Embrittlement in CN3MN Grade Superaustenitic Stainless Steels

Superaustenitic stainless steels (SSS) are widely used in extreme environments such as off-shore oil wells, chemical and food processing equipment, and seawater systems due to their excellent corrosion resistance and superior toughness. The design of the corresponding heat treatment process is crucial to create better mechanical properties. In this respect, the short-term annealing behavior of CN3MN grade SSS was investigated by a combined study of Charpy impact tests, hardness measurements, scanning and transmission electron microscopy. Specimens were heat treated at 1200 K (927 A degrees C) for up to 16 minutes annealing time and their impact strengths and hardnesses were tested. The impact toughness was found to decrease to less than the half of the initial values while hardness stayed the same. Detailed fracture surface analyses revealed a ductile to brittle failure transition for relatively short annealing times. Brittle fracture occurred in both intergranular and transgranular modes. SEM and TEM indicated precipitation of nano-sized intermetallics, accounting for the intergranular embrittlement, along the grain boundaries with respect to annealing time. The transgranular fracture originated from linear defects seen to exist within the grains. Close observation of such defects revealed stacking-fault type imperfections, which lead to step-like cracking observed in microlength scales

CN3MN süperostenitli paslanmaz çeliklerde gevrek kırılma mekanizmaları.

Steel is one of the most widely used materials in almost every major engineering application. Their uses extend from the large constructions in the capitals to the small kitchenware products in houses. Steel has been known since the ancient times, and the properties of steel have been enhanced for hundreds of years by the help of the improvements in metallurgy and material science. Steel has been almost at the center to the discovery-based development of new alloys with notable properties. The steel family has been still growing with the innovations in compositions, heat-treatments and production techniques. Attributed primarily to its remarkable deteriorative properties, CN3MN, is one of the significant Super Austenitic Stainless Steel (SSS), which is a subclass of austenitic stainless steels. Main application areas of CN3MN are marine constructions and oil wells due to their excellent corrosion resistance and superior toughness. However, according to the results of recent time temperature transformation (TTT) diagrams, incorrect heat-treatments for as short as 15 minutes causes fracture toughness of the alloy to decrease by 50% as embrittlement occurs. Wrought steel pieces are typically smaller in size and can be heat-treated and cooled rapidly in order to avoid embrittlement. However, in the case of cast superaustenitic stainless steel, cooling rate is much slower so the regions suffered from such embrittlement is extended. The focus of this study is to understand the mechanisms of the embrittlement problem after annealing heat-treatments for very short times i.e. 30 seconds to 16 minutes at 927°C. The fracture surfaces were investigated by using scanning electron microscope (SEM). Failure analysis results showed a ductile to brittle failure transition for relatively short annealing times. The first reason for embrittlement was defined as the precipitates formed at the grain boundaries. Since the intermetallic precipitates are very brittle, they deteriorate the mechanical properties when the brittle network is formed on the grain boundaries. Average precipitate size in 30-second annealed specimen was found to be about 400 nm with a high number density. The crystal structure of the precipitates formed in short-term annealing could not be matched with previously determined secondary phases in austenitic stainless steel. The kinetics of the nucleation and growth was determined to be very sluggish in the previous studies. In this study, it is shown that the newly determined precipitates have relatively high nucleation rates. This is related to inhomogenous distribution of Mo after the homogenization. These Mo-rich regions are thought to trigger a rapid precipitation. Second mechanism observed in CN3MN is transgranular embrittlement. The heat-treated specimens contained some cracks with zigzag pattern. Close observation of such defects revealed stacking-fault type imperfections, which lead to step-like cracking observed in micron-length scales. The reason of having such faulted regions was connected to having low stacking fault energy. Stacking fault energy of austenite phase in CN3MN was calculated to be between 12-30 mJ/m2 which is quite low for a metallic alloy. Another implication for low SFE was the dislocation distribution. Low SFE promotes entangled dislocation structure instead of subgrain formation, which limits the dislocation motion. The low SFE suppress the crosslinking and climb processes, which are the most active phenomena to maintain dislocation motion. The stacking-fault observed in austenitic matrix changes the FCC by HCP sequence within the defected nano-metric regions. Therefore, the vast majority of transgranular cracks were formed within the brittle HCP-sites resulting into catastrophic transgranular fracture.M.S. - Master of Scienc

FEN BİLİMLERİ ENSTİTÜSÜ/LİSANSÜSTÜ TEZ PROJESİ

CN3MN SÜPERÖSTENİTLİ PASLANMAZ ÇELİKLERDE GEVREK KIRILMA MEKANİZMALAR

Effects of pH and cobalt concentration on the properties of nickel cobalt alloy plating

Nickel-Cobalt alloys are among the most prominent materials due to their magnetic properties, wear resistance, thermal conductivity and electrocatalytic activities. Although nickel and/or nickel-cobalt alloys have been plated from Watts solution for a long time, today, especially for the engineering applications, nickel sulfamate based solutions are commonly preferred due to the advantages of low internal stress and high deposition rate. The properties of nickelcobalt alloy platings are directly related to the pH and the cobalt amount of the solution. In the literature, the optimum pH range was determined as 3.0-5.0, but the effects of the changes within this range on the properties of the coatings have not been investigated deeply. Three different solutions were prepared to perform coatings from solutions containing different pH and amounts of cobalt in this study. The coatings were characterized in terms of grain size and shape, composition, and hardness properties

Optimization of Mechanical Properties of Nickel-Cobalt Coatings from Sulphamate Baths

Nickel-cobalt alloys are important coating materials due to their magnetic properties [1-2], wear resistance [3-4] and electrocatalytic properties [5]. Their deposition from sulfamate electrolytes has many advantages including low internal stress, homogenous deposition at high current densities, high current efficiencies and higher hardness as compared to the conventional Watts electrolyte [6]. A statistical experimental design was used to study the effects of boric acid and nickel chloride concentrations, bath temperature and applied current density on nickel coatings in sulphamate baths. After the optimization of the electrolyte composition, effects of pH and Co additions to the electrolyte were investigated. The coating thickness and its distribution were measured by XRF techniques. Microhardness, surface morphology and internal stress of coatings were also determined. It was found that Co addition to the electrolyte and operation at higher pH yielded deposits of higher Co content and enhanced hardness up to a certain point

Embrittlement in CN3MN Grade Superaustenitic Stainless Steels

Superaustenitic stainless steels (SSS) are widely used in extreme environments such as off-shore oil wells, chemical and food processing equipment, and seawater systems due to their excellent corrosion resistance and superior toughness. The design of the corresponding heat treatment process is crucial to create better mechanical properties. In this respect, the short-term annealing behavior of CN3MN grade SSS was investigated by a combined study of Charpy impact tests, hardness measurements, scanning and transmission electron microscopy. Specimens were heat treated at 1200 K (927 °C) for up to 16 minutes annealing time and their impact strengths and hardnesses were tested. The impact toughness was found to decrease to less than the half of the initial values while hardness stayed the same. Detailed fracture surface analyses revealed a ductile to brittle failure transition for relatively short annealing times. Brittle fracture occurred in both intergranular and transgranular modes. SEM and TEM indicated precipitation of nano-sized intermetallics, accounting for the intergranular embrittlement, along the grain boundaries with respect to annealing time. The transgranular fracture originated from linear defects seen to exist within the grains. Close observation of such defects revealed stacking-fault type imperfections, which lead to step-like cracking observed in microlength scales.This article is from Metallurgical and Materials Transactions A 45 (2014): 2405-2411, doi: 10.1007/s11661-013-2179-8. Posted with permission.</p