Tool Steel Alloys: Composition and Alloying Elements
Introduction
Tool steels comprise a versatile category of engineering alloys specially optimized for manufacturing cutting, forming, and shaping tools. Their unique properties are derived from careful control of chemical composition and sophisticated heat treatment.
The various tool steel grades available today contain a range of alloying elements that influence their hardness, strength, toughness, and high temperature behavior. This article will provide an in-depth look at the major alloying additions utilized in tool steels and how they impact properties and performance.
Importance of Alloying Elements in Tool Steels
Unlike plain carbon steels, tool steels depend on significant amounts of alloying elements to achieve their characteristic properties:
- Hardness – High hardness for cutting, shearing, and forming is enabled by carbide-forming alloys
- Wear resistance – Hard carbides of vanadium, molybdenum, and chromium impart excellent abrasion resistance
- Toughness – Elements like manganese, nickel, and cobalt affect the ductility and fracture resistance
- High temperature strength – Hot hardness at elevated temperatures is provided by tungsten, molybdenum, and cobalt
Very precise control over the types, forms, and amounts of alloying additions allows tool steel properties to be tailored as needed for a wide array of tooling applications.
Major Alloying Elements in Tool Steels
The following are the primary alloying elements utilized in tool steel compositions:
Carbon
The most essential hardening element, typically present at 0.7-1.5% levels in tool steels. Fundamental to developing hardness through heat treatment. Also combines with other elements to form hard carbides.
Tungsten
Added in levels up to 18% in high speed tool steels. Provides hot hardness and toughness. Forms hard tungsten carbides. Enhances abrasion resistance at high temperatures.
Molybdenum
Added up to 10% in many tool steel alloys. Increases hardenability and strength at elevated temperatures. Contributes to wear resistance through molybdenum carbides.
Chromium
Added up to 5% to increase hardenability, impart corrosion resistance, and improve wear resistance through chromium carbides.
Vanadium
Added up to 4% to form extremely hard vanadium carbides which enhance abrasion resistance and help maintain hardness.
Manganese
Present up to 1.4% to increase hardenability while also improving toughness. Combines with sulfur to form manganese sulfides.
Cobalt
Added up to 10% in certain alloys for additional high temperature strength and hot hardness.
Nickel
Added in some alloys up to 5% to increase toughness and impact strength. Also enhances hardenability in lean alloys.
Effects of Key Alloying Elements
In addition to their primary functions, some other effects of the main alloying elements include:
Carbon – Raises tensile strength but also increases brittleness. Reduces weldability.
Chromium – Contributes to tempering resistance and secondary hardening. Slightly lowers thermal conductivity.
Manganese – Acts as a mild deoxidizer during steelmaking. Improves machining characteristics.
Molybdenum – Helps resist softening at high temperatures. Promotes fine grained microstructure.
Vanadium – Strong carbide former to control grain size. Creates clean steel.
Tungsten – Provides resistance to thermal shock cracking. Enhances hardenability.
Cobalt – Augments tensile strength and heat treat response. Intensifies hardness.
Nickel – Boosts toughness. Adds corrosion resistance. Improves hardenability in lean alloys.
The optimal mix of these and other elements creates the balanced combination of hardness, toughness, and hot strength needed.
Alloying Elements in Specific Tool Steel Grades
Looking closer at a few of the most common tool steel types illustrates how alloy content is adjusted to meet the requirements of different applications:
A2 Air Hardening Tool Steel
- 1.0% carbon, 5% chromium – deep hardening alloy steel
- Molybdenum, vanadium, and iron
- High hardness capability with good toughness
D2 High Carbon High Chromium Tool Steel
- 1.5% carbon, 11-12% chromium – very high hardness
- Molybdenum, vanadium, tungsten
- Excellent wear resistance and dimensional stability when hardened
M2 High Speed Tool Steel
- 0.85% carbon, 4% chromium, 5% molybdenum, 6% tungsten
- 2% vanadium, traces of other alloys
- Outstanding resistance to softening at elevated temperatures
H13 Hot Work Tool Steel
- 0.4% carbon, 5% chromium, 1.5% molybdenum, 1% vanadium
- Resistance to thermal fatigue and thermal shock cracking
S7 Shock Resisting Tool Steel
- 1% carbon, 3.25% chromium, 2% manganese, 1% silicon
- Superior toughness and ability to resist impact and shock loading
The unique alloy makeup of each grade tailors properties to specific tooling needs.
Effects of Impurities in Tool Steels
While tool steels focus mainly on alloy content, impurities can also influence properties:
Sulfur – Causes brittleness and hot shortness. Typically kept below 0.03%.
Phosphorus – Degrades low temperature impact toughness. Maintained below 0.03%.
Oxygen – Affects cleanliness. Kept to 5-25 ppm range.
Nitrogen – Can improve strength when controlled in PM tool steels. Usually limited to 50-100 ppm.
Hydrogen – Causes brittleness and cracking. Minimized through vacuum degassing.
Controlling impurity levels through modern steelmaking is crucial for achieving clean tool steel chemistries.
Key Points on Tool Steel Alloy Compositions
In summary, some important points regarding tool steel alloys:
- Alloying elements enable key properties like hardness, strength, toughness and high temperature behavior
- Carbon is the primary hardening constituent along with other carbide formers
- High speed steels utilize large amounts of tungsten, molybdenum, and cobalt
- Cold work steels contain chromium, molybdenum, and vanadium additions
- Alloy content must be carefully balanced to create an optimal combination of properties
- Even trace residuals like sulfur and phosphorus are kept low to prevent embrittlement
- Each tool steel grade has a tailored chemistry matched to its intended function
Understanding how alloy composition impacts tool steel performance allows the identification of the proper grade for an application.
Innovations in Tool Steel Alloy Development
Advancements in tool steel alloys focus on creating grades with improved combinations of hardness, toughness, and high temperature resistance:
- Powder metallurgy – Allows cleaner compositions with finer carbides
- Microalloying – Small additions of elements like niobium or titanium for enhanced properties
- Coating alloys – New grades designed for optimum compatibility with protective coatings
- Nitrogen alloying – Careful nitrogen additions improve strength and toughness
- Alloy modeling – Computer modeling predicts effects of alloy changes before testing
- Laves phase control – Optimizing Laves formation improves toughness in PM tool steels
- Forging vs ingot cast – Different solidification results in altered alloy performance
Ongoing alloy research and development continues to push the property profiles of tool steels to new levels.
Applications Determining Optimal Tool Steel Alloys
The wide range of tool steel applications necessitates different alloy requirements:
Cold Forming Tools
Require maximum hardness for shear strength and wear resistance. Often utilize D2, A2 or M2 alloys.
Hot Forming Dies
Need retained hardness and strength at elevated temperatures. H13, H11 or H19 tool steels commonly used.
Metal Cutting Tools
Demand hot hardness along with fracture resistance. High speed steels like M2 or M42 employed.
Molds and Dies
Need good polishability along with hardness for mold finish. P20 or 420 stainless steels often selected.
Blanking and Punching
Require high toughness for shock resistance. S7 alloy commonly utilized.
Matching tool steel alloy compositions to the demands of each unique function ensures success.
Frequently Asked Questions About Tool Steel Alloys
What are the main differences between tool steels and carbon steels?
Tool steels contain significant alloying elements including chromium, tungsten, molybdenum, vanadium and cobalt to provide hardness, strength and wear resistance that carbon steels do not offer.
What primary factors determine the alloy content of a tool steel?
The primary desired properties like hardenability, toughness, high temperature behavior, as well as considerations like cost and availability drive the optimal alloy selection.
How do alloys like tungsten improve tool steel hot hardness?
Tungsten forms hard tungsten carbides that help maintain hardness at elevated temperatures encountered in hot metal forming and other tooling uses.
What are secondary effects of common tool steel alloys?
In addition to their primary purposes, alloying elements like nickel, manganese, and chromium also influence properties like toughness, hardenability, machinability, and corrosion resistance.
How are impurities like sulfur and phosphorus controlled in tool steels?
Careful selection of scrap materials and melting practices, along with ladle metallurgy and inclusion shape control methods, minimize impurity levels.
What innovations are improving tool steel alloy development?
Powder metallurgy, computer modeling, microalloying, new carbide engineering methods, and advanced processing are enabling alloy improvements.
I hope this overview provides helpful knowledge on the critical role alloying plays in designing tool steel grades to meet demanding tooling applications! Please let me know if you have any other questions.
