November 3, 2026 · Indumor
In this chapter
A 1045 comes out of a water quench at 58 HRC and so brittle it will snap if dropped on the floor. Tempered at 200 °C it drops to 55 and takes a blow; at 600 °C it sits at about 20 and machines without fuss. The same steel, three different parts depending on how far it was heated and how it was cooled. That is what this chapter is about.
Everything turns on two temperatures. At 727 °C (A1) steel starts transforming into austenite; at A3, which falls with carbon from 912 °C in pure iron to 727 °C at 0.77 %, the transformation is complete. To harden, you go 30 to 50 °C past A3 and cool faster than the austenite needs to turn back into pearlite: what you get is martensite, the hard structure. Steels above 0.77 % (bearing and tool steels) are hardened just above A1, not Acm. Spheroidize annealing sits just below A1; carburizing goes to 900 to 950 °C, where austenite takes in more carbon.
The figure is for plain carbon steel. Alloying elements move the lines: nickel and manganese lower A3, chromium, silicon and molybdenum raise it; that is why 4140 is hardened at 830 to 870 °C (about 1550 °F). What each steel contains is in chapter 19.
Thirteen names that show up on drawings and heat-treat orders, with their temperature and what they do. Soak time comes from the rule of one hour per 25 mm of section (one hour per inch), with minimums.
| Treatment | Typical temperature °C | Cooling | Which steels | What it achieves |
|---|---|---|---|---|
| Full annealing | A3 + 30 to 50 °C (carbon steels: 800 to 900) | In the furnace, 10 to 20 °C/h down to 600, then air | Carbon and alloy steels, forgings, weldments | Maximum softness and ductility; refined grain; for cold forming |
| Spheroidize annealing | 680 to 720 (just below A1), 4 to 20 h, or cycling about A1 | Slow, in the furnace | Over 0.5 % C: tool and bearing steels (52100, O1, D2) | Globular cementite: best machinability; the +AC condition they are bought in |
| Normalizing | A3 + 30 to 80 (1045: 840 to 880; 4140: 850 to 880) | Still air | Carbon and low-alloy up to 0.5 % C; forgings, castings, weldments | Fine, uniform grain; a bit harder than annealed; erases the thermal history |
| Hardening (quenching) | A3 + 30 to 50 (medium carbon: 820 to 880; tool steels: 790 to 1230 by type) | Water, brine, oil, polymer, air or salt | Over 0.3 % C: medium carbon, alloy, tool and martensitic stainless | Martensite: maximum hardness, brittle; always followed by tempering |
| Tempering | 150 to 250 (tools); 500 to 680 (Q&T); 2 h minimum | Air | Every hardened steel | Removes brittleness and sets the hardness; high-alloy tool steels get 2 or 3 tempers |
| Stress relieving | 550 to 650 (carbon steels); on Q&T parts, 30 °C below their tempering temperature | Slow in the furnace to 300, then air | Weldments, heavy roughing, castings | Lowers residual stress without changing hardness; stability before finishing |
| Quench and temper (Q&T) | Quench from 820 to 880 + temper at 540 to 680 | Oil or water, then air | 1045, 4140, 4340, 8640, 42CrMo4, 34CrNiMo6 | 25 to 40 HRC with toughness: shafts, grade 8.8 to 12.9 bolts, con rods, spindles |
| Carburizing | 880 to 950 (gas: 900 to 930), 2 to 24 h | Direct oil quench, or reheat and quench from 780 to 820 | Up to 0.25 % C: 1010 to 1020, 8620, 16MnCr5, 20MnCr5 | 0.5 to 2 mm case at 58 to 62 HRC over a tough core; temper at 150 to 200 |
| Carbonitriding | 780 to 880 in gas with ammonia, 1 to 4 h | Oil | Low carbon: 1010 to 1020, 12L14, sheet parts | 0.1 to 0.6 mm case, 58 to 62 HRC, less distortion; small parts in volume |
| Nitriding | 500 to 570, 10 to 90 h in gas; salt bath 570 for 1 to 3 h; plasma 400 to 550 | Slow in the furnace or air; no quench | With Al, Cr, Mo or V: 41CrAlMo7, 4140, 4340, H13, martensitic stainless | 0.1 to 0.6 mm case at 600 to 1200 HV depending on the steel; minimal distortion; done after Q&T and machining |
| Induction or flame hardening | 850 to 950 in seconds, surface only | Water or polymer spray | Medium carbon: 1045, 1055, 4140, 1144; ductile iron | 1 to 5 mm case (flame up to 10) at 50 to 60 HRC; untouched core; temper at 150 to 200 |
| Austempering | Austenitize at 850 to 900; salt bath at 260 to 400 until transformed | Air, no tempering | 0.5 to 1.0 % C in thin sections (springs, blades); ductile iron (ADI) | Bainite: 40 to 55 HRC with toughness; little distortion and no cracks |
| Martempering | Austenitize; salt at 180 to 250 until the section evens out; then air and a normal temper | Salt, then air | Alloy and tool steels of awkward shape (52100, D2) | Same hardness as a direct quench with minimal distortion and cracking risk |
e: largest section of the part (mm). Minimums: 0.5 h for austenitizing, 2 h for tempering, 1 h for stress relief. Counted from when the part reaches temperature, not from when the furnace was loaded.
| Largest section mm | Austenitizing h | Tempering h | Stress relief h |
|---|---|---|---|
| 6 | 0.5 | 2 | 1 |
| 12 | 0.5 | 2 | 1 |
| 25 | 1 | 2 | 1 |
| 40 | 1.6 | 2 | 1.6 |
| 50 | 2 | 2 | 2 |
| 75 | 3 | 3 | 3 |
| 100 | 4 | 4 | 4 |
| 150 | 6 | 6 | 6 |
| 200 | 8 | 8 | 8 |
Counted from the moment the part reaches temperature. High-alloy tool steels need only 20 to 30 min at austenitizing temperature; tempering, never less than 2 h.
Maximum hardness is set by carbon and nothing else: 0.2 % gives 45 HRC, 0.45 % gives 58, and from 0.6 % up you reach 64 or 65. Alloying elements decide something different, how deep that hardness goes and with which quenchant. A 1045 needs water and still will not harden at the center of a 40 mm bar; a 4140 is oil-quenched and hardens through at 50 mm; a 4340, at 100 mm.
| C % | Maximum hardness HRC | At 50 % martensite HRC | Example steels |
|---|---|---|---|
| 0.10 | 38 | 26 | 1010 (carburized only) |
| 0.15 | 42 | 29 | 1015 |
| 0.20 | 45 | 31 | 1020, 8620 (core) |
| 0.25 | 48 | 34 | 1025, 25CrMo4 |
| 0.30 | 51 | 37 | 1030, 4130 |
| 0.35 | 54 | 40 | 1035, C35E |
| 0.40 | 56 | 43 | 4140, 4340, 1040 |
| 0.45 | 58 | 45 | 1045, C45E |
| 0.50 | 60 | 47 | 1050, 6150 |
| 0.55 | 62 | 50 | 55Cr3 |
| 0.60 | 64 | 51 | 1060, 5160 |
| 0.70 | 65 | 54 | 1070 |
| 0.80 | 66 | 54 | 1080, 1084 |
| 1.00 | 66 | – | 52100, W1 (retained austenite) |
| Medium | Severity H | Speed | Which steels | Watch out for |
|---|---|---|---|---|
| Brine (water with 10 % salt), 20 °C | 2 (still) to 5 (agitated) | Very fast | Low-hardenability carbon steels, simple shapes, W1 | High cracking risk; corrodes the tank |
| Water, 20 to 40 °C | 1 (still) to 1.5 (agitated) | Fast | 1035 to 1060, induction-hardened 1045, parts without abrupt section changes | Distortion and cracking on complex shapes; not for alloy steels |
| Quenching oil, 40 to 80 °C | 0.25 (still) to 0.5 (agitated) | Medium | Alloy steels (4140, 4340, 8620, 52100) and O1, D2 tool steels | Smoke and fire: extraction and a lid; bath volume matters |
| Polymer (PAG), 5 to 30 % | 0.2 to 1 by concentration | Between oil and water | Oil replacement without fire; induction; thick 1045 and 4140 | Check concentration and temperature; drag-out with the parts |
| Still or forced air | 0.02 to 0.08 | Slow | Air-hardening: A2, D2, H13, M2 in vacuum, small 420 | Minimal distortion; limited section |
| Pressurized gas in a vacuum furnace (N₂, 2 to 10 bar) | 0.1 to 0.5 by pressure | Slow to medium | Tool and high-speed steels: D2, H13, M2; clean parts | No scale or decarburization; done by the heat-treater |
| Molten salt, 180 to 400 °C | No H value: the part evens out before transforming | Stepped | Martempering and austempering: 52100, D2, springs | Hazardous bath (nitrates); heat-treaters only |
Indicative H (in 1/inch), from Grossmann's tables for brine, water and oil; it depends on agitation and bath temperature, and today's fast oils exceed those values.
The temperatures in the next table come from the standards and the mill catalogs; the final hardness depends on section, furnace and temper, which is why it is a range. D2, H13 and M2 are tempered two or three times: the first temper turns retained austenite into fresh martensite that has to be tempered again. To go from HRC to Brinell or Vickers, see chapter 21.
| Steel | Austenitize °C | Quench | As-quenched HRC | Temper °C | Final hardness HRC | Notes |
|---|---|---|---|---|---|---|
| 1045 / C45E | 820-860 | Water (oil for thin parts) | 55-58 | 550-650 | 17-24 | Will not through-harden over 25 mm; induction gives 55-58 HRC on the case |
| 4140 / 42CrMo4 | 830-870 | Oil | 54-58 | 540-680 | 28-36 | Through-hardens to 50 mm; nitrides well |
| 4340 / 34CrNiMo6 | 830-860 | Oil | 55-58 | 540-660 | 30-40 | Through-hardens to 100 mm |
| 8620 carburized / 20NiCrMo2-2 | Carburize at 900-930; quench from 780-840 | Oil | Case 62-65 | 150-200 | Case 58-62; core 20-40 | Usual case 0.8 to 1.5 mm |
| 52100 / 100Cr6 | 830-860 | Oil (or salt at 180-220) | 64-66 | 150-180 | 60-64 | Bearings: 62 ± 2 HRC; 15 to 30 min at temperature |
| O1 / 1.2510 | 790-820 | Oil | 63-65 | 150-250 | 57-63 | 20 to 30 min at temperature; low distortion |
| D2 / 1.2379 | 1000-1040 | Air, oil or pressurized gas | 62-64 | 180-250 (or 500-530) | 58-61 | Preheat at 650 and 850; temper twice |
| H13 / 1.2344 | 1020-1050 | Air, oil or gas | 50-54 | 550-620, 2 or 3 times | 44-52 | Preheat in two steps; double temper is mandatory |
| M2 / 1.3343 | 1190-1230 | Salt at 550 then air, or vacuum with gas | 63-65 | 540-570, 3 times for 1 h | 63-66 | Secondary hardening: tempering raises the hardness |
| 420 stainless / X20Cr13 to X46Cr13 | 980-1050 | Oil or air | 48-56 | 150-300 | 46-54 | Do not temper between 425 and 600: corrosion resistance and toughness drop; hardness depends on carbon |
| 17-4PH H900 | Solution 1040 + 482 °C, 1 h | Air (solution: air or oil) | 33-36 (condition A) | 482 | 40-44 | ASTM A564: 40 HRC minimum, Rm 1310 MPa |
| 17-4PH H1025 | Solution 1040 + 552 °C, 4 h | Air | 33-36 (condition A) | 552 | 35-38 | Tougher; 35 HRC minimum, Rm 1070 MPa |
| 17-4PH H1150 | Solution 1040 + 621 °C, 4 h | Air | 33-36 (condition A) | 621 | 28-33 | Toughest and most machinable; 28 HRC minimum |
Guide values at the surface of parts up to 50 mm; in larger sections the core ends up softer depending on hardenability.
Worked example · a 4140 shaft quenched and tempered to 28-32 HRC
A 4140 shaft, 60 mm by 900 mm, ordered quenched and tempered to 28 to 32 HRC. Soak at 850 °C: 60 ÷ 25 = 2.4 h, rounded to 2.5 h from the moment the furnace is back at temperature. Quench in agitated oil, hung vertically so it does not bow. Temper at 600 °C for 2 h, twice: by the table, that leaves about 30 HRC, and the shaft is ground afterwards.
Hot steel glows, from a red that is barely visible at 550 °C to white at 1300 °C. A smith hardens a chisel "at cherry red", about 780 °C, judging the part in the shade: in direct sunlight the same color shows up 50 °C later. Temper colors are a different thing, a very thin oxide film that grows on polished steel between 200 and 350 °C and shifts from straw to blue as it thickens. They let you temper carbon-steel tools without a furnace: polish one face, heat slowly from the opposite side and quench when the color you want reaches the edge.
| Color | Temperature °C | Color | Temperature °C |
|---|---|---|---|
| faint red, barely visible | 550 | light red | 850 |
| brown red | 630 | orange (yellowish red) | 900 |
| dark red | 680 | dark yellow | 1000 |
| dark cherry red | 740 | light yellow | 1100 |
| cherry red | 780 | yellowish white | 1200 |
| light cherry red | 810 | white | 1300 |
Indicative: charts differ from one another by up to 50 to 100 °C, and the color depends on the light in the shop.
| Color | Temperature °C | Tools |
|---|---|---|
| pale straw | 200 | Scrapers, carbon-steel lathe tools, gauges |
| dark straw | 220 | Taps, dies, drills, reamers, milling cutters |
| golden yellow | 230 | Precision punches, die knives |
| golden brown | 240 | Blanking dies, hammer faces, punches |
| reddish brown | 250 | Wood bits, small cold chisels |
| purple | 270 | Cold chisels for steel, center punches, butcher knives |
| violet | 280 | Axes, wood chisels, press tools |
| dark blue | 290 | Screwdrivers, wrenches, table knives |
| light blue | 320 | Springs, wood saw blades |
| blue gray | 340 | Leaf springs, parts that need toughness over hardness |
Indicative: other charts put purple at 280 °C and dark blue at 310 °C; the color also depends on the time at temperature and on the steel.
When a part needs a hard surface over a tough core, only the case is hardened. Carburizing adds carbon to a steel that has little of it (8620, 1020) and then quenches it; depth grows with the square root of time. Nitriding runs at 500 to 570 °C, needs no quench and barely distorts, so it is done on a part already quenched, tempered and machined; it needs steels with chromium, molybdenum, vanadium or aluminum. Induction hardening heats the case in seconds and quenches it with a spray: it is the treatment for 1045 and 4140 shafts that carry bearings.
d: case depth (mm); t: hours at temperature; K: 0.45 at 870 °C, 0.54 at 900 °C, 0.635 at 925 °C (0.025 in/√h), 0.75 at 955 °C (Harris). Total case, not effective: the effective case (to 550 HV, about 50 HRC) is 25 to 40 % less.
| Time h | 870 °C mm | 900 °C mm | 925 °C mm | 955 °C mm |
|---|---|---|---|---|
| 1 | 0.45 | 0.54 | 0.64 | 0.75 |
| 2 | 0.64 | 0.76 | 0.90 | 1.06 |
| 4 | 0.90 | 1.08 | 1.27 | 1.50 |
| 6 | 1.10 | 1.32 | 1.56 | 1.84 |
| 8 | 1.27 | 1.53 | 1.80 | 2.12 |
| 12 | 1.56 | 1.87 | 2.20 | 2.60 |
| 16 | 1.80 | 2.16 | 2.54 | 3.00 |
| 24 | 2.20 | 2.65 | 3.11 | 3.67 |
Harris equation with the K values from the formula. Total case; the effective case is 25 to 40 % less.
Worked example · an 8620 pinion with a 1 mm case
An 8620 pinion that calls for a 1.0 mm effective case. Since the effective case is about 25 % less than the total, aim for 1.3 mm total. At 925 °C: t = (1.3 ÷ 0.635)² = 4.2 h, plus the time to bring the load up; at 900 °C it would be 5.8 h. If the flanks are ground after hardening, add what will be removed, 0.1 to 0.2 mm per flank.
The 1 h per 25 mm rule with the minimums from the table.
The heat treater does what the paper says. A complete order carries the steel with its standard and the certificate if there is one; the condition it arrives in; the treatment by name (quench and temper, carburize and harden, nitride); the hardness with its tolerance and where it is measured; the case depth and how it is measured (effective, to 550 HV); the areas to mask from carburizing, such as threads and holes, and the stock left for grinding. A 60 by 900 mm shaft can come out of the oil with 0.3 mm of bow: 0.2 mm per side for grinding is normal, and after nitriding a polishing pass is enough. Grinding allowances are in chapter 14.
Between 820 and 860 °C (about 1500 to 1580 °F), quenched in water (or oil for thin parts) and then tempered. It comes out at 55 to 58 HRC on the surface; tempered at 550 to 650 °C it ends up at 17 to 24 HRC (210 to 245 HB).
Cherry red. At 700 °C it is dark red, at 900 °C orange and at 1100 °C light yellow. Judge the colors in the shade; in strong light they look darker and you underestimate the temperature.
Hardening is the fast cool from austenite that leaves the steel hard and brittle. Quench and temper adds a high temper (500 to 680 °C): the steel ends up less hard, 25 to 40 HRC, but tough, which is the condition shafts and quality bolts are used in.
Sources: ASM Handbook, vol. 4, Heat Treating (1991); EN 10083-2 and EN 10083-3 (hardening and tempering temperatures); EN 10084 (case hardening); ISO 683-1, 683-2 and 683-3; ISO 4957 (tool steels); ASTM A564 (17-4PH); SAE J1268 (hardenability bands) and SAE J406; Hodge and Orehoski, Trans. AIME 167 (1946); DIN 17022 (withdrawn; old reference for heat-treating practice); Harris, F. E., Case Depth: An Attempt at a Practical Definition, Metal Progress (1943); Tabellenbuch Metall (heat and temper colors); Machinery's Handbook (temper colors).
We machine them, manage the heat treatment and grind them to size in our shop in Córdoba, Argentina.
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