Stress-corrosion cracking in copper refrigerant tubing

A metallurgical failure analysis was conducted to determine the cause of cracking in several sections of copper refrigeration tubing. The tubing in question was part of a new mechanical design, implemented to mitigate fatigue failures of solder joints that had occurred in tubing systems fabricated under the previous design. A comprehensive metallurgical evaluation revealed intergranular fracture of the copper in a region of the tubing that had been significantly cold worked during manufacture. On discovery of a source of moist ammonia in the system, associated with the location of failure, intergranular stress-corrosion cracking (SCC) was identified as the failure mechanism. A modified design, incorporating annealing of the formed copper tube section, was recommended to avoid future failures.     

Stress-corrosion cracking of a monel 400 tube

A bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery installation, failed by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid (HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, electron dispersive analysis by X-ray (EDXA) indicated significant segregation of sulfur and chlorine on grain boundaries. Failure was attributed to hypochlorous-acid (HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O₂, and the oxygen entered the tube during shutdowns/startups of the installation. Residual stresses, originating from the in situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation of sulfur on grain boundaries enhanced the susceptibility of the material to cracking.     

Fracture mechanical properties in controlled rolled CMn thermomechanically treated steels with splittings

Fracture mechanical properties of a thermomechanically treated CMn steel were investigated in both longitudinal and transverse directions relative to the rolling direction. The CTOD fracture toughness testing was performed at three deformation rates in the temperature range from −60 to +40°C. Fracture initiation was investigated at room temperature. The CTOD fracture toughness depended very much on the specimen orientation with respect to sulphide inclusions. In the transverse specimens, maximum load was reached just after yielding, hence the recorded CTOD_m values were nearly independent of the rate of deformation and testing temperature.     

Stress-corrosion cracking of reactor feed pump wear rings

Stress-corrosion cracking (SCC) is observed in machine components when an optimal combination of stress levels and environmental conditions is present in susceptible material. An interesting case history of intergranular SCC failure is presented in this paper. This failure occurred in wear rings that were shrink-fitted on the impellers of a multistage horizontal feed pump during the project procurement phase. After manufacture and assembly, two pumps were performance-tested in the supplier's test loop for a combined total of approximately 600 h. The test duration lasted for approximately three months. During this duration, the two pump internals were not absolutely dry and were exposed to an indoor environment in the supplier's test plant. After the completion of the tests, the pump internals were inspected, cleaned, dried, and packaged before air freighting to the reactor site. The pumps were in storage at the site for approximately two months. When the pumps were opened for inspection, two wear rings on each of the pumps were cracked. This paper summarizes the results of the studies to evaluate the root cause of the wear ring failures and the corrective actions implemented to avoid similar failures.     

Stress-corrosion cracking and its propagation in aligned short-fibre composites

Stress-corrosion tests are conducted on aligned short-fibre composites to assess their performance in both air and 1 N H₂SO₄ environments. The materials used are 3 mm E-glass, AS4 carbon, and their hybrid fibre reinforced epoxy composites. Several crack propagation models are developed to describe different cracking behaviour; the observed composite fracture modes verify these proposed models. Pre-immersion has significant effects onK _l-t _f and crack arrest of hybrid composites. A comparison of theoretical and experimentalK _l-t _f curves indicates good agreement of time to failure. The present research has quantified the susceptibility of these composites to environmental attack.     

Studies of a thermomechanically treated Al-Zn-Mg alloy

The influence of subgrain structure on the precipitation process in an Al-4.2 wt% Zn-1.6 wt% Mg alloy was studied and correlated with its mechanical properties for ageing temperatures of 110, 142 and 165° C.     

Stress-corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices

Stress corrosion cracking of transparent conductive layers of indium tin oxide (ITO), sputtered on polyethylene terephthalate (PET) substrates, is an issue of paramount importance in flexible optoelectronic devices. These components, when used in flexible device stacks, can be in contact with acid containing pressure-sensitive adhesives or with conductive polymers doped in acids. Acids can corrode the brittle ITO layer, stress can cause cracking and delamination, and stress-corrosion cracking can cause more rapid failure than corrosion alone.The combined effect of an externally-applied mechanical stress to bend the device and the corrosive environment provided by the acid is investigated in this work. We show that acrylic acid which is contained in many pressure-sensitive adhesives can cause corrosion of ITO coatings on PET. We also investigate and report on the combined effect of external mechanical stress and corrosion on ITO-coated PET composite films. Also, it is shown that the combination of stress and corrosion by acrylic acid can cause ITO cracking to occur at stresses less than a quarter of those needed for failure with no corrosion. In addition, the time to failure, under ~ 1% tensile strain can reduce the total time to failure by as much as a third.     

Corrosion cracking of high-strength structural steels

A study of the mechanism of corrosion cracking of high-strength carbon steels showed that the stress dependence of time-to-rupture of steels and copper, magnesium and aluminum alloys tested in tension in various media is described by the same equation. Hence it was concluded that the basic phenomena involved in corrosion cracking are common for all alloys.     

Creep rupture of thermomechanically treated Mo-5%W alloy

The effect of thermomechanical treatment such as high temperature swaging on the creep failure mechanism of the Mo-5%W alloy was investigated. It was found that heavily swaged specimens fail after a relatively large amount of creep deformation, while slightly swaged specimens fail after a few percent of strain. In this paper the above-mentioned behaviour is explained on the basis of a semiquantitative model.     

Hydrogen-induced cracking in two linepipe steels

Linepipe steels are susceptible to hydrogen-induced cracking (HIC) in wet, sour gas environments. Two commercially produced linepipe steels were investigated with regard to HIC on cathodic charging. Both steels, B and C, showed a high banded microstructure consisting of alternative layers of polygonal ferrite and a mixture of non-ferritic constituents (pearlite, bainite, and martensite-austenite). The degree of banding was higher in Steel B than in Steel C. Also present were elongated inclusions in Steel B, while in Steel C they were more or less equiaxed. On cathodically hydrogen-charging in the absence of external stress, microvoids formed at low current densities at or around inclusions. On prolonged charging, these voids grew and propagated parallel to the bands, running along the interface between ferrite/non-ferrite constituents, along inclusions lodged in the non-ferritic consitituents, and at places through the non-ferritic constituents. Steel B, not unexpectedly, showed more severe permanent microstructural damage than Steel C, leading to the conclusion that a high banded structure and/or the presence of elongated inclusions is deleterious to resistance against HIC.     

Hydrogen assisted intergranular cracking in steels

McMahon suggested that interface decohesion at grain-boundary carbides and precipitates is the mechanism of hydrogen assisted intergranular cracking, HAIC, in high strength steels. In general, cleavage of grain-boundary carbides, adhesion failure or interface decohesion at grain-boundary carbides and precipitates, and crack-tip shear slip along the grain boundary could be the mechanisms of HAIC. Hydrogen reduces cleavage strength, adhesion strength and the resistance to shear slip; therefore, hydrogen assists intergranular cracking. A method of identifying such mechanisms is suggested. A generalized theory of hydrogen assisted cracking is deduced. Brittle crystals cleave on their cleavage planes. Cleavage cracking of such crystals is anisotropic. When the crack-tip stress intensity factor, K, is low, the tortuous cracking process from the anisotropy results in rapidly increasing Stage-I crack growth rate with respect to K. The mechanism of the crack growth threshold, K_TH, is also discussed.     

Common features of fretting-fatigue cracking in steels

Fretting can occur when repeated loading on a structure or part causes repetitive relative movement at contacting metallic surfaces. The fretting process may cause local metal loss and impact the initiation and/or propagation of fatigue cracks. There are some features of fretting-fatigue cracking that are unique. Several of these features are illustrated in three case histories described in this paper: the failure of a splined papermaking refiner shaft, the failure of two coal-pulverizer shafts, and the cracking of a crankshaft flange from a ship’s engine. Fretting fatigue usually results in recognizable damage to at least one of the contacting surfaces. The fretted areas are roughened and, in steel, are usually decorated with reddishbrown deposits. Cracks may be initiated in the damaged region but are located close to the boundary between the damaged areas. Cracking normally starts at an angle of less than 90° to the surface. The geometric stress concentrations present on the component may be overridden because of fretting, and cracks may initiate on previously smooth surfaces. Some cracks may initiate and grow to only a shallow depth before ceasing propagation.     

Sulfide cracking of structural steels and alloys

We present the result of investigation of some pipe and structural steels as well as their welded joints on the resistance to sulfide cracking. We show that refinement of pearlitic pipe steels by known methods can improve their resistance to the level of the best hydrosulfide-resistant steels of foreign firms. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences. L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 1, pp. 83–88, January–February, 1998.     

Crack resistance and micromechanisms of fracture of thermomechanically treated high-strength steel

We study the influence of thermomechanical treatment with deformation by the method of hydrostatic extrusion on the parameters of crack resistance of 45KhN2MFSh high-strength steel and plot the dependences of the critical stress intensity factorK _Ic and critical crack opening displacements δ_c on temperature. It is shown that these curves have the threshold character. The results of microfractographic analysis demonstrate that changes in crack resistance observed as temperature decreases are accompanied by changes in the micromechanisms of fracture in the regions of the onset of crack propagation, which may take place under the condition of changes in the second-order stress-strain state. We show that the temperature curves of the parameters of crack resistance can be efficiently used in determining the temperature of brittle-ductile transition. In the considered case, this temperature does not depend on the size of the specimen and the loading mode and characterizes the structural state of the cracked material. As compared to conventional modes of thermal treatment, thermomechanical treatment guarantees much higher values of crack resistance, especially at low temperatures, and decreases the threshold of cold brittleness for 45KhN2MFSh steel by 20°C. The indicated increase in crack resistance is explained by the hereditary influence of the deformational substructure on the structural and morphological parameters of martensite. Khar'kov State Technical University of Automobiles and Roads, Khar'kov. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 82–86, May–June, 1997.