HEAT TREATMENT

The purpose of heat treating plain-carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, and impact resistance.

Types of heat treatments

Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours.The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel.

Full annealing: Plain-carbon steel is heated to approximately 40 °C above Ac3 or Ac1 for 1 hour.The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. Fully annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.

Process annealing: A process used to relieve stress in a cold-worked plain-carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C.

Normalizing: Plain-carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this assures the steel completely transforms to austenite. The steel is then air cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.

Quenching: Plain-carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature.This crystalline structure has a very high amount of internal stress. Due to these internal stress quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three (lower carbon content) to four (high carbon content) times harder than normalized steel.

CARBON STEEL

Carbon steel, is a metal alloy, a combination of two elements, iron and carbon, where other elements are present in quantities too small to affect the properties. The only other alloying elements allowed in plain-carbon steel are manganese (1.65% max), silicon (0.60% max), and copper (0.60% max).

Steel with a low carbon content has the same properties as iron, soft but easily formed. As carbon content rises the metal becomes harder and stronger but less ductile and more difficult to weld. Higher carbon content lowers steel's melting point and its temperature resistance in general.

Types of carbon steel

Mild (low carbon) steel: approximately 0.05–0.29% carbon content (e.g. AISI 1018 steel). Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased through carburizing.

Medium carbon steel: approximately 0.30–0.59% carbon content (e.g. AISI 1040 steel). Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.

High carbon steel: approximately 0.6–0.99% carbon content. Very strong, used for springs and high-strength wires.

Ultra-high carbon steel: approximately 1.0–2.0% carbon content. Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 1.2% carbon content are made using powder metallurgy and usually fall in the category of high alloy carbon steels.

Stress is the internal distribution of force per unit area that balances and reacts to external loads applied to a body.

Strain is the geometrical expression of deformation caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape. If strain is equal over all parts of a body, it is referred to as homogeneous strain; otherwise, it is inhomogeneous strain.

TENSILE STRENGTH

The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure.

There are three typical definitions of tensile strength:

Yield Strength/Yield Point: The stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently. Knowledge of the yield point is vital when designing a component since it generally represents an upper limit to the load that can be applied
Ultimate strength: The maximum stress a material can withstand.
Breaking strength: The stress coordinate on the stress-strain curve at the point of rupture.

Necking: After a metal has been loaded to its yield strength it begins to "neck" as the cross-sectional area of the specimen decreases due to plastic flow. When necking becomes substantial, it may cause a reversal of the engineering stress-strain curve, where decreasing stress correlates to increasing strain because of geometric effects.This is because the engineering stress and engineering strain are calculated assuming the original cross-sectional area before necking.

STRESS Vs STRAIN GRAPH

Stress vs. Strain curve for structural steel

1 - Ultimate Strength
2 - Yield Strength
3 - Rupture
4 - Strain hardening region
5 - Necking region