Practices in CNC Machining Martensitic Stainless Steel Components

Martensitic stainless steel, known for its high strength and hardness, presents machining challenges that require precise process control and tool selection. This article explores the CNC machining process for low-volume parts of SS17-4 PH, detailing the key considerations, machining challenges, and solutions to ensure batch production.

Project Background

• Material: SS17-4 PH；
• Order Quantity: 500 pieces；
• Precision Requirements:Hole diameter tolerances, 1 location at ±0.001in, 2 locations at ±0.002in; Surface A includes 4 × #4-40 UNC-2B threaded holes with a depth of 0.35in; Internal hole surface roughness: Ra 0.8; external surface roughness: Ra 1.6；
• Applications: Components of energy extraction equipment；

Material Analysis

SS17-4 PH, a martensitic stainless steel, offers high strength and hardness, with a tensile strength range of 1,310–1,450 MPa and hardness of HRC 40–44. Its overall performance surpasses austenitic stainless steels. While its corrosion resistance is inferior to austenitic grades, it remains sufficient for weak corrosive environments such as atmospheric or freshwater exposure.

For this project, tolerance control is moderately challenging but requires consistency and stability across batch production. The internal hole surface finish (Ra 0.8) indicates its use for assembly to ensure smooth operation, while the external surface finish (Ra 1.6) meets general standards for non-functional areas.

SS17-4 PH, valued for its corrosion resistance, high strength, and aesthetics, is widely used in aerospace structures, automotive engine components, energy equipment, and medical devices.

Machining Challenges for SS17-4 PH Parts

Compared to austenitic stainless steel, SS17-4 PH exhibits higher strength and hardness but lower toughness and ductility, leading to poor machinability. Key challenges include:

• SS17-4 PH components would experience severe surface hardening during machining.
• Difficult chip control, affecting machining stability.
• Poor thermal conductivity causes elevated temperatures at the cutting surface and tool.
• Surface quality assurance requires optimized parameters.

To address these, it is necessary to select cutting tools with high hardness and excellent wear resistance, and reasonably adjust the cutting parameters to enhance machining efficiency and quality.

Process Route for Low-Volume Production

A precise and structured machining process is essential to achieve the required precision and surface quality. The following steps outline the machining approach for these SS17-4 PH components.

Step 1: Material Cutting

• Procure a square stock (2.5m length).
• Cut along the length direction with a tolerance of +0.2mm.

Step 2: 4-Axis CNC Machining for Surface B and Side Ear Structures

• Fixture design: Steel fixture with 1-mold-5-cavity layout.
• Clamping method: Toad-style clamping jaws with 2.0mm thickness at clamping points.
• Precision control: Upgrade dimensions #5 and #9 in the control drawing to positioning dimensions for subsequent steps, adjusting tolerance to ±0.015mm.

Step 3: 4-Axis CNC Machining for Surface A and Side Hole Structures

• Fixture design: Steel material with 1-mold-5-cavity layout.
• Clamping method: Aluminum single-ear expansion block for side-push clamping.
• Optimization: The side-push block is aligned with the product’s center hole and incorporates a crescent-shaped relief structure to prevent distortion of the center hole’s roundness during clamping.

Tool Selection

Selecting the right tools is critical for efficient machining and extended tool life, especially when working with hardened materials like SS17-4 PH.

Drill Bits

Pre-drill center holes for the three large holes on Surfaces A/B before rough milling.

• Hole diameters: ∅9.78mm and ∅10.0mm.
• Tool material: Solid carbide internal-cooling drill bits.
• Advantages: The sharp cutting edges, combined with high-pressure internal water cooling at high rotational speeds, effectively manage thermal buildup. In the same time, the flushing chips are done through pressurized water flow, meeting the drilling requirements for stainless steel materials.

Milling Cutters

• Tool type: Coated carbide cutters specialized for stainless steel.
• Hardness: HRC 65.
• Design: Unequal helix angles (1° difference) with spiral chip flutes for enhanced chip breaking and evacuation.
• Coating: Ceramic coating (Al₂O₃, TiO₂, TiN).
• Properties: High hardness and exceptional wear resistance. For instance, alumina coatings can achieve hardness ratings between 2000-3000HV, effectively resisting abrasive wear on stainless steel surfaces during machining or operational use. Additionally, ceramic coatings exhibit superior chemical stability, capable of withstanding corrosion from acids, alkalies, salts, and other chemical substances while protecting the stainless steel substrate.

Programming Strategy

• Roughing: UG’s adaptive dynamic high-efficiency roughing.
• Benefits: Full-edge cutting for higher material removal rates, large depth-of-cut and feed rates for efficiency, and balanced tool load distribution to reduce wear.
• Finishing: Both the roughing and finishing processes employ a side cutting edge machining strategy to enhance machining accuracy and surface finish quality.

Final Implementation and Process Validation

Through precise process planning, tool selection, and programming, this machining solution ensures stable and efficient production of SS17-4 PH components.

Efficient and Stable Process

This solution employs 4-axis CNC machining, which has been validated through practical applications to demonstrate its advantages in efficiency, stability, and reliability.

Alternative process configurations can also be developed, including a 5-axis + 3-axis machining process or pure 3-axis machining. The final selection among these options will depend on the machine shop’s available equipment resources and the required process stability.

Fixture Design for Batch Production

Given the client’s annual recurring orders (1-2 times per year, 400-500 units per order), we have invested in premium materials and advanced techniques for the fixture manufacturing to ensure a long service life and sustained reusability across multiple production cycles.

Standardized process documentation provides detailed and consistent engineering guidelines for each production batch, guaranteeing repeatable manufacturing outcomes.

Through this case sharing, we hope to demonstrate WayKen’s professional capabilities in engineering project development. Thank you for your attention, and we look forward to in-depth collaborations on new project developments!