Acta Metallurgica Sinica, 2017, 53(5): 531-538
doi: 10.11900/0412.1961.2016.00321
热加工对Z3CN20-09M双相不锈钢组织及热老化冲击断裂行为的影响

Effects of Hot Working on the Microstructure and Thermal Ageing Impact Fracture Behaviors of Z3CN20-09MDuplex Stainless Steel
张海, 李时磊, 刘刚, 王艳丽

摘要:

铸态Z3CN20-09M双相不锈钢经1200 ℃热锻后,采用400 ℃热老化100、1000、3000 h处理。利用SEM和EBSD分析了铸态及锻态Z3CN20-09M双相不锈钢的显微组织和热老化1000、3000 h的冲击断口,采用纳米力学探针和冲击试验机测试了铸态及锻态Z3CN20-09M双相不锈钢热老化0、100、1000、3000 h微区力学性能和冲击性能。研究表明,经热加工后铁素体晶粒的取向呈现无序态,奥氏体晶粒由粗大的柱状晶经再结晶后变成细小的等轴晶。随热老化时间延长,铸态和锻态材料的冲击功都呈现下降趋势。热老化1000 h,铸态和锻态材料均呈现微孔聚集型断裂,断口出现大量韧窝花样。热老化3000 h,铸态和锻态材料均呈现韧窝/解理混合型断裂特征,铁素体发生脆性解理断裂, 奥氏体以撕裂或呈微孔聚集型断裂。铁素体区域内取向不同导致锻态材料冲击断口解理特征明显少于铸态材料。

关键词: 双相不锈钢 ; 热加工 ; 热老化 ; 断裂 ; 晶粒取向

Abstract:

Duplex stainless steels are widely used in nuclear industry for their excellent mechanical behavior, good weldability and superior ressistance to corrosion, the fracture toughness of which will be deteriorated with ageing time, as they are exposed to a certain temperature (204~538 ℃). In the present work, hot forging will be employed to induce the change of ferrite grain orientation and refinement of austenite grains; it is expected to improve the impact toughness after long-term thermal ageing. The microstructure and impact surface morphology of Z3CN20-09M duplex stainless steel were investigated by using SEM and EBSD. The micro-mechanical properties and impact properties of Z3CN20-09M duplex stainless steel at different thermal ageing time were tested by a nano-indenter and an instrumented impact tester. The results show that the crystal orientation of ferrite changes obviously and the austenite is changed from the original coarse columnar grains to the fine equiaxed grains after hot working. The im pact toughness of cast materials and forged materials decreases greatly with ageing time. The charpy impact energy of both aged and unaged forged-materials is higher than that of cast material. Cast material and forged material exhibit microvoid coalescence fracture in the early of thermal ageing; after 3000 h thermal ageing, the impact fracture features changes from ductile dimples to brittle cleavages in ferrites and tearings or dimples in austenites. However, cleavage features in forged material are significantly less than those in cast material due to the difference in ferrite crystal orientation.

Key words: duplex stainless steel ; hot working ; thermal ageing ; fracture ; grain orientation

双相不锈钢具有优良的综合力学性能(足够的强度、高的塑性和韧性、抗疲劳性能)、焊接性能和耐腐蚀性能,被广泛应用于核工业[1~3],如核电站的阀体、主冷却剂管道和核电海水循环泵的叶轮[4]。双相不锈钢部件在一定温度下(204~538 ℃)长期服役[5],断裂韧性会随着时间的延长而下降,这种现象称为热老化,热老化会严重威胁部件的结构完整性,进而给设备安全运行带来潜在风险,因此关于双相不锈钢热老化的研究是非常有意义的。

对长期热老化处理后的双相不锈钢研究发现:随热老化时间延长,材料的Charpy冲击功下降[6,7],屈服强度变化不大,抗拉强度小幅度增加,塑性下降[7,8],铁素体相硬度增加,而奥氏体硬度变化不大[9,10]。据文献[6~8,9~12]报道,出现上述现象的主要原因是热老化后铁素体内部发生了调幅分解生成了富Fe的α相和富Cr的α′相,并析出了G相,而奥氏体相几乎没有变化。目前国内外对双相不锈钢的热老化机制的研究较成熟,后期研究工作主要针对如何消除或者缓解此类热老化。

前期研究[13]发现,Z3CN20-09M双相不锈钢组织凝固模式为铁素体奥氏体型,即铁素体先从液相中析出,随后从液相析出的奥氏体将铁素体包在其中。铁素体枝晶和奥氏体柱状晶晶粒粗大,奥氏体晶粒内的铁素体晶粒取向相同,导致材料经长期热老化后,Charpy冲击功迅速下降,冲击断口形貌中铁素体解理特征的比例远高于铁素体在材料中的体积分数[14]。基于前期的研究工作,本工作引入热锻工艺,通过热锻既可以将取向相同的铁素体晶粒变为无序晶粒取向,也可以把奥氏体粗大的柱状晶变为细小的等轴晶,使得两相组织得到优化,从而提高热老化双相不锈钢原始和热老化后的冲击性能,并结合断口形貌观察以及电子背散射衍射(EBSD)结果,分析铁素体晶粒取向对双相不锈钢冲击断裂行为的影响。

1 实验方法

实验材料为 Z3CN20-09M 双相不锈钢,其化学成分(质量分数,%)为:C 0.021,S 0.002,Si 1.04,Mn 0.86,P 0.021,Cr 20.4,Ni 9.72,Mo 0.13,Cu 0.18,Co 0.09,N 0.04,Fe余量。原始工艺为静态铸造。热加工工艺为自由锻,每个应变方向变形量约为20%;变形后采用水淬冷却方式;随后进行固溶处理,固溶处理的温度为1050 ℃,保温时间为1 h,冷却方式为水冷;在400 ℃下对材料进行热老化处理,热老化时间分别为100、1000 和3000 h。

分别从原始铸态、热锻未固溶态、固溶态样品切取10 mm×10 mm×3 mm方块,研磨抛光后采用10%(体积分数) HClO4 酒精溶液进行电解抛光,电解抛光电压为30 V,电流保持在1.0 A左右,电解时间为15 s,随后使用SUPRA 55型扫描电镜(SEM)进行EBSD测试,采用Image-Pro Plus 6.0软件统计铸态材料和热锻+固溶1 h材料中铁素体的含量。分别从热老化0、100、1000和3000 h样品切取10 mm×10 mm×3 mm方块,经研磨抛光后经5 g FeCl3+100 mL HCl+100 mL酒精+100 mL H2O溶液侵蚀,采用MTS Nano Indenter XP纳米力学探针对试样中铁素体进行5次纳米压入实验,取平均值,压入深度为500 nm。采用MTS ZBS-450型冲击试验机进行Charpy冲击功测试,冲击试样尺寸为10 mm×10 mm×55 mm,利用SUPRA 55型SEM观察冲击断口形貌。

2 实验结果
2.1 组织形貌

图1是Z3CN20-09M双相不锈钢铸态和热锻+固溶态试样微观组织的SEM-BSE像。从BSE像可以看出,铸态组织由铁素体和奥氏体组成(图1a),经热锻+固溶后铁素体的形态并未发生明显变化,铁素体仍以岛状分布在奥氏体基体中,原始铸态铁素体含量约为11.7% (体积分数),经热锻+固溶处理后铁素体含量略有降低,约为11.1% (图1b)。

图1 Z3CN20-09M双相不锈钢铸态和热锻+固溶态的SEM-BSE像

Fig.1 SEM-BSE images of Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b)

图2为Z3CN20-09M双相不锈钢铸态和热变形态的EBSD像。可见,铸态组织为晶粒内分布着岛状铁素体的粗大奥氏体,铁素体具有相同的晶体学取向,且其尺寸远远小于奥氏体晶粒尺寸(图2a)。经热锻后,奥氏体由粗大的柱状晶变成了细小的等轴晶粒,铁素体的晶粒尺寸和奥氏体的晶粒尺寸属同一数量级(图2b)。此外,原先一个奥氏体晶粒通过变形被破碎为取向不同的细小晶粒,而奥氏体总体晶粒尺寸也由原来的300 μm左右变为30 μm,奥氏体中出现了大量小角度晶界(白色),其比例大约为20%。

图2 Z3CN20-09M双相不锈钢铸态及热锻态的EBSD像

Fig.2 EBSD images of Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction (b)

图3为铸态Z3CN20-09M双相不锈钢热锻后经1050 ℃保温1 h的EBSD像。可见小角度晶界和畸变晶粒消失,奥氏体晶界平直化和奥氏体晶粒尺寸均匀化,说明材料在固溶处理过程中发生了静态再结晶。经热锻后,铁素体在奥氏体内的分布也发生了变化,由原来铸态分布在晶内变为锻态分布在晶界上。图3c为晶界分布图,绿色为铁素体,黑色为大角度随机晶界,其余为特殊晶界(coincident site lattice boundary,简称CSL晶界),特殊晶界所占比例约为80%,一小部分特殊晶界是热锻过程中形成,其余大部分为固溶处理中形成的。对于CSL晶界的研究主要集中于奥氏体钢,如316 L、304[15,16],对双相不锈钢中奥氏体特殊晶界的讨论特别少。CSL晶界是fcc金属在变形+退火过程形成的[17],Z3CN20-09M双相不锈钢中奥氏体在1000~1200 ℃热锻过程中发生了变形,随后在1050 ℃固溶处理时形成了大量CSL晶界。文献[18,19]报道,CSL晶界的增加有利于材料的晶间耐腐蚀性能,其原因是特殊晶界能破坏大角度随机晶界的连接性。铸态Z3CN20-09M双相不锈钢经热锻固溶后,其奥氏体晶粒尺寸得到大量细化,晶界的长度增加,由于晶界能量较高,原子处于不稳定状态,以及晶界富集杂质原子的缘故,与晶内相比,晶界的腐蚀速率一般较快[20];而大量CSL晶界能提高其耐腐蚀性能,双相不锈钢经热加工后腐蚀性能的变化还有待进一步研究。

图3 Z3CN20-09M双相不锈钢经热锻+固溶处理后的EBSD像

Fig.3 EBSD images of Z3CN20-09M duplex stainless steel after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h(a) phase distribution map(b) inverse pole figure(c) image of grain boundary character distribu- tion

2.2 力学性能

Z3CN20-09M双相不锈钢纳米硬度随热老化时间延长变化趋势如图4所示。由图可见,铸态及锻态Z3CN20-09M双相不锈钢中铁素体纳米硬度都随热老化时间延长而升高,经3000 h热老化后,铁素体纳米硬度由未热老化的3.6 GPa左右上升为6.5 GPa左右,而奥氏体纳米硬度变化则不明显。另外,Z3CN20-09M双相不锈钢铁素体的载荷-位移曲线如图5所示。从图5可以看出,随热老化时间延长,保持同一个纳米压入深度500 nm,铁素体的载荷-位移曲线向左上方移动,未热老化铸态和锻态Z3CN20-09M双相不锈钢中铁素体的最大压入载荷分别为23和24 mN,而经3000 h热老化后,其最大压入载荷分别为40和39 mN。上述现象表明:铁素体随热老化时间延长,塑性变形能力越来越差,且铸态和锻态Z3CN20-09M双相不锈钢在热老化过程中铁素体微区力学性能变化趋势一致。

图4 Z3CN20-09M双相不锈钢铁素体和奥氏体纳米硬度随热老化时间的变化

Fig.4 Change of nano-hardness with ageing time for austenite and ferrite in Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b)

图5 Z3CN20-09M双相不锈钢铁素体压入载荷-位移曲线

Fig.5 Load- displacement curves of ferrite in Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b) at different ageing time

图6 铸态和锻态Z3CN20-09M双相不锈钢Charpy冲击功随热老化时间的变化趋势

Fig.6 Change of Charpy-impact energy with ageing time for Z3CN20-09M duplex stainless steel before and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h

铸态和锻态Z3CN20-09M 双相不锈钢 的 Charpy冲击功随热老化时间延长的变化趋势如图6所示。铸态材料经热加工后冲击功由原来的327 J变为365 J,热老化100 h前的材料在量程为450 J的冲击实验中均未冲断,这是因为未经热老化时,铁素体和奥氏体的变形能力相当,奥氏体晶粒的细化导致未热老化材料冲击韧性的提高。铸态和锻态材料随着热老化时间的延长,冲击功均呈现下降趋势。热老化1000和3000 h材料的冲击断口形貌如图7所示。可见,热老化1000 h,铸态和锻态材料都呈现微孔聚集型断裂,断口呈现韧窝花样(图7a和b)。而热老化3000 h时,铸态和锻态材料冲击断口均发生韧窝/解理混合型断裂,锻态材料的冲击断口韧窝花样较多,铁素体解理特征较少,而铸态材料冲击断口铁素体解理特征较多,奥氏体撕裂特征较少(图7c和d),这与材料经热老化3000 h后的冲击功结果一致。

图7 热老化1000和3000 h铸态和锻态Z3CN20-09M双相不锈钢的冲击断口表面形貌

Fig.7 Fracture surface morphologies of Z3CN20-09M duplex stainless steel before (a, c) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b, d) aged for 1000 h (a, b) and 3000 h (c, d)

3 分析讨论

铁素体的含量会对热老化性能产生影响[21,22],此外,铁素体的形态和分布均会对热老化性能产生影响[23],铁素体在热老化脆化后,会成为裂纹快速扩展的通道,细长的铁素体会成为材料脆化的通道。如图2所示,铸态和锻态材料中铁素体形态并未发生明显变化。Pettersson等[24]报道双相不锈钢冲击功的下降与铁素体的硬度增加呈线性关系。从本实验纳米压痕的结果可以看出,铸态和锻态材料热老化前后铁素体微区力学性能基本保持一致。因此,铁素体的含量、硬度、形态对铸态及锻态热老化材料的冲击断裂行为的影响相同。

采用小变形量对双相不锈钢Z3CN20-09M进行热锻时,由于铁素体含量较低,因此奥氏体承担了绝大部分变形,奥氏体晶粒也由原来粗大的柱状晶发生再结晶变成了细小的等轴晶,由于固溶时间较短,所以铁素体很难溶于奥氏体中。铸态材料在变形时,铁素体位于奥氏体晶内,在奥氏体晶粒变形的同时,铁素体晶粒也会跟着转动,导致铁素体的晶粒取向发生了变化。

图8 铸态Z3CN20-09M双相不锈钢中铁素体的EBSD像

Fig.8 EBSD image of ferrite in cast Z3CN20-09M duplex stainless steel

图9 热老化铸态Z3CN20-09M双相不锈钢冲击断裂过程示意图

Fig.9 Illustration of the impact fracture process of cast Z3CN20-09M duplex stainless steel after thermal ageing (Arrows mark ferrite cleavage along (001) plane, and white dotted lines mark tearing in austenite)

图8为铸态Z3CN20-09M双相不锈钢中铁素体的EBSD像。可见,铸态Z3CN20-09M双相不锈钢中一个奥氏体晶粒或区域内铁素体的晶粒取向相同。图9为铸态Z3CN20-09M双相不锈钢的冲击断裂示意图。可见,主裂纹会优先沿着铁素体扩展,断口处发生解理断裂的铁素体含量高于材料中铁素体的平均含量,在主裂纹到达铁素体之前,铁素体已经沿着(001)发生解理断裂[25~27],图10断口表面黄线区域内的解理小平面就是由晶粒取向相同的铁素体断裂形成的。由于奥氏体在热老化过程中硬度变化不大,变形能力很强,位于铁素体之间的奥氏体会在很大的剪切作用下被撕裂。

图10 Z3CN20-09M双相不锈钢中取向相同的铁素体在断裂时形成的解理小平面

Fig.10 Cleavage facets formed by fracturing of ferrites with the same orientation (as circled by yollow lines) in Z3CN20-09M duplex stainless steel

图11 锻态Z3CN20-09M双相不锈钢中铁素体的EBSD像

Fig.11 EBSD image of ferrite in Z3CN20-09M duplex stainless steel after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h

图11为锻态Z3CN20-09M双相不锈钢铁素体的EBSD像。可见,铸态Z3CN20-09M双相不锈钢经热锻后,铁素体的晶粒取向发生了明显的变化,大部分相邻铁素体的晶粒取向都不同。图12为锻态Z3CN20-09M双相不锈钢的冲击断裂示意图。与铸态材料冲击断裂类似,在主裂纹未到达铁素体之前,铁素体已经沿(001)发生解理断裂。可见,主裂纹如果沿着某一晶粒取向的铁素体扩展,就会绕过其相邻不同晶粒取向的铁素体,这就解释了锻态材料冲击断口铁素体解理特征远小于铸态材料的原因。锻态材料在冲击断裂过程中,主裂纹所穿过解理断裂的铁素体较少,奥氏体应力集中较小,所以奥氏体呈现微孔聚集型断裂。

图12 热老化锻态Z3CN20-09M双相不锈钢冲击断裂过程示意图

Fig.12 Illustration of the impact fracture process of thermal aged Z3CN20-09M duplex stainless steel after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (Arrows mark ferrite cleavage along (001) plane, and white dotted lines mark microvoid coalescence in austenite)

4 结论

(1) Z3CN20-09M双相不锈钢经热锻固溶1 h处理后,铁素体的含量和形态并未发生明显变化,但铁素体晶粒取向发生了显著的变化;奥氏体发生了再结晶并形成了大量CSL晶界。

(2) 锻态材料和铸态材料铁素体的纳米硬度和最大压入载荷随热老化时间的延长而升高,铁素体变形能力下降,且锻态和铸态材料铁素体的纳米硬度变化趋势一致;而奥氏体纳米硬度变化不大。

(3) Z3CN20-09M双相不锈钢经热加工后,未热老化和热老化3000 h的冲击性能均高于铸态材料;热老化前期,铸态材料和锻态材料均呈现微孔聚集型断裂,热老化3000 h后,断裂机理为铁素体发生解理断裂,奥氏体撕裂或呈微孔聚集型断裂;主裂纹如果沿着某一晶粒取向的铁素体扩展,就会绕过相邻不同晶粒取向的铁素体,导致锻态材料断口解理特征少于铸态材料。

The authors have declared that no competing interests exist.

参考文献

[1] Wu J.Duplex Stainless Steel [M]. Beijing: Metallurgy Industry Press, 1991: 1
[本文引用:1]
(吴玖. 双相不锈钢 [M]. 北京: 冶金工业出版社, 1991: 1)
[2] Chung H M.Aging and life prediction of cast duplex stainless steel components[J]. Int. J. Pressure Vessels Piping, 1992, 50: 179
ABSTRACT Case duplex stainless steels, used extensively in nuclear, chemical, and petroleum industries because of higher strength, better weldability, higher resistance to stress corrosion cracking, and soundness of casting, are susceptible to thermal aging embrittlement during service at temperatures as low as 鈭250掳C. Recent advances in understanding the aging mechanisms, kinetics, and mechanical properties are presented, with emphasis on application of the material in safety-significant components in a nuclear reactor. Aging embrittlement is primarily due to spinodal decomposition of ferrite involving segregation of Fe, Cr, and Ni, and precipitation of M23C6 on ferrite-austenite boundaries or in ferrite. Aging kinetics are strongly influenced by synergistic effects of other metallurgical reactions that occur in parallel with the spinodal decomposition, i.e. clustering of Ni, Mo, and Si and G-phase precipitation in ferrite. A number of methods are outlined for estimating end-of-life aging, depending on several factors such as degree of permissible conservatism, availability of component archive material, and methods of estimating and verifying the activation energy of aging.
DOI:10.1016/0308-0161(92)90037-G      URL     [本文引用:0]
[3] Haušild P, Berdin C, Bompard P, et al.Ductile fracture of duplex stainless steel with casting defects[J]. Int. J. Pressure Vessels Piping, 2001, 78: 607
Experimental and numerical studies were carried out on CF8M steel with special regard to the influence of casting defects i.e. shrinkage cavities on fracture behaviour. As-received and thermally aged materials were studied. Physical steps of damage in specimens containing casting defects were observed using in-situ tensile tests in a scanning electron microscope. Mechanical properties of duplex stainless steel were characterised using three-point bend tests of three types of specimens 鈥 defect free, pre-cracked, and containing shrinkage cavities. These mechanical tests were simulated using Gurson's model including crack nucleation. A modelling of the shrinkage cavities is proposed in order to study their influence on the mechanical response of the specimen.
DOI:10.1016/S0308-0161(01)00069-2      URL     [本文引用:1]
[4] Clark C A, Guha P.Improvements in corrosion resistance, mechanical properties, and weldability of duplex austenitic/ferritic steels[J]. Mater. Corros., 1983, 34: 27
ABSTRACT The corrosion behaviour, toughness and weldability of duplex stainless steel can be improved by controlling the composition and austenite: ferrite ratio in the base metal and in the weld deposits. Addition of nitrogen is beneficial to ductility and restistance to pitting corrosion; the latter is also improved by increased chromium contents which stabilize the passive film. This effect is further enhanced by the addition of molybdenum which, however, should not exceed sigma phase at high temperatures. Copper in amounts up to 1.5% improves the resistance to marine environments; however, a certain upper limit should not be exceeded because of adverse effects on hot ductility. In view of the function of nickel concerning the austenite: ferrite ratio (which should be about 50:50) the nickel content should be appropriately selected and should be higher in the filter metal.Verbesserung der Korrosionsbestndigkeit, der mechanischen Eigenschaften und der Schwei08barkeit von austenitisch-ferritischen SthlenKorrosionsverhalten, Festigkeit und Schwei08barkeit von nichtrostenden Sthlen mit Duplex-Gefüge k02nnen durch entsprechende Zusammensetzung und Einstellung des Austenit: Ferrit-Verhltnisses im Grundwerkstoff und im Schwei08gut verbessert werden. Zusatz von Stickstoff verbessert die Duktilitt und die Lochkorrosionsbestndigkeit; diese wird ebenfalls verbessert durch erh02hte Chromgehalte, welche die Passivierungsschicht stabilisieren. Diese Wirkung wird noch verstrkt durch Zusatz von Molybdn, das jedoch bestimmte Grenzen nicht überschreiten darf, da sonst die Gefahr der Sigma-Phasen-Spr02digkeit bei h02heren Temperaturen besteht. Auch Kupfer in Mengen bis 1,5% verbessert die Korrosionsbestndigkeit besonders in Meerwasser, doch sollte wegen der abtrglichen Wirkung auf die Warmzhigkeit ein bestimmter Gehalt nicht überschritten werden. Wegen der wichtigen Rolle von Nickel für das Austenit:Ferrit-Verhltnis (vorzugsweise 50:50) mu08 der Nickelgehalt entsprechend gewhlt werden und sollte im Schwei08gut etwas erh02ht werden.
DOI:10.1002/maco.19830340105      URL     [本文引用:1]
[5] Tucker J D, Miller M K, Young G A.Assessment of thermal embrittlement in duplex stainless steels 2003 and 2205 for nuclear power applications[J]. Acta Mater., 2015, 87: 15
Duplex stainless steels are desirable for use in power generation systems due to their attractive combination of strength, corrosion resistance, and cost. However, thermal embrittlement at intermediate homologous temperatures of ~887°F (475°C) and below, via spinodal decomposition, limits upper service temperatures for many applications. New lean grade duplex alloys have improved thermal stability over standard grades and potentially increase the upper service temperature or the lifetime at a given temperature for this class of material. The present work compares the thermal stability of lean grade, alloy 2003 to standard grade, alloy 2205, through a series of isothermal agings between 500°F (260°C) and 900°F (482°C) for times between 1 and 10,000 hours. Aged samples were characterized by changes in microhardness and impact toughness. Additionally, atom probe tomography was performed to illustrate the evolution of the α-α' phase separation in both alloys at select conditions. Atom probe tomography confirmed that phase separation occurs via spinodal decomposition for both alloys and identified the formation of Ni-Cu-Si-Mn-P clusters in alloy 2205 that may contribute to embrittlement of this alloy. The impact toughness model predictions for upper service temperature show that alloy 2003 can be considered for use in 550°F applications for 80 year service
DOI:10.1016/j.actamat.2014.12.012      URL     [本文引用:1]
[6] Weng K L, Chen H R, Yang J R.The low-temperature aging embrittlement in a 2205 duplex stainless steel[J]. Mater. Sci. Eng., 2004, A379: 119
Abstract The effect of isothermal treatment (at temperatures ranging between 400 and 500 °C) on the embrittlement of a 2205 duplex stainless steel (with 45 ferrite–55 austenite, vol.%) has been investigated. The impact toughness and hardness of the aged specimens were measured, while the corresponding fractography was studied. The results show that the steel is susceptible to severe embrittlement when exposed at 475 °C; this aging embrittlement is analogous with that of the ferritic stainless steels, which is ascribed to the degenerated ferrite phase. High-resolution transmission electron microscopy reveals that an isotropic spinodal decomposition occurred during aging at 475 °C in the steel studied; the original δ-ferrite decomposed into a nanometer-scaled modulated structure with a complex interconnected network, which contained an iron-rich BCC phase (α) and a chromium-enriched BCC phase (α′). It is suggested that the locking of dislocations in the modulated structure leads to the severe embrittlement.
DOI:10.1016/j.msea.2003.12.051      URL     [本文引用:1]
[7] Yao Y H, Wei J F, Wang Z P, et al.Effect of long-term thermal aging on the mechanical properties of casting duplex stainless steels[J]. Mater. Sci. Eng., 2012, A551: 116
Abstract Casting duplex stainless steels (CDSSs) used for pressurized water reactor (PWR) pipes are susceptible to thermal aging brittleness during long-term service at its working temperature from 288 掳C to 327 掳C. In order to investigate its thermal aging behavior, Z3CN20-09M CDSSs have been thermally aged at 400 掳C up to 15,000 h. The micro-hardness of austenite and ferrite phases, conventional tensile properties, impact properties and SPT properties at different aging duration have been measured. The results show that the micro-hardness in ferrite gradually increases with increase of long thermal aging time, while the effect of the long aging time on the micro-hardness in austenite is negligible. The tensile strength and yield strength progressively and slightly increase with the long aging time, respectively. The impact property test confirms that there is the same change tendency as the percentage of elongation which decreases with the long aging time. The changes of SPT ultimate strength, SPT yield strength and SPT specific fracture energy by aging individually show that there is an almost same tendency as the tensile strength, the yield strength and the percentage of elongation with the thermal aging time. All above the mechanical properties changes of Z3CN20-09M CDSSs are associated with the changes of the dislocation configurations in austenite, the precipitation of G-phase on the dislocation line and in ferrite phase, the spinodal decomposition and the coarsening of the spinodally decomposed structure in ferrite after different thermal aging time.
DOI:10.1016/j.msea.2012.04.105      URL     [本文引用:2]
[8] Hereñú S, Sennour M, Balbi M, et al.Influence of dislocation glide on the spinodal decomposition of fatigued duplex stainless steels[J]. Mater. Sci. Eng., 2011, A528: 7636
The present work is focused on assessing the influence of dislocation movement on spinodal decomposition through scanning transmission electron microscopy (STEM) in combination with energy dispersive X-ray spectroscopy (EDS) analysis in aged duplex stainless steel (DSS) S32750. Dislocation bands and microbands are the prominent dislocation arrangements observed in fatigue tested aged samples. By EDS measurements it was found that the spinodal decomposition was dissolved inside these dislocations structures. Therefore, the mechanism of microband formation developed in the ferritic phase during cycling seems to be responsible for the demodulation of the spinodal decomposition and cyclic softening of the aged DSS.
DOI:10.1016/j.msea.2011.06.070      URL     [本文引用:1]
[9] Li S L, Wang Y L, Li S X, et al.Microstructures and mechanical properties of cast austenite stainless steels after long-term thermal aging at low temperature[J]. Mater. Des., 2013, 50: 886
ABSTRACT The cast austenite stainless steels were investigate in order to understand the microstructural evolution and mechanical properties in the long-term thermal aging at 400. C for up to 20,000. h. Spinodal decomposition and G-phase precipitation in ferrite after long-term thermal aging lead to the degradation of mechanical properties. Ferrite hardness increases with aging time, but the austenite hardness does not change. Tensile strength is not strongly affected by aging time, but the plasticity has a significant decrease after long-term aging. Under impact with high strain rate, the ferrite phases deform by the way of deformation twinning. High stress concentration on the phase boundaries cause the phase boundary separating and the austenite's tearing off. 2013 Elsevier Ltd.
DOI:10.1016/j.matdes.2013.02.061      URL     [本文引用:1]
[10] Miller M K, Hyde J M, Cerezo A, et al.Comparison of low temperature decomposition in Fe-Cr and duplex stainless steels[J].
[本文引用:1]
Appl. Surf. Sci., 1995, 87-88: 323
[11] Sahu J K, Krupp U, Ghosh R N, et al.Effect of 475 ℃ embrittlement on the mechanical properties of duplex stainless steel[J]. Mater. Sci. Eng., 2009, A508: 1
The binary iron–chromium alloy embrittles in the temperature range of 280–50002°C limiting its applications to temperatures below 28002°C. The embrittlement is caused by the decomposition of the alloy to chromium-rich phase, α′ and iron-rich phase, α. This phenomenon is termed 47502°C embrittlement as the rate of embrittlement is highest at 47502°C. Primarily the investigations on 47502°C embrittlement were confined to binary iron–chromium alloys and ferritic stainless steels. Duplex stainless steel grades contain varying proportions of ferrite and austenite in the microstructure and the ferritic phase is highly alloyed. Moreover, this grade of steel has several variants depending on the alloy composition and processing route. This modifies the precipitation behaviour and the resulting change in mechanical properties in duplex stainless steels when embrittled at 47502°C as compared to binary iron chromium systems. The precipitation behaviour of duplex stainless steel at 47502°C and the effect on tensile, fracture and fatigue behaviour are reviewed in this article.
DOI:10.1016/j.msea.2009.01.039      URL     [本文引用:0]
[12] Kawaguchi S, Sakamoto N, Takano G, et al.Microstructural changes and fracture behavior of CF8M duplex stainless steels after long-term aging[J]. Nucl. Eng. Des., 1997, 174: 273
ABSTRACT Microstructural changes and fracture behavior in cast CF8M duplex stainless steel after aging at 300–450°C for 300–1064000 h have been investigated. Both, optical microscopical and transmission electron microscopical analyses, hardness and ferrite content measurements have been carried out in this study. Strengthening and aging phenomena of the ferrite phase have been identified by hardness measurements. Spinodal decomposition and heterogeneous precipitation of G-phase were found to be responsible for strengthening of the ferrite phase after aging with a temper parameter (see Appendix A) in the range ca. 1.8–4.5. Three different fracture modes, dimples, cleavage and α/γ grain boundary separation, have been observed for Charpy V-notch and CT test specimens fractured at 20°C.
DOI:10.1016/S0029-5493(97)00126-X      URL     [本文引用:0]
[13] Li S L.Thermal aging of Z3CN20-09M steels in the primary circuit piping of nuclear power plants [D]. Beijing: University of Science and Technology Beijing, 2013
[本文引用:1]
(李时磊. 核电站一回路管道Z3CN20-09M不锈钢的热老化研究 [D]. 北京: 北京科技大学, 2013)
[14] Li S L, Wang Y L, Zhang H L, et al.Microstructure evolution and impact fracture behaviors of Z3CN20-09M stainless steels after long-term thermal aging[J]. J. Nucl. Mater., 2013, 433: 41
Z3CN20-09M steels of primary circuit piping in Daya Bay Nuclear Power Plant were studied on the microstructure evolution and fracture behaviors after thermal aging at 400 掳C for up to 20,000 h. The impact toughness of aged materials decreases a lot with aging time, and the impact fracture features change from ductile dimples into brittle cleavages in ferrite and tearing in austenite. Nano-indentation tests indicate that hardness in ferrite continuously increases with aging time. After long-term aging, ferrite decomposes into coherent Cr-rich and Fe-rich domains, and extensive G-phases precipitate homogeneously in ferrite, but no precipitate in austenite. Spinodal decomposition in ferrite leads to the thermal aging embrittlement in CASS. G-phase, with the Fm-3m space group and the lattice parameter of 1.14 nm, adopts a cube-on-cube orientation relationship with the ferrite matrix.
DOI:10.1016/j.jnucmat.2012.09.004      URL     [本文引用:1]
[15] Kokawa H, Shimada M, Michiuchi M, et al.Arrest of weld-decay in 304 austenitic stainless steel by twin-induced grain boundary engineering[J]. Acta Mater., 2007, 55: 5401
A twin-induced grain boundary engineered 304 austenitic stainless steel with a high frequency of coincidence site lattice boundaries produced by a one-step thermomechanical process was arc-welded. The heat-affected zone was examined by orientation imaging microscopy and corrosion tests. The results indicated that the intergranular corrosion due to sensitization in the heat-affected zone were significantly suppressed, because the high frequency of coincidence site lattice boundaries and the optimum grain boundary character distribution were stable in the grain boundary engineered stainless steel during arc-welding. The grain boundary engineering can arrest the weld-decay of austenitic stainless steel effectively.
DOI:10.1016/j.actamat.2007.06.005      URL     [本文引用:1]
[16] West E A, Was G S.IGSCC of grain boundary engineered 316L and 690 in supercritical water[J]. J. Nucl. Mater., 2009, 392: 264
This study evaluated the influence of a high fraction of special grain boundaries on the intergranular stress corrosion cracking susceptibility of 316L stainless steel and nickel base alloy 690 in supercritical water. By thermomechanically processing the alloys to create specimens with largely different special boundary fractions, it was possible to isolate the effects of the grain boundary structure on the intergranular stress corrosion cracking behavior. Constant extension rate tensile experiments were performed in 500 掳C deaerated supercritical water, and SEM analysis of the cracking behavior was performed on the gage surfaces of the specimens. Results indicate that the fraction of cracked grain boundary length in the specimens with higher fractions of special boundaries is reduced for 316L and 690 by factors of 9 and 5 at 15% strain, and 3 and 2 at 25% strain, respectively. This reduction is due to the special boundaries, which at 25% strain have a frequency of cracking that is 9-18 times lower than that for a random high angle boundary.
DOI:10.1016/j.jnucmat.2009.03.008      URL     [本文引用:1]
[17] Shi F, Li X W, H Y T, et al. Optimization of grain boundary character distribution in Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel[J]. Acta Metall. Sin.(Engl. Lett.), 2013, 26: 497
ABSTRACT Grain boundary engineering (GBE) is a practice of improving resistance to grain boundary failure of the material through increasing the proportion of low 危 coincidence site lattice (CSL) grain boundaries (special grain boundaries) in the grain boundary character distribution (GBCD). The GBCD in a cold rolled and annealed Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel was analyzed by electron back scatter diffraction (EBSD). The results show that the optimization process of GBE in the conventional austenitic stainless steel cannot be well applied to this high-nitrogen austenitic stainless steel. The percentage of low 危CSL grain boundaries could increase from 47.3% for the solid solution treated high-nitrogen austenitic stainless steel specimen to 82.0% for the specimen after 5% cold rolling reduction and then annealing at 1423 K for 10 min. These special boundaries of high proportion effectively interrupt the connectivity of conventional high angle grain boundary network and thus achieve the GBCD optimization for the high-nitrogen austenitic stainless steel.
DOI:10.1007/s40195-013-0323-5      URL     [本文引用:1]
[18] Shimada M, Kokawa H, Wang Z J, et al.Optimization of grain boundary character distribution for intergranular corrosion resistant 304 stainless steel by twin-induced grain boundary engineering[J]. Acta Mater., 2002, 50: 2331
ABSTRACT The effects of process parameters, pre-strain, annealing temperature, time, etc. on grain boundary character distribution (GBCD) and intergranular corrosion in thermomechanical treatment were examined during grain boundary engineering of type 304 austenitic stainless steel. Slight pre-strain annealing at a relatively low temperature resulted in excellent intergranular corrosion resistance due to optimized GBCD, i.e. the uniform distribution of a high frequency of coincidence site lattice boundaries and consequent discontinuity of random boundary network in the material. The optimum distribution can be formed by introduction of low energy segments on migrating random boundaries during twin emission and boundary鈥揵oundary reactions in the grain growth without generation of new random boundaries.
DOI:10.1016/S1359-6454(02)00064-2      URL     [本文引用:0]
[19] Tsurekawa S, Nakamichi S, Watanabe T.Correlation of grain boundary connectivity with grain boundary character distribution in austenitic stainless steel[J]. Acta Mater., 2006, 54: 3617
The connectivity of grain boundaries can be an important microstructural parameter governing the bulk properties of polycrystalline materials as well as the grain boundary character distribution. On the basis of the percolation theory, the connectivity of random boundaries that are susceptible to percolative phenomena like corrosion in a polycrystal has been quantitatively evaluated and examined in connection with the grain boundary character distribution in austenitic stainless steel. The percolation threshold has been found to occur at a fraction of approximately 7002±025% coincident site lattice (CSL) boundaries (3002±025% random boundaries). The maximum random boundary cluster length drastically decreases and the frequency of resistant triple junctions increases with increasing frequency of the CSL boundaries near the percolation threshold. An increase in the frequency of resistant triple junctions can enhance corrosion resistance of polycrystalline austenitic stainless steel even if the grain boundary character distribution is the same.
DOI:10.1016/j.actamat.2006.03.048      URL     [本文引用:0]
[20] Hu G X, Cai X, Rong Y H.Fundamentals of Materials Science [M]. 3rd Ed., Shanghai: Shanghai Jiao Tong University Press, 2010: 3
[本文引用:1]
(胡赓祥, 蔡珣, 戎咏华. 材料科学基础 [M]. 第3版, 上海: 上海交通大学出版社, 2010: 3)
[21] Li S L, Wang Y L, Wang X T, et al.Effects of ferrite content on the mechanical properties of thermal aged duplex stainless steels[J]. Mater. Sci. Eng., 2015, A625: 186
ABSTRACT The duplex stainless steels (DSS), solution treated at 1080 掳C for different hours, were further thermal aged at 400 掳C for as long as 3000 h to evaluate the effects of ferrite content on the thermal aging behaviors. For the unaged DSS, a high content of ferrite is helpful in enhancing the mechanical properties. After thermal aging for 3000 h, the impact energies of DSS are found to decrease with increase in ferrite content. Extending solution time in the single-phase austenitic region can decline the ferrite content and improve the distribution and shape of the ferrite phases. The compositions of undissolved ferrite during this solution process have no obvious change, and thus the thermal aging kinetics are not affected. For the long-time served DSS components, solution treatment in the single-phase austenitic region for a relatively longer time is helpful in enhancing the resistance of thermal aging embrittlement.
DOI:10.1016/j.msea.2014.11.065      URL     [本文引用:1]
[22] Timofeev B T, Nikolaev Y K.About the prediction and assessment of thermal embrittlement of Cr-Ni austenitic-ferritic weld metal and castings at the ageing temperatures 260-425 ℃[J]. Int. J. Pressure Vessels Piping, 1999, 76: 849
This paper presents impact test results of austenitic–ferritic weld metal of the type 10X18H9F3C2 (12% ferrite) and cast base metal of the type 08X20H10M3C2 (10–32% ferrite) after a prolonged thermal ageing at temperatures 250–450°C. The obtained experimental data give the possibility to predict impact strength degradation of austenitic–ferritic metal after 10–10 h exposure and to determine thermal embrittlement.
DOI:10.1016/S0308-0161(99)00055-1      URL     [本文引用:1]
[23] Li S L, Wang Y L, Zhang H L, et al.Effects of prior solution treatment on thermal aging behavior of duplex stainless steels[J]. J. Nucl. Mater., 2013, 441: 337
The influence of solution temperature on thermal aging behavior was studied in duplex stainless steels. With increasing solution temperature, the ferrite contents remarkably increase, Cr and Ni elements redistribute. During thermal aging, the impact properties of higher solution temperature treated materials suffer a serious degradation, which is not only related with ferrite content but also the alloy compositions in ferrite. Enrichment of Ni in ferrite can accelerate the spinodal decomposition kinetics. Thermal aging-inducing strain fields in ferrite cause the embrittlement of DSS.
DOI:10.1016/j.jnucmat.2013.06.017      URL     [本文引用:1]
[24] Pettersson N, Wessman S, Thuvander M, et al.Nanostructure evolution and mechanical property changes during aging of a super duplex stainless steel at 300 ℃[J]. Mater. Sci. Eng., 2016, A647: 241
The nanostructure evolution and the corresponding changes in mechanical properties of a super duplex stainless steel 2507 (UNS S32750) during aging at 300. 掳C up to 12,000. h have been investigated. Microstructural studies using transmission electron microscopy and atom probe tomography show that subtle Cr concentration fluctuations develop during aging. The amplitude of the concentration fluctuations is proportional to the hardness of the ferrite phase, and it is also proportional to the decrease in room temperature impact toughness during aging. The fracture behaviour of the alloy changes gradually from ductile to cleavage fracture, upon aging. The cracks were found to propagate through the ferrite phase, partly along deformation twin interfaces, and delamination between the austenite and ferrite phases was observed.
DOI:10.1016/j.msea.2015.09.009      URL     [本文引用:1]
[25] Li S L, Wang Y L, Li S X, et al.Effect of long term aging on the microstructure and mechanical properties of cast austenitic stainless steels[J]. Acta Metall. Sin., 2010, 46: 1186
[本文引用:1]
(李时磊, 王艳丽, 李树肖. 长期热老化对铸造奥氏体不锈钢组织和性能的影响[J]. 金属学报, 2010, 46: 1186)
将铸造奥氏体不锈钢在400℃下时效至104h,通过力学性能测试和组织观察,分析了老化时间对力学性能和组织结构的影响.结果表明铁素体相的体积分数随老化时间没有变化;长期老化后材料的磁导率降低,同时铁素体相的纳米压入硬度增加;材料的Charpy冲击功随老化时间迅速下降,冲击断口形貌也随之不断变化;通过TEM观察发现,铁素体相在长期老化后析出了弥散的α'相和少量的G相,α'相是由铁素体发生调幅分解而生成的,是导致铸造奥氏体不锈钢热老化脆化的主要原因.
DOI:10.3724/sp.j.1037.2010.00250      URL    
[26] Zheng K, Wang Y L, Li S L, et al.The microstructure and tensile fracture behavior of long term thermal aged Z3CN20-09M stainless steel[J]. Acta Metall. Sin., 2013, 49: 175
[本文引用:0]
(郑凯, 王艳丽, 李时磊. 长期热老化后Z3CN20-09M不锈钢的微观组织与拉伸断裂行为[J]. 金属学报, 2013, 49: 175)
利用TEM和HRTEM研究了400℃热老化2×10~4 h后Z3CN20-09M不锈钢的微观组织,用纳米力学探针研究了微区力学性能.结果表明:相比于原始状态,经400℃热老化2×10~4 h后,Z3CN20-09M不锈钢中的铁素体发生了调幅分解并且析出了G相,导致铁素体的硬度增加和塑性变形能力下降.利用原位疲劳试验机、SEM和电子探针研究了400℃热老化2×10~ 4 h后Z3CN20-09M不锈钢的微型平板试样拉伸行为.结果表明:热老化后,不锈钢的屈服强度和断裂强度升高,延伸率下降.热老化不锈钢中铁素体发生解理断裂,微裂纹萌生于相界并向铁素体内扩展;奥氏体主要发生微孔聚集型韧性断裂,并且在局部区域发生撕裂.调幅分解是铁素体发生硬脆化和热老化前后Z3CN20-09M不锈钢拉伸断裂行为发生改变的根本原因.
DOI:10.3724/SP.J.1037.2012.00560      URL    
[27] Ghosh A, Kundu S, Chakrabarti D.Effect of crystallographic texture on the cleavage fracture mechanism and effective grain size of ferritic steel[J]. Scr. Mater., 2014, 81: 8
The effect of crystallographic texture on impact transition behavior has been studied in a low-carbon steel. Crystallographic texture was found to influence the general yield temperature through its effect on the plastic constraint factor. The effective grain size depends on the angle between the {001} cleavage planes of the neighbouring crystals, rather than the grain boundary misorientation angle as determined from electron backscattered diffraction analysis considering the angle–axis pair.
DOI:10.1016/j.scriptamat.2014.02.007      URL     [本文引用:1]
资源
PDF下载数    
RichHTML 浏览数    
摘要点击数    

分享
导出

相关文章:
关键词(key words)
双相不锈钢
热加工
热老化
断裂
晶粒取向

duplex stainless steel
hot working
thermal ageing
fracture
grain orientation

作者
张海
李时磊
刘刚
王艳丽

ZHANG Hai
LI Shilei
LIU Gang
WANG Yanli