CNC Machining 38CrMoAl (41CrAlMo7): Parameters, Tooling & Surface Finish Tips

The first time a batch of 38CrMoAl bar stock hit incoming inspection, the drawing said 41CrAlMo7, but the heat number read 38CrMoAl. A quick call confirmed the grades were equivalent for the part—but only if the shop understood the aluminum content, the nitriding response, and how this steel behaves under a carbide insert. That conversation shapes everything below.

Reading the Material Certificate Before You Cut

38CrMoAl is a Chinese nitriding steel covered by GB/T 3077. Its European counterpart is 41CrAlMo7 under EN 10085. Both belong to a small family of steels deliberately alloyed with aluminum to promote a hard, stable nitride case during gas, ion, or salt-bath nitriding. The aluminum is not there for machinability—it actually complicates chip formation slightly—but it is essential for achieving surface hardness in the 900–1100 HV range after nitriding.

Chemical composition boundaries for 38CrMoAl according to GB/T 3077 are shown below. Single values are maximums. This table is a useful incoming inspection reference because a heat that drifts high on aluminum or low on chromium will respond differently in the nitriding furnace, even if it machines acceptably.

For comparison, the EN 10085 grade 41CrAlMo7-10 (material number 1.8509) typically tightens the chromium band to 1.50–1.80% and allows molybdenum up to 0.40%. If your drawing lists 41CrAlMo7 and you receive a 38CrMoAl cert, check the heat analysis against the more restrictive of the two standards before accepting the substitution. When in doubt, our CNC machining steel reference covers other nitriding and case-hardening grades used for similar work.

Mechanical Properties in Two Distinct Conditions

Machinists often see 38CrMoAl in the quenched and tempered (QT) condition, because that is how it is delivered for pre-nitriding machining. The core properties below apply to round bar in the 16–40 mm diameter range after oil quenching from 940°C and tempering at approximately 640°C, per GB/T 3077. These are minimum specified values unless otherwise indicated. For larger sections, hardenability limits will reduce core strength at the center.

Two numbers matter most for planning. First, the QT core hardness of 250–300 HBW is still machinable with coated carbide, but it is not free-cutting. Second, the nitrided surface hardness of 900–1100 HV is far beyond what any conventional cutting tool can remove economically. That single fact dictates the entire manufacturing sequence: machine in the soft state, nitride, then finish by grinding, honing, or lapping.

CNC Machining Parameters for the Pre-Hardened State

The table below gives starting speeds, feeds, and depths of cut for 38CrMoAl in the annealed or as-rolled condition, approximately 170–229 HBW. Use coated carbide inserts with a hard substrate and a TiAlN or AlCrN PVD coating. If the stock is in the QT condition at 250–300 HBW, reduce surface speed by 20–30% and keep feed rates in the lower half of the ranges to control heat at the cutting edge.

What Happens If You Try to Machine After Nitriding

Once the white layer and diffusion zone are present, the surface is typically 900 HV or higher. A carbide insert at 60 m/min will fail within seconds, not minutes. The only practical material-removal processes are cylindrical grinding, internal grinding, surface grinding, honing, or lapping. Leave 0.05–0.10 mm per side on functional surfaces for post-nitriding finishing. If threads, grooves, or seal areas must remain soft, mask them before nitriding; re-cutting a nitrided thread is not a production option.

Shop Floor Pitfalls That Show Up Without Warning

• Built-up edge from the aluminum content. The 0.70–1.10% Al can promote adhesion on the cutting edge at lower speeds. Use positive rake geometry, polished chip grooves, and adequate coolant concentration. If you see a shiny aluminum-colored smear on the insert, increase speed or change to a sharper edge preparation.
• Surface tearing on finishing passes. A dull tool or insufficient depth of cut can smear the surface instead of cutting cleanly. Keep finishing DOC at 0.3 mm or more to stay below the work-hardened layer left from roughing.
• Distortion after nitriding. Parts with uneven cross-sections can move 0.02–0.05 mm during nitriding. Stress-relieve before finish machining, and leave enough stock on critical dimensions. Do not assume the nitrided part will be drop-in ready without a final grind.
• Forgetting that the core is still only 250–300 HBW. The hard case is thin. If the part sees high contact stress in a gear root or spline, the core can yield under the case. This is a design consideration, not a machining fix.

Where 38CrMoAl and 41CrAlMo7 Actually Show Up

This grade is not a general-purpose alloy steel. It gets specified when a component needs a wear-resistant surface, good fatigue strength, and a moderately tough core—without the distortion risk of carburizing. The aluminum-containing nitriding steel produces a harder case than many plain Cr-Mo steels, which is why designers select it for sliding and rotating contact conditions.

• Plastic injection molding screws and barrels. Glass-filled nylon or PBT wears plain 4140 quickly. A nitrided 38CrMoAl screw with a 0.3–0.4 mm case routinely outlasts a hard-chrome-plated screw in abrasive compounds.
• Extruder shafts for rubber and polymer processing. The combination of core toughness and surface hardness prevents fatigue cracking at keyways while resisting abrasive wear from fillers.
• Hydraulic piston rods and guide rails. When induction hardening is impractical for long, slender parts, nitriding 38CrMoAl gives a uniform case along the full length with minimal distortion.
• Textile machinery spindles and cam plates. High-cycle, low-lubrication contact points benefit from the low friction and high hardness of a nitrided surface.
• Gearbox shafts and pump rotors in demanding service. The nitrided surface improves scuffing resistance, while the QT core handles torsional loads.

For a broader comparison with carburizing and induction-hardening steels, our materials reference outlines where each heat treatment route makes sense.

Pre-Nitriding Machining Sequence Worth Following

If you are quoting a 38CrMoAl part with a 0.30 mm case depth and a 0.8 µm Ra ground finish, the plan is not the same as machining 4140. A reliable sequence looks like this:

• Rough machine all surfaces, leaving 1.5–3.0 mm stock on functional faces and 0.3–0.5 mm on non-critical dimensions.
• Stress relieve at 550–600°C for 2–4 hours if the part has thin walls, deep pockets, or asymmetric features. Cool in air.
• Semi-finish and finish machine to within 0.05–0.10 mm per side of final dimensions. Leave the pre-nitriding surface at Ra 1.6 µm or better; rough surfaces nitride unevenly.
• Mask threads, seal grooves, and any area that must remain soft. Copper plating or a commercial masking paste works for gas nitriding.
• Nitride at 510–530°C for the specified time. Keep temperature uniform to limit distortion.
• Finish grind or hone critical diameters and faces. Remove no more than 0.05–0.10 mm per side to stay within the hard case.

This sequence is standard for nitriding work, but small shops sometimes skip the stress relief and then wonder why a long shaft bows after nitriding. The extra furnace cycle is cheap insurance on a part that may already have 10–15 machining operations invested. See our surface treatment notes for more on how nitriding interacts with final dimensions.

Engineering Judgement on Grade Selection

38CrMoAl is not always the right answer. It costs more than 42CrMo4 or 40Cr, and the aluminum content can make deep-hole drilling and threading more sensitive to tool condition. If your part only needs a wear surface and does not need the high hardness of a full nitriding steel, a lower-cost Cr-Mo steel with a shorter nitriding cycle may be sufficient. But when the drawing calls for a 0.3 mm case at 950 HV minimum and the part will run millions of cycles against a polymer melt or a sliding seal, 38CrMoAl earns its keep.

For machinists, the biggest lesson is to respect the aluminum. It is not a free-machining additive. It changes chip flow, promotes built-up edge at low speeds, and makes post-nitriding machining impossible. Plan the job around the soft state, leave the right grinding allowance, and treat the nitriding furnace as part of the manufacturing process—not an afterthought.

If you have a drawing with 38CrMoAl, 41CrAlMo7, or a similar nitriding steel and need production machining, send the material specification, quantities, tolerances, and surface finish requirements. Our team will review the part geometry and heat treatment plan before quoting. Request a CNC machining quote here and include the drawing so we can assess the pre- and post-nitriding operations in one workflow.