Iron, carbon and the phases
Steel is iron with up to about 2 percent carbon and other alloying elements. At room temperature the soft, ductile phase is ferrite (almost pure iron) with hard iron carbide (cementite) in layers called pearlite; above the critical temperature (about 727 °C for plain carbon steel, 800 to 850 °C for the usual hardening temperatures) the structure changes to austenite, which dissolves the carbon. Cool austenite slowly and it turns back to ferrite and pearlite (soft); cool it fast enough (quench) and the carbon is trapped, giving martensite: very hard, strong and brittle. The hardness of martensite depends almost only on the carbon content, roughly HRC ≈ 20 + 60 √C: 45 for a 0.18 percent steel (too soft to harden usefully), 58 HRC for 0.40 percent (4140) and 74 HRC for 0.80 percent (a tool steel).
The steel families
| Steel | Type | Typical use | Strength (MPa) | Notes |
|---|---|---|---|---|
| 1018 | low carbon, 0.18 % C | bar, shafts, carburizing | 370 to 440 | not through-hardenable; case harden |
| 1045 | medium carbon, 0.45 % C | shafts, gears, general | 600 to 750 (Q&T) | water or oil quench; shallow hardening |
| 4140 | Cr-Mo, 0.40 % C | rods, shafts, bolts, tooling | 900 to 1 100 (Q&T) | oil quench; hardens through about 50 mm |
| 4340 | Ni-Cr-Mo, 0.40 % C | aircraft shafts, heavy sections | 1 000 to 1 300 (Q&T) | deep hardenability; tough |
| A36 / S355 | structural, 0.2 % C | frames, plate, sections | 250 / 355 yield | weldable; not heat treated |
| 304 / 316 | austenitic stainless | food, chemical, marine | 215 to 290 yield (annealed) | work hardens; no quench hardening |
| 17-4 PH | precipitation hardening | shafts, valves, aerospace | 1 000 to 1 200 | age at 480 to 620 °C |
| D2, M2, O1 | tool steels | dies, cutters | 60 to 64 HRC | high carbon and alloy; careful tempering |
The four-digit AISI/SAE code gives the family and the carbon content: 10xx plain carbon, 41xx chromium–molybdenum, 43xx nickel–chromium–molybdenum, the last two digits the carbon in hundredths of a percent (4140 is 0.40 percent). European names use the yield strength for structural steels (S355 = 355 MPa) or the carbon and alloys for others (42CrMo4 ≈ 4140). Always check the exact grade and condition on the mill certificate; strength figures depend on the heat treatment and the section.
The heat treatments
| Treatment | What is done | What it gives |
|---|---|---|
| Annealing | heat to austenite, cool very slowly in the furnace | soft, easily machined, stress-free |
| Normalizing | heat above the critical temperature, cool in still air | fine, uniform grain; the usual condition of structural steel |
| Stress relieving | hold at 550 to 650 °C, cool slowly | removes welding and machining stresses with little strength change |
| Hardening (quenching) | heat to austenite (840 to 870 °C for 4140), quench in oil, water or polymer | martensite: hard and brittle |
| Tempering | re-heat to 150 to 700 °C after quenching | trades hardness for toughness (see below) |
| Case hardening | add carbon (carburizing) or nitrogen (nitriding) to the surface, then harden | hard wear-resistant skin, tough core |
| Induction / flame hardening | heat only the surface rapidly, quench | local hardness, low distortion |
Quenching and tempering
Martensite as quenched is too brittle to use, so it is tempered: heated to a moderate temperature, which lets some carbon out as fine carbides. The higher the tempering temperature, the softer and tougher the steel. The result is a smooth trade between hardness (and strength) and toughness.
Time and temperature are interchangeable to a degree. The Hollomon–Jaffe tempering parameter P = T (20 + log10 t), with T in kelvin and t in hours, has the same value for equal results: tempering at 540 °C for 2 hours gives P = 16 508, equal to 10 hours at 513 °C or 1 hour at 552 °C. A short hot temper, a long cooler one: the same hardness, within the accuracy of the method.
Worked example: the press piston rod
Worked example: the 45 mm piston rod of the hydraulic press
- The rod (Cylinders and Actuators) carries 80.4 kN in compression: stress 51 MPa. That is small, but the rod is also hard-chromed, runs through a seal and takes side loads, so a medium-carbon alloy steel is chosen: 4140, quenched in oil from 850 °C and tempered at 540 °C for 2 hours.
- From the tempering curve: 36 HRC ≈ 336 HB, so the tensile strength is about 3.45 × 336 = 1 159 MPa and the yield about 85 percent of that: 985 MPa (typical).
- Safety factor on yield for the 80 kN load: 985 ÷ 51 = 19. The real limits are buckling (see the cylinder page), fatigue from pressure cycling and surface damage, not static strength.
- Surface: chrome plating 20 to 30 µm on a 0.2 µm Ra ground rod; the 36 HRC core supports the hard plating without denting (a soft core makes the plating crack).
- Section size: a 45 mm bar of 4140 hardens through in oil (see below), so the strength is uniform to the center.
Hardenability and section size
A quenched piece cools fastest at the surface and slowest at the core. If the core cools too slowly it forms pearlite or bainite instead of martensite and stays soft. Hardenability is the ability of a steel to form martensite at depth: alloys (chromium, molybdenum, manganese, nickel) slow the transformation and so raise it, which is why 4140 hardens through a 45 mm bar in oil while a 1045 plain carbon steel of the same size would harden only a few millimeters deep. The Jominy end-quench test measures the hardness along a bar quenched at one end and is the standard way of rating a steel; the ideal critical diameter DI of 4140 is about 4.5 inches (115 mm) against 0.9 in (23 mm) for 1045. A faster quench (water) hardens deeper but risks distortion and cracking; a slower one (oil, polymer) is gentler. Always quench in the medium the grade is designed for.
Case hardening and surface treatments
- Carburizing: low-carbon steel (1018, 8620) is heated to about 925 °C in a carbon-rich atmosphere. The case depth grows with the square root of time: roughly 0.635 √t mm with t in hours, so 8 hours gives 1.8 mm. The part is then quenched and tempered: surface 58 to 62 HRC over a tough core. Used for gears, pins and shafts.
- Nitriding: nitrogen is diffused in at 500 to 550 °C without quenching, giving a thin (0.1 to 0.6 mm) very hard skin with minimal distortion: good for finished parts and molds.
- Induction hardening: a coil heats the surface in seconds; fast and localized, for crankshafts, rails and spindles. The part must be medium carbon steel.
- Coatings: hard chrome, electroless nickel, PVD (TiN, DLC) and black oxide add wear or corrosion resistance but do not change the core.
Heat treatment brings distortion (parts grow, shrink and warp, so grind or machine after hardening), quench cracks at sharp corners and sudden changes of section (round every corner, avoid thin fins) and decarburization. Leave allowance (0.2 to 0.5 mm) for finish grinding and design the shape for even cooling. Safety: quench oil can ignite, hot parts burn, salt baths explode with water, and furnace atmospheres can be toxic: heat treat only with training and the proper equipment.
Related
- Next: Aluminum, Copper, Titanium and Other Alloys.
- Material Properties and the Material Properties Table.
- Fatigue and Failure.
- Materials and Processes: the whole guide.
FAQ
Why temper after quenching?
Untempered martensite is extremely hard but brittle and full of internal stress: it can crack on its own. Tempering relieves the stress and restores toughness at the price of some hardness.
Can I harden a 1018 bar?
Not usefully: with 0.18 percent carbon, the martensite reaches only about 45 HRC. Case harden it (add carbon first), or choose a steel with more carbon.
How do I check the heat treatment?
A hardness test (Rockwell C) at several points, compared with the specification, and a microstructure check on critical parts. Hardness converts to tensile strength by about UTS = 3.45 HB for steels.
Does welding affect heat-treated steel?
Yes: the heat-affected zone is re-hardened or softened. Weld before heat treatment where possible and use a procedure for hardenable steels (Welding Processes and Weld Design).
References
- ASM Handbook, Volume 4: Heat Treating, ASM International; Krauss, G., Steels: Processing, Structure, and Performance, ASM International.
- Callister, W. D., Rethwisch, D. G., Materials Science and Engineering: An Introduction, Wiley; Machinery’s Handbook, Industrial Press (steel properties and heat treatment).
- ASTM A370 (mechanical testing of steel), ASTM E18 (Rockwell hardness) and ASTM E140 (hardness conversion tables).
Last reviewed: October 2026. Hardness curves and strengths are typical; use the mill certificate and the heat-treater’s data.