Failure mechanisms in tool steels

Manufacturing Engineering, Jan 1999 by Miller, Patricia

Understanding why tools fail is the first step in achieving longer tool life

In an ideal world, one grade of tool steel would do the job for every application. In the real world, toolmakers and manufacturing engineers must examine different grades of steel to extend and enhance the life of each tool steel in every specific application.

Users commonly start the process of choosing a tool steel by looking at the factors that limit the life of the steel currently used. Luckily, industry has a good understanding of the basic failure mechanisms that can occur in tooling and the means available to deal with them.

Cold work tooling fails because of wear, sticking, plastic deformation, fatigue, and gross cracking. A certain amount of wear always occurs; when it becomes excessive and the part no longer meets print requirements, users must do something about it. Wear in cold work tooling involves two distinctive, wellunderstood mechanisms: abrasion and adhesion.

Abrasive wear occurs when hard particles scour the tool surface. Very hard work materials can cause this abrasion, as can coatings on their surfaces in cold work applications. Improper lubrication or lack of lubrication and insufficient clearances can cause yet more abrasive wear. In practice, the "cutting particles" shear through the tool substrate.

To combat abrasive wear, use tool steels with high hardness, and increase the hardness and content of the carbide within the steel. Classic grades like AISI D2 or AISI D6 contain large, blocky chromium carbides that act as hard barriers to the softer cutting particles and increase the wear resistance of steel. Chromium carbides fall in the range of 1700 HV (Hardness Vickers).

Newer tool steels have more regularly shaped carbides of higher hardness: vanadium carbides in Uddeholm's Vanadis 4 and 10 grades provide a hardness in the range of 2800 HV. Carbide content in these grades ranges from 13 to 24%, as compared to 13% in AISI D2. The regular shape of the carbides (produced via a powder metallurgical process) yields a uniform response to heat treatment, significantly increased toughness, and less chipping.

Adhesive wear is the microwelding of two moving surfaces in contact with each other. It can pull small fragments of tool material from the tool surface. These fragments can stick to the work material and cause adhesive wear on the tool surface. Adhesive wear increases when:

tools cut a material that's similar to tool steel and of similar hardness;

tools cut thick, gummy work materials like aluminum, copper, stainless steel, and low-carbon steels with hardnesses less than 100 HB (Harness Brinnell);

and under conditions of high friction or heavy loading.

The tougher the steel, the better it can withstand the tearing that occurs. If this wear takes place because two similar steels are in contact, then never mate two simi lar materials of the same hardness. Vary the hardness and use surface coatings to reduce friction. In areas where sliding contact occurs but the parts in question don't need high hardness, consider using a copper alloy. Because of its higher thermal conductivity, the copper alloy will pull localized heat from the surface more rapidly than a steel part.

Plastic deformation, also known as mushrooming or indentation, occurs when a process exceeds the yield strength of steel, defined as the point where 0.2% plastic strain has taken place. Until the load exceeds its yield strength, the steel can withstand the load on the tool and return to its original shape. When the steel's yield strength is exceeded, the load on the area causes permanent deformation. The harder and thicker the work material, the more likely it is that the tool will deform.

Plastic deformation can happen in any application. In cold work tooling, the process can damage the tool's working surfaces and produce shape changes. Using a steel with a higher yield strength minimizes plastic deformation, but to get higher yield strength you must increase the hardness of the tool steel you currently use. If you can do so without degrading its toughness, it can do the job. Otherwise, choose a grade of tool steel that can be used at a higher hardness level.

A coating will not help this situation. Compare a coating to the crust on a loaf of bread: the softer core will yield under continued pressure. Heat treatment can also influence the yield strength of steel. Highly alloyed tool steels like D2 will exhibit higher compressive yield strength when the tool is tempered at a temperature that is higher than 950degF (510degC).

Temperature and chemistry affect the yield strength of steel. As the temperature of a steel increases, its yield strength will decreaseunless the steel contains elements that help retain its strength at elevated temperature (hot hardness). Tool steels containing molybdenum, tungsten, or cobalt will retain their strength at elevated temperature.

Fatigue failures are produced under repeated cyclical stress at loads less than the ultimate tensile strength of the material. In cold work applications, fatigue might appear as fine chipping at the cutting edge. Surface imperfections at highly stressed corners or regions can initiate cracks that, over time, will propagate through the tool. Grinding marks, EDM defects, heavy stamp marks, or surface changes caused by heat treatment can cause these imperfections.