Fundamentals

To understand the internal processes of steel during heat treatment, knowledge about the iron-carbon equilibrium diagram and the related terms is essential. They are briefly explained on this page and are required to understand the sections about grain structures and heat treatment. They are also summarized in the glossary for reference.

Grain Structure

The grain structure is substantially responsible for the properties of steel and is influenced by alloying elements as well as by the heat treatment. Steel has a crystalline structure, an assembly of many crystals, the grains. Each grain consists of a crystal lattice, built up of unit cells. The size of the grains is in the range of microns, the unit cell being built from a few iron- and substitution-atoms has a size in the range of Angströms.

Iron-atoms can buildt a unit cell by two different arrangements: the face-centered cubic gamma-iron and the body-centered cubic alpha-iron. The face-centered cubic unit-cell of the gamma-iron is larger than the unit-cell of the alpha-iron and can therefore store more carbon on interstices.

The arrangement of the iron atoms of the unit-cell depends on the temperature as well as on the alloying elements. Pure iron at room temperature arranges as alpha-iron, the so called ferrite. At 1663F it transforms into austenite, also called gamma-iron. At a temperature of 2554F it changes into body-centered cubic delta-iron, which melts at
2782F. While forging Damascene steel, temperatures of above gamma-iron are of no interest. The transformation temperatures are called A1, A2, etc. They show a hysteresis between heating and cooling, this being indicated by the indices
c (chauffage=heating) respectively r (refroidissement=cooling).

Carbon

Carbon is the most important alloying element, making steel hardenable. The carbon atom is just a little bigger than the accommodating interstices. Due to the size of the unit cell, alpha-iron can dissolve 0.018% carbon and gamma-iron up to 1.7%. If there is more carbon present than can be dissolved, iron carbide (Fe3C), also known as cementite, is segregated.

The transformation temperatures and grain structures being in plain carbon can be read from the iron-carbon equilibrium diagram. The displayed structure types are based on slow cooling rates, necessary for the equilibrium.

Perlite is a laminated composite of ferrite and cementite. At 0.83%C this phase is in eutectoidic equilibrium. Another equilibrium (eutectic) is found at 4.3%C, the smelt solidifies directly to ledeburite at this carbon content, a mix of primary cementite and carbon-saturated austenite. Cementite formed from liquidus is called primary cementite, segregated from solidus it is called secondary cementite.

When fast-cooling gamma-iron, the carbon does not have time to build Fe3C, be it in form of perlite or globular or grain boundary cementite. The resulting grain structure is a hardening structure, martensite or bainite.

forge-welding

Forge-welding steel is based on diffusion. Two clean and oxide free surfaces are brought into contact at high temperature and pressure. Atoms from the one surface penetrate the other surface, forming a quasi homogenous joint. Often small impurities, mostly oxides, remain in the weld. Further investigations on their influence in pattern-welded steel have to be done.

Since heated steel will form an oxide layer when exposed to air, a flux has to be used during welding. Formerly silica sand was used. Nowadays, mostly borax –often with additives– is used. The flux is solid at room temperature but melts on the hot steel, forming a glass like coating, keeping the oxygen away from the steel. Borax dissolves oxides, removing thinner oxidation, therefore being more effective than silica sand. While forge-welding, the flux is driven out of the weld, rinsing particles out of the weld.

Heating steel to welding temperature requires some experience. Welding at a too low temperature will cause a weak bond, often cleaving later on. When heating too high, the risk of burning the steel and immense grain-growth will be the result. The process of oxygen invading the steel and forming crack of oxide is called burning the steel. The welding temperature depends on how the steel is alloyed, making it difficult or impossible to forge-weld certain combinations and types of steel.

If a steel billet is at welding temperature, the pressure necessary for diffusion is produced by fast blows of the hammer. Starting on one end so that the flux is driven out of the weld, the whole billet is forge-welded. Often more than one heating is necessary to complete the weld.



© 2005 G.v.Tardy