Forge welding

Forge welding is a solid-state welding process^[1] that joins two pieces of metal by heating them to a high temperature and then hammering them together.^[2] The process is one of the simplest methods of joining metals and has been used since ancient times. Forge welding is versatile, being able to join a host of similar and dissimilar metals. With the invention of electrical and gas welding methods during the Industrial Revolution, forge welding has been largely replaced.

History

Sponge iron used to forge a Japanese katana.

The history of joining metals goes back to the Bronze age, where bronzes of different hardness were often joined by casting one into the other, such as the blade of a sword into a handle or the tang of an arrowhead into the tip. Brazing and soldering were also common during the Bronze age.^[3] The act of welding (joining two solid parts through diffusion) began with iron. The first welding process was forge welding, which started when humans learned to smelt iron from iron ore; most likely in Anatolia (Turkey) around 1800 BC. Ancient people could not create temperatures high enough to melt iron fully, so the bloomery process that was used for smelting iron produced a lump (bloom) of iron grains sintered together, small amounts of steel, slag, and other impurities, referred to as sponge iron because of its porosity. After smelting the sponge iron needed to be welded, or "wrought," into a solid block (billet) to squeeze out air pockets and excess slag. Many items made of wrought iron have been found by archeologists, that show evidence of forge welding, which date from before 1000 BC. Because iron was typically made in small amounts, any large object, such as the Delhi Pillar, needed to be forged welded out of smaller billets.^[4][5]

Forge welding grew from a trial-and-error method, becoming more refined over the centuries.^[6] Due to the poor quality of ancient metals, it was commonly employed in making composite steels, by joining high-carbon steels, that would resist deformation but break easily, with low-carbon steels, which resist fracture but bend too easily, creating an object with greater toughness and strength than the could be produced with a single alloy. This method of pattern welding first appeared around 700 BC, and was primarily used for making weapons such as swords, with the most widely known examples being from Damascus, Japanese, and Merovingian swords.^[7][8]

Materials

Many metals can be forge welded, with the most common being both high and low-carbon steels. Iron and even some hypoeutectic cast-irons can be forge welded. Some aluminum alloys can also be forge welded.^[9] Metals such as copper, bronze and brass do not forge weld readily. Although it is possible to forge weld copper-based alloys, it is often with great difficulty due to copper's tendency to absorb oxygen during the heating.^[10] Copper and its alloys are usually better joined with cold welding, explosion welding, or other pressure-welding techniques. With iron or steel, the presence of even small amounts of copper will severely reduce the alloy's ability to forge weld.^[11][12]

Forge welding between similar materials is caused by solid-state diffusion. This results in a weld that consists of only the welded materials without any fillers or bridging materials. Forge welding between dissimilar materials is caused by the formation of a lower melting temperature eutectic between the materials. Due to this the weld is often stronger than the individual metals.

Processes

A mechanized trip hammer.

The most well-known and oldest forge-welding process is the manual-hammering method. Manual hammering is done by heating the metal to the proper temperature, overlapping the weld surfaces, and then striking the joint repeatedly with a hand-held hammer. The weld surfaces will usually be formed for the proper joint, and then struck with a hammer to join them. The joint is often formed to allow space for the flux to flow out, by beveling or rounding the surfaces slightly, and hammered in a successively outward fashion to squeeze the flux out. The hammer blows are typically not as hard as those used for shaping, preventing the flux from being blasted out of the joint at the first blow.

When mechanical hammers were developed, forge welding could be accomplished by heating the metal, and then placing it between the mechanized hammer and the anvil. Originally powered by waterwheels, modern mechanical-hammers can also be operated by compressed air, electricity, steam, gas engines, and many other ways. Another method is forge welding with a die, whereas the pieces of metal are heated and then forced into a die which both provides the pressure for the weld and keeps the joint at the finished shape. Roll welding is another forge welding process, where the heated metals are overlapped and passed through rollers at high pressures to create the weld.^[13][14]

Temperature

The temperature required to forge weld is typically 50 to 90 percent of the melting temperature. Iron can be welded when it surpasses the critical temperature (the A₄ temperature) where its allotrope changes from gamma iron (face-centered cubic) to delta iron (body-centered cubic). Since the critical temperatures are affected by alloying agents like carbon, steel welds at a lower temperature-range than iron. As the carbon content in the steel increases, the welding temperature-range decreases in a linear fashion. Iron, different steels, and even cast-iron can be welded together, provided that their carbon content is close enough that the welding ranges overlap. Pure iron can be welded when nearly white hot; between 2,500 °F (1,370 °C) and 2,700 °F (1,480 °C). Steel with a carbon content of 2.0% can be welded when orangish-yellow, between 1,700 °F (930 °C) and 2,000 °F (1,090 °C). Common steel, between 0.2 and 0.8% carbon, is typically welded at a bright yellow heat.^[15]

When the steel reaches the proper temperature, it will begin to weld very readily, so a thin rod or nail heated to the same temperature will tend to stick at first contact, requiring it to be bent or twisted loose. One of the simplest ways to tell if the iron or steel is hot enough is to stick a magnet to it. When iron or steel cross the A₂ critical temperature, it begins to change into the allotrope called gamma iron. When this happens, the steel or iron becomes non-magnetic. In steel, the carbon will begin to mix with gamma iron at the A₃ temperature, forming a solid solution called austenite. When it crosses the A₄ critical temperature, it changes into delta iron, which is magnetic. Therefore, a blacksmith can tell when the welding temperature is reached by placing a magnet in contact with the metal. When red or orange-hot, a magnet will not stick to the metal, but when the welding temperature is crossed, the magnet will again stick to it. The metal may take on a glossy or wet appearance at the welding temperature. Care must be taken to avoid overheating the metal to the point that it gives off sparks from rapid oxidation (burning), or else the weld will be poor and brittle.^[16]

Decarburization

When steel is heated to an austenizing temperature, the carbon begins to diffuse through the iron. The higher the temperature; the greater the rate of diffusion. At such high temperatures, carbon will readily combine with oxygen to form carbon dioxide, so the carbon can easily diffuse out of the steel and into the surrounding air. By the end of a blacksmithing job, the steel will be of a lower carbon content than it was prior to heating. Therefore, most blacksmithing operations are done as quickly as possible to prevent too much decarburization, preventing the steel from becoming too soft.

To produce the right amount of hardness in the finished product, the smith generally begins with steel that has a carbon content that is higher than desired. In ancient times, forging often began with steel that had a carbon content much too high for normal use. Most ancient forge-welding began with hypereutectoid steel, containing a carbon content sometimes well above 1.0%. Hypereutectoid steels are typically too brittle to be useful in a finished product, but by the end of forging the steel typically had a high carbon-content ranging from 0.8% (eutectoid tool-steel) to 0.5% (hypoeutectoid spring-steel).^[17]

Applications

One of the most famous applications of forge welding involves the production of pattern-welded blades. During this process a smith repeatedly draws out a billet of steel, folds it back and welds it upon itself.

Another application was the manufacture of shotgun barrels. Metal wire was spooled onto a mandrel, and then forged into a barrel that was thin, uniform, and strong.

In some cases the forge-welded objects are acid-etched to expose the underlying pattern of metal, which is unique to each item and provides aesthetic appeal.

Flux

Forge welding requires the weld surfaces to be extremely clean, or the metal will not join properly, if at all. Oxides tend to form on the surface while impurities like phosphorus and sulfur tend to migrate to the surface. Often a flux is used to keep the welding surfaces from oxidizing, which would produce a poor quality weld, and to extract other impurities from the metal. The flux mixes with the oxides that form and lowers the melting temperature and the viscosity of the oxides. This enables the oxides to flow out of the joint when the two pieces are beaten together. A simple flux can be made from borax, sometimes with the addition of powdered iron-filings.^[18]

The oldest flux used for forge welding was fine silica sand. The iron or steel would be heated in a reducing environment within the coals of the forge. Devoid of oxygen, the metal forms a layer of iron-oxide called wustite on its surface. When the metal is hot enough, but below the welding temperature, the smith sprinkles some sand onto the metal, which reacts with the wustite to form fayalite, which melts just below the welding temperature. This produced a very effective flux which helped to make a strong weld.^[19]

Early examples of flux used different combinations and various amounts of iron fillings, borax, sal ammoniac, balsam of copaiba, cyanide of potash, and soda phosphate. The 1920 edition of Scientific American book of facts and formulae indicates a frequently offered trade secret as using copperas, saltpeter, common salt, black oxide of manganese, prussiate of potash, and "nice welding sand" (silicate).

See also

• Pattern welding
• Friction Welding
• Friction stud welding

References

1. Shirzadi, Amir, Diffusion Bonding, archived from the original on 2010-02-12, retrieved 2010-02-12.
2. Nauman, Dan (2004), "Forge welding" (PDF), Hammer's blow: 10–15.
3. Introduction to Welding and Brazing by R. L. Apps, D. R. Milner -- Pergamon Press 1994 Page x1
4. Welding by Richard Lofting -- Crowood Press 2013 Page 1
5. History of Humanity: From the seventh century B.C. to the seventh century A.D. by Sigfried J. de Laet, Joachim Herrmann -- Routledge 1996 Page 36--37
6. Introduction to Welding and Brazing by R. L. Apps, D. R. Milner -- Pergamon Press 1994 Page xi
7. The History of Hardening by Hans Berns -- Harterei Gerster AG 2013 Page 48--49
8. A History of Metallography by Cyril Stanley Smith -- MIT Press 1960 Page 3--5
9. Principles of Welding: Processes, Physics, Chemistry, and Metallurgy by Robert W. Messler, Jr. -- Wiley VCH 2008 Page 102
10. CDA Publication Issue 12 by the Copper Development Association -- CDA 1951 Page 40
11. Alloying: Understanding the Basics by Joseph R. Davis -- ASM International 2001 Page 139
12. Joining of Materials and Structures: From Pragmatic Process to Enabling by Robert W. Messler -- Elsevier 2004 Page 333
13. Metal Casting and Joining by K. C. John -- PHI Learning 2015 Page 392
14. New Edge of the Anvil: A Resource Book for the Blacksmith by Jack Andrews --Shipjack Press 1994 Page 93--96
15. New Edge of the Anvil: A Resource Book for the Blacksmith by Jack Andrews --Shipjack Press 1994 Page 93--96
16. New Edge of the Anvil: A Resource Book for the Blacksmith by Jack Andrews --Shipjack Press 1994 Page 93--96
17. The History of Hardening by Hans Berns -- Harterei Gerster AG 2013 Page 48--49
18. Bladesmithing with Murray Carter: Modern Application of Traditional Techniques by Murray Carter -- F+W Media 2011 Page 40
19. Iron and Steel in Ancient Times By Vagn Fabritius Buchwald -- Det Kongelige Danske Videnskabernes Selskab 2005 Page 65

External links