S. Ahila

The effect of ultrasonic vibration on nickel electrodeposition

Abstract The effect of ultrasonic vibration on microhardness, residual stresses and surface finish of nickel electrodeposits was investigated. A comparison was made between the results obtained with an ultrasonically agitated bath and those with a still bath. Ultrasonic agitation increased the deposition rate and limited current density. The deposit from the bath with ultrasound gave a smoother surface and exhibited increased microhardness. The residual stresses of the deposits were reduced with an ultrasonically agitated bath.

Fatigue strength of nickel electrodeposits prepared in ultrasonically agitated bath

Nickel coatings are widely used 1:o prevent wear of structural steels resulting from abrasion and corrosion. When used for this purpose it may be expected that the coatings should also influence other characteristics such as mechanical woperties. As the plating alters the surface characteristics of the base metal, the fatigue properties of steels may be markedly affected by the presence of such coatings. Fatigue crack initiation generally occurs at the surface of a specimen. The internal stress of the deposit, its inherent fatigue strength relative to that of the substrate, its hardness and its thickness have all been shown to affect the fatigue limit [1-3]. In recent years, the effect of ultrasonic agitation during electrodeposition has gained popularity [4-6]. Because of the catastrophic failure due to hydrogen embrittlement in many plated highstrength steels, any effort to improve plating should attempt reduction, if not total eJimination, of this undesirable feature. Research is being continuously carried out with this as the main objective. Ultrasonics provide a solution to this problem. The present work is concerned with studying the effect of ultrasonics of the fatigue properties of nickel electrodeposits. A comparison has been made between the deposits obtained from a still bath and an ultrasonically agitated bath. The composition of a nickel Watts bath used for the present study is given in Table I. Fatigue tests were carried out in a rotating, bending fatigue testing machine of type PUNZ. Mild steel specimens prepared according to DIN 50113 standards (Fig. 1) and coated with nickel (thickness 20/~m) were employed. The mechanical properties of the base metal were: yield strength 475 MPa, tensile strength 650 MPa and hardness 194 Vickers Hardness Number (VHN). All the fatigue tests were carried out in air at room temperature (25 °C). The thickness of deposit was measured using the gauge, Mikrotest III. The surface roughness of the plated specimens was measured by a perthometer, Perthon S5P. Fig. 2 shows the fatigue life of the specimens at different applied stresses. Three samples were tested

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.

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.

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.

Fusion characteristics of the Na2SO4-K2S2O7 and K2SO4-K2S2O7 systems

Corrosion of high-temperature alloys has been a problem in power plants. Ash and salts are deposited on the surface of boiler tubes in certain temperature ranges and cause accelerated corrosion of the material. Rapid corrosion is often associated with the presence of molten salts on the surface. Although spectrochemical analysis shows a number of chemical elements in the accumulated deposits, the corrosion problem is concerned mainly with the sulphates of sodium and potassium. Under suitable conditions the sulphates of these alkali metals absorb SO3, forming pyrosulphates according to

Assessment of high temperature performance of induction pressure welded 2.25Cr-1Mo steel in corrosive environment

In this article, an attempt has been made to evaluate the effect of hot corrosion due to potassium sulfate and sodium chloride mixture on creep rupture of induction pressure welded (IPW) 2.25 Cr-1Mo steel. Based on the results obtained from creep and hot corrosion tests at 550 and 600 C, the following conclusions are drawn: (1) The coating of 40% K[sub 2]SO[sub 4] - 60% NaCl led to lowering of rupture life of the material. (2) Coated samples showed localized oxidation and sulfidation along the grain boundary thereby weakening it. The enhanced oxidation due to the coating was attributed to continuous supply of oxygen to the metal through the liquid phase of salt coating. (3) Scale failure occurs due to the development of stress on the scale on continuous reaction of salt coating with the base metal. (4) Once the scale spalled off, fresh metal is exposed for further attack leading to early failure.