Which carbon or alloy steel should you choose?

Yield strength across conditions and forms. (minimums) marks specified minimums.

Which carbon or alloy steel has the highest yield strength?

On guaranteed minimums, Alloy steel AISI 4340 is highest at 1,125 MPa in condition Z at 30 mm, then Alloy steel AISI 4140 at 740 MPa for 10 to 100 mm. Of the plain steels, cold-drawn Carbon steel 1045 guarantees 500 to 540 MPa and A36 250 MPa under 8 in thick. Test specimens go higher but are not guarantees: AISI H13 hot-work tool steel reaches 1,520 MPa (52 HRC, Uddeholm Orvar Supreme), Alloy steel AISI 4130 1,330 MPa (a hydrogen-study table) and 4140 1,235 MPa (measured in 69 MPa helium). D2 has only a compressive yield (2,200 MPa at 62 HRC), a different test.

The plain steels sit lower. A36 structural carbon steel is 250 MPa under 8 in thick and 220 MPa over. Carbon steel 1010 runs 170 to 345 MPa across hot-rolled, cold-drawn and tube forms. 1018 gives 240 MPa hot rolled and 370 MPa cold drawn, and 1215 shows 415 MPa cold drawn. For Carbon steel 1045, datasheet minimums run from 300 MPa hot rolled to 540 MPa cold drawn under 16 mm, with no quenched-and-tempered number.

How much ductility do you give up for strength?

You give up a lot, and 4340 shows it inside one grade. In condition T its minimum elongation is 13 % with tensile strength of 850 to 1,000 MPa. In condition Z the minimum is 5 % with tensile strength above 1,550 MPa. These are datasheet minimums.

The plain steels keep more stretch at lower strength. 1010 bar shows 22 to 30 %. A36 plate is specified at a minimum of 21 % in a 2 in gauge and 18 % in an 8 in gauge, for plate up to 24 in wide; the SSAB footnote lowers the requirement by two percentage points for wider plate. 4140 shows a 12 % minimum at 930 to 1,080 MPa tensile in its 10 to 100 mm sections. One laboratory test of high-strength 4140 in 69 MPa hydrogen gas gave only 2.6 % elongation, which is one test and not typical service.

What does heat treatment do to 4130, 4140, 4340 and 1045?

It moves strength a long way in the alloy steels. 4130 normalised at 870 °C has 435 MPa yield, 670 MPa tensile and 197 HB. Quenched and tempered with a temper near 635 °C it has 635 MPa yield and 820 MPa tensile. The same source gives two higher strength levels, 1,050 and 1,330 MPa yield, for tempers anywhere between about 250 and 540 °C, and does not say which temper gives which.

4140 is quenched in oil from 830 to 860 °C and tempered at 540 to 680 °C, and the supplier says to avoid tempering between 200 and 420 °C because of temper brittleness. 4340 is hardened at 815 to 845 °C in oil or polymer and tempered at 455 to 650 °C.

The 4140 and 4340 comparison is narrower than it looks. The datasheets give the same minimums at 100 mm (4340 condition U, 740 MPa) and at 150 mm (condition T, 665 MPa). The 4340 advantage in our data is its higher conditions: 835 MPa at 63 mm (condition V) and 925 to 1,125 MPa at 30 mm (conditions W to Z), against 4140's 740 MPa for the whole 10 to 100 mm range. The cost is toughness: the 4340 Charpy minimum falls from 42 J at U to 9 J at Z. As general engineering knowledge, not in our data, 4140 and 4340 are chosen over 1045 for thicker sections because they harden deeper, which is hardenability.

1045 can be hardened too. The datasheets give normalising at 850 °C, hardening at 800 to 830 °C with a water quench, and tempering at 150 to 200 °C, but our data has no strength or hardness for the quenched-and-tempered state. The low-carbon 1010 and 1018 are not through-hardened in our data; their profiles name only surface treatments.

How hard can these steels get, and what are tool steels for?

Tool steels are the hard end. D2 works at 58 to 61 HRC when tempered at 200 to 500 °C, against 24 to 38 HRC for 4340 bars (quenched and tempered); condition Z is 444 HB, above that. Its 63 to 65 HRC is the state before tempering and not a working hardness. Tempering at 600 °C leaves 51 HRC and at 700 °C only 35 HRC; the tempering tables do not state the hardening temperature. AISI D2 cold-work tool steel is a cold-work steel for blanking and cutting tools; its datasheet calls its cracking resistance rather limited.

AISI H13 hot-work tool steel is the hot-work steel for die-casting, extrusion and forging tools. The figures here are Uddeholm Orvar Supreme. At a 610 °C temper it measures 45 HRC, and its modulus falls from 210 GPa at 20 °C to 140 GPa at 600 °C. The supplier warns that tempering between 425 and 550 °C causes temper brittleness.

P20 plastic mould steel is the pre-hardened mould steel. It is supplied at 28 to 34 HRC (280 to 325 HB), so blocks are machined and polished without a further heat treatment. A reheat above the supply temper softens it: 42 HRC at 500 °C, 36 HRC at 600 °C, 28 HRC at 700 °C. It rusts unless protected, and very thick blocks harden less through.

Which of these steels weld and machine well?

Our profile says A36 is readily welded by all welding processes, and 1018 is described as easy to weld. 1215 is the opposite: the supplier says it is difficult to weld and not recommended for case hardening.

As carbon and alloy content rise, the weld needs more control. 1045 needs a preheat of about 200 to 300 °C and a post-weld heat treatment at 550 to 660 °C, kept below the tempering temperature. 4130 needs the same preheat, baked consumables, ER80S-D2 filler and slow cooling, because it is prone to hydrogen cracking. The tool steels need hot work: D2 at a working temperature of 200 to 250 °C, H13 at 325 to 375 °C, and P20 preheated to 315 to 370 °C and kept under 565 °C.

For machining, 1215 is the free-machining grade at 135 % on a basis of 1212 = 100, against 70 % for 4130 annealed and cold drawn. P20 is rated 60 to 65 % against a 1 percent carbon steel, a different reference that cannot be compared with those two.

What traps catch people buying these steels?

The first trap is not asking what condition the bar arrives in. For 4340, conditions X, Y and Z are typically ordered annealed for machining and heat treated afterwards. As general engineering knowledge, not in our data, 4140 and 4340 are often bought annealed or pre-hardened, so settle who heat treats the part, and to what hardness, before you cut.

Several traps are general engineering knowledge, not in our data. Cold-drawn bar can move when keyways are cut. Leaded free-machining alternatives to 1215 exist, and our 1215 profile holds no lead value. Welded A36 and 1018 structures can fracture in a brittle way in the cold, and 4140 and 4340 are harder to weld because their carbon and alloy content raises hardening and cracking risk. A36 lists chemistry only as maximums, so two plates can differ in carbon and manganese.

Higher strength raises hydrogen risk: in one study of 4140 in hydrogen gas, the lower-strength condition lost about 15 percent of its notched tensile strength and the high-strength condition about 60 percent. Tool steels can crack or distort in the quench; the H13 profile recommends an interrupted quench. Our data holds no welding procedure for 4140 or 4340, so qualify one before welding either.

Terms used

Yield strength

the stress where a metal starts to deform for good. Design loads are usually set below it.

Quenching and tempering

heating steel until it hardens, cooling it fast, then reheating it to a lower temperature to trade some hardness for toughness.

Ruling section

the bar thickness that a strength guarantee applies to. Datasheets give lower minimums for thicker sections, which cool more slowly in the quench (general engineering knowledge, not in our data).

Condition letters T to Z

the codes one 4340 datasheet uses for strength levels. Each letter has its own ruling section, minimum strength and impact energy.

Charpy or Izod impact energy

the energy a notched bar absorbs when a pendulum breaks it. More energy means tougher. The two tests are not interchangeable.

Preheat

warming the joint before welding so it cools slowly and does not crack.

What to check before you order

1. Ask for the grade and the condition together: hot rolled, cold drawn, normalised, annealed, or quenched and tempered, with the temper temperature. Ask for the mill test certificate.
2. Quote the specification: AMS 6346 bar, 6350 sheet or 6371 tube for 4130; AMS 6349 or 6382 for 4140; AMS 6414 or 6415 for 4340.
3. For 4140 and 4340, ask for the condition letter and ruling section, and for minimum yield, tensile, elongation and Charpy energy at that section. Say whether numbers are minimums or typical values.
4. For welded parts, ask for the preheat, filler and post-weld heat treatment, and check them against a qualified welding procedure.
5. For P20, D2 and H13, ask for the delivered hardness and the tempering temperature, and avoid reheating above the supply temper.

What do we not know yet?

• 1045 has no quenched-and-tempered strength or hardness in our data. 4140 has no annealed strength and 4340 no normalised strength.
• No welding or machinability data is stored for 4140 or 4340, and no hardenability or carbon-equivalent figure for any grade. 1010 and 1045 hardness is stored under a different property name, so no single Brinell chart can include them.
• D2 has no tensile or yield strength and no toughness number, H13 has no elongation, and no wear rate is stored for any tool steel.
• Impact energy exists for 4130, 4140 and 4340 only, mixing Izod and Charpy values with no test temperature, so toughness cannot be compared across the carbon steels.
• Welding preheats, fillers and machinability ratings come from commercial guides and datasheets, not from welding procedure specifications.
• Cost, fatigue and most corrosion rates are not stored, and the cast irons are outside this article.

Questions people ask