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

GALVANNEALED

Galvannealed or Galvanneal, Is the result from the combined process of galvanizing and annealing the steel. Galvanneal does not flake off its galvanized coatin when formed, stamped and bent. The very fine matte finish acts like a printer and easily adheres to paint and is very rust proof, only white to dark grey marks appear if it comes in contact with water. Galvanneal sheets offers good paintability, weldability, corrosion resisttance, and formability. It is extensively used in the automotive, signage, electric equipment, and other industries requiring good paintability and long reliable service life.

Galvannealed sheet is carbon steel sheet coated with zinc on both sides by the continous hot-dip process described in production methods. Immediately as the strip exits the coating bath, the molten zinc coating is subjected to an in-line heat treatment that converts the entire coating to a zinc-iron alloy.

ANNEALING

Annealing, in metallurgy and material science, is a heat treatment wherein the microstructure of a material is altered, causing changes in its poperties such as strength and hardness. It is a process that produces equilibrium conditions by heating and maintaining at a suitable temperature and then cooling very slowly. It is used to induce softness, relieve internal stresses, refine the structure and improve the cold working properties. There are three stages in the annealing process, with the first being the recovery phase, which results in the softening of the metal through removal of crystal defects and the internal stresses which they cause. The second phase is recrystallization, where new grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties.

DUTY CALCULATION

GALVANIZATION

In current use, it typically means hot dip galvanizing, a metallurgical process that is used to coat steel or iron with zinc. This is done to prevent corrosion of the ferrous item; while it is accompalished by non-electrochemical means, it serves as electrochemical purpose.

HOT DIP GALVANIZING is a form of galvanization. It is the process of coating iron or steel with a thin zinc layer, by passing the steel through a molten bath of zinc at a temperature of around 460 °C. When exposed to the atmosphere, pure zinc reacts with oxygen to form zinc oxide, which further reacts with carbon dioxide to form zinc carbonate, a dull grey, fairly strong material that stops further corrosion in many circumstances. Galvanized steel is widely used in applications where rust resistance is needed.

TOP 7 STEEL PRODUCERS

1) 77.5 Mton Arcelor-Mittal (Global)

2) 31.3 Mton Baosteel Group (China)
3) 31.1 Mton POSCO (South Korea)

4) 26.5 Mton Nippon Steel (Japan)
5) 25.8 Mton JFE (Japan)

6) 20.5 Mton Jiangsu Shagang (China)

7) 20.5 Mton Tata Steel-Corus (India)

COLD ROLLING PROCESS

Cold rolling is a metalurgical process in which metal is passed through a pair of rollers at a temperature below its recrystallization temperature. This process hardens the metal, by compressing and stretching the metal crystals. After the rolling process, the metal is annealed by heating it above the recrystallization temperature after every few rollings, to prevent it from becoming brittle and cracking.

HOT ROLLING PROCESS

Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties. This is achieved by heating a component or material to its upper critical tempearture and then applying controlled load which forms the material to a desired specification or size.

STEEL MAKING PROCESS

Steel is an alloy of iron and carbon. It is produced in two-stage process.

First, iron ore is reduced or smelted with coke and limestone in a blast furnace, producing molten iron which is either cast into pig iron or carried to the next stage as molten iron.

In the second stage, known as steel making, impurities such as sulpur, phosphorus, and excess carbon are removed and alloying elements such as manganese, nickel, chromium and vanadium added to produce the exact steel required.

STEEL ALLOYING ELEMENTS AND RESULTANT PROPERTIES

Other materials are often added to the iron-caron mixture to tailor the resulting properties.

NICKEL AND MANGANESE
When added in steel, it add to its tensile strength and make austenite more chemically stable.

CHROMIUM
It increases the hardness and melting temperature.

VANADIUM
It also increases the hardness while reducing the effects of metal fatigue.

CHROMIUM AND NICKEL
Lare amount of chromium and nickel (often 18% and 8% respectively) are added to stainless steel so that a hard oxide forms on the metal surface to inibit corrosion.

TUNGSTEN
Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel.

On the other hand Sulphur, nitrogen and phosphorusmake steel more brittle , so these comonly found elements must be removed from the ore during processing.

STEEL

Steel is an alloy element primarily made of iron, with caron conten of .02% to 1.7% by weight.

Carbon and other agent act as hardening agent, preventing the dislocations in iron atom crystal lattice fron sliding past one another.

Varying the amount of alloying elements and their distribution in the steel controls qualities such as the hardness, elasticity, ductility and tensile strength of resultant steel.

Steel with higher carbon content can be made harder and stronger than iron, but is also more brittle. The maximum solubility of carbon in iron is 1.7% by wieght at 1130 °C