Tool Steels Table 2c. Common Tool Faults, Failures, and Cures Heat-Treatment Faults Machinery's Handbook, 31st Edition
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Fault Description
Probable Failure
Possible Cure
Improper preparation for heat treatment. Certain tools may require stress relieving or annealing, and often preheating as well
Tools highly stressed during machining or forming, unless stress-relieved, may aggravate the thermal stresses of heat treatment, thus causing cracks. Excessive temperature gradients developed in nonpreheated tools with different section thicknesses can cause warpage. Causes grain coarsening and a sensitivity to cracking that is more pronounced in tools with drastic section changes. The tool may not harden at all, or in its outer portion only, thereby setting up stresses that can lead to cracks. Water-hardening tool steels are particularly sensitive to inadequate quenching media, which can cause soft spots or even violent cracking. Cracking, particularly of tools with sharp corners, during the heat treatment can result from holding the part too long in the quench or from incorrectly applied tempering.
Stress relieve, when needed, before hardening. Anneal prior to heavy machining or cold forming (e.g., hobbing). Preheat tools (a) having substantial section thickness variations or (b) requiring high quenching temperatures, as those made of high-speed tool steels. Overheated tools have a characteristic microstructure that aids recognition of the cause of failure and indicates the need for improved temperature control. Controlling both the temperature of the furnace and the time of holding the tool at quenching temperature will prevent this not too frequent deficiency. For water-hardening tool steels, use water free of dissolved air and contaminants, also assure sufficient quantity and proper agitation of the quench. Following the steel producer’s specifications is a safe way to assure proper heat-treatment handling. In general, the tool should be left in the quench until it reaches a temperature of 150–200 ° F (66–93°C), and should then be transferred promptly into a warm tempering furnace. Double temper highly alloyed tool steel of the high-speed, hot-work, and high-chromium categories to remove stresses caused by martensite formed during the first tempering phase. Second temper also increases hardness of most high-speed steels. Heating in neutral atmosphere or well-maintained salt bath and controlling the furnace temperature and the time during which the tool is subjected to heating can usually keep the carbon imbalance within acceptable limits.
Overheating during hardening; quenching from too high a temperature
Low hardening temperature
Inadequate composition or condition of the quenching media
Improper handling during and after quenching
Insufficient tempering
Omission of double tempering for steel types that require it may cause early failure by heat checking in hot-work steels or make the tool abnormally sensitive to grinding checks.
Decarburization and carburization Unless hardened in a neutral
atmosphere the original carbon content of the tool surface may be changed: Reduced carbon (decarburization) causes a soft layer that wears rapidly. Increased carbon (carburization) when excessive may cause brittleness.
The addition of more than one element to a steel often produces what is called a synergis tic effect. Thus, the combined effects of two or more alloy elements may be greater than the sum of the individual effects of each element. Classification of Tool Steels.— Steels for tools must satisfy a number of different, often conflicting, requirements. The need for specific steel properties arising from widely vary ing applications has led to the development of many compositions of tool steels, each intended to meet a particular combination of applicational requirements. The diversity of tool steels, their number being continually expanded by the addition of new develop - ments, makes it extremely difficult for the user to select the type best suited to his needs or to find equivalent alternatives for specific types available from particular sources. As a cooperative industrial effort under the sponsorship of AISI and SAE, a tool classifi cation system has been developed in which the commonly used tool steels are grouped into seven major categories. These categories, several of which contain more than a single group, are listed in Table 3 with the letter symbols used for their identification. The indi vidual types of tool steels within each category are identified by suffix numbers following the letter symbols.
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