A355A most closely matches ASTM A355, "Standard Specification for Steel Bars, Alloy, for Nitriding," Class A. A note on how we got there: the data behind the previous version of this page was internally inconsistent — a commented-out composition table that didn't match any real A355 chemistry, a Werkstoff number that actually belongs to an unrelated stainless alloy, and boilerplate text describing it as a plain low-carbon steel. Rather than repeat any of that, this identification is based on independently sourced ASTM A355 Class A chemistry, which lines up closely with this page's designation. We're presenting it with that context so you can weigh it accordingly rather than treating it as beyond question.

What makes a nitriding steel like this one distinct is aluminum. A355 bars carry a deliberate 0.80–1.20% aluminum addition alongside chromium and molybdenum, because aluminum reacts readily with nitrogen during gas or salt-bath nitriding to form a dense layer of very hard, very fine aluminum nitride particles at the surface — routinely reaching case hardness in the 65–72 HRC range without any quench-and-temper cycle. That gives finished parts an exceptionally hard, wear-resistant surface over a tough, machinable core, which is why this family of steel shows up in crankshafts, camshafts, cylinder liners, gears, and precision spindles that need to resist wear and fatigue without the distortion risk of conventional through-hardening.

Before nitriding, A355A machines like a mid-carbon chromium-molybdenum alloy steel in the 0.38–0.43% carbon range — broadly comparable to 4140 in the annealed or normalized-and-tempered condition. The aluminum addition that defines this grade doesn't come into play until the nitriding step itself; at the bulk-hardness level a machinist encounters before heat treatment, it has little effect on cutting forces or tool wear. All machining should be completed before nitriding, since the nitrided case is thin, extremely hard, and not intended to be cut — post-nitride work is limited to grinding or lapping if any stock removal is needed at all.

Chip formation is continuous to segmented with a properly matched chipbreaker, similar to other Cr-Mo alloy steels in this hardness range, and tool wear is driven mainly by the chromium-bearing carbides rather than by anything unusual in the alloy. As with 4140 and similar grades, light cuts or a dull edge can burnish the surface and leave it harder to engage cleanly on the next pass, so keeping the tool sharp and the feed rate consistent matters more than raw cutting speed for tool life and finish.

A coated carbide grade with good hot hardness handles this material well across turning, drilling, and milling; there's nothing about the pre-nitride condition that calls for specialized tooling beyond what a shop would already use on a comparable alloy steel.