Amy's Deck Flashcards
manufacturing processes for ferrous metals (forging)
Forging involves the compression of material at an elevated temperature between surfaces (dies). When the forging hammer hits the surface, the maximum force on the material occurs.
Open die forging:
Dies are in a simple geometric shape that doesn’t entirely enclose the material.
Closed die forging:
Materials are entirely enclosed by the die. Excess material is extracted into the thin space around the cavity, creating a seam line that is later removed.
Produces a stronger component as the grain flow coincides with the shape of the object.
Manufacturing processes for ferrous metals (rolling)
Rolling is an extensively used technique for the plastic deformation of metals. Metals are pressed into shape through rollers to achieve a simple standard size.
Hot Rolling:
The ingots of the required metal are passed through rollers to produce the required thickness. The metal’s temperature is above the recrystallisation temperature.
- Unstressed finished product
- Easier to do than cold rolling
- Favourable directional grain flow
= smaller grain structure - refined
Cold Rolling:
Metal’s temperature is below the recrystallisation temperature.
- Produces are more dimensionally accurate products, with elongated coarse grains
Grain flow = harder, stronger less ductile product
Manufacturing processes for ferrous metals (casting)
Casting: pouring molten metal into a mould for a specific shape.
- rapid and cost-effective.
Sand Casting:
Sand is packed around a pattern of the finished product. The mould is in two halves to allow the pattern to be removed. Once metal solidifies, sand is removed, reconstituted and ready for use again.
- High dimensional accuracy
- Useful for engine blocks and heads
- The final surface finish is poor and inaccurate.
Die Casting:
- Metal forced into a mould cavity under pressure
Manufacturing processes for ferrous metals (extrusion)
Extrusion is a process where a material is placed in a container and forced to pass through an opening at one end. The opening shape determines the shape of the extrusion.
- complexity of shapes
- good surface quality and dimensional accuracy
- expensive
Direct extrusion requires more effort and is used with more ductile materials whereas indirect is used for alloys. Both are hot working processes.
Impact extrusion is a cold working process that involves the use of hammer impact to extrude the shape.
The punch goes into the die and the material is forced from the die around the punch.
Manufacturing processes for ferrous metals (powder forming)
Powder forming is the process of creating metal powders and then blending and compacting the mixture in a die. They are then pressed into the mould to form the shape required and the shapes are then heated and sintered in an atmosphere. During sintering, the pressed powder particles fuse together, forming metallurgical bonds. After the item is sintered at a temperature to allow atoms to diffuse between grains, producing a homogeneous grain structure.
Manufacturing processes for ferrous metals (welding)
Welding is a permanent process that involves the heating of metal to be joined until melting and intermixing at the joint.
Heat at a high temperature causes a weld pool of molten material which cools to form the joint, which can be stronger than the parent metal. Pressure can also be used to produce a weld, either alongside the heat or by itself.
It can also use a shielding gas to protect the melted and filler metals from becoming contaminated or oxidised.
Types of welding
- Butt joint
A connection between the ends or edges of two parts making an angle to one another of 135-180° inclusive in the region of the joint.
- T joint
A connection between the end or edge of one part and the face of the other part, the parts making an angle to one another of more than 5 up to and including 90° in the region of the joint.
- Corner joint
A connection between the ends or edges of two parts makes an angle to one another of more than 30 but less than 135° in the joint region.
- Edge joint
A connection between the edges of two parts making an angle to one another of 0 to 30° inclusive in the region of the joint.
Changes in the Macrostructure of cast iron
The macrostructure of cast iron can change through different heat treatment processes. If cast iron is slowly cooled, it can produce a grey iron with a graphite microstructure. However, if it is rapidly cooled, it can form a white iron with a carbon microstructure. The microstructure of cast iron can also change due to the addition of alloying elements, such as silicon, magnesium, and nickel. These additions can improve the strength and corrosion resistance of the material.
Changes in the microstructure of cast iron
Cooling rate: The cooling rate during solidification plays a significant role in determining the grain size of cast iron. Faster cooling rates result in smaller grain sizes, whereas slower cooling rates result in larger grain sizes.
Alloying elements: The addition of certain alloying elements like nickel, manganese, and molybdenum can refine the grain size of cast iron.
Heat treatment: Heat treatment involves heating the cast iron to a specific temperature and holding it for a specific time to change its microstructure. Annealing or normalizing can be used to increase the grain size, whereas quenching and tempering can be used to decrease the grain size.
Changes in the Macrostructure of steel
One common change in macrostructure is the formation of grain boundaries. During the solidification process, molten steel forms into grains, which are separated by boundaries. These boundaries can become more pronounced or can change in shape and orientation due to various factors such as heat treatment, welding, or mechanical deformation.
The macrostructure of steel changes depending on the heat treatment process it undergoes, such as annealing, quenching, or tempering. During annealing, the steel is heated and then slowly cooled, which results in a coarse-grained structure. This structure makes the steel more ductile and less brittle. In contrast, quenching involves rapidly cooling the steel in a liquid bath, which results in a fine-grained structure. This structure makes the steel harder and more brittle. Tempering involves reheating the steel after quenching to obtain a balance between hardness and ductility. The heating and cooling processes also affect the crystal structure of the steel, with different crystal structures having different mechanical properties. and the formation of martensitic microstructures due to rapid cooling rates
Changes in the Microstructure of Steel
Heat Treatment: Depending on the temperature and time of exposure to heat, the grain size of steel can be altered. For instance, heating steel to a high temperature and then quenching rapidly will result in a fine-grained structure, whereas slow cooling leads to a coarse-grained structure.
Cold Working: Cold working results in the formation of smaller grains and can lead to an increase in strength and hardness.
Welding: During welding, the heat input causes the structure of the steel to change, leading to a zone of altered microstructure around the weld. The grain size in this zone is typically smaller than in the base material.
changes in properties of ferrous metals
- Corrosion Resistance: Ferrous metals have poor corrosion resistance, meaning they are prone to rust and degradation when exposed to moisture and air. However, by adding alloying elements such as chromium, nickel, and molybdenum, the corrosion resistance can be greatly improved.
- Strength: Ferrous metals are typically strong and durable, making them suitable for use in heavy-duty applications such as construction, automotive, and industrial machinery. With the addition of more carbon or other strengthening elements, their strength can be further improved.
- Ductility: Ductility is the ability of a material to be deformed without breaking. Ferrous metals are generally ductile, allowing them to be shaped and formed into various shapes. However, high-carbon steels can become brittle and lose their ductility.
- Magnetic Properties: Ferrous metals are typically magnetic, which makes them useful in various industries. Magnetic materials are used in electric motors, generators, and other electrical devices.
- Melting Point: The melting point of ferrous metals varies depending on the type of metal and alloying elements in the mix. Generally, ferrous metals have high melting points which makes them suitable for use in high-temperature applications.
- Weldability: Ferrous metals are typically easy to weld and join together, making them ideal for use in construction and manufacturing. However, the quality of the weld can depend on the type of metal and welding process used.
