Selecting High-Performance Tool Steels for Metalforming Tools

By Edward Tarney, Director of Technology
Crucible Service Centers, A Division of Crucible Materials Corporation, Camillus, NY

By comparing the levels of metallurgical properties offered by different steels, tool users can determine which tool steels are best suited for resisting performance problems or enhancing tool performance.

The success of a metalforming tool depends upon optimizing all of the factors affecting its performance. Usually, operating conditions (applied loads, abrasive environments, impacts and other factors) determine how well a tool holds up.

Most tool failures are related to mechanical causes. However, with the variety of tool steels available for manufacturing metalforming tools, it often is possible to choose a tool steel with a favorable combination of properties for a particular application. The properties of tool steels that have a direct influence on tool performance include hardness, toughness and wear resistance.

Historically, tool steels used for stamping and forming tools have included A2 and D2, with occasional use of the high speed steel (HSS) M2. A2 and D2 are familiar to most tool builders and tool users as common, general purpose cold work tool steels. They combine good all-around performance properties for stamping and forming with low cost, wide availability and relatively easy fabrication. However, sometimes they do not provide the level of performance needed for high-volume production.

Where long runs and infrequent regrinding are desirable, higher-alloy tool steels or carbide might be chosen to upgrade from the commonly used tool steels, Fig 1. Other properties, such as impact resistance, may be sacrificed in order to gain higher wear properties. Conversely, steels chosen for their resistance to impact or breakage may not be capable of high-wear resistance. An understanding of these tool steel properties and related issues permits selection of the optimum steel for most applications.

Fig. 1--Tool steels commonly used in cold forming applications.

Properties of Tool Steels

Before discussing specific grades, it will be useful to discuss the general properties of tool materials. The primary properties important to cold work tools are hardness, toughness and wear resistance, Fig 2. To some extent, each of these properties may be varied independently in tool steels, so it makes sense to consider each separately. In fact, the same properties would be important to consider in carbide materials as well as in steels.

Fig. 2--The three major properties of tool steels.

An understanding of tool material properties, combined with an understanding of which factors limit tool life for a particular tool (breakage, wear, deformation, etc.), will allow tool users to specify the best performing grade for nearly any application. Tool users can examine failed tools to determine which properties may have been lacking in a tool, which properties should be improved and which other properties must be considered in alternate materials.

Hardness

Hardness is a measure of resistance to deformation. In tool steels, hardness most commonly is measured using the Rockwell C test. Hardened cold work tool steels generally are about 58-64 hardness Rockwell C (HRC), depending upon grade. Most are about 60-62 HRC. Occasionally, some in use are up to about 66 HRC.

Hardness testers work by using a standardized load to make an indentation in the test piece, then measuring the size of the indentation. A large indentation indicates low hardness. A small indentation indicates high hardness. Thus, the resistance of a material to deforming (by compression indentation) is a direct indication of its hardness.

Fig. 3--Hardness versus compressive strength.

When different steels measure at similar hardnesses, it is because the hardness tester made the same size impression in each. Thus, at the same hardness, different steels have similar resistance to deformation, Fig. 3. This is because the hardness test is nearly independent of the grade of steel tested.

Tools that plastically deform in service possess insufficient hardness. Permanent bending of cutting edges, mushrooming of punch faces or indenting of die surfaces (peening), indicate insufficient hardness. Because the resistance of a steel to indentation is directly related to the hardness, not the grade, corrective actions for deformation may include increasing hardness or decreasing operating loads. Changing grades will not help a deformation problem, unless the new grade is capable of higher hardness.

Small differences in hardness usually do not have a significant effect on wear life of tool steels. Different tool steels are used at similar hardnesses, yet offer significant differences in expected wear life. Thus, hardness usually is not a primary factor in wear resistance--only in deformation resistance. The wear resistance of tool steels is affected more directly by chemical composition (grade).

Toughness

Toughness (as considered for tooling materials), is the relative resistance of a material to breakage, chipping or cracking under impact or stress. Toughness may be thought of as the opposite of brittleness. Toughness testing is not as standardized as hardness testing. It may be difficult to correlate the results of different test methods. Common toughness tests include Charpy impact test and bend fracture tests, Fig. 4.

Fig. 4--Methods of toughness testing.

During impact testing, a small sample is held in a fixture and fractured by a moving impacter--such as a calibrated weight on a pendulum. Toughness is reported as the amount of energy that the sample absorbs before it breaks. This energy usually is expressed in foot-pounds or joules.

Brittle materials will absorb little energy before fracturing. In bend fracture testing, a fixtured sample is subjected to gradually increasing amounts of pressure--usually side or bending pressure--until it breaks.

Most tool steels are notch sensitive. This means that any small notch present in the sample will permit it to fracture at much lower applied impacts. Solid carbide is even more notch sensitive than tool steels. Thus, in addition to inherent material properties, the impact resistance of tooling components is impaired significantly by notches, undercuts, geometry changes and other common features of metalforming tools and dies.

In service, wear failures usually are less troublesome than toughness failures (breakage). Breakage failures can be unpredictable, catastrophic, interruptive to production and might present a safety concern. Conversely, wear failures usually are gradual and can be anticipated and planned for.

Fig. 5--Choosing tool steels for toughness.

Toughness failures may be the result of inadequate material toughness or a number of other factors, including heat treatment, fabrication (EDMing) or a multitude of operating conditions (alignment, feed, etc.). Toughness data is useful to predict which steels may be more or less prone to chipping or breakage than other steels (Fig. 5), but toughness data cannot predict performance life of tools.

Wear Resistance

Wear resistance is the ability of material to resist being abraded or eroded by contact with work material, other tools or outside influences (scale, grit, etc.). Wear resistance is provided both by the hardness level and the chemistry of the tool. Wear tests are quite specific to the environment creating the wear and the application of the tool.

Most wear tests involve creating a moving contact between the surface of a sample and some destructive environment. There are two basic mechanisms for wear damage in tooling.

Wear involving erosion or rounding of edges, as from scale or oxide, is called abrasive wear. Abrasive wear does not require high pressures. Abrasive wear testing may involve sand, sandpaper or various slurries or powders.

Wear from intimate contact between two relatively smooth surfaces, such as steel on steel, carbide on steel, etc., is called adhesive wear. Adhesive wear may involve actual tearing of the material at points of high pressure contact due to friction.

We sometimes intuitively expect that a harder tool will resist wear better than a softer tool. However, different material grades, used at the same hardness, provide varying wear resistance. For instance, O1, A2, D2 and M2 would be expected to show increasingly longer wear performance, even if all were used at 60 HRC. In fact, in some environments, lower hardness, high alloy grades may outwear higher hardness, lower alloy grades. Thus, factors other than hardness must contribute to wear properties.

Effect of Chemistry on Wear

Tool steels contain the element carbon, in levels from about 0.5 percent to more than 2.0 percent. The minimum level of about 0.5 percent is required to allow the steels to harden to the 60 HRC level during heat treating. The excess carbon above 0.5 percent plays little role in the hardening of the steels. Instead, it is intended to combine with other elements in the steel to form hard particles called carbides.

Fig. 6--Tool steel chemistry can have a major effect on wear.

Tool steels contain elements such as chromium, molybdenum, tungsten and vanadium. These elements combine with the excess carbon to form chromium carbides, tungsten carbides, vanadium carbides, etc., Fig. 6. These carbide particles are microscopic in size and constitute from about 5 percent to 20 percent of the total volume of the microstructure of the steel. The actual hardness of individual carbide particles depends upon their chemical composition. Chromium carbides are about 65-70 HRC. Molybdenum and tungsten carbides are about 75 HRC. Vanadium carbides are 80-85 HRC.

How Carbides Affect Wear

Embedded carbide particles function like the cobblestones in a cobblestone street. They are harder than the steel matrix around them and can help to prevent the matrix from being worn away in service. The amount and type of carbide present in a particular grade of steel largely is responsible for differences in wear resistance.

At similar hardnesses, steels with greater amounts of carbides, or carbides of a higher hardness, will show better resistance to wear. This accounts for differences in wear resistance among, say O1, A2, D2 and M4 steels. Ideally, tool steels would contain as much carbide volume as needed for the desired wear performance. In fact, "solid carbide" tooling consists of 85 percent or 90 percent tungsten carbide particles, in a matrix of 15 percent or 10 percent cobalt to hold them together.

Chemically, the microscopic carbide particles in tool steels are similar to the carbide particles in solid carbide tools. However, very high amounts of carbide particles can lead to problems in grinding, or lower toughness. More comments on the effect of carbides on toughness and grind ability will be discussed under the section subtitled, "Effect of Steel Manufacturing on Properties."

Because of their high hardness, vanadium carbides are particularly beneficial for wear resistance. When present in significant amounts, vanadium carbide tends to dominate other types of carbide when it comes to affecting wear properties. For instance, the chemical content of M4 HSS is nearly identical to that of M2 HSS, except M4 contains 4 percent vanadium instead of 2 percent. Despite the high levels of molybdenum and tungsten carbides (about 6 percent tungsten and 5 percent molybdenum) in each grade, the difference in vanadium content gives M4 nearly twice the wear life of M2 in many environments.

Fig. 7--Effects of carbide content on wear resistance.

By taking into account the alloy content and the affinity of each element for forming carbides, a rudimentary estimate may be made as to the likely wear life of a tool steel. Steels with high volumes of carbide particles or high hardness types of particles, usually exhibit the best wear resistance. Vanadium carbides, because of their hardness and chemistry, are the most effective at enhancing wear properties. Chromium carbides are among the least effective. Based on this information, relative factors may be assigned to the alloying elements, and a "vanadium equivalent" number generated. This vanadium equivalent gives an estimate of the relative amount and effectiveness of carbides present in a tool steel. As shown in Fig. 7, there is a reasonable correlation between this number, the results of wear testing and field performance.

Effect of Steel Manufacturing on Properties

The maximum practical limit of the amount of carbide-forming elements that may be added to a steel for wear resistance depends upon the ability to maintain a reasonable distribution of the resultant carbides throughout the microstructure of the steel. When steels are manufactured, they are melted in large batches containing the desired chemical composition. Batches are poured into ingot molds and solidify into castings that subsequently are forged or rolled into bars.

Carbides are formed during the solidification process. Under conditions of long, slow solidification, these carbides form large, segregated networks because they do not stay dissolved in the liquid steel. In addition, larger amounts of carbide particles will result in greater segregation and less uniformity in the steel microstructure.

Segregation causes two basic problems. First, areas of high concentrations of carbide particles may be difficult to grind--resulting in fabrication difficulties. Second, when segregated areas are physically elongated during rolling or forging, they result in a directionally oriented microstructure and reduce material toughness along the grain direction. Vanadium levels above about 3 percent are high enough to cause particular grinding and toughness difficulties. For this reason, despite its benefits for wear resistance, vanadium usually is limited to about 2.5 percent or less in conventionally manufactured tool steels.

P/M Tool Steels

In order to manufacture tool steels with high-wear resistance, without encountering the serious drawbacks caused by segregation, particle metallurgy processes are used to produce P/M tool steels having high vanadium content. P/M tool steels are atomized into fine droplets and solidified from liquid steel so rapidly that the carbides are prevented from forming into large segregated networks. The droplets form a powder, which is then consolidated into cans, bonded together under high pressure and forged or rolled into steel bars.

Fig. 8--Microstructure of P/M versus conventional tool steels

Carbides formed during the extremely rapid solidification of P/M tool steels are fine in size (2 to 4 microns), and uniformly distributed throughout the structure. This is in contrast to larger carbides (up to 50 microns or more in size), and the characteristic alloy segregation or banding in conventional tool steels. This is why a characteristic feature of P/M tool steels is their near-complete freedom from carbide segregation, Fig. 8.

Properties of tool steels

Fig. 9--Effects of carbide content on wear resistance.

Because the distribution of carbides in the microstructure of P/M steels is so fine and uniform, higher amounts of carbide-forming elements may be added. Thus, higher wear resistance may be developed, without the toughness and grindability limitations inherent in conventional steel making. The P/M process has allowed the development of grades containing 4, 5, 10 and even 15 percent vanadium--offering far greater resistance to wear than previous, conventionally produced tool steels, Fig. 9. Because of their high-wear resistance, these grades are particularly suitable for high-production operations.

Fig. 10--P/M tool steel toughness versus conventional steels.

In addition, the uniformity of the CPM microstructure provides improved toughness in CPM versions of conventional tool steels, Fig. 10. The CPM versions of grades are more resistant to brittle failures. In fact, most CPM grades designed for metalforming tools have impact resistance comparable to the lower wear resistance grades such as D2. Thus, in some cases, CPM steels may be used to provide both wear and toughness improvements over other tool steels.

Heat Treating High-Alloy Tool Steels

The heat treating process used to harden steels consists of heating them up to a high temperature (usually 1700 to 2200 deg. F), then quenching to near room temperature, and finally reheating to some intermediate temperature for tempering (300 to 1100 deg. F). A characteristic of low- to medium-alloy steels (A2, D2) is that they soften somewhat, from their maximum hardness, during the tempering process. The amount of softening depends upon the temperature exposure and the individual grade characteristics. To retain maximum hardness (above 58 HRC), A2 and D2 usually are tempered around 400 to 500 deg. 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. These steels are tempered above 1000 deg. F and retain their full hardness during this exposure.

Fig. 11--Hardness versus tempering temperature.

Many beneficial surface treatments, including nitriding, titanium nitride coating, etc., may be applied to tool steels to provide lower friction, better wear resistance or other enhancements. Most of these coatings are applied at temperatures in the range of 850 to 1050 deg. F. Thus, the treatment process can limit the service hardness of low- or medium-alloy steels. However, because the higher alloy content steels retain their maximum hardness after such exposures, normal surface-treatment temperatures have no effect on their hardness, Fig. 11. Thus, tools can be treated without fear of dimensional or hardness changes.

Choosing Tool Steels Based Upon Properties

As mentioned before, A2 and D2 are common steels used for metalforming tools. More highly alloyed grades offer better wear resistance. When choosing tool steel for a stamping 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 enhanced? What trade-offs may be required?

Hardness should be a concern for tools requiring high resistance to plastic deformation. Tools used for stamping steel generally need to be about 56-58 HRC minimum, although some form tools and tools for nonferrous material may be softer.

Most tool steels are capable of reaching roughly similar hardness levels (low 60s HRC), and thus will have similar abilities to resist plastic deformation. However, some high speed steels 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 generally is not an effective method for increasing the wear life of tools, but only for minimizing deformation.

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, even the lowest impact toughness tool steels are many times higher than solid carbide. (The toughness of carbide materials often is measured in inch-pounds, whereas the toughness of tool steels is measured in foot-pounds.) However, within the families of tool steels, some are much better for impact resistance than others.

Shock-resisting steels, such as S7 and A9, are formulated to offer optimum resistance to breakage. However, there is a basic difference in the heat treating process used for the two grades. S7 generally cannot be coated for improved surface wear properties because of its low tempering temperature. A9 typically is tempered above 900 deg. 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 T-15 or Rex 76 when high hardness is needed. These grades provide the closest wear and hardness properties to carbide, while offering the toughness properties of tool steels.

Other Factors to Consider

There are several other factors beside inherent material properties that often contribute to chipping or breakage failures. Tool steels are notch-sensitive materials. The presence of notches, undercuts, radii, changes in section or any geometric features will concentrate any applied stress in one area and exaggerate the tendency of a material to fracture. All reasonable precautions should be taken to avoid unnecessarily sharp radii.

In addition, in heat treated and EDMed tools, the EDMing operation can leave the wire-burned surface in a condition prone to chipping. Where EDMed tools are subject to chronic chipping or breaking problems, they should be stress relieved (tempered) after EDMing and before being placed into service. When practical, the EDM-affected layer should be removed.

Hardness and toughness may be considered "step" functions. This means that as long as the property is high enough to prevent damage (indentation or breakage), there is no advantage to increasing the property further.

However, wear resistance may be considered a "continuous" function. This means that continuing increases in the wear resistance of the steel will result in continuing increases in the life of the tool Thus, upgrading for wear resistance may always offer benefits, provided that other properties are not compromised.

When long-term abrasive wear resistance is desired in a tool, a steel with higher wear properties is in order. 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 A2 or D2 and carbide. In working with abrasive media, the CPM steels offer very high resistance to wear.

In applications that generate 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 nonmetallic coating (titanium nitride, titanium carbonitride or other related ceramic coatings). These coatings can reduce the coefficient of friction between the workpiece and the tool. This will reduce the risk of welding or galling wear. MF

Crucible CPM, 3V, 9V, 10V, 15V, Rex 76 and CruWear are trademarks or service marks of the Crucible Materials Corporation.