Gunvant N. Maniar

Effect of gamma-gamma prime mismatch, volume fraction gamma prime, and gamma prime morphology on elevated temperature properties of Ni, 20 Cr, 5.5 Mo, Ti, Al alloys

This study was designed to provide a critical test for the postulate that the mismatch between the lattices of austenite (γ) and the age-hardening gamma-prime (γ′) precipitates and the resultant coherency strains have a significant influence on the elevated temperature, particularly stress rupture, properties of a nickel-base superalloy. Two experimental alloys with a base analysis of Ni, 20 Cr, 5.5 Mo were designed with variable titanium and aluminum additions. To discern the effect of mismatch, an alloy without molybdenum was also experimented with. By manipulating the mismatch and volume fraction γ′ by heat treatment and chemistry, it was shown that a lower γ-γ′ mismatch indeed is beneficial to stress rupture life. Importance of volume fraction γ′ on this elevated temperature was also established.

Carbides in M-50 high speed steel

The carbides in M-50 high speed tool steel were studied in detail. The dissolution of carbides as a function of austenitizing temperature, and their precipitation as a function of tempering temperature were characterized by X-ray diffraction and microchemical analysis. The carbides in the annealed steel are M23C6, M6C, M2C, and MC. Upon austenitizing, with increasing temperatures, the carbides dissolve in the order: M23C6, metastable M2C, M6C, and MC. The residual carbides in the heat treated steel are MC and stable M2C. The solvus temperatures of M23C6 and M6C were determined. Upon tempering the hardened steel, with increasing tempering temperatures, carbides precipitate in the order: M23C6, metastable M2C, MC, and M6C. It is shown that the composition of the precipitated metastable M2C is different from that of the residual stable M2C and it varies with the tempering temperature.

Heat treatment of 706 alloy for optimum 1200 °F stress-rupture properties

Evaluation of a commercial heat treatment for 706 alloy indicated that it resulted in relatively low 1200° F stress rupture ductility. It was determined that this was caused by a solution treatment which dissolved all of the age-hardening phases in the alloy and caused a coarse grain size and supersaturated matrix condition. Based upon extensive fine structure study of the 706 alloy as well as previous experience with 718 alloy and other Fe−Ni-base superalloys, a heat treatment is developed which effectively optimizes the 1200°F stress-rupture properties of the alloy. The key to best properties was found to be the precipitation of globular to plate-like Ni3Cb/Ni3Ti at the grain boundaries in conjunction with maintaining a fine as-forged grain structure.

The microstructure of 706, a new Fe−Ni-base superalloy

Alloy 706 is a precipitation-strengthened Fe−Ni-base superalloy which has been reported to have improved machining characteristics over other similar alloys. The present work was conducted to document the phase relationships of the alloy in comparison with other Fe−Ni-base alloys. The precipitated phases identified in the alloy include γ′, Ni3Cb, Ni3Ti, M23C6, and MC. A small amount of Laves phase was also detected. The effects of some composition variations on solvus relationships for γ′ and Ni3Cb/Ni3Ti are documented. Means to take advantage of the structural response of the alloy to develop desired properties are suggested. It is shown that on the basis of chemistry and microstructural response, 706 alloy can be considered to be intermediate to the well-known alloys, 718 and 901.

Correlation of gamma-gamma prime mismatch and strengthening in Ni/Fe-Ni base alloys containing aluminum and titanium as hardeners

Various authors have invoked coherency strains and disregistry between the crystal lattices of the matrix and γ′ phase to account for considerable hardening in γ′-strengthened superalloys. Hagel and Beattie correlated the mode of precipitation with the degree of its lattice mismatch. Heydt and Whitney used this approach during the development of an Fe-Ni base high temperature alloy. To understand the role of such a relationship, an investigation of a few experimental Ni-base/Fe-Ni base alloys was carried out. These alloys were strengthened by variable titanium, aluminum, and molybdenum additions and contained chromium. Lattice parameters of the solution treated and aged samples were measured. The γ′ phase was electrolytically extracted for lattice parameter determinations, and γ−γ′ mismatch calculated. The γ−γ′ mismatch calculated. The γ−γ′ mismatch was correlated with room temperature hardness and stress rupture properties at 1200°F. The influence of alloying additions, matrix and γ′ lattice parameters were interrelated.

Microstructure of thermally embrittled 18 Ni (250 KSI) marage steel

Abstract The thermal embrittlement in high-strength 18 Ni maraging steel, induced by processing and fabrication, has been attributed to coarse grains and the precipitation of platelike carbides in the prior austenite boundaries. The literature on the subject discusses a heat containing higher than recommended residuals (carbon, phosphorous, sulfur, and nitrogen) [1]. In the same study, the authors failed to discern the effect of grain size from that of the precipitates in a low residual heat. This paper describes a controlled study involving a production heat containing very low residuals, carried out to discern the effects of the grain size and the precipitates. By refining the grain size by a triple annealing treatment, it is shown that even the high-purity material may be embrittled if subjected to very high temperature followed by an exposure at an intermediate temperature where grain boundary precipitation is facilitated.

Effect of Reverted Austenite on the Mechanical Properties and Toughness of a High Strength Maraging Stainless Steel Custom 450

One of the problems confronted by a metallurgist, whether in Quality Assurance or Research, is that of the control of austenite in ferritic or martensitic steels. The martensitic transformation in many alloys does not reach 100% completion with the structure containing some amount of “retained Austenite”. Similarly during aging of the maraging stainless steel alloys, “reverted austenite” is formed concurrent with the precipitation of an age-hardening phase. The amount of austenite, retained or reverted, has a marked effect on the properties of these alloys. It should be also mentioned, that in austenitic steels, the formation of delta ferrite during high temperature anneals and the pseudo-martensite formed during cold working, has a marked effect on properties. Therefore, it is desirable to have a means to rapidly and accurately measure the quantity of austenite for quality control and for research problems.