Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel

Sources of steel:

4140/4340 – Forklift Tines
5160 or 1095 – Truck Coil Springs
1085 – Truck Leaf Springs
4360 or 1095 – Auto Coil Springs
5160 or 1085 – Auto Leaf Springs
1095  or W-1 Tool Steel or W2 Tool Steel – Files

SAE Series Type of Steel
10XX Plain Carbon Steel
11XX Some Manganese is present
13XX Manganese
2XXX Nickel steel with 0.5% Nickel
23XX 3.5% Nickel
25XX 5.0% Nickel
3XXX Nickel Chromium steel. 33XX has somewhat higher amounts of both.
4XXX Molybdenum
40XX Carbon/Molybdenum
41XX Chromium/Molybdenum
43XX Chromium/Molybdenum/Nickel
46/48XX Molybdenum/Nickel
5XXX Chromium
51XX Low Chromium
515XX Corrosion and heat resistant steel (stainless)
52XX Medium Chromium
53XX High Chromium
6XXX Chromium/Vanadium Steel
71XX Tungsten Steel
86/87XX Nickel/Chromium/Molybdenum Steel
92XX Manganese/Silicon Steel

ASI Code Type of Steel
A Air hardening
D Die Steel
F Carbon/Tungsten
H Hot work alloys
L Low alloy
M Molybdenum
O Oil hardening
P Casting Steel
S Shock resistant
T Tungsten
W Water hardening

Spring steel grades

SAE grade (ASTM grade)	Composition	Yield strength	Typical hardness [HRC]	Maximum hardness [HRC]	Comments
1074/1075[2]	0.70-0.80% C, 0.50-0.80% Mn, max. 0.040% P, max. 0.050% S[3]		44–50[4]	50	Scaleless blue steel
1095 (A684)[2]	0.90-1.03% C, 0.30-0.50% Mn, max. 0.040% P, max. 0.050% S[3]	60–75 ksi (413-517 MPa) Annealed	48–51[4]	59	Blue spring steel
5160 (A689)[5]	0.55-0.65% C, 0.75-1.00% Mn, 0.70-0.90%Cr[3]	97 ksi (669 MPa)		63	Chrome-silicon spring steel; fatigue-resistant
9255	0.50-0.60% C, 0.70-0.95% Mn, 1.80-2.20%Si[3]
301 Spring-temperedstainless steel (A666)[6]	0.08-0.15% C, max. 2.00% Mn, 16.00-18.00% Cr, 6.00-8.00% Ni[3]	147 ksi (1014 MPa)		42

Principal low-alloy steels
SAE designation	Composition
13xx	Mn 1.75%
40xx	Mo 0.20% or 0.25% or 0.25% Mo & 0.042% S
41xx	Cr 0.50% or 0.80% or 0.95%, Mo 0.12% or 0.20% or 0.25% or 0.30%
43xx	Ni 1.82%, Cr 0.50% to 0.80%, Mo 0.25%
44xx	Mo 0.40% or 0.52%
46xx	Ni 0.85% or 1.82%, Mo 0.20% or 0.25%
47xx	Ni 1.05%, Cr 0.45%, Mo 0.20% or 0.35%
48xx	Ni 3.50%, Mo 0.25%
50xx	Cr 0.27% or 0.40% or 0.50% or 0.65%
50xxx	Cr 0.50%, C 1.00% min
50Bxx	Cr 0.28% or 0.50%
51xx	Cr 0.80% or 0.87% or 0.92% or 1.00% or 1.05%
51xxx	Cr 1.02%, C 1.00% min
51Bxx	Cr 0.80%
52xxx	Cr 1.45%, C 1.00% min
61xx	Cr 0.60% or 0.80% or 0.95%, V 0.10% or 0.15% min
86xx	Ni 0.55%, Cr 0.50%, Mo 0.20%
87xx	Ni 0.55%, Cr 0.50%, Mo 0.25%
88xx	Ni 0.55%, Cr 0.50%, Mo 0.35%
92xx	Si 1.40% or 2.00%, Mn 0.65% or 0.82% or 0.85%, Cr 0.00% or 0.65%
94Bxx	Ni 0.45%, Cr 0.40%, Mo 0.12%
ES-1	Ni 5%, Cr 2%, Si 1.25%, W 1%, Mn 0.85%, Mo 0.55%, Cu 0.5%, Cr 0.40%, C 0.2%, V 0.1%

 Principal effects of major alloying elements for steel

Element	Percentage	Primary function
Aluminium	0.95–1.30	Alloying element in nitriding steels
Bismuth	–	Improves machinability
Boron	0.001–0.003	A powerful hardenability agent
Chromium	0.5–2	Increases hardenability
	4–18	Increases corrosion resistance
Copper	0.1–0.4	Corrosion resistance
Lead	–	Improved machinability
Manganese	0.25–0.40	Combines with sulfur and with phosphorus to reduce the brittleness. Also helps to remove excess oxygen from molten steel.
	>1	Increases hardenability by lowering transformation points and causing transformations to be sluggish
Molybdenum	0.2–5	Stable carbides; inhibits grain growth. Increases the toughness of steel, thus making molybdenum a very valuable alloy metal for making the cutting parts of machine tools and also the turbine blades of turbojet engines. Also used in rocket motors.
Nickel	2–5	Toughener
	12–20	Increases corrosion resistance
Silicon	0.2–0.7	Increases strength
	2.0	Spring steels
	Higher percentages	Improves magnetic properties
Sulfur	0.08–0.15	Free-machining properties
Titanium	–	Fixes carbon in inert particles; reduces martensitic hardness in chromium steels
Tungsten	–	Also increases the melting point.
Vanadium	0.15	Stable carbides; increases strength while retaining ductility; promotes fine grain structure. Increases the toughness at high temperatures

Iron Classifications

Pig iron – the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux.Charcoal and anthracite have also been used as fuel. Pig iron has a very high carbon content, typically 3.5–4.5%,^[1] which makes it very brittle and not useful directly as a material except for limited applications.

Wrought iron – the purest form of commercial iron, containing 0.10% to 0.25% of carbon and less than 0.25% of impurities like sulfur, phosphorus, silicon and manganes

Steel Classifications

Medium carbon steel – Approximately 0.30–0.59% carbon content. Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.

High carbon steel – Approximately 0.6–0.99% carbon content. Very strong, used for springs and high-strength wires.

Ultra-high carbon steel – Approximately 1.0–2.0% carbon content. Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles orpunches. Most steels with more than 1.2% carbon content are made using powder metallurgy. Note that steel with a carbon content above 2.0% is considered cast iron.

Heat Treatment Terms

• Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel. The image to the right shows where spheroidizing usually occurs.^[11]
• Full annealing: Carbon steel is heated to approximately 40 °C above Ac₃ or Ac₁ for 1 hour; this assures all the ferrite transforms into austenite (although cementitemight still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 20°C (68.4°F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the “bands” of pearlite are thick. Fully annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.^[12]
• Process annealing: A process used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. The image rightward shows the area where process annealing occurs.
• Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method rids any temperature gradient.
• Normalizing: Carbon steel is heated to approximately 55 °C above Ac₃ or Ac_m for 1 hour; this assures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100.4 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.^[13]
• Quenching: Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature. The critical temperature is dependent on the carbon content, but as a general rule is lower as the carbon content increases. This results in a martensitic structure; a form of steel that possesses a super-saturated carbon content in a deformed body-centered cubic (BCC) crystalline structure, properly termed body-centered tetragonal (BCT), with much internal stress. Thus quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three to four (with more carbon) fold harder than normalized steel.^[14]
• Martempering (Marquenching): Martempering is not actually a tempering procedure, hence the term “marquenching”. It is a form of isothermal heat treatment applied after an initial quench of typically in a molten salt bath at a temperature right above the “martensite start temperature”. At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained austenite which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer marquenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer temperatures equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process reduce internal stresses and stress cracks, but it also increases the impact resistance.^[15]

• Quench and tempering: This is the most common heat treatment encountered, because the final properties can be precisely determined by the temperature and time of the tempering. Tempering involves reheating quenched steel to a temperature below the eutectoid temperature then cooling. The elevated temperature allows very small amounts of spheroidite to form, which restores ductility, but reduces hardness. Actual temperatures and times are carefully chosen for each composition.^[16]

• Austempering: The austempering process is the same as martempering, except the steel is held in the molten salt bath through the bainite transformation temperatures, and then moderately cooled. The resulting bainite steel has a greater ductility, higher impact resistance, and less distortion. The disadvantage of austempering is it can only be used on a few steels, and it requires a special salt bath.