Tool Steel Grades

Steel tools empower humanity to shape wood, plastic and metal into the billions of items that enhance the quality of life. With tool steel, two words loom large: edge and impact. Steel alloys with relatively high carbon content are easier to sharpen to a fine edge, and this sharpness can endure the rigors of production. Conversely, low-carbon steel is difficult to bring to a fine edge but stands up to repeated impacts. To select the correct tool steel type for a project, it is helpful to understand the grades and their place in the manufacturing process.

Types of Tool Steel

Like all steels, tool steel contains iron, carbon, manganese, phosphorus and sulfur. From that starting point, alloying metals and a range of manufacturing processes combine to yield tool steel grades in six letter-coded categories:

  • W-Grade: Water Hardening
  • O-Grade: Oil Hardening
  • A-Grade: Air Hardening
  • D-Grade: D Type
  • S-Grade: Shock Resisting
  • H-Grade: Hot-Working

Each letter grade in turn contains several alloy types denoted by numerals, for example, A2 and O1.

Tool Steel Grades

Depending on a project’s budget and use case, any one of these many types may prove to be the best fit.

Water Hardening: W-Grade

The oldest and least expensive tool steel grade takes its name from the water-quenching step of the manufacturing process. Quenching is the essential rapid cooling stage that unites alloy and iron molecules in hardened steel. W grade types include none of the alloy elements present in other tool grades, and the water hardening process thus produces steel with a soft core and high hardness. With careful annealing, W-grade steel boasts the highest machinability of all tool steels. Thanks to their high carbon content, some W-grade types need no further heat treatment. W-grade types soften above 300 degrees F and are poorly suited for high-temperature uses.

Uses of W-Grade tool steel include:

  • Cutlery
  • Embossing taps
  • Engraving stamps
  • Drill bits
  • Circular cutters
  • Razor blades
  • Wood lathe tools

Oil Hardening: O-Grade

Similar to the W grades, O grades are a relatively inexpensive high-carbon steel. O-grade types typically contain traces of chromium and manganese to aid hardening. Compared to water quenching, oil quenching allows for slower cooling in the production stage. This gradual cooling produces a tool steel much less likely to distort in the heat treatment production phase. The resulting alloy is also less brittle.

Type O1, the most popular O-grade alloy, is a general-purpose tool steel that holds an edge when sharpened. In a manufacturing setting, this alloy is popular for jigs, guides, punches and trim dies. O1 steel is weldable, but the alloy requires careful heating to avoid cracks.

The type O6 alloy includes silicon and a higher chromium content. This highly machinable alloy exhibits exceptional metal-to-metal wear performance. Type O6 also retains oil when lubricated, making the alloy an ideal choice for thread cutting. The O6 alloy works well for cams, bushings, shear blades and granulator plates.

Air Hardening: A-Grade

Air-hardened tool steels offer an admirable balance of price, strength, toughness and machinability. With diameters up to 100 mm, a 5% chromium content allows the A-grade tool steel types to harden by air or pressurized gas. Air-hardened tool steel types maintain low distortion during the heat treatment phase of their production. This technique results in a steel that offers better wear resistance than O-grade types with a lower cost than the high-chromium D-grade.

A2 is the most-used A-grade tool steel, and its typical applications include woodworking cutting tools, medium-duty punches and hammers. Type A2 also serves in plastic injection molding for less complex molds. Type A7 uses chrome and vanadium to produce wear resistance with a 50% improvement on its A2 cousin. Able to withstand wear from sand, gravel and slurry, type A7 is well-suited for powder compaction tooling, ceramic extrusion tools and brick mold liners.

Other uses for A-grade types include:

  • Lathe center knives
  • Industrial die bending forms
  • Gauges
  • Industrial knives
  • Blanking dies
  • Mandrels

D-Types: D-Grade

With 1% more carbon and nearly triple the chromium content of A-grade tool steel, D-grade types excel in high-temperature applications. Some D-grade alloys include molybdenum, cobalt or vanadium to push thermal performance even higher. Able to maintain their hardness in temperatures approaching 800 degrees F, the D-grade tool steels are ideally suited for die casting, die bending, and forging dies. Along with high heat resistance, the D types resist wear from sliding contact with other metals. This abrasion resistance enables D-grade tool steel to function as drawing dies for bars, tubes and wires. Steel producers employ both air and oil quenching for their D-grade products.

Other applications for the D types include:

  • Forging dies
  • Automobile engine valves
  • Lamination tools
  • Knurling tools
  • Food processing knives

Shock Resisting: S-Grade

A low-carbon steel designed to stand up to constant high impacts, S-grade tool steel is the material of choice for jackhammer bits and heavy-duty punches. Silicon, chromium and tungsten give the S-grade alloys their impact resistance, but at the price of low abrasion resistance. Steel producers use oil hardening for S-grade steel up to 60 mm in diameter, and water hardening for larger parts. Beyond impact resistance, S-grade tool steels also excel across a broad temperature range and offer above-average machinability.

Type S7 steel is a sound choice for cold work tools including shear blades, chisels and punches. Type S7 is also an option for tooling applications below 1,000 degrees F. With the highest impact resistance among S-grade steels, type S5 is the premier choice for pneumatic tool clutches and stamping dies. Other applications for the S-grade types include:

  • Tableware dies
  • Nail gun hammers
  • Wood chipper knives
  • Power drill chuck jaws
  • Swaging dies
  • Quarrying implements

Hot-Working: H-Grade

The H-grade types are low-carbon tool steels well suited to extreme temperature demands. That quality derives from the steel’s unique production step. The hot-working process heats steel above the metal’s recrystallization temperature. The molecular changes induced by this heating produce tool steels with exceptional uniformity. While the hot-working process does not improve the metal’s innate tensile strength, the uniformity substantially reduces the risk of cracks with H-grade steel.

Among tool steels, H grade types are the champions of red hardness, the quality of retaining hardness when heated to a dull red color. Three alloying metals fine-tune the H-grade types for different manufacturing missions:

  • Chromium: Types H1 to H19 feature 5% chromium, yielding an alloy able to withstand rapid heating and cooling cycles. This resistance to thermal fatigue makes H-grade chromium alloys the metal of choice for hot forging dies. In the die casting process, these alloys perform as inserts, cores and cavities. With plastic injection molding, chromium-alloyed H-grade tool steel excels for cavity molds.
  • Tungsten:Types H20 to H39 use tungsten content ranging as high as 18%. This alloy can withstand prolonged contact with hot metals, enabling its use in extrusion dies for brass, nickel and steel. The tungsten alloy H-grades shine for mandrel production.
  • Molybdenum:With types H40 to H59, 5% molybdenum content delivers an alloy for extreme temperature conditions. With high corrosion resistance and excellent abrasion resistance, the molybdenum alloys excel in forming dies and punches.

Conclusion

Choosing the correct tool steel grades is vital for a project’s success. The choice of tool manufacturer is equally as important. Special Tool & Engineering‘s in-house capabilities have enabled us to meet the tooling needs of the world’s most demanding manufacturers for more than two decades.

 

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