Characterisation of severely deformed austenitic stainless steel wire

Abstract

The microstructure of 8 μm diameter wire produced by the severe deformation of 316L austenitic stainless steel has been examined using TEM and X-ray diffraction. The deformation imparted amounts to a true strain of 6·3. Data from previous studies on strain induced transformation of this steel have been combined with new results to show that true strains >2 are required in order to observe mechanical stabilisation, i.e. the cessation of martensitic transformation when the martensite/austenite interfaces are unable to propagate through the dislocation debris created in the austenite.     

Dynamic strain aging effects in niobium microalloyed steel

Abstract

The dynamic strain aging behaviour of a niobium microalloyed steel has been examined. Hot tensile testing was carried out on heat treated and as received specimens. Heat treated specimens were austenitised at 1000°C for 1 h, and then cooled in air or in a stainless steel cylinder to obtain various amounts of free or uncombined interstitial solutes in solid solution, to examine the effect on the dynamic strain aging behaviour of the steel. It was found that dynamic strain aging takes place in niobium microalloyed steel during tensile testing at temperatures ranging from ambient temperature to 450°C at a crosshead displacement rate of 2 mm min^-1. As a result, the ultimate tensile strength and initial work hardening rate exhibit maximum values at temperatures between 200 and 350°C. Also, load-extension graphs for tested specimens show serrated behaviour and yield points at 200, 250, and 300°C. It is believed that dynamic strain aging in niobium steel is caused by interaction between dislocations and interstitial solutes (nitrogen and carbon) or solute pairs consisting of one interstitial and one substitutional solute atom (for example Mn-C and Mn-N).     

Measuring and modelling residual stresses in butt welded P91 steel pipe including effects of phase transformations

Abstract

Residual stresses in a circumferentially butt welded steel pipe have been measured and numerically predicted. The pipe, containing the circumferential weld, has an outer diameter of 290 mm and a wall thickness of 55 mm, typical of components in power generation plants. An axisymmetric thermomechanical finite element (FE) simulation has been performed to obtain the residual stress field induced by the fusion welding of the pipe, taking solid state phase transformation effects into account and using temperature dependent material property data. Residual stresses have been measured using the X-ray diffraction and deep hole drilling techniques. Good correlation has been demonstrated with the predictions of the FE model. The paper demonstrates that a mixed experimental and numerical approach is useful for determining the residual stress distribution in welded joints.     

Influence of calcium aluminate cement in magnesia castable on total oxygen content of interstitial free steel

Abstract

Samples of interstitial free (IF) steel buried in MgO castable bonded by calcium aluminate cement (CA) in graphite crucibles were heated at 1600°C for 90 min. Total oxygen content (TOC) of the steel was examined after heating and the refractory was investigated by SEM and EDS. It was found that TOC was higher in IF steel samples in contact with MgO castables containing 3 or 5 wt-% CA than with castables containing 7 wt-% CA or without CA. A liquid layer formed between refractory oxide and molten steel separates the refractory oxides from molten steel and inhibits direct dissolution of oxides in the molten steel. Transfer of oxygen between the refractory oxide and molten steel occurs by the formation of CaO.Fe₂O₃ at the boundary between the refractory oxide and the liquid layer, diffusion of CaO.Fe₂O₃ in the liquid phase layer, decomposition of CaO.Fe₂O₃ and dissolution of FeO into the molten steel. W ith increasing CA content in MgO based castables the CaO content in molten steel increases, but iron oxide content decreases, leading to the result mentioned above.     

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.     

Characteristics of Acoustic Emission Signal Specific to Isolated Cracks Triggered by Scratch Tests in Stainless Steel Coatings

The tribological process during contact between two surfaces involves mechanical and tribochemical changes as well as material transfer. PVD coatings have a complex structure, and there is a need to characterize the failure of such coatings and this has been carried out by a non-destructive method, namely acoustic emission. In this paper simple scratch tests are described in order to study some characteristics of nitrogen-doped stainless steel coatings on 40CrMo4 construction steel. Scratch tests were performed in order to induce a mechanical failure in the coatings. The aim of this paper is to show that the nucleation of isolated cracks mechanism that leads to failure can be monitored by certain characteristics of the acoustic emission signal.     

Effect of additions of Ti,B and Ce on microstructural stability,creep strength and creep damage in austenitic stainless steel

Abstract

Optical and transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis of bulk extracted precipitate residues were carried out on long term (more than 80 000 h) creep tested (at 1023 K) type 304 austenitic stainless steels with different levels of Ti content to assess the microstructural stability and creep strength. B and Ce were added to the steels to suppress the creep cavitation. Finer Ti(C,N) particles with higher density and narrower size distribution were observed in steels with a higher Ti content, resulting in an increase in the creep rupture strength. However, higher Ti content increased the intergranular precipitation of the σ phase on longer creep exposure, resulting in the increase in creep cavitation and in the decrease in creep rupture strength. The study indicated an optimum level of Ti and {C + (6/7)N} content with the Ti/{C + (6/7)N} ratio close to the stoichiometric value of the Ti(C,N) precipitate particles that should also be close to their solubility limit at the solution heat treatment temperature.     

Optimum model for predicting temperature settings on hot dip galvanising line

Abstract

Controlling the annealing cycle in a hot dip galvanising line (HDGL) is vital if each coil treated is to be properly galvanised and the steel is to have the right properties. Current HDGL furnace control models usually take into account the dimensions of the coil to be dipped and, in some cases, the type of steel. This paper presents a new model for monitoring furnace temperature settings, which considers not just the coil dimensions but also the chemical composition of the steel. This enables the model to be adjusted more suitably to each type of steel to be dipped, so that the HDGL annealing cycle is optimised and rendered more efficient in dealing with new products. The ultimate aim is to find a model that is equally efficient for new types of steel coil that have not been processed before and whose dimensions and chemical compositions are different from coils processed previously. To find the best model, this paper compares various new and classical algorithms for developing a precise and efficient prediction model capable of determining the three temperature settings for heating on an HDGL located in Avilés (Spain) on the basis of the physical and chemical characteristics of the coils to be processed and the preset process conditions.     

Solidification cracking in low alloy steel welds

Abstract

The high solidification cracking susceptibility of low C steel weld metals was investigated using pure Fe model alloys containing 0–0·23%C, 0–5%Ni and 0–0·0144%B. In addition, a few Fe–C–Ni ternary alloys were also tested. Solidification cracking susceptibility was tested using longitudinal varestraint and transvarestraint tests. Cracking was evaluated using crack length and brittleness temperature range criteria. The Fe–C alloys showed high cracking tendency in two regimes, the first in the ultralow carbon range of 0·03–0·05%C and the second in a narrow band close to 0·1%C. The cracking was much more than that attributable to solute segregation. In Fe–Ni and Fe–B alloys, cracking was a function of alloy content. Solidification cracking due to C and Ni was higher in the ferritic mode of solidification compared to the austenitic, unlike in stainless steels, where the ferritic mode provides high resistance to cracking. In Fe-C-Ni ternary alloys, cracking could be better related to composition in terms of a variable coefficient for C in the Ni equivalent. In the vicinity of 0·1%C, cracking was attributable to shrinkage due to solid state transformation from δ to γ in the brittle temperature range, and is similar to that occurring during continuous casting of steel. However, this factor did not appear to play a role in cracking in the ultralow C range of 0·03–0·05%C.     

Distribution Around the Oxide-Substrate Interface of Alloying Elements in Low-Carbon Steel

In order to study the physicochemical evolutionduring oxidation of Fe-C-X_i alloyssurfaces (X_i = Cu, Ni, Al, Si, S, withX_i% < 0.5 wt.%, C% < 0.1 wt.%), anoriginal analysis method has been used. After separating the oxide from the metalsubstrate, the first atomic layers of both innersurfaces have been observed by Auger ElectronSpectroscopy (AES). The depth profiles obtained, around0.4 m thick on both sides of the oxide-metal interface,have been compared for four steels of differentcomposition. Significant differences have been observedand described in this paper. A qualitativeinterpretation of diffusion processes has been proposed. Ametallographic study illustrates some physicalconsequences of alloying elements, especially on thescale thickness.