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Combating tool wear with coatings. When will a coating help?By
Steve Chamberlain Modern toolmaking methods and materials have reduced production time, cost, and headaches significantly over the last 20 years. But the rapid pace of the tooling revolution, coupled with extremely complex manufacturing technology, has left many people confused and misinformed. The two main types of tool wear are surface and substrate. Surface Wear Coatings are effective when used properly to solve surface problems such as adhesive and abrasive wear. Adhesive Wear. Adhesive wear is the localized bonding of metals, also known as galling. Galling has two causes: the natural attraction of like materials to bond and rough surface finish. Galling often occurs when D-2 tools are used to work stainless steel. D-2 contains about 12 percent chrome, which doesn't match well with the 18 to 24 percent chrome in stainless steel. Separating the materials solves the problem. The microscopic surface finish plays a very large role in galling. The surface of steel may feel smooth, but it isn't. A microscope view shows a mountain range of sharp ridges and valleys. Small particles of the work material stick to these tiny scratches and imperfections in the tool surface and eventually build up to visible galling. Abrasive Wear. Abrasive wear is caused by hard particles in the work material plowing into and through the tool surface. These hard particles, or carbides, scratch and pit the tool steel because they are harder. In addition to the naturally occurring carbides, man-made problems like welded blanks or tubes, burrs, and work hardening eat away at the steel surface. Coatings often can solve both adhesive and abrasive wear simultaneously. Placing an extremely hard surface barrier between the tool and work material seals the surface and prevents adhesion. As long as the surface treatment is pure and very dense, it will prevent hard particles in the work material from damaging the substrate surface. Additionally, coatings often have much lower coefficients of friction than hardened steel. However, thin-film coatings don't cover up the underlying surface profile, so critical areas of metal flow must be mirror-finished before coating. After mirror finishing the part, using a hard coating will protect it from abrasive and adhesive wear. Substrate Wear Substrate problems, including fatigue cracking, heat cracking, fracture, and plastic deformation, often are much more difficult to evaluate. Fatigue Cracking and Plastic Deformation. When a tool fails from fatigue or plastic deformation, the problem probably can't be solved with coatings alone. Plastic deformation occurs when the compressive force of the action overwhelms and deforms the tool itself. One way to overcome fatigue or plastic deformation is to use a harder steel substrate or reduce the pressure in the operation. The forming radius causes half of these problems. The one problem area that can be improved the most for the least cost and hassle is the forming radius of a die. When under pressure, metal acts like a liquid and flows over this radius. Making sure the forming radius is perfect will increase metal flow and decrease tool stress and wear. The radius should be opened as much as possible, and it must be a true radius that is ground correctly. Removing any transitions or flats and polishing to a mirror finish also helps. A mirror finish is achieved by finishing the radius by hand with a 900 stone or diamond paste and a felt bob. Once the radius is perfect, a coating can keep it in that condition. Heat Cracking. Heat cracking of the substrate can be caused by heat generated when the tool is made or run. If it is caused by operation, a coating often can help. By decreasing the coefficient of friction, sealing the surface, and providing a barrier between like elements, a coating can go a long way toward reducing thermal stress. A coating may help reduce the cost and amount of lubricant required, but reductions should be incremental and closely monitored. Another good idea is to use more heat-resistant tool steels, such as M series or particle metals, in high-stress applications. Fracturing. Fracturing, or chipping (see Figure 1), probably indicates the need for a tougher, not harder, substrate. Often tools are hardened beyond their capability in an attempt to prevent surface wear. A high-performance coating that can prevent surface wear allows underhardening of the substrate, resulting in a softer but tougher and more durable substrate.
Cost Factor Correct tool design and manufacture are critically important to reducing tool wear. It is amazing how many toolmakers choose tool steel based on the cost of the steel. Hundreds of options are available for tool steel substrates, and most tool and die designers use two or three a year, at most. Many shops tend to use standardized A-2 and D-2 steel because they find it difficult to keep track of materials and hardening specifications accurately. Both are excellent general-purpose steels. They are common, and easy and inexpensive to harden. Unfortunately, some metal forming problems can't be solved with these materials. The single most important factor in selecting steel is the job requirement, not the initial cost. Nowhere is the old adage "you get what you pay for" more true than with tool and die. In reality, the cost of the steel is a tiny fraction of the lifetime tooling cost. For example, a block of D-2 may cost $50 and a particle metal block of similar size may cost $300, for a real cost increase of $250, or 600 percent. Yes, that will make the accountants sit up and take notice! But the total tool cost may be $5,000, so the increase in steel cost really amounts only to 5 percent of the total tool cost. When the steel is considered as part of a complete die, upgrading the steel in a few problem areas is cost-effective. Heat Treating Options As much as toolmakers deal with heat treating, many don't understand it well. While heat treating is a science and can get complex, the basics are worth learning. Often companies don't have a specification for how to treat the tools and never check the hardness or ask for certifications. Many options are available for heat treating various steels, and fabricators need to make sure the process they use fits their particular application. If persistent chipping or cracking is the problem, one option is to underharden the tool. Full hardness for D-2 is 58 to 60 HRC, which is what most toolmakers use. Small parts can be hardened slightly more, but D-2 becomes brittle higher than 60 HRC. Many applications don't require that much hardness. Heat treating D-2 to 55 to 57 HRC usually results in a material with adequate hardness and toughness. If 55-HRC D-2 forms a good part and doesn't deform but still shows wear, one option is to add a good coating to increase surface hardness and reduce wear. If 57 HRC is not enough compressive strength and the tool shape deforms, the next step might be to move up to a tool steel that can be hardened safely higher than 61 HRC, such as M series, modified D-2, or particle metal. These all can be underhardened to 58 to 60 HRC to become tough, durable substrates while providing the extra hardness needed. Sometimes people find that steel they hated at full hardness is wonderful when underhardened. If full hardness is required to address a surface wear problem but not to form the part, then a coating should be used for the surface wear, and hardness should become less important. Another trick is stress relieving the tool. Stresses build up in tooling over time and use and often show up as large cracks. Stress relieving the tool involves retempering it at 25 degrees F below the original tempering temperature, following standard tempering instructions, and then allowing the material to cool to room temperature. Doing this on a regular basis can greatly extend tool life. However, some tool coatings cannot stand up to high heat. High-performance, thin-film coatings can save cost and time if they are used appropriately. However, coatings are just one part of an overall tooling project. Other factors include tooling material, proper -- not always maximum -- hardening, coatings, stress relieving, and examining a tool's failure mode. Steve Chamberlain is a business development manager with the TD Center, 2020 15th Street, Columbus, IN 46220, phone 812-378-1669, fax 812-378-1591. The TD center supplies pure carbide coatings to automotive, appliance, furniture, and electrical industries. |
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TD Center |
Today's
high-performance, thin-film tool coatings are designed to prolong
the life of tooling while reducing part marking, lubricant cost
and volume, heat buildup, and maintenance and increasing shelf life,
lubricity, and dimensional control. Coatings can't, however, solve
every tool problem. The best solution depends on the type and cause
of tool wear.
