Formation of residual stress in steels and titanium alloys during ion nitriding

An X-ray diffraction method is developed for determining residual stress in the surface layers of metal samples characterized by a variable chemical composition across the layer depth. The method includes the determination of interplanar spacings for different diffraction reflections and the separation of the contributions to the interplanar spacing from elastic stresses and a solid-solution composition. The separation is based on the elastic anisotropies of the crystal lattices of metals with cubic and hexagonal structures. The efficiency of this method is demonstrated for EI961 and 12Kh18N10T steels and PT7M titanium alloy upon ion nitriding.     

Dispersion strengthening austenitic stainless steels by nitriding

The internal nitridation of thin sections of austenitic Fe−Cr−Ni−Ti alloys containing up to 2 pct Ti was studied over the temperature range 1600° to 2210°F in order to develop a method of strengthening the alloys through the introduction of a dispersoid of stable titanium nitrides. The interparticle spacing (IPS) of the nitrides was found to increase linearly with depth from the external surface; the effects of various parameters on the rate of change of IPS vs depth are presented. The mechanical properties of these alloys at room and elevated temperatures were markedly improved by internally nitriding. Useful mechanical properties were obtained up to 2200°F, with typical properties at 2000°F of 10 to 20 ksi 0.2 pct offset yield strength and 15 to 25 ksi ultimate tensile strength, but section thickness was limited to about 10 mils because of the increase in IPS with depth and the long nitriding times needed for thicker material. In order to produce a small interparticle spacing in a heavier section, internally nitrided 5 mil strip was consolidated by hot roll bonding and evaluated at a 60 mil thickness by tensile and rupture testing at 2000°F. It is demonstrated that the approach taken in this work offers a feasible technique for making a high temperature alloy having useful engineering properties.     

Simulation of alloying-element diffusion during electrodiffusion heat treatment of steels

The effect of an electric current on the diffusion of alloy-forming components and vacancies in alloy steels subjected to electrodiffusion heat treatment has been studied. The diffusion coefficients of chromium, silicon, and aluminum are shown to increase by one to three orders of magnitude as compared to the reported data.     

Residual stresses in ground steels

A new X-ray method for the evaluation of three dimensional (residual) stress states is demonstrated by studies of the effect of grinding on Armco iron and a medium carbon steel. Although the penetration depth of the Cr-radiation employed in this study is only 5 μm, there is evidence of residual stresses normal to the surface (normal and shear components). In the past it has been assumed that these stress components can be neglected. Shear stresses normal to the surface are small in Armco iron, but significant (± 60 MPa) in steel. From the sign of the shears, the direction of final grinding can be determined. Cooling decreases the tensile stresses parallel to the surface in steel; surprisingly, the opposite result is found in Armco iron.     

Gas nitriding of high vanadium steels—experiments and simulations

Four experimental high vanadium alloys were gas nitrided in an ammonia-nitrogen atmosphere. The microstructure and concentration gradients have been investigated by means of several techniques. The nitriding process has been tentatively simulated using the DICTRA software. A precise process simulation does not seem possible at present; the reason for this is discussed. Instead, bounds for the carbon and nitrogen concentration profiles were obtained by applying different simulation conditions.     

Effect of low-pressure carburizing and plasma nitriding on mechanical properties and fatigue endurance limits of low alloy sintered steels

ABSTRACT

Thermochemical treatments like plasma nitriding or surface carburizing are commonly used to enhance surface hardness of steel components. An important difference between these treatments is the temperature at which they are carried out. In the present paper, the surface carburizing was carried out following a recently reported non-isothermal low pressure carburizing (LPC) treatment. In order to gain a comparative view of the effect of different treatments on the microstructure, microhardness, fatigue and impact properties, materials with distinct hardenability and widely used in the industrial production were evaluated. Tests were also carried out using industrially processed components aimed to an application demanding high wear resistance. The microstructural evolution during case hardening was studied by optical and electron microscopy.     

Solidification of high-speed tool steels

Gradient solidification and differential thermal analysis (DTA) experiments were used to study the process of solidification and the solidification microstructure of 11 alloys comprising the composition range of customary commercial high-speed steels (with the exception of cobalt-alloyed grades). Also included are a number of experimental high-speed steels alloyed with niobium. The results include the effects of alloy composition and cooling rate on the width of the solidification interval and on the sequence of the solidification reactions; the types of eutectics formed (austenite with M₆C, M₂C, or MC) and their volume fractions; the chemical compositions of the ledeburitic and primary carbides; and the relation between the chemistry of the carbides and that of the melt. Special attention is given to the formation and composition of heterogeneously nucleated primary MC particles and to the chemistry and stability of eutectic M₂C, which is important as a precursor to MC and M₆C in the microstructure of finished (hot-worked and heat-treated) material.     

后期渗氮工艺制备取向硅钢的组织转变

后期渗氮工艺是一种低温板坯加热生产技术,广泛应用于取向硅钢生产企业。通过金相＋ODF织构分析对工艺各阶段组织进行研究。实验结果表明：热轧及常化织构板沿板厚方向分布不均匀,{110}＜001＞织构在距热轧板表面1/4厚度处分布最强;冷轧板织构主要是α线和γ线织构;脱碳退火后α线织构大量减弱,{111}＜112＞织构得到加强,而渗氮后织构变化不明显。二次再结晶组织大晶粒内部出现少量小尺寸的孤岛小晶粒,{001}＜100＞位向孤岛小晶粒是 Goss晶粒无法吞并的。     

Grain size of high-speed tool steels

Influence of the carbide population on austenite formation during hardening of high speed tool steels has been investigated. It was established that austenite grain size varies directly with the mean primary carbide size or interparticle spacing. The type of annealing treatment—temper annealing or transformation annealing—given prior to hardening has an additional effect on the subsequent austenite grain size. This effect is related to the characteristics of the secondary carbide population. Austenite grain refinement was found to significantly improve the performance of high speed steel tools operating under intermittent cutting conditions. The observed tool wear during intermittent cutting is explained in terms of a combination of mild wear and “microspalling” mechanism at the cutting edge.     

Diffusion processes during plasma spraying of nickel-chromium coatings on powder steels

Translated from Poroshkovaya Metallurgiya, No. 5(341), pp. 33–37, May, 1991.     

Maximum plastic strengthening in tool steels

A method is proposed for calculating the maximum strengthening in tool steels in plastic shaping. The method is based on the permissible deformational damage of the material. In the proposed approach, the influence of the stress state is taken into account in calculating the plastic strengthening and deformational damage of the material. The relation between the deformational damage and plastic strengthening is studied. The influence of plastic strengthening on the mechanical properties of tool studies is established.     

合金工具钢标准探讨

据生产实践,对合金工具钢标准进行了分析和探讨,提出新标准中一些问题的同时,还提出些建议内容.     

New high-nitrogen corrosion-resistant tool and high-speed steels

The methods of nitriding of metals in traditional technological via ingot processes are briefly described, and their disadvantages are noted. The variations of nitriding powder metallurgy are given, and their advantages are discussed. The world experience in the production of nickel-free, corrosion-resistant steel with 1% N by powder metallurgy is analyzed, and experience in the production and operation of powdered high-nitrogen tool and high-speed steels is considered.     

Spray forming bypasses PM for tool steels

High quality tool-steel billets are the result when steel powders are compacted by spraying and then forged to eliminate porosity…     

Surface hardening of nickel alloys by means of plasma nitriding

Surface hardening of Ni alloys by plasma nitriding was investigated by using tentative Ni binary alloys contained nitride forming elements such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Mn, Fe, Al, or Si at the nitriding temperature from 673 to 1073 K. Surface hardness was different depending on the types of alloying elements, their contents, and their nitriding temperatures. Higher hardness than HV500 was obtained in Ti, V, Nb, and Cr containing alloys at 823 to 873 K, but other alloys showed lower surface hardness than HV400. The elements Ti, V, Nb, and Cr were the effective alloying elements for the surface hardening of nitrided Ni alloys. From transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis, the nitrided layer was composed of fine precipitate particles in the matrix of the nitrided layer. At the lower nitriding temperature, these particles were metastable fine particles or Ginier-Preston (GP) zone having coherency with the matrix, and these fine particles induced large microstrain in the matrix. However, at the higher nitriding temperature, equilibrium nitride particles were precipitated and coherency with the matrix was decreased. Therefore, the hardening of Ni alloys by plasma nitriding was due to the microstrain induced in the nitrided layer by the precipitation of metastable particles or GP.     

Selecting elements for diffusional metallization of tool steels

The influence of coating elements used in diffusional metallization on tool performance is analyzed. Recommendations are made for tools operating in different conditions and cutting different materials, with different lubricants.     

Simulation of diffusion controlled phase transformations in steels

Phase transformations in steels are currently treated with empirical approaches which make use of the vast amount of existing knowledge and experience. The aim of this paper is to show that more fundamental approaches are complementary, and mandatory in order to arrive at predictive capabilities. Such approaches have to take into account the local conditions on a microscopic level such as inhomogeneities, element distributions, driving forces, thermodynamic properties of the phases. With this sophistication it becomes possible to treat complex situations such as the long term stability of microstructures under service conditions, giving room for new optimizations.