S. Ramakrishna Iyer

A comparative study of oxidation and hot corrosion of a cast nickel base superalloy in different corrosive environments

Oxidation and hot corrosion in sulphate, chloride and vanadate environment of a cast nickel base superalloy are discussed. Weight gain studies are carried out in air for the uncoated and coated samples. The corrosion products are analysed and the possible mechanisms of corrosion based on the corrosion compounds are discussed. The variation of weight gain with time indicated a parabolic growth of oxides. The outer layers consisted of oxides and in case of hot corrosion, in addition to oxide outer layer, internal sulphidation was also noticed.

A simple four-point bend creep testing apparatus for brittle ceramic materials

A knowledge of the creep behaviour of ceramic materials is important in computing the lifetime at high temperatures. Very often creep tests are performed as bending tests rather than tensile creep, because it is relatively difficult to perform a satisfactory tensile creep test on brittle materials. The other common method is compression creep testing. Davies and Sinha Ray [1] built apparatus for carrying out constant-load creep tests in tension on polycrystalline ceramics at temperatures up to 1750 °C. A special type of specimen grip-universal joint assembly was used to hold the specimen accurately on the tensile axis. The strain was measured using two cathetometers, which focused on fine platinum wire tied to the specimen ends. Also, a dial gauge and linear-displacement transducer were used for the same purpose. It is reported that these three methods of measuring strain give results that are in good agreement with each other. Many investigators such as Morrell [2], Kossowsky et al. [3], Lange et al. [4], Govila [5] and Wakai et al. [6] have used tensile creep tests to a great extent. In tensile creep tests the cost of test fixtures and specimen preparation are two of the limiting factors. Carrol and Wiederhorn [7] developed an inexpensive technique for measuring the tensile creep and a simple method used to produce test specimens. Flags with platinum wire were attached to measure the creep deformation. Carroll et al. [8] later measured the tensile creep deformation, using a laser extensometer to monitor the relative displacement of flags that were attached to the gauge section of the specimen. Kandil and Dyson [9] discussed in detail the development of tensile creep facilities for ceramics. Hulse and Copley [10] developed three apparatuses for obtaining stress-strain and creep data in compression at elevated temperatures. Many types of compressive creep testing equipment have been described in the literature [11-13]. Some are rather complex and bulky in design. Smith and Moore [14] made a relatively simple and inexpensive compression creep testing apparatus which can be used at temperatures up to 1550 °C. A linear variable differential transducer was used to measure the strain. Recently, Carroll and Weiderhorn [7] reported a compressive creep testing equipment which consists of a screw-driven testing machine which applies the load to the specimen through a series of rounded pedestals and loading blocks. In general, the compressive creep test is relatively simple and has been frequently used to characterize the creep

Carbide precipitation in 2.25 Cr-1 Mo steel and its weldments during creep testing

Abstract Elevated temperature behaviour of 2.25 Cr-1 Mo steel (UW — unwelded) and weldments, namely, manual metal arc (MMA) and induction pressure (IP) welded joints, was assessed. The creep rupture time was found to follow the order: IPW>UW>MMAW. The nature, size and distribution of carbides, as evidenced by transmission electron microscopic (TEM) studies, were found to have a bearing on the rupture time of the specimens.

Creep of porcelain-containing silica and alumina

Hard porcelain ceramics find many applications because of their high hardness, high mechanical strength and moderate thermal-shock resistance. The addition of alumina as a filler to porcelain increases its strength at room temperature. In the present investigation, four-point-bend creep tests were carried out for porcelain-containing silica (SP-1) and alumina (AP-3) at 800, 900 and 1000°C. The creep data were analysed using a power-law creep, and the stress exponents were estimated. The activation energy for these two materials was found to be 45 kcal mol−1. The viscosity of the feldspar glassy phase was also determined from the creep tests. The test samples were analysed by scanning electron microscopy (SEM). The X-ray diffraction results (XRD) show that the amount of crystalline phase in the material increases after creep testing.

A comparative study of hot corrosion of welded and unwelded 2.25 Cr-1Mo steel

Abstract Hot corrosion of unwelded and welded 2.25 Cr-1Mo steel coated with a potassium sulphate and sodium chloride mixture is discussed. The coating composition is a eutectic mixture with 40 wt% K 2 SO 4 and 60 wt% NaCl. Weight gain studies were done for welded specimens containing only weld metal and composite specimens containing both weld metal and heat-affected zone. The results indicated that the coated specimens were more corroded than uncoated specimens. Also welded samples showed less attack than unwelded samples.

Measurement of hot corrosion of 2.25Cr1Mo steel-347H steel welded joint in K2SO4NaCl mixture

Abstract 2.25Cr-1Mo steel-347H dissimilar metal welded joint prepared using a nickel-based filler metal (INCO 82) as a transition joint was assessed for its oxidation and corrosion performance in a mixture containing 40% K 2 SO 4 and 60% NaCl. Hot corrosion attack led to the formation of oxides of iron on the surface and sulphides of iron and chromium on the subsurface.

Assessment of high temperature performance of a cast nickel base superalloy in corrosive environment

Turbine blades are subjected to creep deformation through their continuous centrifugal loading during operation. In addition to this, hot corrosion in gas turbines limits the durability of even the best blade materials at present available. In this article, an attempt has been made to evaluate the effect of hot corrosion due to sodium sulphate, sodium chloride and sodium metavanadate salt mixture on creep rupture of a cast nickel base turbine blade superalloy.

High temperature stability of 2.25Cr-1Mo steel during creep

The creep rapture behaviour of 2.25Cr—1Mo steel in air and in a salt mixture was studied. The salt coating, which can form a liquid phase at the test temperatures, increased the creep rate and reduced the rupture life of the material. The coating reduced the available cross-section of the material by removing the surface layers, thereby resulting in a reduction of the rupture life. Cross-sections of coated samples showed an outer oxide layer comprising oxide of the metal and precipitates of sulphide at the metal/oxide interface. This subsurface penetration of the corrodants was responsible for the early failure of the coated samples. This is typical of hot corrosion mechanisms. The formation of various carbides like M23C6 and M6C, as observed by transmission electron microscopy, during creep reduced the creep strength of the material both in air and in the coated state. Increasing temperature enhanced the formation of these carbides with a consequent decrease in creep strength. Applied stress did not seem...

Creep deformation of induction pressure welded 2·25Cr-1Mo steel

Abstract In this paper, experimental results involving the effect of stress and temperature on creep behaviour of induction pressure welded (IPW) 2·25Cr-1Mo steel are presented. Creep rupture tests were conducted at 550–700°C in steps of 50°C over a stress range of 112·5–180 MPa. Above 650°C failure of the specimen was enhanced due to the microstructural instability. Failure in the specimens occurred invariably in the heat affected zones (HAZ), and the fracture surfaces indicated ductile failure.

Comparative study of creep behaviour of 2·25Cr-1Mo steel and its weldments

Abstract Creep rupture tests under uniaxial loading condition were performed in atmospheric air on 2·25Cr-1Mo steel and its welded joints, namely Induction Pressure Welded (IPW) and Manual Metal Arc Welded (MMAW) joints, at test temperatures of 550 and 600°C and stresses of 110–180 MPa. The data obtained were analysed and the creep behaviour of unwelded material (UW) and the welded joint were compared. Under the given conditions, the IPW joints performed better than the base metal, but the MMAW joints showed less resistance to creep than the unwelded 2·25Cr-1Mo steel.