• PDF / 2,769,994 Bytes
• 12 Pages / 593.972 x 792 pts Page_size
• 102 Downloads / 272 Views

DOWNLOAD

REPORT

---

18 percent nickel maraging steels possess ultrahigh strength combined with excellent toughness. The toughness of these steels, particularly of 18Ni 250 grade is distinctly superior to conventional low alloy steels at the same strength. This is achieved through age hardening of relatively soft low carbon martensitic structure; hence, the name maraging is derived. This unique characteristic along with ease of fabrication has made 18Ni 250 steel a preferred choice in aerospace world. Since 1982 when maraging steel indigenization program was established in India, this grade is being manufactured indigenously and is in use for the fabrication of solid rocket motor casings of satellite launch vehicles. The Indian satellite launch vehicles, Polar Satellite Launch Vehicle (PSLV), and Geosynchronous Satellite Launch Vehicle (GSLV) use 18Ni 250 grade maraging steel for the solid rocket boosters. Indian defence sector also has inducted this grade in some of the programs.

RENU N. GUPTA and M.K. MUKHERJEE are with the Larsen & Toubro Limited, Heavy Engineering IC, Powai Campus, Mumbai 400 072, India. V.S. RAJA is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Mumbai 400 072, India. Contact e-mail: [email protected] S.V.S. NARAYANA MURTY is with the Materials and Metallurgy Group, Vikram Sarabhai Space Center, Trivandrum 695 022, India. Manuscript submitted March 8, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

One of the major challenges in welding of 18Ni 250 grade maraging steel is to achieve weldment mechanical properties comparable to that of the base metal. Microstructurally maraging steel weldment contains different zones and their influence on the overall efficiency of the weld is complex. In the fusion zone, during aging treatment, austenite pools formation take place due to the presence of enriched segregation of Ni, Mo, and Ti alloying elements. Presence of these pools reduce the weld fracture toughness as studied by Kenyon and Maoyan.[1,2] Heat affected zone (HAZ) of maraging steel shows three distinct bands; coarse grained near fusion line, light etched, and dark etched band. The light etched zone is heated into fully austenitic region during welding and thus on cooling forms martensitic structure similar to the base metal. The dark etched band which is heated into the two phase region contains martensite with fine dispersion of stable and soft reverted austenite transformed during welding. Presence of this soft austenite improves fracture toughness of HAZ, however, reduces weldment strength.[3,4] The numerous experimental results have shown that weldment strength up to 90 to 95 pct of the base metal and toughness between 70 to 90 pct of the base metal could be attained with gas tungsten arc welding (GTAW) with modified filler metal composition.[1–5] Due to this peculiar welding metallurgy, maraging steel weldment properties are affected by a local repair due to additional heat input during repair. A few studies were conducted on microstructural changes due to l