1020 vs 1045: Low-Carbon and Medium-Carbon Steels Compared

The chemistry gap

	SAE 1020	SAE 1045
Carbon	0.17–0.23%	0.43–0.50%
As-rolled strength	~350–450 MPa	~600 MPa class
Hardening route	Case hardening only	Through / surface hardening
Weldability	Excellent	Needs preheat and care

What 1020 does well

1020 is a welding and forming steel: machine bases, brackets, pins and shafts that see modest loads. It cannot be through-hardened — there is not enough carbon — but it is an excellent carburizing steel. After carburizing and hardening, the case reaches roughly 58–62 HRC while the low-carbon core stays soft and shock-tolerant: the classic recipe for pins, bushings and light gears. It also bends, deep-draws and welds without special procedure, which is why it dominates general fabrication.

What 1045 does well

1045 carries enough carbon to harden throughout in moderate sections — realistically a few tens of millimetres with an oil or water quench — and responds well to induction or flame hardening, giving cases around 55–60 HRC over a tough normalised or quenched-and-tempered core. That combination makes it the default shaft, axle, gear-blank and bolt steel of medium-duty machinery.

The weldability trade

The same carbon that makes 1045 hardenable raises its carbon equivalent and its cracking risk. Welding 1045 typically calls for preheat (often in the 150–250 °C range depending on thickness and restraint), low-hydrogen consumables and sometimes a stress-relief. None of that applies to 1020, which is routinely welded with no preheat at all.

Choosing

• Welded structures, formed parts, carburized wear parts → 1020.
• Shafts and machine parts needing hardened wear surfaces or Q&T strength → 1045.
• Thick sections needing uniform hardness through the section → neither; step up to a chromium-molybdenum steel such as 4140, where hardenability comes from alloying rather than carbon.

Condition is half the specification

Both grades are sold hot-rolled, normalised, cold-drawn and (for 1045) quenched-and-tempered, and properties move more with condition than with the small chemistry differences between mills. 1045 in the Q&T condition reaches tensile strengths in the ~700–850 MPa region with far better ductility and fatigue behaviour than the as-rolled bar of identical chemistry — the reason shaft specifications name a condition, not just a grade. Cold drawing raises strength and improves finish on both steels at some cost to ductility, and is the standard supply form for small turned parts. When comparing quotations, compare grade + condition + standard together; "1045" alone describes a chemistry, not a performance.

Near-names to watch

Specifications often arrive with neighbouring grades: 1018 and 1022 behave essentially like 1020 (the differences are manganese and small carbon shifts), while C45 is the EN counterpart of 1045 and 1045 itself overlaps the GB 45 grade. These substitutions are normally safe for general engineering, but the governing standard should still be named on the order, because delivery-condition definitions and test requirements — not chemistry — are where near-equivalents quietly diverge.