Eva Mazancová

Statistical model of cleavage crack propagation in spheroidized steels

Abstract This contribution presents a model of cleavage crack propagation in spheroidized steels. Microcrack initiation is considered in precipitated carbide particles when the local stress exceed cleavage strength σf of the carbide given by the Griffith criterion. A major crack is formed by microcrack linking leading to the final fracture instability. Using crack transition probabilities the probability of crack arrest at any given point within the active volume was calculated. The model suggests that the susceptibility of a microstructure to cleavage failure can be assessed by the least value of ( x s σ 2 y K 2 IC ) parameter at which the probability of crack arrest Pii is equal to unity.

Prediction of fracture toughness for a pressure vessel steel with mixed bainite-ferritic microstructure

Abstract This paper presents a model for fracture toughness prediction of a 10MnNi2Mo steel having mixed bainite-ferritic microstructure as used for the pressure components of a nuclear reactor primary circuit. In addition to cleavage microcrack nucleation in carbide particles, consideration is given to a dislocation mechanism for nucleation of microcracks in ferrite as proposed by Cottrell. The presented model showed good agreement between the predicted and experimentally determined values of stress intensity factor.

Local structure of explosively welded titanium–stainless steel bimetal

Abstract Bimetals targeted for industrial applications are usually designed to combine the properties of two dissimilar metals, e.g. high strength, high hardness and low cost of steels with chemically resistive titanium. In this work the structure of one particular, but for industries very interesting, bimetallic system – Cr/Ni stainless steel clad with titanium has been studied. The material was prepared by explosion welding, a method capable of joining a wide variety of similar or dissimilar materials. Our analysis is based on X-ray micro-diffraction experimentation utilizing hard monochromatic X-rays focused down to micrometer size. In this way the bimetal in bulk form was analyzed and microstructural differences between the joined materials and their interface were determined.

Cleavage fracture path prediction for a pressure vessel steel

Abstract This contribution presents a model for fracture path prediction applied for pressure vessel 10MnNi2Mo steel with mixed bainite-ferritic microstructure. Microcrack initiation is considered both in precipitated carbide particles and in ferrite grains. A major crack is formed by microcrack linking leading to the final fracture instability. Using crack transition probabilities the probability of crack stability in the active volume was calculated. Considering the fracture toughness as a controlling parameter of crack stability the temperature dependence of predicted fracture toughness has been set. A very good agreement with experimentally obtained temperature dependence of K IC values has been found.

Model of ductile fracture initiation in spheroidized steel

Abstract Statistical analysis of the microstructure of spheroidized steel has been harnessed to evolve a new method for assessing the final stage of the ductile fracture process, the coalescence of voids. For a known particle size distribution and a known spatial distribution of second phase particles in the steel matrix, the Garrison and Thompson coalescence criterion is applied to determine the probability density of local fracture deformations, and the dependence of the probability of ductile fracture initiation upon the parameter expressing the distance ahead of the crack tip, x/S. This newly devised model of ductile fracture has been applied to a low-carbon niobium-microalloyed steel, containing 0.043% by weight of niobium; the results confirm that the probability of ductile fracture initiation ahead of a crack diminishes as the distance from the crack tip grows. As the coalescence of voids ahead of a crack tip governs the onset of stable crack propagation, the new model is expected to prove useful chiefly in the statistical prediction of upper bound of fracture toughness in engineering steels.

Hydrogen Response of 304 SS and Ti Weld Realised by Explosion

Welds of thick plates (304 SS) cladded with Ti of commercial purity both in as received state and subsequently heat treated (HT, 600°C/1.5 h/air), charged by hydrogen, were investigated after fatigue tests (tension-pressure) with amplitude of 20 Hz. Simultaneously, samples of above mentioned welds were also exposed in H2S in accord with NACE Standard TM0284-2011 to find hydrogen induced cracking (HIC) response. Charged welds after the HT showed by 15 MPa higher threshold level than the welds without the HT. The HIC tests generally demonstrated favourable results both after as weld state and after subsequent HT. Short and thin cracks were observed, exclusively located in intermetallic phase areas, where approx. 16-20 wt. % of Ti was revealed. Fracture surfaces of fatigue bars showed maximal failure in areas with 42-97 wt. % of Ti.

The instability of plastic deformation in as-quenched martensite

Abstract The paper reports on an investigation of the temperature dependence of the rate sensitivity parameter in as-quenched martensite of Fe-24.1wt.%Ni-0.40wt.%C alloy. Most of the ascertained values of the rate sensitivity parameter were negative. Apparently, the rates of dislocation motion surpassed the rates at which the interaction forces attained their maxima. Simultaneously, the temperature ranges of the plastic deformation instabilities were determined. At low temperatures, the activation parameters of plastic deformation U 0 = 0.31 eV and v * = 10 b 3 were determined. The probable mechanism of plastic deformation in this temperature range was proposed.

Influence of sour environment on hydrogen embrittlement of steel for oil country tubular goods (OCTG)

This paper reports a study on the susceptibility to hydrogen embrittlement in 12 heats of an engineering steel, used for the production of drill pipes, which differed in their sulphur contents and in their microalloying with titanium, niobium and/or vanadium. Tests were performed in the presence of H2S in a NACE (National Association of Corrosion Engineers) solution complying with Specification TM-01-77 Section, 2. The findings prove that life forecasts must always take into account a whole set of microstructural factors, as well as the conditions that govern an uniform distribution of hydrogen atoms in the material.

Changes of Microstructure and Grainsize of Martensitic Stainless Steel during the Processes of Hot Reversed Extrusion, Broaching and Heat Treating

The field of high pressure steel cylinders (HPSC) used for the variety of applications, such as storage of technical gases, compressed natural gas (CNG), medical gases and special applications like scuba diving cylinders is still significantly expanding. With increasing safety requirements, the need of new techniques applied in field of HPSC structural and materials innovations comes into place.Speaking mainly about the field of scuba diving cylinders, the need of corrosion protection of inner surface comes in place due to the possible risk of valve blockage by metal corroded particles of inner surface. Even such a risk occurs in extremely rare occasions, there was until now no generally applied solution (such as inner wet, powder or Teflon coating, etc.) due to significantly high costs of such additional inner surface protection. Another problem that occurs so far are significant visual imperfections of scuba diving cylinders caused by the rough treating by scuba divers during the use and manipulation that causes chipping of outer surface painting and subsequent corroded areas.The breakthrough solution is the scuba diving cylinder made of a stainless steel by the processes of reversed extrusion and broaching from billet without any need of an additional coating. No such a product was made so far by these methods of production and no evidence of an evolution of the microstructure and grainsize changes during such manufacturing with subsequent heat treating was ever documented and analysed.

Changes of Mechanical Properties in Response to Changes in the Microstructure of High Pressure Steel Cylinders Obtained by the Various Heat Treatments

The field of high-pressure steel cylinders (HPSC) and vessels used for the variety of applications especially for the CNG (Compressed Natural Gas) in automotive industry is still expanding. With increasing safety requirements, a need of new techniques applied in field of HPSC structural innovations comes into place. Conventional treatment provides generally sufficient parameters of HPSC, but the long-term goal is to achieve as high as possible strength properties vs. as high as possible notch impact strength, which is currently difficult fully accessible, because of the banded structures (segregation banding) created during the process of reversed extrusion and reversed hot rolling.