PSI - Issue 17

Flavio Pereira de Moraes et al. / Procedia Structural Integrity 17 (2019) 131–137 Flavio Pereira de Moraes et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Stainless steels play a very significant role in the modern world and it is difficult to imagine the existence of several industrial sectors, such as the chemical, petrochemical, food and nuclear industries, without stainless steels. Among the several groups of stainless steels, the austenitic ones are the most used and represent about 2/3 of the total production of stainless steels. The austenitic stainless steel AISI 316L is a stainless steel mostly used at higher temperatures (Plaut et al, 2007). During service or heat treatment in the temperature range of 550 to 900ºC, apart from small quantities of carbides M 23 C 6 (FCC, NaCl type , M = Cr, Fe, Mo) and M 6 C (FCC, diamond type , M = Fe, Mo, Cr), precipitation of undesirable intermetallic phases may occur in the austenitic stainless steels (Padilha and Rios, 2002). The intermetallic phases most frequently observed in austenitic stainless steels containing Mo, which is the case of the AISI 316L, are: sigma, σ , (TCC, D8b, Fe-Cr- Mo); chi, χ, (Cubic, α -Mn type, A12, Fe-Cr-Mo) and the Laves phase, η, (HCP, C14, Fe 2 M, M = Mo, Ti). TTT diagrams of the precipitation of these phases in the AISI 316 and 316L have been determined by Weiss and Stickler (1972). The occurrence of the above-mentioned intermetallic phases generally results in the loss of ductility as experimentally shown by Hull (1973), apart from leading to the impoverishment of the matrix solid solution of important alloying elements, mainly Cr and Mo, causing loss in corrosion resistance as has been showed by Terada et al. (2008). Therefore, studying the occurrence of these phases is of great interest, mainly for the correct usage of this steel, as well for the development of new compositions of austenitic stainless steels. Recently an AISI 316L pipe which has been operational in the petrochemical industry for about fifteen years (including stops and plant maintenance interruptions) became available for analysis and a microstructure characterization program, related to the mechanical properties and corrosion, has been started and is in progress at our department. The main objective of the present work is to present the most significant results related to the microstructural changes, in the mechanical properties and in the intergranular corrosion resistance of an 8-inch nominal dia. pipe of austenitic AISI 316L, exposed for 100,700 hours. at 640ºC in the petrochemical industry. Therefore, some complementary techniques of microstructural analysis, hardness, tensile and impact mechanical tests and the corrosion test known as practice “A”, of the ASTM A262 (2008) standard, have been used. 2. Experiment The samples for analysis have been taken from the exit side of the pipe system, near the butt weld area. The reactor exit temperature is controlled by the flow of the cooling liquid kept constant at 640 °C, with an internal pressure of 4.5 MPa. The pipework, which started operations in the year 2002, has a nominal diameter of 8 inches (200 mm) and 0.59 inches (15 mm) wall thickness, specified by the ASTM A358 Grade TP316. In this paper only results of the bulk area of the pipe will be presented, leaving the welded area for a later paper, due to shortage of publication space. The chemical compositions (mass %) of the pipe material and the, by the producer, specified one are presented in Table1. Comparing the values obtained with the specified ones one may affirm that the material used can be considered as being of the AISI 316L type.

Table 1. Chemical composition (mass %) of the pipe and the AISI 316L specification. Element C Si Mn Cr Mo

Ni

Pipe

0.018 ≤ 0.03

0.28

1.69

17.03

2.32

12.58

Si ≤ 1.00

Mn ≤ 2.00

16.0 – 18.0

2.0 – 3.0

10.0 – 14.0

Specification

To perform the metallographic analysis (optical and scanning electron microcopy), the samples have been ground, polished mechanically, and etched with the Villela or Glyceregia solutions, in accordance to traditional methodology. The analysis for scanning electron microscopy (SEM) and the chemical analysis using energy dispersion spectrometry (EDS) have been performed on a FEI Quanta 450 FEG microscope. Apart from these, additional analysis has been performed with the SEM on the fracture surfaces of the Charpy samples. The X-ray diffraction analysis has been performed in the range between 20 and 120º using a Philips X’PERT -MDP diffractometer using CuK α 1 radiation. In order to evaluate the sensitization degree (intergranular corrosion), the test known as practice “A”, in accordance with