Crater wear

Fig. 2. Tool wear mechanisms, (a) Crater wear on a cemented carbide tool produced during machining plain carbon steel, (b) Abrasive wear on the flank face of a cemented carbide tool produced during machining gray cast iron, (c) Built-up edge produced during low speed machining of a nickel-based alloy.

Grater Wear. Crater wear (Fig. 2a) is caused by a chemical interaction between the rake face of a metal-cutting insert and the hot metal chip flowing over the tool. This interaction may involve diffusion or dissolution of the tool material into the chip. 

Built-up Edge. At relatively low speeds in metal-cutting operations, the tool tip does not get hot enough for crater wear to be significant. Under these conditions the metal may, however, become welded to the tool tip as built-up edge (Fig. 2c). 

Straight WC—Co tools are not suitable for machining steels that produce long chips because straight grades undergo crater wear from diffusion of WC into the steel chip surface. However, soHd solutions of WC—TiC, WC—TiC—TaC, etc, resist this type of chemical attack. In addition, tantalum carbide can improve thermal-shock resistance. Steel cutting compositions thus typically contain WC—TiC—(Ta,Nb)C—Co. Tantalum carbide is often added as (Ta,Nb)C because the chemical similarity between TaC and NbC makes their separation expensive. 

Hardness and solubiUty for other carbides make TiC an important component of siatered cemented carbides. Although the addition of TiC or WTiC2 to WC—Co alloys imparts crater wear resistance, it also reduces the transverse mpture strength and fracture toughness of these alloys. Therefore, the amount of TiC or WTiC2 added to WC—Co alloys for steel machining is kept to a minimum, typically no greater than 10 wt %. The TiC-based cermets, on the other hand, may contain 30—85 wt % TiC. 

Other wear mechanisms are flank wear and crater wear which occur mostly with cemented-carbide tools. Flank wear refers to the depression that is formed below the cutting edge on the side of the tool caused by the abrasive wear of the cemented carbide. TiC is particularly effective in reducing it. Crater wear occurs in the form of small depressions on the rake face behind the point of contact of the tool with the workpiece. Diffusion of the cobalt binder into the cutting chip usually occurs with crater wear. TiN is effective in reducing both diffusion and crater wear.PI... 

Of the various tool failure mechanisms, fracture is least desirable because it is unpredictable. Most tool material development work is focused on minimizing flank wear and retarding unwanted tool failure modes such as catastrophic fracture, gross plastic deformation, BUE, crater wear, and DOCN. 

HSS and TiN-coated HSS all materials of low—medium strength and hardness turning, drilling, milling, broaching medium speed flank wear, crater wear low hot hardness, limited hardenability and wear resistance, low to medium cutting speed, low- to medium-strength materials... 

Dwaihl (1940), Trent (1952) and Trigger and Cho (1956) conducted a number of fundamental studies on various aspects of tool wear of cemented carbide tools. There are mainly two types of wear, viz. flank wear and crater wear. Crater wear starts on the rake face at some distance away from the tool nose, as the maximum temperature is attained at this point. It is a diffusion-dominant wear, and temperature plays an excessive role in it. The flank wear occurs at the flank surface and affects the dimensional accuracy to a great extent. Recently, Astakhov (2004) has argued that the existing measures of flank wear are insufficient for its characterization and he has proposed new concepts. 

Trigger, K. J., Chao, B. T. (1956). The mechanism of crater wear of cemented carbide tools. Transactions on ASME, 78, 1119-1126. 

Mainly occurring types of wear in broaching is comer wear (CW), flank wear (VBa), rake face wear (VBy), and rounding of the cutting edge. Due to relatively low cutting speeds and therefore little development of temperature compared to other machining processes, crater wear does not occur (Hoffmann 1976). 

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