Materials Performance

MAR 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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58 MARCH 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS SELECTION & DESIGN S Because of its superior corrosion re- sistance, austenitic stainless steel se- ries Type 316L (UNS S31603) is a good material for petrochemical industrial equipment. In some weld areas, local- ized and stress corrosion cracking de- fects can develop in this material. The major cause of this cracking is precipi- tation of chromium in austenite grain boundaries. Heat treatment is a com- mon approach to address this prob- lem, but it is not possible for in situ equipment. Research shows that in- creasing trace elements in weld mate- rials can control chromium precipita- tion. The effect of different alloy elements on welding properties was investigated for three filler metals. The results show that precipitation of tita- nium and niobium in the grains can inhibit chromium carbide (Cr 3 C 2 ) pre- cipitation and improve cracking resis- tance and corrosion resistance in sul- fur environments. S t r e s s c o r r o s i o n c r a c k i n g ( S C C ) normally occurs in standard and high- carbon (0.10% max) grades of austenitic stainless steels (SS) and other austenitic alloys that are sensitized by either weld fabrication or operation in the sensitized temperature range. 1 The principal variables that affect the SCC mechanism are tensile stress, solution environment, and metal properties. More- over, austenitic SS are susceptible to SCC when exposed to seawater, hydrogen sul- fide (H 2 S), sodium hydroxide (NaOH)-H 2 S, magnesium chloride (MgCl 2 ), and barium chloride (BaCl 2 ) solutions. 2 Sulfur ions and chloride ions are significant contributors to SS corrosion. In sulfur and chloride solu- tions, pitting and SCC corrosion initiate with sulfur ions. 3 Failures due to SCC occur under critical stress, which is dependent on environmen- tal factors and the degree of sensitization, which, in turn, depends on temperature and time of sensitization. Increasing the degree of sensitization changes the fracture mode from intergranular to transgranular. At the sensitization temperature, chro- mium carbide (Cr 3 C 2 ) precipitates at the grain boundaries and depletes the chro- mium in the grain body, thus lowering SCC resistance. 4 Precipitation of Cr 3 C 2 in an austenitic SS is dependent on carbon diffusion and solubility of carbon in the austenite. Each of these factors is strongly temperature- dependent—both diffusion rate and car- bon solubility increase with increasing temperature. 4 Increasing temperature results in chro- mium diffusion into the Cr-depleted grain boundar y zone. This treatment relieves over half of the residual stress and thus re- duces susceptibility to SCC. This operation is not possible in situ or for large equipment, but with good welding procedures, use of suitable filler metal, and reduced heat input, SCC susceptibility can still be controlled. Some researchers investigated inter- granular corrosion behavior of AISI Type 316Ti (UNS S31635) and Type 321 (UNS S32100) austenitic SS, and found that the presence of Ti in Type 316Ti SS reduces chromium-rich carbide precipitation and increases the stability of titanium carbide (TiC), which tends to replace chromium in Corrosion Resistance of Welds in Type 316L Stainless Steel m o H ammad e S mailian, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran

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