Effect of twinning on microstructural evolution during dynamic recrystallisation of hot deformed as-cast austenitic stainless steel

An as-cast austenitic stainless steel was hot deformed at 1173 K, 1223 K, and 1373 K (900 °C, 950 °C, and 1100 °C) to a strain of 1 with a strain rate of 0.5 or 5 s⁻¹. The recrystallised fraction is observed to be dependent on dynamic recrystallisation (DRX). DRX grains nucleated at the initial stages of recrystallization have similar orientation to that of the deformed grains. With increasing deformation, Cube texture dominates, mainly due to multiple twinning and grain rotation during deformation.

New insights into the mechanism of low-temperature active-screen plasma nitriding of austenitic stainless steel

Low-temperature active-screen plasma nitriding is an effective surface engineering technology to improve the wear and corrosion resistance of austenitic stainless steel through the formation of expanded austenite. The material sputtered from the active screen and redeposited on the specimens has been suggested to play an important role in the nitriding mechanism involved. This paper reports a patterned deposition layer, which is in correlation with the grain orientation of polycrystalline specimens. This has provided new insights into the nitriding mechanism. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Dry sliding wear of active screen plasma carburised austenitic stainless steel

Low temperature diffusion treatments with nitrogen and carbon have been widely used to increase the tribological performance of austenitic stainless steels. These processes produce a layer of supersaturated austenite, usually called expanded austenite or S-phase, which exhibits good corrosion and wear resistance. The novel active screen technology is said to provide benefits over the conventional DC plasma technology. The improvements result from the reduction in the electric potential applied to the treated components, and the elimination of such defects and processing instabilities as edge effects, hollow cathode effects and arcing. In this study, AISI 316 coupon samples were plasma carburised in DC and active screen plasma furnaces. The respective layers of carbon expanded austenite were characterised and their tribological performance was studied and compared. Detailed post-test examinations included SEM observations of the wear tracks and of the wear debris, EDX mapping of the wear track, EBSD crystal orientation mapping of the cross sections of the wear tracks, and cross-sectional TEM. Based on the results of wear tests and post-test examinations, the wear mechanisms involved are discussed.

Study of active screen plasma processing conditions for carburising and nitriding austenitic stainless steel

Active screen (AS) is an advanced technology for plasma surface engineering, which offers some advantages over conventional direct current (DC) plasma treatments. Such surface defects and process instabilities as arcing, edge and hollow cathode effects can be minimised or completely eliminated by the AS technique, with consequent improvements in surface quality and material properties. However, the lack of information and thorough understanding of the process mechanisms generate scepticism in industrial practitioners. In this project, AISI 316 specimens were plasma carburised and plasma nitrided at low temperature in AS and DC furnaces, and the treated samples were comparatively analysed. Two diagnostic techniques were used to study the plasma: optical fibre assisted optical emission spectroscopy, and a planar electrostatic probe. Optimum windows of treatment conditions for AS plasma nitriding and AS plasma carburising of austenitic stainless steel were identified and some evidence was obtained on the working principles of AS furnaces. These include the sputtering of material from the cathodic mesh and its deposition on the worktable, the generation of additional active species, and the electrostatic confinement of the plasma within the operative volume of the furnace.

Numerical simulations on warm forming of stainless steel with trip-effect

In steels with TRIP-effect, a phase transformation from the retained-austenite to martensite occurs during forming, and it significantly affects hardening behaviours. Such an effect is sensitive to the amount of strain as well as the temperature variation. For materials with a strong TRIP-effect, new forming techniques are needed to develop that can lead to lighter and stronger components in automotive industry. This paper presents a coupled thermo-mechanical finite element modelling and simulation of a warm deep drawing of austenitic stainless steel (including a TRIP-effect) using LS-DYNA and temperature effect on forming process of such materials is investigated.

The restoration of the passivity of stainless steel Weldments in pickling solutions observed using electrochemical and surface analytical methods

In this study, a solution containing ammonium fluoride (NH4F) and nitric acid (HNO3) was used as an alternative to the conventional highly toxic pickling solution HF/HNO3 for pickling weldments of selected stainless steels including Type 316 stainless steel (UNS S31600), duplex stainless steel 2205 (UNS S32205), and super duplex stainless steel 2507 (UNS S32750). Electrochemical and surface analytical methods were used to understand the effects of pickling on the stainless steel weldments. Cyclic potentiodynamic polarization (CPP) test results indicated that the restoration of passivity of stainless steel weldments could be achieved by pickling the weldments in both HF/HNO3 solution and NH4F/HNO3 solutions. Scanning electron microscopy observation of the UNS S32750 weldment surface revealed that both the HF/HNO3 solution and the NH4F/HNO3 solution could remove the heat tint on the weldment. X-ray photoelectron spectroscopy analysis indicated that treatment in these two pickling solutions produced passive films with similar characteristics. Thus, this work suggests that the NH4F/HNO3 solution is a promising alternative to HF/HNO3 solution for the pickling of stainless steel weldments, and that the CPP test approach can be used in conjunction with surface analytical methods for further development of safer and environmentally friendly picklingsolutions.

Corrosion behaviour of direct current and active screen plasma carburised AISI 316 stainless steel in boiling sulphuric acid solutions

Active screen plasma is a recently developed plasma surface alloying technique, which has shown potential for addressing some drawbacks associated with conventional direct current plasma processes. In this study, the corrosion performance of untreated, direct current and active screen plasma carburised AISI 316 was investigated by immersion in a boiling solution of sulphuric acid. The experimental results show that the corrosion behaviour of expanded austenite produced by low temperature plasma carburising is controlled by the type and density of surface defects; the corrosion properties of the active screen plasma carburised material are superior to that produced by direct current plasma because of the significantly reduced edge effect and surface defects; and the bias level used in the active screen carburising treatment has a profound effect on the corrosion performance of the material. Based on the experimental results, the corrosion mechanisms involved are discussed.

Microstructure evolution and softening processes in hot deformed austenitic and duplex stainless steels

The microstructure evolution and softening processes occurring in 22Cr-19Ni-3Mo austenitic and 21Cr-10Ni-3Mo duplex stainless steels deformed in torsion at 900 and 1200 °C were studied in the present work. Austenite was observed to soften in both steels via dynamic recovery (DRV) and dynamic recrystallisation (DRX) for the low and high deformation temperatures, respectively. At 900 °C, an "organised", self-screening austenite deformation substructure largely comprising microbands, locally accompanied by micro-shear bands, was formed. By contrast, a "random", accommodating austenite deformation substructure composed of equiaxed subgrains formed at 1200 °C. In the single-phase steel, DRX of austenite largely occurred through straininduced grain boundary migration accompanied by (multiple) twinning. In the duplex steel, this softening mechanism was complemented by the formation of DRX grains through subgrain growth in the austenite/ferrite interface regions and by large-scale subgrain coalescence. At 900 °C, the duplex steel displayed limited stress-assisted phase transformations between austenite and ferrite, characterised by the dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the transformed regions with strain. The softening process within ferrite was classified as "extended DRV", characterised by a continuous increase in misorientations across the sub-boundaries with strain, for both deformation temperatures.

Microstructural evolution during hot deformation of duplex stainless steel

The microstructure evolution during hot deformation of a 23Cr-5Ni-3Mo duplex stainless steel was investigated in torsion. The presence of a soft δ ferrite phase in the vicinity of austenite caused strain partitioning, with accommodation of more strain in the δ ferrite. Furthermore, owing to the limited number of austenite/austenite grain boundaries, the kinetics of dynamic recrystallisation (DRX) in austenite was very slow. The first DRX grains in the austenite phase formed at a strain beyond the peak and proceeded to <15% of the microstructure at the rupture strain of the sample. On the other hand, the microstructure evolution in δ ferrite started by formation of low angle grain boundaries at low strains and the density of these boundaries increased with increasing strain. There was clear evidence of continuous dynamic recrystallisation in this phase at strains beyond the peak. However, in the δ ferrite phase at high strains, most grains consisted of δ/δ and δ/γ boundaries.

An effective approach to grow boron nitride nanowires directly on stainless-steel substrates

When growing one-dimensional (1D) nanomaterials via the vapour–liquid–solid (VLS) model, the substrates usually need to be coated with a layer of catalyst film. In this study, however, an effective approach for the synthesis of boron nitride (BN) nanowires directly onto commercial stainless-steel foils has been demonstrated. Growth occurs by heating boron and zinc oxide (ZnO) powders at 1100 °C under a mixture of nitrogen and hydrogen gas flow (200 ml min−1). The stainless-steel foils played an additional role of catalyst besides substrate during the VLS growth of these BN nanowires. The as-synthesized nanowires emit strong photoluminescence (PL) bands at 515, 535 and 728 nm. In addition, we found that the gas flow rate and the hydrogen content in the gas mixture strongly affected the diameter and yield of the nanowires by changing the relative concentration of the nanowire growth species in the chamber.

Selective oxidation synthesis of MnCr2O4 spinel nanowires from Commercial stainless steel foil

For the first time, MnCr₂O₄ spinel single-crystalline nanowires were simply synthesized by heating commercial stainless steel foil (Cr_0.19Fe_0.70Ni_0.11) under a reducing atmosphere. The nanowires have an average diameter of 50 nm and a length of about 10 μm. Some nanowires are sheathed with a thin layer of amorphous silicon oxide. Photoluminescence measurements revealed that the nanowires exhibit an emission band at 435 nm, which resulted from the oxygen-related defects in the silicon oxide sheath. It was found that the reducing atmosphere plays a key role for the nanowire growth. In the reducing atmosphere, the Mn and Cr elements in the stainless steel could be selectively oxidized because of their higher affinity for oxygen than the Fe and Ni elements. The Fe and Ni elements in the stainless steel, however, acted as the catalyst for the vapor–liquid–solid (VLS) growth of the MnCr₂O₄ nanowires.

Experimental and numerical investigation of low pressure tube hydroforming on 409 stainless steel

Tube hydroforming has been widely used to produce automotive structural components due to the superior properties of the hydroformed parts in terms of their light weight and structural rigidity. Compared to the traditional manufacturing process for a closed-section member including stamping and followed by welding, tube-hydro forming leads to cost savings due to reduced tooling and material handling. However, the high pressure pumps and high tonnage press required in hydroforming, lead to increased capital investment reducing the cost benefits. This study explores low pressure tube hydro forming which reduces the internal fluid pressure and die closing force required to produce the hydroformed part. The experimental and numerical analysis was for low pressure hydro formed stainless steel tubes. Die filling conditions and thickness distributions are measured and critically analysed.