7CrSiMnMoV钢的马氏体形态及其强韧性研究SCIEI

本文用金相及电子显微分析研究了7CrSiMnMoV钢的马氏体形态及亚结构,并对不同形态的马氏体转化规律进行了分析。研究发现该钢冷却产生的马氏体有三种类型,包括晶界上优先产生的具有位错亚结构的宽大板条马氏体,晶内较细小的板条马氏体以及内部少量的孪晶马氏体片。基于上述结果,本工作试用短时等温淬火形成的下贝氏体来代替晶界马氏体,证明晶界组织形态对材料的强韧性有着很大的影响。     

980MPa级高强钢焊接接头HAZ的组织和性能

采用焊接热模拟的方法,研究一种980 MPa级低碳贝氏体高强钢焊接接头HAZ不同区域,通过对各个区域的组织及相的分析,以及相应的拉伸及冲击试验研究了此类高强钢的组织和性能.结果表明,粗晶区的冲击性能最好,细晶区的冲击性能最低,为热影响区的薄弱环节.粗晶区组织为均匀粗细相间的板条贝氏体组织;在板条贝氏体上弥散析出碳化物;板条贝氏体界面上的奥氏体薄膜的存在是粗晶区韧性提高的原因.细晶区为孪晶马氏体+少量的板条马氏体,孪晶马氏体是导致细晶区性能下降的主要原因.

Abstract：

Microstructure and mechanical properties of HAZ of 980 MPa low carbon bainite high strength steel joints were studied . The different regions of welded joint HAZ were simulated by welding thermal simulation technique. Microstructure observation, phase analysis, and corresponding tensile and impact test for different regions of welded joint HAZ were taken. The results indicate: the impact property of coarse grain zone is the best, while the impact property of fine grain zone is the worst. So the fine grain zone is the weakest part of the welded joint. Microstructure of coarse grain zone is uniform interweaved coarse and fine lath martensites, and precipitation acicular ferrite are distributed between the lath martensites. Toughness of coarse grain zone is increased owing to the austenite thin film adherent lath martensite interface. Microstructure of fine grain zone are twin martensite and a small number of lath maarten-site, only minor acicular ferrite are distributed in the twin martensite . Toughness of fine-grained zone was decreased owing to twin martensite.     

Comparison of thermally induced and deformation induced martensite in Fe–29% Ni–2% Mn alloy

The morphology and crystallography of martensite either formed by cooling to subzero temperatures (thermal effect) or by compression deformation were compared for different austenite grain sizes of Fe–29% Ni–2% Mn alloy by transmission electron microscope (TEM). TEM observations revealed both and ′ martensite formation within large grained austenite phases by thermal effect whereas only ′ martensite formation was observed in small grained austenite phases. On the other hand, compression deformation effect caused only ′ martensite formation in both small and large grained austenite phases of Fe–29% Ni–2% Mn alloy. Thermally induced ′ martensite exhibited a lenticular morphology with partial twinnings that are peculiar to this kind of morphology. The crystallographic orientation relationship between austenite and thermally induced ′ lenticular martensites was found to be as Kurdjumov–Sachs (K–S) type relationship.     

NUCLEATION AND GROWTH OF FERROUS MARTENSITES ALONG BOUNDARIES

Martensites distributed along the austenite grain boundaries and twin boundaries have beenexamined in Fe-C,Fe-Ni-C and Fe-Cr-Mn-Mo-C alloys.The martensites may nucleatepreferentially and grow easily along these boundaries.In the mixed martensites,thepreferentially formed one is plate or butterfly martensite     

铁基马氏体的界面形核与长大SCIEI

对分布在Fe-C,Fe-Ni-C及Fe-Cr-Mn-Mo-C合金中奥氏体晶粒界及孪晶界上的马氏体进行了观察分析。结果发现,马氏体易于在奥氏体晶粒界及孪晶界优先形核,并易于沿这些界面长大;在混合马氏体组织中,其界面优先形成的是片状马氏体或蝶状马氏体。     

板条状马氏体的亚结构及形成机制

研究马氏体亚结构及形成机制具有重要理论意义和应用价值.应用35CrMo等材料,淬火为马氏体组织,采用JEM-2100电镜观察马氏体形貌和亚结构,发现35CrMo钢马氏体为板条状,亚结构除了高密度缠结位错外,还有高密度层错.结合2Cr13、Fe-15 Ni-0.6C合金试样的浮凸试验,应用隧道扫描显微镜观察表面马氏体浮雕形貌,测定浮凸的微细尺寸.综合上述试验结果,分析位错、层错形成机制.发现浮凸效应中没有出现位错滑移迹象,认为切变模型不能解释高密度位错和层错的形成.最后应用马氏体相变新机制分析了高密度缠结位错、精细层错的形成.     

Deformation-induced martensitic transformation behavior in cold-rolled and cold-drawn type 316 stainless steels

We investigate deformation-induced martensitic transformation behavior in cold-rolled and cold-drawn specimens of type 316 stainless steel. Deformation-induced martensite preferentially nucleates at the twin boundary between the austenite matrix and a deformation twin. In the cold-rolled specimen, martensite formed at the twin boundary has a Kurdjumov–Sachs (K–S) relationship with both the austenite matrix and the deformation twin (“double K–S relationship”). In the cold-drawn specimen, two kinds of deformation twins with different twin planes are typically formed, and therefore deformation-induced martensites are formed where the deformation twin boundaries intersect: martensite thus has an imperfect “triple K–S relationship” with the austenite matrix and the two deformation twins. The complicated crystallographic orientation relationship between austenite and martensite grains strongly restricts the formation of some variants of deformation-induced martensites. Because of the difference in number of nucleation sites in the cold-drawn and cold-rolled specimens, martensitic transformation is more enhanced in the former than in the latter.     

Twinned Martensitic Substructure in a Water Quenched Fe-1.0 wt% C Alloy

A Fe-1.0 wt% C alloy was quenched into water from 1100 °C, leading to lath martensite and plate martensite of body-centered tetragonal structure. Both these two martensites have the twinned substructure that generates mirror symmetric diffraction patterns with extra diffraction spots around n /3 (112). The twinned substructure has the origin from twinned martensitic variants, namely twin-related crystals separated by {110}, rather than {112}<111> deformation twins. Tetragonality effect on the electron double diffraction of twinned variants was discussed.     

Effect of grain boundary character on the martensitic transformation in Fe–32at.%Ni bicrystals

The effect of grain boundary character on the martensitic transformation was examined in two types of Fe–32at.%Ni bicrystals containing a 90°<211> tilt or a 90°{211} twist boundary focusing on the martensite-start temperature (Ms) during cooling, the morphology of the martensite and the variant selection. The Ms of bicrystals with a tilt boundary was higher than that of single crystals, while bicrystals with a twist boundary showed no significant difference from that of single crystals. Coarse lenticular martensites formed symmetrically in neighbouring grains around the tilt boundary. In contrast, tiny martensites were uniformly distributed in bicrystals with a twist boundary, and in single crystals. In the vicinity of the tilt and twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants. Moreover, since the equivalent variants in neighbouring grains at the tilt boundary are selected, strain compatibility of the shape strain of martensite across the tilt boundary is satisfied, resulting in an increased Ms. The effect of pre-strain on the heterogeneous nucleation of martensite at the boundaries was also investigated.     

时效温度对SLM 18Ni300马氏体时效钢显微组织和力学性能的影响

采用激光选区熔化(SLM)技术制备了18Ni300马氏体时效钢,结合拉伸试验、硬度测试和显微组织表征等手段,研究了时效温度(390, 490, 590℃)对SLM 18Ni300马氏体时效钢显微组织和力学性能的影响。结果表明,SLM成形试样主要由Fe-Ni马氏体基体和胞状亚结构组成,经时效处理后,试样微观组织发生显著变化。随着时效温度的升高,胞状亚结构逐渐分解,马氏体逆转变成为奥氏体,Σ3晶界占比下降。同时,Ni₃X(X=Ti, Al, Mo)纳米相弥散析出,并在590℃时粗化。随着时效温度的升高,SLM 18Ni300马氏体时效钢的强度和硬度均先增加后下降,伸长率先降低后增加。其中,490℃时效的SLM马氏体时效钢兼具超高强度和较好塑性,这与其基体中弥散分布的纳米析出相、适量的奥氏体含量和较低的Σ3晶界占比有关。     

Substructures of lenticular martensites with different martensite start temperatures in ferrous alloys

This study investigated the substructures of lenticular martensites with different martensite start temperatures (Ms) by transmission electron microscopy. Observation of Fe–33Ni revealed a substructural change from fine transformation twins in the midrib and twinned region to several sets of screw dislocations in the untwinned region during growth. Tangled and curved dislocations also appeared near the martensite–austenite interface of the untwinned region, as the martensite inherited the dislocations in the surrounding austenite. In contrast, curved and tangled dislocations appeared in the entire untwinned region in Fe–31Ni and in the whole martensite plate in Fe–20.5Ni–35Co, as the higher Ms temperatures facilitated the plastic deformation of the surrounding austenite. Thermally transformed thin plate martensite in Fe–31Ni–10Co–3Ti grew into a lenticular shape accompanied by a substructure with dislocations after deformation at temperatures above the Ms temperature. The change in the substructure of lenticular martensite presumably resulted from the local temperature rise in the martensite plate.     

The compound formation mechanism of butterfly martensite

The butterfly martensites formed in Fe-1.20% C, CrMn, GCr18Mo, and 25Cr2Ni4WA steels have been investigated by optical and transmission electron microscopy. It has been found that the mid-rib of a butterfly martensite is a thin-plate martensite and that the formation of butterfly martensite is a multi-stage process. In the initial stage, crossed or kinked thin-plate martensites are formed. Subsequently, these become mid-ribs, grow more, then create butterfly martensite in various forms. Crossed thin-plate martensite thickens into butterfly martensite with tails, while the kinked form transforms to tailless butterfly martensite by thickening on only one side of the wings. The formation of butterfly martensite may not be a single continuous process, but rather a compound one comprised of separate stages.     

马氏体相变理论研究的最新进展

马氏体相变的切变机制存在误区,研究马氏体相变的形核、长大、组织形貌及亚结构等具有重要理论价值和实际意义。本文综合整理了近年来马氏体相变试验研究的新成果,指出马氏体在晶界、相界面、位错等缺陷处形核,并非切变形核;发现板条状马氏体中存在层错亚结构。位错、孪晶亚结构的形成也非切变所致;马氏体组织形貌的演化与应变能有密切关系;马氏体表面浮凸是相变比体积增大所致,N型切变缺乏试验依据。     

Effect of Hierarchical Microstructures of Lath Martensite on the Transitional Behavior of Fatigue Crack Growth Rate

In this study, the fatigue-crack growth (FCG) behavior of 20CrMTiH steel with different substructure sizes was investigated. The results showed that coarsen microstructures exhibit excellent growth resistance. Moreover, two transitional behaviors were observed in the FCG curves of all specimens. The first transition point occurs in the near-threshold regime, whereas the second transition point occurs in the Paris regime. A comparison of substructure size to cyclic plastic size showed that the block size is almost equal to cyclic plastic size at ?K_T1, indicating that block size is an effective grain size to control the first transitional behavior of fatigue-crack propagation, whereas the second transitional behavior is related to the packet width or grain size. According to the fracture morphology, the fracture mechanism above and below the transition point responsible for the above phenomenon were distinguished. In addition, two prediction models based on microstructure size were established for lath martensite to evaluate the threshold and stress intensity factor range at the transition point.     

超高温双重淬火对40Cr钢显微组织和冲击韧性的影响

研究了双重淬火工艺对40Cr钢的奥氏体晶粒大小、显微组织和冲击韧性的影响。发现双重淬火促进混晶出现和超高温双重淬火引起第二次淬火时的奥氏体晶粒粗化和产生粗大晶粒的遗传,从而使其冲击韧性下降。并指出,高温淬火引起40Cr钢形成的单一色调的束状马氏体不是板条马氏体,而是片状马氏体的一种新形态—束状薄片马氏体。超高温预先淬火并不能使40Cr钢生成板条马氏体,而只能生成束状薄片马氏体。     

Thermal stability of nanocrystalline layer prepared by surface mechanical attrition in 0Cr18Ni9Ti stainless steel

By means of surface mechanical attrition (SMA), a nanostructured surface layer was formed on a 0Cr18Ni9Ti austenite stainless steel plate. A strain-induced martensite transformation was observed during SMA treatment, and a single magnetic martensite phase layer with thickness of about 30 μm was gotten. The grain growth and phase transformations in the nanocrystalline layer are investigated during heating. The grain growth exponent for nanocrystalline polycrystalline steel is estimated. The kinetics mechanism governing the grain growth in the nanocrystalline layer is discussed. The martensite in the surface layer is quite stable and the temperature at which the reverse transformation of martensite to austenite starts during heating is about 500 ℃.     

Microstructural Effects on the Mechanical Integrity of a TRIP-800 Steel Welded by Laser-CO2 Process

In this study, a TRIP-800 steel was welded using a Laser CO₂ process, and the resultant microstructures were characterized by optical, scanning, and transmission electron microscopy (TEM) means. It was found that the microstructure of the steel in the as-received condition consisted of ferrite, bainite, and retained austenite (RA), including some martensite. In particular, TEM observations indicated that the developed martensites were high carbon twinned martensites. It was found that laser beam welding (LBW) promoted the development of up to 23% martensite in the fusion zone (FZ) and up to 30% in the heat-affected zone (HAZ). In addition, determinations of RA using x-ray diffraction indicated that the amount of RA developed in the FZ was relatively small (<6%). Confirmation for the relatively large amounts of martensite in both the FZ and HAZ was indirectly made by the shape of microhardness profiles, which resembled a “top hat.” Tensile testing in welded strips indicated a loss of strength and ductility. An examination of the fracture surfaces indicated that the steel fractured in a brittle fashion at the HAZ-BM interface. Apparently, the development of relatively large amounts of martensite in the HAZ reduced the steel toughness. In turn, this indicated that LBW leads to martensite embrittlement in the HAZ regions, but not at the parting line of the FZ.     

马氏体组织对SFPB处理双相钢表面纳米结构及力学性能的影响

目的 通过对不同微观组织铁素体/马氏体双相钢进行表面纳米化处理,探究材料表面晶粒细化和塑性变形机理。方法 采用超音速微粒轰击（SFPB）技术对经临界区退火（IA）、中间淬火（IQ）和分级淬火（SQ）后的双相钢进行纳米化处理,采用SEM、OM和XRD研究试验钢表面SFPB前后的微观组织特征,采用显微硬度仪测试其表面硬度,采用拉伸实验测试其力学性能。结果 热处理后,IA、IQ和SQ试样马氏体组织分别呈岛状、纤维状和块状,IQ试样平均晶粒尺寸最小,但马氏体体积分数最大。SFPB工艺处理后,双相钢表面形成了一定厚度的梯度纳米晶层（GNS）,该晶层内的晶粒尺寸均达到纳米级别,且随距离表面深度的增大而增大。IA-GNS、IQ-GNS和SQ-GNS试样表面硬度分别为285.9、266.7、382.1HV,抗拉强度分别为771.30、820.02、663.81 MPa,延伸率分别为8.89%、14.70%、10.04%。IQ-GNS试样断口以韧窝为主,SQ-GNS和IA-GNS试样断口韧窝较少,有明显裂纹开口。结论 表面产生强烈塑性变形时,由于位错的分割作用,表面晶粒尺寸细化至纳米级,材料强度大幅提高,同时纳米级纤维状马氏体微观组织的存在使得IQ-GNS试样保持了较高的塑韧性。     

IN SITU METALLOGRAPHIC OBSERVAION OF THERMAL CYCLING EFFECTS ON PHASE TRANSFORMATION IN Cu-Zn-Al SHAPE MEMORY ALLOY

1.IntroductionTheproperties0fshapememoryalloys(SMA)aredegradedt0differentdegreesbyage-ing,thermalcycling,stresscycling0rprestrain.Thedegradation0fshapememoryeffecthasbeenstudied[1-6]ands0mepr0blemsaren0wunderstoodandthishasallowedsomedevel0pmentsinthepracticalusage0fCu-Zn-AlSMA.H0wever,theunstabletransf0rma-tiontemperaturesandthestabilization0fthermalmartensitesduringthermalcyclinghaveseriouslyrestraineditsprogressinusageandithasbec0meurgenttos0lvetheremainingproblems-Frommanyrecentstudies…     

A NEW MORPHOLOGY OF MARTENSITE

A new “mid-relief” plate martensite was observed together with lenticular plate and thin plate martensites in Fe-29.37Ni-0.34C alloy after twice stepped cooling:-100/-120℃, -120/-140℃,-155/-165℃ and -165/-196℃,by means of colour metallography and TEM.The morphology of martensites changes with decreasing M_s temperature.The “mid-relief” plate martensite is characterized by the intensive mid-relie and the substructures of symmetric double twins or half twins and half dislocations,the formation of this martensite can be interpreted by a stress-induced co-generated mechanism.