1018 vs 1045 steel: which carbon steel should a machined part be made from?

What does the extra carbon actually change in 1018 vs 1045?

Carbon is the only designed difference between the two grades, and it controls almost everything else: strength, the ability to harden by quenching, weldability and machinability. 1045 carries between two and a half and three times the carbon of 1018 with the same manganese range.

Both are plain carbon steels in the AISI/SAE 10xx series, where the last two digits give the nominal carbon in hundredths of a percent. Their composition limits are listed in the grade tables of ASTM A29/A29M and in SAE J403: per ASTM A29, 1018 (UNS G10180) runs 0.15–0.20% carbon and 1045 (UNS G10450) runs 0.43–0.50% carbon, both with 0.60–0.90% manganese. Neither grade has a specified minimum strength of its own. The grade designation fixes the chemistry; the strength you get depends on the bar condition you buy, which is why a drawing that says only “1045” leaves the most important property open.

That distinction matters for three reasons. First, the carbon in 1045 is enough to form hard martensite when the steel is quenched from the austenitizing temperature, so the part can be hardened where it wears; 1018 cannot, unless carbon is first added to the surface by carburizing. Second, the same carbon makes the heat-affected zone of a weld harden and crack, so 1045 is not a casual welding steel. Third, more carbon means more pearlite in the microstructure, which raises cutting forces and tool wear.

Which one can be hardened: carburized 1018 or induction-hardened 1045?

Both can end up with a hard wear surface, by different routes: 1018 has to be carburized first because it lacks the carbon to harden, while 1045 can be induction or flame hardened as supplied. Neither grade through-hardens in a thick section, so a requirement for a hard core needs an alloy steel instead.

Carburizing 1018. The part is held in a carbon-rich atmosphere so carbon diffuses into the surface, then quenched. The result is a high-carbon case over a soft, tough core, and as industry guidance a carburized and quenched 1018 surface typically reaches about 58–62 HRC. The whole part goes through the furnace, so every surface grows and moves; threads and bores that must stay soft or on size need a stop-off (copper plating or paint) called out on the drawing. Carburizing is the right answer for small pins, bushings and cam followers that need a hard skin and can tolerate whole-part distortion.

Induction or flame hardening 1045. Only the zone that needs to be hard is heated and quenched, so a journal, a seal diameter or a cam lobe can be hardened while the rest of the shaft stays as drawn. A typical induction-hardened 1045 surface lands around 50–58 HRC, with case depth set by the frequency and heating time. Because the heat is local, distortion is usually smaller than with carburizing, which is why 1045 is the default for shafts that need hard bearing or seal journals.

Through hardening. Plain carbon steel is shallow hardening. The ASTM A255 end-quench (Jominy) test shows hardness in 1045 falling off within a short distance of the quenched end, so in a solid shaft of any real diameter the center stays soft even after a water quench. Water quenching 1045 also invites cracks at cross holes, keyways and sharp shoulders. If the part must be hard through its section, or must be quenched and tempered to a guaranteed core strength, specify an alloy grade; and if it must be 1045 with a known hardening response, buy 1045H to ASTM A304, which adds a guaranteed hardenability band to the chemistry.

One combination to avoid: carburizing 1045. It already has more carbon than a carburizing grade, and adding more makes the case brittle and prone to retained austenite and grinding cracks. Carburize a low carbon grade, induction harden a medium carbon grade.

How do 1018 and 1045 differ in machining, distortion and plating?

1018 cuts faster and with less tool wear, but its softness makes it gummy, with long stringy chips and built-up edge at low speed. 1045 cuts slower and wears tools faster, yet often gives a cleaner finish and holds a sharp edge on threads better.

The machinability ratings in the table are relative values against 1212 free-machining steel, and they are a fair guide to cycle time: 1045 typically runs at lower surface speed than 1018 for the same tool life. Neither grade is a free-machining steel. If cycle time dominates and the part is not welded or hardened, a resulfurized grade may serve better: C1117 behaves like a free-cutting 1018 and still carburizes, and 1144 plays the same role against 1045. The sulfur that helps the chip hurts weldability and transverse ductility, so the substitution should be approved, not assumed.

Distortion deserves attention on long shafts. Cold drawn bar of either grade carries residual stress from drawing, and a keyway or a flat milled along one side can let it bow; why that happens and how to specify stress relieved bar is covered in the hot rolled vs cold rolled steel guide. For induction-hardened features, leave grind stock on the journal and grind after hardening if the final tolerance is tight, since the hardened zone moves.

Plating behaves the same on both grades while they are soft. Once either grade has been hardened, by carburizing or induction, the hard zone moves into the range where electroplating can cause hydrogen embrittlement. ASTM B633 includes post-plating treatment to reduce that risk, and its scope says steel stronger than 1700 MPa (about 46 HRC) should not be zinc electroplated to it at all, which is below the hardness a carburized case or an induction-hardened journal usually reaches. A soft 1018 spacer normally needs no bake; on a hardened part, either mask the hardened zone from plating or agree the finish and the bake with the plater before the drawing is released. The hydrogen embrittlement baking guide covers the callout, and the zinc plating vs galvanizing guide covers the finish choice itself.

What should you know before sending a 1018 or 1045 part to this plant?

Neither 1018 nor 1045 is on the list of bar grades the CNC turning page shows running today, and the factory page does not list heat treatment among the processes at the lead CNC plant. Say in the RFQ whether a listed grade is acceptable and how the hardening requirement should be handled.

The CNC turning page lists the steel bar running at this plant as 52100, C1117 and 12L14, plus ASTM A513 DOM tube. For a soft turned spacer or bushing drawn in 1018, the useful question for the RFQ is whether C1117 or 12L14 is an acceptable substitute; for a carburized pin, C1117 is the closer match because it is itself a carburizing grade. There is no listed grade that stands in for 1045, and 52100 is a high-carbon bearing steel rather than a substitute, so a 1045 shaft should be quoted to the grade on the drawing and the RFQ should ask how the bar would be sourced.

CNC turning and milling run at our lead CNC plant; die casting, stamping and surface finishing go through partner plants we manage, as the factory page explains. Hardening is not on that list, so a carburized or induction-hardened part needs the requirement written on the drawing and confirmed in the quote rather than assumed. The CNC milling page is explicit that milling here is a second operation on castings, not general billet machining, so a 1018 bracket cut entirely from bar or plate is better placed with a dedicated machining shop. On finishing, zinc plating, RoHS-compliant blue trivalent chromate and oil-dip rust protection are on the surface finishing menu our partner plants apply, and the quote names where each step is done. Material certification is available on request and a first-article inspection report covers new parts, as listed on the quality page. When the drawing is ready, send it for a quote.