Fourth Year Final Presentation

Metallographic Examination and

Micro Hardness Examinationof Multi-pass Welds

By Joseph Stynes

Supervised ByBrian O’Brien

Purpose of Project• Metallographic examination & Vickers Micro Hardness testing of multi-pass welds using selected carbon steels to observe the effects of welding, using MMA and TIG welding processes.

Main Objectives• To analyze the effects of welding on the microstructure of the parent metal, the

HAZ, and the weldment of mild steel, API 5L, medium carbon steel (EN 8), and O1 tool steel, using photographic images.

• To conduct Vickers Micro Hardness tests on the weld specimens and ascertain whether or not the hardness changes across the weld specimen.

• To identify and explain the causes of any weld defects present, such as porosity, slag inclusions, lack of fusion.

- To calculate the arc energy and heat input caused by the different welding processes, MMA and TIG and determine the effects on the

microstructure.- To estimate the yield strength of the weld specimens so as to determine if the weld specimens differ in yield strength from that of the raw materials and if that is a concern regarding design of welded components.- To calculate the Carbon equivalent of the selected steels so as to

determine their weldability.

Background and justification for project

• It is important to obtain sound accurate information regarding the mechanical properties of specific materials.

• To observe how and why the hardness of carbon steel changes when subject to welding, and how this is related to carbon content.

• To determine if it is feasible to or advisable to weld specific carbon steels. This would be calculated using the International Institute of Welding carbon equivalent formula.

• If a value above 0.4 is calculated then it is accepted that the material can be difficult to weld without precaution or heat treatment.

• Mild steel = 0.33 Max. *These are Maximum values for the • API 5L = 0.48 Max. * Implications of not following recommended• EN 8 = 0.606 Max. Procedures can result in brittle failure. • O1 Tool steel = 1.158 Max.

Average Vickers Micro Hardness results

• The values highlighted in yellow, for example, represent a typical hardness profile across all zones for MMA welded mild steel. See graph 1 on the following slide.

• The values Highlighted in red, however, are the HV values for hardness in the Heat Affected Zone in the medium carbon steel (EN 8). See graph 3 on the following slide.

• These results were not expected, a rise in hardness from the parent metal to the Heat Affected Zone is what was expected.

• This anomaly was due to the inadequate size of the weld specimen coupled with excessive growth of ferrite in the HAZ due to the high chromium content of the filler material.

• The EN 8 weld specimens became saturated with heat which made it impossible for the material to retain its expected mechanical properties.

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

Graphs of Vickers Micro Hardness of the weld specimens

Mild steel, API 5L, EN 8, O1 Tool steel

MMA Welded Mild Steel Plate MMA Welded API 5L (Seamless Pipe)

MMA Welded EN 8 (Medium Carbon) MMA Welded O1 Tool Steel (Annealed)

Macroscopic imaging of defect freeweld specimens

• The images of MMA & TIG welded mild steel illustrate sound defect free weld specimens.

• The grain structure visible in the MMA specimen is more coarse, suggesting localized annealing due to the lower heat input.

• The grain structure visible in the TIG specimen is more refined, suggesting a more generalized annealing throughout the weld specimen.

Macroscopic imaging of defective weld specimens

SLAG INCLUSION

LACK OF PENETRATION

LACK OF FUSION

LACK OF SIDEWALL FUSION

POROSITY

Weld defects in weld specimens caused by:

• Inadequate welding current

• No side wall wetting

• Inadequate gas shielding

• Improper welding technique

MMA WELDED MILD STEEL TIG WELDED MILD STEEL

The variation in Vickers Hardnessin multi pass welds

• There is a rise in Vickers Hardness vertically from root to cap in multi pass welds.

• Welds are subsequently annealed after every pass.

• The cap weld is exposed to the cool ambient air and therefore cools much more rapidly than the previous weld passes.

Weld specimen microstructure

MILD STEEL PARENT METAL 50 x 10 MILD STEEL HEAT AFFECTED ZONE 50 x 10

EN 8 PARENT METAL 50 x 10 EN 8 HEAT AFFECTED ZONE 50 x 10

Large white Ferrite grains with black Pearlite at the grain boundaries

Smaller white Ferrite grains with black Pearlite at the grain boundaries

White Ferrite grains with larger amountof Black Pearlite at the grain boundaries

Dark Martensitic structure interspersedwith Ferrite

Arc Energy and Heat Input

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%

• Arc Energy and Heat Input are methods used to determine the heat energy imparted to a material during the electric arc welding process.

• MMA and TIG welding processes impart different levels of heat input into the weld specimen.

• Higher heat input leads to a slower cooling rate which promotes excessive grain growth as shown in the TIG welded EN 8 specimen below.

TIG Welded EN 8 MMA Welded EN 8

HAZ of EN 8 HAZ of EN 8

Yield strength of weld specimens• Yield strength is directly related to hardness.

• Yield strength is equal to 3.17 x Vickers Hardness (only applicable to Carbon Steel).

• This is an important consideration regarding the design of welded components because of the need to know if welding alters the raw material to an unacceptable level.

Defect Free MMA Welded O1 Tool Steel

Annealed

Defective TIG Welded O1 Tool Steel

Normalized

Defective TIG Welded O1 Tool Steel

Room Temp

Conclusions• As predicted there is an increase in hardness across the parent

metal, through the HAZ, and then a decrease in hardness in the weldment.

• Along with the reduction in grain size comes an increase in hardness, equally there is a decrease in hardness with excessive grain growth.

• There is a significant difference in the value of heat input between the TIG welding process and the MMA welding process. The higher heat input means a prolonged cooling rate promoting excessive grain growth.

• For the weld specimens tested, it was discovered that with an increase in hardness came an increase in yield strength. This becomes a problem within the HAZ of high carbon containing materials because it could lead to brittle fracture.

Thank you for listeningAny questions ???