Machinery's Handbook, 31st Edition
Tool Steels
437
Table 2d. Common Tool Faults, Failures, and Cures Grinding Damages
Fault Description
Probable Failure
Possible Cure
Grinding Damages
Excessive stock removal rate causing heating of the part surface beyond the applied tempering temperature
Prevention: by reducing speed and feed, or using coarser, softer, more openstructured grinding wheel, with ample coolant. Correction: eliminate the discolored layer by subsequent light stock removal. Not always a cure, because the effects of abusive grinding may not be corrected. Prevention: by correcting the grinding wheel specifications. Correction: in shallow (0.002–0.004-inch, or 0.05–0.1 mm) cracks, by removing the damaged layer, when permitted by the design of the tool, using very light grinding passes. Dress wheel with sharper diamond and faster diamond advance to produce coarser wheel surface. Alternate dressing methods, like crush-dressing, can improve wheel surface conditions. Dress wheel regularly to avoid loading or glazing of the wheel surface. Improve coolant supply and quality, or reduce stock removal rate to reduce generation of heat in grinding. Reduce rate of advance; adopt wheel specifications better suited for the job. Use ample coolant or, when harmful effect not eliminated, replace abrasive cutoff by some cooler-acting stock separation method (e.g., sawing or lathe cutoff) unless damaged surface is being removed by subsequent machining.
Scorched tool surface displaying temper colors varying from yellow to purple, depending on the degree of heat, causes softening of the ground surface. When coolant is used, a local rehardening can take place, often resulting in cracks. Intense localized heating during grinding may set up surface stresses causing grinding cracks. These cracks are either parallel but at right angles to the direction of grinding or, when more advanced, form a network. May need cold etch or magnetic particle testing to become recognizable. Heating of the work surface can cause scorching or cracking. Incorrect dressing can also cause a poor finish of the ground work surface. Introducing into the tool surface heat that is not adequately dissipated or absorbed by the coolant can cause softening or even the development of cracks. The intensive heat developed during this process can cause a hardening of the steel surface, or may even result in cracks.
Improper grinding wheel specifications; grain too fine or bond too hard
Incorrectly dressed or loaded grinding wheel
Inadequate coolant, with regard to composition, amount, distribution, and cleanliness
Damage caused by abusive abrasive cutoff
Table 3. Classification of Tool Steels
Category Designation
Letter Symbol
Group Designation Molybdenum types Intermediate types Tungsten types Chromium types Tungsten types Molybdenum types High-carbon, high-chromium types Medium-alloy, air-hardening types Oil-hardening types
Application Type
M
High-speed tool steels
M50 - M52
Cutting Tools
T
H1 - H19 H20 - H39 H40 - H59
Hot-work tool steels
Hot-work
D
Cold-work
Cold-work tool steels
A O
Cold-work; hot-work (some grades)
Shock-resistant tool steels
S P L F
— —
Low-carbon tool and mold steels Special-purpose tool steels Water-hardening tool steels
Cold-work Cold-work
Low-alloy types
Carbon-tungsten types
W Cold-work The following detailed discussion of tool steels will be in agreement with these categories, showing for each type the percentages of the major alloying elements. However, these values are for identification only; elements in tool steels of different producers in the mean analysis of the individual types may deviate from the listed percentages. -
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online