Iron in its natural state is very soft and not useful for tools and components unless it is combined with carbon to make steel. The percentage of carbon determining important characteristics of the final product; the lower the carbon content, the softer the product and the higher the carbon content, the harder the product. Premium steel consists of a carbon content between 0.02%-1.7% by weight. In early horology steel was difficult to produce with the methods available, which led to the usage of wrought iron in the make-up of most tower clocks.
Metallurgy is a domain of service that studies the physical and chemical behavior of metallic elements, their compounds and their mixtures that are called alloys. It is also the technology of metal: the way in which science is applied to their product use. The first evidence of human metallurgy dates from the 5th and 6th century millenium BC in Serbia, but as with much in prehistoric times, ultimate beginnings cannot be clearly defined. Egyptian “diggers” in iron date from 300 BC. The secret of countries defeating each other in war was extracting and working iron in producing weaponry.
Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into a purer form. In order to convert a metal oxide or sulfide to a purer metal, the ore must be reduced physically, chemically or electrologically.
Extractive metallurgists are interested in three primary strains; feed, concentrate (valuable metal oxide/sulfide) and tailings (waste). After mining large pieces of the ore, they are broken through crushing and/or grinding in order to obtain particles small enough where each is mostly valuable or waste.
Mining may not be necessary if the ore body and physical environment are conducive to leaching. Leaching dissolves minerals in an ore body and results in an enriched solution. The solution is collected and processed to extract valuable metals.
Ore bodies often contain more then one valuable metal. Tailings may be used to extract for a secondary process.
Common engineering metals include aluminum, chromium, copper, iron, magnesium, nickel, zinc and of course brass. Much effort has been placed on understanding the iron-carbon alloy system, which includes steels and cast irons. Plain carbon steels are used in low cost, high strength applications where weight and corrosion are not a problem. Cast irons, including ductile iron are also part of the iron-carbon system.
Stainless steel or galvanized steel are used where resistance to corrosion is important.
In production engineering, metallurgy is concerned with the production metallic components for use in consumer or engineering products. This involves the production of alloys, shaping, heat treatment and surface treatment. The goal is to achieve a balance between material properties such as cost, weight, strength, toughness, hardness, corrosion and fatigue resistance and performance in temperature extremes. In salt water, ferrous metals and some aluminum alloys corrode quickly.Metals exposed to cold or cryogenic conditions may endure a ductile to brittle transition and lose their toughness, becoming more brittle and prone to cracking. Metals under continual cyclic loading can suffer from metal fatigue. Metals under constant stress at elevated temperatures can creep.
Metal Working Processes
Metals are shaped by processes such as:
-casting-molten metal is poured into a shaped mold
-forging-a red hot billet is hammered into shape
-rolling-a billet is passed through successively narrower rollers to create a sheet
-extrusion-a hot malleable metal is forced under pressure through a die, which shapes it before it cools
-machining-lathes, milling machines and drills cut the cold metal to shape
-fabrication-sheets of metal are cut with guillotines or gas cutters and bent into shape
-cold working processes shape is altered by rolling, fabrication or other processes while it is cold, this process increases strength (work hardening). Work hardening creates microscopic defects in the metal, which resist further change in shape.
Metal can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion.
-Annealing-a process that softens metal by allowing recovery of cold work and grain growth
-Quenching is used to harden alloy steels to trap dissolved solute atoms in solution
-Tempering will cause the dissolved alloyed elements to precipitate, or also to improve impact strength and ductile properties
Electroplating is a common surface treatment technique. It involves bonding a thin layer of another metal such as gold, silver etc. to the surface of the product. It is used to reduce corrosion and also enhance the products aesthetic appearance
Scientists study the microscopic and macroscopic properties under the microscope. The sample in question is ground flat then polished. It is then viewed under the electron microscope showing its composition, mechanical properties and processing history
This is the production of metals and minerals from raw materials such as ores and waste materials. Its theoretical underpinning involves science contributions of physics, analytical chemistry and mineralogy
Mineral processing involves the process used to manipulate the particle size of solid raw materials and to separate valuable materials from materials of no value, referred to as gangue. These physical properties can include density, particle size and shape, electrical and magnetic properties and surface properties.
Copper is a ductile metal with very high thermal and electrical conductivity. It is used as a thermal conductor, an electrical conductor and a constituent of various alloys (i.e. Brass). Copper has been mined since the Roman era, first from Cyprus, hence the name. Corroded copper compounds are often encountered as salts of CU2+, which often import blue or green colors to minerals such as turquoise and historically widely used as pigments. Copper when oxidized, externally corrodes to a characteristically green patina. The oldest artifact from copper is a 8700BC pendant. Gold was the only metal used before copper. Brass and bronze were alloyed soon after the discovery of copper itself. Bronze-brass alloys became so widespread in Europe approximately 2500-60 BC, that it became named the Bronze Age. Zinc was first alloyed with copper in the 10th century BC in Palestine.
Iron smelting, originally produced in bloomers, furnaces where bellows were used to force air through a pile of iron ore and burning charcoal. The carbon monoxide produced by the charcoal reduced the iron oxide from the ore to metallic iron. However the blooming was not hot enough to melt the iron, so the metal collected in the bottom of the furnace as a spongy mass or “Bloom”, whose pores were filled with ash and slag. The bloom then had to be reheated to soften the iron and melt the slag, and then repeatedly beaten and folded to force the molten slag out of it. The result of this time consuming and laborious process was wrought iron, a malleable but fairly soft alloy. Carbonization, the process of adding carbon to wrought iron. While the iron bloom contained some carbon, the subsequent hot-working oxidizes most of it. Smiths in the Middle East discovered that wrought iron could be turned into a much harder product by heating the shaped pieces in a bed of charcoal for some time and then grinding it in water or oil. This procedure turned the outer layers of the piece into steel, hence the movement of the charcoals carbon into the wrought iron, producing a much harder and less brittle product. This was the beginning of steel-making, further technologies and heating would follow to make the piece of wrought iron fully congruent with carbon.
Welding is a fabrication process that joins materials, usually metals or thermoplastics by causing coalescence. This is achieved by melting the workpiece and adding a filler material to form a pool of molten material that cools to become a strong joint. Many energy sources can be used for welding, including a gas flame, an electric arc, a lazier, an electron beam, friction and ultrasound. It can be done in many different environments including open air, underwater and in space. The process is dangerous, one must avoid burns, electric shock, poisonous fumes and overexposure to ultraviolet light.
Brazing is a joining process in which a filler metal is melted and drawn into a capillary formed by the assembly of two or more work pieces. The filler metal reacts metallurgically with the work piece and solidifies in the capillary forming a strong joint. Unlike welding, the work piece is not melted. Brazing is similar to soldering, but occurs at temperatures in excess of 450°C (842°F). Brazing produces less thermal stresses than welding and brazed assemblies tend to be more ductile then weld because alloying elements can not segregate and precipitate.
Soldering is a joining process that occurs at temperatures below 450°C (847°F). It is similar to brazing in that the filler is melted and drawn into a capillary to form a joint, although at a lower temperature. Because of the lower temperatures and difficult alloys used as fillers, the metallurgical reaction between fillers and work pieces is minimal resulting in a weaker joint.
Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. Common processes include annealing, perceptive strengthening, quenching and tempering. The annealing process softens the metal by the allowing of cold work and grain growth. Quenching is used to harden steel alloys, or in precipitation hard-enable alloys, to trap solute atoms in solutions. Tempering will cause the dissolved alloying elements to precipitate or in the case of quenched steels, improve impact strength and ductile properties.
Electroplating is a common surface treatment technique. It involves bonding a thin layer of another metal such as gold, silver, zinc or rhodium to the surface of the product. It is used to reduce corrosion as well as to improve the products aesthetic appearance.
Plating covers a surface by depositing metal on a conductive surface. After being used for a hundred years, plating decorates objects, for corrosion inhibition, to improve solder ability, to harden, to improve wear ability, to reduce friction and for other purposes. There are plating methods; in one a solid surface is covered with a metal sheet and then heat and pressure are applied to fuse them (a version of this is Sheffield plate). Other plating techniques include vapor deposition under vacuum and sputter deposition. More recently plating used liquid. Mellatizing refers to coating metal on non-metallic objects.
In electroplating, an ionic metal is supplied with electrons to form a non-ionic coating on a substrate. A common system involves a chemical solution with the ionic form of the metal an anode (positive charge) which may consist of the metal being plated ( a soluble anode) or an insoluble anode (usually carbon, platinum, radium) and finally, a cattrode (negative charged) where electrons are supplied to produce a film of non-ionic metal.
Electro less Plating
Electro less plating, also known as chemical or auto-catalytic plating is a non-galvanic type of plating method that involves several simultaneous reactions in an aqueous solution which occur with the use of power.
For specimen examination the object is prepared by grinding, polishing and etching. Once prepared it is amalganized by optical or electron microscopy. Mechanical preparation is the most common, using finer and finer successive abrasion to remove scratches until polished. Metallographic specimens are mounted using a hot compression thermosetting resin. Metallographic are mounted using a hot compression thermosetting resin. After curing specimen is wet, ground using abrasives. Under magnification certain microstructures consistently can be seen (i.e. inclusions and nitrides). Non cubic microstructures can be reheated.
Heat treatment is a method used to alter the physical and sometimes chemical properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. Treatments also occur incidentally during hot forging or welding.
Metallic materials consist of a microstructure of small crystals called “grains” or crystallites. The nature of the grains (i.e. grain size and composition) is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling the rate of diffusion and the rate of cooling within the microstructure.
Complex heat treating schedules are often devised by metallurgists to optimize an alloys mechanical properties.
Annealing is a technique used to recover cold work and relax stresses within a metal. Annealing typically results in soft, ductile metal. When an annealed part is allowed to cool in the furnace, it is called a “full heat treatment”. When an annealed part is removed from the furnace and allowed to cool in the air, it is called a heat treatment. A stress relief annealing is when only the first stage of annealing is performed.
The second stage of annealing is recrystallization, where new stress-free grains grow. The third stage is grain growth, which causes the existing grains to grow.
Hardening and Tempering (Quenching and Tempering)
To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other gas (such as nitrogen) oil, polymer dissolved in water or brine. Upon being rapidly cooled, a portion of the austenite (dependent on alloy composition) will transform to martensite, a hard, brittle crystalline structure. The quenched hardness of a metal depends on its chemical composition and quenching method. Cooling speeds from fastest to slowest go from polymer, brine, fresh water, oil and forced air. However, quenching a certain steel too fast can result in cracking, which is why high tensile steels such as AISI4140 should be quenched in oil, tool steels such as 2767 or H13 hot worked tool steel should be quenched in forced air, and low alloy or medium-tensile steels such as XK1320 or AISI1040 should be quenched in brine or water.
When a precipitation hardening alloy is quenched, its alloying elements will be trapped in a solution, resulting in a soft metal. Aging a solutionized metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age “naturally”, meaning that the precipitator form at room temperature or they may age “artificially” when precipitates only form at elevated temperatures. In some applications, no naturally aging alloys may be stored in a freezer to prevent hardening until further operations.
Some technologies allow different areas of a single object to receive different heat treatments. This is called differential hardening. It is common for high quality service and swords.
Case hardening is a process in which an alloying element, most commonly carbon or nitrogen, diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.
Depth of Hardening
The Rockwell scale is used to compare part specifications for case depth hardening. For cases less then .38mm the Rockwell scale is not reliable.
Bell Metal – Four parts copper and one part tin, with small quantities of zinc, lead, iron, bismuth and antimony. Used for metal laps and steps to obtain a high polish on steel.
Beryllium – A metal used in small concentrations with nickel and steel to produce non-magnetic, non-corrosive balance and temperature compensated springs.
Brass – A soft yellow alloy composed of copper and zinc in various proportions. An average horological composition is 65% copper and 35% zinc. A mix of 70% and 30% respectively, is considered to be a soft brass found on antique clock plates for its workability. Gear wheels and hardened parts are a 60/40 ratio, which is stronger due to added zinc. Other metals can be added to the alloy in small amounts to modify hardness, color and other properties. Brass becomes hardened when rolled or hammered. Its strength and hardness are dependent upon alloying and/or the cold work process.
Bronze – An alloy of copper and tin that is generally modified with small amounts of zinc, lead or other elements. It is a stronger and harder material than brass and sometimes used for bushings. It becomes hardened when rolled or hammered. Its strength and properties vary widely and are dependent upon alloying and/or the cold work process.
Elinvar – An alloy of nickel, titanium, chromium, aluminum, manganese, silicon, carbon and iron. It is used for some hair springs and chronometer due to its low coefficient of expansion and constant elasticity under temperature change. It is non-magnetic and corrosion resistant.
Invar – An alloy made up of 36% nickel and 54% cobalt with a thermal expansion of nearly zero. Used for balance wheels and compensated pendulum rods.
Nivarex- An alloy used for balance springs; consisting of nickel, iron, beryllium, molybdenium, tungsten and chromium. It is non-magnetic, rust-proof and has a low rate of expansion and contraction.
Steel – An alloy produced by refining molten pig-iron.
Iron is purified and a certain amount of carbon is added. Steel is classified on the amount of carbon content, which is directly related to hardness. After a certain point of adding carbon its strength declines. Safe steel contains less than .25% carbon, medium steel contains .25-.6% carbon and hard steel contains .6-1.7% carbon.
Alloy- A metallic substance resulting from the fusion, mixture or combination of 2 or more metals.
By combining metals of specific proportions, the composition of the resulting metal (alloy) can be conditioned for a specific purpose. It can be formulated to increase or decrease its density, hardness, linear expansion, corrosion and conductivity.
Brittleness– The ability to break with a relatively smooth fracture.
Cast Iron – A generic name for a group of metals which are alloys of iron, carbon and silicon.
Coefficient of expansion – A comparative measure of volume change with heat application.
Contraction – To shrink or become smaller
Conductor – A medium that transmits energy such as heat or electricity.
Corrosion – Gradual deterioration, usually caused by a chemical reaction.
Density – The mass per unit of volume
Ductile – The ability to undergo a change of form without breaking.
Elastic – The ability of returning to its original state after being stretched.
Element – A substance that cannot be separated by chemical means into a substance different from itself.
Expansion – To increase in volume or spread out.
Ferrous – A method used to classify metals based on a relationship to iron. All other metals are considered to be nonferrous. Ferrous metals used in horology are iron and steel.
Feasible – Capable of being melted.
Galvanize – A coating for metals to prevent corrosion.
Hardness – Resistance to indentation, deformation or machining.
Linear – Extended or arranged in a line.
Luster – The quality of shining by reflecting light
Magnetism – The property of attraction and repulsion of certain substances.
Malleable – The capobility of being shaped or extended by hammering.
Metal – A substance in its pure state, characterized by ductility, conductivity and luster.
Slag – Vitrified matter separated during the reduction of a metal from its ore.
Strength – The capability to resist force and wear.
Tool steel – A high carbon steel used for making tools.
Tenacious – The condition of being tough, stubborn and adhesive.
Tensile strength – The maximum load per unit of original cross section that a material is able to withstand.
Pig ore – Iron ore obtained from the first smelting in a blast furnace.
Precious Metals – Gold, silver and platinum; these have limited horological use, but one should have knowledge of use ie. gold is found on some high grade pocket watch movements. It was used for the center wheel and other components-giving smoother action and low friction.
Gold – An element having a yellow luster. It is soft, malleable, ductile and relatively non-corrosive.
Silver – A pure white, very soft metal, harder than gold with brilliant luster. It is malleable, ductile, resistant to corrosives and an excellent conductor.
Platinum – A gray-white metal, heavier than gold, ductile, malleable and resistant to corrosives.
Wrought Iron – A low carbon steel containing much slag.
Addendum (Steel Heat Treatment)
A “Critical Temperature” is the one which steel must be heated so it will harden properly when cooled by quenching. The higher the carbon content, the lower the “critical temperature” needed for hardening. In sampling, steel cooled at “white heat” is very brittle due to a coarse molecular structure found on cooling; steel cooled at “bright red” heat is somewhat less brittle due to a finer structure formed on cooling; steel cooled at “dull red” heat is less brittle yet due to a fine, smooth molecular structure when cooled.
Chart Color – Temperature Relationships Used in Hardening Steel
Color Temperature (degrees F)
Dull Red 1280
Dark Cherry Red 1470
Cherry Red 1650
Clear Cherry Red 1830
Deep Orange 2010
Clear Orange 2190
Brite White 2550
Steel that has been heated for hardening then cooled by submerging in water is hard and brittle. The same steel cooled by dunking it in oil tends to be somewhat less brittle. Specific oils, formulated for this purpose, with a safe flash point, must be used to avoid a possible fire or explosion.
When heating, heat to a “dull red” then drop into water or oil quickly to cool it. To prevent the oxidizing effect of heat and air from forming a hard scale on the surface of the part, heat it slightly at first then dip it into liquid soap or roll it on a common bar of laundry soap until it is fully covered. This coating will flash off when the part is heated to “dull red” and will drop off into the oil or water.
The item in question to prevent uneven contraction and warping must be dunked or dropped quickly into the cooling medium with limited “air time”.
A thin or small part can be wrapped in a fine iron wire in the shape of a spring for heating.
Use the “file test”. Draw a file over the surface, if it does not make a scratch, the piece is considered hard, if a scratch occurs, the soft part will have to be reheated.
Polish the hardened parts surface with emery cloth so color changes during heating can be observed. Heat the piece in a direct flame until it assumes the color indicating the color temperature desired. Drop immediately into water to cool it.
In general, as a piece of steel is heated it first turns “pale straw” (tool edge brittleness) and if the heat-air oxidation contents goes all the way to “pale blue” which is used for springs (steel quite soft) with temper almost being completely “drawn”.
Note: Color does not always indicate temper. Any piece of steel or soft iron will show the same colors when heated (surface heat oxidation reaction) but no intermolecular changes will occur unless the tempering hot steel has been previously hardened to glass hard.
(Purples to Blues Being used for Only the Color Themselves)
Steel screws and small parts to be blued (or tempered) can be placed on a bluing slab or on an even bed-pan of brass shavings, which would hold the piece to insure that they are heated evenly to achieve a uniform color. When blueing, parts clean and polish before heating. Care should be taken not to touch the polished surface with the fingers, for this finger print will defile a color. The fancy and brighter the polish pretempering, the better and more even the color will be.
Hard steel must be annealed for shaping and machining. This technique removes all the hardness. It is accomplished by heating the piece above its critical range “cherry red” and allowing it to cool very slowly. This can be achieved in air or an annealing pan filled with sand. Sand pan annealing allows the heated steel to become even more softer then air, thus allowing it to cool while remaining in the sand, which takes considerably more time.
Finishing Replacement Components
For antique clocks and watches parts are no longer available and fabrication is obligatory. The replacement part should be equal in appearance and quality of the original. Select metal is sawed, filed and ground to the same size and shape; then it is hardened and tempered and the surface is ground and polished to match the original finish.
A stoned finish is satisfactory for small parts of most clock movements. Start by stroking the part lengthwise on a flat combination of carborumdum stone covered with a very thin coating of light oil. 100 grid black wet/dry paper cemented to a piece of melamine will work fine,
The next level in surface finishing is grinding. Oilstone powder (isolux 50, natural oily white stone powder), rotten stone, pumice and vienna lime can be used. The powders are mixed with a light oil to form a slurry, using a flat block of Boxwood, float glass or lead is preferable. Also, aluminium oxide abrasive papers can be used.
After a desired ground finish is achieved, a further step can be polishing. The part in this stage can be taken to a brilliant finish. Use the same blocks used in the grinding stage but with fine abrasive powders such as; diamantine, sapphirine, rouge, diamond powder or an equivalent substitute mixed with oil. Other abrasives “Raybrite B”, 410 emery paper, a flat jasper stone.
Note: With all metal surfacing procedures avoid cross-contamination.
1. Grinding out turning tool marks
A. Select a slip out of iron and charge with oilstone slurry
B. Contact charged slip with pivot
C. With lathe at a moderate speed, apply light pressure. Marry back and forth. Replenish abrasive, if area on slip turns black. Clean with pith or cork.
1. A. Select a boxwood or bell metal slip adding aluminum oxide or similar abrasive paste to a full luster
B. Instead of above step its possible to polish or burnish by using a flat burnisher, this also compresses the metal resulting in a smooth hard surface