This document summarizes a student project that investigated the microstructure and properties of welded steel joints. The project involved welding and analyzing mild steel, API steel, medium carbon steel, and tool steel using MMA and TIG welding. Microscopic analysis revealed the grain structures in the base metal, heat-affected zone, and weld metal. Vickers hardness testing showed expected hardness profiles but unexpected results for medium carbon steel. Heat input and thermal cycling affected the microstructure and properties differently between welding methods and materials. The project provides analysis of microstructure, hardness values, yield strength calculations, and comparisons between welding techniques and materials.

1. Student Name: Mr. Joseph Stynes
Student Number: X00105837
• Multi-pass welds are different in nature to single
pass welds, not simply because of the number of
weld passes but because of the effect that repeated
thermal cycling has on the final microstructure.
Because the initial weld is not exposed to the
cooling ambient air for any length of time, as
would be the case with a single pass weld, its final
mechanical properties are quite different.
• The nature of multi-pass welds guarantee a level
of preheat into the material and, upon completion
of the welding operation, slows the cooling rate
down and greatly reduces the likelihood of the
formation of cracks within the welds and the
parent material. When the final weld is completed,
whether it is a multi-pass fillet weld or a multi-
pass butt weld, its microstructure differs
significantly from the previous welds. This is
because the final weld is exposed to the cool
ambient air, unlike the previous welds. This
ensures that the final welds have a different
cooling rate than the underlying welds and
therefore a higher level of hardness.
• To investigate and analyse the effects of welding
on the microstructure of mild steel, API 5L carbon
pipe, medium carbon steel (EN8), and O1 tool
steel. The microstructure from parent metal to
Heat Affected Zone (HAZ) and weldment are to
be identified.
• To analyse, interpret and compare the structures
of the mild steel, API 5L carbon pipe, medium
carbon steel (EN8), and O1 tool steel welded
coupons, and discuss their differences.
• To conduct Rockwell hardness and Vickers Micro
Hardness tests on mild steel, API 5L carbon pipe,
medium carbon steel (EN8), and 01 tool steel
welded coupons to ascertain if the hardness
changes across the zones from parent metal to
HAZ to weldment.
• To investigate and compare the effects of heat
input caused by utilizing different welding
processes, namely MMA & TIG.
• To identify and explain the causes of any weld
defects present. Different weld defects will occur
using different welding processes, this will be
achieved utilizing poor operator technique.
• Phase one – This phase included the cutting,
machining, and welding of a selection of
steels.
• Phase two – This phase encompassed both the
sectioning and mounting of the welded
samples as well as the planar grinding and
polishing of the samples.
• Phase three – The final phase was the
microscopic examination and photographic
imaging of the material microstructures as
well as the Vickers micro hardness testing of
welded samples.
• My supervisor Brian O’Brien for his ongoing
support and encouragement throughout this
project.
• Elaine McGeough for patiently training me on the
use of the metallographic equipment in the
materials lab.
• Brian O’Donnchadha for his extensive help on the
use of the hardness testing equipement.in the
metrology lab.
• Alan Somers for allowing me access to the
manufacturing lab anytime that I needed it.
• Chris Keogh for his help with a critical part of this
project.
This project has multiple aspects of investigation
and analysis. These aspects of investigation include:
• The Microscopic Analysis of the weld specimens
allowed for the close examination of the zones of
interest such as the parent metal, the HAZ, and
the weldment.
• The Photographic Imaging showed graphically
the variation in grain structure within the
different zones. The grain structures within these
zones changed due to the heating effect of the
thermal cycles that they had been subject to
because of welding . Reference sample, on the
other hand, had undergone specific heat
treatment processes which also changed their
microstructure.
• The Vickers Micro Hardness test was used to
determine the hardness of the weld specimens
and the surrounding material. The tests revealed
typical hardness profiles of carbon steel, for
example, mild steel. However there were some
unforeseen results regarding hardness within the
HAZ of the medium carbon steel (EN 8). Instead
of an increase in hardness within the HAZ, a
significant decrease was recorded.
• The Heat Input imparted to the weld specimens
by the MMA and TIG welding processes differ
significantly and this has an effect on the final
mechanical properties of the weld and its
surrounding HAZ and parent metal.
• The Yield Strength of the weld specimens was
calculated by multiplying the Vickers HV value
by 3.17. Determining if the yield strength of the
material has changed to a critical point is an
extremely important consideration for the design
of welded components . This became a problem
within the O1 Tool Steel samples .Due to its
very high carbon content, the specimens that
were improperly heat treated experienced
multiple cracks.
• The Thermal Cycling of the Multi Pass Welds
continually alter the microstructure of the weld
specimens because the subsequent welds are
welding over previous weld passes and in effect
normalising the weldment. The final weld is
exposed to the air upon cooling and has a
different hardness value and different
microstructure due to the quicker rate of cooling
than the previous welds.
Variation in Hardness from Root to Cap
Defect free MMA and TIG welded Mild Steel Specimens
Four types of steel welded with MMA,
and the corresponding Vickers Micro Hardness Plots
The table below is a sample of the Average hardness results
obtained while conducting the Vickers Micro Hardness tests.
PROCESS ARCV(Volts)WELDINGI(Amps) EFFICIENCY(η) WELDTESTL(mm) TIME(Seconds)TRAVELSPEED(mm/min) AE(kJ/mm) HI(kJ/mm) %DIFFERENCE
MMA 19 75 0.8 50 17 176.471 0.4845 0.3876
TIG 75 120 0.6 50 22 136.364 3.96 2.376
TABLEOFRESULTSFORARCENERGYANDHEATINPUT
513.00%
Table of results for Arc Energy and Heat Input
Graph of Yield Strength of Weld Specimens
Examples of Carbon Steel Microstructure
Annealed EN 8 Normalised EN 8
Quenched EN 8 Tempered EN 8
MATERIAL JOINTTYPE WELDING PROCESS HVINPARENTMETAL HVINHEATAFFECTEDZONE HVINWELD
MILDSSTEEL BUTT MMA 135.133 165 154.6
MILDSSTEEL BUTT TIG 146.1 179.55 182.7
API5LCARBONPIPE BUTT MMA 176.25 189.1 162.35
API5LCARBONPIPE BUTT TIG 156.675 193.5 175
EN8MEDIUMCARBONSTEEL BUTT MMA 202.45 145.75 227.433
EN8MEDIUMCARBONSTEEL BUTT TIG 176.85 169.85 481.75
01TOOLSTEEL(ANNEALED) BUTT MMA 184.5 399 230.5
01TOOLSTEEL(ANNEALED) BUTT TIG 184.5 312 207.5
TABLEOFAVERAGEVICKERSMICROHARDNESSVALUESACROSSTESTSAMPLES