C. C. Degnan

Reactive sintering of stainless steel

Abstract Pressed and sintered 316L type stainless steel compacts have been produced using an innovative process termed reactothermitic sintering RTS. This technique has the ability to produce full density, near net shape parts using conventional compaction and mesh belt furnace practices. It utilises chemical reactions at the surface of the stainless steel powders in which the energy balance is matched to provide transient liquid films that can be frozen at high cooling rates to consolidate the material without slumping. Small quantities of elemental aluminium and nickel powders are added before compaction that react, during sintering, both with themselves and with oxide layers present on the bulk stainless steel powder surfaces. In addition to the naturally occurring oxides present on the surface of stainless steel powders, artificial oxide layers, deposited by sol-gel and direct thermal oxidation, have been used to aid investigations into the nature of the reactions involved. The RTS technique leads to the generation of novel microstructures in the systems described that are characterised by negligible interconnected porosity. The present paper details the experimental programme undertaken to ascertain and substantiate these phenomena together with a model of how porosity is eliminated in this system.

Influence of reinforcement volume fraction on sliding wear behaviour of SHS derived ferrous based metal matrix composite

Abstract Steel matrix composites reinforced with (W,Ti)C particles were processed by directaddition of an innovative powder to molten 0.4 wt-%Clow alloy steel.Thispowder was producedusing a self-propagatinghightemperature synthesis(SHS) reaction and consisted of a dispersion of fine (W,Ti)C particles (5-10 μm) in an iron binder. Dry unidirectional sliding wear behaviour ofthe compositematerialanditsunreinforced counterpartwas investigated atroomtemperature against a white cast iron counterface. In situ monitoring of wear and friction, in conjunction with electron microscopy, has been used tointerpretwearscar microstructures observed intermsofwear mechanisms. Wear experimentswere performedat a sliding speed of 1.5 m s-1 at different test loads. It was found that the wear resistance of the composite material was superior to that of their unreinforced counterparts over the entire range of loading employed during this investigation. The unreinforced base alloy exhibited a transition from mild to severe wear at a load of approximately 70 N. No such transition was found to occur for the composite materials. Instead, after a transient period of running in wear, steady state conditions were observed. At the highest level of carbide addition this transient period did not occur and the composite wore in a regime of mild wear over the full spectrum of loading utilised.

Comparison of the green strength of warm compacted Astaloy CrM and Distaloy AE Densmix* powder compacts

Abstract It is important that the mechanical properties of green warm compacted powder parts be accurately determined if advanced manufacturing techniques, such green machining, are to be exploited successfully. In this study, the green strength of Astaloy CrM Densmix and Distaloy AE Densmix*, two powder mixes designed specifically for such applications, was measured using the Brazilian disc compression test. Fracture statistics were employed to compare the strengths under the same compaction regime, and the Weibull modulus calculated for each case. Further tests were performed in which the warm compaction temperature and pressure were varied. It was found that a strong relationship exists between the green strength of each material and its compressibility. This relationship was explained in terms of pore size and pore distribution with respect to the stress field developed within the compact.

Measurement of green strength of warm pressed Distaloy PM compacts: influence of specimen geometry and test method

Abstract It is important that the green strength properties of powder compacts be accurately determined if advanced manufacturing techniques, such as green machining, are to be exploited successfully. In this study, the green strength of Distaloy AE Densmix, a powder mix designed for such applications, was measured using several test methods. Fracture statistics were correlated with compact density and test method employed, and the Weibull modulus calculated for each case. It was found that Weibull analysis accurately predicted the green strength dependence on the test configuration utilised.

Relationship between physical structure and machinability of green compacts

Abstract The dependence of green machinability on compact density and strength was investigated for room temperature and warm compacted steel powder compacts containing two different types of lubricant. Brazilian disc compression tests were employed to determine green strength, while machinability was assessed in terms of response to drilling. For the room temperature compacted materials, it was found that high compact densities and strength were not, in most cases, associated with improvements in machinability. Furthermore, it was shown that lubrication (both type and quantity) and compaction pressure plays a critical role in determining the level of breakouts observed. In contrast, the use of warm compaction, in conjunction with specially designed lubricants, has been shown to be a suitable method of producing high density, high strength compacts while retaining good green machining characteristics. Mechanisms responsible for the observed behaviours of both the room temperature and warm compacted specimens have been forwarded in the present paper.

Properties of reacto-thermitically sintered stainless steel

Abstract Reacto-thermitic sintering (RTS) is a novel process capable of producing near net shape powder metallurgical stainless steel components with negligible interconnected porosity at sintering temperatures of 1150°C. It utilises chemical reactions between oxides present on the surface of conventional stainless steel powders and small quantities of reactive additives to produce a transient liquid phase. This liquid can be frozen during cooling to consolidate the component without slumping. Previous work by the authors has studied the fundamental mechanisms underpinning the evolution of this liquid phase and its subsequent behaviour in eliminating porosity. The present paper investigates the effect of varying the quantity of reactive additive on the dimensional stability, mechanical properties, and corrosion resistance of stainless steel specimens produced by RTS in the light of this work.

Reactive sintering in Fe-Co system

Abstract Low porosity powder metallurgy compacts have been manufactured from treated elemental iron and cobalt powders sintered at 1150°C under an H2(g) atmosphere. Their microstructures consist of an interconnected mixed oxide network which encapsulates both the iron and cobalt phases. The production technique employed is an innovative process termed reacto-thermitic sintering (RTS), which leads to near full density and near net shape parts utilising conventional uniaxial compaction and mesh belt furnace practices. The RTS technique relies on microscale exothermic reaction between small quantities of added elemental Al and oxides present on the surface of the bulk powder, together with the bulk powder itself. This results in the production of a transient liquid phase which freezes rapidly and consolidates the compact without slumping. In order to generate an interconnected mixed oxide network, experiments were designed such that the Al powder reacts with the cobalt and the surface of the iron powder which is artificially doped with Fe and Cr oxides. Differential thermal analysis (DTA) and energy balance calculations revealed that the Al and the oxide coating reaction does not proceed directly. Instead the main contribution to the exothermic process is the reaction between Al and Co/Fe. The system does not exhibit true RTS behaviour and the interconnected network of mixed Al, Cr, and Fe oxides is created by subsequent reaction of Co-Al and Fe-Al intermetallics with the artificial Fe-Cr oxide coating on the Fe. The microstructure obtained exhibits negligible porosity with the metallic particles on the whole fully encapsulated by the oxide.