CHAPTER 3 : METAL WORKS, CASTING PROCESS AND HEAT TREATMENT ON STEEL

COLD WORK

INTRODUCTION OF COLD WORK

• Definition : Cold work is a process of plastic deformation which carried out below recrystallization temperature at room temperature.

• Grains become deformed and elongated.

COLD WORK PROCESS

ROLLING

WIRE AND TUBE

DRAWING

ROLLING• Definition :

- Process of reducing the thickness (or changing the cross-section) of a long workpiece by compressive forces applied through a set of rolls and carried out at room temperature.

• Produce : - Plates, Sheets and Foils.

• Advantages :- Better surface finish.- Good dimensional accuracy.- Increase tensile strength and toughness.

• Diagram :

WIRE AND TUBE DRAWING• Definition :

- Process of reducing or changing the shape/cross section of a round rod or wire by pulling it through a die.

• Produce : - Wire, rod and tube.

• Advantages :- Good surface quality.- Good dimensional accuracy.

• Diagram :

ADVANTAGES AND DISADVANTAGES OF COLD WORK

1. Better surface finish.

2. Good dimensional accuracy

Advantages

1. High cost

2. Has to be annealed to remove brittleness.

Disadvantages

HOT WORK

INTRODUCTION OF HOT WORK

• Definition : Hot work is a process of plastic deformation which carried out above recrystallization temperature.

• Grain recrystallization.

GRAIN RECRYSTALLIZATION• Process by which deformed grains are replaced by a new set of

undeformed grains that nucleate and grow until the original grains have been entirely consumed.

TYPES OF HOT WORK

FORGING

HOT ROLLING

EXTRUSION

FORGING• Definition :

- Process which the workpiece is shaped by compressive forces applied through various dies and tools.

• Produce :Discrete parts

• Advantages :- Good strength and toughness.

• Diagram :

HOT ROLLING• Definition :

- Process of reducing the thickness (or changing the cross-section) of a long workpiece by compressive forces applied through a set of rolls and carried out above recrystallization temperature.

• Produce : - Wire, rod and tube.

• Advantages :

• Diagram :

EXTRUSION• Definition :

- Process when hot billet is forced through an opening die by hydraulic force.

• Types :- Direct/Forward : billet forced by ram through die.- Indirect/Inverted : die moves toward the billet.

• Produce :- Cylindrical bars or hollow tubes.

• Advantages : - Able to produce complex shape.- Have high dimensional accuracy.- Have a good surface finish.

• Diagram :

ADVANTAGES AND DISADVANTAGES OF HOT WORK

Advantages

Porosity of the metal is minimised

Increase in ductility

Disadvantages

Poor surface finish

Less precise tolerances

CASTING

INTRODUCTION• Definition of Casting :

Is a process of pouring molten metal into a mould, allowing it to cool and removing the metal from the mould after solidify.

CASTING PROCESS

1. SAND CASTING 2. LOST

WAX/INVESTMENT CASTING

3. PRESSURE DIE CASTING

SAND CASTING• Is a method of casting metals in sand mould.• Consist of :

1. Filling the resulting cavity with molten

metal.

2. Allowing the molten metal to

solidify.

3. Breaking away the sand mould.

4. Removing the casting.

SAND CASTING MOULD

• Flask :- A mould frame which consists of two parts: Cope (the upper part) Drag (the lower part).

• Gating System : - Channels which a molten metal flows through to the mould cavity.- Consists of :i) Pouring cup – which the molten metal is poured.

ii) Sprue – which the molten metal flows downward.

iii) Runner – a channel which the melt is supplied to the gates.

iv) Gates – inlets to the mould cavity.

v) Riser – which supply additional metal to the casting as it shrinks during solidification.

• Vent :Remove air within the mould cavity and gases formed when a molten metal contacts the mould surface.

• Cores :A separate insert made from sand which are placed in the mould to form the interior surface of casting.

SANDS FACTORSProperties FactorsCasting surface finish

Fine and round grain sand which can be closely packed

Gas permeability Fine grain sands allow gases and steam evolved during casting to escape easily.

Mould strength Fine grain sands.Collapsibility Allow for the casting to shrink while

cooling in order to avoid defects in the casting.

TYPES OF SAND MOULD• Sand moulds are characterized by ;

- The types of sand that comprise- The methods used to produce

• 3 Types of sand:Green sand - The green sand is prepared from silica sand, water and a certain quantity of a clay (bentonite, kaolin). Resin bonded sand - a mixture of silica sand with a polymeric resin as the bonding agent. Sodium silicate bonded sand - a mixture of silica sand with 3-4% of sodium silicate.

PROCESS OF SAND CASTING• Mould-making

- A sand mould is formed by packing sand into each half of the mould. - The sand is packed around the pattern, which is a replica of the external shape of the casting. - When the pattern is removed, the cavity that will form the casting remains.

• Clamping - The surface of the mould cavity is first lubricated to facilitate the removal of the casting. - Then, the cores are positioned and the mould halves are closed and securely clamped together.

• Pouring - The molten metal is ladled from its holding container in the furnace and poured into the mould. - Enough molten metal must be poured to fill the entire cavity and all channels in the mould.

• Cooling - The molten metal that is poured into the mould will begin to cool and solidify once it enters the cavity. - When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed.

• Removal - After the predetermined solidification time has passed, the sand mould can simply be broken, and the casting removed.- This step, sometimes called shakeout, is typically performed by a vibrating machine that shakes the sand and casting out of the flask.

• Trimming - During cooling, the material from the channels in the mould solidifies attached to the part. - This excess material must be trimmed from the casting either manually via cutting or sawing, or using a trimming press.

ADVANTAGES & DISADVANTAGES OF SAND CASTING

ADVANTAGES

Low cost of mould materials and equipment

Large casting dimensions may be obtained.

Wide variety of metals and alloys may be cast.

DISADVANTAGES

Rough surface.

Poor dimensional accuracy.

Limited wall thickness: not higher than 2.5-5

mm.

LOST /WAX INVESTMENT CASTING

• Is a casting process where molten metal is poured into an expendable ceramic mould.

PROCESS OF INVESTMENT CASTING

• Pattern creation - A central wax gating system (sprue, runners, and risers), form a tree-like assembly. - The gating system forms the channels through which the molten metal will flow to the mould cavity.

• Mould creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. - This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. - The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell.

• Pouring - The mould is preheated in a furnace and the molten metal is poured from a ladle into the gating system of the mould, filling the mould cavity.

• Cooling - After the mould has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. - Cooling time depends on the thickness of the part, thickness of the mould, and the material used.

• Casting removal - After the molten metal has cooled, the mould can be broken and the casting removed. - Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen).

• Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. - Heat treatment is also sometimes used to harden the final part.

ADVANTAGES & DISADVANTAGES OF INVESTMENT CASTING

ADVANTAGES

- Can form complex shapes and fine details.- Many material options.- High strength parts.- Very good surface finish and accuracy.

DISADVANTAGES

- Time-consuming process.- High labour cost.- High tooling cost.- Long lead time.

PRESSURE DIE CASTING• The molten metal is forced into the die cavity at high pressure.

PROCESS OF PRESSURE DIE CASTING

• Clamping - The first step is the preparation and clamping of the two halves of the die. - Each die half is first cleaned from the previous injection and then lubricated to facilitate the ejection of the next part. - After lubrication, the two die halves, which are attached inside the die casting machine, are closed and securely clamped together.

• Injection - The molten metal, which is maintained at a set temperature in the furnace, is next transferred into a chamber where it can be injected into the die. - The method of transferring the molten metal is dependent upon the type of die casting machine, whether a hot chamber or cold chamber machine is being used.- Once transferred, the molten metal is injected at high pressures into the die.

• Cooling - The molten metal that is injected into the die will begin to cool and solidify once it enters the die cavity. - When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed.

• Ejection - After the predetermined cooling time has passed, the die halves can be opened and an ejection mechanism can push the casting out of the die cavity. - The ejection mechanism must apply some force to eject the part because during cooling the part shrinks and adheres to the die.

• Trimming - During cooling, the material in the channels of the die will solidify attached to the casting. - This excess material, along with any flash that has occurred, must be trimmed from the casting either manually via cutting or sawing, or using a trimming press.

ADVANTAGES & DISADVANTAGES OF PRESSURE DIE CASTING

ADVANTAGES

- Can produce large parts.- Can form complex shapes.- Very good surface finish and accuracy.- High production rate- Low labor cost

DISADVANTAGES

- Trimming is required.- High tooling and equipment cost.- Long lead time