Crack Incubation Research Articles

Effect of boriding surface treatment on hydrogen embrittlement of UNI 30NiCrMo12

The influence of boriding on hydrogen embrittlement was studied by means of delayed failure tests performed on borided UNI 30NiCrMo12 steel. The samples were borided at 900°C for 8 hours with powders ( B 4 C=30%, KBF 4=5%, SiC=65% and quenched at the end of the thermochemical treatment. Hydrogen was introduced into the samples under galvanostatic conditions (d.c. 10 mA/cm 2) by means of cathodic charge in a 0.1 N sulphuric acid solution kept at a constant temperature of 22° C±1. The tests carried out on borided specimens showed an appreciable increase in both the threshold load value and the crack incubation times with respect to the untreated steel. This behaviour is due to the barrier effect of the borided layer. With this coating, hydrogen entry into the base metal was delayed. As a consequence the critical hydrogen concentration in a potential fault was reached later and the crack nucleation and the fracture propagation were delayed. Furthermore the load needed to cause failure within a fixed time was higher for borided samples than for untreated ones.

Factors in hydrogen embrittlement of high strength steels

The effect of pH and current density on a hydrogen induced fracture on UNI 39NiCrMo3 steel was studied using the delayed failure test at a constant load. The hydrogen was introduced into the material by means of cathodic polarization in a sulphuric acid solution at different pH values. The current density imposed on the samples during the embrittlement test ranged from 5 to 20 mA/cm 2. The results obtained showed a noticeable influence of the pH and current density on threshold load values and crack incubation times. This behaviour was attributed to the effect of the pH and current density on rate at which atomic hydrogen adsorbs on the material surface.

The effect of temperature on hydrogen induced fracture in UNI 40NiCrM07 steel

The influence of the temperature on hydrogen embrittlement in UNI 40NiCrMo7 steel was evaluated by means of delayed failure tests. The tests were carried out in the temperature interval 2°–60°C using samples which had undergone two different heat treatments. Hydrogen was introduced into the samples in intensiostatic conditions (d.d.c. 10mA/cm 2) by means of cathodic charge in a 0.1N sulphuric acid solution thermostatically maintained at the test temperatures. The results obtained showed a noticeable influence of the temperature on threshold load values and crack incubation times in the tests carried out on quenched material, whereas this influence was found to be negligible in the tests performed on tempered material. This different behaviour was attributed to the effect of the temperature on movement kinetics in the dislocations, responsible for the accumulation and transport of hydrogen into the zones of maximum stress, and on hydrogen diffusion in the metallic lattice.

ChemInform Abstract: Stress Corrosion Cracking at Constant Load of 304L Stainless Steel in Molten NaCl‐CaCl2 at 570 °C.

AbstractCreep tests of 304L stainless steel in molten chloride solution involves intergranular stress corrosion cracking.

Interaction entre le fluage et la corrosion electrochimique en milieu de sels fondus: Cas de l'acier inoxydable AISI 304L dans NaCl-CaCl 2 fondu à 570 °C

Creep tests at a constant load have been performed in molten NaCl-CaCl 2 at 570 °C. A tensile apparatus was set up to determine the electrochemical parameters (the potential vs. an Ag⋎Ag + reference electrode, and the current density) during mechanical tests. The fractured area, observed by means of a scanning electron microscope, showed intergranular stress corrosion cracking; when the temperature was kept at 570 °C, precipitation of M 23C 6 induced crack propagation. The crack incubation time t i and crack propagation time t p were determined from creep rate curves ≐ = f(t). t i and t p were functions of the applied stress. The crack propagation rate constant was slightly dependent on the applied stress. The test piece potential varied during mechanical testing. Anodic dissolution at the bottom of cracks seemed to play an important part in the corrosion cracking process.

EXAMPLE OF AN APPLICATION OF AN ASSESSMENT PROCEDURE FOR DEFECTS IN PLANT OPERATING IN THE CREEP RANGE

AbstractA worked example is presented to illustrate the application of a CEGB procedure for the assessment of defects in plant that operates within the creep range. The example chosen is a large cylindrical pressure vessel with a fully circumferential crack for which experimental results are available. The example demonstrates that it is straightforward to perform the calculations required in the procedure for the time for structural failure by continuum damage mechanisms, the time for crack incubation and the time for crack growth. It is predicted that crack incubation in the pressure vessel occurs at about one‐third of life with the remaining life being taken up by crack growth. The predictions are in close agreement with the experimentally observed incubation, growth and final failure times.

Stress corrosion cracking at constant load of 304L stainless steel in molten NaCl-CaCl 2 at 570°C

The incubation and propagation times of cracks in 304L in molten NaCl-CaCl 2 at 570°C were related to the applied stress value, from creep and creep rate curves. Rest potential versus time curves were recorded simultaneously. The results showed intergranular stress corrosion cracking. When the temperature was kept at 570°C, precipitation of chromium carbide M 23C 6 which promoted cracking propagation, was induced. Determination of the crack rate shows that anodic dissolution at the bottom of the cracks is the main process during the stress corrosion crack propagation of 304L stainless steel in the stress range used.

The dynamic behaviour of an oblique edge-crack under impact loading

When a stress wave (tensile or compressive) impinges on a crack existing in an elastic medium, reflection, refraction and diffraction-phenomena take place. A result of diffraction is the loading of the crack. While compressive stress-waves do not create any stress concentration at the tip of an existing crack, tensile stress-waves develop stresses at the tip which may cause a propagation of the crack. If the tensile pulse is weak the crack may propagate by steps under the action only of successive tensile stress-pulses, whereas intermediate compressive-stress pulses do not have any influence. A complete study of the phenomena of incubation, initiation and propagation of cracks in thin plates, when they are subjected to a compressive pulse, which is subsequently reflected from the free boundaries of the plate and changed to complicated wave-trains, was undertaken in this paper, based on the method of caustics. Interesting results that were derived from this experimental study are presented.

Strip yield models of creep crack incubation and growth

The strip yield model of Dugdale and Bilby, Cottrell and Swinden is applied to analyse the incubation and growth of creep cracks. This extends previous work by Vitek (on incubation) and by Heaton and Chan (following To) on growth. Following both Vitek and Heaton and Chan, the main criterion of crack growth used is a COD-based one. The crack is assumed to grow at constant crack opening displacement (COD) φ=φc and the creep strain ∈t at the crack tip is assumed to be φ/h, where the gauge length is a material constant. The COD-based theory indicates that the incubation and failure timest i,t fare controlled by the stress intensity factorK at large stresses, but by the net stress at very low stresses. A weakness of the theory is that it neglects primary-creep effects: these have to be absorbed by modifying the secondary-creep index. If this is done, then comparison with two independent experiments shows moderately good agreement with the COD-based theory for botht iandt f.

Effect of Grain Size on incubation and Propagation of Stress Corrosion Cracks in Type 302 Steel

Abstract Effect of grain size on the incubation and propagation of stress-corrosion cracks in annealed Type 302 steel exposed to 42 w/o MgCl2 was investigated. Incubation period was grain-size dependent; incubation times of 15 and 40 minutes were determined for materials of grain size ASTM No. 5 and 9, respectively. Propagation rates were considered to be nearly independent of grain size.