eq - paper 3 Flashcards
Describe the essential features of a quality control plan and explain the advantages of using such a document for a welded fabrication
List four conditions that are necessary to promote (start) Hydrogen cracking.
a.) Sufficient hydrogen
b.) Susceptible microstructure (Hard Brittle Structure)
c.) Sufficient stress
d.) Temperature in the range 100 to 200/300°C
What effect does carbon have on the strength and weld ability of a material (3)
a.) Carbon increases the strength but reduces the weldability.
Contamination from ferritic steel requires carful control when welding s/s. name 4 precautions that must be taken to avoid contamination
a.) Use Stainless Steel cleaning brush.
b.) Use tools reserved for Stainless Steel.
c.) Use aluminum oxide grinding wheels reserved only for Stainless Steel.
d.) Carefully clean off grease and dirt before welding.
explain the procedure when welding carbon to s/s ?
Butter the carbon steel with a 309 electrode first to form a layer of austenitic weld metal then complete the weld as a conventional stainless weld.
The buttered carbon steel may need to be stress relieved prior to completing the joint.
In this way carbon steel can be stress relieved without exposing the stainless steel to a heat treatment that may sensitise it.
Describe how stress corrosion cracking occurs (8)
a.) The corrosion resistance of metal or alloy is decreased and general corrosion occurs. This is called stress-accelerated corrosion.
b.) This results in cracking or brittle failure of the metal.
c.) The stress corrosion cracking or SCC is a type of brittle fracture in a ductile material.
d.) Stress corrosion should be referred to as failure by cracking resulting from the combined effects of corrosion and stress.
e.) These two must work together for failure to occur.
Pneumatic testing is considered, under certain circumstances, to be parlicularly hazardous type of test. Explain why this is so and detail the circumstances under which a test of this nature is sometimes carried out in place of hydrostatic testing?
pneumatic testing, which involves pressurizing a system or component with air or gas, can be particularly hazardous because:
- High pressure: Pneumatic testing often involves higher pressures than hydrostatic testing, which can lead to more severe consequences in case of a failure.
- Rapid failure*: Air or gas is compressible, so if a failure occurs, the pressure can drop rapidly, causing a violent explosion or rapid expansion.
- Greater stored energy: The compressed air or gas contains more stored energy than water, making the potential for damage greater.
Despite these hazards, pneumatic testing is sometimes preferred over hydrostatic testing in certain circumstances:
- Leak detection: Pneumatic testing is more sensitive for detecting leaks, as the compressed gas can escape more easily than water.
- System design: Some systems, like pneumatic or hydraulic systems, are designed to operate with air or gas, making pneumatic testing more representative of actual operating conditions.
- Lighter weight: Pneumatic testing may be preferred for lighter-weight components or systems that cannot withstand the weight of water.
- Cleanliness: Pneumatic testing is often used in applications where water cannot be used due to cleanliness or contamination concerns.
- Specialized equipment: Some equipment, like pneumatic valves or actuators, require pneumatic testing to ensure proper function.
Pneumatic testing is often used in industries like:
- Aerospace
- Automotive
- Oil and gas
- Power generation
- Process industries
In these cases, pneumatic testing is typically carried out with proper safety precautions, such as:
- Containment vessels
- Remote operation
- Pressure relief devices
- Emergency shutdown systems
- Trained personnel
It’s important to note that pneumatic testing should only be performed by experienced personnel, with proper equipment and safety measures in place, to minimize the risks associated with this type of testing.
what Criteria should be considered when selecting a fillermetal for a weld joint
Describe CTOD testing
a.) The test is performed by placing the specimen into a three point bending and measuring the amount of crack opening.
b.) This is done by means of a strain gauge attached to a clip placed between two accurately positioned knife edges at the mouth of the machined notch.
c.) As bending proceeds, the crack tip plastically deforms until a critical point is reached when the crack has opened sufficiently to initiate a cleavage crack.
d.) As a rule of thumb, a CTOD value of 0.1mm and 0.2mm at the minimum service temperature is regarded as demonstrating adequate toughness.
- If a material has a Carbon Equivalent (< 0.4) will preheat be normally required? (4)
a.) Readily weldable, preheat generally not required if low hydrogen processes or electrodes are used.
b.) Preheat may be required when welding thick sections material, high restraint and with higher levels of hydrogen being generated.
Why is hydrogen a concern in welding
- Hazards of Hydrogen-Induced Cracking: Hydrogen poses a significant threat to weld integrity, leading to cracking and potential catastrophic failures.
- Hydrogen Embrittlement: Hydrogen accumulates at high-stress regions, causing embrittlement and reducing mechanical properties.
- Sources of Hydrogen: Hydrogen can enter welds through moisture, electrode coatings, shielding gases, and contaminants like grease and oil.
- Delayed Cracking: Hydrogen-assisted cracking can occur hours or days after welding, making it challenging to detect.
- Industry Standards: The American Welding Society (AWS) and International Institute of Welding (IIW) provide guidelines to minimize hydrogen-related issues.
- Material Susceptibility: Certain materials, like high-strength steels, are more prone to hydrogen embrittlement and cracking.
- Mitigation Strategies: Low-hydrogen electrodes, proper storage, and drying techniques can help reduce hydrogen content
Give 2 reasons why preheat is used when welding a high carbon equivalent steel. list 2 methords of applying preheat?
Explain why Hardness tests are carried out in welding in the Fabrication environment. (8)
a.) To determine if the weld metal matches that of the parent metal and if a cross section of a joint is subjected to hardness tests, the relative values in the weld, the HAZ and parent metal can be investigated.
b.) High values in the HAZ will indicate a fast quench condition and may suggest the necessity of changes in the welding technique in order to increase the heat input and reduce the quenching effect and hardness.
c.) The result my demand a change in either parent metal or electrode.
List 2 adv and 2 disadvantages of RT,MT and PT?
Explain the GTAW Dressing technique to improve the fatigue life of a welded component
GTAW (Gas Tungsten Arc Welding) dressing, also known as weld toe dressing or weld profiling, is a technique used to improve the fatigue life of a welded component by modifying the weld toe geometry.
The process involves using a GTAW torch to precisely melt and reshape the weld toe, creating a smooth, rounded profile. This technique aims to:
- Reduce stress concentrations: By removing sharp angles and notches, stress concentrations are minimized, reducing the risk of fatigue cracks.
- Improve weld toe geometry: The rounded profile helps to distribute stresses more evenly, improving the overall fatigue performance.
- Remove defects: Any micro-defects or discontinuities are removed, further enhancing the weld’s integrity.
The GTAW dressing technique involves:
- Cleaning the weld area
- Adjusting the GTAW torch settings for a precise, controlled arc
- Meltting and reshaping the weld toe to achieve a smooth, rounded profile
- Inspecting and verifying the modified weld toe geometry
