EFFECT OF WELDING CONSUMABLES ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A WELDED SA 530 GR 70 STEEL

ABSTRACT

The main purpose of this work is to study the effect of welding consumables on the microstructure and mechanical properties of a welded SA 530 GR 70 steel. Steel is an alloy of iron and carbon and it is usually cast into malleable form and it can be changed in shape by forging, rolling and can be joined using different joining process like welding etc. welding is the process of coalescing materials such as metals or thermoplastic in order to seamlessly join them. The following welding processes under taken are Flux Cored arc Welding (FCAW)  and the filler type was mild steel filler wire, Shielded Metal arc Welding (SMAW) was used and the electrodes used were E6010, E6013 and E7018 and Submerged Arc Welding (SAW) was used with a mild steel filler metal. The test carried out are Magnetic particle test and ultrasonic test which are non-destructive test, mechanical test done are tensile test and hardness test, and microstructural test. SA 530 GR 70 steel shows increase in hardness especially on the heat affected zone followed by the fusion zone before the base metal due to type of cooling which takes place and the type of grains formed during the welding process. There is a change on microstructure where the base metal changes and create dendrite shape at weldment area and columnar grains at the heat affected zone. The result showed that different welding processes and consumables will give different strength and must be importantly considered by the welder.

 

 

 

CHAPTER ONE

INTRODUCTION

1.1 BACKGROUND

Steel is arguably the world’s most ‘advanced” material. It is a very versatile material with a wide range of attractive properties which can be produced at a very competitive cost. It has a diverse range of applications, and is second only to concrete in its annual production tonnage. Steel is not a new invention which leads to a common misperception that “everything is known about steel” amongst those outside its field. Steel is generally defined as a ferrous alloy containing less than 2.0wt%C. The complexity of steel arises with the introduction of further alloying  elements into the iron-carbon alloy system. The optimization of alloying content in the iron-carbon system, combined with different mechanical and heat treatment leads to immense opportunities for parameter variations and these are continuously been developed.

Steel is an alloy of iron and carbon and it is initially cast into a malleable form, and it can be changed in shape by forging, rolling or other mechanical processes. The difference between steel and cast iron is that steel do not contain graphite or free carbon. Carbon exist in small quantity in ferrite and majority in cementite. There are different types of steel but we are to deal majorly on mild steel.

Figure 1:1 The iron-iron carbide phase diagram

Mild steel is a type of steel containing a small percentage of carbon, strong and tough but not readily tempered. It is also known as plain carbon steel and low carbon steel. It is the most common type of steel because its price is relatively low while the material properties are acceptable for many applications. Mild steel contains approximately 0.05-0.25% carbon, making it malleable and ductile. It has a relatively low tensile strength but it is cheap and easy to form, its surface hardness can be increased through carburizing.

It’s often used when large quantities of steel are needed for example structural steel. The density of mild steel is approximately 7.85g/cm and the Young modulus is 200GPa. Low carbon steel contains less carbon than other steel and are easier to cold form, making them easier to handle. This has given them an advantage compared to other nation since it was used in making weapons (Sacks and Bonhart, 2005). Low carbon steel is a type of steel that contains fine grains and it started being designed since the 19^th century. Low carbon steel can be classified when the carbon content is lower than 0.2 percent (American Society for Testing and Materials 2001). Low carbon steel is widely used in fabrication industry due to excellent to weight ratio and one of the applications are in automobile industry (Khodabakhshi et al, 2011). This material is suitable to use in automotive industry because it can absorb high impact force without cracking. This happen because it has low carbon which makes it a ductile material compared to high carbon steel that is more brittle and easy to crack although it has more strength.

Steel is an important engineering material. It has found applications in many areas such as vehicle parts, truck bed floors, automobile doors, domestic appliances etc. It is capable of presenting economically a very wide range of mechanical and other properties. Traditionally, mechanical components has been joined through fasteners, rivet joint etc. In other to reduce time for manufacturing, weight reduction and improvement in mechanical properties, welding processes is usually adopted.

Welding is define today as a process of coalescing materials such as metals or thermoplastics in order to seamlessly join them. In this process, melting of the base metal takes place due to high heat generation during welding process. A filler metal is used to join the metal by forming a pool of molten metal. When it cools it forms the joint and it becomes stronger in strength than the base metal. Welding process can be done in different types of environments such as open air, under water etc. During welding process some precautions have to be taken in order to avoid damages to human as high heat is produced and the intensity of the arc is high and also it produces fumes which may be hazardous to human beings.

Welding is extensively used as a fabrication process for joining material in a wide range of compositions, parts, shapes and sizes. It is an important joining process because of high joint efficiency set up, flexibility and low fabrication costs. Welding is an efficient, dependable and economic process. Welded joint finds applications in critical components in which if failure occur are catastrophic. Hence, inspection methods and adherence to acceptable standards are increasing. These acceptable standards represent the minimum weld quality which is based upon test carried out on welded specimen containing some discontinuities.

Welding involves a wide range of variables such as time, temperature, electrode, power input and welding speed that influence the eventual properties of the weld metal. Concerning the welding of low carbon steels, it has been shown that the grain coarsened zone and heat affected zone are very critical since embrittlement is concentrated in these areas. It is also known that the final microstructures and mechanical properties of welded steel depends on some parameters like percentage of carbon and presence of other elements such as sulfur or phosphorous. Low carbon steel that has less than 0.25% carbon, display good weldability, because they can be generally welded without special precautions using most of the available welding processes. There are different types of welding processes and they are:

(a) Shielded metal arc welding

(b) Fluxed core arc welding

(c) Submerged arc welding

(d) Tungsten arc welding

(e) Manual inert gas welding

These are few types of welding processes that are commonly used and these processes uses electrodes and it can be divided into two namely: consumables and non-consumables.

1.2 WELDING ELECTRODE

An electrode is a metal wire that is coated. It is made out of materials with a similar composition to the metal being welded. There are covered electrodes and also bare electrodes. Tungsten electrodes are not part of covered electrodes, it contains 2% cerium or thorium and have better electron emissivity, current-carrying capacity and resistance to contamination than pure tungsten electrodes. Tungsten inert gas (TIG) electrodes are non-consumables as they do not melt and become part of the weld, and it require the use of a welding rod. The manual inert gas (MIG) welding electrode is a continuously fed wire referred to as wire.

As a result, arc starting is easier and the arc is more stable. The electron emissivity refers to the ability of the electrode tip to emit electrons. A lower electron emissivity implies a higher electrode tip temperature is required to emit electron and  hence a greater risk of melting the tip. Covering electrodes are electrodes that are used in shielded metal arc welding process (SMAW) and they are covered with flux. This type of electrodes are consumable, meaning they become part of the weld, and their function are as follows:

Functions of Electrode Covering

The covering of the electrode contains various chemicals and even metal powder in order to perform one or more of the functions described below.

(a) Protection: It provides a gaseous shield to protect the molten metal from air. For a cellulose- type electrode, the covering contains cellulose, (C₆H₁₀ O₅) _X. A large volume of gas mixture of H₂, CO, H₂O and CO₂ is produced when cellulose in the electrode covering is heated and decomposes. For a limestone (CaCO₃) type electrode, on the other hand CO₂ gas and CaO slag form when the limestone decomposes. The limestone type electrode is a low hydrogen type electrode because it produces a gaseous shield low in hydrogen. It is often used for welding metals that are susceptible to hydrogen cracking, such as high strength steels.

(b) De-oxidation: It provides deoxidizers and fluxing agents to deoxidize and cleanse the weld metal. The solid slag formed also protects the already solidified but still hot weld metal from oxidation.

(c) Arc Stabilization: It provides arc stabilizers to help maintain a stable arc. The arc is an ionic gas (a plasma) that conducts the electric current. Arc stabilizers are compounds that decompose readily into ions in the arc such as potassium oxalate and lithium carbonate. They increase the electrical conductivity of the arc and help the arc conduct the electric current more smoothly.

(d) Metal Addition: It provides alloying elements and/or metal powder to the weld pool. The former helps control the composition of the weld metal while the latter helps increase the deposition rate.

1.3 AIM OF STUDY

The aim of this work is to study the effect of welding consumables on the microstructural and mechanical properties of a welded SA 530 GR 70 steel.

1.4 OBJECTIVES

1. To study the microstructure of the weldment using metallurgical microscope or optical microscopy.
2. To use standard mechanical test to study the effects of different welding conditions on the weld.
3. To study the effect of the different welding types on the microstructure and mechanical properties on the weld.
4. To study the different types of microstructure obtained using different welding variables.
5. To use non-destructive test to study the effect of each consumables on a welded mild steel.

1.5 SIGNIFICANCE

The study is significance because its contribution deepens the knowledge of welding consumables and their effect on weldment and welding processes especially on SA 530 GR 70 steel.

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