The heat treating process used to harden steels consists of heating them up to a high temperature (usually 1700/2200°F), then quenching to near room temperature, and finally reheating to some intermediate temperature for tempering (300/1100°F). A characteristic of low to medium alloy steels (A2, O1, D2) is that they soften from their maximum hardness somewhat during tempering. The amount of softening depends on the temperature exposure and the individual grade characteristics. To retain maximum hardness (over about 58 HRC), A2 and D2 are usually tempered around 400/500°F. Higher exposures result in lower hardness. A side benefit of high alloy content, typical of high speed steels, and most of the high wear resistance CPM steels, is that the tempering characteristics are changed because of the alloy content. They are tempered over 1000 F, yet retain their full hardness during this exposure.
Coatings and Surface Treatments
Beneficial surface treatments, including nitriding, titanium nitride coating, etc., are often applied to tool steels to permit lower friction, better wear resistance, or other properties. Most of these coatings are applied at temperatures of about 850/1050F. Thus, the treatment process can limit the service hardness of low or medium alloy steels. However, the higher alloy content steels such as M2, M4 as well as CPM 3V, 9V, 10V, 15V retain their maximum hardness after such exposures. Thus, normal surface treatment temperatures have no effect on their hardness, and tools may be treated without fear of dimensional or hardness changes. The additional wear protection of a surface treatment may be added, without sacrificing deformation resistance. The CPM grades provide excellent substrates for all types of surface treatments.
Choosing Tool Steels Based on Properties
As mentioned above, A2 and D2 are common steels used for metal forming tools. More highly alloyed grades offer better wear resistance. When choosing the tool steel for any tool, the required properties for the application should be considered. What is the workpiece? What is the historical failure mode for current or similar tooling? Which properties should be increased? What trade-offs may be required?
For tools requiring high resistance to plastic deformation, hardness should be a concern. Tools for stamping steel generally need to be about 56/58 HRC minimum, although some form tools, and tools for non-ferrous work material, may be softer. Most tool steels are capable of reaching roughly similar hardness levels (low 60’s HRC), and thus will have similar abilities to resist plastic deformation. However, some high speed steels, such as CPM Rex T15 and Rex 76, can achieve hardnesses approaching 70 Rockwell C. Keep in mind that in tool steels, the major mechanism controlling wear properties is the type and amount of carbide particles present. For this reason, increasing the hardness is not generally an effective method for increasing the wear life of tools, but only for minimizing deformation.
For better deformation resistance than A2 or D2 tools (60/62 HRC)
|— M2, Cru Wear – (62/63 HRC)
|— CPM M4 – (63/64 HRC)
|— CPM T15 – (64/66 HRC)
|— CPM Rex 76 – (64/67 HRC)
For tools needing high resistance to chipping or breakage, for instance where frail geometries or thin projections or sharp notches are a problem, high impact toughness is required. In general, tool steels, even those with low impact toughness, are many times tougher than solid carbide. (The toughness of carbide materials is often measured in inch-pounds, where tool steels are measured in foot-pounds.) Within the families of tool steels, there is some variation in impact resistance. Shock-resisting steels, like S7 and A9, are both designed to offer optimum resistance to breakage. However, they differ in their heat treating process. S7 cannot generally be coated for improved surface wear properties, because of its low tempering temperature. A9 is typically tempered at over 900 F, and thus may be coated by any of the common commercial coating processes. The maximum hardness of both grades is approximately 58/59 HRC. In examining alternatives to carbide tools, where chipping is the normal failure mode, the toughness comparisons among steels are usually moot. In these cases, the normal recommendation is to use CPM 10V or 15V instead of carbide in most applications, or Rex T15, Rex 76 or Rex 121 when high hardness is needed. These grades provide the closest wear and hardness properties to carbide, while offering the toughness properties of tool steels.
There are several other factors beside inherent material properties which often contribute to chipping or breakage failures. Tool steels are notch-sensitive materials. The presence of notches, undercuts, sharp radii, changes in section, or any geometric features may concentrate applied stress and exaggerate the material’s tendency to break. All reasonable precautions to avoid unnecessarily sharp radii should be exercised. In addition, in heat treated and EDM’d tools, the EDM operation can leave the surface in a condition prone to chipping. Where EDM’d tools are experiencing chronic chipping or breaking problems, they should be stress relieved (tempered) after EDM before going into operation, and if practical the EDM layer should be removed as well (stoned, polished, etc.).
For better impact toughness than D2 tools (20 ft-lbs)
|— A2 – 45 ft-lbs
|— CPM 3V – 55/80 ft-lbs
|— CPM 9V (if lower hardness OK) – 50/70 ft-lbs
|— A9 (coated or nitrided for wear) – 80/100 ft-lbs
|— S7 ( low wear resistance) – 100/125 ft-lbs
Hardness and toughness may be considered “step” or “threshold” functions; that is, as long as the property is high enough to prevent damage (indentation or breakage), there is no further advantage to increasing the property even higher. However, wear resistance may be considered a “continuous” function; that is, continual increases in the wear resistance of the steel will result in increases in the life of the tool. Thus, upgrading for wear resistance may always offer benefits, provided other properties are not compromised. When long-term abrasive wear resistance is desired in a tool (that is, when the basic tool runs well, but a longer in-service time is desired), a steel with higher wear properties is appropriate. In this case, nearly all the choices for upgrading will involve steel of higher alloy content. Several of the high-alloy CPM steels offer wear properties midway between conventional tool steels, and carbide. In working with abrasive media, the CPM steels offer very high resistance to wear. However, in situations generating severe metal-to-metal wear (adhesive wear, or galling), the best solution is to separate the two metal surfaces. This may involve a lubricant, or commonly a non-metallic coating (titanium nitride, titanium carbonitride, or other related ceramic coatings). These coatings reduce the coefficient of friction between the workpiece and the tool, and reduce the risk of welding or galling wear. When coatings are not practical, materials offering a combination of high toughness, high hardness, and resistance to abrasion, such as CPM 3V or CPM M4, are suggested.
For better wear resistance than D2 tools
|— CruWear, M2, CPM 3V (2-3% V)
|— CPM M4, T15 (4-5% V)
|— CPM 10V, 15V (max V)