Fluidized bed CrN coating formation on prenitrocarburized plain carbon steel

CrN coatings were formed on plain carbon steel by prenitrocarburizing, followed by thermoreactive deposition and diffusion (TRD) in a fluidized bed furnace at 570 °C. During TRD, Cr was transferred from Cr powder in the fluidized bed to the nitrocarburized substrates by gas-phase reactions initiated by reaction of HCl gas with the Cr. The microstructural processes occurring in the white layer, caused by N diffusion toward the surface during this stage were studied. This study compares TRD atmospheres employing inert gas and HCl or inert gas, H2, and HCl. Surface characterization was performed by scanning electron microscopy (SEM), x-ray diffraction (XRD), and glow-discharge optical-emission spectroscopy (GDOES).

Cr(N,C) diffusion coating formation on pre-nitrocarburised H13 tool steel

The microstructural processes of Cr(N,C) coating formation by thermoreactive deposition and diffusion (TRD) on pre-nitrocarburised H13 tool steel were studied. Both nitrocarburising and TRD were performed in fluidized bed furnaces at 570 °C. During TRD, chromium was transferred from chromium powder in the fluidized bed, to the nitrocarburised substrates by gas-phase reactions initiated by reaction of HCl gas with the chromium. Addition of 30% H2 to the input inert gas was found to increase the rate of coating formation, although hydrogen reduction resulted in rapid loss of nitrogen to the surface. The reason for the increased rate of coating formation could not be established without further investigation, although several possible explanations have been proposed. It was found that porosity and the formation of an iron nitride ‘cover layer’ during nitrocarburising were the biggest influences on the microstructure of the Cr(N,C) coating. Microstructural characterization of the coatings was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and glow discharge optical emission spectroscopy (GDOES).

Characterisation of galvanneal coatings on strip steel

Galvanneal is a form of zinc-coated sheet steel, where steel is dipped in molten zinc, and then heat treated in a furnace to produce a complex iron-zinc coating. Many industries, such as automotive, use galvanneal for components fabricated from sheet steel. The microstructural properties of galvanneal have a significant influence on how well the sheet metal changes shape on stamping. By means of optical microscopy, scanning electron microscopy, and glow-discharge optical emission spectrometry, we present a study of the microstructure of several galvanneal samples, both stamped and unformed, relating the phases and morphology of the coatings to performance in stamping operations. Samples of galvanneal were subjected to different heat-treatment temperatures. The frequency of defects in stamped components was found to be related to the average alloy content in the coatings, which varied with furnace temperature. An increased average iron content in the coatings was related to increased powdering defects in stamping operations that use galvanneal coated sheet steel.

Influence of the geometry of an immersed steel workpiece on mass transfer coefficient in a chemical heat treatment fluidised bed

The mass transfer during carburising in a fluidised bed and in a steel workpiece has been studied experimentally in this work. This involved carburising experiment in an electrically heated fluidised bed at 900–970°C with natural gas and air as the atmosphere. A steel workpiece was designed to provide a range of carbon transfer surfaces of different geometries in the fluidised bed, and the carbon transfer coefficient was measured at these surfaces. The carbon transfer coefficient was determined from the carbon distribution within the diffusion layer of the sample. An empirical relationship of the carbon potential as a function of carburising atmosphere, bed temperature and fluidising velocity was determined, based on the understanding of the mass transfer mechanism and analysis of the experimental results.

The static, dynamic and metadynamic recrystallisation of a medium carbon steel

Grain refinement during and after hot isothermal deformation of a medium carbon steel has been investigated. The average austenite grain size decreased with an increase in strain for the hot deformed and recrystallised material, with refinement extending beyond the strain for the peak stress. A window of strain that corresponds to transition from classical static to metadynamic recrystallisation was observed in respect to the recrystallised material. Within this post-dynamic transition window the strain at which strain independent softening occurs was different for different volume fractions of the recrystallised material. This led to a new terminology corresponding to initiation of strain independent softening. For the alloy of this study, strain independent softening for the start of post-deformation recrystallisation occurred near the strain to the peak stress. The strain corresponding to complete metadynamic recrystallisation, which was defined as when all levels of recrystallisation were strain independent, was much greater than the strain for the peak stress.

Recrystallization in 304 austenitic stainless steel

A 304 austenitic stainless steel was deformed using hot torsion to study the evolution of dynamic recrystallization (DRX). The initial nucleation of dynamically recrystallization occurred by the bulging of pre-existing high angle grain boundaries at a strain much lower than the peak strain. At the
peak stress, only a low fraction of the prior grain boundaries were covered with new DRX grains. Beyond the peak stress, new DRX grains formed layers near the initial DRX and a necklace structure was developed. Several different mechanisms appeared to be operative in the formation of new high angle boundaries and grains. The recrystallization behaviour after deformation showed a classic transition from strain dependent to strain independent softening. This occurred at a strain beyond the
peak, where the fraction of dynamic recrystallization was only 50%.

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

In the present study, wedge-shape samples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.

Grain size effect on the warm deformation behaviour of a Ti-IF steel

The effect of grain size on the warm deformation behaviour of a titanium stabilized interstitial free (IF) steel was investigated using hot torsion. The initial work hardening regime is followed by the development of a broad stress peak after which work softening occurs. The hypothetical saturation stress (Estrin–Mecking model) and the stress at final strain were relatively insensitive to grain size. However, the strain to the peak stress was strongly dependent on the grain size at low values of the Zener–Hollomon parameter. A simple phenomenological approach, using a combined Estrin–Mecking model and an Avrami type equation, was used to model the flow curves. The hypothetical saturation stress, the stress at final strain and the strain to peak stress were modelled using three different hyperbolic sine laws. A comparison with independent data from the literature shows that the apparent activation energy of deformation determined in this work (Q=372 kJ/mol) can be used to rationalize the steady-state stress in compression data found in the literature.

The static and metadynamic recrystallization behaviour of an X60 Nb Microalloyed Steel

A rapid method has been developed to determine recrystallization kinetics of Nb microalloyed steels by interrupted hot torsion test. The softening behaviour was achieved as a function of different processing parameters. The method clearly identified three regions, where the strain dependency of the recrystallization rate varied. Firstly, at large strains the rate of recrystallization was not a function of strain; this is generally ascribed to metadynamic recrystallization. At lower strains the time to 50% recrystallization showed a power low relationship with strain, characteristic of static recrystallization. A further break point exists on the time for 50% softening curve when strain induced precipitation occurs in the material. The onset of strain induced precipitation was at strains below the strain to the peak stress at temperatures below 900°C. The experimental results were used to estimate the time for 50% softening and to anticipate the onset of the strain induced precipitation for the alloy of this study. Grain refinement of the recrystallized austenite continued to strains significantly beyond the peak stress and beyond the static to metadynamic recrystallization rate transition.