Martensitic Stainless 420 and 440C: Korloy Machining Strategy for Hardened Cutlery Steels

Martensitic stainless steels occupy a unique position in the machining world: they combine the corrosion resistance of stainless steel with the ability to be hardened to levels approaching tool steel. This dual nature means the same material demands completely different machining approaches depending on whether it has been heat-treated. Korloy offers specific solutions for both conditions across the 420 and 440C grades most commonly encountered in industry.

Understanding the Two Alloys

420 Stainless Steel

With 12-14% chromium and 0.15-0.40% carbon, 420 stainless achieves moderate hardness of 50-52 HRC after heat treatment. Its primary applications include plastic injection mold cavities, surgical instruments, valve components, and turbine blades. Most machining occurs in the annealed state (approximately 200 HB), with finish machining sometimes required after hardening for mold applications demanding precise dimensions post-heat-treatment.

440C Stainless Steel

The highest-carbon martensitic stainless (0.95-1.20% C, 16-18% Cr), 440C reaches 58-60 HRC when properly hardened. It is the standard choice for stainless steel bearings, high-quality knife blades, medical cutting instruments, and precision valve seats. The high carbon content forms large chromium carbide particles within the matrix, creating an inherently abrasive microstructure that poses unique tooling challenges.

Machining in the Annealed Condition

Both 420 and 440C in the annealed state behave similarly to austenitic stainless in terms of chip formation characteristics: they produce continuous chips, generate significant heat at the cutting edge, and tend toward built-up edge at lower speeds. However, their machinability rating is actually better than 304/316 grades due to the absence of extreme work-hardening.

The recommended Korloy grade for annealed martensitic stainless is PC9530. This PVD-coated carbide grade offers the combination of sharp cutting edge geometry, heat resistance, and anti-adhesion properties needed for stainless steel machining. Cutting speeds of 120-180 m/min are appropriate for both alloys in the soft condition, with positive rake chipbreakers providing clean chip control.

Machining in the Hardened Condition

420 at 50-52 HRC

At this moderate hardness level, coated carbide remains viable. Korloy PC5300, with its multi-layer CVD coating providing superior hot hardness and crater wear resistance, handles 420 at full hardness when speeds are reduced to 80-120 m/min. The key is accepting the speed reduction rather than forcing a carbide grade beyond its thermal limits. PC5300’s thick alumina layer acts as a thermal barrier, protecting the carbide substrate from the elevated temperatures generated by the hardened workpiece.

440C at 58-60 HRC

At this hardness level, conventional carbide fails rapidly due to the combined effect of high hardness and the abrasive chromium carbide particles embedded throughout the microstructure. Korloy KBN10M (CBN) is the correct solution for 440C in the hardened condition. CBN provides the hot hardness to machine at 58-60 HRC while resisting the abrasive wear that destroys carbide flanks within minutes.

Cutting speeds with KBN10M on hardened 440C range from 100-150 m/min for finishing operations, with depths of cut limited to 0.05-0.25mm. The stainless nature of the workpiece means dry cutting is preferred, though light mist coolant is acceptable for 440C since cutting temperatures are slightly lower than plain carbon hardened steels at equivalent hardness.

Chipbreaker Selection: NM Geometry

For hardened condition machining of both alloys, the Korloy NM chipbreaker is the mandatory choice. This geometry features a narrow negative land with controlled chip flow designed specifically for light depths of cut on hard materials. The NM profile keeps cutting forces directed into the insert body rather than creating bending stress at the edge, which is critical when machining materials above 50 HRC.

In the annealed condition, the MM chipbreaker (medium machining) provides better chip control for the longer, more continuous chips produced at higher speeds and feeds. Switch to NM only when transitioning to hardened workpiece machining.

Comparison Table: Annealed vs Hardened Approach

Common Failure Mode: Edge Chipping from Carbide Stringers in 440C

The most frustrating failure mode when machining 440C, particularly in the hardened condition, is random edge chipping that appears unrelated to parameters or tool life. The root cause is almost always chromium carbide stringers, elongated clusters of extremely hard carbide particles aligned along the rolling direction of the bar stock.

These carbide stringers act like embedded cutting tools in reverse: when the insert encounters a dense cluster, the localized impact stress at the cutting edge exceeds the fracture toughness of the tool material, causing a small chip to break away. The problem is worst in lower-quality 440C stock where carbide distribution is uneven.

Solutions for Carbide Stringer Problems

Edge honing: Apply a light hone (0.02-0.04mm K-land) to the cutting edge before use. This micro-chamfer distributes impact stress over a wider area, preventing the point-loading that initiates chips. For CBN inserts, request factory-prepared edges with appropriate hone specifications.

Tougher grade selection: If KBN10M chips persistently on 440C, move to KBN25M. The higher CBN content and metallic binder provide significantly better impact resistance at the cost of slightly rougher surface finish. For many 440C applications (bearing races, valve seats), the finish from KBN25M still meets requirements.

Speed adjustment: Moderately increasing speed (10-15% above baseline) can help by increasing cutting temperature enough to locally soften the matrix surrounding carbide particles, reducing impact severity. However, this must be balanced against accelerated overall wear.

Practical Tips for Production

When machining 420 for mold applications, perform roughing in the annealed state whenever possible, leaving only 0.2-0.3mm stock for finish machining after heat treatment. This approach maximizes tool life by keeping the heavy cutting in soft material while achieving final dimensions on the hardened workpiece with minimal CBN or premium carbide consumption.

For 440C bearing components, always verify the carbide distribution quality of incoming bar stock. Request ESR (electroslag remelted) or premium-grade material for critical applications. The improved carbide uniformity in premium stock dramatically improves tool life consistency and reduces the random chipping events that disrupt production planning.

Monitor tool wear patterns carefully. Progressive, even flank wear indicates correct parameters. Irregular chipping, cratering behind the cutting edge, or notching at the depth-of-cut line each indicate specific parameter adjustments needed. Korloy technical support can analyze worn inserts and recommend corrections specific to your operation.