Being a welder is a great occupation
What is welding, and how does it work? Not many people know what welding is or how it works. According to Mary Bonk“welding is the process of heating and melting metal parts to join them together” (Bonk,3). A welding machine uses electricity to melt the electrode, fusing one piece of metal to another. The welding machine uses a ground that connects to the surface that allows the welder to create a flowing currant passing through the electrode and the metal that the welder is wanting to fuse together. Electrodes (or welding rods) can come in many different sizes and with different types of flux. The flux is the protective coating that is around the welding rod protecting the weld as it is being melted to the metal.
There are a few different ways to learn welding. High schools offer welding classes, there are also colleges with available welding curses, or there is on the job training. High school level welding classes are mainly teaching the students the basics of welding to prepare them for colleges and future jobs. “A high school diploma is preferred but not required. High school courses in mathematics and physics are recommended.”(Bonk,2) Many companies will provide on the job training, most of the skills that are learned are learned in the field where improvising is necessary
There are many different needs for a welder in many different fields. “Construction companies or manufacturing plants that employ welders may have job opportunities.” (Bonk,3) Construction company’s often have jobs for welders. When a construction company builds bridges they need a good welder to put the steel frame together. Without a good welder that knows how to produce a good weld the steel might not hold up as well, making it difficult to build the bridge or making it unsafe. Fabrication/manufacturing company’s use welders to build their products. Now days products that used to be made by hand.
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Name Instructor Name Welding 219 06 June 2013 Welding “Welding is a process critical to our present state of civilization and technical advancement, yet little understood and most often taken for granted” (Haynes and Storer 1). We are constantly hearing through television and other media sources that the job market for people trained in some sort of vocational skill is in very high demand. Media advertisements are encouraging students to consider a vocational skill when looking at their future. Welding is one vocational skill that has been and will continue to be in high demand. It is estimated that today, 50% of the gross national product relates to welding and is a part of just about everything you see, including the making of airplanes, ships and different manufactured products, from lawn mowers to big machinery. Welders hold approximately 452,000 jobs nationally with most jobs related to the manufacturing industy (Welding Career Guidance). Welding also plays a big role in providing energy. Welders are involved in the maintenance and building of offshore oil rigs, pipelines, power plants and even wind turbines (Salary Information). According to James Hunt, “Most companies and factories are looking to hire people who have some training with regards to welding .” To better understand the welding profession, we need to understand the.
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ARC WELDING ME 353 Presentation 11-3-2000 Presented by Damon Ogden OUTLINE • • • • • • What is Arc Welding The four most common types Non destructive testing Design considerations Strength Safety Feel free to ask questions at any questions at any time. Arc Welding • Welcome to the world of WELDING What is Arc Welding . An electric arc between the and electrode and the work piece generates heat. Sufficient heat is generated to melt the work pieces together. ELECTRODE ARC WORK PIECES Electricity The range of welding current used can be from 5 to 500 amps. The voltage ranges from 20 to 30 volts, AC or DC. Both are determined by the material thickness. A 60 watt light bulb draws .5 amps. Four Common Types of Welding •Stick SMAW (shielded metal arc welding ) •Mig GMAW (Gas Metal Arc Welding ) •Flux-Core FCAW (Flux-core Arc Welding ) •Tig GTAW (Gas Tungsten Arc Welding ) Characteristics Each welding process has unique traits that make it more suited for particular processes Some Terms Electrode This is where the current passes from the welder to the work piece. There are two types: 1) Contact/consumable 2) Non Contact More Terms Atmospheric protection There are 2 types: 1) Shielding Gas 2) Flux Stick Weld Schematic Current Stick Welding • Uses a.
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three types of welding are Mig, Tig, and Stick. All three of these welding styles have special purposes and are widely used in the world. Mig or Metal Inert Gas welding is the most common type of welding . Tig, or Tungsten Inert Gas, welding is the hardest method to learn, but it has the most satisfying finish welds. Stick or Shielded Metal Arc welding can be done under water, to repair large channel boats that aren’t able to be lifted out of the water. Mig welding uses a supply of argon gas, to form a shield around a solid steel wire. The steel wire is feed through a mig gun by pressing a trigger on the handle. As the wire comes out, a surge of electricity melts the wire. When this is done, a pool of weld begins to form and it connects two pieces of metal together. One more type of wire is called flux coated steel. Flux wire is similar to steel wire, just coated in a white flux casing. The reason for the flux is to promote the two types of metal to create a clean bond so the weld doesn’t snap under force. Also, this wire can be used without argon gas and is safe to use inside of a ventilated facility. Mig welding is most commonly used on mild steel, but can be used on other metals such as stainless steel and aluminum. Items such as cars, trains, furniture, and anything that has steel can be welded. An assembly line in the Detroit Michigan Ford factory.
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covers processes such as welding . brazing, soldering, adhesive bonding, and mechanical joining. These processes are an important and necessary aspect of manufacturing operations. This paper deals with one topic in particular ‘Welding ’. Welding is a process for joining similar metals. Welding joins metals by melting and fusing the base metals being joined and the filler metal applied. Welding employs pinpointed, localized heat input. Most welding involves ferrous-based metals such as steel and stainless steel. Welding covers a temperature range of 1500º F - 3000º F. Weld joints are usually stronger than or as strong as the base metals being joined. Typically, welding is used for forging, farrier, blacksmithing, oil pipelines, and food equipment applications. Now, let us go into Welding in more detail. There are a number of different types of welding . Oxyfuel gas welding (OFW) includes any welding operation that uses combustion with oxygen as a heating medium. Gas tungsten arc welding (TIG) which is used for nonferrous materials like aluminum. Gas metal arc welding (MIG) is used in industries because it’s easier and fast to use. Shielded metal arc welding (STICK) which is the most type of welding method used. For welders blueprint reading is.
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What is MIG Welding . Metal Inert Gas (MIG) welding . also sometimes called Gas Metal Arc Welding (GMAW) is a process that was developed in the 1940s for welding aluminum and other non-ferrous metals. MIG welding is an automatic or semi-automatic process in which a wire connected to a source of direct current acts as an electrode to join two pieces of metal as it is continuously passed through a welding gun. A flow of an inert gas, originally argon, is also passed through the welding gun at the same time as the wire electrode. This inert gas acts as a shield, keeping airborne contaminants away from the weld zone. Advantages and Disadvantages of MIG Welding The primary advantage of MIG welding is that it allows metal to be welded much more quickly than traditional "stick welding " techniques. This makes it ideal for welding softer metals such as aluminum. When this method was first developed, the cost of the inert gas made the process too expensive for welding steel. Over the years, the process has evolved, however, and semi-inert gases such as carbon dioxide can now be used to provide the shielding function, which now makes MIG welding cost-effective for welding steel. Besides providing the capability to weld non-ferrous metals, MIG welding has other advantages: .
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Gas Tungsten Arc Welding The Process Gas Tungsten Arc Welding . also known as Tungsten Inert Gas Welding . is an arc welding process that uses a non-consumable tungsten electrode to produce a weld. The weld area is protected from atmospheric contamination by an inert shielding gas, such as argon or helium, and filler metal is normally used, except in autogenous welds. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding . and allows for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. How it works Manual Gas Tungsten Arc Welding is considered the most difficult of all the welding processes commonly used in the industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. GTAW normally requires two hands, since most.
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millennia, called forge welding . with the earliest examples of welding from the Bronze Age and the Iron Age in Europe and the Middle East. The ancient Greek historian Herodotus states in The Histories of the 5th century BC that Glaucus of Chios "was the man who single-handedly invented iron-welding ." Welding was used in the construction of the iron pillar in Delhi, India, erected about 310 AD and weighing 5.4 metric tons. The Middle Ages brought advances in forge welding . in which blacksmiths pounded heated metal repeatedly until bonding occurred. In 1540, Vannoccio Biringuccio published De la pirotechnia, which includes descriptions of the forging operation. Renaissance craftsmen were skilled in the process, and the industry continued to grow during the following centuries. In 1802, Russian scientist Vasily Petrov discovered the electric arc and subsequently proposed its possible practical applications, including welding . In 1881–82 a Russian inventor Nikolai Benardos created the first electric arc welding method known as carbon arc welding . using carbon electrodes. The advances in arc welding continued with the invention of metal electrodes in the late 1800s by a Russian, Nikolai Slavyanov (1888), and an American, C. L. Coffin (1890). Around 1900, A. P. Strohmenger released a coated metal electrode in Britain, which gave a more.
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Welding Safety Types of Welding Hazards Radiation exposure •Can cause retinal burning and cataracts •Proper lenses with the appropriate shading must always be worn •UV radiation cannot be sensed by heat or brightness Types of Welding Hazards Electric shock •Two kinds of electric shock: primary voltage shock and secondary voltage shock •Primary voltage shock involves 230 or 460 volts and is caused by touching both the lead inside the welding equipment and the welding equipment case or other grounded metal while the equipment is powered ON Types of Welding Hazards Electric shock •Secondary voltage shock involves 60 to 100 watts and is caused by touching a part of the electrode circuit and the side of the welding circuit. The basic arc-welding circuit Types of Welding Hazards Electric shock •Do the following to avoid electric shock: ➤ Keep dry and wear dry gloves. ➤ Stand or lie on plywood, rubber mats or other insulation. ➤ Do not rest any part of the body on the workpiece. ➤ Keep electrodes and electrode holders in good condition. ➤ Do not touch electrodes or metal parts with either the skin or wet clothing. Types of Welding Hazards Fires and explosions •The welding process can produce extreme heat; however, fire hazards are not caused by the heat but by the effect of the heat on the workpiece, such as sparks.
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The Outreach Coordinator, are responsible for planning meetings and other activities geared toward supporting the engagement for the people of Mobile County. The Coordinator works to eliminate barriers and ensure that the communities have access to health screenings. In addition, the Outreach Coordinator works closely with the administrators to ensure that communities are informed about the benefits and services offered.
The Outreach Coordinator, Community and works as part of the Family Services Team to promote a healthy community.
Eligibility, Recruitment, Selection, Enrollment and Attendance
* Develops school specific recruitment plans aimed at informing communities and maintaining full enrollment of the District’s highest-needed assistance for families.
* Participates in annual citywide health events. Encourage eligible families to apply for affordable health services.
* Works with city and statewide organizations to host events.
* Implements strategies aimed at increasing parent engagement and eliminating any barriers to participation in their child’s education.
* Assists families in accessing needed services and develops needed support systems.
* Develops and implements comprehensive classroom level family development plans through strong engagement with parents.
* Regularly quantifies and assesses achievement of classroom level family development plans; and communicates goal achievement with parents.
* Refers to, and collaborates with, Case Manager Specialist for family or child crisis intervention and resources referral services that require ongoing follow-up and/or support.
* Collaborates with Administrative staff and offer support and communicate regularly with the people in the community.
* Implements and maintains an effective and efficient record-keeping, reporting and monitoring system that ensures compliance with all HIPPA and state regulations.
* Facilitates parent training and support.
Published: 23rd March, 2015 Last Edited: 23rd March, 2015
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Background to this assignment
This unit provides the background knowledge and understanding required by those working in
the welding industry as practitioners or in a support role. The unit covers the behaviour of
materials during welding together with features and effects of the welding processes. It covers
the effects that welding has on materials, such as cracking and the weldability of metals.
This unit provides opportunities to develop key skills in application of number and
Assignment 1: Physical Features of Welding
1 Physical features of welding
The arc: voltage distribution across the arc, heat generation at the cathode and anode, arc characteristics and control, temperature distribution in the arc and effects, influence of magnetic fields, limits of application
Shielding gases: tungsten-inert gas JIG) welding, metal inert gas/metal active gas (MIG/MAG) welding, plasma welding, argon, helium. inert gas mixtures
Electrode coverings and fluxes: manual metal arc (MMA) welding, submerged arc welding, flux-cored welding rutile, basic, cellulosic, iron powder, fused, agglomerated, mixed, flux cored, iron cored, gas welding, brazing and soldering fluxes, effects of fluxes and electrode coverings on welding brazing and soldering
Oxy-acetylene welding combustion: composition of inner cone and outer envelope, products of combustion, effects of different flame types, heat distribution in flame
2. Behaviour during welding
Distortion. expansion, contraction, distortion, control of distortion, rectification of distortion, residual stress, pre and post weld heat treatment, stress relief
Heat distribution during welding. thermal gradients, heat flow, the weld thermal cycle, effects on the structure of the weld metal, effects on the structure of the parent metal, heat affected zones (HAZ), HAZ sub-zones
Structure and properties of pure metals. crystalline structure, crystal structure types, micro structures of metals, solid state transformation, elastic/plastic deformation, recrystallisation, cold and hot deformation, work-hardening, mechanical properties (influence of temperature
Structure of the welded joint. thermal field, heat input, peak temperature, cooling rate, dilution, weld metal, solidification of weld pool, structure of the weld, fusion line, HAZ, microstructure of the HAZ, grain growth, relationship grain size-toughness, weldability (definitions), single and multi-pass welding
Testing and examination of materials and welded joints. destructive testing, testing welded joints, tensile and bend tests, notch impact tests (ductile and brittle fracture, transition temperature), hardness tests, fatigue strength tests, metallographic examinations, specimen preparation, macro and micro examination
Alloys and phase diagrams. metals and alloys, alloying elements, solidification, solid solutions, structure of alloys, types of structures, hardening mechanisms (cold working, solid solution, grain size, solid state transformation), intermetallic phases, ageing, basic types of phase diagrams (non-, fully- and partly mixable components), iron-carbon (Fe-C) diagram, influence of elements on the Fe-C-diagram, mechanical properties
Heat treatments of base materials and welded joints. normalising, hardening, quenching and tempering, solution annealing, homogenisation, stress-relieving, recrystallisation annealing, precipitation hardening, heat treatment in practice, heat treatment equipment,
regulations, heat-treatment-diagrams, temperature measurement/recording
3. Weldability of metals
Types of and applicable welding processes for: plain carbon and carbon-manganese steels; fine-grained steels
Thermomechanically treated steels: application of structural and high strength steels; low alloy steels for very low temperature application; low alloy creep resistant steels; creep resistant and heat resistant steels; copper and copper alloys; nickel and nickel alloys; aluminium and aluminium alloys; stainless steels Other metals and alloys: titanium, magnesium
4 Defects in welds
Types and causes of defects in welds: lack of fusion, porosity, solid inclusions, lack of penetration, undercut, piping, craters, oxidation, underfil/concavity, overlap, burn-through excessive weld metal, types of cracks: longitudinal, transverse, edge, crater, centreline,
fusion zone, underbead, weld metal or parent metal; dimensional inaccuracies, remedial action
Crack types and features of: cold-cracking due to hydrogen in steels; Lamellar tearing; hot cracking; reheat cracking
Study the notes given, reading and learning materials listed below
Attempt to answer questions shown on Question sheet.
You should submit your work for feedback from your tutor within three weeks of starting this assignment.
Contact your tutor to arrange for a tutorial on your work.
Act upon the comments and feedback given to you by your tutor and submit the work for formal assessment within one week of your tutorial.
Sign the declaration on the cover sheet of this assignment and complete and submit the student review sheet at the end of this assignment
Recommended reading and study materials
The course makes essential use of the following textbook
The science and Practice of Welding Volume 2 - The Practice of Welding by A.C. Davies Tenth Edition. 1993, Cambridge University Press. 0-521-43566-8
Other references are as follows:
Non-destructive Testing, 1987, by R. Halmshaw, Edward Arnold (Publishers) Ltd, ISBN: 0-7131-3634-0
Reference book - Engineering Materials by R L Timings, Volume 1, Addison Wesley Longman, ISBN: 0582319285.
Weldability of Metals
In order to fulfill the PASS criteria you must successfully answer the following questions:
Question 1 Determine the weldability of common engineering metals
The weldability of a material is how easy it is to weld without producing undesirable properties or defects.
Plain Carbon and Carbon Manganese steels
One way to determine a steels weldabilty is to use a carbon equivalent formula. The carbon equivalent formula gives an indication of the carbon content (plus additional alloys) which would contribute to an equivalent level of hardenability for that of steel (which increases hydrogen cracking susceptibility). High carbon equivalent steels tend to have high strength, but lower toughness and poorer weldability.
Below is the carbon equivalent formula:
A carbon equivalent of less than 0.4 means good weldability, above 0.5 the weldability is considered poor.
Mild or low carbon steels have a carbon equivalent of less then 0.4. They are easily welded with arc, gas or resistance welding processes. These steels have low hardenability and a low susceptibility to hydrogen cracking.
Carbon Manganese steels with Ceq between 0.4 and 0.5 can be welded easily using low hydrogen electrodes. Even though low hydrogen consumables are used, there is still a risk of hydrogen cracking, but weldability is still considered to be good. For sections greater than 25mm preheat is required (40 - 75C) and in some cases PWHT (post weld heat treatment) is required in order to stress relieve.
The weldability of steels with a Ceq above 0.5 is dramatically reduced due to the hard martensitic phase which is formed which results in a high risk of hydrogen cracking and low toughness in the weld. To weld steels with a Ceq above 0.5, a low hydrogen process must be used. Depending on carbon content, preheat of 150 - 250C should be maintained, and after weldingPWHT must be carried out to provide slow cooling. Providing correct consumables, and pre and post heat treatment is adequately applied, high carbon steels can be welded with arc welding processes.
Aluminium and aluminium alloys
Welded aluminium finds a wide range of applications in pressure vessels, gas pipelines, furniture, armour vehicles, aerospace and shipbuilding industry.
The main processes for welding aluminium and its alloys are TIG and MIG. It is possible to weld aluminium with MMA, but this is rarely carried out. TIG is used for welding thinner sections using AC current which also provides a cathodic cleaning action to remove the layer of Aluminium Oxide (AlO).
The surface of AlO affects the weldability of aluminium and its alloys in two ways. Firstly, AlO can hold moisture and grease which are sources of hydrogen that can cause porosity in the welded structure. Secondly, particles of AlO remain solid during welding which may enter the weld resulting in lack of fusion.
MIG is used for welding thicker sections, although it is possible to weld pure aluminium with gas welding (oxy-acetylene).
When welding aluminium, the heat from the welding zone is quickly extracted away due to aluminium's high thermal conductivity. This results in a wide HAZ, and problems with lack of fusion. Residual stresses are increased due to aluminium's high thermal expansion coefficient which causes uneven expansion and contraction. Aluminium doesn't change colour before it melts making it harder for the welder to assess the temperature during welding.
Due to these reasons, aluminium and its alloys are increasingly welded using newer welding processes such as laser welding and friction stir welding. Lasers have a concentrated heat source which counters the high thermal conductivity of aluminium, and reduces distortion. Friction stir welding is a solid welding process which therefore avoids the problems of porosity and cracking in welds.
Titanium is used in a wide range of applications such as aerospace, chemical plant, power generation and oil and gas extraction. This is due to its high strength, low weight and outstanding corrosion resistance.
α - β titanium alloys have a two phase structure and have medium to high strength, but the weldability is sensitive to the ration of α and β phases. TIG is the most common method of welding titanium, but it is also possible to use plasma welding, laser welding, electron beam welding or resistance welding. Matching filler is usually used when arc welding, and adequate gas shielding is required to protect the hot weld and HAZ from the atmosphere since at high temperature, the corrosion resistance of the weld is reduced. The main issues are from contamination of the weld due to atmospheric oxygen and moisture, nitrides, oxides or hydrides which may result in weld embrittlement. Porosity is also possible from dissolved hydrogen from filler metal contamination.
Magnesium alloys are used for a number of applications, ie automotive applications. Magnesium is the lightest structural metal, equal to that of miuld steel. Magnesium alloys can be heat treated and work hardened, however they have a low melting point, high thermal conductivity, and high thermal expansion coefficient, which can mean difficulties with getting good fusion when welded, and high distortion.
Creep resistant steels
Creep resistant low alloy steels usually contain 0.5 - 1% Mo for enhanced creep strength along with Cr contents between 0.5 and 9% for improved corrosion resistance. Due to these alloying elements, the steels offer greater tensile and creep strength at elevated temperatures compared with carbon steels. The alloying elements of Cr and Mo increase hardenability and reduce weldability. As the alloy element content increases, hardenability increases and therefore weldability decreases. This can causes hydrogen cracking, loss of toughness from grain growth in the coarse grained HAZ, reheat cracking and sometimes hot cracking. Common welding processes used for creep resistant steels are MMA, TIG, MAG, FCAW, and SAW. To minimise the above weld defects, preheat should be applied to thicknesses greater than 12mm. As the Cr content increases, the preheat temperature should be increased.
It is essential that PWHT is carried out after welding to reduce residual stress and to allow hydrogen to escape. Generally the consumables have lower carbon content than the parent metals and are low in hydrogen.
Cryogenic steels are alloys used in applications for low temperatures particularly in the transportation and storage of liquefied gases. Cryogenic steels are designed to show good fracture toughness at low temperatures, which is done mainly from the composition and microstructure. Adequate toughness can be obtained by using fine grained C-Mn steels where the fine grain size is obtained from additions of aluminium, titanium and nickel which imparts good toughness at low temperature.
When welding low nickel cryogenic steels (1 - 3.5% Ni) a low heat input or pulsed welding techniques should be used to prevent hot cracking due to the presence of impurities such as sulphur and phosphorus.
In 2 - 3.5% Ni steels, there is a risk of cold cracking in the HAZ or weld metal due to high hardenability. Hydrogen levels should be controlled and preheat (150 - 250C), and PWHT (580 - 620C) should be applied. All arc welding processes can be used and MMA electrodes are either matching composition or more usually high Ni inconel electrodes.
Similar to 1 - 3.5% nickel steels, 5 - 6% nickel steels have a ferrite and pearlite / martensite structure (QT steel) which shows reduced toughness in the HAZ due to an increase in grain size in the area heated to over 850C and therefore a low hydrogen process is essential. A pulsed welding technique should be used to keep the heat input to a minimum. PWHT is carried out at 650C followed by rapid cooling. Typical filler materials are Ni - based eg Inconel 82, Inconel 625.
9% nickel steels show a ductile martensitic structure with 5% austenitic retained. The austenite structure acts as a sink for any hydrogen present, so there is no danger of hydrogen cracking. Therefore no preheat is required for thicknesses up to 50mm.
9% nickel steels must have a very low sulphur and phosphorus content to avoid solidification cracking and low toughness.
The weld metal strength under matches the parent metal strength because nickel based filler materials are required. Fully austenitic fillers are not allowed due to their high coefficient of thermal expansion and the formation of brittle martensite near the fusion line. AC current and demagnetisation is required to avoid arc blow during welding as 9% nickel steels are highly magnetised. The weld pool is also considered as 'sluggish'.
HSLA steels (High Strength Low Alloy)
Conventional carbon manganese steels are limited in strength by the reduced weldability at higher levels of carbon and manganese. HSLA steels are designed to give improved strength at lower carbon levels. The high strength and toughness comes from the fine grain size, which can be produced through micro-alloying and thermo-mechanically controlled processing (TMCP). This gives HSLA steels extremely good weldability.
However, it is difficult to match the strength of the parent metal in the weld metal. Therefore the weld metal needs to be over-alloyed to compensate. This causes a risk of hydrogen cracking in the weld rather than in the parent plate so preheat should be based on the weld metal composition. PWHT is required for thicknesses greater than 40mm.
One of the main welding problems is to achieve adequate fracture toughness in the HAZ and weld metal. High heat inputs can result in the HAZ softening, so heat input and maximum interpass temperature may need to be restricted. High strength grade steel offers potential benefits from being able to use higher operating stresses finding a wide range of applications such as pipelines, structural steels for buildings and bridges, pressure vessels, cranes, and submarine hull construction.
The main weldability problems for stainless steels are:
High temperature grain coarsening resulting in embrittlement. This affects ferritic and duplex stainless steels.
Low temperature hydrogen cracking. This affects martensitic and some precipitation hardened stainless steels.
Hot (solidification) cracking which is mainly a problem for austenitic stainless steels.
Sigma (σ) phase embrittlement. This is a high chromium brittle intermetallic phase occurring between 500 and 1000C. This can be a problem in elevated temperature service for ferritic and duplex stainless steel, and in the weld metal of austenitic stainless steels where there is some ferrite. To prevent this, only preheat if necessary, keep the interpass temperature below 150C and avoid PWHT.
Weld decay (sensitisation). This is where carbon diffuses to the grain boundaries and combines with chromium to form carbides. This leaves a chromium depleted layer along the grain boundaries which is susceptible to corrosion. This is a problem for austenitic stainless steels.
Nickel and nickel alloys
Nickel has a stable face centred cubic (fcc) structure at all temperatures from cryogenic up to its melting point (1455C). Nickel has no phase transformation so the grain size can only be refined by cold working followed by annealing. The thermal expansion of nickel is equal to carbon steel and is used in a wide range of applications due to its excellent corrosion resistance, high and low temperature properties and heat resistance.
Pure nickel has extremely good weldability and can be welded using MMA, TIG, MIG, and SAW preheating to ambient temperature. The precipitation hardened grades are more difficult to weld because of their lower ductility and susceptibility to weld / HAZ cracking. When welding nickel alloys, the filler material matches the parent material and contains oxygen - removing elements to help prevent porosity and cracking.
Nickel alloys can also be welded using resistance welding, laser welding, and electron beam welding.
The defects that can occur when welding nickel and its alloys are:
Solidification cracking - Segregation of impurities such as sulphur and phosphorus in the centre of the weld, just before final solidification leads to solidification cracking. This can be prevented by reducing the welding speed to improve depth-to-width ration of weld bead.
Porosity - Caused by gases from atmospheric or surface contamination entering the weld pool during welding. Can be prevented by cleanliness of joint, gas shielding, and preheating to remove any moisture in the joint. Adding oxygen-removing agents, such as aluminium or titanium can also help reduce porosity, but these elements can form small slag spots so interpass cleaning is critical.
Oxide inclusions - Oxides of titanium, niobium, chromium, and nickel have a much higher melting temperature than the base metal forming inclusions in the weld pool. This can be prevented by grinding or maching the layer of surface oxide.
Lack of side wall fusion - Caused by a weld pool which is hard to control. Bevel angle may need to be increased when welding nickel and its alloys to prevent this.
Copper and copper alloys
Copper and its alloys have good corrosion resistance, good electrical and thermal conductivity, extremely tough, very ductile, has a thermal expansion coefficient of 1.3 times that of steel and melts at 1083C. Due to coppers high thermal conductivity, preheating is required for thicknesses over 5mm to produce a fluid weld pool and avoid fusion defects.
The process used for welding copper and its alloys are usually MIG and TIG using either argon, or argon-helium shielding gases. However electron beam welding is useful for welding thick sections.
Porosity when welding copper and its alloys can be avoided by using appropriate filler wire containing deoxidants (Al, Mn, Si, P, and Ti) and by drying electrodes and fluxes before welding.
Copper nickel alloys (cupronickels) contain between 5 and 30% nickel, 90/10 and 70/30 CuNi alloys are commonly welded grades. They can be welded using inert gas processes using matching filler wire. They have a thermal conductivity similar to low carbon steel and can normally be fusion welded without preheat.
CuNi alloys do not contain deoxidants, therefore autogenous welding is not recommended due to the risk of porosity. To help prevent porosity, filler wires contain between 0.2 - 0.5% titanium which acts as a deoxidant. Argon gas is used for both welding and back purging, especially in pipe welding to produce an oxide-free root.
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