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Effect of grain size on mechanical properties of Inconel 718

Inconel 600 for acid processing

Inconel 718 is a widely used superalloy that is relatively economical however has good mechanical characteristics. Inconel 718 is heat processed at different temperatures. The grain sizes were measured on both samples by using Light Optical Microscope. The mechanical test results show the damage of tensile strength and hardness while enhancement of creep life with an increase in grain size. The stress relaxation results offer inkling of the effect of heat processing on Inconel 718. The most common way of changing the microstructure of a material is through heat processing. The mechanical characteristics that are of interest are strength, toughness, ductility, creep and fatigue. Two different sets of samples are heat processed at different temperatures and effect of heat processing on the grain size is evaluated. For better understanding of effect of various elements on the overall properties of the material. The tensile, creep and stress relaxation tests were performed and the results were evaluated.

There are different kinds of super alloys with their own special characteristics, but Inconel 718 is different because of its supreme mechanical characteristics and supreme corrosion resistance. The most commonly used property is its structural stability that remains stable even at its melting point. The effect of heat processing on the mechanical characteristics.  There are several supealloys made by different manufacturers that found their applications in aircraft industry, chemical plant system, power production and nuclear power units. They are available in cast or wrought forms. The chemical composition of each of them varies, depending on the required characteristics.

These alloys are reinforced by an intermetallic compound precipitation in a FCC matrix. The common precipitate is gamma prime. Inconel 718 as gamma prime and gamma double prime precipitates as iron-nickel base due to their higher iron content. Precipitation or age hardening is done to achieve desirable strengthening precipitates and control other secondary phases. They include carbides, borides and TCP. The aging process increases hardness and strength however at the cost of reduction in ductility.

These heat treatments are followed by chemical process for example metallographic etching. It is a technique that is commonly used to highlight on microscopic characteristics such as grains, grain boundaries, impurities and even phases. These etchants are used to develop contrast between elements in Inconel 718 sample by corroding some elements.

Grain size plays a major role in achieving the required properties for instance creep, tensile strength, low cycle fatigue and crack growth. To investigate the above mechanical properties and their retain to grain size a careful preparation of the test sample was done.

The crystalline solids are an amalgamation of various crystals often called polycrystalline.  Each and every crystal is named a grain. The demarcation between grains are named as grain. The demarcation between grains are named grain boundaries.

Superalloy Inconel 718 wire samples are solution heat processed at 960oC for two hours and at 1050oC for two hours, after aging at 720oC for eight hours and after at 620oC for 8 hours for samples. Average grain size measurement has been done after ASTM E112- 12 and ISO 643 standards. There are different methods for grain size measurements, for example intercept method. SEM is a relatively new method that facilitate in sample imaging used basically for surface topographic studies where the sample surface was tested in three dimensions with high resolution and high magnification. The grain size that increases due to heat treatment results in loss of strength of Inconel 718.