V.M. Radhakrishnan

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.

Creep crack growth behavior at 1033K of directionally solidified CM 247 LC — a cast nickel-base superalloy

Creep crack growth (CCG) studies on metals and alloys have gained importance and momentum in the recent years. In the present study, the creep crack growth behavior of a directionally solidified (DS) cast nickel-base superalloy has been investigated at 1,033K and the results obtained are reported here. The alloy studied is commonly used for producing turbine blades and vanes for aeroengine applications.

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.

Fatigue and fracture behavior of a nickel-chromium free austenitic steel

Abstract The austenitic steels based on the Fe-Mn-Al-C system are relatively less expensive than those from the Fe-Cr-Ni-C system due to their being nickel-chromium free stainless steels. The fatigue crack growth (FCG) behavior and elastic-plastic fracture toughness ( J- integral ) of an alloy of the Fe-Mn-Al-C system have been studied at room temperature. This fatigue crack growth study revealed a bilinear form of behavior in relation to the Paris approach. Fractographic studies confirmed that the crack growth micromechanism changes from being microstructure-sensitive to being structure-insensitive when the reversed plastic zone size exceeds the grain size of the material. This change is accompanied by a change in crack closure mechanism from a roughness-induced to a plasticity-induced one. The Ni-Cr free austenitic steel offers better fatigue crack growth resistance as compared to that for the conventional 304L type stainless steel containing Ni-Cr. It also is found to possess a high fracture toughness ( J Ic ) value of 187 kJ/m 2 comparable with other grades of stainless steel containing nickel and chromium.

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.

Effect of temperature on crack growth in type 304 stainless steel

Abstract Investigations have been carried out to study crack growth behaviour in type 304 stainless steel under constant-load creep conditions in the temperature range 550–650°C. At 550 and 600°C the load point deflection rate bears a unique relation to the creep crack growth rate, independent of the load. However, the results at 650°C show that the relation is load dependent. The parameters, stress intensity factor and net section stress, do not describe the crack growth rate well and there is wide scatter. The energy rate line integral C∗ appears to be applicable at 550 and 600°C but not at 650°C where it gives a systematic segregation of data. On the basis of a phenomenological approach a new parameter is derived which appears to correlate well with the creep crack growth at all the three temperatures investigated. This approach also provides an index which can differentiate between creep brittle and creep ductile materials.

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...