Effects of Mn and Ti on microstructure and inclusions in weld metal of high strength low alloy steel

Abstract

The influences of alloying elements on chemical composition of non-metallic inclusions, impact toughness and microstructure in weld metals of high strength low alloy steels have been studied. Results indicated that microstructure had changed from a mixture of acicular ferrite, proeutectoid ferrite, ferrite side plates and microphases to a mixture of acicular ferrite, bainite and microphases due to the addition of Mn and Ti. The impact toughness of weld metal was improved correspondingly. The volume fraction and composition of inclusions both influenced the proportion of acicular ferrite. Mn and Si based oxide globular inclusions located at the boundary of acicular ferrite plates in the weld metal produced using C–Mn–Si–Cu wire. When Mn and Ti were added to welding wires, the inclusions within acicular ferrite plates permitted fewer primary acicular ferrite plates to grow into relatively larger dimensions. Secondary acicular ferrites nucleating on pre-existing ferrite plates refined microstructure effectively.     

高碳钢超塑性形变组织的研究

本文研究了高碳钢在893k 温度下超塑形变过程中组织结构的变化。尽管含碳1.2wt％的钢形变前的原始组织中存在一些片层状珠光体,但以0.31mm/min 的速度拉伸后仍获得了442％的延伸率。由于这些片层状珠光体在形变过程中迅速球化,因而对钢的超塑性能无明显妨害。在形变中,球状渗碳体发生彼此相连,含碳1.6wt％的钢中的渗碳体形成沿晶界的条状和网状,考虑到钢的室温力学性能,作者建议用于超塑成形的钢的含碳量选择在0.8～1.2wt％范围。     

45#钢高速冲击穿孔的显微组织

借助烧结的W合金圆柱体垂直冲击60mm厚的45^#热轧钢板实验,利用扫描电镜和光学金相显微镜研究了45^#钢高速冲击穿孔的显微组织。冲击波使穿孔周围的晶粒破碎,冲击引起的材料变形功转变为热能,以及侵彻过程中的摩擦作用,使穿孔表面熔化,靠近熔化区的晶粒发生再结晶。从穿孔表面到钢板内部可分为：熔化快凝层、再结晶细晶层、变形细晶层、形变层和正常基体组织。在细晶层和形变层中,铁素体晶粒的变形量要远远大于珠光体,部分珠光体开裂,其周围形成微裂纹和微孔洞。钢中的硫化锰夹杂在变形中被剧烈拉长。由于铁素体的塑性,钢板的破坏方式为延性扩孔。     

夹杂物和晶粒尺寸对洁净车轮钢室温冲击韧度的影响

对洁净车轮钢在不同温度下进行正火处理,得到不同晶粒尺寸的显微组织,然后对车轮钢进行室温冲击试验,利用扫描电镜对冲击试样断口形貌进行观察,研究了夹杂物和晶粒尺寸对洁净车轮钢室温冲击韧度的影响。结果表明：部分车轮钢冲击试样以夹杂物起裂,夹杂物类型为Ti（C,N）,另一部分冲击试样断口起裂源处则未发现有夹杂物,但尺寸在10μm以下的Ti（C,N）夹杂物对车轮钢的冲击韧度没有明显的影响,而晶粒尺寸对车轮钢的冲击韧度有明显影响;其主要原因是室温下车轮钢冲击断裂的临界事件是微裂纹穿过晶界扩展引发解理断裂,因此晶粒尺寸是决定洁净车轮钢冲击韧度的主要因素。     

高硅压铸铝合金拨叉断裂原因分析

目的 研究高硅压铸铝合金的金相组织对零件强度的影响。方法 采用金相显微镜和电镜对比,分析正常件和断裂件以及故障零件断口附近的组织形态。结果 在故障件中发现了大量的脆性相β-AlFeSi相,呈现出含有微裂纹的条状或块状。对于耐磨Si相,同样也观察到了对基体产生不利影响的微裂纹,这些裂纹是导致零件失效的主因。结论 采用高Si含量铝合金时,要严格控制Fe含量,过程监控脆性相的含量及形态,必要时可以采用晶粒细化剂,避免零件内部微裂纹的产生。     

X70管线钢焊缝金属弯曲脆断分析

用光学显微镜、扫描电镜和能谱仪分析了X70管线钢焊缝金属弯曲脆断的原因。结果表明，焊缝金属中存在的非金属夹杂物，在低应力作用下形成微裂纹成为裂源，加之焊缝组织中存在大量的M-A结构群，从而发生焊缝金属在室温下的解理断裂。     

Oxide formation during electric resistance welding of low carbon steels

The influence of Mn and Cr on the formation of surface oxides and their entrapment during electric resistance welding of Si containing low carbon steels was studied using thermodynamic calculations and experimental investigation of the microstructure of the different oxide inclusions within welds on electric resistance welding-fabricated pipes. The process of oxidation during the welding process, its dependence on oxygen content, temperature of oxidation and the composition of ferrite within the parent steel were analysed. It was shown that the most important influence on the type of oxide inclusions formed during welding is the Mn/Si ratio, while Cr has an indirect influence via carbide formation, which increases the relative content of Mn in ferrite.     

60Si2Mn弹条断裂失效分析

采用化学分析,断口扫描,硬度测定和金相检验方法,对60Si2Mn弹条在试用过程中出现的频繁断裂进行了分析。确认,弹条脆断的原因是由于热处理不当,使淬火组织过烧,造成弹条综合性能差而引起的。     

A Finite-Element Analysis on the Rheorolling Process of Semi-Solid Spring Steel

With a geometrical model of porous material, a 3D finite-element analysis on the rolling process of spring steel60Si2Mn in the semi-solid state is carried out using software MARC. In terms of flat and groove rolling conditions,stress field and strain fiel     

High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

The fatigue fracture behavior of four ultrahigh strength steels with different melting processes and therefore different inclusion sizes were studied by using a rotating bar two-point bending fatigue machine in the high-cycle regime up to 107 cycles of loading. The fracture surfaces were observed by field emission scanning electron microscopy （FESEM）. It was found that the size of inclusion has significant effect on the fatigue behavior. For AtSI 4340 steel in which the inclusion size is smaller than 5.5 μm, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 μm, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 pro. In the case of internal inclusion-induced fractures at cycles beyond about 1×10^6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the fish-eye and a granular bright facet （GBF） was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1×10^6. Neither fish-eye nor GBF was observed for Aermet 100 steel in the present study.     

Crystallography of brittle fracture and deformation twinning in ferritic steels

Abstract

The crystallography of brittle fracture and deformation twinning in ferritic steels is difficult to study experimentally, because of its three-dimensional aspects. The present paper reports the development of methodologies to study the phenomenon via customisation of various electron backscatter diffraction and SEM routes. It is shown that both direct (from the fracture surface) and indirect (from an adjacent polished side) measurements yield valuable information on crystallographic aspects of the fracture processes. Specifically, brittle fracture in three ferritic steels is studied: a C–Mn weld metal, a low alloy Mn–Mo–Ni steel similar to grade A533 and an ultralow carbon (0·002 wt-%C, 0·058 wt-%P) steel plate. The main conclusions resulting from development of the experimental techniques are that cleavage fracture occurs only on {001} planes, and that intergranular accommodation is present at the fracture surface. Further observations suggest that a cleavage side crack, initially deflected by a deformation twin, eventually blunts at the intersection of two deformation twins. This provides a mechanism for limiting the mean length of microcracks during brittle fracture.     

The determination of steel cleanliness in the as-cast steel ingot 26NiCrMoV145

The content and the type of non-metallic inclusions in the as-cast ingot for open-die forgings made from 26NiCrMoV145 steel were determined. The standard methods for the determination of non-metallic inclusions are suitable only for the wrought steels, so a modified standard method for the determination of the non-metallic inclusions in the as-cast steel was performed. Analyzed samples were taken from three different positions on the ingot. The determination of the type of the non-metallic inclusions was made by EDS analysis. The content of the non-metallic inclusions was determined using an optical microscope.     

Q345B中厚钢板分层缺陷的形成原因

Q345B中厚钢板在拉伸试验后断口出现分层现象，选择分层缺陷严重的部位，采用光学显微镜、扫描电镜、能谱仪等对其显微组织、缺陷形态和微区成分进行了分析。结果表明：Q345B钢板拉伸试样分层的主要原因是元素偏析；连铸时，硫、锰、铌和钛等元素在钢带心部偏析形成了硫化锰、碳化铌和碳化钛等夹杂物；轧制过程中，这些夹杂物使心部组织变形成条状或带状组织，从而引起了钢板的分层。     

高强度弹簧不锈钢的强化机理研究

本文在所开发的3YC系列高强度奥氏体弹簧不锈钢材料的研究基础上,进一步研究了材料在固溶处理、冷加工变形和时效强化等3个工艺过程中的组织与性能的关系,建立了材料强度与化学成份,冷变形量之间的定量关系。     

Fatigue studies on dual-phase low carbon steel

Low cycle fatigue studies have been carried out on 2 wt% Mn, 2 wt% Si and 0.1 wt% C steels with dual-phase and tempered martensitic structures. Fatigue crack initiation and propagation were investigated using scanning electron microscopy as well as optical microscopy. In addition, taper-section and cross-section techniques were also performed for more detail studies on the correlation of crack initiation with the internal microstructures of the testing samples. Internal microstructures were also investigated on the dual-phase steel sample before and after fatigue fracture by transmission electron microscopy.     

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part 1: influence of hydrogen trapped by inclusions

The fracture surfaces of specimens of a heat-treated hard steel, namely Cr–Mo steel SCM435, which failed in the regime of N = 10⁵ to 5 × 10⁸ cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat-treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.     

Slurry Preparation and Rolling of Semi-solid 60Si2Mn Spring Steel

The nondendritic semi-solid slurry preparation of 60Si2Mn spring steel has been studied in this paper. The experiments have shown that when stirred for 2 min on the test condition, the semi-solid slurry with 50%～60% fraction solid and spherical primary austenitic grains in the size of 100～300μm can be obtained and is easy to be discharged from the bottom little hole of the stirring chamber. The nondendritic slurry of 60Si2Mn spring steel can be rolled into given plate form successfully, but the solid phase and liquid phase is easy to be separated in rolling process so that the solid primary austenite is concentrated in the center and the liquid is near the edge of the rolled plate. The rupture strength and elongation of the plate rolled only once with semi-solid slurry are lower than that of the traditionally repeated hot-rolled plate of 60Si2Mn spring steel.     

Effects of Mn on fracture behaviour of DO3 Fe3Al-based intermetallic alloy

The fracture behaviour of DO3 Fe3Al-based intermetallic alloys with and without Mn (1.5 at%) were investigated by tensile tests (TT), transmission electron microscope (TEM) and scanning electron microscope (SEM). The results show that the addition of Mn could improve mechanical properties of the alloy, including room temperature ductility and high temperature strength. DO3 Fe–28Al fractured in a transgranular cleavage mode at room temperature and it gradually changed to a ductile style with temperature increasing, whereas the sample with addition of Mn fractured mainly in a mixed intergranular–transgranular cleavage mode. Three major factors are considered to have effect of Mn on fracture behaviour of the alloys: reducing grain size, promoting slip and cross slip and enhancing cleavage strength. © 1998 Chapman & Hall     

原位观察稀土镧对低合金高强度钢焊接热影响区晶粒细化的影响

使用高温激光共聚焦扫描显微镜原位观察了含0.016％(质量分数,下同)的La和不含La的低合金高强钢模拟焊接热影响区中高温阶段奥氏体的长大和组织转变行为,并使用光学显微镜和电子扫描显微镜对比分析了添加0.016％的La对粗晶热影响区晶粒细化的影响.结果 表明:添加0.016％的La使低合金高强钢中的夹杂物由Al-Mg-...