Microstructure and Salt Fog Corrosion of Wrought Mg-Al-Zn and Mg-RE Alloys

Wrought magnesium alloys have received attention due to their potential application as lightweight materials. However, their use is limited by their poor corrosion resistance. Rare earth additions have the potential to enhance corrosion resistance. The present work included a microstructural investigation and corrosion testing of the alloy WE-43, containing Nd and Y, which was compared against the more conventional compositions of AZ31 and AZ61 alloys. All three alloys exhibited a recrystallized equiaxed structure after hot rolling with the presence of second phases—precipitates. The WE-43 alloy exhibited a better corrosion resistance than AZ31 and AZ61 under salt fog testing, indicated by the lower depth of attack and lower weight loss. The second phases in the microstructure of AZ31 and AZ61 alloys determined their corrosion resistance. The second phases in the AZ31 and AZ61 alloys (based on Al-Mg and Al-Mn phases) were nobler than the Mg matrix and catholically acted, thus sacrificing the Mg matrix. The superior corrosion resistance of WE43 was due to the incorporation of Y in the oxide/hydroxide film. In addition, the second phases in the WE43 consisted of Nd and Y and were less noble than the Mg-matrix. Thus, they acted as anodic sites protecting the Mg-matrix. The above results show the beneficial effect of rare earth additions to wrought Mg alloys towards increased corrosion resistance

Creep rupture in HP-Nb refractory steel tubes due to short-term overheating

The premature creep rupture of cast reformer tubes made of HP-Nb steel due to short-term overheating, was investigated. Investigation was performed on specimens from ruptured, cracked, and uncracked tubes and included metallography, mechanical testing, and SEM/EDX analysis. It was found that accidental short-term overheating, after 5 years of normal operation, caused rapid tertiary creep, manifested by the formation of dense cavities and cracking, which eventually led to the premature creep rupture of several tubes of the reformer. Creep cavities formed preferentially at M7C3 carbides but also at M23C6 and MC carbides while crack propagation took place along the interdendritic carbide network. It appears that the radial direction of the columnar grains from the inside to outside diameter orients the grain boundaries perpendicular to the hoop stress, a situation which degrades creep rupture resistance. An analysis based on the Larson–Miller parameter confirmed the premature creep rupture of the tubes due to overheating

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel

Investigation of Stress-Oriented Hydrogen-Induced Cracking (SOHIC) in an Amine Absorber Column of an Oil Refinery

Stress-oriented hydrogen-induced cracking (SOHIC) of an amine absorber column made of a Hydrogen Induced Cracking (HIC) resistant steel and operating under wet H2S service was investigated. SOHIC was not related to welds in the column and evolved in two steps: initiation of HIC cracks in the rolling plane and through-thickness linking of the HIC cracks. Both the original HIC cracks as well as the linking cracks propagated with a cleavage mechanism. The key factors identified were periods with high hydrogen charging conditions, manifested by high H2S/amine ratio, and stress triaxiality, imposed by the relatively large thickness of the plate. In addition, the mechanical properties of the steel away from cracked regions were unaffected, indicating the localized nature of SOHIC