Two body type abrasion wear behaviour in hypoeutectic 16%Cr cast irons with Mo

Abstract

Hypoeutectic 16%Cr cast irons, both Mo free and 1–3%Mo containing specimens were prepared to investigate their abrasion wear behaviour. Annealed specimens were hardened at 1323 K and then tempered at three temperatures from 673 to 873 K for 7·2 ks, the temperature giving the maximum hardness (H_Tmax) and the lower and higher temperature, (L-H_Tmax, H-H_Tmax). The abrasion wear behaviour was investigated using a two body type Suga abrasion wear tester. A linear relation was obtained between wear loss and wear distance. The highest wear resistance or the lowest wear rate (R _W) was obtained in H _Tmax specimens except for the Mo free specimen. The lowest wear resistance or the highest R _W was obtained in H-H_Tmax specimens. The R _W was decreased with an increase in macrohardness. The lowest R _W appeared around 25% retained austenite (V _γ ). The R _W was decreased with an increase of Mo content, and the V _γ value at the minimum R _W shifted to the high V _γ side.     

Current status of supersaturated surface engineered S phase materials

Abstract

The present paper reviews the scientific development of the understanding of S phase. It is now known that S phase formation is an example of paraequilibrium phenomena. A necessary but not sufficient condition for S phase formation is the presence of a face centred cubic (fcc) structure at least in part with structure in the starting alloy. An essential requirement is for a nitride forming element to be present, particularly Cr. After surface engineering with carbon, nitrogen or carbon and nitrogen to generate supersaturated solid solutions, the various tribological, corrosion, mechanical and microstructural studies are reviewed for the various alloy systems. The current industrial status of S phase technology on an international basis is examined and the potential for its acceptance in China is discussed.     

Martensitic steels with carbide free microstructures containing retained austenite

Abstract

The concept of employing martensitic microstructures containing controlled quantities of carbon enriched retained austenite as a new route to high tonnage steels with improved properties is developed through an introduction to the novel non-equilibrium heat treatment procedure which has become known as quenching and partitioning (Q&P). This builds upon the earlier development of steels containing carbide free bainite and retained austenite, enabled by alloying with Si to suppress carbide precipitation, embracing a philosophy to produce refined ferritic microstructures by transformation to acicular forms, with the ferrite protected from the more damaging effects of carbon, which is partitioned to the austenite phase where it acts as a chemical stabiliser. This route offers enhanced strength from the martensite structure along with the promise of new properties from the behaviour of the retained austenite phase which could potentially contain a very high controlled concentration of carbon.     

Secondary austenite and chromium nitride precipitation in simulated heat affected zones of duplex stainless steels

Abstract

Primary and secondary intragranular austenite precipitation and its relationship with chromium nitride (Cr₂N) were studied in a simulated multipass heat affected zone (HAZ) of five duplex stainless steel alloys (UNS S32304, S32205, S32550, S32750, and S32760). The Gleeble thermal-mechanical simulator was used to perform short duration and high cooling rate ferritisation and reheating heat treatments. TEM and FEG-SEM analysis, coupled with a specially developed electrolytic etching technique, revealed the cooperative growth of secondary austenite and Cr₂N precipitation along the ferrite/austenite (α/γ) interfaces. Additionally, the observed close coexistence of intragranular nitride (Cr₂N) and intragranular secondary austenite suggests the heterogeneous nucleation of secondary austenite from the nitrides as supported by previously reported low energy nitride/austenite (Cr₂N/γ) interfaces for the observed orientation relationship between both phases. Based on these observations, a new mechanism is proposed for intragranular secondary austenite nucleation related to the intragranular nitride precipitates.     

Ferrite to austenite reverse transformation process in B containing 9%Cr heat resistant steel HAZ

Abstract

Creep properties of the high Cr heat resistant steel welded joint can be improved by adding B due to prevention of the grain refinement in heat affected zone (HAZ). In the present study, phase transformation behaviour of the B steel HAZ has been investigated to understand suppression mechanism of the grain refinement. During reverse transformation, fine austenite was formed through diffusional transformation at the prior austenite grain boundary in the first stage, and then coarse austenite was formed at the same location of the original austenite. The volume fraction of the fine austenite increased with increasing perk temperature of the weld thermal cycle. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was observed during the reverse transformation by a confocal laser microscope. These results indicate that martensitic or displacive reverse transformation takes place during welding and it prevents the grain refinement in HAZ.     

Analysis of martensite transformation behaviour in welded joint using low transformation temperature welding wire

Abstract

Low transformation temperature welding (LTTW) wire has been found to improve the fatigue strength in welded joints. In the present study, the temperature dependence of the mechanical properties in a dual phase microstructure of austenite and martensite was estimated using the properties of full austenite and full martensite in numerical analyses. A welding method effective for residual stress reduction and fatigue strength improvement was shown by applying a calculation method under transformation superplasticity and transformation induced plasticity in high strength steel welded joints. With this method, the influence of the welding pass sequence on the residual stress distribution and fatigue strength was examined in a boxing fillet welded joint using LTTW. The transformation tensile residual stress in the weld toe was decreased by sectioned welding, and the fatigue limit by sectioned welding with LTTW improved in comparison with the fatigue limit of a joint welded with conventional wire in the same process.     

Mechanomaking of nanostructured hypereutectic white irons: powder fabrication

Abstract

White irons containing 5·7 wt-%C were produced by room temperature, high energy milling from iron and carbon elemental powders. Both iron and cementite phases have crystal sizes of less than 10 nm in the powders which have fully dense particles of 50 μm mean size. The gas content is 0·5 wt-% and the powder is stable up to 550°C. Above 550°C decomposition of cementite occurs, involving a decrease in combined carbon content from 5·7 to 3·5 wt-% (annealing at 1050°C). The crystal growth kinetics has two temperature regions with lower activation energies in the high temperature (above 800°C) region. Crystal sizes still below 100 nm were observed for annealing up to 800°C. The size distribution in mechanosynthesised powders was modified by tumbling (dry) and attritor (wet) milling down to mean sizes of 4 μm and 1·4 μm respectively. Whereas tumbling milling does not alter the properties of MS powders, wet attritor milling produced higher gas content (7 wt-%) and much decreased thermal stability. Mechanosynthesised and tumbling milled powders can be degassed prior to consolidation, while wet attritor milled ones cannot.     

Determination of initial interface debonding in tungsten fibre reinforced epoxy composite using photon emission and electric current fluctuation

Abstract

Photon emission from an interface and electric current fluctuation between fibre and matrix were studied using a tungsten fibre reinforced epoxy pullout specimen, to determine the initial interface debonding event. Direct observation of the interfacial debonding behaviour was carried out separately, for comparison with photon emission results and electric current measurements. Photon emission and electric current fluctuation were observed simultaneously, and the load at these events was nearly the same as that found at the onset of interface debonding by direct observation. Both techniques were found to be effective tools for determination of initial interface debonding, because the behaviours originated from a discharge at the interface which occurred just after the onset of debonding.     

Integrated modelling of thermal cycles,austenite formation,grain growth and decomposition in the heat affected zone of carbon steel

Abstract

The microstructure evolution in the heat affected zone (HAZ) of 1005 low carbon steel during gas tungsten arc welding (GTAW) was quantitatively investigated using a combination of several numerical models. In particular, the α ferrite→γ austenite phase transformation during heating was studied using a Johnson–Mehl–Avrami (JMA) analysis, the γ grain growth was calculated using a Monte Carlo simulation, and the γ→α transformation during cooling was examined using an austenite decomposition model. In addition, the phase equilibria of the 1005 steel were calculated using computational thermodynamics software, Thermo-Calc, while the necessary temperature v. time data for all the microstructure models were obtained from a thermofluid model. These models were then used to calculate the extent of austenitisation with time during heating, the γ grain growth, and the volume fractions of various microconstituents of the final microstructure in the HAZ. It was found that a considerable amount of superheat was required for the initiation and completion of the α→γ transformation under the heating rates typical of arc welding. Significant γ grain growth was found to take place in the HAZ, particularly in the vicinity of the fusion zone (FZ) boundary, where the computed maximum γ grain size was about eight times greater than that of the base metal. The predicted final microstructure in the HAZ was predominantly allotriomorphic and Widmanstatten ferrites, which was consistent with the post-weld metallographic measurements. Overall, the computed microstructure evolution in the HAZ using the multiphenomena models was consistent with the available experimental data. The results reported here indicate that it is now possible to develop a quantitative model of complex weld microstructure evolution with the recent advances in transport phenomena and phase transformation models.     

Effect of prior austenite characteristics on mechanical properties of thermomechanically processed multiphase TRIP assisted steels

Abstract

To develop any multiphase transformation induced plasticity (TRIP) assisted steel through thermomechanical processing routes, the effects of materials and process variables are to be thoroughly identified. Accordingly, the present work was programmed to study the influence of the state of prior austenite on the subsequent transformations and the related final phase combinations. In this framework, using a low carbon TRIP assisted multiphase steel, the compression specimens were subjected to different thermomechanical processing schedules to generate different prior austenite characteristics. The results indicated that the characteristics of martensite/austenite (M/A) microcomponent in the final microstructure were dictated by the state of prior austenite. In fact, increasing prior austenite grain size led to large decrease in the amount of M/A phase and this, in turn, resulted to higher strength without sacrificing the ductility. It was also found that static recrystallisation of austenite brings about more percentage of M/A phase compared with dynamic recrystallisation.