by Ångström
For those of us who extensively scratch build, kit build, or even just build the occasional model-railway item, the question of working in metal, and as to the best metal to use must occur. Often the best metal to use is not the obvious one, so it is the purpose of this article to list some of the more important properties of metals frequently encountered in the hobby, together with the author's opinions as to what use can be made of them.
Many of the metals listed are alloys, which means that they consist of a mixture of more than one metal, or are a mixture of at least one metal together with small quantities of non-metallic elements. It must be appreciated that some alloys, such as stainless steel, come in a variety of differing compositions and mechanical properties, and it would be beyond the bounds of this article to discuss them in detail. In such cases, the variety listed is stated as typical.
The composition of all the metals discussed is given at the start of each paragraph on the particular metal. The normal chemical symbols for the elements are used, and the meaning of each symbol is listed below.
List of Symbols
The following symbols and abbreviations are used in this
article:
Al = Aluminium
Bi = Bismuth
C = Carbon
C# = Carbon in the form of iron carbide
Co = Cobalt
Cr = Chromium
Cu = Copper
Fe = Iron
Mn = Manganese
Ni = Nickel
P = Phosphorus
Pb = Lead
Sb = Antimony
Si = Silicon
Sn = Tin
V = Vanadium
W = Tungsten
Zn = Zinc
Notes on Physical Properties
In order to understand some of the terminology used in this article, the following notes may be of assistance. Metric units are used throughout.
1. Density is the weight per unit volume of the material.
2. Most alloys containing copper become very much harder and stiffer when rolled or worked, a process called "work hardening". Upon heating to red heat, they become soft again, a process called "annealing".
3. Machineability is a measure of the amount of energy needed to cut metal while turning, milling, drilling etc.
4. Springiness is the measure of how much the metal can be deformed yet still spring back to its original shape when the deforming load is removed.
5. The ability of the metal to conduct heat and electricity is expressed as a percentage of the conductivity of copper, as copper is the best of all common metals in both these respects.
6. Ferromagnetic is the ability of a material to transmit magnetic flux without becoming permanently magnetized.
Aluminium: 100% Al.
Aluminium is too soft and too weak to be of much use in modelling. Also, it cannot be soft soldered by normal means. However paint and adhesives stick to it very well, and it is easy to machine and bend. Therefore its light weight of 2.6gm/cc makes it a good material for rolling-stock superstructure where strength is of little importance.
I understand that a flux is now available that will allow aluminium to be soldered using ordinary soft solder. Here are the details: LA-CO aluminum flux paste for use with 50/50,60/40,95/5 or tin/silver soft solders. Part No. 22404. Four ounces. Available from LA-CO Industries 1201 Pratt Blvd. Elk Grove Village, IL 60007, USA. I hope this helps. - Phil
Brass Sheet or Tube: 67% Cu, 33% Zn (typical).
Brass has too high a conductivity of heat (27% Cu) to make it a good modelling material in sheet form. This high conductivity makes it difficult to solder, since the heat is drained away from the iron too quickly; or, when attempts are made to solder another item to the model, parts already soldered become unsoldered. Brass tube also has a poor machineability although it files easily.
However, in its hard-rolled form, brass sheet is easy to work with, is cheap, and its easy acquisition makes it an acceptable and popular material, even though it requires considerable skill to solder and machine. Due to the soft oxide that forms on brass, paint and adhesives do not stick very well, although the use of etch primers improves this situation.
Brass Rod and Bar: 62% Cu, 35% Zn, 3% Pb (typical).
Brass rod and bar is cast rather than rolled or extruded, and is known as "free-machining brass". Its machineability is excellent, but its solderability is equally bad as brass sheet. However, its easy acquisition makes it the most commonly used material for machined fittings. Brass has a relatively high coefficient of friction and therefore, if its yellow colour can be tolerated, it is a good material for traction tyres. Brass rod and bar is equally as poor to paint as brass sheet.
Cast Iron: 96% Fe, 4% C.
The free carbon as graphite in cast iron, and its good machineability makes it an excellent material where the load on the bearings is low, as in live-steam cylinders and piston rings. Cast iron is much softer than steel, very brittle, strong in compression, but weak in tension. Where strength is of little importance, cast iron is extensively used in cast components due to its cheapness. However, cast iron rusts very readily, and is dirty to work with.
Copper: 100% Cu.
Copper is an almost useless material for the small-scale modeller due to its softness, weakness, and, due to its very high heat conductivity, the difficulty in soldering it. For the live-steam modeller, copper is the best material to use for boilers since it has a great resistance to corrosion, particularly when hot. Copper is a very good electrical conductor, and makes good electrical contact with itself as in wiping contacts. However, its lack of springiness makes copper useless for sprung pick-ups.
Duralumin: 96% Al, 4% Cu (typical).
Aluminium alloy, commonly called "Duralumin" or just "Alloy", is about four times as strong as aluminium itself, and is therefore a useful material. It cannot be soft soldered, but has good machineability and corrosion resistance, while its white colour makes it an acceptable substitute for steel, where the approximate colour of steel needs to be retained. Duralumin has a relatively high coefficient of friction (similar to brass), so makes a good material for traction tyres.
Gunmetal: ( 85% Cu, 5% Zn, 5% Sn, 5% Pb ) typical.
Its excellent machineability and comparatively low conductivity of heat (12% Cu), makes gunmetal one of the best materials for machined fittings which have to be soldered in place. Also, it makes a very good bearing material for use with steel journals. It is easy to cast, and most live-steam castings which are loosely said so be brass, are in fact gunmetal.
High Speed Steel: 76% Fe, 18% W, 4% Cr, 1% V, 0.7% C#.
The best drills and cutting tools are made from high speed steel, which is an extremely hard and stiff metal. Small (broken) drills come in handy for use as piston rods or other rodding where strength and rigidity are called for. High speed steel can only be soft soldered with the use of a phosphoric acid flux and even then it is very difficult. The metal, as it cannot be annealed by normal means, can only be cut by grinding. High speed steel is often wrongly referred to as "tool steel" which is reviewed later.
Lead: 100% Pb.
Lead is extremely soft and weak, and has no use in modelling except for weighting. Its low melting point (327°C) makes it easy to cast, and at 11.3gm/cc it is the heaviest of all common metals.
Mazak: 95% Zn, 4% Al, 1% Cu (typical).
Mazak is encountered as a die-casting alloy. It is a dreadful material to drill and tap, and its only useful characteristic, apart from diecasting properties, is that it has a high coefficient of friction and therefore makes an excellent traction tyre.
Some manufacturers use Mazak for wheels, and such wheels have a bad reputation for picking up dirt. However, like aluminium and other soft materials, this only happens when sparking is allowed to occur, as when an electrically-driven locomotive does not have enough pick-ups.
Mild Steel: 99.8% Fe, 0.2% C#.
The ease with which mild steel rusts makes it an unattractive modelling material for many purposes, except when it is used to give the appearance of the real thing like in coupling rods. Also, it is relatively difficult to solder except with an acid flux, which compounds the rusting problem.
When coated with tin in the form of "tin plate", these objections are eliminated, but an additional disadvantage of mild steel is that it is ferromagnetic, which can cause the magnet of a nearby electric motor to become weakened. Mild steel is a good material for traction tyres although it makes comparatively poor electrical pick-up.
Nickel Silver: 65% Cu, 18% Ni, 17% Zn (typical).
Nickel silver is the ideal modelling material. Solder takes to it very readily, and with a conductivity of heat of only 6% Cu, even very low wattage irons can be used with ease. In its hard-rolled form, it is very stiff, springy and comparatively hard, yet relatively easy to file. However when annealed, it becomes soft like copper.
Most leading modellers swear by nickel silver, but it is difficult to get in sheet form in Australia. It can be obtained in bulk from N.F. Tresidder, 47 Hepburn St, Lithgow, NSW, or from many UK model shops by mail order. In rod form, nickel silver is easy to machine, but not as easy as cast brass. The author is unaware of anywhere that nickel silver rod can be bought in Australia.
Nickel silver has a low coefficient of friction making it undesirable for traction tyres. However, it makes excellent electrical contact, even though its electrical conductivity at 6% Cu is low. Nickel silver tarnishes very little and has a white-to-slightly-yellow appearance making it an acceptable substitute for steel, such as with rails, coupling rods and valve gear. Paint sticks to nickel silver moderately well but etch primers should be used.
Phosphor Bronze: 95% Cu. 5% Sn, less than 0.1% P.
Phosphor bronze has similar properties to nickel silver, but it is more difficult to file and solder. Its outstanding property is its springiness, and together with the fact that it makes good electrical contact, it is the ideal material for sprung electrical pick-ups. Phosphor bronze is moderately difficult to machine, but makes an excellent bearing material for steel journals. Paint sticks poorly to phosphor bronze but etch primers help.
Piano Wire: 98.7% Fe, 0.8% C#, 0.3% Mn, 0.2% Si.
Piano wire is a very hard, stiff and springy metal. It is relatively resistant to rust and therefore is the ideal material for modelling hand rails and delicate pipe work that can be damaged easily with handling. Piano wire is moderately easy to solder, but an acid flux helps greatly in this respect.
Silver Steel: 99% Fe, 1% C#.
Silver steel is a very hard material and therefore has good wear resisting properties. It is difficult to machine and hard to file, but is used almost exclusively for axles and other rotating parts. When heated to red heat and then quenched quickly in oil or water, it becomes so hard that it cannot be machined or filed. Thus, in its hardened state, it can be used for form tools or other hand-made cutting tools.
Solder: 50% Pb, 50% Sn (typical).
Apart from its use for soldering, this alloy is useful in that its grey appearance resembles steel, and with the skilled use of a soldering iron, items made from brass or other yellow metals can be plated and thus made to have a steel-like appearance. The melting point of solder is 216°C, and electrical conductivity 11% cu.
Spring Steel: 98% Fe, 1% Mn, 1% C#.
As its name implies, spring steel is ideal whenever spring is required. However, it makes poor electrical contact, and on its own is very poor for sprung electrical pick-ups. Its use in this respect can be greatly improved if copper or nickel silver is soldered on to the actual contact areas.
Stainless steel: 70% Fe, 20% Cr, 10% Ni (typical).
Stainless steel is similar to nickel silver in many ways except that it is harder to file, and almost impossible to solder with resin flux. If phosphoric acid is used as a flux, and the surfaces are well cleaned, it becomes very easy to solder, and what is more, a stronger soldered bond is made than with brass. Therefore it is good to use for delicate bits and pieces that might be knocked off. Like aluminium, a very hard and tenacious oxide is formed on stainless steel, and this allows paint to stick very well indeed. Etch primers should be avoided for they will not etch stainless steel and could worsen the adherence of the paint. Stainless steel is a poor conductor, both of heat (6% Cu) and electricity (3% Cu), but makes good electrical contacts for modelling purposes. Stainless steel has good machineability, but, because of its work-hardening property, requires so much skill to drilling it, that most modellers find it unacceptable.
Tool Steel: 87% Fe, 7% Cr, 4% W, l% V, 0.6% C#, 0.4% Mn.
Tool steel is a more sophisticated and expensive equivalent of silver steel. Tool steel can be machined in its annealed condition, and then hardened by heating to red heat and quenching in oil or water. In all respects, tool steel has better mechanical properties than silver steel; is very resistant to rusting; and therefore is the ideal metal for making special forming and cutting tools.
White Metal: 51% Sn, 33% Pb, 13% Sb, 3% Cu (typical).
With a melting point of only 240°C, white metal can be cast into silastic-rubber moulds, and is encountered in cast metal parts in rolling-stock kits and accessories. As a metal to work with it has nothing to recommend it, for it is soft, brittle and dreadful to file.
Woods Metal: 50% Bi, 25% Pb, 13% Sn, 12% Cd.
Also called "Cerobend", woods metal is as dreadful as white metal to work with, but with a density of 9.6 gm/cc, it is almost as heavy as lead. With a melting point of only 70°C, it is excellent for adding weight into inaccessible places in locomotives. However, its low melting point can be a disadvantage, but when mixed about 50/50 with lead, its melting point can be raised to about 105°C, and the extra lead makes it an even better weighting material. Either on its own, or mixed with ordinary solder, woods metal can be used as a low melting-point solder for white metal kits.
Tungsten:
Since this article was written, Daniel Sieber of Zurich, Switzerland, has added a comment concerning the use of tungsten and its alloys. Please follow this link to read Daniel's most interesting and helpful contribution.