Tips for Choosing Tool Steels, Heat Treatment, and Surface Treatments

Tips for choosing tool steels, heat treatment, and surface treatments

By Nick Tarkany made on a much smaller scale with strict The chart in Figure 1 contains some of quality procedures and possess qualities the commonly used tool steels and their electing the proper tool steels, necessary to perform a specific task, such alloy content. While each alloy element heat treatment, and surface treat- as machining or perforating. listed in the table contributes to a specific S ments for stamping of coated Many different qualities in tool steels characteristic in the finished steel, it also materials can be a complex and confusing are sought based on a particular appli- can create an undesirable side effect, par- process. To simplify this process, you first cation need. These needs can be met by ticularly when used in excessive amounts. must understand a few basic facts about the adding a particular alloy along with the Additionally, alloy elements can react with available choices. appropriate amount of carbon. The alloy each other, which either can enhance or, in combines with carbon to enhance the some cases, detract from the final result. Tool Steel Analysis steel’s wear, strength, or toughness char- Figure 2 compares the three tool steel and Characteristics acteristics. These alloys also contribute characteristics necessary for stamping Tool steels are vastly different from to the steel’s ability to resist thermal and applications: toughness, wear resistance, steel used in consumer goods. They are mechanical stress. and compressive strength. While some Article reprinted with permission from the May/June 2000 issue of Stamping Journal®. steels possess exceptional values for one characteristic, they tend to have low val- Steel Type C Mn Si Cr W Mo V ues for one or both of the other two characteristics. A2 1.0 0.9 0.3 5.25 0 1.15 0.3 High-load stamping applications such as stainless steel; spring steel; and high- D2 1.5 0.5 0.3 12.0 0 0.9 0.9 strength, low-alloy steel call for tool steels that have a combination of shock resis- M2 0.85 0.3 0.3 4.15 6.4 5.0 1.9 tance and high compressive strength. M2 or PM-M4 tend to work the best in these M4 1.4 0.3 0.3 4.0 5.75 4.5 4.0 applications. Toughness. If toughness were the CPM 10V 2.45 0.5 0.9 5.25 0 1.3 9.75 only factor to consider in choosing a tool steel, S7 would be the obvious choice Figure 1 (see Figure 2). Unfortunately, this is not Alloys are combined with carbon to produce characteristics in common tool steels. This table lists the case. Tool steel toughness tends to the percentages of alloy content for some commonly used tool steels. decrease as alloy content increases. This increase in alloy content also demands a Heat-Treat Considerations higher price. 1. Segregating by size and material type. Toughness also is affected by the man- Each grade of tool steel has specific 2. Fixturing. ufacturing process of the steel. The par- heat-treat guidelines to acquire optimum 3. Preheating. ticle metallurgy (PM) process can enhance results for a given application. Stamping 4. Soaking (austenitizing). the toughness of a given grade of tool steel operations place a higher demand on 5. Quenching (martensite transforma- over its conventional counterpart. Note the toughness than do cutting operations. This tion). difference in toughness between M4 and means that a given grade of tool steel 6. Tempering. PM-M4 in Figure 2. should be heat-treated differently if it is to 7. Freezing (cryogenics). Wear Resistance. Increased alloy be used as a stamping tool versus a cutting content typically means increased wear tool. Segregation by size is extremely resistance, as illustrated in Figure 2. Per- Tool steels are only as good as the heat important, because items of different sizes forating coated materials places a high treat they receive. The keys to achieving require adjustments in preheat, soak, and demand on abrasive resistance. High- optimum results in heat treat include: quench rates. Fixturing ensures even sup- speed steels such as M2 and PM-M4, as port and uniform exposure to heating and well as high-alloy grades such as CPM- 10V, can provide the necessary wear resis- 10 tance. These steels also serve as a good 9 substrate for wear-resistant coatings. Toughness 8 Carbides are hard particles that provide Wear Resistance 7 wear resistance. They are suspended in Compressive Strength the matrix structure of alloy tool steels. 6 The majority of carbides are formed when 5 alloy additives such as vanadium, molyb- 4 denum, and chrome combine with carbon 3 as the molten steel begins to solidify. Greater amounts of carbide improve wear 2 resistance but reduce toughness. 1 O1 A2 S7 D2 M2 CPM-10V M4 Compressive Strength. Two factors 0 PM-M4 CPM-10V

— — — — — affect compressive strength: alloy content — HRC 60 HRC 61 HRC 60 HRC 62 HRC 62 HRC 58

and punch material hardness. Alloy ele- — HRC 62 ments such as molybdenum and tungsten — — HRC 60 HRC 63 contribute a great deal to the compressive strength. Additionally, the higher the hardness of a given grade of steel, the higher that steel’s compressive strength. Figure 2 Choosing the proper tool steel for a particular application involves balancing toughness, wear resistance, and compressive strength. This chart compares these properties on a conceptual scale of 1 to 10.

FABRICATORS & MANUFACTURERS ASSOCIATION, INTL. • 833 FEATHERSTONE RD. • ROCKFORD, IL 61107-6302 815-399-8700 • Fax: 815-484-7700 • Web Site: www.fmametalfab.org • E-mail: [email protected] cooling during the heat-treat process. Cold-work Tool Steel. During preheat- 4,000 ing of cold-work tool steels (A2, D2, etc.), parts are heated to just below the critical 3,500 austenitizing temperature of about 800 3,000 degrees C (1,450 degrees F) long enough to allow the part to be heated evenly. This 2,500 dness is necessary because when the part enters 2,000 the austenitizing temperature range, the s Har steel is restructured on an atomic level, ker 1,500 HRC 75 creating a volume expansion. If this vol- Vic ume expansion does not occur uniformly, 1,000 HRC 69 the part will distort and possibly crack. 500 Soaking (austenitizing) is heating the part into the carbide phase region for a 0 specified period of time. The intention is to Nitride TiN PVD TiN CVD TiCN PVD CrN PVD TiC CVD TD force some of the alloy elements into the matrix. Soaking cold-work steels such as Figure 3 A2 or D2 at temperatures in the high end Common surface treatments add extra amounts of hardness to a tool steel of their austenitizing range (overheating) or higher produces excessive levels of retained austenite and generates a coarse have good temper resistance, which processes. Fluidized bed, salt bath, and grain structure. This results in inferior allows them to be tempered at higher tem- gas are the most common and economi- toughness in the finished product. peratures. After quenching, the steels con- cal processes to apply nitride. Ion nitride Quenching is the sudden cooling of tain a majority of untempered martensite is a good process, but it tends to be more parts from the austenitizing temperature and about 30 percent retained austenite. expensive. Nitride surface treatments work through the martensite transfer range. This Retained austenite and untempered mar- in a broad range of applications. Salt bath transforms the steel from austenite to martensite contain a great deal of stress that nitride is the best from an application tensite, hardening the parts. Unfortunately, must be relieved or the tool will fail. point of view but has lost favor because of tool steels have a transformation range Tempering at 550 degrees C (1,000 environmental concerns. that is well below room temperature. This degrees F) or higher tempers the untem- Titanium nitride, titanium carbonitride, is one reason why cold-work steels benefit pered martensite and converts about half and chrome nitride applied using the phys- from freezing (cryogenics). of the retained austenite into untempered ical vapor deposition (PVD) work well on Tempering is necessary to remove martensite without reducing the part hard- precision tooling when it is used in specific stress associated with the hardening pro- ness below HRC 60. Because the higher applications. Titanium nitride offers better cess. Cold-work tool steels generally are tempering temperatures are enough to con- wear resistance than does nitride; however, tempered at 200 degrees C (400 degrees F) vert retained austenite to martensite, the it will encounter some difficulties when or less. Because of the low tempering tem- need for cryogenic treatment is reduced working with copper and stainless steel perature involved, one temper generally is significantly. applications. Titanium carbonitride provides adequate for cold-work tool steels. Standard heat-treat practice for high- greater wear resistance in a narrower range High-speed Tool Steel. High-speed speed tool steels calls for at least two tem- of applications. and high-alloy steels such as M2, PM-M4, pers; however, three tempers are needed Titanium nitride and titanium carbide, and CPM-10V require a somewhat differ- to bring the amounts of retained austenite applied using the chemical vapor deposi- ent approach to heat treating. Although and untempered martensite to acceptable tion (CVD) process and thermal diffusion the process initially appears similar to cold levels for stamping operations. (TD), work best in forming applications working of tool steels, the temperatures and that do not require high levels of preci- number of tempers differ. Surface Treatment sion. Because of the high processing Preheat temperatures begin at about Considerations temperatures involved, distortion and size 830 degrees C (1,525 degrees F), and soak Surface treatments often are used changes occur that limit the precision with temperatures can be higher than 1,100 to prolong tool life. These treatments which these tools perform. degrees C (2,000 degrees F). increase surface hardness and wear Surface Treatment Process Tempera- Because the soak temperature resistance while reducing the coefficient tures. Surface treatments can be applied approaches the melting point, control of friction. to a variety of substrate materials (tool of both time and temperature is critical. Common Surface Treatments. There steels) with varying results. Oversoaking a part can result in incipient are many surface treatments and treatment Cold-work tool steels such as A2 and melting—the alloys with lower melting processes from which to choose. D2 have tempering temperatures below points begin to melt within the structure, Nitride is a treatment that case-hard- PVD and nitride processing temperatures. damaging the steel’s grain structure. ens the surface of the substrate material. Exposure to these temperatures will draw High-speed and high-alloy tool steels This treatment can be applied by numerous down the hardness of cold-work tool steels to below HRC 58, which leaves the Surface treatments Coatings with higher hardness values, substrate steel vulnerable to deformation such as titanium carbide and TD, tend �� beneath the coating, creating adhesion to be thicker and must be applied with a can be applied to a �� problems for coating.�� Part growth and �� great deal of heat, which prevents their use �� distortion also� �become �� factors that affect �� in many applications. variety of substrate �� assembly of the tooling and precision of ��Figure 3 lists hardness values for the finished product.�� materials (tool steels) ��a number of coatings. Because these CVD process� � ��coatings are applied at �� coatings are extremely thin and nearly the high end of the austenitizing range for ��with varying results. ��undetectable on the HRC scale, their val- cold-work�� tool steels. Coarse grain struc- ues cannot be measured with a Rockwell ture�� and size changes should be expected �� hardness tester. in these circumstances, which will affect � � ��× �� × �� precision and toughness adversely. Coating�� Thickness. Coating thickness Conclusion TD is× a� �� unique process that uses becomes�� an issue in high-precision appli- Creating the best tool for a stamping the carbon� �� content × �� within × � the�� substrate cations.�� operation involves analyzing tool steels material� to �� form the coating and quenches ��Nitride is a process of case hardening to find one that provides the proper bal- the tool as part of its coating cycle. It can the existing surface of the part. Although ance of wear, strength, and toughness �� be applied to D2 steel. However, the sub- nitride�� does not build up on the sur- characteristics for a particular application. strate�� hardness typically falls below HRC face, the heat involved may cause slight Whatever grade of tool steel is selected  58,�� potentially reducing the strength of the growth�� in the part as a result of the matrix must be heat-treated properly to capital- part. accommodating the diffusion of nitrogen. ize on those qualities and provide optimal PVD process coatings and nitride work PVD coatings are relatively thin and results in production. well with high-speed and high-alloy tool cover only areas within the line of sight of A variety of surface treatments also are steels such as M2, M4, and CPM-10V. the coating source. Precision of the coated available to increase surface hardness and The PVD process temperatures fall more area generally is maintained, and the fit at wear resistance while reducing the coef- than 30 degrees C (50 degrees F) below tool assembly typically is not affected by ficient of friction of tool steels, helping to the tempering temperature of high-speed this process. prolong tool life. Understanding the avail- and high-alloy tool steels, nearly eliminat- CVD and TD coatings are thicker and able tool steel options is the first step in ing distortion and growth in the part. cover an entire part, affecting the preci- achieving quality results. Because of the high processing tem- sion of the working end and its fit in a perature of CVD coatings and TD, post retainer. Portions of punches may need to Nick Tarkany is Director, Research & heat treat commonly is required to achieve be stripped and reworked to assemble the Technical Education, with Dayton Progress acceptable hardness of the substrate tooling. Corp., 500 Progress Road, Dayton, Ohio material. Distortion and growth are to be Surface Treatment Hardness. Hard- 45449, phone 937-859-5111, fax 937-859- 5353, Web site www. daytonprogress.com. expected. ness provides an indication of wear resis- Dayton Progress Corp. has been a producer Because it uses carbon that is already tance and lubricity for a given coating. of high-precision punch and die components present in the substrate material, TD works PVD process coatings and nitride enhance for more than 50 years. Originally presented well for steels with high carbon content, the life of precision high-speed and high- at FABTECH® International 1999, November such as M4 and CPM-10V, in low- to alloy tool steels but will not cure wear 14-18, 1999, Chicago, Illinois, co-sponsored by Fabricators & Manufacturers Association, moderate-precision applications. TD is not problems caused by tight die clearances or International, and Society of Manufacturing recommended on M2 because of that tool prevent punches from bending under high Engineers. steel’s relatively low carbon content. loads.