Cutting tool material development

Aluminum is getting a lot of press in automotive circles. To many, the recently announced Ford F-150 represents the advent of the Aluminum Age of automotive construction. But that’s not the entire story. While body panels and engines make the shift toward aluminum, many components within motor vehicles will remain stalwartly in the steel camp for the foreseeable future.

Drivetrain components such as drive shafts and transmission gears continue to be made of steel. Even with the 2015 F-150, the body panels might be aluminum, but the suspension uses significantly more high-strength, difficult-to-cut steel than the outgoing model.

With more high-strength steel content and tighter tolerances on parts, cutting steel is going to continue to be challenging for automotive component manufacturers, especially with materials such as ISO P25, a steel with hardness from 150 to 350 Brinell.

Since it was introduced in 1970, the P25-coated, cemented-carbide grade insert has been known for its versatility and ability to handle a wide range of steel-types, parts, operations, and volumes. Six generations of P25 grades have been developed in the four decades since the insert’s launch.
 

Tougher materials

Steel cutting is difficult, regardless of the form of the metal or the alloy in question. The cutting edge of the tool must be able to withstand a combination of high force (203,100psi to 449,600psi or 1,400N/mm² to 3,100N/mm²) and high temperature (reaching up to 1,832°F or 1,000°C). Tool inserts that can handle those stresses will offer longer tool lives and higher cutting speeds. Sandvik Coromant engineers designed new inserts to deal with the wide range of conditions that steel-cutting manufacturers are likely to face. The GC4325 offers new edge-treatment processes and controls; a stronger, more balanced insert substrate; and more advanced post-treatment processes of inserts.

Steel turning, the most common type of machining, involves a range of considerations. Shops deal with a variety of materials including unalloyed steels, low-alloyed, and high-alloyed steels – from soft and sticky to hard and abrasive, from low-spec to strong. The World Steel Association estimates that more than 3,500 different grades of steel exist, each offering unique physical, chemical, and environmental properties.

In addition to the wide variety of materials, machinability ratings differ with bar, tube, forgings, castings, rolled, drawn, untreated, hardened/tempered, and pre-machined workpieces. Some automotive components come from bar stock; many more are from forgings.
 

Making the cut

In the cutting process, workpiece material first seizes as it approaches the edge, at the primary shear zone. Then, after the edge-line, the top (rake) face is host to the flow of the chip. This is the secondary shear zone where considerable energy is transformed when the material is forced to yield.

The combination of cutting-edge geometry and tool material, along the short distance from the edge-line through the chipbreaker on the indexable insert, determines the effect of shear stress. The concentrated load of the chip is forced across the face, ideally transporting away most of heat generated in the process. Friction is a major factor making the design of the contact with the chip and the texture of the insert’s surface instrumental in performance.

The destructive mechanisms in that flow zone are mainly chemical dissolution and some plastic deformation. Diffusion wear is a major characteristic of turning at higher cutting speeds along with abrasive wear, mainly on the clearance face of the insert. These wear types are continuous and controllable. What should not be tolerated is uncontrolled, discontinuous wear such as dominant plastic deformation, thermal cracking, and notch wear.
 

Coating innovation

In designing the new steel-turning grade, Sandvik Coromant considered a number of factors: raw materials, hard-particle mix, binder, grain-size, substrate-gradient design, and coating technology.

One of the main tool-material innovations has been in the coating’s structure and adhesion. Alumina, a well-established tool-material, is an oxide-based ceramic and a proven coating material. An effective barrier to the insert, it is chemically inert and has low thermal conductivity making it resistant to crater wear through diffusion. It has a high melting point, more than 3,632°F (2,000°C), and its close-packed crystal structure makes it very hard.

Conventional aluminum-oxide coating uses a randomized crystal direction. These coatings provide satisfactory machining properties, but leave the path open for some weaknesses when subjected to hostilities at the edge line and rake face.

Sandvik Coromant’s Inveio is a new alumina-coating tool material with uni-directional, not randomized, crystal orientation. By controlling the growth of crystals in a uniform way throughout the chemical vapor deposition (CVD) process, the aluminum-carbide crystals line up, facing the same way. This gives the coating a strong, uniform structure that better withstands the forces and temperatures of the flow-zone. In effect, a new type of rake-face contact surface has been created.

Tightly packed atom-planes, directed toward the contact-zone with the chip, improve crater-wear resistance – the main diffusion-wear mechanisms caused by heat and pressure in steel turning. Heat can be dissipated more efficiently along the crystal-plane, out of the flow-zone with the chip.
 

Further benefits

Another effect was resistance to micro-cracking. Cracks grow along the weakest part, horizontally, improving the durability of the coating through slow, uniform and controlled wear. False crater wear from flaking also has been reduced. The inner coating, a fine-grained titanium carbonitride with a columnar structure, resists abrasive wear. This leads to longer tool-life, improved process security, and a potential for higher cutting speed. The edge-line of the insert, a major determiner in both part quality and the rate of edge breakdown, is more resilient, giving improved predictability of the insert capability and part results.

The insert substrate furthers counter impression tendencies, giving higher resistance to plastic deformation. The edge withstands higher temperatures without weakening, and a new substrate-gradient type acts as a micro-crack blocker to improve durability.

Micro-geometry shape and sizes at the edge have been engineered for smoother cutting action. The edge-line has become tougher and more resilient to wear, particularly for intermittent cuts.

This new generation in coated-insert technology represents substantial progress in tool-material technology, one that could fundamentally alter turning tool performance in automotive machining.

Sandvik Coromant
www.sandvik.coromant.com

IMTS 2014 booth #W-1500