Reverse transformation mechanism of martensite to austenite and amount of retained austenite after reverse transformation in Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel

Abstract

The reverse transformation mechanism of martensite to austenite and the volume fraction of retained austenite have been studied in an Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel by means of dilatometry, transmission electron microscopy and X-ray diffraction. Below a heating rate of 10 K s^-1, the reverse transformation of α' to γ occurs by diffusion, whereas it occurs by a diffusionless shear mechanism above 10 K s^-1. After reversion treatment at low temperatures, filmlike retained austenite is observed along α' lath boundaries, while reversion treatment at high temperatures produces granular retained austenite inside the α' laths in addition to filmlike retained austenite. The volume fraction of retained austenite at room temperature increases with increasing reversion treatment temperature, exhibiting a maximum at ~625^° C, above which it decreases with increasing reversion temperature.     

Preferred orientations in cast metals

Abstract

The origin of the preferred orientations observed in the columnar grains of fcc and hcp metals under conditions of both planar and dendritic interface growth is considered. The preferred orientations are explained in terms of competitive growth of chill grains normal to the chill surface and by assuming that the chill grains grow in a dendritic manner.     

Investigation of a black oxide layer on the surface of pressure-diecasting dies

Detailed experiments have been carried out to study the effectiveness of two methods of generating a black oxide layer on a pressure-diecasting die in order to prevent soldering of molten aluminium. Three types of steel surface were examined: bare, black oxidised with oil and black oxidised using an acidic anti-solder grease.

As a result of experiments involving the pressure- diecasting of aluminium alloys, it was determined that the black oxidised surface obtained with the acidic anti-solder grease increased the yield of an insert by three times compared to a similar insert oxidised with oil, and by six times when compared with a bare steel insert.

This effect was confirmed in laboratory tests. The strength of the interface between aluminium alloy and a steel surface oxidised with an acidic anti-solder grease was six times lower than that of the interface between the aluminium alloy and bare steel. The oxide layer obtained with an acidic anti-solder grease is, therefore, seen to provide effective protection from soldering. This fact was also confirmed by a special laboratory test designed to determine the tendency of various aluminium alloys to solder to a die surface.

The treated surfaces of the die cavity were examined by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy. It was found that the blue-black oxide layer was twice as deep when oxidised using an acidic anti-solder grease than when oxidised in oil.     

Crack Initiation and Propagation in Spheroidal-graphite Cast Iron with Different Micromorphologies

The increasing adoption of spheroidal-graphite (s.-g.) cast irons for use in safety critical parts calls for a study of the materials' fracture mechanics. It is becoming necessary to analyse the reliability criteria of these materials on a more rigorous basis. This paper describes studies based upon qualitative and quantitative analysis of crack initiation and propagation under the action of fatigue and three-point bending loads on s.-g. cast irons with the same graphite nodularisation characteristics, but having matrices exhibiting different quantities of ferrite, pearlite and cellular-contour carbides.     

Effect of particle size variations of gypsum bonded investment powders on metallurgical quality of investment castings

Abstract

In the present work, the effect of particle size distribution parameters of investment powder on the metallurgical characteristics of castings produced by flask mould investment casting process was investigated. The main aim of this study is to establish the relationships between the casting quality and investment powder characteristics. For this purpose, models for tensile, density and surface roughness specimens were prepared in accordance with the standards and these models were moulded by gypsum bonded investment powders. Specimens of sterling silver (92·5Ag–7·5Cu alloy) were subsequently cast into these moulds by centrifugal casting process. After the completion of solidification process, the specimens were tested and results were analysed. The preliminary results illustrate that fineness of investment powders play an important role in determining the metallurgical quality of castings produced by investment casting process.