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Alloy Steel

Alloy steels are medium-carbon steels (0.25–0.50 % C) with 1–3 % of chromium, molybdenum, nickel, manganese or boron added for hardenability: the alloy slows the transformation so the part hardens through the section in an oil quench instead of only at the surface, and the molybdenum resists softening on tempering. Quench-and-tempered they cover 28–50 HRC and 700–1,500 MPa yield, which makes 4140, 4340 and their EN / JIS twins 42CrMo4 and SCM440 the default for shafts, gears, bolts, tooling and any highly loaded part that is heat-treated.

Popular alloy steel grades

Compare in the explorer
GradeYield, MPaTensile, MPaDensity, g/cm³MachinabilityWeldabilityBest for
41404806907.85GoodFairDefault Q&T shafts, bolts, fixtures; 28–50 HRC
43406209507.85FairPoorLarge sections, high toughness, fatigue-critical
41304407007.85GoodGoodWeldable tube and sheet; aircraft, roll cages
41505208607.85FairPoorMore carbon for harder wear surfaces
42CrMo45508507.85FairFairEN twin of 4140 on European drawings
34CrNiMo65609007.85FairPoorEN twin of 4340; heavy shafts and gears
86405708607.85GoodFairNi-Cr-Mo bolts and medium-section parts

All 232 alloy steel grades

Choose alloy steel for

Shafts, spindles, axles, gears, splines, high-strength bolts and studs, hydraulic rods, fixtures, machine-tool components, weldable high-strength tubing (4130) and any part where a specified hardness of 28–50 HRC through the section, with toughness and fatigue strength to match, is the requirement.

Look elsewhere when

The part is never heat-treated and 1018 or 1045 does the job, only the surface needs to be hard (carburized 8620 or 16MnCr5, or induction-hardened 1045), the requirement is above 55 HRC or wear resistance (tool steel A2, D2, or 52100), it must stay strong above about 450 °C (H13, or the Cr-Mo pressure steels), or corrosion resistance is needed (17-4 PH and 410 stainless give 4140-class strength).

Yield strength against elongation at break, Alloy steel (through-hardening) among every steel grade

No materials in view have both of these properties.

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Alloy steel by application

Shafts and spindles
4140 pre-hardened (28–32 HRC) machined to size, or quench-and-tempered to 36–42 HRC then ground. Over about 75 mm diameter, or when the shaft is fatigue-critical, 4340 / 34CrNiMo6 hardens through where 4140 cannot.
Gears without carburizing
4140 or 4150 through-hardened to 38–45 HRC for moderate loads, or 4140 nitrided for a 60 HRC skin with no quench distortion; 4340 when the tooth must survive shock. Carburized 8620 or 18CrNiMo7-6 remains the answer for high-contact-stress gearing.
High-strength bolts and studs
4140 / 42CrMo4 for property class 10.9 and 12.9 and ASTM A193 B7 studs, 4340 or 8740 for 1,030 MPa (150 ksi) aerospace-type fasteners; roll the threads after heat treatment for fatigue.
Welded high-strength structure
4130 normalized: welds with ER80S-D2 or ER70S-2 at low preheat, then stress-relieve; used for tubular frames, roll cages and aircraft engine mounts. 4140 weldments need 250–300 °C preheat and a post-weld temper.

Working with alloy steel

Machining
Best machined pre-hardened at 28–32 HRC (4140 PH, 42CrMo4 +QT): the chip breaks, the finish is good and there is no post-machining quench to distort the part. Annealed stock is gummy; above 40 HRC use CBN or ceramic inserts and plan on grinding. Machinability is about 65 % of 1212 for 4140, less for 4340. Leave 0.2–0.4 mm for grinding if hardening after machining.
Welding
4130 welds readily (ER80S-D2, preheat 100–150 °C for thick sections). 4140 and 4340 are weldable but crack-prone: preheat 250–300 °C, low-hydrogen filler, slow cool and a temper afterwards — and the weld zone will not match the base hardness. Never weld a part after final heat treatment except for repair.
Heat treatment
Austenitize 830–870 °C, oil quench (4140 can be polymer-quenched in small sections), temper 200–650 °C to hit the hardness: for 4140 about 50 HRC at 200 °C, 40 HRC at 450 °C, 30 HRC at 600 °C. Avoid tempering 4140 / 4340 in the 260–370 °C embrittlement range. Nitriding (500–550 °C, 0.2–0.5 mm case, 55–65 HRC surface) suits parts already tempered above that temperature.
Finishing
Black oxide, phosphate, zinc or zinc-nickel plating, hard chrome for hydraulic rods; bake plated parts above 40 HRC within hours to drive out hydrogen (hydrogen embrittlement is real in 12.9 bolts). Nitriding and QPQ double as corrosion protection. 4140 rusts like 1018 — there is no bright-and-bare option.

Other steel series

Alloy Steel — frequently asked

What is alloy steel?

Alloy steels are medium-carbon steels (0.25–0.50 % C) with 1–3 % of chromium, molybdenum, nickel, manganese or boron added for hardenability: the alloy slows the transformation so the part hardens through the section in an oil quench instead of only at the surface, and the molybdenum resists softening on tempering. Quench-and-tempered they cover 28–50 HRC and 700–1,500 MPa yield, which makes 4140, 4340 and their EN / JIS twins 42CrMo4 and SCM440 the default for shafts, gears, bolts, tooling and any highly loaded part that is heat-treated.

Which grade of alloy steel should I use?

4140 (42CrMo4, SCM440, EN19) for most shafts, spindles, bolts and fixtures: through-hardens to about 60 mm, is sold pre-hardened at 28–32 HRC, machines well in that state and tempers anywhere from 22 to 50 HRC. 4340 (34CrNiMo6, EN24) when the section is larger, toughness at high strength matters or fatigue is the failure mode — the nickel gives hardenability past 100 mm and better impact at 40+ HRC. 4130 (25CrMo4) is the weldable one: lower carbon, used normalized for tubing, roll cages and aircraft structure. 8640 and 8740 are the triple-alloy Ni-Cr-Mo grades for bolts and medium sections; 4150 and 4145 add carbon for harder wear surfaces; 4330V and 300M are the modified 4340s for the heaviest loads. 40Cr, 35CrMo and 42CrMo are the GB equivalents of 5140, 4135 and 4140.

Which grade of alloy steel is the strongest?

52SiCrNi5 has the highest yield strength in the series at 1,700 MPa; 30NiCrMo16 follows at 1,620 MPa. Values are for each grade's default condition.

Which grade of alloy steel is easiest to machine?

4024 (machinability very good, 72/100), then 4028 (72/100).

Is alloy steel heat-treatable?

Yes. These grades gain their strength from heat treatment; the conditions and their properties are listed on each grade page.

4140 or 4340?

For sections up to about 60–75 mm and hardness to 45 HRC, 4140 — lower alloy content, easier to machine and weld. 4340 adds about 1.8 % nickel, which pushes through-hardening past 100 mm and improves toughness and fatigue life at 40–50 HRC; it is the choice for large shafts, crankshafts, landing gear (as 4340M / 300M) and anything shock-loaded at high strength. Both are ordered quench-and-tempered and machined pre-hardened.

What does pre-hardened 4140 mean?

Bar quench-and-tempered at the mill to 28–32 HRC (about 750–900 MPa yield), so it can be machined to final size with no further heat treatment and no quench distortion. On EN drawings it is 42CrMo4 +QT. Use it when the finished part is happy at 30 HRC; if 40 HRC or more is needed, rough-machine, harden, then grind.