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.
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 19th 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.
