Surface hardening of low-carbon martensitic steels by boronizing

A large group of low-carbon martensitic steels developed recently possesses some advantages that make them widely applicable in the industry. The main merit of these steels consists in the possibility of deformation-free quenching in air and welding in a nonheat-treated state. These features are accompanied by a deep hardenability and high strength characteristics of the products and semiproducts. However, when these steels are used to produce parts operating under the conditions of contact friction their surface hardness and wear resistance should be increased. Since such steels are relatively novel, the methods of their surface hardening are still being developed. The present work is devoted to determining the possibility of optimizing the process of diffusion boronizing of carbon martensitic steels. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 6–9, July, 1998.     

Homogenität und Korrosionsbeständigkeit hochlegierter Stähle

Homogeneity and corrosion resistance of high alloy steels A major number of case histories in the chemical industry are due to local corrosion the origin of which can be attributed to the inhomogeneity of the steels produced according to conventional melting process. Special processes such as electro slag remelting may give rise to a considerable increase in structural homogeneity of corrosion resistant alloys. Typical examples are increased resistance to nitric acid, Streicher's solution, seawater or reaction mixtures of urea synthesis. These results clearly demonstrate the superiority of the material which is largely free from inhomogeneities such as segregations which give rise to local corrosion phenomena.     

Slurry Erosion Studies on Surface Modified 13Cr-4Ni Steels: Effect of Angle of Impingement and Particle Size

Hydroturbine steels, such as 13Cr-4Ni martensitic steels, are generally subjected to heavy-erosive wear and loss of efficiency due to solid particulate entrainment in the water. Surface-modified steels have proven to give better performance in terms of erosive wear resistance. In the present study, an attempt is made to investigate the effect of angle of impingement and particle size on slurry-jet erosion behavior of pulsed plasma nitrided and laser hardened 13Cr-4Ni steels. Laser hardening process has shown good performance at all angles of impingement due to martensitic transformation of retained austenite. Plastic deformation mode of material removal was also an evident feature of all laser-hardened surface damage locations. However, pulsed-plasma nitrided steels have exhibited chip formation and micro-cutting mode of erosive wear. Erosion with 150-300 μm size was twice compared to 150 μm size slurry particulates.     

Study of hyper-interfacial bonding of ultrafine-grained steels

To obtain so-called super-steels providing a good combination of high strength and high toughness, efforts are being made to develop ultrafine-grained steels involving intense grain size refinement with retention of a mild steel-equivalent low-alloy composition in order to lessen their environmental impact and reduce total life cycle costs.^1-5 Within this context, it is imperative to develop suitable welding and joining technologies for structural applications of ultrafine-grained steels; a number of previous studies have addressed their welding characteristics.^6-8 Developed ultrafine-grained steels, however, face a number of important problems such as grain coarsening, HAZ softening and microstructural changes due to the thermal effect of the welding process itself. Such microstructural changes are predicted to exert a significant effect on joint characteristics, particularly deterioration of welded joint performance in ultrafinegrained steels as steels subjected to a high degree of grain size control. Microstructural changes in the joints of ultrafine-grained steels, however, have not yet been satisfactorily investigated.     

The effect of thermal cycles and welding technology on the structure and mechanical properties of joints made in HCM12 and HCM12A steels

Abstract

Previously developed and practically employed low-carbon low-alloy high-strength steels for welded structures are broadly classified into: 1) steels containing few alloying elements and manufactured with an as-hot-rolled or normalised structure (such as eg SM steels); 2) thermomechanically processed steels with a reduced content of alloying elements (such as eg TMCP steels); 3) steels strengthened by addition of alloying elements and conditioning treatment (eg high-strength low-carbon low-alloy steels such as HT780 steel). Previously used 800 MPa class steels for welded structures, while manufactured by the technique noted in (3), face problems such as weldability, cost, recyclability, etc. To overcome these problems, research has been increasingly focused on the development of 800 MPa class high-strength steels for welded structures with mild steel-equivalent contents of alloying elements.^1–3 The basic concept underlying high strengthening involves the ferrite grains being ultra-refined to a grain size of less than 1 ìm in accordance with the Hall–Petch rule.^4–8     

A new interpretation of the corrosion loss processes for weathering steels in marine atmospheres

Most available data sets for the long-term corrosion loss of various grades of weathering steel exposed to marine atmospheric environments are demonstrated to be consistent with the multi-phase corrosion model previously proposed for steels exposed to marine environments. This means that the early corrosion of weathering steels by oxidation is gradually inhibited by the build-up of corrosion products. These produce anoxic and sub-oxic conditions that may permit microbiological activity to govern the longer-term corrosion loss process. This new interpretation for the long-term corrosion of weathering steels may have implications for the design of such steels.     

A damage model for assessing pipeline safety in corrosion environments

Considering such practical pipeline service conditions in the petroleum transportation field as corrosion from harmful media (H₂S, CO₂ etc.) and load from the pressure of flowing petroleum as key factors, the degradation of the mechanical properties of pipeline steels and the corresponding variations in stress states are studied. A damage variable is introduced to reflect the degradation in the mechanical properties of the pipeline due to corrosion. A diluted solution approach is proposed to model the corrosion process of harmful media within the pipeline and the distribution of the damage variable along the radial direction of the pipeline is derived. An approach to the safety evaluation of damaged pipeline is proposed by comparing the instant properties of pipeline steels with servicing conditions.     

Defect-free joining of zinc-coated steels by bifocal hybrid laser welding

Laser welding of zinc-coated steels in an overlap setup is prone to weld defects and seam expulsion reducing in particular the properties in mechanical loading and in general the deployability of such weldments in industry. Several laser welding process technologies failed to created defect-free welds in zinc-coated steels. This paper renders the welding of zinc-coated steels by the novel technology of bifocal hybrid laser welding. The zinc-coated steels under consideration are DX56D + Z, DC04 + ZE, and HXT700D. The bifocal hybrid laser system is realised by combining an Nd:YAG laser with a high power diode laser, both of 3 kW maximum output power. The beam parameter product (BPP) of the employed Nd:YAG laser of 25 mm mrad translates with an optical system of focal length f = 150 mm into a circular focus of diameter 0.45 mm, whereas the BPP of the HPDL of 85 mm × 200 mm mrad can achieve a rectangular focus of 0.9 mm × 3.7 mm. The optical system allows the respective focal plane and relative position of the foci to be independently vertically and horizontally positioned. This paper presents research into the causes of instabilities in laser welding of zinc-coated steels. Experimental evidence is considered and presented to establish the need for an empirical process model for stable laser welding of zinc-coated steels. The increase of process robustness is discussed.     

Copper brazing response of some advanced high strength steel grades

Sheet steels are commonly used in automotive powertrain applications wherein formed and assembled components undergo a copper brazing operation as part of the manufacturing process. For high performance applications including transmission pump housings and turbines, specially processed microalloyed steels have been developed and specified in order to develop the desired mechanical properties after exposure to brazing temperatures of ～1100 °C. New families of advanced high strength steel (AHSS) sheet are currently being developed and implemented. These are typified by increased alloy levels and exhibit unique property combinations that are particularly suited to auto body applications requiring the dual characteristics of high formability and high strength. Here it is considered that such grades may also be suited to powertrain components requiring high initial formability coupled with high final strength after copper brazing. That is, transformation strengthening during cooling after brazing is envisioned as a potential strengthening mechanism available with these new steels. In this work, initial screening results are reported to elucidate the response of some experimental AHSS to a simulated copper brazing operation during which the steels are taken into the austenitic temperature range and subsequently cooled at moderate rates to ambient conditions.     

Simulation of κ-Carbide Precipitation Kinetics in Aged Low-Density Fe–Mn–Al–C Steels and Its Effects on Strengthening

Fe–Al–Mn–C alloy systems are low-density austenite-based steels that show excellent mechanical properties. After aging such steels at adequate temperatures for adequate time, nano-scale precipitates such as κ-carbide form, which have profound effects on the mechanical properties. Therefore, it is important to predict the amount and size of the generated κ-carbide precipitates in order to control the mechanical properties of low-density steels. In this study, the microstructure and mechanical properties of aged low-density austenitic steel were characterized. Thermo-kinetic simulations of the aging process were used to predict the size and phase fraction of κ-carbide after different aging periods, and these results were validated by comparison with experimental data derived from dark-field transmission electron microscopy images. Based on these results, models for precipitation strengthening based on different mechanisms were assessed. The measured increase in the strength of aged specimens was compared with that calculated from the models to determine the exact precipitation strengthening mechanism.     

Friction stir welding of duplex stainless steels

Duplex stainless steels are successful in a variety of applications such as the food industry, petrochemicals and plants for desalination of seawater, where high corrosion resistance and high mechanical strength are required. However, the beneficial microstructure may change during fusion welding steps, and it can compromise the performance of these materials. Friction stir welding is a solid-state process avoiding typical problems concerning solidification such as solidification cracks, liquation and segregation of alloying elements. Superduplex stainless steels can avoid unbalanced proportions of ferrite and austenite, formation of secondary deleterious phases and grain growth of ferrite in the heat-affected zone. Consolidated friction stir welded joints with full penetration 6 mm thick were obtained for UNS S32101 and S32205 duplex and S32750 and S32760 superduplex stainless steels. The friction stir welds were submitted to tensile tests indicating an improvement of strength in welded joints, showing increased yield and tensile strength for all studied cases. Regarding the microstructural characterization, an outstanding grain refinement was observed in the welded joint, achieving grain sizes as small as 1 μm. This refinement was associated with the combination of microstructural restoration mechanisms in the dual-phase microstructure promoted by severe deformation associated with a high temperature during the welding process.     

Martensite microstructure of 9–12%Cr steels weld metals

Several new grades of steel have been developed in recent years, which have been based on the high-strength 9–12%Cr steels. These steels were developed to meet the proposed demand by power generation companies to increase efficiency to operating at higher temperatures by and thus burning less fossil fuel, principally coal, and thereby reducing costs and also meeting the increasingly stringent environmental requirements.

The 9–12Cr steels used for high temperature components in power plants are generally required to possess good mechanical properties, corrosion resistance, creep strength and fabricability. Although such steels normally have a fully martensite microstructure, they are also susceptible to the formation of delta ferrite, mainly during the welding process. Delta ferrite has several detrimental effects on such properties as creep, ductility and toughness. Thus, it is important to avoid its formation.

In this study the formation of delta ferrite in the weld metal of high-strength 9–12%Cr steels has been analysed for several samples with variations of key alloying elements. The results indicate that the most effective way to avoid delta ferrite in the weld metal is to reduce the chromium equivalent value to as low a value as possible. A fully martensite microstructure is obtained when both the Schneider chromium equivalent is lower than 13.5 and the difference between the chromium and nickel equivalent is lower than 8.     

New aspects of the study of polymer quenchants

Conclusions Preferable for the tempering of steels with low hardenability (such as steel 45) are the polymeric media modifications A-25 and A-8, for the steels with moderate hardenability (for example, 40Kh) the UZSP-1 medium is better, for the steels with high hardenability (such as 34KhN3MFA) the modification A-23 is best.VNIITMASh Scientific-Production Organization, Volgograd. Yarsintez Scientific-Production Organization, Yarosiavl'. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 6–8, May, 1993.     

Characterization of the Trace Phosphorus Segregation and Mechanical Properties of Dual-Phase Steels

The segregation behavior of trace amount of phosphorus (P) and the mechanical properties of dual-phase (DP) steels have been systematically studied. The microstructure of DP steels is mainly composed of martensite, ferrite and nanoscale carbides. For the DP steels with different trace amounts of P (≤ 0.015 wt%), P has almost no effect on the mechanical properties. Atom probe technology (APT) analyses confirm that P segregation was only found at the precipitate/matrix interface. Moreover, the precipitates of (Ti, Mo) C are widely distributed in the ferrite, martensite and ferrite/martensite interface regions. The special segregation feature of P would not concentrate at specific regions such as ferrite/martensite interface and/or martensite lath interface, which reveals that trace amounts of P (≤ 0.015wt%) have almost no effect on the mechanical properties of DP steels. It is proved for the first time that the MC-type carbides of (Ti, Mo) C can reduce or eliminate the damage effect of P on the mechanical properties of steels, which provides a new way for the design of alloys to reduce P damage. This work will promote to increase the P content control standard in DP steels from 0.01 to 0.015 wt%, which will not change the mechanical properties, but greatly reduce the scrap rate and increase the energy efficiency of the manufacturing process.     

钢表面铸渗烧结涂层的研究

铸渗技术是提高钢铁材料耐磨性的一种有效表面防护技术,铸渗技术的研究与应用具有重要的理论意义和工业应用价值.概述了铸渗技术改善钢铁材料耐磨性的研究进展,综述了目前采用的铸渗材料和方法,如表面合金化、外加强化相表面材料复合、内生强化相表面材料复合(反应铸渗).铸渗工艺的发展历经普通铸造工艺、V法铸渗工艺、V-EPC铸渗工艺、离心铸渗工艺和SHS-casting工艺,指出了该项技术目前存在的问题,并提出了今后的研究发展方向.     

Cold-rolled,high-strength sheet steels for auto applications

Several groups of cold-rolled, high-strength sheet steels have been developed to optimize the required strength and formability levels for automotive applications. Multiphase steels offer new opportunities where high-strength levels are demanded. The future in steel development will be determined by the physical modeling of properties and by adapting new process routes such as thin slab casting and in-line rolling. In this article, developments in traditional strengthening concepts (e.g., microalloying and substitutional hardening with phosphorus) and more recently developed concepts (e.g., bake hardening and strengthening of interstitial-free steels) are reviewed.     

工模具钢的两相渗碳研究

虽然出现了多种多样的表面硬化技术,但渗碳仍然是生产上常用的表面化学热处理方法,它能有效地提高材料表面的耐磨性。而且生产工艺简单,成本低廉,在生产中一直广泛应用。该方法用于低碳钢或低碳合金钢的表面强化。已有相当成熟的工艺［１］。目前已逐渐用于共析钢和过共析钢［２］。由于这些材料本身含碳量很高,在渗碳温度下处于奥氏体和碳化物两相共存状态,所以把这种渗碳方法称之为两相渗碳。１　试验方法试验材料选用三种常见的工模具钢：热作模具钢３Ｃｒ２Ｗ８Ｖ、高速钢Ｗ１８Ｃｒ４Ｖ和Ｔ８钢。在自制的小型滴注式渗碳炉中渗碳…     

Modeling the Gas Nitriding Process of Low Alloy Steels

The effort to simulate the nitriding process has been ongoing for the last 20 years. Most of the work has been done to simulate the nitriding process of pure iron. In the present work a series of experiments have been done to understand the effects of the nitriding process parameters such as the nitriding potential, temperature, and time as well as surface condition on the gas nitriding process for the steels. The compound layer growth model has been developed to simulate the nitriding process of AISI 4140 steel. In this paper the fundamentals of the model are presented and discussed including the kinetics of compound layer growth and the determination of the nitrogen diffusivity in the diffusion zone. The excellent agreements have been achieved for both as-washed and pre-oxided nitrided AISI 4140 between the experimental data and simulation results. The nitrogen diffusivity in the diffusion zone is determined to be constant and only depends on the nitriding temperature, which is ~5 × 10^?9 cm²/s at 548 °C. It proves the concept of utilizing the compound layer growth model in other steels. The nitriding process of various steels can thus be modeled and predicted in the future.     

钢的氢脆的新研究方向

简要回顾了人们对钢的氢脆问题的认识过程和研究历史,概述了钢中氢的存在状态与氢脆的关系,钢的塑性变形与氢脆的关系,其中包括断口形貌和断裂过程。此外还论述了奥氏体不锈钢的氢脆及氢引起的断裂现象等共性问题。     

螺纹刀具的热处理

介绍了用不同钢制作的螺纹刀具的技术要求、热处理工艺,包括T12A、GCr15、9SiCr钢手用丝锥,高速钢机用丝锥,Cr12MoV钢滚丝轮,9SiCr、Cr12MoV、65Nb钢搓丝板,9SiCr钢园板牙等,以及热处理注意事项,如淬火晶粒度控制、原材料质量要求和淬火畸变控制等。