A study of shell growth irregularities in continuously cast 310S stainless steel

Abstract

Growth irregularities in continuous casting are believed to be associated with crack formation and breakouts. Differential thermal analysis on 310S stainless steel samples indicated primary precipitations of both austenite and ferrite during solidification. In tensile tests on solidifying samples, abrupt shrinkages in volume were detected in the peritectic range of temperatures. Micrographic and microsegregation analysis on samples extracted from a breakout shell revealed high ratios of primary-precipitated austenite in the thick sections of the shell, and high ratios of primary-precipitated ferrite in the thin sections. Alternating precipitations of austenite and ferrite are proposed to occur during solidification. Regions of the shell with high ratios of primary austenite remain in contact with the mould and exhibit high growth rates, whereas regions with high ratios of primary ferrite shrink in volume due to the ferrite to austenite transformation, which results in the formation of air gaps between the shell and the mould and reductions in growth rate.     

Tom Bell Young Author Award Winner 2008 Influence of silicon on nitriding behaviour of hot work tool steels

Abstract

Hot work tool steels are widely used for pressure die casting moulds, die inserts, extrusion tools for aluminium processing and for steel forging. Nitriding increases the lifetime of such tools in many cases, yet delivers disappointing results in others. To optimise performance and for knowledge based surface design, it is indispensable to understand the mechanisms which occur in the near surface zone during nitriding. Nitrogen and carbon profiles obtained for X38CrMoV51 (AISI H13) steels with two silicon levels (1·1 and 0·3%), together with high resolution microscopy and electron energy loss spectroscopy, revealed that the secondary carbides are gradually transformed into nitrides during nitriding. Thermodynamic calculations confirmed the experimental observations. The near surface zone can be divided into three subzones: (1) a nitrogen enriched, almost carbon free zone with high nitride precipitation density and high hardness; (2) a nitrogen enriched and carbon depleted zone where the carbide–nitride transformation occurs; (3) a carbon enriched zone where the displaced carbon from zones 1 and 2 reprecipitates. A correlation between microstructure and microhardness and residual stress profiles was observed for all three zones. It was found that silicon, although not directly participating in the formation of nitrides, has a strong impact on the properties of the near surface zone by stabilising the secondary carbides and retarding the carbide–nitride transformation. This results in homogeneous precipitation in the transformation zone, thus avoiding micrometre sized precipitates which can act as defects and promote crack propagation. The conclusions of the present work are in accordance with literature studies on the effect of silicon on the tempering behaviour and the secondary carbide structure of 5%Cr martensitic steels.     

Acoustic technique to discriminate boiling state during quenching

Abstract

In quench hardening, it is important to determine the boiling state (film, nucleate or convection), in order to control the cooling process. However, the boiling state changes with time and with position on the specimen. A new method developed to discriminate the boiling states is described. High frequency induction heating was applied to stabilise the boiling state and the sound of boiling was monitored with an underwater microphone. The results were evaluated by using fast Fourier transform spectroscopy. For the results obtained using high frequency induction heating and various quenching methods, the boiling states could be discriminated by matching the sound frequency of the sample. Consequently, it was possible to discriminate the boiling states from the frequency observed in nucleate boiling, which has a characteristic frequency band and high intensity compared with film and convection states.     

Case hardening of zirconium

Abstract

The poor tribological properties of zirconium limit its application, particularly its use in valves in the nuclear industry. Following up on a successful program to case harden titanium, the authors applied the same techniques to zirconium. Although the two metals are closely related, the diffusing species used for titanium were not found to be suitable for zirconium because of cracking in the sub-ceramic diffusion zone. A modified approach using hydrogen to improve the ductility of the layer successfully reduced its hardness, but not its propensity to cracking.     

Effect of trace Ag on the microstructure and precipitation of an Al–Cu–Mg alloy

Abstract

This paper investigated the effect of different amounts of Ag addition on the microstructure, properties and precipitation processes of Al–4·6Cu–6·9Mg(wt-%) alloy using various analytical methods. It was found that Ag addition stimulated new X′ 9 and Ω phases precipitated finely and dispersively in the matrix, as a result of Mg–Ag co-clusters; the volume fraction of precipitates increased with the content of Ag addition. Such precipitation improved the mechanical performance of the Al–Cu–Mg alloy significantly. The mechanism for the formation of new precipitates is also described in this paper.     

Thermal stability and age hardening of supersaturated AlCrN hard coatings

Abstract

During advanced machining processes (high speed and dry cutting), the temperature at the cutting edge can exceed 1000°C. For modern protective hard coatings, thermal stability is of major interest. Equally important are superior mechanical properties, such as hardness, remaining at a high level over a wide temperature range. AlCrN coatings perform well in cutting tests and show excellent oxidation resistance as well as good tribological behaviour. In this work, supersaturated cubic Al_0.7Cr_0.3N coatings deposited by cathodic arc evaporation are studied. The phase and microstructure evolution of the material is investigated up to 1450°C using a combination of differential scanning calorimetry, thermal gravimetric analysis, mass spectrometry, X-ray diffraction and analytical transmission electron microscopy. During annealing up to 925°C, hexagonal AlN precipitates are formed at grain boundaries. At higher temperatures, a transformation of the remaining cubic AlCrN matrix into Cr via Cr₂N takes place, accompanied by a release of nitrogen. After annealing up to 1450°C, the AlN grains coarsen and coalesce around the Cr and Cr₂N grains. The results explain the superior cutting performance by the formation of precipitates, but also demonstrate the limitations in usage at high temperature regimes due to decomposition. Nevertheless, the substitution of Cr in the CrN lattice by Al has proven to increase the decomposition resistance significantly. Finally, nanoindentation experiments reveal that AlCrN coatings retain hardness beyond the stage of residual stress recovery up to 900°C, demonstrating an age hardening process.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 2 – Mechanical property characterisation

Abstract

Mechanical property characterisation has been carried out on specimens of 16Cr - 5Ni - 1Mo stainless steel, subjected to various aging cycles. The heat treatment cycles involved solution treatment at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, tensile, hardness, impact, and creep tests were conducted. Specimens aged at 475 ° C exhibited maximum values of tensile strength and hardness with minimum values of ductility and impact toughness, while specimens aged at 625 ° C had maximum values of impact toughness and ductility. The results were correlated with the microstructural data presented in Part 1 of this study. Softening of the martensitic matrix at 625 ° C occurs as a result of the elimination of internal stresses, the decrease in the dislocation density, and the high volume fraction of austenite which lead to the drop in values of tensile strength and hardness. The results of the study reveal that aging at 550 ° C for 4 h gives the optimum combination of strength, hardness, ductility and toughness for this steel.     

Tensile and fatigue behaviour of sinter hardened Fe–1·5Mo–2Cu–0·6C steels

Abstract

In the present work the tensile and axial fatigue behaviour of sintered hardened Fe–1·5Mo–2Cu–0·5C at three density levels (6·8, 7·0 and 7·2 g cm^–3) have been studied. The materials were tested under the as sintered condition, and after tempering at 180 and at 240°C. The results show that steels under the as sintered condition posses a high hardness but a brittle tensile deformation and fracture behaviour. Tempering at 180 and 250°C induces the disappearance of brittleness and tensile fracture is thus ductile although very localised at the necks. Fatigue strength is determined by the resistance of the materials to the internal damage evolution due to the nucleation of small cracks at the pores edges, and their coalescence into a long crack. Tempering induces an increase in the fatigue resistance. The greatest fatigue strength at 2 × 10⁶ cycles is displayed by the steel with a density of 7·2 g cm^–3 and tempered at 180°C.     

Thermochemical surface treatment of titanium

Abstract

Titanium has very poor tribological properties. Various coatings can be used, for example, TiN, to improve them, but the loading is limited by the low strength of the substrate. Thermochemical treatments produce a layer that is sufficiently thick to support a load, but have to be carried out at high temperatures, 950 and 1050°C, for oxidation and nitriding processes respectively, degrading the core properties. An alternative treatment is desirable that could produce a substantive load bearing layer with good wear properties at 850°C or lower. The obvious candidate species were oxygen, nitrogen and carbon to form hardened diffusion layers, under a thin TiO₂, TiN or TiC surface layer. Nitrogen was not found to be sufficiently active at lower treatment temperature to have any beneficial effects and tended to block the diffusion of other species. Layers formed using various sources of nascent nitrogen, carbon and oxygen were studied. It was found that all the species forming hardened surface layers with the highest values of surface hardness and case depth (72 μm and 922 HV after 24 h at 850°C) were produced using carbon monoxide.     

Bulk high hardness Al90Ce2Mn8 alloy prepared by powder metallurgy

Abstract

Using powder metallurgy, bulk high strength Al₉₀Ce₂Mn₈ alloy 25 mm in diameter by 10 mm with near nil porosity has been obtained under certain pressing and heating conditions. The conditions for the best mechanical properties are a pressing temperature of 753-793 K, a pressing time of 30 min, and pressing stress of 1·2 GPa. The compression strength reaches 895 MPa with a hardness of 26 HRC when the alloy is pressed at 753 K. The strength increase is attributed to second phase strengthening and fine grain strengthening.