Shop Note

Notes on various Metals, and Hardening/Annealing them

General Notes:

• All metals oxidize more rapidly as temperature increases; rapid cooling (e.g., quenching) arrests oxidization without any other harmful effects, except in the case of steel (see below).
• Some metals and alloys are "hot-short" - they become "pasty" or "crumbly" at high temperatures, and must be worked at low temperatures.

Aluminum:

• Will work-harden, but is liable to melt near red heat - hold at lower temperature for a longer time gives the same effect.
• No way to harden except by working the metal (e.g., twisting or hammering).
• Often hot-short.

Cupreous Alloys:

• Oxidizes readily.
• Cooling slowly or quickly has no effect on the result, except for surface oxidation.
• Heating only softens the metal; working (e.g., hammering, twisting, stretching) hardens it.
• Anneal by heating to red heat.

Copper:

Very soft unless work-hardened; then may become brittle.

Copper Alloys (e.g., brass, bronze):

• Severe work-hardening; most allows are hot-short.

Lead/Tin alloys:

• Used primarily for castings, and as low temperature ("soft") solder.
• Melt at relatively low temperatures (200°-700°F)
• Difficult to solder, because they melt at almost the same temperature as the solder.
• Lead and it's alloys are TOXIC!
• Lead and it's alloys decompose readily in the presence of acetic acid (common in some woods, adhesives).
• Brittiania metal is typically 80-90% Tin, 10-20% Copper (Lead free).
• "Gun metal" is 5% Tin, 5% Lead, 5% Zinc, 85% Copper (machines freely).
• "Soft" solder is typically 30-50% Tin, 50-70% Lead.
• "Pot metal" is typically 85-90% Lead, 10-15% Copper.

Ferrous metals:

Iron:

(Also called "mild steel" - less than .25% carbon.)

• Iron can't be hardened in the fire.
  
• It can be work-hardened, and then must be heated to reduce the stresses to keep from fracturing.

Cast Iron:

(More than 2% carbon.)

• Not much use in modeling, except for operating machinery (e.g., working steam engines).

Actual steel oxydation colors - right end heated.

Steel (carbon):

("High-carbon steel") Carbon content .25% to 1.75% carbon content; useful for making tools.

• Changes state when at red heat (greater than 1400 degrees; a magnet won't stick when in this state)
  
• If cooled slowly (burying it the forge's ash pile) it will be soft enough to machine (e.g., filing).
• If cooled rapidly (plunging into water, brine, or oil, depending on the alloy - the initial shock of the cooling liquid is the same for all, but the cooling rate differs) it gets extremely hard and brittle (like glass), may actually shatter from the shock if not careful...can't be filed.

  Oxidation colors		Use for:
  faint straw	400°F	razor
  straw	440°F	ax
  deep straw	475°F	cold chisel
  bronze	520°F	hammered chisel
  peacock	540°F	carving gouge
  purple	550°F	spring
  full blue	590°F	gun barrel
  light blue	640°F	no springiness
  (color observed on polished metal)

• In order to reduce the brittleness (and soften the metal), it is "annealed" - polish the surface, then heat the work until (soap-bubble like) Oxidation colors appear, then quench again.
• A "straw" color is suitable for cutting wood; "purple" is better for a spring.
  
• Since annealing involves temperatures of 500-600°F, your bake-oven can reach those temperatures even if your other sources of heat can't.
  
• Heating to white heat will burn the steel, and make it useless for future tools.
• High-carbon steel for making tools is readily available as "Drill rod", "Piano wire", and "Concrete nails".

Alloy Steels (e.g., Stainless"):

Distinguished from "high-carbon steel" by the introdiction of other metals (e.g., Chromium). There are so many different alloys that any general rule will not apply to at least some of them!

• Dental tools are often stainless steel.
• Other alloys are used for cutting tools (e.g., hacksaw blades - must be able to retain hardness even when hot).

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