class week 6 Flashcards
- Explain why hardness tests are carried out in a welding - fabrication environment. [8]
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 heat affected zone and the parent metal can be investigated.
High values in the heat affected zone 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.
The results may demand a change in either parent metal or electrode
- Define hardness. (2]
The ability to withstand indentation
- List three types of hardness testing. [3]
Brinell, Rockwell and vickers
- Explain why charpy impact tests are employed in a welding/fabrication environment and how the test is carried out. Draw the graph related to charpy impact testing. [10]
Impact tests are employed to determine the behaviour of materials when subjected to shock loads. For most engineering purposes it is desirable that a metal should possess some measure of toughness, eg, that it should absorb some energy before it fractures.
The test applied is one in which a notched specimen is broken by a swinging hammer and the amount of energy required to break the specimen is recorded. The notch in the specimen will be on the opposite side to the swinging hammer
5.Fully explain CTOD testing. (8)
The test is performed by placing the specimen into three point bending and measuring the amount of crack opening. 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.
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.
As a rule of thumb, a CTOD value of between 0.1mm and 0.2mm at the minimum service temperature is regarded as demonstrating adequate toughness.
- Nickel based electrodes are sometimes used for welding cast iron materials, what are the advantages of these electrodes, [4]
a.) Nickel does not have a tendency to form carbides with carbon.
b.) Nickel based electrodes has a low solubility for carbon, there for as the weld metal cools, the solubility of carbon in nickel decreases and the carbon is rejected from the liquid metal as graphite.
c.) Nickel based electrodes reduces shrinkage stresses and possible cracking.
- Explain how sensitization occurs in stainless steels. [6]
When stainless steels are heated or cooled through the temperature range 500° c to 850°c, carbon, in solution in the steel, forms carbides with the chromium and precipitates out as small particles. These carbides form preferentially at the grain boundaries and cause the interior of the grain to be depleted of chromium. The lower chromium results in a loss of corrosion resistance and the steel is sensitised
- Fully describe how the thermal severity test is carried out with the aid of sketches. [12]
The CTS Test consists of two plates, one square and the other rectangular. The square plate, 3” on a side, is fastened by a ½” bolt through its centre to the rectangular plate and anchored by welding.
The test weld at the right side of the specimen is called the bithermal weld because the plates supply two paths for heat flow, one in each plate.
The test on the left side is called the trithermal weld since heat can flow in one direction in the top plate and in two directions in the bottom plate, or a total of three paths.
The contacting surfaces are ground before assembly to ensure efficient heat flow. After the specimen is assembled and has cooled to room temperature, the bithermal weld is deposited, the specimen is allowed to cool to room temperature, the bithermal weld is deposited, the specimen is all allowed to re – cool and the trithermal weld is deposited.
After a hold for 72 hours at room temperature, the degree of cracking is determined by measuring the crack length of three metallographic specimens cut from cross – sections of the weld.
- A dished end has been manufactured and delivered to your company to be welded onto a pressure vessel. Your Q.C. Dept. has drawn up a QCP for checking the dished end. List the important points to ensure that the dished end meets the quality requirements of the Q.C. Dept.
- Explain the difference between Q.C. & Q.A.
- List 10.off items you would consider essential for PWHT procedure.
- Why is pneumatic testing considered more dangerous than hydrostatic testing and under what circumstances would pneumatic testing be substituted for hydrostatic testing.
- A hydrostatic test is to be carried out on a pressure vessel. Explain in detail how it should be done to meet code requirement standards.
- Why are single pass welds inferior to multi pass welds with regards to mechanical properties?
a.) They produce columnar grain structure with inferior properties, a multi pass weld will heat treat the weld below it with an increase in mechanical properties because the weld has an equiaxed grain structure.
- List four sources of hydrogen that may have a detrimental effect on a weldment.
Moisture in the flux coating (smaw)
Moisture in the saw flux
Moisture in the Atmosphere
Wet Material
- Name five basic strengthening mechanisms for fine grained steels.
a.) Precipitation hardening.
b.) Cold Strain hardening.
c.) Solid Solution hardening.
d.) Grain refinement.
e.) Quench and Temper.
- Explain the term 18/5.
a.) Time taken to cool from 800°C to 500°C.
b.) It is the temperature / time cycle of an individual weld run and is the time taken during cooling for a weld run and its heat affected zone to pass through the temperature from 800°C to 500°C
- List three items, which can describe corrosion as a deterioration of a substance.
a.) Eating away of metals.
b.) Destruction of metal by chemical or electrochemical reaction with its surrounding environment.
c.) Destruction of materials by means of wear, abrasion and grinding.
d.) Rust is materials natural state.
- Why is aluminium regarded as a corrosion resistant material?
a.) It forms a thin surface film (Al2 O3) on exposure to the atmosphere which tends to insulate the aluminum metal or ally from the environment.
- What is meant by the “carbon equivalent” of commercial steel?
The carbon equivalent (CE) of commercial steel is a measure of the steel’s overall carbon content, taking into account the effects of other alloying elements that affect the steel’s hardness and weldability. It’s a way to estimate the steel’s carbon content based on its chemical composition.
- Define “stainless steel”. How is the “stainless” property achieved? List the four main groups of stainless steel.
a.) Austenitic Stainless Steels (300 series).
b.) Martensitic Stainless Steels (400 series).
c.) Ferritic Stainless Steels.
d.) Duplex
- What is creep? Give an example of creep resistant material.
CREEP IS THE TERM GIVEN TO THE MATERIAL DEFORMATION THAT OCCURS AS A RESULT OF LONG TERM EXPOSURE TO LEVELS OF STRESS THAT ARE BELOW THE YIELD OR ULTIMATE STRENGTH. RATHER THAN FAILING SUDDENLY WITH A FRACTURE, THE MATERIAL PERMANENTLY STRAINS OVER A LONGER PERIOD OF TIME UNTIL IT FINALLY FAILS.
CREEP DOES NOT HAPPEN UPON SUDDEN LOADING BUT THE ACCUMULATION OF CREEP STRAINS IN LONGER PERIODS CAUSES FAILURE OF THE MATERIAL. THIS MAKES CREEP DEFORMATION A TIME – DEPENDENT DEFORMATION OF THE MATERIAL.
THE RATE OF THIS DAMAGE IS A FUNCTION OF THE MATERIAL PROPERTIES AND THE EXPOSURE TIME, EXPOSURE TEMPERATURE AND THE APPLIED LOAD (STRESS).
DEPENDING ON THE MAGNITUDE OF THE APPLIED STRESS AND ITS DURATION, THE DEFORMATION MAY BECOME SO LARGE THAT A COMPONENT CAN NO LONGER PERFORM ITS FUNCTION.
Examples of creep-resistant materials include:
- Nickel-based superalloys (e.g., Inconel, Incoloy)
- Cobalt-based superalloys (e.g., Haynes 230)
- Titanium alloys (e.g., Ti-6Al-4V)
- Stainless steels (e.g., 304H, 310H)
- Refractory metals (e.g., Tungsten, Molybdenum)
- Ceramic materials (e.g., Silicon Carbide, Alumina)
- High-temperature steel alloys (e.g., 9Cr-1Mo, 12Cr-1MoV)
- Why is hydrogen a concern in welding?
Hydrogen is a concern in welding because of the risk of hydrogen-induced cracking ¹ ²:
- Hydrogen can enter the weld pool in the form of moisture from the atmosphere, the base material, or welding filler metals.
- Once in the weld pool, hydrogen can diffuse into the metal and accumulate in areas of high stress, such as the grain boundaries and heat-affected zones.
- As hydrogen builds up, it can cause stress and potentially lead to cracking, especially in materials with higher carbon and alloy content.
- Hydrogen-induced cracking can occur at temperatures below 600 degrees Fahrenheit and can happen immediately or up to several weeks after welding.
- This type of cracking is dangerous, as it can lead to catastrophic failures if not addressed before a part is put into service.
- Why is preheat sometimes required before welding?
- Reduce residual stresses and distortion control
- Prevent hydrogen cracking (cold cracking/delayed cracking)
- Reduce the propensity of hard microstructure formation
- Eliminate stress-oriented weld cracking and minimize welding porosity
- Prevent rapid heating and cooling, which can lead to cracks
- Reduce shrinkage stress
- Eliminate moisture and humidity in the welding area, which eliminates the source of hydrogen gas
- Explain the following terms with reference to a metal:
a) Fatigue strength
b) Fatigue life
a) Fatigue strength:
Fatigue strength refers to the maximum stress a metal can withstand without failing due to repeated loading and unloading cycles. It’s the stress level below which the metal will not fail, even after a large number of cycles. Fatigue strength is typically determined by testing a sample under cyclic loading until failure occurs.
b) Fatigue life:
Fatigue life is the number of cycles a metal can withstand before failing due to repeated loading and unloading. It’s a measure of how long the metal will last under cyclic stress before cracking or breaking. Fatigue life is often expressed in terms of the number of cycles to failure (Nf) or the number of cycles to a specific crack size.
- List three detrimental effects of notches in a welded structure.
- Stress Concentration
- Fatigue Crack Initiation
- Hydrogen-Induced Cracking (HIC)
- What are the “axial stress” and “hoop stress” in a pressure vessel?
- Axial stress (also known as longitudinal stress):
This stress acts parallel to the longitudinal axis of the vessel, along its length. It is caused by the force exerted by the internal pressure along the axis of the vessel. Axial stress tries to stretch the vessel in the longitudinal direction. - Hoop stress (also known as circumferential stress):
This stress acts around the circumference of the vessel, perpendicular to the longitudinal axis. It is caused by the force exerted by the internal pressure on the vessel’s walls, trying to expand its circumference. Hoop stress tries to stretch the vessel in the radial direction.
- Explain the “GTAW dressing” technique to improve the fatigue life of welded components.
The “GTAW dressing” technique, also known as weld toe dressing or weld profiling, is a method used to enhance the fatigue life of welded components by modifying the weld toe geometry. This process involves the following steps ¹:
- Using a GTAW torch to precisely melt and reshape the weld toe
- Creating a smooth, rounded profile
This technique aims to:
- Enhance the overall fatigue life of the welded component
- Improve the durability and reliability of the component
- Minimize the risk of cracking and failure under cyclic loading conditions
The GTAW dressing technique is considered a post-weld treatment method that can effectively improve the fatigue strength of welded joints, particularly in high-strength steel components
- Describe the essential features of a quality control plan and explain the advantages of using such document for a welded fabrication.
