STUDIES ON TRIBOLOG1CAL CHARACTERISTICS OF DUAL PHASE STEELS

Abstract

The physical and chemical nature of the interaction between the mating surfaces in relative motion needs a thorough understanding for the development of wear resistant materials. Strength, toughness and ductility are undoubtedly the most important properties of the materials but the major difficulty in optimising these properties arises from the fact that strength is usually inversely related to toughness and ductility. The two phase materials are the best means for optimising these mutually exclusive properties. The underlying principle is to utilise the beneficial effects of the second phase and at the same time mitigating the less desirable features by the presence of the other constituent phase. The size, distribution, shape and volume fraction of the second phase critically controls the mechanical properties of the two phase systems. It is a well established fact that the friction and wear behaviour of the materials is affected by their mechanical properties which in turn are the function of the microstructure of the materials. Hence, a study of the morphology of the second phase in a microstructure and its influence on the friction and wear behaviour is critically important in two-phase wear resistant materials. Metals and alloys are the most common materials employed in engineering for wear resistant applications. Thus, despite growing interest in ceramics and polymeric composites as engineering tribo-materials, much of the wear research being conducted is directed towards metallic materials. Although, aluminium and magnesium alloys and composites based on these matrix materials containing particles or fibers, are being used or developed for wear resistant applications but the steels have retained their place intact in wide ranging wear resistant applications. The steels offer a unique advantage of tailoring their properties by development of a variety of microstructure and properties by simple heat treatment ii techniques. In the quest for developing new wear resistant materials, the dual phase steels are the most promising among the two phase metallic materials because they possess some unique mechanical properties viz., continuous yielding, high tensile strength, high rate of work hardening and a high uniform and total elongation. In addition, the microstructure has relatively softer ferrite matrix containing islands of hard martensite phase. Hence, the present investigation is aimed to analyse the friction and wear behaviour of the dual phase steels under dry sliding conditions using a pin-on-disc wear testing machine in the specific context of the role of microstructure. In the present study, the plain carbon steel containing 0.42 wt pet carbon have been intercritically annealed in the (a+y) region of Fe-C phase diagram at a constant temperature of 740°C for different holding times followed by water quenching to develop dual phase structures having four different martensite content. Normalised steel having the same carbon content has been used as a reference material. Armco iron and hardened 0.42 wt pct carbon steel with fully martensitic structure have been used to simulate the properties of the constituent phases of the dual phase steel. The plain carbon steel containing 0.14 wt pct carbon has also been intercritically annealed for five minutes followed by water quenching to develop the dual phase structure. The microstructures and the mechanical properties of all the different materials used in the present investigation have been characterised. The friction and wear behaviour under dry sliding condition has been determined for all these materials having different microstructures. The present study also focuses on the synergy between the matrix of ferrite and hard martensite islands during dry sliding of dual phase steels and its effect on the observed friction and wear behaviour which may enrich our understanding of the material aspects of friction and wear. Chapter-1 contains the introductory remarks highlighting the technological importance of the problem under investigation. iii Chapter-2 begins with a critical review of the existing literature on the techniques of production and mechanical properties of dual phase steels. It is followed by an exhaustive survey on the various aspects of the friction and wear behaviour of the metallic materials in general, and of steels in particular. The different types of mechanisms giving rise to wear have been outlined. The existing models for different types of wear mechanisms are presented in the literature. The effects of microstructure, mechanical properties, normal load, sliding velocity, surface roughness and environments on the friction and wear behaviour of the steels have also been reviewed. The limited knowledge on the role of microstructure and the normal load on the friction and wear characteristics of dual phase steels has been given a special attention as the role of these two variables has,been particularly investigated in the present study. In the end the formulation of problem is presented. Chapter-3 outlines the experimental procedures followed in the present investigation. The method used to determine the chemical composition is given. The details of the vertical tube furnace used for intercritical annealing heat treatment for the development of both the dual phase and the fully martensitic structure have been described along with the patimeters used for the heat treatment schedule. The procedures followed for the study of microstructure, hardness and uniaxial tensile tests have also been described in this chapter. Fracture surfaces of the specimens have been examined under scanning electron microscope. The friction and dry sliding wear in the Armco iron, normalised steels both low (LCN) and medium carbon (N), dual phase (DP) steels and fully martensitic steel (FMS) have been determined by tests carried out on pin-on-disc machine against a counterface of En-32 steel hardened to HRC 62 to 65, following procedures outlined in this section. Pin weight losses have been measured at different intervals of time. Five normal loads viz., 14.7, 19.6, 24.5, 29.4 and 34.3 N have been used for Armco iron and 0.42 wt. pct carbon steels whereas, the low carbon normalised and dual phase steel containing 0.14 wt pct carbon have been tested under the normal loads of iv 14.7, 24.5 and 34.3 N in the present investigation. For all the friction and wear tests the sliding speed has been maintained at a constant value of 1.15 m/s. The wear debris generated during sliding has been observed under stereo-optical microscope. The X-ray diffraction analysis of the wear debris collected during sliding has been carried out to identify the phase constituents following the method described in this chapter. The wear surfaces as well as the subsurface of the pin after sliding have been examined under scanning electron microscope (SEM). The method of determining the rise in temperature of the pin surface has been presented. The method to examine the microstructure of the deformed subsurface of the pin after the sliding also forms a part of this chapter. Chapter-4 describes a model developed to predict the oxidative wear rate of two phase materials like the dual phase steels. It has been assumed that oxidation of both the phases take place with the same kinetics but the hardness of the underlying phases are different. The oxide layer builds up over successive contacts and the oxide under the contact breaks up into wear debris once the oxide builds upto a critical level of thickness. The calculated results for wear rate assuming parabolic law of oxide growth have been compared with those observed experimentally under different loads. It is observed that this model has consistently overestimated the wear rate. However, if the critical thickness of the oxide is taken a little higher or the oxidation rate constant is taken a little lower than those assumed on the basis of reported values, the calculated wear rate may match the observed ones. In another model, where the critical thickness of the oxide layers, for their removal as debris particle have been estimated separately from the experimental results of the fully ferritic Armco iron and fully martensitic steel, the calculated wear rates for dual phase steels have given an underestimation of results. Chapter-5 describes the results on microstructure and mechanical properties characterising Armco iron, low and medium carbon normalised steels, dual phase steels and fully martensitic steel (EMS). The martensite content in dual phase steels and the microstructure of all the materials have been characterised by optical microscopy. The increasing time of holding during intercritical annealing followed by water quenching results in increasing volume fraction of martensite in medium carbon dual phase steels (DPI, DP2, DP3 and DP4). The distribution of martensite in dual phase steels has generally been homogeneous. The Brinell hardness measurements point towards an increasing hardness with martensite volume fraction. The dual phase steels show a continuous yielding behaviour under tension confirmed by the absence of yield point in the load-extension curve while a clear yield point phenomenon is visible in the load-extension curve of normalised steel. The Armco iron simulating ferrite phase in DP steel shows typically ductile mode of fracture whereas the fully martensitic steel shows a typically brittle fracture. The dual phase steels show the mixed mode (ductile + brittle) of fracture with an increasing dominance of brittle mode with the martensite content. The UTS has the highest value for fully martensitic steel and the lowest for Armco iron with those for normalised and dual phase steels falling in between. The UTS in DP'steels increases with martensite content. The percentage elongation decreases with increasing volume fraction of martensite in the dual phase steels. The strain hardening coeficient decreases linearly with increasing martensite content. Chapter-6 contains the results and discussion pertaining to the friction and wear characteristics of the Armco iron, medium carbon normalised (N) and dual phase steels (DPI, DP2, DP3 and DP4) and fully martensitic steel (FMS). The variation of wear volume at a given load and sliding velocity is generally found to be linear and the wear rate at that normal load has been determined from the linear least square fit of the variation of wear volume with sliding distance. If the variation of the wear rate with normal load is linear, it implies that Archard's law is being obeyed. The wear coefficient, KA, has been determined using Archard's equation as given below. vi V II K= A LS (1) Where, V is the cumulative volume loss under a normal load of L after a sliding distance of S. II is the initial hardness of the softer one (pin) of two mating materials sliding against each other. The wear coefficient has been estimated from the slope of the linear variation of wear rate with load, V/SL, by multiplying it with the initial hardness of the corresponding pin material. The variation of cumulative wear volume has been analysed using two separate stages (run-in and steady state) of wear behaviour. TWo linear segments will also allow remaining within the framework of Archard's law. The change in slope has been observed after the first six experimental points (first stage-run-in), fitted by one line, and the latter six points (steady state) have been fitted with another line with the sixth point common between them. Both the lines have been determined by the linear least square fit and their slopes give the corresponding wear rate. The variation of the cumulative wear volume with sliding distance has been found to be linear in both the segments i.e., run-in and steady state, and the cumulative volume loss increases with load. All the materials having different microstructures, used in the present investigation have followed this common trend. The cumulative wear volume is found to be the highest for Armco iron and the lowest for fully martensitic steel with those for the N steel and the dual phase steels lying in-between. Thus, at a given load, the cumulative volume loss decreases with increasing martensite volume fraction. The wear rate for the Armco iron is observed to be the highest whereas it is found to be the lowest for fully martensitic steel in both the run-in and the steady state stages of wear. The wear rates for the normalised steel and all the dual phase steels are considerably lower than those for Armco iron but higher as compared to those of fully martensitic steel vii