Alloy Steel CNC Processing Service

The grades on our floor are 1018, 1045, 4130, 4140, 4340, A36 and tool steel. They are not interchangeable. 1018 is low-carbon, machines like butter, and carries almost no hardenability. Use it for brackets and covers where strength is not the argument. 1045 gives you more carbon and roughly double the tensile strength, and it takes a decent induction-hardened surface on a shaft.

Chromoly 4130 sits in the middle. It welds well, responds to normalizing, and suits tube-and-fitting work, landing gear links and airframe brackets. When a part has to carry load through a thin wall, 4130 is often the cleanest answer. It machines at around 180–220 m/min with coated carbide, which is not slow.

The 4140 and 4340 pair is where most of our quarry comes from. Both are through-hardening grades with chromium and molybdenum, and 4340 adds nickel for deeper toughness. Pre-hard 4140 at 28–32 HRC is a common starting point for mold bases, hydraulic blocks and gripper jaws, because you machine it near final size and skip the quench. Fully hardened 4140 or 4340 at 45–55 HRC is a different job entirely, and we plan it accordingly.

A36 and tool steel cover the ends. A36 is structural plate, easy to cut, poor for anything with a fit. Tool steel like D2 or H13 shows up when the part itself is a die insert or a wear pad. H13 at 48–52 HRC cuts cleanly with the right setup, but it punishes light finishing passes and dull tools.

Hardness is the first number to check, not the alloy family. At 28–32 HRC, pre-hard 4140 cuts like a medium carbon steel. Above about 40 HRC, the cutting edge starts to see a hard, abrasive chip and heat goes into the tool rather than the part. Above 50 HRC, you want cubic boron nitride or ceramic inserts, smaller depths of cut, and a rigid setup.

Surface speed is the second lever. For 4130 and 4140 in the normalized or pre-hard condition, we run coated carbide at roughly 150–220 m/min with a feed of 0.1–0.25 mm per tooth. In hardened 4340 at 50 HRC, that drops to 60–100 m/min and the feed stays modest so the edge does not chip. If a shop quotes the same numbers for both, question the quote.

Chip control matters more than most people expect. Alloy steel produces a stringy, hot chip at low carbon and a segmented, brittle chip at high hardness. We adjust the depth of cut and use high-pressure coolant so the chip breaks and leaves the pocket. A nest of stringy chips around a boring bar causes chatter, and chatter is what eats the tolerance.

Heat treatment moves dimensions. A quench-and-temper cycle can shift a 200 mm shaft by a few hundredths of a millimeter, and that is not a machining error. For tight parts we machine in the annealed or pre-hard state, let the heat treater do the cycle, and then finish-grind or hard-mill the critical diameters. That sequence is what keeps a ±0.005 mm callout real instead of optimistic.

Five-axis work solves most of the hard geometry. A 4140 hydraulic block with cross-drilled galleries and angled faces is a single setup on one of our 16 simultaneous 5-axis machining centers, which removes the re-fixturing error that shows up on a 3-axis sequence. We also run 16 mill-turn centers for shaft-type work, so a turned diameter and a milled flat come off the same machine without a second datum.

Size limits are real, so check them early. The largest travel we offer is 4,000 × 400 × 150 mm for long, slender parts. Mid-size work goes on 750 × 1,150 × 550 mm and 600 × 600 × 600 mm machines. Compact housings run on 500 × 500 × 450 mm and 500 × 310 × 200 mm platforms, and a Ø400 mm rotary table handles round features that need indexing.

Pre-hard 4140 is our default for structural parts that need strength without a post-heat-treat step. It saves a cycle and a straightening operation. If a drawing calls for 55 HRC through the core, we will say so and quote hard milling plus grinding, because pretending it machines like 30 HRC helps nobody.

Inspection is where the tolerance is proven. We check raw material certificates on receipt, monitor in-process on the critical bores, and run a final inspection on every part before it ships. Reports are available on request. Our historical qualification rate is 99.99%, and we would rather flag a risky callout during DFM than discover it at final inspection.

Do not specify 4140 for a part that never sees load. If a cover plate only keeps dust out, 6061 aluminum is lighter, cheaper to cut and finishes faster. We see 4140 on cosmetic brackets, and it is a waste of both material and cycle time.

Hardened material is a poor fit for thin walls and deep pockets. A 45 HRC wall under 2 mm will deflect under cutting pressure and may need a grinding step that doubles the cost. If the design allows, keep hardenable sections thick and put the tight tolerance on a ground diameter instead of a milled pocket.

Post-machining heat treatment is a schedule risk. A quench-and-temper cycle adds days and can introduce distortion that forces rework. For prototypes, pre-hard material usually gets you a functional part faster. Reserve full hardening for the production run once the geometry is frozen.

Welded assemblies with machined interfaces need a plan. Weld, stress relieve, then machine the interface. If you machine first and weld after, the joint pulls the bore out of position. That single sequencing decision accounts for more scrap on 4130 frames than tool wear does.