Metal Forming Operations

Metal Forming

• Metal forming operations are used to produce a new shape or improve the properties of a metal.
• Shaping in the solid state can be divided into non-cutting shaping (e.g., forging, rolling, pressing) and cutting shaping (e.g., machining operations).

Plastic Deformation

• Plastic deformation occurs when a metal is deformed beyond its elastic range.
• The main objectives of metal working processes are to provide the desired shape and size, improve mechanical properties, and reduce internal voids or cavities.
• Metals are commonly worked by plastic deformation, which can be achieved by applying mechanical force or heating the metal and then applying a small force.

Strain Hardening

• Strain hardening (also known as work-hardening or cold-working) is the process of making a metal harder and stronger through plastic deformation.
• Dislocations move and interact with each other, resulting in a decrease in their mobility and a strengthening of the material.
• Strain hardening can be demonstrated by bending a wire or paper clip repeatedly.

Effects of Elevated Temperature on Strain Hardened Materials

• When strain hardened materials are exposed to elevated temperatures, the strengthening that resulted from the plastic deformation can be lost.
• Heat treatment can be used to remove the effects of strain hardening, which involves three stages: recovery, recrystallization, and grain growth.

Recovery

• Recovery occurs when a stain hardened material is held at an elevated temperature, allowing atoms to move to unstrained positions and relieve internal strain energy.
• This results in a reduction in internal residual stresses and a movement of dislocations to lower-energy positions.

Recrystallization

• Recrystallization occurs at a higher temperature, where new, strain-free grains nucleate and grow inside the old distorted grains and at the grain boundaries.
• This results in a return to the original weaker and more ductile states of the material.

Grain Growth

• Grain growth occurs when a specimen is left at a high temperature beyond the time needed for complete recrystallization.
• This results in a decrease in the strength and toughness of the material.

Embossing

• Embossing is a process that involves stretching a metal sheet to shape under pressure using a punch and a die.
• This process gives a stiffening effect to the metal and can produce ornamental wares.

Coining

• Coining is a cold working operation that involves producing shallow configurations on a metal surface using a die and a punch.
• This process is used for producing important articles such as medals, coins, and stickers.

Bending

• Bending is a common process in the manufacturing industry that involves the plastic deformation of a metal sheet over an axis.
• This process changes the shape of the workpiece while maintaining its volume.

Hot Working

• Hot working is a mechanical working process that occurs above the recrystallization temperature of the metal.
• This process is used to produce large deformations and can result in a refined grain structure and improved mechanical properties.

Effects of Hot Working on Mechanical Properties of Metals

• Hot working can produce a refined grain structure and improve mechanical properties.
• It can also reduce the stresses required to produce deformations and increase the possible amount of deformation.
• Hot working can result in a uniform distribution of impurities and improve the surface finish of the metal.

Merits of Hot Working

• Hot working can be used to achieve large deformations without strain hardening.
• It can improve the grain structure and mechanical properties of the metal.
• It can also reduce the stresses required to produce deformations.

Demerits of Hot Working

• Hot working can result in a poor surface finish due to oxidation and scaling.
• It can also lead to a loss of carbon from the surface of the steel, resulting in a loss of strength.

Cold Working

• Cold working is a mechanical working process that occurs below the recrystallization temperature of the metal.
• It is used to obtain better surface finish, increase mechanical properties, and produce thinner materials.

Purpose of Cold Working

• Cold working is used to obtain better surface finish and increase mechanical properties.
• It is also used to produce thinner materials and improve the dimensional accuracy of parts.

Limitations of Cold Working

• Cold working can result in a decrease in ductility and an increase in strain hardening.
• It requires higher forces and more powerful equipment than hot working.
• It can also result in a distortion of the grain structure and the formation of internal stresses.

Advantages of Cold Working

• Cold working can produce a smooth surface finish and accurate dimensions.
• It can increase the strength and hardness of the material.
• It can also result in better dimensional accuracy and a reduction in distortion.

Disadvantages of Cold Working

• Cold working can result in a distortion of the grain structure and the formation of internal stresses.
• It can also limit the amount of deformation that can be given to the material.
• It requires higher forces and more powerful equipment than hot working.

Rolling

• Rolling is a metal forming operation that involves the plastic deformation of a material caused by compressive force applied through a set of rolls.
• It is used to produce a wide range of products, including blooms, billets, plates, and sheets.
• Rolling can be done at high or low temperatures, depending on the desired outcome.### Rolling Process
• Hot rolling produces residual stress-free products, but scaling is a major problem, affecting dimensional accuracy.
• Cold rolling is gaining importance for sheets, foils, etc., due to high accuracy and lack of oxide scaling, and also strengthens the product through work hardening.

Rolling Mills

• Rolling is a major manufacturing process for sheets and other cross-sections of large length, such as I beams, railroads, etc.
• It involves compressing a metal workpiece between a set of rolls, reducing its cross-section area and increasing its length.
• This process offers high production rates, surface finish, and grain structure, but has high setup costs.

Terminology

• Ingot: a cast metal with porosity and blowholes, soaked at 1200°C and rolled into blooms, billets, or slabs.
• Blooms: the first rolled product, made by rolling ingot at high temperature, with a cross-section area ≥ 230 cm².
• Slab: made by hot rolling of ingot, with a cross-section area ≥ 100 cm² and width ≥ 3 times its thickness.
• Billets: made by hot rolling of blooms, with a cross-section area ≥ 40 cm², used to roll into pipes, bars, wire, etc.
• Plate: a product of further rolling of slab, with a thickness > 6 mm.
• Sheet: a product with a thickness < 6 mm and width > 60 cm.
• Strip: a product with a thickness < 6 mm and width < 60 cm.

Types of Rolling Mills

• Two high reversing mill: rolls rotate in one direction and then in the other, allowing the metal to pass back and forth through the rolls several times.
• Two high non-reversing mill: rolls revolve continuously in one direction, requiring smaller and less costly motive power.
• Three high rolling mill: consists of three parallel rolls, with adjacent rolls rotating in opposite directions, reducing thickness at each pass.
• Four high rolling mill: used for hot rolling of armor and other plates, as well as cold rolling of plates, sheets, and strips.
• Tandem rolling mills: used for rolling multiple stands in a continuous process.