Low temperature nitriding of medium carbon steel

The authors present the results of nitriding of medium carbon steel at a low temperature of 573 K by using the surface-alternating current nanocrystalline treatment (SACNT), which is much lower than conventional nitriding temperature (about 773 K). The SACNT induces electrovalent bond to part by means of the iron ion transgressing, and the surface evolves to ultrafine grain layer. The nanostructured surface layer enhances the nitriding kinetics of pre-treated medium carbon steel. The samples were characterized by metallographic testing (scanning electron microscopy (SEM) and optical microscope (OM)), microhardness tester and X-ray diffraction.     

Microstructure of tool steel after low temperature ion nitriding

Abstract

The microstructural development in H13 tool steel upon nitriding by an ion beam process was investigated. The nitriding experiments were performed at a relatively low temperature of ～400°C and at constant ion beam energy (400 eV) of different doses in a high vacuum preparation chamber; the ion source was fed with high purity nitrogen gas. The specimens were characterised by X-ray photoelectron spectroscopy, electron probe microanalysis, scanning and transmission electron microscopy, and grazing incidence and Bragg–Brentano X-ray diffractometry. In particular, the influence of the nitrogen surface concentration on the development of the nitrogen concentration depth profile and the possible precipitation of alloying element nitrides were discussed.     

The effect of low temperature annealing on the mechanical behavior of cold rolled dual-phase Twinning-Induced Plasticity steel

In recent years, Twinning-Induced Plasticity (TWIP) steels with high specific strength have been developed to mainly address the unsaturated demands of transportation industries for weight reduction. To achieve the exclusive mechanical properties of TWIP steels, the understanding of their thermomechanical processing (TMP) behavior is highly necessitated. In the present work, the influence of cold rolling and post-annealing treatments on the mechanical behavior of a new dual phase (γ + α) TWIP steel have been studied. The microstructural studies indicated the presence of deformation twins in the deformed state of material. Annealing the as-rolled experimental alloy could result in the formation of Widmanstätten austenite within the ferrite grains at 500 °C. The nearly constant yield stress at high annealing durations was attributed to the opposite effects of recovery and Widmanstätten austenite formation.     

Enhancement of paint adhesion to cold rolled steel in salt bath by low temperature Ar-H2 plasmas

This study investigates the enhanced adhesion of paint to cold rolled steel (CRS) in salt bath by Ar-H₂ plasmas. It is found that the surface treatment on CRS, prior to painting, by low temperature plasma cleaning with argon-hydrogen plasmas at room temperature (23 °C), can be used for improving the adhesion of paint to CRS in salt bath. The tape test (ASTM 3359 Method) demonstrated this improvement, with a rating of “0” for untreated CRS for aging at a 3.5 wt% NaCl solution for 6 days and “5” for argon-hydrogen plasma-cleaned CRS at certain plasma conditions even for aging at a 3.5 wt% NaCl solution for 6 days. The adhesion performance of paint to CRS in salt bath is highly dependent on the surface characteristics of the CRS and the work of adhesion of paint to CRS.     

Recrystallisation textures in cold rolled low carbon steel containing ferritic and bainitic microstructures

Abstract

Low carbon steel strip was heat treated to generate four different starting microstructures (fine and coarse polygonal ferrite, acicular ferrite and bainite) for investigating their influence on texture development during cold rolling and annealing. The starting materials were cold rolled to 50–90% reduction and annealed for various times in the temperature range 853–953 K. The resultant microstructures and textures were examined mainly by electron backscatter diffraction and X-ray diffraction. The initial microstructure strongly influenced the crystallographic rotation paths during cold rolling, whereby high strain deformation generated strong {223}〈110〉 texture components in the polygonal ferritic microstructures, whereas a strong {001}〈110〉 texture was produced in the acicular/bainitic microstructures. Subsequent annealing generated, to varying degrees, the classic {111}〈uvw〉 (γ-fibre) recrystallisation texture in all materials. Unexpectedly, coarse polygonal ferrite produced the strongest γ-fibre recrystallisation texture after 70–90% cold rolling reduction. Based on arguments involving the effect of carbon in solution, initial grain size and deformation textures on recrystallisation texture development, it was shown that a strong γ-fibre texture can indeed be generated in coarse polygonal ferrite.     

The effect of temperature on plasma nitriding behaviour of DIN 1.6959 low alloy steel

A series of experiments have been conducted on DIN 1.6959 low-alloy steel using a 5 kVA DC plasma nitriding apparatus with the aim of elucidating the role of treatment temperature in plasma nitriding process. Treatments were carried out in 75%N₂-25%H₂ atmosphere of 4 mbar for 5 h at temperatures ranging from 350 to 550 °C. Optical microscopy, scanning electron microscopy, X-ray diffraction, along with surface roughness and microhardness measurements were utilized to characterize the treated samples. The depth, microstructure, hardness profile and phase constituents of the nitrided layers as well as the surface roughness of the samples were assessed as a function of treatment temperature. The results suggested that the compound layers were mostly dual phase consisting of gamma prime and epsilon iron nitride phases. Increasing treatment temperature increases compound layer and diffusion layer thicknesses. However, maximum surface hardness and roughness were found on the samples treated at 500 and 550 °C, respectively.     

Dynamic recovery and recrystallization in a cold rolled ultra low carbon steel plate: a neutron and X-ray diffraction study

The cold rolling of ultra low carbon steel is a complex process involving the formation of texture and the development of residual stresses. The mechanism for both texture and residual stress development is dislocation multiplication. Both texture and residual stresses can be determined by means of neutron and X-ray diffraction. In this paper, results of such measurements on a cold rolled plate (final thickness 10 mm) are presented along with theoretical predictions based on finite element calculations of the residual stress state. The results show that during cold rolling not only the formation and multiplication of dislocations play a role. It is seen that besides the creation of dislocations, two dislocation annihilation processes are involved, namely recovery and recrystallization. These happen despite the fact that the cold rolling takes place at relatively low temperatures (20相似文献   

不锈钢低温盐浴表面氮化技术发展与现状

不锈钢具有良好的耐蚀性、韧性和可加工性,但其硬度低、耐磨性差,严重制约了其在有磨损腐蚀要求环境体系中的应用.为了提高不锈钢的抗磨损性能,通过低温活性盐浴表面处理技术提高其耐磨性的同时不降低其耐蚀性,使得其在具有磨损的腐蚀介质环境中能够提高其抗磨损性能而不被腐蚀,可延长其使用寿命.本文系统地介绍了低温活性盐浴表面氮化处理技术的发展、原理及应用和处理后表面氮化层的特点、组织和表面性能.低温活性盐浴表面处理后,表层活性N原子扩散在奥氏体间隙中形成过饱和固溶体的S-相,产生特殊的位错-应力FCC结构,使得奥氏体晶格发生严重变形,不但提高了其腐蚀性,同时大大提高了其硬度,使得不锈钢硬度能够达到1 000 HV0.1,其抗磨损性能亦得到了显著提高.     

Microstructure and mechanical properties of the modified layer obtained by low temperature plasma nitriding of nanocrystallized 18Ni maraging steel

In the present study, low temperature plasma nitriding of nanocrystallized 18Ni maraging steel has been carried out at 360 °C from 1 to 24 h in a mixed gas of 25%N₂ + 75%H₂. The surface phase constitutions and microstructures of the nitrided layer have been investigated by X-ray diffraction analysis, transmission electron microscopy and optical microscopy. Nanoindentation and microhardness tests have been performed to determine the surface hardness and the hardness profile in the nitrided layer. The plasticity of the nitrided surface has been analyzed based on the nanoindentation results. The results show that at the initial stage of nitriding, the surface phase consists of a solid solution of nitrogen in α-Fe, and nanoscale nitrides and aging phase are formed with increasing of treatment time. The surfaces nitrided for 8 and 16 h possess the highest hardness. The plasticity factor calculations suggest that the nitrided surfaces have a good wear resistance and possess excellent plasticity.     

Effect of treatment time on low temperature plasma nitriding of stainless steel by saddle field neutral fast atom beam source

Recent research carried out in laboratories showed that Saddle field neutral fast atom beam source is a promising method for nitriding of stainless steel. In the present work, the effect of treatment time on the microstructural and mechanical properties of plasma-nitrided stainless steel sample was investigated by this new method. Plasma nitriding was carried out at 420 °C and at a pressure of 0.1 Pa for a time range of 1 to 12 h. SEM-EDX, microhardness tests, optical microscopy and X-ray diffraction (XRD) were used to evaluate the mechanical and structural properties of the nitrided layer. It was found that nitriding time has a pronounced effect on the structural and mechanical properties of low-temperature plasma-nitrided samples and produced a precipitation-free thin hard nitrided layer within a short processing time.     

冷轧带肋钢筋应力松弛试验机的研制

针对冷轧带肋钢筋材料在高应力(高于松弛下限)作用下会产生流变(晶格变形、滑移),应力将随时间衰减的特性,研制该电子应力松弛试验机。该机采用MaxTest标准化模块软件系统进行应力、变形和位移闭环的自动控制,同时在结构设计上通过使用三爪夹具保证防滑,通过结构力学的核算来保证其刚度,从而具有防止试样打滑、精确地负荷和形变控制、连续并适时地绘制松驰力-试验时间曲线的特点,并设计在意外因素(停电、断电)下继续进行试验的措施,充分满足标准要求和实际工作需要。     

Development of carbon—Low alloy steel grades for low temperature applications

Low alloy steels are processed to fulfill the requirements of low temperature applications. Besides the chemical composition, the steel should receive a suitable heat treatment to ensure the targeted mechanical properties at low temperature. In other words, the steels are designed to delay the ductile to brittle transition temperature to resist dynamic loading at subzero temperatures. Steel alloys processed for liquefied gas pipeline fittings are examples for applications that need deep subzero impact transition temperature (ITT).The main purpose of the present work was to find a suitable heat treatment sequence for alloys LC2 and LC2-1. Further, it aimed to correlate the impact toughness with the microstructure and the fracture surface at different sub-zero temperatures.The steels under investigation are carbon-low alloy grades alloyed with Ni, Cr and Mo. LC2 steel alloy has been successfully processed and then modified to LC2-1 alloy by addition of Cr and Mo. Oil quenching from 900 °C followed by tempering at 595 °C was used for toughness improvements. Hardness, tensile and impact tests at room temperature have been carried out. Further impact tests at subzero temperatures were conducted to characterize alloys behavior. Metallographic as well as SEM fractographic coupled with XRD qualitative analysis are also carried out.Non-homogenous martensite-ferrite cast structure in LC2 was altered to homogeneous tempered martensite structure using quenching-tempering treatment, which is leading to shift the ITT down to −73 °C. Addition of Cr and Mo creates a very fine martensitic structure in LC2-1 alloy. Quenching-tempering of LC2-1 accelerates ITT to −30 °C. It is expected that the steel was subjected to temper embrittlement as a result of phosphorus segregation on the grain boundary due to Cr and Mo alloying, as it was concluded in reference no. [6].     

Randomization of texture during recrystallization of austenite in a cold rolled metastable austenitic stainless steel

In the present investigation, attempt has been made to study the evolution of the texture during annealing at temperatures ranging from 600 to 1000 °C on a 95% cold rolled AISI 304L austenitic stainless steel. Major components are centered on Goss orientation and Cu component {1 1 2} 〈1 1 1〉 as well as the BR component {2 3 6} 〈3 8 5〉. With increase in annealing temperature the textural evolution shows emergence of weak texture. The evolution of texture can be correlated with the deformation texture through twin relationship.     

低成本热轧双相钢焊接性能研究

研究了DP590低成本热轧双相钢采用合适的焊丝、焊剂及合理的焊接工艺后,钢板焊接接头的拉伸性能、冲击韧性及硬度的变化情况。证明了实验钢具有优良的抗软化能力和冷弯成型性能且淬硬倾向不明显。实验表明,开发的DP590低成本热轧双相钢焊接性能优良,适于焊接制造汽车车轮、横梁、纵梁等构件。     

不锈钢母材及其焊缝金属材料单拉本构关系研究

对12个奥氏体型及12个双相型不锈钢正面角焊缝和侧面角焊缝连接试件进行了单调拉伸试验,考察了不同焊接工艺对角焊缝连接力学性能的影响。结果表明：采用氩弧焊焊接工艺的不锈钢角焊缝试件破坏面与电弧焊焊接工艺的试件破坏面形状相差较大,后者破坏面更加光滑;同时由于受到复杂应力的作用,正面角焊缝试件的真实破坏角度并不为相关规范规定的理论值45°;对于奥氏体型不锈钢角焊缝,氩弧焊试件与电弧焊试件的强度比分别为1.03 (正面角焊缝试件)及1.13 (侧面角焊缝试件),相对变形量之比为1.46及1.11;而对于双相型不锈钢角焊缝,两者的强度比分别为1.12和1.04,相对变形量之比为1.66及1.45;氩弧焊试件表现出了更好的力学性能。对于两种不锈钢材料,正面角焊缝强度均远大于侧面角焊缝的强度,建议在工程设计和相关规范的编制/修订中考虑正面角焊缝强度提高的影响。     

Effects of various nitriding parameters on active screen plasma nitriding behavior of a low-alloy steel

Active screen plasma nitriding (ASPN) is a novel surface modification technique that has many capabilities over the conventional DC plasma nitriding (CPN). In this study, 30CrNiMo8 low-alloy steel was active screen plasma-nitrided under various nitriding parameters such as active screen set-up parameters (different screen hole sizes, mesh sheet and plate top lids) and treatment temperature (520, 550 and 580 °C), in the gas mixture of 75% N₂+25% H₂ and chamber pressure of 500 Pa for 5 h. The properties of the nitrided specimens have been assessed by evaluating composition of phases, surface hardness, compound layer thickness and case depth using X-ray diffraction (XRD), microhardness measurements and scanning electron microscopy (SEM). It was found that the screen hole size and top lid type (mesh or plate) play an important role in transition of active species (nitrogen ions and neutrals) toward the sample surface, which in turn can affect the nitrided layer hardness and thickness. Treatment at higher temperature with bigger screen hole size resulted in a thicker compound layer and higher layer hardness. The compound layers developed on the samples treated under different conditions were dual phase consisting of γ′-Fe₄N and ε-Fe_2-3N phases.     

CSP热轧1.0mm超薄规格低碳钢板的组织及性能

采用光学显微镜、扫描电镜和拉伸实验等方法研究了CSP线轧制工艺与薄规格低碳钢板显微组织、力学性能特征之间的关系.显微组织观察和力学性能实测结果表明:CSP线生产的低碳钢连铸坯铸态组织比较均匀,靠近表面层的枝晶宽度与心部区域差别很小,皆为较细的树枝晶,枝晶宽度在几微米至30μm之间;成品1.0mm低碳钢薄板的组织很细,约为5μm,但轧向与横向组织中的铁素体晶粒形貌和尺寸存在差异;由于晶粒组织细小且钢中有害元素、夹杂物的含量低,故板的强度和延伸率高.     

SCP热轧1.0mm超薄规格低碳钢板的组织及性能

采用光学显微镜、扫描电镜和拉伸实验等方法研究了CSP线轧制工艺与薄规格低碳钢板显微组织、力学性能特征之间的关系。显微组织观察和力学性能实测结果表明：CSP线生产的低碳钢连铸坯铸态组织比较均匀，靠近表面层的枝晶宽度与心部区域差别很小，皆为较细的树枝晶，枝晶宽度在几微米至30μm之间；成品1.0mm低岩石钢玺板的组织很细，约为5μm，但轧向与横向组织中的铁素体晶粒形貌和尺寸存在差异；由于晶粒组织细小且钢中有害元素、夹杂物的含量低，故板的强度和延伸率高。     

Effect of NaBr on the corrosion of cold rolled steel in 1.0 M phosphoric acid

The inhibition of NaBr on the corrosion of cold rolled steel in 1.0 M phosphoric acid was studied by using weight loss method and polarization method. It was found that the adsorption of bromide ion can prevent steel from corrosion and the adsorption follows the Langmuir adsorption isotherm. Polarization studies showed that NaBr is a mixed-type inhibitor for steel corrosion in 1.0 M phosphoric acid. Thermodynamic parameters such as adsorption heat, adsorption entropy and adsorption free energy were obtained from the experimental data of temperature studies for the inhibition process at four temperatures ranging from 30 to 45°C. The kinetic data such as apparent activation energies and pre-exponential factors at different concentrations of the inhibitor were calculated, and the effects of the apparent activation energy and pre-exponential factor on the corrosion rate of cold rolled steel were discussed. The inhibitive action was satisfactorily explained by using thermodynamic and kinetic models. The results obtained from polarization experiment were in good agreement with those obtained from weight loss measurement.