MICROSTRUCTURAL & MICROHARDNESS ANALYSIS OF DEFECTIVE AND DEFECT FREE

MULTI-PASS WELDED COUPONSStudent Name: Mr. Joseph Stynes

Student Number: X00105837

ABSTRACT

OBJECTIVES

METHODOLOGY

RESULTS DISCUSSION

ACKNOWLEDGEMENTS

• 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 resultsobtained while conducting the Vickers Micro Hardness tests.

PROCESS ARC V (Volts) WELDING I (Amps) EFFICIENCY (η) WELD TEST L (mm) TIME (Seconds) TRAVEL SPEED (mm/min) AE (kJ/mm) HI (kJ/mm) % DIFFERENCEMMA 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

TABLE OF RESULTS FOR ARC ENERGY AND HEAT INPUT

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 JOINT TYPE WELDING PROCESS HV IN PARENT METAL HV IN HEAT AFFECTED ZONE HV IN WELDMILDS STEEL BUTT MMA 135.133 165 154.6MILDS STEEL BUTT TIG 146.1 179.55 182.7

API 5L CARBON PIPE BUTT MMA 176.25 189.1 162.35API 5L CARBON PIPE BUTT TIG 156.675 193.5 175

EN8 MEDIUM CARBON STEEL BUTT MMA 202.45 145.75 227.433EN8 MEDIUM CARBON STEEL BUTT TIG 176.85 169.85 481.7501 TOOL STEEL (ANNEALED) BUTT MMA 184.5 399 230.501 TOOL STEEL (ANNEALED) BUTT TIG 184.5 312 207.5

TABLE OF AVERAGE VICKERS MICROHARDNESS VALUES ACROSS TEST SAMPLES