Abstract
9% Ni and low carbon steel is used in cryogenic services in oil and gas industries. The final mechanical properties are adjusted by quenching and tempering heat treatments. However, the un-correct tempering may cause temper embrittlement, with drastic decrease of toughness at cryogenic temperatures. In this study, specimens tempered at 350°C, 400°C and 450°C showed very low toughness at low temperature (-196°C) due to temper embrittlement. Specimens slowly cooled from the tempering temperature (565°C, 585°C and 605°C) also showed toughness reduction in comparison with specimens tempered at the same temperature and cooled in water. The brittle fracture was characterized by intergranular cracks and cleavage. Magnetic Barkhausen Noise (MBN) inspection was conducted to verify if this technique can be used to detect the temper embrittlement in 9Ni steels. The root mean square (RMS) of the MBN signal was higher in specimens as quenched and in specimens tempered in the temper embrittlement range (350°C-500°C) than in specimens which were correctly tempered (565°C-605°C and water cooled). Comparing specimens tempered at 565 and 585°C range and slowly cooled with those which were water cooled, the RMS(MBN) was higher in the former group, which presented the lower toughness. However, the MBN inspection could not separate specimens tempered at 605°C slowly and rapidly, which can be related to the higher austenite volume fraction measured in the specimen slowly cooled.
Highlights
9% Ni low carbon steels were developed in 1940’s by International Nickel Company as an option to replace materials used in cryogenic applications
* Corresponding author: ssmtavares@terra.com.br https://doi.org/10.10 51/matecconf /202134902020 as a material used in cryogenic applications until temperatures as low as -196°C, through heat treatments according to normative specifications
The observation of the fracture surface by scanning electron microscope (SEM) of specimen quenched and tempered at 400oC (QT400) reveals a brittle aspect with cleavage facets and intergranular cracks (Fig. 3(a–b))
Summary
9% Ni low carbon steels were developed in 1940’s by International Nickel Company as an option to replace materials used in cryogenic applications (such as austenitic stainless steels, nickel alloys and aluminum alloys). Https://doi.org/10.10 51/matecconf /202134902020 as a material used in cryogenic applications until temperatures as low as -196°C, through heat treatments according to normative specifications. The use of this steel reduced productions costs and problems with welding, in addition to producing lighter constructions, while mechanical properties, as mechanical resistance and toughness, are maintained satisfactory in temperatures well below 0°C. [2,3,4] This treatment produces a microstructure of ferrite and iron carbides (tempered martensite), with retained austenite. This combination provides very high impact toughness Charpy at −196°C. MBN inspection was performed as an attempt to identify non-destructively the heat treatments condition in which temper embrittlement is observed
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
