Crack propagation of secondary hardened alloy steels in gaseous hydrogen atmosphere

Abstract

The crack propagation behaviour of secondary hardened alloy steels having various Ni contents and a 18%Ni maraging steel was studied using modified compact tension specimens under 98–784 kPa hydrogen gas pressure p_H2 . The effect of Ni content and retained austenite was examined by comparing oil quenched specimens with those cooled to liquid nitrogen temperature. It was estimated from the effect of p_H2 on the crack propagation rate da/dt that the permeation of hydrogen from the crack tip surface decreased with increasing Ni content in the order 6 or 9, 13, then 18%Ni. It was also estimated that the hydrogen induced embrittlement of grain boundaries decreased with increasing Ni content in the order 6, 9, then 13%Ni and that the embrittlement was greater for the steel containing 18%Ni than for the steel containing 13%Ni. The effect of retained austenite was expected to suppress not the permeation of hydrogen, but the embrittlement of grain boundaries.

MST/757     

Importance of interface chemistry in hipping diffusion bonding of precipitation hardened martensitic stainless steels

Abstract

The effect of processing route on the microstructure and properties of hot isostatically pressed diffusion bonds, manufactured from precipitation hardened martensitic stainless steels, is addressed in this paper. The quality of the diffusion bond was assessed using a range of analytical techniques and a programme of mechanical testing. It is shown that the microstructure and properties of the diffusion bond are strongly influenced by the prebonding processing route used in its manufacture. Using conventional encapsulation techniques the interface is contaminated by oxide particles, whose composition and morphology depends upon the precise chemistry of the steel. These particles prevent grain growth across the interface and frequently result in tensile fracture at the interface. Refinements to the prebonding processing route result in lower particle densities and grain growth across the interface. However, these measures result in the interface becoming weakened by the precipitation of copper at the interface.     

Carbide steels synthesized by metallothermy

We describe the technology of synthesis of materials based on combined (self-propagating high-temperature synthesis + metallothermy) processes. Carbide steels are synthesized by using the developed compositions of charges containing a binder in the form of high-speed steel and tungsten carbides. The characteristics of yield of carbide steel from the charge for micromelting as well as the chemical composition, microstructure, mechanical, and technological properties of the synthesized alloys are experimentally investigated. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 5, pp. 90–94, September–October, 2005.     

Simulation of dislocation glide in precipitation hardened materials

Precipitation hardening is a widely used method for increasing the critical resolved shear stress (CRSS) of a material. Our simulations offer a flexible means to calculate the CRSS as a function of many parameters involved, e.g. the average precipitate size. For this, one or more dislocations are simulated while gliding through obstacle fields of arbitrary type or spatial arrangement. The elastic self-interaction is fully allowed for. Unlike analytical approaches and simulations known from literature, our method covers both shearing and circumventing of obstacles in a single model. To start with, the obstacles used in this contribution were chosen to be spheres with a constant obstacle stress inside; the distribution of the radii and the spatial arrangement suit the case of Ostwald-ripened particles. This corresponds to the case of the nickel base superalloy NIMONIC PE16 where a dislocation has to create an antiphase boundary in order to shear the long range ordered precipitates. Typical examples for dislocation arrangements are presented, and the results for various obstacle concentrations and mean radii are compared with published results.     

Simulations of precipitation in ferritic steels

Abstract

A new technique based on Monte Carlo random sampling has been proposed to simulate the precipitation kinetics in alloys. The new approach employs time dependent nucleation and diffusion laws, considers both intergranular and intra-granular precipitation, and also combines precipitation kinetics with intergranular segregation. The simulation can be used not only to predict the average size of precipitate phase particles, but also to predict particle size distributions, volume fraction, and interparticle spacing. The new approach overcomes the shortcomings of earlier model calculations where only the average size of the precipitate phase is considered. In addition, the proposed simulation overcomes the difficulty of connecting Monte Carlo steps to real time using the Metropolis algorithm. The approach has been used to simulate M₂₃C₆ precipitation kinetics in a creep resistant steel, P92: the results are in good agreement with published experimental measurements, and the model is believed to be applicable to other types of precipitates in different alloys.     

Rolling contact fatigue behavior of sintered and hardened steels

Powder-metal-processed bearings and gears are finding increasing application because of their economical and technical advantages. The residual pores from the sintering operatives act as lubricant pockets and dampen sound and vibration. However, porosity also decreases the mechanical strength and reduces the life of components fabricated by powder processing relative to similar wrought components. The rolling contact fatigue behavior of sintered and heat treated steel rollers was investigated using a fatigue test machine designed and fabricated for that purpose. The powder-metal-processed and the wrought steel rollers that were tested had similar composition and hardness and were mated against wrought steel rollers of high hardness. The contact stress versus number of cycles to failure data showed that the wrought steel had a very high endurance limit under rolling contact fatigue compared to the sintered steels investigated. Rolling contact fatigue behavior was found to depend on the porosity present in the material. Large surface peeling failures and pitting type fatigue failures were observed in the sintered and hardened steels, while only pitting type failures were observed in the wrought steels     

Brittle-to-ductile transition in the fracture of steels hardened by thermomechanical treatment

This article describes the results of a comparative study of the brittle-to-ductile transition observed during the fracture of steel after quench-hardening and after thermomechanical treatment, with respect to the tempering and test temperatures. Substantial differences in the kinetics of the increase in ductility during the brittle-to-ductile transition were revealed by mechanical torsion tests and electron microscopic examination of fracture surfaces.     

Thermodynamic calculations for precipitation in maraging steels

Abstract

In the present work, thermodynamic calculations for several maraging systems have been carried out, and the results are compared with experimental data. The calculations were conducted using ThermoCalc. Excellent agreement is obtained between calculation and experimental measurements using mainly the atom probe. As a highlight, calculated equilibrium phases and their mole fractions in 1RK91 steel recently developed by Sandvik compare extremely well with atom probe microchemistry data, which showed the presence of copper rich particles, mixed Ni₃Al and Ni₃Ti, and molybdenum rich precipitates. Calculations also indicate the thermodynamic stability of μphase in the Fe–Ni–Mo and Fe–Ni–Co–Mo systems, Ni₃Al and Ni₃Ti in a chromium containing steel, and NiMn in a Fe–Ni–Mn system. However, it should be noted that thermodynamic calculations may only be used as a guideline for systems not in equilibrium.     

Dynamic phenomena in finish turning of hardened steels with cBN-based tools

The paper presents some findings of the investigation of finish turning of KhVG hardened steel (60–62 HRC) using a cutting tool with an insert made of a cubic boron nitride based composite (cBN-Si₃N₄ system). The influence of machining process variables on the cutting force components, vibrations, and machined surface roughness is clarified. The authors propose some practical recommendations of how to choose machining modes and conditions.     

Strengthening effects in precipitation and dispersion hardened powder metallurgy copper alloys

The hardening of copper and copper alloy matrix using powder metallurgy (PM) techniques and different ways for dispersoids formation, as well as analysis of their single and combined effects on the strength of obtained material at room and elevated temperatures, have been presented and discussed. Gas atomized Cu–3.8 wt.%Ti and Cu–0.6 wt.%Ti–2.5 wt.%TiB₂ (Cu–Ti–TiB₂) powders and mechanically alloyed powder Cu–4 wt.%TiB₂ were used as starting materials. The powders were consolidated by hot isostatic pressing (HIP) and hot pressing (HP). Optical, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX), as well as transmission electron microscope (TEM) were used for microstructure characterization of the compacts. High strengthening of the Cu–Ti compacts was achieved by thermal treatment (aging) as a consequence of the development of modular structure and precipitation of metastable Cu₄Ti_(m). Hardening in the Cu–Ti–TiB₂ compacts is due to simultaneous influence of the following factors: the development of modular structure, precipitation of metastable Cu₄Ti_(m), and the presence of TiB₂ dispersoid nanoparticles. In case of Cu–TiB₂ compacts, high starting values of hardness and hardness on the elevated temperatures result from the presence of finely distributed TiB₂ particles in copper matrix obtained by mechanical alloying. Cu–Ti–TiB₂ composite yields much higher hardness values compared with the binary Cu–Ti alloys, owing to primary TiB₂ dispersions formed during atomization. Separation of metastable Cu₄Ti precipitate and the presence of significantly finer TiB₂ particles in the copper matrix are the reason for higher hardness values at peak temperatures (400–500 °C) in multiple-hardened copper alloy compared to the dispersion-hardened.     

低碳钛、钒微合金钢中的相间析出

针对目前较少关注的Ti-V微合金体系,在Formaster—FII膨胀仪上进行奥氏体/铁素体两相区等温,在铁素体相变同时获得Ti、V复合相间析出。采用金相、透射电子显微镜对显微组织、尤其是相间析出形貌、及其与铁素体基体的位向关系进行了观察与分析。研究表明,650℃为静态等温相变的鼻温点,相间析出呈层状分布,具有平面和曲面两种形貌,相间析出碳化物与铁素体基体之间存在Baker-Nutting取向关系。     

Aluminium nitride precipitation in multiple microalloyed pipeline steels

Abstract

The precipitation of aluminium nitride has been studied in pipeline steels containing aluminium, niobium, nitrogen, titanium, and vanadium. It was found that the titanium to nitrogen ratio had a dominant influence on the formation of aluminium nitride as reflected in the high value of aluminium in these precipitates when the ratio was low. Thermomechanical deformation of the steels appeared to promote aluminium nitride precipitation which occurred preferentially in the coarser particles. The precipitates of multiple microalloyed steels were found to be complex and their occurrence was interpreted in terms of isomorphology and miscibility. The overall steel analyses have been utilised in charting the sequence of precipitation.

MST/793     

Strain aging and carbide precipitation in aluminium-killed steels

Abstract

The effect of 0·003–0·050 wt-%C additions on strain aging and carbide precipitation in batch-annealed aluminium–killed steels has been investigated. The low–carbon steels exhibited unusual strain–aging characteristics: steels containing 0·020–0·040 wt–%C strain aged at 160 but not at 50°C, whereas those containing 0.007–0·020 wt–%C aged at both temperatures. This behaviour is due to the presence of fine carbide particles, which form as a result of the supersaturation of carbon in the matrix during cooling and which raise the carbon solubility by the Thomson-Freundlich effect. Expressions are derived which give the carbon supersaturation as a function of temperature during cooling, and of the precipitation temperature of each class of carbide. Carbide precipitation alternates between the nucleation of new particles and the growth of existing ones, depending upon the particle spacing and the cooling rate. Strain aging in the 0·05 wt–%C steel is negligible up to 225°C, but then rises to a peak at 325°C, which has been attributed to effects of partition in the dislocation matrix.

MST/141     

TaC precipitation behaviors in reduced activation martensitic steels

The precipitation behaviors of TaC in reduced activation steels are investigated by the means of experimental and theoretical model. A continuous model based on the Langer–Schwartz theory for nucleation, growth, and coarsening was developed for both homogeneous and heterogeneous precipitation. Applying this model, an extensive data set (the precipitation size and density) including experimental and reported results has been well described. The effects of the thermodynamics parameters on the microstructure were discussed. TaC showed a low coarsening rate during tempering as the solubility of tantalum in the ferrite matrix is quite small with a value of 10⁻⁵ at 1023 K.     

Carbide precipitation in certain alloys of the Co-Ni-Cr-C system

An investigation is reported of structural and hardness changes during the precipitation of Cr₂₃C₆ from f c c supersaturated solid solution in various alloys of the Co-Ni-Cr-C system containing 25 wt % Cr and 0.15 to 0.25 wt % C. The work extends results previously reported on a Co-25.3 wt % Cr-0.26 wt % C alloy, by studying the effect of adding nickel to replace cobalt (either wholly or completely) and hence changing the stacking fault energy. The addition of 10% nickel was found to increase the nucleation rate of matrix precipitation; precipitate particles also nucleated on partial dislocations with associated stacking fault formation. With approximately 55% nickel and 0.15% carbon, there was little matrix precipitation; instead precipitation occurred predominantly on dislocations, as rods growing along 1 1 0 directions; the main climb morphology was the formation of dislocation dipoles. Substantial precipitation hardening effects were obtained, particularly in the cobalt-rich alloys with 0.25% carbon. In a ternary Ni-25 wt % Cr-0.25 wt % C alloy, a non-uniform precipitate dispersion formed involving nucleation in the matrix and on dislocations. An investigation was also made of the effect of prior strain (1 to 10%) on the ageing of the Co-25.3 wt % Cr-0.26 wt % C alloy. Deformation of the f c c solution-treated material produced faulting and also the formation of h c p phase. On ageing, Cr₂₃C₆ precipitation occurred within the lamellae; at long ageing times after 10% deformation carbide particle coarsening, and matrix recrystallization occurred concurrently, and the recrystallization was accompanied by the transformation of the f c c matrix to h c p phase.