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Treating Steel

foundry-molten-metal-poured-from-ladle-for-castingTreating steel is the controlled heating and cooling process used to structurally change and alter the physical and mechanical properties of the material. This process will also occur when steel is being welded, or due to manufacturing processes that either heat or cool metals.

The amount of heat is measured by the temperature of the treated steel. Proper heating requires precise control of temperature and uses heat or cold — usually to extremes in either case — in order to achieve a desired result, which is the hardening or softening of a material.

Steels are an iron and carbon alloy, and while all metals and alloys respond in some way to heat treatment, steel is particularly suitable for the process. It is also the most commonly used material commercially than any other material in the industry.

Very few mechanical properties of metal are affected by heat treatment, including the properties that give steel its tensile and impact strength, ductility, and hardness.

Tensile strength refers to the amount of tension a material can endure before break or fail occurs.

Ductile strength tests the material’s ability to deform under tension — in steel’s case, to stretch as thin as wire.

For materials to be considered Malleable, they need to possess the ability to deform under compressed stress — to thin out by hammering or rolling.

Ductility and malleability are different because depending on the material being tested, results can still vary. One material may have a high ductile and malleable stress resistance (gold) while another will achieve low ductile yet high malleable endurance (lead).

Plasticity could be used to describe both mechanical properties — the extent to which a solid material can be deformed before fracturing occurs.

These processes are all dependent on temperature and pressure.

A metal’s ability to withstand mechanical shock without fracturing is known as impact strength.

Hardness is the metal’s ability to resist penetration, deformation, wear, and surface abrasion.

Ferrite is pure iron with a number of inherent properties at room temperature — large grain size, low hardness, good ductility, and easily machinable. It exists in crystal form and at low temperatures. The crystals bind to resemble lattice. When heat is applied to iron, oxygen in the air will react with the iron’s shiny surface and form Iron Oxide. As the temperature increases, the iron and oxygen produce scale.

Austenite occurs once the steel reaches close to 1,350xF. The crystal structure has changed.

Metallics are micro structures of small crystals (grains) sometimes referred to as crystallites. When the molten metal solidifies, crystal structures arrange in patterns and may be face-centered or body-centered cubic structures.

molten-steelThe cool-down process (annealing) is necessary for steel to perform. The cool-down rate for steel determines the micro-structure. Slow or fast Cool-down depends on final output of the material.

Annealing offers a variety of processes depending on the desired end result. It affects the electromagnetic properties of steel and is done to refine grain size, improve strength, and to remove residual stresses.

Normalization is the process of normalizing grain size and typically occurs after drawing, extrusion, forging, or heavy bending operations. Grain growth occurs when steel is heated to elevated temperatures, making it long, coarse, and inconsistent. The process of normalizing will help to improve machining characteristics, reduce residual stresses when rolling and forging, prevent banding, and even out steel during a hardening process.

Diffusion Hardening surrounds a metal part with the element to be diffused into it in either the solid, liquid, or gas phase. For diffusion to occur, the diffusing element concentration surrounding the part needs to be higher than the element concentration inside the part. The metal and surrounding element are heated to a temperature high enough for diffusion to occur. For pack carburizing, 900 °C is necessary. The part then needs to sit for 12 to 72 hours.

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