Turning and Machining of Stellite Alloy Clad Components

Literature Overview

This 2005 technical paper by Liu Guangyao and Sun Changrong from Dongfang Turbine Works addresses the practical machining challenges encountered when producing finished components from Stellite alloy (cobalt-based hardfacing alloy) clad parts. Stellite alloys (typically Stellite 6, Stellite 21, or Stellite 26) are widely used for wear and corrosion resistance in turbine components, pump impellers, valve seats, and other critical rotating equipment. After the cladding process, the hardened overlay layer must be machined to achieve precise dimensional tolerances and surface finishes, which presents significant challenges due to the extreme hardness, work-hardening tendency, and poor thermal conductivity of cobalt-based alloys.

Material Characteristics and Machining Challenges

Stellite alloys exhibit a unique combination of properties that make machining difficult:

The primary machining challenges include: severe tool wear due to high hardness and abrasive carbide particles (Cr23C6, Cr7C3, and Co3W); rapid work-hardening at the machined surface creating a hardened layer 0.1 to 0.5 mm deep that further accelerates tool wear; poor heat dissipation leading to high interface temperatures between the tool and workpiece; and the formation of built-up edge and adhesion of material to the tool rake face.

Cutting Tool Selection and Optimization

Tool Material Selection

The paper discusses various tool material options with their performance characteristics:

For Stellite alloy turning, CBN inserts have proven to be the most effective option for finishing operations, offering tool lives 5 to 10 times that of conventional carbide tools. The recommended CBN grade for Stellite alloys is a fine-grain, high-toughness type (such as SG710 or equivalent) that provides a balance between wear resistance and fracture resistance.

Cutting Parameter Optimization

The optimal cutting parameters for turning Stellite alloy clad components, as derived from the paper's experimental data:

A critical finding from the study is that the depth of cut should not be less than 0.5 mm during roughing to avoid cutting into the work-hardened layer created by the previous pass. Cutting below the work-hardened zone ensures that the tool engages relatively softer material, significantly extending tool life.

Machining Strategy and Process Design

Rough Machining Phase

The rough turning operation removes the bulk of the excess overlay material (typically 2 to 5 mm) with a CBN or coated carbide insert. The key principle is to maintain a depth of cut greater than the work-hardened layer thickness. The recommended insert geometry features a positive rake angle (gamma = 15 to 20 degrees) to reduce cutting forces, a large nose radius (R = 1.5 to 3.0 mm) to distribute the cutting load, and a negative clearance angle (alpha = 5 to 8 degrees) for edge strength.

Semi-Finishing Phase

After rough turning, a semi-finishing pass with a CBN insert removes 0.3 to 1.0 mm to establish near-final dimensions. The cutting speed is increased to 60 to 80 m/min, and the feed rate is reduced to 0.1 to 0.2 mm/rev. The coolant flow rate should be high (10 to 20 L/min) to effectively cool the cutting zone and flush away swarf.

Finishing Phase

The final finishing pass with a fine-grain CBN insert achieves the required surface finish (Ra 0.8 to 1.6 micrometers) and dimensional accuracy (plus or minus 0.05 mm). The cutting speed is increased to 80 to 120 m/min with a very light feed of 0.05 to 0.1 mm/rev and a depth of cut of 0.1 to 0.2 mm.

Coolant and Lubrication Strategy

The paper emphasizes the importance of high-pressure, high-flow coolant delivery in Stellite alloy machining. Conventional flood coolant is insufficient due to the poor thermal conductivity of the alloy, which causes heat to concentrate at the tool-workpiece interface. The recommended approach includes:

• Coolant pressure of 3 to 5 MPa delivered through internal coolant passages in the tool holder
• Coolant temperature maintained at 25 to 35 degrees Celsius
• Coolant composition: 5 to 8 percent soluble oil with extreme pressure additives
• Flow rate: minimum 15 L/min for roughing, 20 L/min for finishing
• Avoidance of dry machining except for specific CBN applications where controlled chip formation is required

Study Reflection

This paper from 2005 captures essential machining knowledge that remains relevant in modern practice. The systematic approach to tool selection, parameter optimization, and process design provides a solid foundation for machining operations on cobalt-based alloy clad components. The emphasis on work-hardening management—maintaining sufficient depth of cut to avoid cutting into the hardened layer—represents a practical insight that cannot be easily derived from theoretical analysis alone. For modern applications involving high-performance turbine components with tighter tolerances and more demanding surface finish requirements, these principles must be extended through the use of advanced CBN grades, adaptive control systems, and in-process monitoring techniques.