Johan Ahlström

Microstructural evaluation and interpretation of the mechanically and thermally affected zone under railway wheel flats

Abstract During a wheel–rail slide, the temperature is high enough to austenitise the material close to the contact surface. The material is rapidly cooled by heat conduction into the rest of the wheel when the wheelset starts rolling again and thus martensite can form in the surface layer. In the present investigation, the mechanically and thermally affected zones (TAZ) under railway wheel flats have been studied by optical microscopy, scanning electron microscopy and hardness testing. The wheel flat samples were generated in full-scale experiments on the last axle of a train consisting of a locomotive pulling three carriages. The axle load, train speed and locking time of the last axle were varied to simulate several different situations that arise in revenue traffic. The entire population of samples has earlier been studied, but the present investigation goes more into detail on a limited number of samples. Several discoveries are made and suggestions of mechanisms are presented. Among the most important findings are stretched MnS inclusions that act as crack nucleation sites, limited grain growth indicating low austenitisation temperatures and reformation of ferrite and pearlite in the deformed surface layers.

Alloy design for intrinsically ductile refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials. However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through controlled alloying. A new ductile RHEA, Hf0.5 Nb 0.5 Ta 0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.

Analytical 1D model for analysis of the thermally affected zone formed during railway wheel skid

Abstract During sliding between a railway wheel and rail, frictional heat is generated that causes a temperature increase in the surface layer high enough to austenitise it. Due to the rapid cooling that follows when the wheel starts to roll again, martensite often forms in the surface layer. In the present paper, an analytical model of heat conduction in the thermally affected zone (TAZ) is established and evaluated. The model is based on metallurgical observations made after full-scale experiments on wheel skid. These observations indicate that the heat flow is close to one-dimensional (1D) in relevant material volumes, and that the contact surface temperature of the wheel rapidly rises and then saturates at a certain value. Comparison between model output and experimental results supports the choice of boundary condition. Evaluation of the model by the aid of experimental results of TAZ thickness indicates saturation surface temperatures of around 900±100°C. Another interesting result is that the cooling rate in temperature ranges relevant for phase transformation is almost independent of depth.

Sample preparation and microstructural characterization of the gamma titanium aluminide Ti-48Al-2W-0.5Si

Preparing samples that faithfully reveal all microstructural features in γ-TiAl-base alloys for both optical and scanning electron microscope studies is fraught with difficulties. This study demonstrates that satisfactory results can be obtained through mechanical grinding, polishing, and proper etching. A preparation recipe is presented. Nine lots of investmentcast γ-TiAl material with the nominal composition of Ti-48Al-2W-0.5Si have been characterized through optical and scanning electron microscope examinations. The study shows that a small depletion in Al content has a large effect on the microstructure. The duplex microstructure with a lamellar Full-size image (<1 K) colony size of about 500μm and a large percentage of singlephase γ grains as large as 200μm is altered to a coarse, nearly lamellar microstructure with a colony size as large as 5000μm and a small percentage of small single-phase γ grains in the colony boundaries.

Thermal degradation of pearlitic steels: influence on mechanical properties including fatigue behaviour

With the aim to predict the durability of railway wheels, thermomechanical damage was studied for two steels with different alloying levels of silicon and manganese in the temperature range of 500–725°C. Softening caused by cementite spheroidisation in pearlite leads to changes in the mechanical behaviour and an accompanying decrease in fatigue lifetimes. It was found that higher contents of Si and Mn lead to better resistance to softening of both virgin and plastically deformed material. Correspondingly, the high Si-Mn alloyed steel loses much less in fatigue lifetime than the lower alloyed steel.

Mechanical Behavior of a Rephosphorized Steel for Car Body Applications: Effects of Temperature, Strain Rate, and Pretreatment

Temperature and strain rate effects on the mechanical behaviour of a commercial rephosphorized, interstitial free steel have been investigated by uniaxial tensile testing, covering applicable temperatures (-60 – +100°C) and strain rates (1•10-4 – 1•102 s-1) experienced in automotive crash situations. The effect of prestraining to 3.5 % with or without successive annealing at 180°C for 30 min has also been evaluated. These treatments were used to simulate pressing of the plates and the paint-bake cycle in the production of car bodies. Yield and ultimate tensile strengths, ductility including uniform and total elongation and area reduction, thermal softening effect at high strain rate and strain rate sensitivity of stress were determined and discussed in all cases. It was found that the Voce equation [σ = σs- (σs -σ0) exp (e/e0)] can be fitted to the experimental true stress-true plastic strain data with good precision. The parameter values in this equation were evaluated and discussed. Furthermore, temperature and strain rate effects were examined in terms of thermal and athermal components of the flow stresses. Finally, a thermal activation analysis was performed.

Characterization of deformed pearlitic rail steel

Pearlitic steels are commonly used for railway rails because they combine good strength and wear properties. During service, the passage of trains results in a large accumulation of shear strains in the surface layer of the rail, leading to crack initiation. Knowledge of the material properties in this region is therefore important for fatigue life prediction. As the strain is limited to a thin surface layer, very large strain gradients can be found. This makes it very difficult to quantify changes in material behavior. In this study hardness measurements were performed close to the surface using the Knoop hardness test method. The orientation of the pearlitic lamellas was measured to give an overview of the deformed microstructure in the surface of the rail. Microstructural characterization of the material was done by optical microscopy and scanning electron microscopy to evaluate the changes in the microstructure due to the large deformation. A strong gradient can be observed in the top 50 μm of the rail, while deeper into the rail the microstructure of the base material is preserved.

Effect of shot peening on the residual stress and mechanical behaviour of low-temperature and high-temperature annealed martensitic gear steel 18CrNiMo7-6

A martensitic gear steel (18CrNiMo7-6) was annealed at 180 °C for 2h and at ∼ 750 °C for 1h to design two different starting microstructures for shot peening. One maintains the original as-transformed martensite while the other contains irregular-shaped sorbite together with ferrite. These two materials were shot peened using two different peening conditions. The softer sorbite + ferrite microstructure was shot peened using 0.6 mm conditioned cut steel shots at an average speed of 25 m/s in a conventional shot peening machine, while the harder tempered martensite steel was shot peened using 1.5 mm steel shots at a speed of 50 m/s in an in-house developed shot peening machine. The shot speeds in the conventional shot peening machine were measured using an in-house lidar set-up. The microstructure of each sample was characterized by optical and scanning electron microscopy, and the mechanical properties examined by microhardness and tensile testing. The residual stresses were measured using an Xstress 3000 G2R diffractometer equipped with a Cr Kα x-ray source. The correspondence between the residual stress profile and the gradient structure produced by shot peening, and the relationship between the microstructure and strength, are analyzed and discussed.

Forest residues gasification integrated with electrolysis for production of SNG – modelling and assessment

This study investigates opportunities for integrating an electrolysis unit with a biomass gasifier for production of synthetic natural gas (SNG). Gasification is a key technology for production of biofuels and chemicals from lignocellulosic biomass, for which an increased demand is expected in the future. H2produced through an electrolyser can be used to increase the output of a gasifier by reaction with CO2to form CH4. Four integrated flowsheet configurations are evaluated with respect to system energy efficiency and process operating revenue. The system energy efficiencies are in the range of 0.55 – 0.8, and the maximum value of operating revenues is 0.245

Electrochemical Properties of Oxide Scale on Steel Exposed in Saturated Calcium Hydroxide Solutions with or without Chlorides

/kWhdry biomass. The results show that feeding the Sabatier reactor with the full product gas flow coming from the methanation unit, and separating the unreacted CO2afterwards, is the most attractive configuration with respect to operating revenue.