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Work hardening--A material kind of problem.

Work hardening is a "silent" kind of shop problem. No one talks about it much and no one can easily define it, but it can affect almost every job. So, what is work hardening? Technically, work hardening is a restructuring of the part material on a molecular level. When heated to a specific range, for a specific length of time, the molecules of the material rearrange themselves to form a new, harder material.

You say this sounds a lot like heat treatment of metals, well you are fight. The same process that heat treats metal is also the process that causes work hardening, but on a smaller scale. When work hardening occurs, the affected area will appear to be shiny and slippery. Although the surface will appear glazed, it is not flat. The material will have a hard, uneven surface, like a fragmented sheet of ice. This area will be very hard and difficult to machine.

How is a part heated to such temperatures that will allow for work hardening? Metal cutting requires great forces and the action of a tool meeting the material is not always smooth. Friction between the tool and the part is the greatest source of heat on the part and this heat causes work hardening.

In everyday terms, work hardening is caused by dull tools, rubbing against the material. There are a number of tips to help you avoid work hardening.

Use sharp tools. Sharp cutting edges will cut the material, not rub.

Use coolant. Coolant will keep the temperature of the cutting action down. Coolant should be used where possible as work hardening will not occur without high temperatures. Drilling can be a problem area, reduce the risk of work hardening by using a coolant-fed drill.

Use the proper cutting speeds. Run at the correct speed for the material being worked. At incorrect speeds, the tool will not cut smoothly, effectively rubbing and increasing the chances for work hardening. Research the type of material being cut and apply the correct surface footage.

Do not hold the tool at one position with a dwell command. This will 'burn' that spot on the part. The heat caused by friction between the tool and part will be focused on that spot and work hardening can occur.

Even following all of these rules, work hardening occur. The #1 variable in predicting work hardening is the type of material being used. Although not all metals are subject to work hardening, most stainless steels, carbon alloys, and superalloys readily work harden. Be aware that certain machining processes will strain the material and tool increasing the chances of work hardening.

Drilling/Tapping A typical situation where work hardening causes problems is tap breakage. When the tap will not cut and breaks in a drilled hole, often the culprit is work hardening caused by the drill and not the tap at all. The drill overheats in the hole arid causes work hardening inside of the hole.

Peck drilling If the drill can make a continuous cut, work hardening is less likely. The increased number of passes increases the chances for work hardening. A good method is to use a coolant-fed drill to cool the cutting point directly and flush chips.

Threading Threading cycles where multiple passes are required again often show work hardening. This is especially true where the cutting is done entirely on the front of the insert. The plunge angle can cause the back of the insert to rub on the downward movement. The standard 29 deg plunge angle can cause work hardening, try reducing the plunge angle to 10-14 deg.

As these examples show, a machining process that requires multiple machining processes or multiple passes is more susceptible to work hardening. Parts that can be created in one operation will be much less likely to experience work hardening. Work hardening not only affects the part, but the tooling as well. It is a cyclic condition: dull tools cause work hardening and work hardening dulls the cutting tool.

The key to avoiding work hardening is maintaining your tools and always being aware that the materials most of us work with are likely to work harden. Steve Rose is a manufacturing consultant and president of Rose Training Systems, Solon, OH, which also offers Internet website development.

Steve Rose is a manufacturing consultant and president of Rose Training Systems, Solon, OH.
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Comment:Work hardening--A material kind of problem.
Author:Rose, Steve
Publication:Tooling & Production
Article Type:Brief Article
Geographic Code:1USA
Date:Jul 1, 1999
Words:729
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