhao P F, et al.

Microstructure of 10%Cr martensitic heat-resistant steel welded joints and type IV cracking behavior during creep rupture at 650oC

[J]. Mater. Sci. Eng., 2015, A638: 30

[本文引用: 1]

Wang Y Y, Kannan R, Li L J.

Correlation between intercritical heat-affected zone and type IV creep damage zone in grade 91 steel

[J]. Metall. Mater. Trans., 2018, 49A: 1264

[本文引用: 4]

Liu Y, Tsukamoto S, Shirane T, et al.

Formation mechanism of type IV failure in high Cr ferritic heat-resistant steel-welded joint

[J]. Metall. Mater. Trans., 2013, 44A: 4626

[本文引用: 3]

Pandey C, Mahapatra M M, Kumar P.

Effect of post weld heat treatments on fracture frontier and type IV cracking nature of the crept P91 welded sample

[J]. Mater. Sci. Eng., 2018, A731: 249

[本文引用: 1]

Khajuria A, Akhtar M, Bedi R, et al.

Microstructural investigations on simulated intercritical heat-affected zone of boron modified P91-steel

[J]. Mater. Sci. Technol., 2020, 36: 1407

[本文引用: 1]

Matsunaga T, Hongo H, Tabuchi M, et al.

Suppression of grain refinement in heat-affected zone of 9Cr-3W-3Co-VNb steels

[J]. Mater. Sci. Eng., 2016, A655: 168

[本文引用: 1]

Yu X, Babu S S, Terasaki H, et al.

Correlation of precipitate stability to increased creep resistance of Cr-Mo steel welds

[J]. Acta Mater., 2013, 61: 2194

[本文引用: 1]

Sakthivel T, Nandeswarudu S M, Shruti P, et al.

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

[J]. Mater. High Temp., 2019, 36: 76

DOI      [本文引用: 1]

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650 degrees C) and 110-100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M23C6 precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M23C6 precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint.The enhanced MX precipitation through TMT processing and reduction in coarsening of M23C6 precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.

Shassere B A, Yamamoto Y, Babu S S.

Toward improving the type IV cracking resistance in Cr-Mo steel weld through thermo-mechanical processing

[J]. Metall. Mater. Trans., 2016, 47A: 2188

[本文引用: 1]

Zhang J W, Yu L M, Liu Y C, et al.

Improving creep strength of the fine grain heat-affected zone of a novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

[J]. Int. J. Miner. Metall. Mater., 2023, doi: 10.1007/s12613-023-2760-0

[本文引用: 5]

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