PSI - Issue 13

M. Newishy et al. / Procedia Structural Integrity 13 (2018) 353–360 M. Newishy / Structural Integrity Procedia 00 (2018) 000–000

354

2

2. Introduction Nearly all engineering structures experience some form of alternating stress and are exposed to harmful environments during their service life. [1-3] The environment plays a significant role in the fatigue of high strength structural materials like steels. Chloride stress corrosion cracking (CLSCC) is one the most common reasons why austenitic stainless steel pipework and vessels deteriorate in the chemical processing and petrochemical industries. [4-6] Deterioration by CLSCC can lead to failures that have the potential to release stored energy and/or hazardous substances. Failures of plant can be prevented by an awareness of the onset and evolution of CLSCC, and by periodic inspection to monitor the extent of cracking. CLSCC initiates from sites of localized pitting or crevice corrosion. [7-11] CLSCC propagation occurs when cracks grow more quickly from the pit or crevice than the rate of corrosion. For fabricated structures containing tensile residual stresses, the critical depth of localized corrosion to initiate CLSCC would be <1mm. The rate of crack propagation is strongly dependent on temperature but is relatively unaffected by stress intensity. [13-15] Rates of CLSCC propagation can vary from 0.6 mm. yr-1 at near ambient temperatures to >30mm.yr-1 at temperatures ~100 0C. In laboratory tests CLSCC has been observed in samples at temperatures between 250 ° C and 40 ° C The majority of the reported practical instances of CLSCC have occurred where temperatures ≥ 60 ºC. However, a significant number of failures below 60°C have also been reported. Although in these instances there appear to have been other contributory factors which include the use of highly cold worked and/or free-machining grades, Iron contamination of the surface and the presence of a highly corrosive atmosphere containing chloride compounds. [16-17] 3. Investigation Methodology The packing sheets were visually examined in the as-received condition. The sheets were cut and sectioned for destructive tests. Chemical analysis of the material was carried out to identify the chemical composition. Macro and microstructures investigations of the failed packing were carried out for different specimens, using well recognized methods for metallographic preparation; mechanical grinding down to 1200 grade emery paper followed by polishing using 0.1µm agglomerated alpha alumina suspension, rinsed and degreased with acetone, and then electro-etched using 10% oxalic acid at 6V for 45 sec. Scanning electron microscope equipped with EDS analysis was used for in-depth examination of the damaged section and existing phases. Hardness measurements were carried out under a load of 10kg for 15 sec. loading time, to determine hardness values for used and unused samples. 4. Results 4.1. Visual inspection and chemical composition General and close up views of damaged packing sheets are shown in Figs. 1-2. The most relevant observation is that the received packing has different colors as a result of the formation of corrosion scale and the black color one showed many perforated attack at the stressed region zones as shown in Fig. 2. The holes in the sheets have irregular shapes. Localized thinning in wall thickness was relatively observed on the black sheet compared with the brown sheet and unused one. Chemical composition by Spark Ignition Spectroscopy of the unused sheet material given in Table 2 showed the conformity of sheet composition to the 316L. It can be noticed that the Cr and Mo contents, characterizing corrosion resistance austenitic stainless steel (316L), lies within the lower specified range. 4.2. Metallographic examinations Metallographic examinations were conducted on cut pieces from the damaged packing. The microstructures of the unused sheet are shown in Fig.3. No localized attack or corrosion scales were observed on the outer surfaces of the packing sheet. Optical microstructure of the un-used sheet material has shown austenitic grains with numerous twins. Elongated inclusions in the rolling direction of the steel plates together with remaining deformation bands are clearly observed. Fig. 4 indicates that there are several other cracks had initiated at the stressed zones. The cracks initiated at the pitting area. The morphology of the cracks resembles the intergranular stress corrosion cracking (IGSCC) in nature. In order to investigate the effect of cold working process during the manufacturing of the sheets on the mechanical characteristics, microhardness measurements were done on several locations at the plate. The average hardness value