Carbon steels are one of the main categories of steel. Steel is divided into groups since it is an iron (Fe) alloy and therefore it may contain numerous concentrations of alloying elements such as chromium, molybdenum, nickel, manganese. Yet, the mechanical properties of steel are affected according to the carbon percentage in the metal. Groover, 2010) (Callister Jr, 2007). Carbon steel is an iron-based, malleable metal, usually containing less than 1% carbon but in some cases may contain up to 2.03% C (plain carbon steels), along with small percentages of silica, sulphur, phosphorus and manganese. (Carbon steel, 1994-2011). The carbon quantity in carbon steel alters the strength and ductility of the metal. By increasing the carbon, hardness and strength are increased whilst reducing ductility. Yet by doing so, brittleness is increased and welding abilities are reduced due to its affinity to form martensite. This is a kind of tug of war between the properties of the metal.
Carbon steels are divided according to the different amounts of carbon content i.e. mainly into three main classes;
Mild & low carbon steel ( 0.16% – 0.29% C . Having microstructures of pealite and ferrite)
This category is also known as mild steel. They are very common and are widely used since they are quite cheap, easy to form and to work with. Having low carbon content, they are ductile and malleable but have a low tensile strength and do not repond to heat treatments, which would form matensite. Their density is of 7.85g/cm^3 and youngs modulus of 210.000MPa. Surface treatment such as carburizing is performed when large amounts of steel requiring increased surface hardening. To further this strength, this steel is cold worked. Such applications identified as structural steel are used in buildings, where the right weldability, formability, combined with improved strength and resistance fracture, through surface treatment are required. (Types of Carbon steel )
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Cross-sections of structural beams made of low carbon steels.
Medium Carbon steels (0.3% – 0.59% C. Microstructures of tempered martensite)
Such steel, having a higher percentage carbon, increases hardness, brittleness and strength at the same time still being ductile, although this is slightly reduced with machinability. This range of carbon is achieved by austenitizing, quenching (i.e. rapid cooling from the outer surface to the inner) and tempering to create consistent tensile strength within the steel (referred to as Martensite) throughout the body. An application of medium carbon steel includes crankshafts since ductility allows it to retain the tensile strength required. Axle shafts and gearing plates are also made from medium-carbon steel (The uses for medium carbon steel, 1999-2012) (Tata steel Europe)
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Crankshaft side and an end view together with a gear which are made of medium carbon steel
High carbon steel (0.6% – 1.4% C)
These are the hardest and strongest of carbon steels and therefore the least ductile. They are ideal for hardening and tempering condition and have good wear resistance. Hardness may be improved by further adding chromium, vanadium, tungsten, and molybdenum to carbon. An ideal application would be a sharp cutting tool such as scissors or a high strength spring which is required to withstand heavy loads. If welded, heat treatment is further required to keep the same mechanical properties. Higher percentage carbon in such steels may also be present in two other classes which are part of the high carbon steels; (Types of Carbon steel ) (Groover, 2010)
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Higher carbon steels (03%- 1.7% C) – Sulphur and manganese are also present to improve hardness which is ideal for cutting tools such as punches, milling machine cutters, industrial knives.
Ultra high Carbon steels (1% – 2% C) – These large amounts of carbon are required for special cases, mainly non-industrial tools and are produced using powder metallurgy. These are at the limit of mild steel since above 2% carbon steel is said to be cast iron. (The uses for medium carbon steel, 1999-2012) (Callister Jr, 2007)
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Typical applications where such steels are utilised are drilling tools used for high speed drilling in substantial hard material.
P7 – One metal forming technique from the chosen industry;
Since part of my previous assignment involved the automotive industry, I decided to further extend my knowledge on this subject and research on a very important forming technique, which is becoming even more used in this industry. The technique, I researched is the hydro-forming technique and the metals involved were steel and aluminium metals
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Nowadays vehicles are designed to operate as fuel efficient as possible yet without sacrificing on speed. These two factors though contradictory have something in common, weight reduction. To attain this, vehicles are continuously shredded from their weight and this is mainly done by using lighter metals and thin grade panels. However such metal forming becomes quite difficult due to formability and elongation problems. For this reason, hydro-forming technique is used where a sheet metal part is formed under water pressure generated by a punch drawing the sheet in a pressurized water chamber. This increases formability and it is mainly due to the water pressure which holds the material in place which is in turn punched in the forming process, shielding the panel from extreme thinning in critical areas. (Cass) (Altan, 2002)
Sheet hydroforming diagram
This technique can also be used to stretch form or deep draw metal. Other variations of the process are:
Active hydro-forming-involves a process in which the blank is pressed against a die contour.
Viscous pressure forming-where a viscous material is used instead of water to pressurize the medium
Flex forming-water pressure used as an elastic polymer membrane that shields around the sheet and the punch. Adapting the Process (Altan, 2002)
Here shown are the basic components making up the punch to carry out the hydro-forming technique. The upper binder or blankholder provide the holding force for the blank. The water chamber is used to hold the initial blank. Pressure chamber provides hydraulic counter pressure to the water. Hydraulic cylinders provide the force to the blank holder which in terms is controlled through a P.L.C controlling system.
The water is then pumped in to the chamber below the die. This is controlled outwards from a relief valve.
As seen above, we can easily observe the results of the panels produced in steel. The difference between steel and aluminium is that aluminium has 30%- 40% less elongation and formability. The answer to aluminium drawing was hydro-forming which has obtained results of 50% more depth than conventional drawing techniques. (Cass)
Hydro-forming is on the increase in the automotive industry and nowadays, one can state that it is the most frequent used technique for all types of body-panels. However this process has its own limitations such as low manufacturing cycle times, highly specialised expensive heavy duty equipment which requires highly skilled operators. On the other hand, its under mentioned advantages are appealing; (Altan, 2002)
Gives better drawing depths with better strain distribution.
Draws complex shapes in one press cycle.
Reduces die costs since one die is used.
Finish is excellent.
P8: Distinguish between fusion & non-fusion welding processes. Select one fusion or non fusion and discuss the principles of operation, parameter of the process, the equipment used, advantages, disadvantages and precautions taken. Also describe one application for the process selected.
Welding, still considered a recent metal working trade, is a process which involves two or more materials, which are required to be joined together at the surfaces in a strong enough bond not only mechanically (using rivets or bolts) but metallurgic ally (involving diffusion). This makes the bond secure and strong, eliminating the process of having to manufacture a new single part from scratch. To ensure that a good weld is formed, the surfaces to be welded must not have any asperities, meaning that any roughness, dirt and pointed ends must be removed to achieve the best weld possible. In order to overcome these difficulties, pressure, heat or both must be present in the process, which helps to bring atoms together and agitate more the microstructure of the materials so as to create a true secure bond. Cleanliness as already mentioned is essential and depending on the degree of dirt, one must utilise the appropriate cleaning tools. Generally, chemicals are used such as degreasers and solvents, which dissolve the oil or dirt, or else mechanically – were abrasion, grinding etc, are used for rougher surfaces to attain the best smooth layers possible. (Groover, 2010) (Callister Jr, 2007)
Since different materials with a vast number of properties and features may require welding, various types of welding exist on the market, each with their own characteristics and methods, differing in the apparatus, temperature and pressure used, type of gas involved ( acting as a shield) when another metal may be present. These welding methods are generally divided into two groups, non-fusion welding and fusion welding.
Non-Fusion Welding:
This type of welding can also be called solid-state welding involving bonding of materials without melting the base metals and no filler material is added. Non-fusion welding involves some of the oldest welding processes and some of the very latest. Also in such cases, pressure or temperature or both can be used to build up sufficient energy to bond the intended surfaces intended to be welded. The most important factors for a successful solid-state weld are that the two surfaces must be very clean and they must be very close to each other to allow atomic bonding. As no melting is involved, non-fusion welding has quite a few advantages over fusion welding. No melting means that there is no heat-affected zone; the metals surrounding the joint retain their original properties. Most of the processes producing welded joints affect the entire contact point between the two parts, instead of a distinct point like most fusion-welding operations. Adding to this, at times some of these processes are used to bond dissimilar metals and it is important to note that these, if melted and re-solidified, may alter their relative thermal expansions, conductivities, and other properties which are very important when applied to a factual application. The drawbacks on the other hand for such welding process when, compared with conventional fusion welding, are that since the welding cycles take more time to complete, it is a more time consuming process and quite unsuitable for restricted sized parts. It is again important to note that surface preparation is essential before actual welding takes place for the surfaces to bond precisely. Yet the major disadvantage of this process is the relatively high initial investments cost in equipment. (Groover, 2010) (Schmid, 2010)
Typical examples of non-fusion welding processes include;
Diffusion welding (DFW). Pressure is used to hold two surfaces together at a high temperature where the parts bond by solid-state diffusion.
Friction welding (FRW). Bonding is achieved creating heat created from friction between two surfaces.
Ultrasonic welding (USW). Two parts with an oscillating motion from an ultrasonic frequency at moderate pressure is used in a direction parallel to the contacting surfaces. This combination of normal and vibratory forces gives shear stresses that removes surface films and achieves atomic bonding at the surface.
Fusion Welding:
This type of welding technique is also known as liquid-state welding and as the name implies, the base metals for this process are melted using heat. In most fusion welding operations, a filler metal is included in the molten pool where the bond is desired. These may be in the form of consumable electrodes or a wire fed into the weld pool. Their main purpose is to improve and ease the process to produce a much stronger weld in terms of metallurgy (atoms are packed closer together creating a much tighter mechanically bond). As a protection against oxidisation, these processes also include a protective layer between the air around the weld and the molten metal. These can be either in the form of a gas shield or as a type of flux which melts to produce a layer on the weld itself that solidifies and is removed afterwards. Fusion processes where no external metal is used are known to be autogenous welds. Advantages offered from fusion welding, makes it the most common and most vast of the welding processes which are mainly the ability to repeat the weld at the same joint without difficulty, which is relatively fast and adequate for most applications. As for the drawbacks, there are two main problems. The changes in the microstructure after repeated heating and rapid cooling could easily alter the properties of the parent metals and the effects of the residual stresses which build up in the parent metals caused by expansion or contraction. These have a long term effect on the weld itself due to the fatigue produces.
The following are the main types of welding processes;
Gas welding –
Oxyacetylene Welding (OAW)
Arc Welding –
Shielded metal arc welded (SMAW)
Gas – tungsten arc welding (GTAW)
Plasma arc welding (PAW)
Gas- metal arc welding (GMAW)
Flux cored arc welding (FCAW)
Submerged arc welding (SAW)
Electro-slag welding (ESW)
High energy beam welding-
Electron beam welding (EBW)
Laser beam welding (LBW)
(Kou, 2003), (Callister Jr, 2007), (Jha)
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Gas – Metal Arc Welding;
This welding process is a fusion welding process and uses the basics of this type of welding, since it melts the metals at the joining area, using elevated temperatures whilst creating an arc between a continuous fed filler wire electrode and the metals to be joined. The weld is constantly shielded using an inert gas. The type of gas used, differs from application to application. Inert (argon gas for example), is used for MIG welding. Other shielding gases used are carbon dioxide, as well as inert/active gas. Mixtures at times are used mainly to weld mild steel alloys (a mixture may be used from argon, carbon dioxide and oxygen). The ideal gas used for shielding, inert or active, is usually chosen according to the alloy composition and the grade of finish desired.
Metal Inert Gas welding (MAGS & MIG);
Another gas-metal arc welding process is the metal inert gas process commonly known as MIG. In common with the tungsten inert gas process (TIG), MIG welding uses a protective gas shield layer over the weld pool projected using a torch shroud. MIG uses electricity to melt and create the welding ‘pool’ that joins pieces of metal together. It may also be referred to as the “hot glue gun” and is known to be one of the easiest welding processes to learn. It was developed in the 1940’s and even nowadays, it still uses the same principles. The electrical current used to melt the metals is used to create a short circuit between a continuous wire fed through a gun to act as the anode and the cathode being the metal being welded. This short circuit which dissipates enough (approx. 4000 F to 6000 F) heat to melt the metal and the non reactive gas, shields the weld being produced. With the metal molten, the two surfaces fuse together becoming one piece and as the heat is removed, the metal cools, solidifying in a unite piece of metal. Being an easy to learn welding process, makes this application popular and moreover, since it can be used with a variety of materials: carbon steel, stainless steel, aluminium, magnesium, copper, nickel, silicon bronze and other alloys.
http://www.personal.psu.edu/jhk5143/edesgn100/project4/welding/pic3.jpg
MIG welder equipment –
A MIG welder is made up of several parts:
The welder;
Here the main equipment is the wire and a series of rollers which are used to push the wire out from the welding gun. The large spool of wire is held on with a tension nut.
The welding gun;
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The most important part takes place. The gun (1) is made up of a trigger that controls the wire feed and the flow of electricity. Here, the wire is passed through a replaceable copper tip (3). Tips vary in size according to the diameter of the wire (5). The outside of the tip of gun is covered by a ceramic (2) or metal cup which is used to protect the electrode (4) and gives direction to the flow of gas.
The Ground Clamp;
this is basically the cathode (-) in the circuit, which is clamped directly to the piece of metal being welding. Good contact is essential on the bare metal.
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Advantages of MIG welding;
Versatile in the sense that, it welds a wide range of metals and thicknesses,
Will weld in any angle and position,
Less cleaning is required,
Has a good weld bead,
Does not splatter the weld,
Long welds can be done without starts and stops,
Easy to learn.
Disadvantages of MIG welding;
It involves bulky equipment,since the gas source is transported in cylinders
Produces a rougher and less controlled weld compared to TIG welding.
Requires Irregular wire feed,
Porosity and burn-back weld finish,
Unstable arc,
Heavily oxidized weld deposits,
Difficulty in starting the weld on certain material.
Precautions;
At first hand, the most important precautions are those regarding the health and safety aspect. Welding involves heat, which may cause fire leading to accidents and injuries. Therefore, safety gear is essential. These include gloves, apron, welding helmet (to protect your eyes from the bright light produced by the electrical current) and a handy fire extinguisher.
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Bright light whilst welding Safety gear
Other important precautions/defects must be taken during the actual process itself. Whilst welding, one might find that holes are produced in the weld, this might be due to too much current which might occur due to over-melting of material. This is resolved by reducing the current load from the welder apparatus. Spurts might also form on the weld. This is due to the wire speed or power settings regulated being too low. What happens is that the gun would be feeding in too much wire which whilst melting would splatter the weld without forming a proper one. A good weld will be achieved when all the settings are properly set resulting in a smooth weld. It should be noted that the sound of a continuous spark indicates the proper quality of the weld.
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MIG (Fusion) Welding for Aluminium and Its Alloys:
Aluminium welding is regarded as a very difficult procedure due its low melting point yet high heat conductivity and which could result in poor penetration and molten holes. Aluminium being a non ferrous metal, is readily available in various product forms. In order to establish a proper welding procedure, it is essential to know the material properties of the aluminium alloy being welded. These can be effected by;
Aluminium Oxide Coating
Thermal Conductivity
Thermal Expansion Coefficient
Melting Characteristics
In normal circumstances, aluminium is welded using TIG and not MIG welding, since it is far more difficult to weld aluminium with the latter. On the other hand, aluminium being a soft metal, requires that the TIG welder has to utilise AC current not just DC current (this is because intermediate welds are required to bond aluminium) and therefore modifications have to be made on the TIG welding equipment, making it more costly then ever. However though it is considered difficult (especially for an inexperienced worker), the majority of such welding is carried out utilising MIG welding with possible reasonable results, if the following procedures are noted:
Essential features to weld aluminium using MIG welding:
Base-metal preparation:
aluminium oxide and hydrocarbon contamination must be removed
Aluminium oxide melts at 3,700 F while the base-material aluminium underneath will melt at 1,200 F.
Preheating:
Preheating the aluminium work piece can help avoid weld cracking
Preheating temperature should not exceed 230 F
The push technique:
pushing the gun away from the weld pool rather than pulling it, will result in better cleaning action
Travel speed:
Aluminium welds need to be “hot and fast”
High thermal conductivity of aluminium implies the need for higher amps and voltage settings and higher weld-travel speeds
If travel speed is too slow, the weld risks excessive burn
Shielding gas:
Argon, which gives good cleaning action and penetration profile, is the most common shielding gas used when welding aluminium
Welding wire:
Aluminium filler wire that has a melting temperature similar to the base material is ideal.
0.035-inch diameter at a low wire-feed speed – 100 to 300 in. /min
Convex-shaped welds:
Aluminium welding causes crater cracking resulting in failure. These are created due to the high rate of thermal expansion of aluminium.
Power-source selection:
Welding current in excess of 350 A, cc produces optimum results.
Wire feeder:
A constant-torque, variable-speed motor in the wire-feed is essential giving constant force and speed through the gun. A high-torque motor in the welding gun pulls the wire through and keeps wire-feed speed and arc length consistent.
Welding guns:
contact tip approximately 0.015 inch larger than the diameter of the filler metal being,
When the welding current exceeds 200 A, a water-cooled gun is used to minimize heat build-up and reduce wire-feeding difficulties.
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