Studies on fatigue corrosion and corrosion fatigue of ductile iron.

Abstract

Ductile iron offers the combined advantages of good castability with high strength and toughness, and during the past few years has become one of the most widely used engineering materials. The failure of a component of a system when subjected to cyclic stresses is generally known as fatigue. In addition to the inherent properties, the fatigue behaviour of a material is affected by many variables such as surface imperfections, stressconcentration points, and the environment. The cumulative effect of a corrosive environment and fatigue loading on a component is known as corrosionfatigue. In view of the increasing applications of ductileiron castings to replace grey iron, malleable iron and even steel castings and forgings, it has become very essential to understand its behaviour under dynamic loading, particularly when exposed to corrosive atmospheres. The literature review revealed that, barring a few isolated attempts to study the fatigue properties of normal ductile irons, hardly any data are available pertaining to their corrosion behaviour and corrosionfatigue strength. A systematic investigation was, therefore, undertaken to assess the fatigue, corrosion, and corrosionfatigue properties of ductile iron as affected by the matrix structure. The experimental work was planned on the following lines:

Production of normal ductileiron test castings, and heat treatment of these to obtain different matrix structures, viz., ferrite, pearlite, tempered martensite and bainite. Assessment of fatigue properties of these ductile irons in air using rotatingbending fatiguetesting equipment. Evaluation of corrosion properties of the above ductile irons in 3% NaCl and 5% HCl solutions using (i) the weightloss method, and (ii) electrochemical methods. Study of corrosionfatigue characteristics (using the rotatingbending machine) in different environments-3% NaCl, 5% HCl and SAE 40 lubricating oil.

The investigation led to the following conclusions:

I. Fatigue a. The fatigue strength of ductile iron is influenced by hardness, whereas the endurance ratio appears to be directly related to ductility. b. The fatigue strength varies from a minimum of 240 MPa for ferritic ductile iron to 510 MPa for bainitic ductile iron. c. The fatigue strength has been found to increase with increasing tensile strength.

II. Corrosion a. The corrosion behaviour of ductile iron changes only marginally with the change in matrix structure. The corrosion rates of ductile irons with different matrix structures are more or less the same in any particular corrosive medium. b. For ductile iron with any given matrix, an acid medium is much more corrosive than a salt solution. c. The electrochemical potential of ductile irons with various matrices differs appreciably. Martensitic ductile irons are the least corrosive, while pearlitic irons are the most corrosive. The corrosion rates of bainitic and ferritic irons lie between these two limits.

III. CorrosionFatigue a. Ductile iron, a typical ferrous material, behaves like a nonferrous material in corrosive media in that the fatigue life continues to increase when the applied stress is reduced. This is true for all matrices. A clearly defined and marked fatigue limit is not observed in ductile irons when tested in corrosive environments. b. The reduction in fatigue strength is quite drastic under corrosive conditions. The strengthreduction factor is much higher for the acid medium compared to the salt solution. c. Fatigue strength is not appreciably affected when exposed to lubricatingoil medium.