الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 157 |  |  |
| | | from | 157 | | |
Abstract
Carbon steel alloys are an important class of engineering alloys that have been widely used in a variety of industries and environments. Most of engineering products are manufactured and produced by several means. Welding represents 90% of the manufacturing methods.
Recently, gas metal arc welding (GMAW) process, with its solid or flux cored welding wires, has gained the most popularity among the different types of welding processes, because its high quality and economical welds which can be obtained. Several research works investigated the effect of shielding gas composition on weld joint properties in case of solid wire. However, in case of flux cored wire (FCW) another effect has to be considered which is the interaction between gas and flux during arcing is. That interaction may affect the properties of the produced weldment.
The main aim of this work is to study the effect of shielding gas composition on the microstructure and mechanical properties of steel 37-3 carbon steel by using a flux cored wire. The shielding gas mixtures used in this study were: (1) Pure argon (Ar), (2) Pure carbon dioxide (CO2), (3) Argon / Carbon dioxide (95% Ar + 5% CO2), (4) Argon / Carbon dioxide (90% Ar + 10% CO2), (5) Argon / Carbon dioxide (82% Ar + 18% CO2), (6) Argon / Carbon dioxide (80% Ar + 20% CO2), (7) Argon / Carbon dioxide (75% Ar + 25% CO2), and (8) Argon / Carbon dioxide (50% Ar + 50% CO2).
To demonstrate the effect of shielding gas compositions used in this study, firstly bead-on-plate (B.O.P) test pieces were being prepared and the following was being carried out:
a. Arc stability by using arc data monitor III device which measures welding current and arc voltage every second during FCAW process. Accordingly, we can observe the variations in both welding current and arc voltage during welding process.
b. Estimation of heat input by using the measured values of welding current and arc voltage as well as the welding travel speed. Also, estimation of arc efficiency by using the estimated theoretical and actual heat inputs.
c. Measurement of deposition efficiency at constant welding parameters and different gas compositions through the weight gain difference between before and after weld bead deposition.
d. Measurement of weld bead geometry by using profile projector machine to measure weld metal width and depth, heat affected zone (HAZ) depth, and reinforcement height.
e. Hardness profile by using Vickers hardness testing machine.
f. Metallographic examination by using stereoscope for macro-examination to reveal the macrostructure of B.O.P specimens as a weld metal, fusion boundary, and HAZ. Also, using optical microscope for microstructure to reveal grain boundaries and grain interior features and also to identify produced phases and its percents –using JMicroVision software – in the weld metal.
g. Chemical analysis of deposited weld metal by using X-ray fluorescent machine.
After that, complete real welds were being carried out through selecting the best results of 4-shielding gas compositions and then heat input (HI) for each specimen was being estimated using the applied welding current, arc voltage, and welding travel speed. Both non-destructive and destructive tests were being performed. The applied non-destructive tests (NDT) were visual test (VT), dye penetrant test (PT) and Ultrasonic test (UT). On the other hand, destructive tests (DT) were transverse tensile test, Vickers hardness test and impact test. Furthermore, metallographic examination of macro and microstructure and fractography were being carried out.
This study showed that the shielding gas composition has a significant effect on microstructure and mechanical properties of the weld metal of steel 37-3 carbon steel. Arc stability that was being measured for each B.O.P test piece revealed that the pure argon shielding gas has excellent arc stability, and the pure CO2 and (50% Ar + 50% CO2) shielding gases gave noisy arc stability (not stable arc). Deposition efficiency that was being measured for each B.O.P test piece revealed that each shielding gas composition has its own characteristics and interaction with flux. However, the mixing ratio (75% Ar + 25% CO2) has the optimum deposition rate among other shielding gas mixtures.
The Charpy impact toughness test of W.M revealed that as the CO2 percent increases in the shielding gas, the absorbed energy (toughness) of the W.M decreases. This is attributed from the microstructures of the W.M which confirm that acicular ferrite (AF) decreases and on the other hand grain boundary ferrite (GF) increases with an increase in CO2 content of the shielding gas. Accordingly, cleavage cracks propagate more easily in the GF than in AF. where AF has a beneficial effect on the toughness in the absence of other brittle zones.
Furthermore, the hardness of deposited weld metal (W.M) in case of pure Ar shielding gas showed the highest values, while the hardness in case of CO2 shielding gas showed the lowest values this is due to the reheating of the lower pass from the upper heated pass. The hardness incase of other mixing gases are lying between them. This could be attributed to the microstructure of deposited weld metal. Also due to the chemical composition variation, some alloying elements such as Mn and Si were being decreased. So that, the ultimate tensile strength (UTS) of W.M decreased and as a result the W.M hardness decreased.