Use of the correct terminology is important.
Frequently the terms ‘weld’ and ‘joint’ are used incorrectly.
Exact definitions are given in AWS A3.0
Welding (Metallurgical Joining)
• Welding is a process for joining two pieces of metals by employing heat input.
• Welding joins metals by melting and fusing of the base metals being joined and the filler metal applied.
• Most welding involves ferrous-based metals such as steel and stainless steel.
• Welding covers a temperature range of 1500º F – 3000º F (800ºC – 1635ºC).
Codes and Standards
There are many types of work which require engineering materials to be joined by welding, for example:
• Pressure vessels
• Structural /Bridges
• Oil rigs
• Storage tanks
• Car bodies
• Food processing plant, etc.
Below are listed some common codes of practice and standards which cover various types of constructions fabricated by welding.
• Code Class of Work
• ASME IX Welding and Brazing Qualification
• ASME VIII American boiler and pressure vessel code
• AWS D1.1 Structural welding code (American)
• API 1104 Standard for welding pipelines and related structures
WPS & PQR
Weld procedures specification (WPS) is a written document that provides direction to the welder or welding operator for making production weld in accordance with code requirements.
The Procedure Qualification Record (PQR) is a record what occurred during welding the test coupon and results of testing of coupon
Welders qualification to ensure a particular welder is capable of welding to a procedure and obtaining a result that meets specification
The following parameters to be considered during procedure qualification:
• Joint Design
• Parent Metal / Base Metal
• Filler Metal
• Welding Process
• Welding Position
• Electrical characteristics
Material Grouping for the purpose of welding and heat treatment:
• P-Numbers are assigned to base metals depending on characteristics such as composition, weldability and mechanical properties where this can logically be done.
• Group Number; classify the metals with P-numbers for the purpose of procedure qualification where notch-toughness requirements are specified
• F-Numbers are assigned to filler metals or welding rods based on essentially on their usability characteristics, which fundamentally determine the ability of welders to make satisfactory welds with a given filler metal
• Essential Variables – An essential variable is a variable that will influence or change the mechanical or metallurgical properties of the welded joint – changes affecting the procedure approval. Any change in an essential variable requires a new welding procedure specification (WPS).
• Supplementary Essential Variable – A change in a welding condition which will affect the notch-toughness properties of a weldment
• Non-Essential Variable – Non-essential variables are those in which a change, as described in the specific variable may be made in the WPS without re-qualification
There are many welding processes were introduced, among of them the following processes are commonly used in construction, fabrication and manufacturing.
• Shielded Metal Arc Welding (SMAW)
• Submerge Arc Welding (SAW)
• Flux Core Arc Welding (FCAW)
• Gas Tungsten Arc Welding (GTAW)
Shielded Metal Arc Welding (SMAW)
Shielded metal arc welding (SMAW) is a welding process in which the joining of metals is produced by the heat of an arc between a covered electrode and the base metal.
Fig-1 Shielded Metal Arc Welding (SMAW)
• Volts – Controls arc length and shape of the weld.
• Amps – Controls penetration.
• Run out length – Controls travel speed.
• Together the above three main welding parameters control heat input.
• Electrode Angles – Slope affects penetration.
SMAW electrodes are available to weld carbon and low alloy steel, stainless steels, cast irons, copper and nickel alloys and for some aluminum applications
Effect of Variation in Procedure:
• Arc too short; Too short an arc length will cause irregular piling of the weld metal.
• Arc too long; Too long an arc length will cause the deposit to be course rippled and flatter than normal.
• Travel too slow; a slow rate of travel gives a wider, thicker deposit, shorter than normal length.
• Travel too fast; A fast rate of travel gives a narrower, thinner deposit, longer than normal length.
• Current too low; A low welding current tends to cause the weld metal to pile up without adequate penetration into the parent metal.
• Current too high; A high welding current gives a deposit that is flatter and wider than normal with excessive penetration into the parent metal.
Submerged Arc Welding (SAW)
The SAW process, an arc is maintained between the end of a bare wire electrode and the work. As the electrode is melted, it is fed into the arc by a set of rolls, driven by a governed motor. Wire feed speed is automatically controlled to equal the rate at which the electrode is melted, thus arc length is constant (similar to MIG/MAG – constant voltage). The arc operates under a layer of granular flux, hence submerged arc. Some of the flux melts to provide a protective blanket over the weld pool. The remainder of the flux is unaffected and can be recovered and re-used, provided it is dry and not contaminated.
Fig-2 Submerged Arc Welding (SAW)
A semi-automatic version is available in which the operator has control of a welding gun that carries a small quantity of flux in a hopper. It is used in beam, boom, tractor and multi-head type rigs.
Gas Tungsten Arc Welding (GTAW)
Gas Tungsten Arc Welding (GTAW), also known as tungsten inert gas (TIG) welding is a process that produces an electric arc maintain between a non-consumable tungsten electrode and the part to be welded.
The heat-affected zone, the molten metal and tungsten electrode are all shielded from atmospheric contamination by blanket of inert gas (usually Argon) fed through the GTAW torch.
Fig-3 Gas Tungsten Arc Welding (GTAW)
GTAW process can produce temperatures of up to 35000 ºF (19426 ºC). Torch contribute only to the work piece. If filler metal is required to make weld, it may be added manually in the same manner as it is added in the oxyacetylene welding process.
• GTAW is used to weld carbon steel, stainless steel, nickel alloys such as Monel and Inconel, titanium, magnesium, cooper, brass, bronze and even gold.
• GTAW also can weld dissimilar metal to one another such as cooper to brass and stainless steel to mild steel.
Flux Core Arc Welding (FCAW)
Fig 4-Flux Core Arc Welding (FCAW)
• Wire feed speed (and current)
• Arc voltage
• Electrode extension
• Travel speed and angle
• Electrode angles
• Electrode wire type
• Shielding gas composition (if required)
FCAW wires that don’t require a shielding gas commonly emit fumes that are extremely toxic; these require adequate ventilation or the use of a sealed mask that will provide the welder with fresh air.
The process is widely used in construction because of its high welding speed and portability.
A weld that does not meet any or all of specific requirements of a particular specification or code is considered a defective weld.
Not all weld discontinuities are defects, so the defect will refer to general classification;
• Related to drawing or dimensional requirements
• Related to weldment and related to discontinuities
• Related to undesired properties of weld metal or joint
• Related to properties of the base metal
The type of defect as follows;
• Incorrect weld size
• Incorrect weld profile
• Incorrect final dimensions
• Excessive weld reinforcement
• Slag inclusion
• Tungsten inclusion
• Incomplete Fusion
• Incomplete / excessive Penetration
• Surface irregularities
• Burn through
Weld Defects-Their Causes and How to Prevent Them
Inspection of Welding
To verify the weld surface condition in accordance with acceptance criteria
Visual inspection provides the basic element for evaluation of structures or components being fabricated. It constitutes an important aspect of practicable quality control for weldments with joints that require testing.
It has been proven in numerous situations that an effective program of visual inspection will result in the discovery of the vast majority of those defects which would be found later using some other more expensive non-destructive test methods.
The root and second pass or hot pass should be inspected in process for the degree of penetration and side wall fusion especially if no further NDT/NDE is required.
The extent of reinforcement and size and position of the welds in relationship to the fitted joint are important factors in the determination as to whether a welding job should be accepted or rejected, because collectively, the above all reflect the quality of the weld.
The finished weld should be inspected for:
• Incomplete Fusion
• Incomplete Penetration
• Under Fill
• Metallic and non-metallic inclusion
• Excessive reinforcement
Non-Destructive Test (NDT)
Inspection of weld which could not verify visually, there are;
• Radiography (RT)
• Ultrasonic (UT)
• Magnetic Particle (MPI)
• Dye Penetrant (PT)
Destructive Test (used for welding qualification)
Destructive tests on welded joints are usually made as part of the approval of a welding procedure or a welder.
The test pieces are cut from the test weld and their location is often specified in the standard. The British standard for the testing of welds is BS 709: 1983 Methods of testing fusion welded joints and weld metal in steel. The areas for test are shown below.
Commonly used destructive tests are:
• Fracture tests
• Macro section
Positive Material Identification (PMI)
Positive Material Identification (PMI) is one of the more specialized non-destructive testing methods. With Positive Material Identification, the alloy composition of materials can be determined. If a material certificate is missing or it is not clear what the composition of a material is, then PMI offers the solution. PMI is particularly used for high-quality metals like stainless steel and high alloy metals. Elements that can be identified using PMI include: Ti, V, Cr, Mn, Co, Fe, Cu, Zn, Ni, Se, Nb, Mo.
The purposes of PMI are:
• To verify that all critical materials conform to project requirements,
• To ensure that dangerously inappropriate alloys are not incorporated in the completed process plant, either by accident or well-meant but misinformed action;
• To provide documentary evidence to authorities or endorsers that reasonable quality control procedures have been used in building any plant where failure could have serious consequences;
• To identify material other than that specified and to allow for an appropriate body to judge its suitability. This avoids accidental incorporation of acceptable substitutes with inappropriate welding procedures and leaves a record for future plant maintenance.
A minimum ferrite content is necessary to avoid hot cracking in stainless steel welds. The amount of ferrite in the weld metal also control the micro structural evolution during high temperature service. Moreover, the amount of ferrite controls the corrosion and stress corrosion resistance
Duplex solidifies initially as ferrite, then transforms on further cooling to a matrix of ferrite and austenite. In modern raw material, the balance should be 50/50 for optimum corrosion resistance, particularly resistance to stress corrosion cracking. However, the materials strength is not significantly affected by the ferrite / austenite phase balance.
Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. The picture shows the yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic grains transform to austenitic grains (“islands”). The result is a microstructure of roughly 50% austenite and 50% ferrite.
Duplex stainless steels have a two-phase microstructure of austenite and ferrite grains.
Low levels of austenite: – Poor toughness and general corrosion resistance.
High levels of austenite: – Some Reduction in strength and reduced resistance to stress corrosion cracking.
The main problem with Duplex is that it very easily forms brittle intermetallic phases, such as Sigma, Chi and Alpha Prime. These phases can form rapidly, typically 100 seconds at 900°C. However shorter exposure has been known to cause a drop-in toughness, this has been attribute to the formation of sigma on a microscopic scale. Prolonged heating in the range 350 to 550°C can cause 475°C temper embrittlement. For this reason, the maximum recommended service temperature for duplex is about 280°C.