Mechanism of Wear in HSS cutting tools

Introduction • Metal cutting puts extreme demands on the tool and tool material

through conditions of high forces, high contact pressures, high temperatures, and intense chemical attack by difficult to cut work materials

• Most often cutting tools are used close to their ultimate resistance against these loads, especially to the limiting thermal and mechanical stresses.

• Tool wear describes the gradual failure of cutting tools due to regular operation

• Presently, no viable theories exist for predicting tool wear on the basis of properties of tool and work material

Why High Speed Steel

• In spite of the increasing use of high performance tool materials high speed steels (HSS) are still frequently used.

• The relatively high toughness and the possibility of economic manufacturing of tools with complicated geometries justify the use of HSS

• Powder metallurgical grades in combination with Electro Slag Heating (ESH) and Physical Vapour Deposition (PVD) coating technologies has further improved the performance of HSS cutting tools

The cutting process in brief

• Through plastic shear of the work material and sliding of work material against the tool flank and rake face a characteristic temperature profile is established

• The principle heat sources are located at the primary shear zone in the forming chip and in the frictional contact between chip and tool (secondary shear zone), and the highest temperature is reached on the rake face at some distance from the edge.

• The cutting edge is forcing its way through the interior of the work piece like a propagating wedge, both surfaces of the opened “crack” represent highly chemically reactive metal where there is no access to external oxygen or cutting fluids to this region

Generally, the over all cutting force F is related to cutting speed and feed as indicated in Figure. It is indicated that a low friction coating can lower the cutting force and thereby giving a lower edge temperature, which can be utilized to increase the productivity

Tool material properties

High temperature strength

• A metal cutting tool must be able to combine high hardness (or high yield strength) with high fracture strength at elevated temperature

• The latter is especially important in Primary shear zone

• High hardness is associated with brittleness, and strengthening by martensitic hardening, dispersion of hard particles, etc. of a metallic materials lowers the fracture strength

Tool material properties contd..

Fracture strength vs. hardness

Work materials for HSS cutting tools

Work material Hardness [HV] Hard particles Ductility Work harden

C-steels 200 - 250 Cementite Yes Yes

Cast irons 200 - 250 Cementite - -

γ−steels 180 - 250 - Yes Yes

Al-alloys 100 - 150 Oxides, AlFeSi Yes -

Ti-alloys 200 - 350 - Yes Yes

Ni-based alloys

200 - 350 Yes Yes Yes

• Generally, the work materials are macroscopically much softer than the tools. However, many work materials contain constituents (carbides, nitrides or oxides) that are harder (HV 1500 – 3000) and more temperature resistant

• Chip formation generally occurs by extremely high shear rates at which hardness of a carbon steel may well match the hardness of the cutting edge

Tool Wear

• Presently, no viable theories exist for predicting tool wear on the basis of properties of tool and work material

• Optical and electron microscopic and auto radiographic observations suggest that the tool wear phenomena occur at microscopic and atomic levels

• Depending on cutting operation, cutting parameters, work and tool material the performance of the tool is limited by nose wear, flank wear, crater wear, edge chippings, or combinations of these.

• Depending on the same parameters, the wear either occurs gradually by abrasive or adhesive wear, through plastic deformation, by more discrete losses of material through discrete fracture mechanisms, or by combinations of these

Lim and Ashby consider that two major forms of wear are commonly observed on a cutting tool: flank wear and crater wear

• Flank wear generally increases cutting force and the interfacial temperature, leading normally to dimensional inaccuracy and vibration which making the cutting operation less efficient

• Crater wear on the rake face is usually formed at some distance from the cutting edge and it is most frequently observed when cutting steels and other high-melting-point metals at relatively high cutting speeds .This crater gradually becomes deeper with time and may lead to the breakage of the cutting edge, rendering the tool useless.

Types of tool wear

For uncoated tools: • Abrasive wear • Adhesive wear• Large scale plastic deformation• Fatigue and fracture• Diffusion wear

For coated tools: • Coating removal due to poor substrate preparation• Coating removal due to thermal softening of the substrate

Types of tool wear mechanism

Abrasive wear

• Harder tool shears away small particles from the softer work material. Softer work material also removes small particles from the tool material but slowly.

• The hard tool particles are caught between the hard tool and soft workpiece, and this causes additional abrasion wear.

• Hard impurites in Tool and workpiece contain hard particles cause abrasion wear during machining.

Abrasive wear contd…

Wear dominates the crater and flank wear of a milling tool. The arrows point at ridges of HSS material relatively resistant to abrasion. There is also evidence of edge fracture. Work material: C-steel.

Paper knife. An extremely fine-scaled abrasion, only resisted by the hard carbides, dominates the tool wear.

Adhesive wear• Adhesive wear is caused by the formation of welded asperity junctions between

the chip and the tool faces and the fracture of the junctions by the shearing force so that tiny fragments of the tool are torn out and adhere to the chip.

• The adhesive component, often referred to as mild adhesive wear, is a tearing of superficial HSS material by high shear forces resulting in a slow drag of the surface layer and removal of small fragments in the direction of chip flow.

• If the tool is used to its upper limit of heat resistance, severe adhesive wear may result as a large scale plastic flow of surface material in the direction of the chip flow

• Adhesive wear dominates the flank and crater wear of HSS tools if the edges reach high temperatures, i.e. at high cutting speed. Adhesive wear is further promoted when cutting chemically aggressive materials.

• Both mild and severe adhesive wear are primarily resisted by the HSS material through its high yield strength at elevated temperature (high hot hardness)

Flank wear

Tool wear because of built-up edge

Large scale plastic deformation• Sometimes, the HSS tool edge is loaded beyond its yield strength and deforms by

large-scale plastic deformation resulting in edge blunting. This has been observed when HSS soften due to annealing during machining

Fatigue and fracture

• Macroscopic fracture of the whole tool can occur but is a rather scarce event. More common is localised chippings of the tool edge, see. The chippings seem to be initiated by grinding marks running parallel to the edge

Diffusion wear

• Diffusion wear characterizes the material loss due to diffusion of atoms of the tool material into the work piece moving over it

• Requirements for diffusion wear are metallurgical bonding of the two surfaces so that atoms can move freely across the

interface, a temperature high enough to make rapid diffusion possible some solubility of the tool material phases in the work material

• Suh and Kramer proposed the wear rate is controlled by the mass diffusion rate

• Sproul Ono and Takeyama have shown that the chemical reaction taking place at the interface has a major effect on wear as oxygen gas accelerates the formation of oxide layers that are continuously torn off resulting in increased wear, while wear is decreased by an environment of argon gas.

• Altintas proposed , the diffusion wear appear when the temperature increase at the contact zones, the atoms in the two materials become restive and migrate to the opposite material where the concentration of the same atom is less

Wear mechanisms of coated tools

• Coating will primarily protect the cutting edge in two ways: – Acting as a shield against abrasive and mild adhesive wear. – Reducing the tool temperature by reducing the friction between tool and work material.

• The coatings combine a superior hardness (abrasive wear resistance) with relatively low chemical reactivity with metallic materials (adhesive wear resistance)

• Coated tools fail by fatigue and discrete delamination/detachment than removal by slow gradual wear. Once the coating is removed, the wear mechanisms are the same as those of uncoated, although more severe because more severe cutting parameters.The mechanisms are:

– Coating removal due to poor substrate preparation– Coating removal due to thermal softening of the substrate

Coating removal due to poor substrate preparation

• There are primarily two ways by which failure in HSS substrate preparation can occur. The surface temperature during grinding/polishing reaches above the austenitisation

temperature resulting in a brittle interlayer of untempered martensite, The resulting substrate surface is too rough

The lateral compressive stresses state σ present in most PVD coatings will generate interfacial stresses S. At the top of e.g. grinding ridges this stress is a tensile “lift off” stress that may reach the same order of magnitude as the residual stress σ [12]. Such ridges can result from rough grinding.

Tin coating detachment along grinding ridges of a HSS cutting tool

Microscopic fatigue cracks observed on the rake face close to the edge of a hob tooth that has been cutting in carbon steel

Coating removal due to thermal softening of the substrate

• Once the HSS substrate material reaches a temperature level of excessive softening, it fails to resist the contact pressure, and the brittle coating fractures.

• The dark etching contrast underneath the coating, which reveals thermal softening due to over tempering. The coating fractures and individual fragments are then detached in the form of small fragments.

Wear mechanism

Cause of wear Counteractive tool properties

Abrasive wear

Hard particles or other hard phases in the work material remove material by a ploughing action.

High matrix hardness, large volume of hard phases, hard coating

Mild and severeadhesive wear

High cutting speed generates high tool surface temperatures that facilitate strong adhesion between work and tool materials. The worst situation prevails for tough, ductile and chemically reactive work materials with low thermal conductivity.

Smooth surface, sharp edge, high hot hardness, high thermal conductivity, chemically inert (anti sticking) coating

Plastic deformation

High cutting speed generates excessive edge temperatures in combination with high loads.

High hot hardness, high thermal Conductivity.

Fracture and fatigue

Interrupted cutting, especially in combination with high cutting speed and use of cutting fluid, a tough and ductile work material. Use of insufficiently sharp tool edges.

Smooth tool surface, high fracture toughness promoted by a defect free HSS with a fine grained structure of both matrix and hard phases

Table shows common wear mechanisms of HSS tools, their cause and how to fight them

Towards better performance of HSS tools

• Improving the HSS material: Hardness, heat resistance and fracture toughness both macroscopically and microscopically are the prerequisites of high tool performance. Recent HSS development has focused on the homogeneity and cleanliness of the HSS steel.

It is possible to further improve the hardness/toughness ratio by further reducing the size of the matrix grains and hard phase particles down to the nanometer range (applying nano-technology) a further step is possible

• Improving the surface integrity: Avoiding deterioration of the superficial HSS material by excessive heat generation. The macroscopic strength and the resistance to edge chipping of HSS materials is very sensitive to surface defects generated by the surface. A smooth tool surface contributes to the resistance against micro cracking and to avoid premature detachment of coatings

References• Söderberg, S., Jacobson S., Olsson, M., Wear Atlas of HSS Cutting Tools, Proceedings

of the 5th International Congress on Tribology (Eurotrib 89), Helsinki, Finland, Finnish Society for Tribology, 1989

• Lim, C.Y.H., Lim, S.C., Lee, K.S., The performance of TiN-coated high speed steel tool inserts in turning, Tribology International 32 (1999) 393-398

• Hogmark, S. Jacobson, S., Larsson, M., Wiklund, U., Mechanical and tribological requirements and evaluation of coating composites, In Modern Tribology 2000. Ed. B. Bushan, Vol II, 931-959 CRC Press 2001

• Le May, I., Principles of mechanical metallurgy, Elsevier 1981 • Larsson, M., Olsson M., Hedenqvist, P., Hogmark, S., Mechanisms of coating failure as

demonstrated by scratch and indentation testing of TiN coated HSS - On the influence of coating thickness, substrate hardness and surface topography, Surface Engineering 16, 5 (2000) 436-444

• Wiklund, U., Gunnars, J., Hogmark, S., Influence of residual stresses on fracture and delamination of thin hard coatings, Wear 232 (1999) 262-269