Vellore Institute of Technology (VIT) is a private university located in&nbsp;Tamil Nadu, India. Founded in 1984, as Vellore Engineering College, the institution offers 20 undergraduate, 34 postgraduate, four integrated and four research programs. It has campuses in Vellore, Amravati, Bhopal and Chennai.

VIT is one of the top ranked private universities in India according to NIRF, THE and QS Rankings.&nbsp;Govt. of India has recognized&nbsp;VIT, Vellore as an&nbsp;Institution of Eminence. This has allowed VIT to take independent quality initiatives and move up in world ranking.

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VIT University

Eighteen percent (18%) Ni maraging steels are ultrahigh strength steels possessing outstanding fracture toughness. They are based on Fe-Ni-Co-Mo system and contain about 18 wt% nickel. The heat treatment consists of (i) solution annealing, which involves holding the material in the austenitic phase field to take all alloying elements into solution, followed by cooling to room temperature and (ii) aging to realize a fine and uniformly dispersed (Ni,Fe,Co)3(Ti,Mo) precipitates and Fe-Mo based precipitates. The aging treatment is simple; it is carried out at a relatively low temperature and causes very little dimensional change.Today the heat treatment of maraging steel has matured to such an extent that it can take care of variations in the preceding hot working phase of manufacture and consistently lead to high levels of strength, ductility and toughness in a variety of product shapes and sizes, including large section sizes. Thermal cycling can cause grain refinement in these steels and this phenomenon has been industrially exploited to produce massive maraging steel products with a fine grain size and optimum combination of mechanical properties.Overaging treatment can lead to higher toughness levels with some drop in strength. However, this treatment has not found favor in industry to realize higher toughness levels, due to poor reproducibility of properties. Adopting lower strength variants of 18% Ni maraging steel in peak aged condition has instead been the preferred approach.Cobalt being an expensive element, cobalt-free 18% Ni maraging steels have been developed. Titanium is present at a higher level in these steels, compared to their cobalt-bearing counterparts. Heat treatment for realizing the high strength condition again consists of solution annealing and aging. (Ni,Fe)3(Ti,Mo) is the dominating strengthening phase, with higher Ti level and lower Mo level in the precipitate, compared to cobalt-bearing grades.Both cobalt-bearing and cobalt-free maraging steels show marked degradation in toughness, if they get exposed to temperatures in the range 780-950oC while cooling from high temperatures (e.g. 1200°C). This phenomenon is termed thermal embrittlement and can cause serious problems in processing components of thick sections. © 2010 Nova Science Publishers, Inc. All rights reserved.

Department of Manufacturing Engineering

School of Mechanical Engineering

Vellore Campus

Heat Treatment: Theory, Techniques and Applications

Heat treatment of 18% nickel maraging steels

Superalloys are heat-resistant alloys based on iron, nickel-iron, nickel or cobalt, exhibiting high creep and rupture strength at elevated temperatures. A number of both wrought (produced through ingot metallurgy route) and cast grades of superalloys have been developed over the years. There is also a group of superalloys produced through powder metallurgy route. This article focuses on the metallurgy, high temperature mechanical properties and applications of conventional wrought superalloys based on iron, nickel and cobalt. A number of wrought superalloy grades have become commercially available over the last seven decades; the emphasis was on the development of alloys with higher and higher creep resistance. A noteworthy feature of wrought nickel base superalloys available today is their heat-resistance upto 80% of their incipient melting temperature, a fraction that is higher than for any other class of engineering alloys. There have been a number of processing discoveries which helped in development of wrought superalloys with higher and higher performance levels. The strengthening mechanisms that come into play in different groups of wrought superalloys have been well understood. Certain trace elements play a major role in superalloys and there have been a number of studies dealing with underlying mechanisms. Considerable progress has been achieved with reference to understanding the metallurgical stability of wrought superalloys under high temperature loading conditions and its relation to chemical composition. While non-aerospace applications have been there, prominently in the power generation sector, aerospace accounts for the major part of the consumption of the wrought superalloys. Developments in wrought nickel-iron and nickel base superalloys over the decades have importantly contributed to the evolution of modern high performance jet engines. © 2011 by Nova Science Publishers, Inc. All rights reserved.

Superalloys: Production, Properties and Applications

Wrought superalloys-science, technology and applications

The superalloy grade 718 is used extensively for aeroengine applications. Since the fabricated structures operate at high temperatures and engine-designer has also to reckon with stress concentrations, notched stress rupture testing is an important acceptance criterion. The specification stipulates a minimum life of 23 hours and minimum elongation of 4%, when testing is done on specimens with a combined notched and unnotched test section at 922 K and stress level of 690 MPa. Solutionising of 718 is carried out in the temperature range - 1311 to 1339 K or 1200 to 1283 K. Solutionising in the upper range produces notch brittleness; for aeroengine applications requiring good notch toughness, the lower range is recommended. For indigenous production of this superalloy for aeroengine application, solutionising was carried out at 1253 K. Morphology, volume fraction and distribution of delta phase in the microstructure importantly influence the notch rupture life of 718. Microstructure may be acceptable or unacceptable, based on the nature of presence of δ phase. The hot working practice adopted for indigenous production of this material always resulted in an acceptable microstructure after solution treatment. Presence of Laves phase in the microstructure can adversely affect the notch rupture life. Indigenous production of this material included adequate high temperature soaking treatments to completely dissolve the Laves phase. In spite of the foregoing, there were instances when the alloy showed unacceptably low levels of notch rupture life. The presentation shall comprehensively review the subject of notch rupture life of Superalloy 718. © 2010 TIIM, India.

Transactions of the Indian Institute of Metals

Factors influencing the notch rupture life of superalloy 718

Slow strain rate testing of notched cylindrical specimens of 18Ni2400 maraging steel has been carried out in air with 30% relative humidity and synthetic seawater environments. Peak-aged condition has been chosen, considering the relevance to engineering applications. Studies have also been carried out with different notch geometries to understand the effect of stress concentration factor. It is concluded from the study that (i) degree of stress concentration at the notch influences the notched tensile strength (ii) mild hydrogen embrittlement seems to occur in air environment, (iii) synthetic seawater environment drastically brings down the notched tensile strength and time to fracture (iv) environmentally assisted cracking occurs in air tests in quasicleavage and microvoid coalescence modes and in seawater tests in intercrystalline mode. © 2009 Elsevier Ltd. All rights reserved.

World University root node

Corrosion Science

Environmentally assisted cracking of 18%Ni maraging steel

Maraging steels based on iron-nickel martensite constitute a very important family of high-strength steels, which distinguishes itself by demonstrating an unparalleled combination of excellent fabricability and high strength and fracture toughness after heat treatment. Heat treatment of these steels has now been perfected to ensure consistently high levels of strength, ductility, and toughness in a variety of product shapes and sizes. Cobalt-free variants have been commercialized as part of efforts to save production costs. Further knowledge has been generated on 18 % nickel maraging steels regarding phases precipitating during aging, thermal embrittlement, thermal cycling and austenite reversion/retention and their effect on mechanical properties. The paper reviews the progress made over the last one and half decades.